Manual - Px script v3.24
1. Purpose
The main purpose of the Px script is to test the mechanical part of the engine, as well as checking the consistency of its subsystems. The work of the script is based on the automatic analysis of the cylinder pressure waveform, obtained using a sensor installed in place of the spark plug. The script allows you to measure characteristics and check the condition of the following vehicle components and systems:
- pneumatic density of the cylinder at idle speed and above
- geometric compression ratio
- real values of opening and closing angles of intake and exhaust valves
- the influence of all components of the intake tract on the amount of air / mixture that remains in the cylinder at the moment the intake valves close
- checking the throughput of the exhaust system for compliance with this particular engine
- checking the correctness of the ignition timing values both on stationary, and in transient modes of engine operation, taking into account the design features of the tested engine
In the process of analyzing the cylinder pressure waveform and the synchronization signal from the ignition event, the Px script generates several report tabs, where it displays a number of measured and calculated engine characteristics. In addition, this tool evaluates the mutual influence of all the above systems and parameters on each other and checks the consistency of their operation within the engine.
If any malfunctions, inconsistencies or deviations from the average statistical norm are found, the Px script automatically displays the appropriate notifications.
2. Recording the waveforms
To obtain the cylinder pressure waveform of a gasoline engine, a Px35 pressure transducer is used, which is installed in place of the spark plug of the diagnosed cylinder. If necessary, you can also use one of the compatible deep well adapters – PxLonger M14, PxLonger M12, or flexible PxFlex extension.
To obtain the most informative and complete report of the Px script safely, the recommendations given here should be followed.
- The end of the spark plug wire / boot or the ignition coil normally connected to the removed spark plug belonging to the cylinder under test must be connected to a high-voltage Spark Gap with a spark gap set to about ~5 mm. Otherwise, the vehicle's ignition system may be damaged. The spark gap housing must be connected to the engine block.
- To avoid overheating of the cylinder pressure transducer the duration of the test should not exceed three minutes.
- If the diagnosed engine is equipped with a multiport fuel injection system, it is recommended to disconnect the connector from the gasoline injector of the tested cylinder and connect a 100 Ω resistor to it. Due to this, during the measurements, fuel will not enter the cylinder being tested, and the connected resistor will avoid setting a trouble code like "Open circuit of the injector № …". If the injector is not disconnected, unburned fuel will get into the cylinder and into the exhaust system, which may cause overheating and damage to the catalytic converter. Besides, if self-ignition of the air-fuel mixture in the cylinder being tested happens, the pressure transducer may be damaged.
If it is not possible to disable fuel delivery to the diagnosed cylinder, then in order to avoid damage to the catalytic converter due to contact with a large amount of unburned fuel, it is recommended to reduce the duration of measurements to a minimum. To reduce the likelihood of self-ignition in the tested cylinder, allow the combustion chamber to cool by removing the spark plug, wait about 5 minutes before installing the pressure transducer into the cylinder. - If the engine is equipped with two spark plugs for each cylinder, then during the test, the end of the spark plug wire/boot or the ignition coil must also be disconnected from the second spark plug of the tested cylinder and connected to another spark gap. This must be done in order to exclude the possibility of ignition of the air-fuel mixture in the tested cylinder. Otherwise, not only incorrect test results may be obtained, but there is also a risk of the pressure transducer being damaged.
- When testing engines equipped with an electronic throttle actuator, to stop the engine at the last step of the measurements, instead of using the ignition switch, it is recommended to use the Remote Power Off fuse switch, which allows you to stop the engine without turning off the ignition switch. This is required so that the throttle can remain fully open from the moment the engine is turned off until the moment when the engine completely stops rotating due to inertia (in this case, the accelerator pedal must be held fully depressed). Following this procedure allows more accurate and complete diagnostic information when analyzing this waveform file.
2.1 Connection order
The connection procedure is shown only for USB Autoscope IV .
- The end of the spark plug wire / boot or the ignition coil normally connected to the removed spark plug belonging to the cylinder under test must be connected to a high-voltage Spark Gap with a spark gap set to about 5 mm. Otherwise, the vehicle's ignition system may be damaged. The spark gap housing must be connected to the engine block.
- Remove the spark plug from the cylinder being tested and install the pressure transducer in its place. If necessary, use an extension.
- Connect the black alligator clip of the power cable of the motor tester to the tested vehicle's body, and the red alligator clip to the "+" terminal of its battery and make sure that there is a secure electrical connection - the "12 V" indicator on the front panel should light up.
- Connect the universal cable to the output connector of the pressure transducer and connect it to input №3 of the motor tester.
- Connect a high voltage Sync probe to the "In Synchro" input on the front panel of the motor tester and attach it to the high-voltage wire of the cylinder being tested.
Figure 1. Connection of the px35 pressure transducer and Sync probe
If the ignition system is using COP (Coil on Plug) an additional plug wire is needed. A plug wire can be made from ignition cable attached to the spark plug adapter or simply use a spare spark plug wire. Using the extra spark plug wire, connect the ignition coil to a spark gap. The Sync probe should be installed on this wire.
Figure 2. Using the extra spark plug wire, connect the ignition coil to a spark gap
If the engine being tested is equipped with an electronic throttle, then it is recommended to use the remote fuse switch additionally (see paragraph 5 of section 2 for more details). The switch must be connected in place of the electrical fuse for the fuel injectors and/or fuel pump of the diagnosed vehicle.
2.2 Waveform recording order
- In the USB Oscilloscope window, call the "Modes => Px" menu and select the appropriate mode (depending on which pressure transducer is used and whether an extension is used)
- In the USB Oscilloscope window click "Record"
- Start the engine and allow it to idle
- After 3…5 seconds slowly raise the engine speed to 3000…5000 RPM using as little throttle as possible, then close the throttle
- After the idle speed has stabilized, perform a snap throttle by depressing the accelerator pedal as quickly as possible. After reaching the engine speed to 3000...5000 RPM, turn off the engine, but at the same time continue to hold the accelerator pedal fully depressed until the rotation of the engine stops completely. At the same time, you can turn off the engine in one of two ways: by turning off the ignition switch or using a remote fuse switch (for more details, see paragraph 5 of section 2)
- To stop recording signals, click "Stop"
- To save the recorded signals, click "File => Save file"
3. Running a script
3.1 Script configuration window
In order to start the analysis of the recorded signals, in the USB Oscilloscope window, click "Analysis => Run script". In this case, the script configuration window will be displayed on the screen.
