Advanced powertrain development

Combustion analysis is an investigation method relying on high-frequency in-cylinder pressure measurement in an internal combustion engine. By capturing and visualising the pressure trace against crankshaft position in a running engine, all the vital thermodynamic processes could be observed. Having the knowledge about such a phenomenon and process gives the developer a much better insight on how does and how effectively the given engine works. With cylinder pressure investigation it is not only possible to analyse the combustion but also the gas exchange process (low pressure phase) as well. That significantly improves the possibility of having the proper calibration or further developing the engine achieving the demanded targets. It could be stated that no other technology provides such wealth information as cylinder pressure indication, or combustion analysis.

It is important to understand, that combustion analysis itself will not increase your engine’s performance, however, it is the perfect tool to monitor your changes in the calibration, and to reveal the path which you should take to perform a safe, efficient and powerful tuning. As a rule of thumb, for maximum performance MFB50 should be around 8-10 degrees after TDC, and the optimal air-to-fuel ratio is around 13.2, assuming complete combustion. However, this is not that simple! Check out our Introduction into Combustion Analysis guide to find out more here!

IMEP is describing the integral of the pressure trace through the cycles, which correlates with the torque level of the given cycles. Any thermodynamic modification that increases the engine’s torque increases the IMEP level as well.

During the combustion phase the spark plug fires the air-fuel mixture, which starts to burn radially. As the volume of combustion products are higher than the unburned mixture, a pressure wave arises between the burned and unburned zone of the combustion chamber. This wave then reflects from the cylinder walls, pistons, and other mechanical components. Detonation occurs when an abnormal, auto-ignited pressure wave collides with a normal one creating a local pressure peak.

The effect of knock could be monitored on the pressure trace, as it being overlaid by a high frequency oscillation. The frequency of it depends mainly on the bore, as it is a superposition of the different armonics of the pressure wave reflections inside the cylinder.

The detonation that makes the pressure peaks of “knock” could be created in the two following ways. The difference between those two phenomena is the generating cause.

Auto-ignition and Pre-ignition

Auto-ignition is the phenomenon usually assigned with knock. The spark generated combustion leads towards increased cylinder pressure and temperature. This phenomenon occurs when the local peak temperature in the unburned zone reaches the auto-ignition temperature of air-fuel mixture and the secondary flame starts to propagate from that spontaneous ignition point. The generated pressure wave collides with the normal one. Auto ignition is triggered by the combustion coming from the spark event, and therefore could be controlled with altering the spark advance.

Pre-ignition is created in the compression stroke before spark would ignite the mixture. In that case a point of the combustion chamber (hot spot) gets too hot and exceeds the mixture’s auto-ignition temperature. Therefore, the combustion starts even without the presence of the spark. As the volume is still decreasing (the engine is before TDC) that type combustion could cause enormously high cylinder pressure. Furthermore, the combustion generated by a pre-ignition process could cause auto-ignition as well. The pre-ignition is not triggered by a spark event, therefore it could not be controlled with altering the spark event’s position. Auto ignition could be distinguished from pre ignition by the location of the generated pressure peaks. The amplitude of pre-ignition is usually magnitudes higher then a standard knock, thus it is commonly named as super-knock.

It is crucial to state that detonation, or knock is not binary! It always happens, the question is only its amplitude, which should be limited for normal engine operation. By using combustion analysis, you will get by far the best tool to monitor your engine’s behaviour and the knock amplitude for every combustion cycle. Having that data, you can easily set your own limitations, and tune the engine respecting those. For more information, check out our Introduction into Combustion Analysis guide here!