Converting small petrol engine to run on propane

[openspaceman]

9 minutes ago, Woodworks said:

I can get the gas for £68 delivered and from what I can find out that should be 92 litres of propane. What economy figures do you think I should get with propane in comparison with petrol? I was working on the fuel economy being 20-25% less on propane

mass for mass propane has a bit more calories than petrol, so buying it by the kg  makes little difference. Of course we are forced to buy petrol by volume.

 

Propane and petrol expand quite a bit with temperature so at 0C there are almost exactly 2 litres LPG per kg, at 15C LPG has dropped to 1.875litres/kg. Here's the rub, most petrol storage is underground and about 10C most of the year, LPG autogas tanks tend to be above ground so their  liquid volume gets worse as the temperature goes up.

 

I reckoned my LR 110 was good for 19-20mpg on petrol and just under 14mpg on LPG. So just over 70% per litre which is about what theory suggests.

 

As I said earlier to get the best out of LPG you need a higher compression ratio to claw back the lower calorific value per litre, so a spark igniton engine at 12:1 should give a 12% better conversion than the same engine at 8:1 recouping about half the difference. Unless the engine has a knock sensor to retard the ignition when on petrol once converted you can only run LPG.

 

What I was attempting to show is that filling at an autogas pump can be cheaper than buying the bottles. I was told that chicken hatcheries and gamekeepers get the best deal on 47kg bottles as they use large quantities.

[mr kipling]

Motor homes have gaslow cylinders that you can fill at the pump not sure if they are portable or fixed to the van though

[Mike Hill]

@openspaceman

 

Did you run a fuel oiler on your landy?

 

I have a pickup on petrol/lpg that has a olier on it.The truck is new and I was told by a guy who works for Rolls Royce that the oiler is a scam on new engines as the valves and valve seats are harder today.

 

Autogas as it's called over here is half the price of petrol,it's great for commuting or long trips but not great for towing in my experiance.

 

If you buy all the parts a gas fitter can make you a tank which you can fill at the pump and refill your propane machines.many forklifts are propane here.

[openspaceman]

54 minutes ago, Mike Hill said:

@openspaceman

 

Did you run a fuel oiler on your landy?

No, although I was advised the valve seats would wear I think this only happens if you thrash the engine.

Mine was a simple conversion of a venturi between each SU and the manifold with jets fed from a demand valve in the vaporiser. The V8 Rover engine was very prone to cam wear at high mileage and I had a spare high compression engine from a later Range Rover which I intended to fit (and lower ratio transfer gears) but the engine never gave any problems to I gave it to a mate when I sold the LR. I didn't tow much and then it was mostly a nifty lift.

54 minutes ago, Mike Hill said:

 

I have a pickup on petrol/lpg that has a olier on it.The truck is new and I was told by a guy who works for Rolls Royce that the oiler is a scam on new engines as the valves and valve seats are harder today.

That makes sense as harder valve seats came in with unleaded petrol.

54 minutes ago, Mike Hill said:

 

Autogas as it's called over here is half the price of petrol,it's great for commuting or long trips but not great for towing in my experiance.

It's called autogas at shell stations here in UK

54 minutes ago, Mike Hill said:

 

If you buy all the parts a gas fitter can make you a tank which you can fill at the pump and refill your propane machines.many forklifts are propane here.

You could fit a tank from a motor home which you can legally and safely fill from the autogas pump. Filling another tank from that needs a bit of thinking about. The vehicle tanks have a float that prevents filling beyond 70% to prevent overpressure if the temperature goes up. If you fill an ordinary bottle it has no such safety and the  space is  allowed for by knowing the mass of lpg filled. It is easy to overfill and leave no space for expansion.

 

Also forklift trucks use a  liquid take off whereas ordinary bottles are vapour take off.

 

You can buy bulk LPG for heating at less than 40p/kg but you cannot own the tank and none of the suppliers I tried would allow a tank with liquid take off, also my garden was too small  for the safety margins from other buildings to have a tank at home (and they won't supply bulk LPG to a domestic dwelling that has a mains gas connection).

 

A director of a well known forestry equipment suppliers was a hot air balloon pilot and 30  years ago he told me he refilled his balloon tanks with LPG from a bulk tank with liquid take off but never mentioned how he bought it.

[Woodworks]

Cant find any problems in theory so I am going to order the kit and will report back on how well it works

[Rob_the_Sparky]

Note: LPG is not the same as Propane.  LPG contains a significant amount of Butane and this is what causes a problem when the tank get cold as it has a boiling point of ~0C.  Propane on the other hand has a boiling point of ~-42C so in the UK I doubt you will have any issues with it remaining liquid.

 

Don't know the details but I know in caravans you have to convert the gas appliances to run specifically on either LPG or Propane so I expect there will be a difference when running an engine on gas as well.

[openspaceman]

1 hour ago, Rob_the_Sparky said:

Note: LPG is not the same as Propane.  LPG contains a significant amount of Butane and this is what causes a problem when the tank get cold as it has a boiling point of ~0C.  Propane on the other hand has a boiling point of ~-42C so in the UK I doubt you will have any issues with it remaining liquid.

That's right and there is always a certain amount of each I imagine that is because they are separated out in the fractionating tower at the refinery. Calor blue bottles are more butane I think whilst the red bottles are predominately propane. Autogas at shell stations is near 100% propane in UK, NB Autogas is owned by Calor.

 

The issue about the gas in the bottle staying liquid is a different thing. Plainly in the summer there is not much of a problem but in winter if it remains liquid  it cannot come out of the top of the bottle  as vapour  whereas it will still flow out as a liquid from the bottom. So for instance as long as the coolant that flows through the evaporator in a fork lift gas truck can vaporise the liquid lpg the truck will keep running.

 

As liquids turn to vapour their hidden energy increases, enthalpy, without the temperature going up. Consider vigorously boiling water never exceeds 100C at normal pressures. This energy has to be supplied form somewhere. What happens with  a vapour take off is that as the liquid turns to vapour it cools the liquid surface, the faster you draw it off the more it cools. Thus the temperature rapidly falls below ambient if the air outside cannot supply heat through the sides of the cylinder. Hence you see a frost ring develop up to the propane level in the tanks as water vapour is frozen from the surrounding air.

 

It's the same if you have ever tried to refill a cigarette lighter and some of the liquid spills onto your fingers, as it evaporates it makes you cold.

 

Thus in cold weather and a bigger engine, like the 13hp Honda even a 47kg cylinder may struggle to supply vapour as the propane level drops.

 

[Dan Maynard]

I have a 24hp air cooled Honda engine on my chipper, think this may put the dampers on the idea.

[Woodworks]

We will see.

 

This is what the instructions for mine say

 

"For short periods of running any size of cylinder may be used, but for long term running particularly in cold conditions smaller cylinders may start to freeze with consequential loss of power. For reliable operation use a cylinder of at least 19kg net capacity. In cold weather it may be necessary to use 47kg cylinders"

[openspaceman]

1 hour ago, Woodworks said:

We will see

Yes we can fairly easily calculate the amount of propane used for a given power output and thus the amount of latent heat that must be supplied from the surroundings but there will be so many variables  that the proof of the pudding...