Engine who makes my engine

[bertsmobile1]

propane gets poor economy in cars converted to run on propane from petrol because the engine is not designed to burn propane .
I have driven propane vehicles with genuine propane engines that run substantially better than their petrol equivalents
They can get better MPG as petrol engines city use particularly as their idle speed is about 1/2 that of a petrol engine
On highway use that drops off a bit because the energy of propane per litre is lower than the energy content of petrol but ALL of the propane burns in a full propane engine where as a quite a bit of petrol goes in the cylinder and strait out the exhaust due to valve overlap which is one reason why modern cars have to be fitted with afterburners on the exhaust system ( called catalytic converters to hide the fact that a lot of what you pay for goes strait out the tailpipe ) .
Propane engine have no valve overlap and by prefference a resonant exhaust and a higher compression ratio

 

[Hammermechanicman]

Years ago briggs sold kits to convert engines to propane. I installed one. No cam change. It basically adds what amounts to a scuba regulator and a venturi. Intake stroke activates a diaphragm to let the propane in and a needle valve on the diaphragm assy regulates the mixture. You really could not tell the difference between gas or propane. It was on a constant speed generator.

 

[bertsmobile1]

The problem with doing this on an intek is the valve overlap and the decompressor on the intake

 

[bmeister]

OK I'm not an engineer, explain valve overlap.

 

[StarTech]

It is simple. It when one valve starts to open before the other fully closes allowing pressure from, in this case, the exhaust port to enter the intake port. This reduces the intake vacuum also pushing fuel mix by toward the carburetor and air cleaner. With wasted spark system which nearly all lawnmower engines use the chance of accidental ignition of this raw fuel being ignited depending on how much of an overlap occurs. Also the larger the overlap the more hot exhaust gases can enter the intake.

 

[bertsmobile1]

As Star stated
The theory of it is the inlet opens before the exhaust closes and the incoming air:fuel mix scours all of the old burned gasses out of the cylinder.
Fine on a car with a distributor because they only spark on the compression stroke
But a standard flywheel magneto fires every time the magnets pass the coil which is every time the piston comes to the top of the cylinder
So you gat a spark while the inlet valve is open and propane is coming into the cylinder
When running on petrol this is not a big problem because it takes quite a bit of time for the fuel to be atomised and even more time for those droplets to evaporate & become gasseous so they can burn
However propane is a gas to start with so is ready to burn way before petrol would be thus you risk the propane initing and back burning through the carburettor to the evaporator .
Add to that propane burns slower than petrol so a designed to run propane engine will be a long stroke engine ( stroke is substantially longer than the bore )
And because it burns slower you generally use more advance than you would on a petrol engine up to around 3000 rpm

This is why converted engines always run poorly , use a lot of gas & produce less power .
Timing is a big problem on gas powered engines
To get the beast out of LPG for instance you need to advance the carb about 5 to 10 degrees at idle
However this leads to burning of the valves

This is why you are always better tobuy a genuine propane engine than to convert a petrol engine to burn propane

 

[Hammermechanicman]

To over simplify it overlap it needed because of inertia of gasses. As engine RPM goes up the exhaust closes later and the intake opens earlier to get more power. OPE engines operate at a relatively low RPM and the valve overlap is fairly small. As you increase valve duration and overlap the first thing you will notice is poor idle and poor low RPM performance.