Welling, O., Moss, J., Williams, J., and Collings, N., "Measuring the Impact of Engine Oils and Fuels on Low-Speed Pre-Ignition in Downsized Engines," SAE Int. J. Fuels Lubr. 7(1):1-8, 2014, doi:10.4271/2014-01-1219.
One of the limits on the maximum fuel efficiency benefit to be gained from turbocharged, downsized gasoline engines is the occurrence of low speed pre-ignition (LSPI). LSPI may lead to high pressures and extreme knock (megaknock or superknock) which can cause severe engine damage. Though the mechanism leading to megaknock is not completely resolved, LSPI is thought to arise from local auto-ignition of areas in the cylinder which are rich in low ignition delay “contaminants” such as engine oil and/or heavy ends of gasoline. These contaminants are introduced to the combustion chamber at various points in the engine cycle (e.g. entering from the top land crevice during blow-down or washed from the cylinder walls during DI wall impingement).This paper describes a method for testing the propensity of different contaminants to cause a local pre-ignition in a gasoline engine. During one cycle, a small amount of contaminant is injected into one cylinder of a 4 cylinder engine. The spark is suppressed during this or the following cycle to allow detection of local pre-ignition events after spark timing. If the contaminant is injected on the cycle before, it is the cycle following the injection that has the missed spark. By detecting auto-ignition events before and after spark timing, it is possible to compare contaminants over a broad range of ignition tendencies. Sensitivities of pre-ignition tendencies to intake pressures and temperatures, the amount of contaminant introduced, and fuelling ratios are discussed. Additionally, the importance of contaminant stratification is shown, and pre-ignition is demonstrated to result from the introduction of contaminant as early as the beginning of blow-down of the preceding cycle.