Abstract
The improvement in thermal efficiency of jet ignited spark ignition engines over normal SI engines is reviewed and the 10 to 25% increase in efficiency possible is identified for ultra lean mixtures. In a variable compression ratio (‘octane rating’ CFR engine), the increase in compression ratio possible with jet ignition is identified at different knock levels defined in terms of incipient, clearly audible and heavy knock that correlate with cylinder pressure derivatives. If NOx formation is to be avoided mixture strengths greater than lambda of 1.8 should always be maintained so it is not desirable to operate jet ignition engines at stoichiometric mixtures. However for maximum power it is shown that the engine can operate at about 2 compression ratios higher than normal SI. At HUCR, HAJI requires about 7 degrees less spark advance because of the ultra fast burn and has about 25% less end gas (mixture burnt in the knocking process) compared with SI at the same level of knock for both ignition systems. At a speed of 1200 rev/min the knock, at HUCR, is clearly audible but at 1800 rev/min HUCR is obtained at a lower level. At the latter speed the engine returns about 4% extra IMEP than SI. The benefit of high compression ratio is much more important under ultra lean burn conditions where HAJI would normally operate to obtain almost zero NOx, because the high compression ratio further increases the already high efficiency.
Keywords: Knock, jet ignition, highest useful compression ratio, thermal efficiency