Figure 3. Script configuration window
Here you will need to select:
- Pressure transducer channel number
- Type of pressure transducer used
- Valve Timing Type
Analysis of signals by the script algorithm will start after clicking the "OK" button.
Note that the script analyzes the entire recorded waveform file. If a part of the waveform is selected, the script will analyze only this selected section of signals.
3.2 Valve timing types
In the drop-down list of the last item of the script configuration window, several different valve timing types are available. Let's consider each of them.
- Engines with fixed valve timing (DOHC / SOHC / OHC / OHV)
These are engines with the simplest designs of the gas exchange system, which were previously common everywhere, but now are used only on budget cars. On these engines, the camshaft(s) are connected to the crankshaft by means of a drive belt, chain or gear system directly, without any additional devices capable of changing the valve timing during engine operation. Thus, the opening and closing angles of the valves are constant here, and do not change during engine operation. This is the reason for both the main advantage of such engines - the simplicity of design, and its main drawback - not the highest rates of power per liter and torque.
When developing classic Otto engines, at the stage of choosing the values of the valves opening and closing angles, the manufacturer is forced to compromise. This is because, on the one hand, early valve timing is required to obtain high torque at low revs, and on the other hand, late valve timing is required to obtain maximum power at high revs. But, since there is no mechanism for changing the valve timing, the manufacturer has to set the average values of the valve timing.
The fact is that when the engine is running at high speeds, when the piston reaches the bottom dead center (BDC), the process of flowing the air-fuel mixture from the intake manifold into the cylinder actively continues, provided that the intake valve remains open. This is due to the inertia of this flow. Closing the intake valve at this moment would not be optimal, because in this case this flow would be forcibly interrupted, and a much smaller amount of the working mixture would be in the cylinder than it could be. And if a smaller amount of fuel in the air mixture burns in the cylinder, then the amount of mechanical energy developed by the engine will be less than it could be. Therefore, on engines that are designed to be operated mainly at high speeds, such as engines in sports cars, the valve timing is chosen so that the intake valves close very late - at the moment when the piston has already traveled a significant part of its path from BDC towards Top Dead Center (TDC). This is done because when the engine is running at high speeds at this moment, the inertial movement of the flow of the air-fuel mixture from the intake manifold to the cylinder stops, and if the intake valves are closed at this moment, the cylinder turns out to be filled with the fresh mixture as much as possible, which means that the engine develops high power at these speeds. This effect is called "Dynamic Boost".
But the effect of dynamic boost is observed only at high speeds. When the same engine with late timing phases is running at low speeds, when the inertia of the air-fuel mixture flow is weak, the opposite effect will be observed: the piston moving from BDC to TDC will push out a significant part of the air or air-fuel mixture (depending on the type of fuel supply system) from the cylinder back to the intake manifold through the intake valve that did not close in time. As a result, the amount of mixture burned in the cylinder will noticeably decrease, which means that the torque developed by the engine at these speeds will also decrease. Therefore, when designing specialized engines that must be operated mainly at low speeds, the valve timing is chosen so that the intake valves close very early – immediately after the piston has passed BDC.
Manufacturers of classic Otto engines for budget cars are forced to set the average valve timing, which will not be too noticeable on the one hand, a lack of torque at low revs, and on the other hand, low engine power at high revs. The optimal opening and closing angles of the valves here will only be when the engine is running at medium speeds and medium loads.
It should be noted that in order to control the amount of cylinder fill, many manufacturers adjust the resonant characteristics of the engine intake tract, selecting the optimal shape and size of its internal cavities, and sometimes equipping it with systems for changing the geometry of the intake tract.
- Otto cycle + Variable Valve Timing (VVT)
These are presently the most popular engines designs. They differ from classic Otto-cycle engines by the presence of one or more VVT timing gears, which allows the control unit to regulate the valve timing directly during engine operation, setting such opening and closing angles of the intake and/or exhaust valves, which will be the most optimal for the current engine operating mode both in terms of engine power and torque, and in terms of fuel economy and pollution levels.
When the engine is running under high load at low speeds, the control unit, using the valve timing control mechanism, sets the early closing angle of the intake valves, thereby preventing part of the air / mixture from being pushed back into the intake manifold from the cylinder, and thus achieving an increase in torque.
When the engine is running under heavy load at high speeds - the engine control unit sets the closing angle of the intake valve late in order to get the most out of dynamic boost in order to achieve an increase in engine power.
When the engine is running under low load the control unit will set the closing angle of the intake valve late, in order to achieve the maximum effect of pushing part of the air / mixture back into the intake manifold. This allows, at the same engine load, to slightly open the throttle, thereby reducing the level of vacuum in the intake manifold, which in turn reduces "Pumping losses", which in conventional gasoline engines are one of the main reasons for low fuel efficiency (economy) compared to diesel engines. Due to this, fuel consumption is reduced when the engine is operated in this mode.
When the control unit changes the closing angle of the intake valve, the opening angle of the intake valve also automatically changes, since the width of the valve timing in this type of engine is a constant value. A change in the opening angle of the intake valve, in turn, leads to a change in the valve overlap phase. The valve overlap phase is the state of the gas exchange system when, at the end of the exhaust phase / the beginning of the intake phase, the exhaust valve has not yet closed, and the intake valve has already begun to open. Thus, during the closing phase, both valves are partially open. Due to this, the inner cavity of the intake manifold is partially connected to the inner cavity of the exhaust manifold (through the ajar intake valve, the combustion chamber and the ajar exhaust valve). The duration and magnitude of this connection affects many engine parameters. This effect is due to two phase-dependent valve overlap phenomena: a process called "Internal Exhaust Gas Recirculation" and resonant vibrations in the intake and exhaust systems.
