So How Does It Work?

Basicly, LPG sits in your tank as a liquid. It travels through a converter which has the job of converting the liquid into a gas. From here it travels to either your carb or throttle body (on EFI vehicles) and is burnt in the regular way. But it has some special characteristics.

Engines need air and fuel. To make more power, you must burn more fuel, so therefore more air is needed. The amount of air available to an engine depends on to efficiency of the intake, porting, exhaust etc. The ability of an engine to pump air is called volumetric efficiency. This is determined by the shape of the intake & exhaust ports, valve lift & duration, combustion chamber shape etc.

If VE is reduced, so too the maximum power output will be reduced by a similar amount, like a dirty air filter reduces air flow, VE, and hence power. Liquid fuels, when atomized, generally consume a very small amount of space in the intake, so have a negligible affect on VE.

Gaseous fuels on the other hand, can consume between 4 & 15 percent of the intake volume. Hence, power output of the engine is reduced. This inherent power loss is not easily overcome. In theory, LPG causes a 4% VE loss actual results will vary.

While forced induction can increase the power output of any engine, this doesn't over come VE loss. The only way to completely overcome VE loss is gaseous EFI. As the name suggests, this works just like EFI, using additional injectors along side the petrol ones. At this point in time, gaseous injection is still being developed, but, hopefully is not that far off.

Power Loss:

Apart from power losses due to reduced VE, comes a problem that stems from the fact that liquid fuels vaporize as they mix with air in the engine's intake manifold, absorbing energy and cooling the intake charge. Gaseous fuels however, are already in vapour form and cannot cool the intake charge. This results in a higher combustion temperature of around 10%. Sparks plugs will need to be of a cooler rating to maintain normal service intervals.

VE is also lost with the fuel hardware itself. Older dual fuel conversions can suffer an additional 10% VE loss due to air flow obstruction. This same hardware may also affect air flow when running on petrol.

The octane rating of a fuel indictates how slow it will burn and therefore resist pre ignition, before the spark fires.

The higher the comp ratio, the more likely the mix is to pre-ignite (knock) higher octane fuels can be burned at higher compression ratios. Higher compression means greater power output with a given amount of fuel. This can make up for loss of VE. However this increases NOx emissions, which is easily combated with increased exhaust gas recirculation.

Application:

Ok now onto application. It should be noted that most shops specilaising in LPG conversions deal with economy concerned fleets and private buyers and many are lost when it comes to extracting maximum performance from a LPG instalation.

LPG works best with EFI style individual intake runner manifolds, for cross flow Falcons that means XE-XF EFI parts. This will mean you will need a dedicated LPG throttle body, which too is a good thing as regular carbs are designed for petrol flow to cool them, and can over heat on LPG only vehicles.

Any problems?, bertrand@omen.net.au.