GM, Robert Bosch Corporation, and Stanford University will team up in a $2.5M effort to accelerate the development
of Homogeneous Charge Compression Ignition, or HCCI. This technology would allow for the use of
compression ignition techniques similar to diesel engines, but would use lower compression ratios, burn a wide range of
fuels, and result in lean-burn operation with reduced emissions. It could be employed in “regular” drivetrains as well
as hybrids.
Lean-burn (air-fuel ratios above the ?optimum? 14.7:1 ratio) technology has been used on spark-ignition engines, but
with mixed results. It?s difficult to properly ignite a lean mixture with a spark, and the process usually results in
excessive NOx emissions (formed when the normally-inert nitrogen comes in contact with leftover oxygen at extreme
temperatures). When performed properly, the net result is increased fuel economy, but not because the engine is
?starved? for fuel with the lean mixture; rather, it comes from the fact that the throttle must be opened to provide
the extra air, and that results in less work performed by the engine to simply pull the combustion air past a
nearly-closed throttle blade. Throttles cause pumping losses, just like a restricted tailpipe, and that prevents
gasoline engines from being as efficient as possible.
Diesels don?t have throttles – they limit power by regulating the fuel delivered to the chamber – so they don?t suffer
from these pumping losses. Consequently, they run extremely lean under most operating conditions; a air/fuel ratio of
60:1 or greater isn?t unusual at idle.
But if diesels run lean, then why all the black smoke that?s indicative of an excessively rich mixture? That?s because
diesels have pockets of rich mixture near the injection point, and it rapidly leans-out near the perimeter of the
cylinder. It ends up being the worst of both worlds from an emissions standpoint, and this is why ?clean? diesels have
been difficult to develop.
HCCI attempts to inject fuel in such as way as to provide a homogeneous mixture throughout the cylinder. This would
eliminate the soot and hydrocarbon emissions caused by pockets of rich mixture, as well as dramatically decrease the
NOx emissions caused by areas of extremely lean mixture. It?s also possible to reduce the flame propagation time
required to burn all of the mixture, resulted in less wasted power compared to igniting the mixture well before the
piston reaches Top Dead Center.
The technique has been explored for at least 25 years, but until recently, the technology required to accurately
inject the fuel directly into the chamber has not been available. I?m sure that common-rail injection systems for
conventional diesels are lending a lot of technology to this new effort, which means that it stands a decent chance of
succeeding.
Comments from GM and Bosch engineers in the article would seem to indicate that additional technology is still
required, though. Many SAE papers have been written in recent times about attempts to monitor combustion conditions
through direct pressure sensing, optical means, or ionization current measurements, and it?s possible that something
like this will be required to make HCCI work in the real world. It?s also likely that HCCI engines will benefit from
intake air heater systems, such as those that have been used on large off-highway diesels for quite some time (the ?06
GM Duramax uses such a heater, indicating likely future use on a variety of passenger-car and light-truck applications
as well).
Given that the peak pressure of HCCI operation is
approximately double that of spark ignition, it?s likely that such an engine would have to be much stronger than a
typical modern engine, but probably not as heavy-duty as a traditional diesel if the burn rate is controlled
better.
For more on HCCI, including an animation, click here. And
here as well.
Here, too. And
here, if you can read German.