Exhaust gas recirculation is the process of diluting the air-fuel mixture with exhaust gas. There is external recirculation (when part of the exhaust gases is directed from the exhaust manifold through a special valve into the intake manifold), and internal (when, after the exhaust valve is closed, a certain volume of exhaust gases remains in the cylinder for several reasons). The proportion of exhaust gases in the composition of the working mixture affects the amount of nitrogen oxides in the exhaust gases and the fuel efficiency of the engine. The amount of nitrogen oxides decreases due to a decrease in the combustion temperature of the working mixture, and the temperature, in turn, decreases with an increase in the proportion of exhaust gases in the mixture. The fuel efficiency of the engine is improved by reducing pumping losses by decreasing the amount of vacuum in the intake manifold. This happens because with an increase in the proportion of exhaust gases in the working mixture composition, in order to maintain the same level of torque on the engine crankshaft, the control unit has to slightly open the throttle. Due to this, despite the fact that the amount of exhaust gases in the mixture has increased, the amount of air in it remains at the same level, and the engine power output is maintained. But, along with positive effects on engine performance, exhaust gas recirculation may also negative effects. Firstly, with an increase in the amount of exhaust gases in the working mixture composition, the ability of this mixture to ignite from an ignition spark decreases, up to the occurrence of misfires, which is an extremely negative phenomenon (fuel is simply pumped into the exhaust system). Secondly, part of the volume of the cylinder, where the air-fuel mixture could be located, is occupied, due to which the total amount of fresh mixture in the cylinder decreases, which means that the engine output power also decreases, which can be critical when the engine is running under high loads. In extreme cases, when due to a particular malfunction, the internal recirculation has increased very much, cyclic misfires can be observed in the operation of the cylinders. At the same time, it seems that one of the cylinders does not work at idle, although in fact - misfires occur in all cylinders with disrupted gas exchange: each cylinder alternates between running better and worse.
Resonant oscillations in the intake system affect dynamic supercharging, while resonant oscillations in the exhaust system influence the degree of cylinder scavenging by utilizing an effect known as "Combustion chamber purge". Proper tuning of each of these processes allows you to achieve an increase in the engine's power per liter. And both of these processes depend on the phase of valve overlap.
It follows from the above that the valve overlap phase control mechanism is an important tool to improve many engine characteristics. This was one of the reasons why manufacturers began to equip engines with an additional VVT timing gear, designed to control only the exhaust camshaft. Thanks to this, the control unit in such engines gained the ability to independently from the intake phase adjust the exhaust phase, as well as the valve overlap phase.
The exhaust phase control mechanism additionally made it possible to set the optimal moment for the start of the opening of the exhaust valves. This allowed shifting the exhaust valve opening moment toward a "Later" timing, further improving the engine's fuel efficiency in operating modes where it was feasible. This was another reason why manufacturers began to equip the exhaust camshaft with an independent VVT timing gear.
- Miller / Atkinson cycle
The main difference between these types of engines is high fuel efficiency. But, due to their structural complexity, Atkinson cycle engines are not mass-produced and exist only in the form of experimental samples. At the same time, Miller cycle engines have recently become widespread. This happened for several reasons. Firstly, Miller cycle engines do achieve high fuel efficiency (but not as high as Atkinson cycle engines). Secondly, the design of the Miller engine is almost identical to the design of modern Otto engines with the VVT system, due to which it became possible to produce them simply as a modification of conventional engines. Thirdly, a technology was introduced into mass production that almost completely eliminates the main drawback of these engines— power per liter: the hybrid powertrain technology (where a gasoline-powered vehicle's powertrain is supplemented with an electric vehicle’s power system, albeit with a relatively small-capacity traction battery). Thus, a Miller-cycle engine within a hybrid gasoline-electric powertrain is a highly viable alternative to an Atkinson-cycle engine, which is now widely used in modern production vehicles.
Let's get back to the main distinguishing feature of the Miller cycle engine. High fuel efficiency is achieved here due to the specific settings of the timing phases: the gas exchange valves here open and close very late. This leads to the fact that the phase of gas compression in the cylinder is very short, while the phase of the power stroke, on the contrary, is very long. The result is almost similar to the Atkinson cycle engine, but achieved by a much simpler design solution. According to the theory of thermodynamic cycles of the internal combustion engine and according to the results of practical tests, engines with a short compression phase and a long stroke phase have high fuel efficiency. But in the Miller engine, this result is achieved due to a strong lengthening of the intake phase. It turns out that at the end of the intake stroke, when the piston reaches BDC and the compression stroke begins, the intake phase is still quite long and the intake valves remain open until the piston has traveled about half of its path from BDC towards TDC. This means that most of the air / mixture that was drawn into the cylinder during the first part of the intake phase during the intake stroke will now be pushed back into the intake manifold during the first half of the compression stroke when the intake phase is still ongoing. As a result, a significantly smaller amount of the air-fuel mixture will remain in the cylinder and combust than what could have burned. This means that much less energy will be produced compared to an engine with the same cylinder displacement but a different type of valve timing system. This is precisely what causes the main drawback of the Miller cycle engine: low power per liter. In practice, this means that a 1500 cm3 Miller cycle engine is capable of delivering power comparable to that of a conventional 1000 cm3 engine. But at the same time, compared with the second, the first will consume noticeably less fuel. So, a car equipped with a Miller engine will have low fuel consumption, but at the same time, it will not be dynamic enough. Therefore, as mentioned above, such engines are installed on cars with a hybrid power plant, where the lack of peak power of the gasoline engine is compensated by the operation of the electric motor, which work during the acceleration of the car simultaneously with the internal combustion engine, and thus increase the overall power of the power plant. As a result, such a car turns out to be quite dynamic, while consuming a relatively small amount of fuel.
- Variable Valve Lift (VVL)
Structurally, these engines have the most complex mechanism for driving the valves of the gas exchange system. This serves as a means not only of almost complete control over the moments of opening and closing of the valves, but also makes it possible to control the duration of the timing phases in the widest possible range simultaneously with the regulation of the valve lift. Due to the fact that the valve lift here can be set at the level of fractions of a millimeter, and the duration of the intake phase can be measured in ten degrees of the crankshaft angle, it has become possible to operate the engine in a throttle-less mode.
