interesting entwood.
I was under the impression that although LPG has a higher RON (100 or there abouts) it has a lower energy content compared to 95 ron unleaded hence the lower power output of an engine that has been converted.
The compression must be raised in the converted engine for LPG to produce the same power.
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You are partly right in that, if you were designing an engine for LPG from scratch, you would run higher compression than for petrol and would probably advance the ignition timing a little. These are not easy to adjust when converting a vehicle, so there is a compromise in doing so that loses you
some power and economy.
In addition, you are injecting LPG into the intake as a vapour whereas petrol is injected as a liquid. The liquid fuel takes up less volume than the vapour. This means that more volume is available to get air into the cylinder when running petrol. More air means more fuel can be burnt in each cycle of the engine which means more power. So, there is always
some loss of power when running a SGI LPG injection setup versus the same engine running on petrol. As someone who runs such a setup I can say it's not a noticeable loss on the road.
A "mixer" style LPG system is much worse, because the vapour is added to the mixture much earlier in the intake and displaces more air. Anyone installing a mixer on a GM V6 is heading for trouble anyway, though, so we'll discount that.
There are
liquid phase LPG injection systems around. They are in their infancy, expensive and not that reliable yet, but they do allow the engine to develop more power on LPG than on petrol because the LPG is kept as a liquid until it reaches the cylinder.
Because the LPG boils at a much lower temperature than petrol, liquid injection of LPG absorbs a lot of heat from the air as it is injected, making that more dense, so it has a kind of "charge cooling" effect.
Kevin
Author
Topic: LPG and BHP (Read 2051 times)