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Originally Posted by ps2fixer
If you switch over to the diesel world, propane injection is known to act like putting NOS in the diesel engine, but it's much cheaper and it doesn't have to be floored if it's metered going in. I've seen claims of around 30% better fuel econ. I've read up a fair bit and that 30% most people don't account for the propane used. The propane is used as a 10-20% mix with diesel and the theory is the propane helps ignite and encourage a more complete burn of the diesel. The other added benefit is more power which is the most common focus for people wanting those systems. I've seen figures of propane vs NOS and they were real similar, propane + NOS had just at tiny bit more power but separately it was a huge jump over just diesel. Of my understanding, the only extra power from propane over NOS is propane is also burning as a fuel while NOS isn't. The guy doing the tests found even at 10% the jump was massive, I don't think he tried under that figure.
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Propane is already at the vapour phase once Diesel starts to ignite, so it will increase the flame spread leading to a more homogeneous and accurate burn of the Diesel fuel. Then, besides decreasing the amount of soot because of a more complete combustion process, more energy is extracted and turned to motive power from the Diesel fuel. Unless it's a computer-controlled setup which would trim the Diesel down when propane is added, as the total AFR gets a little richer there is a higher performance due to the total energy content of both propane and Diesel. Under certain conditions, it may actually lead to fuel savings, as it would allow to cruise on a higher gear while going uphill, or at a lower RPM also with a higher gear.
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Compressed natural gas engines should be somewhat similar I'd think.
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You mean spark-ignition engines such as either dedicated-CNG or the bi-fuel conversions I usually see in my country? On a sidenote, in the last 3 years I have seen more big-rigs with CNG enhancement in my country, and it serves to the same purpose as propane, but since I have only seen it added to trucks with electronically-governed engines the Diesel fuel is trimmed down in order to increase savings more than performance.