Gaseous Fuels in Spark Ignition & Compression Ignition Automotive Engines

The grossly over-simplified answer is that gaseous fuels are easy to feed to spark ignition engines -- thus there are loads of natural gas buses, propane-powered fork lift trucks etc. Generally, mixture is controlled by a device that works somewhat like a carburetor, metering air and fuel in a gas-to-gas ratio, by volume, although mass-to-mass is used as well. Generally, the gases are cleaner burning than petrol. The drawback is the size and weight of tanks.

In compression-ignition engines, the fuel is best metered as a liquid, because of the high injection pressure required to overcome compression pressure. There are other issues as well, such as whether the injected liquid phase gas will self-ignite when vaporized. The most common gas fed into diesels is propane, which is fed as a gas into the intake airstream, and can accelerate combustion of the main fuel, which remains diesel fuel. The timing remains under control of the diesel fuel injection, because the propane does not self-ignite.

The environmental aspects are much too broad to generalize. They can be related to the environmental cost of mining/producing the fuel... all the way to the tailpipe emissions -- and just those two aspects can be complicated. Hydrogen, for example, appears to be clean burning, (although hydrogen-powered internal combustion engines do put out NOx emissions) but the commercially viable production means is from reforming of methane -- a process which gives off CO2. BMW is the only manufacturer who has supplied more than one or two prototypes of a hydrogen burning (spark ignition) car, and if you looked at the whole process of getting the H2 into the tank, and the large losses of H2 if the car sits unused (all the fuel boils off in about two weeks), you'd be environmentally further ahead to fuel it with gasoline, and further ahead yet to drive a diesel-powered version.

There is a fundamental requirement for efficient combustion and maximum developed power--proper timing of ignition of and rate of combustion.

With 'spark ignition', it is easy to control the ignition of the flame front, and mechanical details of combustion chamber shape and intake valve shape ,shrouds,etc. are used to improve rate of burn characteristics with good success.

Spark ignition engines, until this year, have always had vapor fuel-air mixtures ingested through the intake valve system, seeking uniform mixtures. 'Direct Injection' of LIQUID gasoline at very high pressure (>25,000 psi) in the latest engine technology achieves significantly better fuel economy with stratified fuel-air ratios.

Texaco conducted a lot of research for 'multi-fueled' engines for the U.S. Army in the late 60's-early 70's. The stratified charge , direct injection engine would perform well on anything liquid--from gasoline to jet fuel to No 2 diesel. The objective was to simplify logistics of fuel supply for military vehicles.

Compression ignition engines inject liquid fuel as a very fine spray (fog) into the very hot compressed air, where it starts burning, with fuel injection continuing for a significant time. The injected fuel is completely burned AS IT IS INJECTED, and there in no fuel vapor-air mixture as in a spark ignition engine. The initiation of burning and rate of burning is still very important. Mistiming fuel ignition of even 1 degree of crankshaft rotation significantly impacts power output and fuel economy. Therefore , the time between start of fuel injection and start of burning (called Ignition Delay) is an important fuel characteristic. That property is called 'Cetane Number', with higher Cetane meaning less ignition delay and therefore better power and economy, smoother operation, less smoke etc. The ignition delay property is almost the opposite of what is desirable for spark ignition fuels, where one desires the fuel to NOT self ignite until there is a spark, AND to resist auto-ignition in advance of the flame front sweeping the combustion chamber. We call that detonation if the self ignition is after the spark ignition, but label it 'pre-ignition' if it begins before the spark. Pre-ignition causes very high stress on pistons and bearings and can destroy an engine in seconds.

Thus optimum injection (and ignition) timing for compression ignition engines depends heavily on fuel characteristics, with the most paraffinic (waxy) fuels the best, and aromatic fuels the less desirable from ignition delay perspective. It would be very complicated to vary the moment of fuel injection start to adjust for ignition delay characteristics of a fuel with mechanical fuel injection. The latest electronically controlled 'common rail' diesel injection systems would be very easy to control IF one could figure out how to detect excessive 'fuel knock' that results from longer injection delay periods. This results in more fuel in the combustion chamber and burns near instantly when ignition starts, causing a much higher pressure than normal, which we can hear as a knock, but is difficult for instrumentation to pick out of the inherent combustion noise and vibration.

In summary, it is very problematic to feed a compression ignition engine a fuel-air mixture through an intake valve system and achieve reliable and accurate ignition timing without a spark. Gaseous fuels are much more resistant to self ignition from the heat than are kerosene and diesel fuels.

Also, the power output of a compression ignition engine is controlled only by the amount of fuel injected. The intake air is not throttled and there is always a vast surplus of air, which would be too 'lean' to burn as a vapor mixture under all but highest power requirements. If one throttles intake air, the compressed air temperature in the cylinder is much lower , too low to ignite the fuel.