When the engine is running at low and medium loads, the throttle valve is almost completely open, and does not appreciably restrict the flow of air drawn in by the engine. At the same time, the amount of air entering the cylinders is no longer regulated by the throttle valve, but by the intake valves. This minimizes pumping losses. This is due to the fact that the air through the intake valves here moves only in one direction (now part of the air drawn into the cylinder is not pushed back into the intake manifold at the end of the intake stroke), and also due to the fact that the engine does not have to work in this case in vacuum pump mode, preliminarily diluting the air to the level required in a conventional engine before it is drawn into the cylinder. Due to this, part of the extra work is no longer performed, saving fuel.
This design of the timing mechanism allows, depending on the operating conditions of the engine, to smoothly switch from the Miller cycle to the Otto cycle and vice versa, to switch from the standard throttle restriction of air flow to the engine cylinders to a throttle-less mode of operation. Thus, on the one hand, when necessary, a high fuel efficiency of the engine is achieved, and on the other hand, when necessary, the loss of its power per liter is excluded.
4. Script report
After analyzing the recorded signals by the Px script, the user receives a report consisting of several tabs. The report contains not only the measurement results, but also the results of the analysis of the measured parameters and the diagnosis made by the script algorithm. All this information is displayed in text, table, graphic and animation forms.
4.1 Report Tab
This tab contains information not about the tested car, but about the script itself and its settings. Here is shown:
- version of the script algorithm using which the signal analysis was performed
- parameter values set by the user in the script configuration window
- service notifications and recommendations to avoid errors during the test execution
4.2 Results of the analysis Tab
In the upper part of this tab you can see the "Summary conclusion" section, which displays the diagnosis calculated by the script, a table is displayed in the lower part where the values of the measured and calculated engine parameters are given in text form.
Figure 4. Results of the analysis Tab
- Diagnosis
A diagnosis is one or more short messages telling you about the most important analysis results obtained. It displays information about problems in the operation of the cylinder that require immediate elimination, as well as notifications of issues with the testing methodology.
Figure 5. Reluctor wheel test
If the script was able to check a sufficient number of parameters of the tested cylinder and no problems were found, then in this case the diagnosis "Faults are not detected" is provided.
- Table
The table below the diagnosis consists of several sections. Each section contains information about the operation of the subsystems of the tested engine. Each section displays the values of a number of measured parameters, as well as the limits of acceptable values for these parameters.
The first section of the table is called "General characteristics".
Figure 6. General characteristics section of the table
The following parameters are shown at the top of this section.
– Cylinder leakage – parameter consisting of the sum of two other parameters: pressure and thermal losses. Pressure losses are the amount of air that, during its compression and subsequent expansion escapes from the cylinder through its compression rings, leaking intake or exhaust valve(s), leaky cylinder head gasket or cracks in the engine block, cylinder head, or piston.
Heat loss is the amount of heat that passes from the air compressed in the cylinder to the walls of the combustion chamber. The fact is that as a result of rapid compression, the temperature of the gas in the cylinder rises even without ignition to temperatures that are noticeably higher than the surface temperature of the combustion chamber. Therefore, the walls of the combustion chamber begin to cool the heated gas, taking away thermal energy from it. The loss of thermal energy reduces the efficiency of internal combustion engines, and their developers are forced to take various measures to combat the problem. After all, if this is not done, then most of the heat released from fuel combustion will simply heat the engine cooling system, instead of being converted into mechanical energy, and the fuel efficiency of the engine will suffer.
For a new engine in a modern car, the value of the "Cylinder leakage" parameter is typically 15%. For an engine that has been in operation for a long time, but is still in good condition, this value can reach a value of 20%. If the pressure losses of the cylinder approaches 25%, the cylinder will not be able to work at the engine cranking speed with the starter, but works at idle speed and above. A cylinder where cylinder leakage has reached a value of 30% is no longer able to operate at idle, but will still operate at increased speeds. With losses of 35% or more - the cylinder no longer works at all.
– Compression ratio – this is a dimensionless value showing how many times the volume of the internal space in the cylinder decreases in the process of moving the piston from BDC to TDC. For example, for a 4-cylinder engine with a volume of 2000 cm³, the displacement of one of its cylinders will be 500 cm³. If, for example, the volume of its combustion chamber is 50 cm³, then the volume of the internal space of the cylinder at the moment when the piston is at BDC will be equal to the sum of the volume of the combustion chamber and the working volume of the cylinder: 50 cm³ + 500 cm³ = 550 cm³. When the piston reaches TDC, the volume of the internal space of the cylinder will be equal to the volume of the combustion chamber: 50 cm³. Now, having the values of the maximum possible and minimum possible volumes of the internal space of the cylinder, we can calculate the compression ratio:
550 cm³ : 50 cm³ => 11 : 1
Thus, for the given example, the compression ratio will be 11:1.
The value of the compression ratio is the most important characteristic of the engine, primarily affecting its fuel efficiency: the higher the compression ratio, the more economical the engine. The higher compression ratio in diesel engines compared to gasoline engines is one of the main reasons for the high fuel efficiency of diesel engines. But the compression ratio cannot be increased indefinitely. Its upper permissible value mainly depends on the fuel for which the engine is designed. If this value is exceeded, the gas pressure in the combustion chamber can reach the limit of the detonation resistance of the fuel, and then the process of combustion of the working mixture in the cylinder will cease to be smooth and become explosive. The main negative consequence of this phenomenon, called detonation, is the physical destruction of the piston, which is not able to withstand such high loads.
Even though the compression ratio is set at the time the engine is manufactured and, it seems, should remain constant, in fact it can change for several reasons. As discussed above, the compression ratio is determined by two parameters: the working volume of the cylinder, and the volume of the combustion chamber. And both of these values can change during the operation of the engine. Mainly the volume of the combustion chamber changes. The combustion chamber volume may be reduced due to the accumulation of large amounts of carbon deposits on its surfaces. But it can also increase, which most often happens when a large amount of water enters the cylinder. If the volume of water entering the cylinder is greater than the volume of the combustion chamber, when the piston approaches TDC, it "hits" the water, causing the upward movement of the piston, along with the upper part of the connecting rod, to stop. Meanwhile, the crankshaft, rotating by inertia, continues to raise the lower part of the connecting rod. As a result, the connecting rod is compressed so much that it ultimately bends. If it does not break, its working length becomes shorter than the nominal length. As a result, the connecting rod can no longer lift the piston to the previous height, which automatically increases the combustion chamber volume and, therefore, reduces the compression ratio. This usually happens when a car attempting to ford a flood, causing water to be drawn into the intake manifold through the air filter, from where it travels through the entire intake system into the cylinder.
