"Not your usual" question about LPG...

OK, can someone explain a couple of things about LPG to me please?

My belief has always been that on the whole an engine will produce more power and run more efficiently on super unleaded than normal, as it has a higher octane rating - typically 98RON vs 95RON. I know this is true of my BMW 330Ci, although also realise on smaller and older engines the improvements will be less significant and it does require a higher compression engine to make the most of it.

Anyway, following this train of thought and given that LPG is typically around 102-108 RON, why do we get a decrease in performance and economy when running cars on it, rather than an increase? My Range Rover has a single point injection system and I reckon loses around 15-20% power on LPG, and probably a similar decrease in economy. So where do the loses come from?

My only thought is because it is injected as a vapour it burns less efficiently? If this is the case, if you were to inject LPG as a liquid, would you see the improvements I would expect from the higher octane rating?

Finally, if I were to run the car solely on LPG (I try and give it some petrol to run on from time to time for better lubrication of the engine) and increase the compression, would I see a corresponding increase in performance and economy? I have no intention of doing this, just being theoretical and trying to understand it a bit more.

Cheers!

im not sure, my landy is running better and more powerfully on gas than it was on petrol, and more economicaly. although i think thats to do with the carb was fecked. it used a 3rd of a gallon to do under a quater of a mile which is pretty bad!!!!! it was also underpowered and ticking over at around 2000 rpm...... gas its sweet as a nut, cheaper, no smoke, doesent missfire, im pleased with gas

I think you may find that it is something to do with the energy you can get from the fuel itself. Might be calorific value or something similar.

Interesting, I always thought a drop in power and economy was "the norm!"

Mine runs fine on LPG, no complaints at all, I am just interested to know more about it and why it is (usually) less efficient/powerful, when it should be more as far as I can see...!

Octane rating is a theoretical rating of the anti-knock properties of a fuel. the number is a percentage of what would be achieved with pure octane (there isn't any actual octane in petrol anyway). The octane rating hasn't got anything to do with the actual power output of the fuel. I believe lower octane rated fuels are capable of delivering more power at the expense of detonation problems which is offset by using a lower compression ratio. If you increase the compression ratio, the engine gets more efficient. thats where the extra power comes from.
Thats what I understand anyway.

The reason many modern cars run better on super unleaded than regular unleaded is because they have knock sensors fitted. These adjust the ignition timing to prevent fuel detonation, or "knock" As super unleaded has a higher octane rating than regular unleaded, it is less prone to detonation. The knock sensor detects this and advances the timing, which allows the engine to produce more power. So unless your engine is equipped with a knock sensor, you are unlikely to notice any difference in performance.
The reason you get less power from LPG is because it has a lower calorific value. This means it gives off less energy when it burns, so the engine produces less power due to the slower gas expansion rate in the cylinders. However, on some carb fed engines, clogged or poorly adjusted carbs can mean the engine runs better on LPG than petrol. If this is the case on your vehicle, then it may well be worth cleaning and adjusting your carb(s).
To answer your last question, Yes, increasing the compression may increase the performance slighlty, but not really enough to compensate for the lower energy produced by LPG compared to petrol.

Thanks martin, that explains that, then!

MikeV8SE said:

Thanks martin, that explains that, then!

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A modern multipoint LPG system would also yield more power too, as would having a separate map/tune to make the most of the higher octane of LPG.

Running the RR on higher octane fuel won't increase it's power though, only engines capable of adjusting the timing and having a knock sensor will, or cars already setup to run on it.`

300bhp/ton said:

A modern multipoint LPG system would also yield more power too, as would having a separate map/tune to make the most of the higher octane of LPG.

Running the RR on higher octane fuel won't increase it's power though, only engines capable of adjusting the timing and having a knock sensor will, or cars already setup to run on it.`

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Can you convert/update single point injection systems to run multi-point? And is it worth it?

MikeV8SE said:

Can you convert/update single point injection systems to run multi-point? And is it worth it?

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I don't know about upgrading, suspect you'd replace most of the engine bay side parts.

I'm lead to believe multipoint will offer very similar performance and mpg to petrol.

But it goes to follow, a SPI petrol engine (old Rover car engines) where less powerful and fuel efficient than the same engine running an MPI setup.

300bhp/ton said:

A modern multipoint LPG system would also yield more power too, as would having a separate map/tune to make the most of the higher octane of LPG.

Running the RR on higher octane fuel won't increase it's power though, only engines capable of adjusting the timing and having a knock sensor will, or cars already setup to run on it.`

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Agree totally


Your main issue is the single point injection installation. Yes, its 'possible' to change your LPG install to multipoint. Whether its worth it is a matter of choice really, tho would highly recommend multipoint. Yes, I honestly do not notice a difference when running on LPG to petrol, tho the RRC is better at cruising and don't think theres too much of a difference between the two there. But guess if you are flooring the throttle multipoint is be better

Has anyone added a propane system to there diesel's over there yet?

Fur Trapper said:

Has anyone added a propane system to there diesel's over there yet?

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Will be, but it's been done before, so not really covering new ground.

300bhp/ton said:

Will be, but it's been done before, so not really covering new ground.

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What I met to say was a injection system to work with the diesel to increase the HP, by the way of the air intake after the turbo?

Fur Trapper said:

What I met to say was a injection system to work with the diesel to increase the HP, by the way of the air intake after the turbo?

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Do you mean an LPG installation on a diesel? Which helps the diesel burn more efficiently?