Does LPG's higher octane rating compensate for its lower calourific value?

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Yeah, if it's built for unleaded petrol then it'll survive just fine on LPG. Until the rise of the hybrid, every taxi in Oz ran on BBQ juice, without a problem. 3 transmissions to 1 engine was about normal for a Ford Falcon... for older vehicles without hardened valve seats, we used to install a device called a Flashlube.
 
Originally Posted by NICAT
If any of you has used it, would you recommend it ? How much difference is there in fuel consumption ?

I have had 3 cars (1978 Chrysler New Yorker, 1977 Pontiac Parisienne, 2003 Chevrolet Impala) converted to propane and I highly recommend the conversion if the price is sufficiently low and you drive far enough.

Propane has about 75% of the energy on a volume basis than gasoline so a properly-done propane conversion should similarly get about 75% of the vehicle's gasoline economy on propane. I've found that my fumigation conversions were a bit higher (closer to 80%) and the injection conversion had around 80-85% of the Impala's gasoline economy. I can attribute this to better fuel mixture distribution, better propane ignition advance curves, and higher efficiency because I increased the compression ratio to take advantage of propane 105 octane for the fumigation conversions. Although the Impala was dual fuel with no changes to engine, I think its higher fuel economy was attributable to the PCM running the full programmed advance of the PCM.

Originally Posted by NICAT
LPG has less calourific value, which means reduced power and increased fuel consumption.
But it also has higher octane rating, which let engine use it as a premium fuel, so more advanced ignition timing, more thermal efficiency.
So in theory, it may compensate for its lower calourific value at some level. Car's engine is M54, with 10.8:1 compression ratio.

Fuel stoichiometry is based on mass rather than volume and, on a mass-basis, propane has more energy (20,038 vs 18,679 BTU/lb) than conventional gasoline. Although propane has a higher octane number (105 vs 87) for conventional gasoline, propane has a slower flame speed so you can't just increase the timing advance. A propane advance curve is typically higher initially and lower at higher RPMs and a good starting point is the 14-14-14 rule: 14° initial, 14° centrifugal, 14° vacuum). One guy with a 225 slant six pickup truck found that his optimum timing was much lower. See Update on the Truck and My timing map.

Originally Posted by NICAT

But, since gasoline is liquid, when it evaporates, it cools down the cylinders. But "standart" LPG conversions inject LPG as a gas to cylinders, means its higher octane rating wouldn't be as effective as expected.

You are correct. Running a rich fuel mixture on gasoline helps to lower the temperature of the fuel mixture and thereby reduce the likelihood of knock. Because propane is supplied as dry gas (unless you have a liquid injection system), propane does not have this evaporative cooling effect and running richer propane fuel mixtures actually increases the possibility of burned valves. Rich propane mixtures burn slower and hotter so it is safer to run leaner fuel mixtures. See Combustion 101

It is important to get both the fuel mixture and advance curves correct on a propane conversion, (just like a gasoline engine) but you don't have to worry about knock unless your compression ratio is high. Generally, propane can run leaner fuel mixtures (which improves fuel economy) and the advance curve needs to be set so that the pressure peak occurs at the correct point in the combustion cycle to maximize crankshaft torque. Too early, and the engine fights itself, too late and exhaust valves get burned.
 
Originally Posted by Shannow

Guy at work has converted a V-8 Audi to duel fuel, with port liquid propane injection, and the injectors fired off the OEM computer, but pulse width modified by a piggyback set-up...way too complicated for me, but with the phase change cooling the charge air, it goes pretty hard.

Liquid LPG injection is something that I need to do in my lifetime! Turbo, high AKI and high allowable compression ratio
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Originally Posted by fraso


It is important to get both the fuel mixture and advance curves correct on a propane conversion, (just like a gasoline engine) but you don't have to worry about knock unless your compression ratio is high. Generally, propane can run leaner fuel mixtures (which improves fuel economy) and the advance curve needs to be set so that the pressure peak occurs at the correct point in the combustion cycle to maximize crankshaft torque. Too early, and the engine fights itself, too late and exhaust valves get burned.

Such a critical point to all associating gaseous fuels to burned valves. It doesn't need to be that way. Get an EGT/pyrometer if you need to
 
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It takes a long time for exhaust valves to burn if the fuel mixture and/or timing are off. If you aim to maximize your fuel economy and power while you're setting up your conversion, then the majority of the fuel's energy ends up being used for useful work rather than being wasted as way too hot exhaust gas.

The point I was trying to make is that a properly configured conversion is both reliable and fuel-efficient.
 
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Originally Posted by hpb
Yeah, if it's built for unleaded petrol then it'll survive just fine on LPG. Until the rise of the hybrid, every taxi in Oz ran on BBQ juice, without a problem. 3 transmissions to 1 engine was about normal for a Ford Falcon... for older vehicles without hardened valve seats, we used to install a device called a Flashlube.

in eu there is plenty gas installers, praising the flash lube..
i think there is lots of f.u.d. used to sell that thing. but nobody gives the numbers, if lube it neccessary,,,,

actually, my 11:1 engine has no gas lube installed. its a original citroen instalation...
 
Originally Posted by pep808
hpb said:
Yeah, if it's built for unleaded petrol then it'll survive just fine

i think there is lots of f.u.d. used to sell that thing. but nobody gives the numbers, if lube it neccessary,,,,

actually, my 11:1 engine has no gas lube installed. its a original citroen instalation...


Ooh, 2 cycle oil to stop burned valves.

I was going to say I've never heard of an obdii car burning valves on any fuel type, amazing how people believe prehistoric issues are still a modern problem with a mad/map, o2 and knock sensor in tow.
 
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