Another reason for changes in the compression ratio can be unqualified repair or mechanical tuning of the engine. In this case, not only the combustion chamber volume can change, but also the cylinder displacement, due to the installation of parts not intended for this engine in the crankshaft-connecting rod mechanism, or modifications to old engine parts.
– Idle speed. The measured value of this parameter is displayed here. If during the test execution the engine was idling several times, then to measure this parameter, the Px script algorithm selects the part of the waveform of the recording where the engine had the most stable RPM and under the lowest load.
The measured values of the valve timing are displayed at the bottom of the "General characteristics" section:
- – Exhaust valve opening angle
- – Exhaust valve closing angle
- – Intake valve opening angle
- – Intake valve closing angle
The values of these parameters are measured according to the graph of the quantity of gas in the cylinder, which is displayed in the graphic tab of the "Valve timing" report. These and all other parameters displayed in the table are discussed in detail below, together with the graphic tabs of the Px script report.
4.3 "Quantity" and "Valve timing" Tabs
The Quantity tab is deprecated. In the current version of the USB Autoscope software it has been replaced by the Valve Timing tab. The "Quantity" tab has not been removed from the Px script report since some users want to continue using it.
In the "Valve timing" tab - in its left part is displayed a graph of the quantity of gas in the cylinder and on top of it a diagram of the valve timing, which clearly demonstrates the moments of opening and closing of the valves. The right side of this tab displays an animation of the gas exchange processes occurring in the tested cylinder.
To measure the valve timing, the Px script algorithm analyzes a part of the waveform of the cylinder pressure graph in the section of the transition from the speed drop mode to the idle mode (the so-called "Idle entry section"). This is done for several reasons, one of which is the need for the most accurate measurement of the closing angle of the intake valve. For this reason, the measurement process is carried out not in a section with stable engine speed, but in a transient mode, when the process of dynamic boost is constantly changing due to changes in engine speed. Thanks to this, on the graph of the quantity of gas in the cylinder, the end point of the closing of the intake valve becomes more visible.
Figure 7. Graph of the quantity of gas in the cylinder
The graph of the quantity of gas in the cylinder calculated by the Px script is displayed immediately in two tabs of the report - "Quantity" and "Valve timing". This graph is necessary because the process of measuring the opening and closing angles of the valves is carried out by analyzing the gas quantity graph, and not the cylinder pressure graph, since the gas quantity graph is better suited for finding the opening and closing angles of the valves. So, the Px script firstly converts the pressure graph into a graph of the quantity of gas in the cylinder. The valve timing is better seen here because, if we neglect gas losses, then during the compression and expansion phases, when both the intake and exhaust valves are closed, the number of gas molecules in the cylinder does not change. That is, while the timing valves are closed, the pressure and temperature of the gas in the cylinder change, but its quantity does not change (in fact, the quantity of gas gradually decreases due to losses; and this happens mostly when, due to compression, pressure and the temperature in the cylinder rises). As a result, a horizontal fragment is clearly distinguished on such a graph, the left edge of which corresponds to the end of the intake valve closing, and the right edge to the beginning of the exhaust valve opening.
But a part of the graph of the quantity of gas in the cylinder, corresponding to the closed timing valves, looks like a horizontal line only if this graph is displayed in the Cartesian coordinate system, when the position of the graph point along the X axis (horizontally) displays the angle of rotation of the crankshaft, and along the Y axis (along vertical) - the quantity of gas in the cylinder. A similar form of representation is used in the "Quantity" tab. However, the quantity of gas in the cylinder is displayed here not as a graph (when the line moves from left to right and does not return back), but as a diagram (when the line moves from left to right, then from right to left). The left edge of the graph displayed in the Quantity tab corresponds to TDC, and the right edge corresponds to BDC.
Figure 8. Quantity tab
When displaying a graph of the quantity of gas in a cylinder in a polar coordinate system, when the quantity of gas is displayed as the distance of a graph point from the center, and the angle of rotation of the crankshaft corresponds to the angle of deviation of this point from the vertical line, a part of this graph in the area corresponding to closed timing valves looks narrower not as a horizontal line, but as part of a ring. This form of representation is used in the "Valve timing" tab.
All graphic tabs of the Px script report are not static images, but interactive objects. This means that in order to study the diagrams displayed in more detail, the user can, if necessary, change the display scale, turn off or vice versa select one or another part of it, and also use measuring markers. And, as mentioned above, the "Valve timing" tab is also supplemented with an animation of the gas exchange process, which displays the processes occurring in the cylinder at the crankshaft rotation angle specified by the user using the measuring marker A.
Figure 9. Visualization of gas exchange processes in the cylinder of an engine with a variable valve lift
Just as the diagram of the quantity of gas in a cylinder displays the real processes occurring in the tested cylinder, and not general information about the processes occurring in the cylinder of an average engine, animation does not visualize the average gas exchange processes, but precisely those processes that occur specifically in the tested cylinder. In particular, here you can see such processes as internal exhaust gas recirculation (the flow of exhaust gases from the cylinder into the intake manifold at the moment the intake valve starts to open, the flow of exhaust gases from the exhaust manifold back into the cylinder at the moment the exhaust valve starts to open), dynamic boost and other.
It should be noted that the animation uses a generic engine design model that does not always match the design of the engine being tested. For example, an engine may not be equipped with two camshafts, as shown in the animation, but with one common camshaft that controls both intake and exhaust valves at the same time. And in the case of visualization of gas exchange processes in the cylinder of an engine with a variable valve lift operating in a non-throttle mode, instead of the complex design of the gas distribution mechanism, the animation will show a very small intake cam.
