|
Getting back to the direct injection discussion (if you don't mind me rewinding the thread a bit here), the reason that diesels can run lean mixtures more easily is because the intake air charge is compressed to a temperature that is above the autoignition temperature of the fuel.
Since diesels are essentially unthrottled, a full air charge enters the combustion chamber with every intake stroke, no matter what the power setting is. In this sense, where spark ignited engines vary the intake charge (and compression ratio) - maintaining a constant air/fuel ratio, compression ignited engines vary the air/fuel ratio - not the intake charge.
Listen carefully next time you hear a diesel vehicle accelerate, then coast. They are quite loud with that traditional diesel "knock" during acceleration, but the combustion becomes almost inaudible during deceleration. This is because diesels do not need to maintain a constant air/fuel ratio in the cylinders, and can basically cut the fuel entirely when none is needed. DFCO (deceleration fuel cutoff) is an integral part of compression ignition engine operation. If you gun a diesel engine from idle, you'll hear it rev up loud, then wind down very quietly until it reaches the set idle speed, then sort of "whoosh" back into steady-state idle.
There is a limit to how lean of a mixture spark-ignited engines can run because beyond certain air/fuel ratio (somewhere around 18:1 for gasoline, I think) the fuel molecules are simply too sparse in the air charge to fully combust from being ignited by a spark. This is not a problem in the compression-ignition engine because the intake charge is always hot enough to ignite any amount of fuel.
So, from what I gather, it's the spark-ignition which is the limiting factor preventing gasoline engines from being able to direct-inject like diesels do. A diesel engine is actually closer in operation to a steam engine, partly because the higher energy content of fuel requires more heat and burns slower than gasoline does. In regards to diesel fuel injection, the duration of injection is one of the main factors that determines the amount of power generated in the cylinder, very similar to a steam engine.
As far as the compression discussion is concerned, I'm no expert, but my normally aspirated gasoline engine gives me more miles per gallon accelerating at only 75% load than at 100% load. Perhaps this is simply due to the design of the engine and transmission. But since the compression ratio is lower at lower throttle settings, due to smaller intake air charge, this would mean that my engine is more efficient at a lower-than-maximum compression ratio. Just my thoughts...
__________________
|