generic propane forklift engine question

Bill D · Dec 26, 2019

In general forklifts that run on propane are they just low compression gasoline engines with a different carburetor. Or, are they higher compression engines designed to run efficiently on propane, probably converted from diesel.
I ask because I had asked about propane fueled big rigs and found that propane likes to run much higher compression the gasoline engine. As in Diesel compression pressure ranges.
Bill D.

Rob F. · Dec 26, 2019

Some forklifts can use gasoline and propane. switch from one to the other. you can easily ADD propane to a diesel but a bit more work to totally convert one.

dalmatiangirl61 · Dec 26, 2019

My guess would be that propane forklift engines would be plain gassers with a propane system for fuel, I doubt they are higher compression, but could be wrong. For an over the road truck, where efficiency is more critical, things are probably different.

Scottl · Dec 26, 2019

Most are basically gasoline engines with a propane fuel system instead of gasoline.

The dual fuel units mentioned by Rob F. were generally gasoline lift trucks with a propane carburetor in series with the gas one. They tended to have issues due to mostly being run on propane so when switched over to gasoline they often had issues caused by gaskets and seals in the gasoline carburetor drying out after long periods of disuse. If you ever own one it's a good idea to run gasoline stabilizer and run on gas at least once a week to keep things wet.

john.k · Dec 26, 2019

Biggest issue with dual fuel is rusting thru of the disused gas tank......A lot of the propane mixers simply sit on top of the gasoline carby and rely on the throttle valve of the carby.

Bondo · Dec 26, 2019

The engine in my 1 customers Combi lift is just a S10 engine with a propane port drilled into it. It's the perfect placement of the propane port that is very important.

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dkmc · Dec 26, 2019

Propane is generally around 130 octane. Years back when gasoline fueled flat head industrial engines were around 7:1 compression ratio, the propane versions typically used higher compression pistons to take advantage of the higher octane, as that low of a CR would yield poor performance and maybe hard starting in cold weather. More modern forlklift engines are derived from higher compression auto engines (why re-invent the wheel), which are already higher compression, maybe in the range of 9:1 or even 10:1.
Propane is also less volatile than gasoline, and requires a strong spark to ignite.

ewlsey · Dec 26, 2019

I know the Continental L head engines that were used in tons and tons of forklifts were designed to run on almost any kind of fuel you like. They all have the same heads, same pistons, etc. They all have hardened exhaust seats, which is pretty critical for use with propane.

Some engines spec a slightly different ignition timing for propane. Others, like the Continentals, don't seem to care.

I'm not sure about propane fueled big rigs, but several big rig engine builders make CNG versions of the diesel engines they normally sell. Cummins and Cat for example. They have completely different heads and obviously different fuel systems.

Dieseldoc · Dec 28, 2019

Another issue with switching gas engines back and forth, is the fact that spark timing must be changed for either to run properly after switching over. Some large diesel engines converted to run on natural gas have a mag and spark plugs installed for ignition.

m16ty · Jan 1, 2020

Years ago, when farm tractors could be had with propane. There were dedicated propane tractors from the factory with higher compression. The difference in a propane engine and gas engine was higher compression heads. One of the antique tractor pulling tricks was to put a propane head onto a gas tractor. You then had to run high octane gas, but it gave you a noticeable power increase. Then you had the tractors that were setup to run on kerosene or distillate, they had lower compression than the gas models, and usually had a separate small tank to start on gas and switch to kerosene or distillate once the engine warmed up.

I don't think things like forklifts ever ran higher compression, dedicated propane engines. On something like a forklift that don't require all that much hp anyway, it was never a noticeable power loss with running propane on low compression, and the engine makers could save money by just providing one engine for gas and propane.

dkmc · Jan 1, 2020

You might be right. I was told the propane engines were higher compression by a mechanic at a Clark dealership back in the late 1970s. Doesn't mean its true. A different head would be simpler than different pistons. But I think he said pistons.

m16ty said:
I don't think things like forklifts ever ran higher compression, dedicated propane engines. On something like a forklift that don't require all that much hp anyway, it was never a noticeable power loss with running propane on low compression, and the engine makers could save money by just providing one engine for gas and propane.

m16ty · Jan 1, 2020

dkmc said:
You might be right. I was told the propane engines were higher compression by a mechanic at a Clark dealership back in the late 1970s. Doesn't mean its true. A different head would be simpler than different pistons. But I think he said pistons.

Years ago, there may have been different engines for propane and gas, but I’m pretty sure all forklifts in the past 30 years or so are all the same.

dkmc · Jan 2, 2020

Yes, the engines of the past 30 years are mostly based on automotive designs which are inherently much higher compression.

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