Figure 10. Visualization of gas exchange processes in the cylinder of an engine with a variable valve lift
Here it is necessary to remind once again that the valve timing is displayed for the idle entry section. If the engine is equipped with a variable valve timing system, then in other modes of its operation, the phases may be different. But during the entry into idle, the engine control unit usually leaves the valve timing in its initial position, not shifting it to either "Earlier" or "Later" side. Due to this, it is the initial position of the camshafts that is displayed in the "Valve timing" tab. However, this is not always the case, as some control units continue to adjust the angle of rotation of the camshafts also at the entry to idle. Therefore, if the task is to accurately measure the initial position of the camshafts on a running engine, then before performing the Px test, it is recommended to disconnect the electrical connectors from the VVT valves that control the angle of rotation of the VVT timing gear. In addition, if necessary, the timing phases can be measured in a section different from the idle entry section. To do this, before running the script, you need to select about ten cycles of the engine in that part of the recorded signals where it is required to measure the valve timing.
4.4 "Intake System" tab
The "Intake system" graphical tab from the Px script report is the source of information for the table section, which is displayed in the "Results of the analysis" tab. In both tabs the same information is displayed, but in different forms of presentation. The table displays information about filling the cylinder with air while the engine is idling, as well as under maximum load at various speeds.
Figure 11. Intake system graphical tab from the Px script report
The cylinder filling level when the engine is idling depends on several parameters. Increased cylinder filling at idle may indicate an incorrect composition of air-fuel mixture (too rich or too lean mixture), too much exhaust gas in the mixture (incorrect internal or external recirculation), too late ignition timing, clogged exhaust engine path, or simply too much power take-off from the engine crankshaft. Some of the listed possible causes of these problems are automatically rechecked by the Px script algorithm, and if the assumption is confirmed, the user receives a corresponding notification in the script diagnosis. Another part of the possible causes of this violation requires rechecking by other diagnostic methods, about which the user also receives a corresponding notification.
The cylinder filling level with air when the engine is running under maximum load is controlled by the Px script separately for each rpm value in 500 RPM steps, but by default it is displayed in the table in 1000 RPM steps. The maximum possible filling of the cylinder with air at certain speeds is the most important characteristic of the engine, since the maximum power and torque on the crankshaft that this engine is capable to develop at these speeds primarily depends on it. After all, if the cylinder cannot draw in a lot of air, then the engine management system will be forced to inject a correspondingly smaller amount of fuel so as not to disturb the air-fuel ratio. And since a smaller amount of fuel burns in the cylinder, then the engine will generate less energy. Therefore, the developers are constantly improving the design of the engine, primarily for the sake of improving the maximum filling of the cylinder with air.
In the "Intake system" graphical tab of the Px script, information about the filling of the cylinder with air is presented in the form of a diagram.
Figure 12. Information about the filling of the cylinder with air is presented in the form of a diagram
Each cycle of the cylinder operation here corresponds to one point of the diagram. The color of the point depends on the load under which the engine was operating at that moment, its location along the Y axis depends on the cylinder filling (the greater the filling, the higher the point is located on the diagram), and its location along the X axis depends on the engine speed (the higher speed, the closer to the right edge of the diagram is the point). Due to the fact that this diagram displays all the cylinder operation cycles analyzed by the script, here you can see not only the filling of the cylinder under maximum load (red segment of the diagram) and at idle (a cluster of green dots over the green rectangle), but also during engine acceleration time under minimum load (green segment of the diagram) and during deceleration after snap throttle (blue segment of the diagram). This information is harder to understand, but allows you to perform a deeper analysis of the engine intake system.
Please note that the location of a single point on the diagram here does not represent the total quantity of gas in the cylinder, but the amount of fresh air entering the cylinder. That is, before calculating the data for this diagram, the Px script first calculates the amount of exhaust gases remaining in the cylinder that failed to leave the cylinder, and the total quantity of gases in the cylinder, measured at the moment immediately after the intake valve closed. After that, it subtracts from the total amount of gases the proportion of exhaust gases remaining in the cylinder, resulting in the value of the amount of fresh air entering the cylinder from the intake system.
This graph then displays the Volumetric Efficiency of the engine.
You should pay attention to one more interesting feature of the graph of the maximum filling of the cylinder with fresh air. The shape of this graph matches the shape of the engine torque graph, which requires a Dynamometer to measure directly. This test bench allows the extraction of rotational energy from a car's wheels using special rollers while measuring the amount of this energy and ultimately converting it into power and torque graphs of the engine. Due to the high cost of Dynamometers, they are commonly used only in those workshops that specialize in the tuning of engines. They are necessary in order to be able to understand exactly how the replacement or adjustment of a particular component of the car and / or engine or its control system affected the power output and torque. The simplest example of such tuning is the replacement of a standard air filter with a zero resistance filter. At the same time, without seeing how the engine torque curve has changed, it is quite difficult to understand if there is any benefit from such a replacement.
There are many engine and vehicle components that affect the amount of air and fuel in the cylinder and ultimately the power output and torque of the engine. Examples are the air filter, as already mentioned, the throttle valve, the duct connecting the air filter to the throttle valve, and the shape of the duct, as well as the presence / configuration of resonant chambers attached. Turbochargers/mechanical supercharger, as well as intercoolers. The intake manifold, its volume, the shape and size of the channels and internal cavities, the presence and type of the system for changing the geometry of the intake system. Also the number and size of intake valves, valve timing and valve lift. In addition, cylinder scavenging. Many of these operating parameters are controlled by the engine control unit. In order to check the effect of replacing / retuning any of the listed components, you need to take measurements using the Dynamometer, then compare the result with the results of previous measurements made before making changes to the engine. Alternatively, you can use the Px script, and compare the shape of the graph of the maximum filling of the cylinder before and after making changes to the engine. Such a measurement will not provide absolute values of the engine torque, but it will allow you to see how and at what speed it has changed.
Please note that both the results obtained using the Dyno bench and the graph of the maximum filling of the cylinder obtained using the Px script depend on the height above sea level of the workshop in which the measurements were taken. This happens because as altitude increases, air density decreases (atmospheric pressure decreases). At lover atmospheric pressures, the engine will consume the same volume of air, but since the density is less, the mass of the air consumed is less than under normal atmospheric pressure. However, the developers of the Px script are already working on making changes to its algorithm in order to get automatic correction of the results depending on the height above sea level.
