The science of engine combustion.

[skippyc]

This is for the experts. Think big single cyclinder engine
Questions 1 Which burns faster? A lean or a rich mixture.
2. What is the science behind a backfire through a carb?
3. When there is no crossover on the valves, what could cause a puff of air to come out of the inlet before it sucks again.

[skippyc]

Found a picture to add.

[wilcom]

This is for the experts. Think big single cyclinder engine
Questions 1 Which burns faster? A lean or a rich mixture.
2. What is the science behind a backfire through a carb?
3. When there is no crossover on the valves, what could cause a puff of air to come out of the inlet before it sucks again.

If the cylinder fired prematurly and did not have enough inertia to "get over the top", might cause it to shove the piston down again and that would put it back on the intake stroke and right out the carb...…...

[skippyc]

If the cylinder fired prematurly and did not have enough inertia to "get over the top", might cause it to shove the piston down again and that would put it back on the intake stroke and right out the carb...…...

It happens all the time when it is running.
My guess is a restricted exhaust which leaves some pressure still in the cylinder when the inlet opens. It does have 3 mufflers and a long pipe out to the outside.

[wilcom]

It does have 3 mufflers and a long pipe out to the outside.

Would it be feasable to crack an exhaust joint and give it a test?

[skippyc]

Would it be feasable to crack an exhaust joint and give it a test?

Yes But the other club members aren't interested in improving how things run and won't listen to other people.

[wilcom]

What kind of engine is it? What kind of "valves"?

[Tony Smith]

Lean burns faster than rich.

Your inlet valve leaks


The combustion chamber is still heated and gas in there is also rather turbulent.  When the inlet valve is slammed open - usually when the piston is  either"on the rock" or even prior to the actual intake stroke (usually due to wear in the cam/actuating lever path) there is local heating and therefore expansion of the air in the inlet tube plus some intermixing with the turbulent heated gas in the combustion chamber which causes a momentary "puff" of air out the carb.

The last is nothing to worry about - in fact it happens in our own beloved boxers (and for that fact any 4-stroke engine) - it is a question of degree and all the old stationary engines I've worked on have suffered from wear in the cam and wear in the path of levers, rods etc. leading to the valve.

Backfiring should be looked at as if the cause is what I suspect (leaky intake valve) eventually bad things may happen.

Lastly remember that any "pump" fuel you buy now is goign to be a lot higher octane than what the thing was designed to run on - I used to try and work out the octane of the fuel an engine was designed to run on and then cut pump petrol with kerosene to get approximately the same - at least is going to be "worked" - if it was purely display, running at little over idle and connected to a nominal load, I didn't bother.

a consequence of modern fast-burning fuel can be backfiring and also excessive heat in the combustion chamber - you can measure that with a thermocouple under the spark plug

An alternative is to run them on compressed natural gas if you can buy that cheaply in your jurisdiction, it is very kind to old engines being slow burning

[skippyc]

Thanks Tony The engines were designed to run on suction gas. There are 4 of them actually and they are Richard Hornsbys. Vintage 1911 to 1913.
They have 3 valves, exhaust, inlet and fuel which opens into the inlet manifold. They are running on premium petrol through a homemade Austral type carb which is so bad that fuel drips out of the fuel manifold when the have been running a little while. To get them to run like this the air inlet is almost closed off. When the governor cuts in it stops the fuel valve from opening so the engine sucks on the inlet air passage which is closed and sucked up the exhaust valve as one of the springs had broken and now the valve is leaking.