Engine Operating Condition and Gasoline Fuel Composition Effects on Low-Speed Pre-Ignition in High-Performance Spark Ignited Gasoline Engines

Downsizing is an important concept to reduce fuel consumption as well as emissions of spark ignition engines. Engine displacement is reduced in order to shift operating points from lower part load into regions of the operating map with higher efficiency and thus lower specific fuel consumption [ 1 ]. Since maximum power in full load operation decreases due to the reduction of displacement, engines are boosted (turbocharging or supercharging), which leads to a higher specific loading of the engines. Hence, a new combustion phenomenon has been observed at high loads and low engine speed and is referred to as Low-Speed Pre-Ignition or LSPI. In cycles with LSPI, the air/fuel mixture is ignited prior to the spark which results in the initial flame propagation quickly transforming into heavy engine knock. Very high pressure rise rates and peak cylinder pressures could exceed design pressure limits, which in turn could lead to degradation of the engine. Due to this potential, LSPI is considered a key consideration for further downsizing and improvement in engine efficiency. While some countermeasures exist that OEMs can use to avoid LSPI, such as load limiting, further study is required to formulate better countermeasures.

In the attempt to gain a better understanding of the causes of and potential mitigation methods for LSPI, several engine controls factors and operating conditions were investigated with respect to their effects on LSPI. It was recognized that the two dominant factors in influencing the LSPI occurrence frequency are engine load as governed by fueling rate (energy flux) and in-cylinder air/fuel ratio. When maintaining a constant fueling rate (not BMEP or torque), all other factors such as spark timing, MAT, coolant temp, etc. only played a minor but not necessarily eliding role in their effect on LSPI activity.

From exhaust emission and exhaust port air/fuel ratio measurements it was also recognized that a spike in HC emissions and a significant increase in Lambda (air/fuel ratio enrichment) was associated with LSPI. Furthermore, when inducing a LSPI-like combustion event by using large spark advance for a short duration, HC emission and exhaust port Lambda were significantly lower than during ‘true’ LSPI events. It was concluded that a hydrocarbon based accumulation occurs in the combustion chamber over time. These additional HC are consumed during LSPI events. When combined with the results of other researchers in this field [ 8 , 2 , 5 , 6 ], one might concluded that the leading cause of LSPI is lubricant and/or fuel based HC accumulation in the top land piston crevices volume.

To investigate the source for LSPI from a fuels' perspective, four gasoline fuel blends with similar properties, such as Octane rating, boiling point distribution and RVP but vastly different composition were tested for their effects on LSPI in a modern turbo-charged, DI gasoline engine. From this study, it was recognized that the fuel chemical composition strongly influences the likelihood and magnitude of LSPI. Fuel blends with high levels of aromatics increase the frequency at which LSPI occurs somewhat where as oxygenated fuels and, especially, low aromatic blends reduced the LSPI frequency. It was also learned that despite very similar RON & MON ratings, the knock and auto-ignition characteristics of the test fuels in the DISI engine were different. In particular the low-aromatics fuel blends showed an increase tendency to auto-ignition and knock (traditional SI engine knock or end-gas knock) characterized by the presence of a low-temperature heat release regime prior to the main combustion phase.