Based on the above it is obvious that more air in the cylinders, the more the power and torque is developed by the engine, which is generally preferred by both vehicle manufacturers and buyers. However, increasing the amount of air entering the cylinders indefinitely is also not possible, as it can lead to physical damage to the engine due to detonation combustion of the mixture in the cylinders of an engine operating under full load. That said, such a situation can only arise as a result of improper design or tuning of a racing engine. The fact is that to achieve a significant increase in engine power and torque, high-performance air compressors (superchargers or turbochargers) are installed. As a result, peak pressure in the cylinder rises and may exceed the allowable threshold determined by the fuel's detonation resistance. In such cases, the compression ratio has to be reduced by increasing the combustion chamber volume. However, a significant reduction in the compression ratio, in turn, lowers the engine's power output, which is a critical parameter for a racing engine. Thus, on one hand, reducing the compression ratio is necessary to prevent engine failure, but on the other hand, excessive reduction leads to an unjustified loss of power. Finding the optimal balance can be very difficult, which is why engine failures in racing competitions are not uncommon. The compatibility of the compression ratio and forced air boost is monitored by the Px script. If a risk of engine failure due to over-boosting is detected, an appropriate diagnosis is made.
Figure 13. BMW 850hp racing engine
Figure 14. Px script report
Please note that it is not the standard algorithm for controlling the so-called over-boost that is used here, when peak boost pressure is simply controlled, but a special algorithm for analyzing the compatibility of parameters such as compression ratio and the amount of air in the cylinder.
4.5 "Exhaust system" tab
In the Px script report table, there is a section called "Exhaust system". The data for filling in this section is taken from the graphic tab of the report with the same name. The information displayed here serves to check the operation of the cylinder exhaust system. The piston expends energy to push the exhaust gases out of the cylinder into the exhaust system, which it takes from the crankshaft. The rotational energy of the crankshaft arises due to the combustion of fuel in the engine cylinders. Therefore, the less energy spent on exhaust gases, the less fuel the engine consumes, and the better its fuel efficiency. Also, the more completely the cylinder is scavenged of exhaust gases, the more space will be available for fuel and air and the more maximum power the engine will eventually be able to develop. The Px script allows you to estimate the numerical value of the power consumption for the release of exhaust gases from the cylinder at all engine speeds that the engine developed during the test.
Figure 15. Exhaust system tab from the Px script report
Pay attention to the fact that the estimation of energy costs for exhaust by the value of the pressure in the exhaust manifold is not correct, since the pressure here is not stationary, but pulsating. This circumstance has a very important influence. The fact is that the main part of the energy expended on the release is spent at the moment when the piston moves at maximum speed, that is, in the middle of the exhaust stroke. Therefore, the back-pressure should be measured exactly at this moment, and not in the center of the exhaust manifold, but near the exhaust valve of the tested cylinder, since due to resonance processes at the same time, the gas pressure in different parts of the exhaust manifold will be different. That is, even a significant increase in the pressure of the exhaust gases on the working surface of the piston at the moment when it has almost reached TDC is not critical in terms of energy costs, since the piston is barely moving here, which means it does almost no work. However, even a slight increase in pressure in the cylinder in the middle of the exhaust stroke leads to significant losses in the rotational energy of the crankshaft. All these features are taken into account by the Px script algorithm when calculating exhaust costs.
In the graphic tab "Exhaust system" each cycle of the engine corresponds to one point on the diagram. The lower the point is, the less energy in a given cycle of operation was expended by the engine to clean the cylinder from exhaust gases. The higher the engine speed was, the closer to the right edge of the diagram is the point. Moreover, as it was done in the "Intake system" tab, the color of the point in the "Exhaust system" diagram shows what engine load was at that moment.
Figure 16. Color of the point in the "Exhaust system" diagram shows what engine load was at that moment
Below the diagram, you can see the area colored in light red. It shows where the chart points should be placed. The upper limit of this area indicates the permissible limit of energy costs for scavenging the cylinder of exhaust gases. This limit is not a fixed line, it is automatically calculated individually for each tested cylinder. An increase in the level of energy consumption for exhaust is allowed only if there is an increased filling of the cylinder with air. The fact is that the filling of the cylinder depends not only on the operation of the intake system, but also on the operation of the exhaust system. So, the correct tuning of the resonant processes in the exhaust system allows you to achieve a noticeable increase in the filling of the cylinder with air, but at the same time, the energy consumption of the engine for the exhaust gases also increases somewhat. Therefore, in such a situation, the script automatically raises the limit of permissible energy costs in the "Exhaust system" tab. The turbocharger, if so equipped, serves to force air into the engine's intake manifold, also works by taking energy from the exhaust gas stream, thereby raising the pressure in the cylinder during the exhaust stroke. This circumstance is also taken into account by the Px script algorithm.
The most common reason leading to an increase in energy costs for exhaust gas is a decrease in the throughput of the exhaust system due to restriction in the catalytic converter. The restriction can be caused by melting of the substrate, coating with carbon or ashes, or physical damage. Restrictions that hamper the passage of exhaust gases through the exhaust system become more significant with higher engine speed, since with an increase in speed, the flow of exhaust gases also increases, and with it, the back pressure in the exhaust manifold also increases. In extreme cases, this leads to a phenomenon where the engine can no longer increase RPM above a certain threshold. This happens because all 100% of the power generated by the engine is spent on pushing the exhaust gases out of the cylinders. In the example below, this rev limit occurs at an engine speed just above 3000 RPM.
Please note that it is not the standard algorithm for controlling the so-called over-boost that is used here, when peak boost pressure is simply controlled, but a special algorithm for analyzing the compatibility of parameters such as compression ratio and the amount of air in the cylinder.
Figure 17. An example of clogged exhaust system
A less common cause of exhaust system malfunction is the replacement of its components with non-original parts. The consequences of such a replacement are far from straightforward. For example, installing what appears to be a more efficient, larger-diameter component can often have the opposite effect, as it disrupts the resonance processes occurring in the exhaust manifold. As a result, the energy required to expel exhaust gases may reach or even exceed the upper permissible limit. This leads to reduced vehicle dynamics and increased fuel consumption. However, the opposite can also be true—a well-executed exhaust system upgrade can reduce the energy required for exhaust gas expulsion, thereby improving engine performance. This once again confirms the fact that automakers do not always pay enough attention to exhaust system tuning, especially in budget vehicles.
