The dashed white line shows what the pressure graph would look like if no fuel was supplied to the cylinder. It is clear that in this case, the peak pressure in the cylinder reached 48 Bar.

Additionally, it is visible how the pressure curve from the air compression by the piston is overlaid by the pressure from fuel combustion. It is noticeable that the pressure in the cylinder increased three times due to fuel combustion — twice by a small amount and once more significantly. The first two pressure increases were caused by the combustion of very small amounts of fuel, known as pre-injection. The final pressure increase in the cylinder resulted from the combustion of the main fuel dose.

Upon closer examination, it can be seen that just before the pressure increase from fuel combustion, the cylinder pressure initially decreased slightly, and only after that did ignition occur, leading to a rise in pressure. This happens because after diesel fuel is injected into the cylinder, it first undergoes "preparation" for ignition. Immediately after being injected into the cylinder, the fuel begins to absorb the thermal energy of the compressed air, which is used to heat the fuel to its boiling point and then for its vaporization. As a result of the thermal energy absorption, the temperature of the pre-compressed air decreases slightly, leading to a drop in cylinder pressure. Only after the fuel vapor and the air mix at the necessary temperature does ignition occur, causing a rise in cylinder pressure.

The value of pressure increase depends on the amount of fuel burned. However, this does not mean that it is directly proportional to the amount of injected fuel. For example, if a fault causes too large a dose of fuel to be injected into the cylinder, instead of a large pressure increase, the opposite effect might occur. In such cases, the energy required to heat a large amount of fuel to its boiling point might be so great that the temperature in the cylinder drops below the ignition temperature of the diesel fuel. As a result, the fuel vapor may not ignite, leading to a missed ignition and the expulsion of diesel vapor into the exhaust system as bluish smoke. Such situations can arise either due to incorrect pre-injection fuel dosing (resulting from mechanical wear of the injector or incorrect injector coding, which could lead to excessive or completely absent pre-injection), or due to reduced cylinder filling with air (caused by malfunctioning of the turbocharger or EGR system), or due to reduced compression ratio (due to low compression or reduced compression resulting from a hydrostatic impact and bending of the connecting rod).

This is just one example of how the pressure graph can help a technician better understand the processes occurring in the cylinder of the diesel engine being diagnosed and make a diagnosis more quickly and accurately.

Similar measurements can also be conducted on vehicles equipped with traditional glow plugs that do not have built-in pressure sensors. To do this, you need to replace the traditional glow plug with one that has a built-in pressure sensor. If the dimensions of your existing Beru glow plug do not match the dimensions of the plugs used in the test engine, you can use an adapter.