The supercharged spark ignition engine with direct fuel supply system in cylinder (SI engine) has a problem on abnormal combustion at low engine speed. It is called the LSPI (Low Speed Pre-Ignition)[1].
This research focuses on one of the source of abnormal combustion which is the autoignition of lubricating oil from piston crown in cylinder, here especially, frequency of autoignition in cylinder[2,3,4].
In this experiment, the test engine operates without spark ignition as motoring operation. The advantage of this method is to avoid the effect of gasoline dilution[5,6]. Namely, it is able to reveal the essence of abnormal combustion experimentally. The 2 kinds of lubricating oil are tested.
The measured data show that the frequency of autoignition in this research is 1/10 of that of lubricating oil scattering from piston crown. The abnormal combustion occurs the frequency of 1 time in each 10000 cycles approximately.
The special behavior of the LSPI has been measured. The autoignition timing is ATDC in the initial stage. When the cycle of autoignition proceed, its timing is advance to BTDC.
On the other hand, the time history of pressure and temperature in cylinder are able to evaluate the autoignition by Livengood-Wu integral[7]. The activation energy E which is estimated by the integral means the quantitative evaluation of autoignition in cylinder. The dangerous activation energy E for the LSPI is under 6452(× 104 J/mol)in this research. The activation energy E would be able to take advantage of the engine design and simulation to prevent from the LSPI.