4.6 "Ignition Timing" Tab
As you know, the combustion of the mixture in the cylinders of a piston engine does not occur instantly, but gradually. Consequently, the increase in pressure in the cylinder from its combustion also occurs gradually. But the pressure in the cylinder is also affected by the volume of the space above the piston, which is constantly changing due to the movement of the piston. So, during the compression stroke, the piston moves towards the TDC, reducing the volume in the cylinder, and thereby increasing the pressure, and during the power stroke, it moves away from the TDC, increasing the volume and lowering the pressure in the cylinder. The superposition of these processes one on another forms a pressure curve in the cylinder, which eventually acquires a rather complex shape. This is also superimposed by the fact that the speed of the piston depends on the speed of the crankshaft. In any case, after some time after the air-fuel mixture has been ignited, a pressure peak occurs in the cylinder from the combustion of the mixture. The efficiency of the engine directly depends on the position of the piston at the moment this pressure peak occurs.
In piston engines running on gasoline, gas, alcohol, or mixtures thereof, ignition of the working mixture in the cylinders is initiated by an electrical ignition spark that is discharged between the electrodes of the spark plug. Controlling the moment of occurrence of the ignition spark allows you to control the moment of occurrence in the cylinder of a pressure peak from the combustion of the mixture. Thus, having the correct ignition timing allows you to achieve maximum engine efficiency. Calculating the best moment of ignition is fairly difficult since for each mode of engine operation, the optimal ignition timing will be different. The starting point for measuring the ignition moment is TDC 0°, since this is where the power cycle begins. The offset of the ignition timing relative to TDC 0° is measured in degrees of the crankshaft angle and is called the Ignition Advance Angle. So, for example, the value of this parameter equal to 0° means that the ignition spark occurred at the moment when the piston was in the TDC position 0°, that is, at the end of the compression stroke / the beginning of the power stroke; and the Ignition Advanc Angle value of 10° means that the spark occurred slightly earlier than in the previous example: from the moment the spark occurs, the piston will reach TDC 0° only after the engine's crankshaft rotates another 10°. Negative Ignition Advance Angle values indicate that the ignition spark occurs after the piston has passed TDC 0° and started moving towards BDC.
Since Ignition Timing has a profound effect on engine performance, it is always worth paying attention to and check this parameter during engine diagnostics (for example, when diagnostics are performed to determine the causes of deterioration in economy and / or power / torque). And the Ignition Timing should be checked in those engine operating modes in which it is actually operated, that is - not only when the engine is idling, but also at medium and high speeds, and during low, medium, and high load.
The standard method for checking Ignition Timing involves measuring this parameter only when the engine is idling. Therefore, in order for the technician to be able to perform a full-fledged check of the operation of the Ignition Timing adjustment subsystem, the Px script report table was supplemented with the "Ignition Timing" section.
Figure 18. Ignition timing tab from the Px script report
The data for filling in this section is taken from the graphic tab of the report with the same name.
Figure 19. Ignition timing graphic tab from the Px script report
As in the "Intake" and "Exhaust" tabs, here one colored dot in the diagram corresponds to one cycle of the cylinder being tested. The location of this point vertically depends on the moment of occurrence of a spark discharge between the electrodes of the spark plug serving the tested cylinder, horizontally - on the engine speed, and the color - on the load under which the engine was operating at that moment.
As mentioned above, for each mode of engine operation, its own value of the Ignition Timing will be optimal. For example, when idling, the ignition timing value is usually 10°, and with increasing speed, the ignition timing increases. But this only happens when the engine is not heavily loaded. If the engine is running under high load, then in order to prevent the detonation combustion of the mixture, the ignition timing value should be reduced, sometimes to negative values. Another difficulty here is that for each engine the optimal values of the ignition timing are different, since they depend on many of its design features. In addition, there is also a dependence on fuel, since different types of it have their own combustion rate.
In most modern vehicles the ignition timing is controlled by an electronic engine control unit. In old engines, the initial value of the ignition timing was set by rotating the mechanical ignition distributor housing. A standard procedure during “tune ups” of older vehicles is to check the ignition timing adjustment. The mechanical ignition distributor also performed the ignition timing correction, depending on the operation mode of the engine. To do this, it was equipped with two mechanisms such as a centrifugal regulator (advancing the timing with engine RPM) and a vacuum ignition timing corrector (More timing advance with increasing engine vacuum, less timing advance with lesser engine vacuum). Since the characteristics of these mechanisms very much influenced the power and efficiency of the engine, they were specifically designed by the manufacturer for each engine model and vehicle. When checking the ignition timing at the repair shop, only the performance of these mechanisms was usually checked, but not their characteristics.
The Px script not only measures and displays the values of the Ignition Timing in all modes in which the engine worked during the test, but also checks whether these values of the Ignition Timing are suitable for this tested engine. At the same time, it takes into account not only the engine operating modes, but also its characteristics as the compression degree (the greater the degree of compression, the later the ignition timing should be) and the maximum filling of the cylinder with air (the greater the filling, the later the ignition timing should be). If the script detects the operating mode of the engine, where the ignition timing value goes beyond the permissible limits, it shall notify the technician using a text message in the "Conclusion" section. The script does not take into account the type of fuel on which the engine works: the Px script algorithm suggests that during the test, the engine worked on gasoline with an octane number of 95. Therefore, for example, when setting up high performance engines operating on special fuel (for example, on gasoline with octane number 102), you can ignore the messages of the script type "Ignition Timing early...". And vice versa: if the engine is operated on a fuel with a low octane number, you can ignore messages like "Ignition Timing late...". But if the script displays a message like "Over advanced ignition timing" or "Too late ignition timing" then a malfunction is present.
In the mechanical ignition distributors, if necessary, it was possible to change the characteristics of both the centrifugal regulator and the vacuum corrector. On contemporary vehicles, the Ignition Timing setting can only be performed by modifying the software of the electronic control unit. And, as strange as it may seem, this necessity arises quite often in modern vehicles. When testing engines equipped with advanced electronic control systems capable of adjusting Ignition Timing based on the actual anti-knock properties of the fuel using a knock sensor, the Px script frequently detects suboptimal ignition timing adjustment settings or sometimes even outright incorrect ones. This issue is observed not only in lower-end vehicles but also in mid-range ones.
