The science of engine combustion.

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The science of engine combustion.

Post by skippy » Wed Dec 12, 2018 10:23 pm

This is for the experts. Think big single cylinder 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.
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Re: The science of engine combustion.

Post by Jack Gifford » Thu Dec 13, 2018 6:59 am

Okay, I'll be a sucker and try, although I'm definitely not an "expert"...

1> In terms of speed of fuel mass burning speed, I think that a rich mixture is faster. However, the greater mass of fuel to burn causes the flame front speed to be slower, which explains why a "fat" mixture is "safer" regarding detonation potential.

2> Again, I think it occurs at slow engine speeds as an intake valve is closing, when hot (or burning) mixture is pushed back into the intake tract and ignites mixture there.

3> Just a guess- that inertia of the incoming air mass causes it to "bounce" off the closed valve?
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Re: The science of engine combustion.

Post by minetymenace » Thu Dec 13, 2018 9:08 am

4. The pressure of the hot gasses in the chamber must be greater than atmosphere when the inlet valves open. Caused by valve timing, exhaust velocity too low.

I would guess that your valve timing is not optimal (normally adjustable on something like that). If the inlet inlet valve is operated by engine suction against atmos, could be exhaust gas velocity too big and/or spring too weak.

Studying the photo, I would also suggest that it wouldn't do any harm to loose a few pounds from the girth................I'm talking about the flywheel, the flywheel! :oops:
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Re: The science of engine combustion.

Post by skippy » Thu Dec 13, 2018 10:43 pm

That was an old school friend from Tasmania.
I also suspect exhaust restriction for the air coming out of the intake, as it has a long way to travel to the atmosphere.
I noted once that it backfired a lot when the ignition was left retarded.
Sadly the old farmers running these engines at the moment are not willing to experiment to solve the problems they are having, it's alright it's running.
They have stuffed up the exhaust valve and are to silly to see why it happened.
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Re: The science of engine combustion.

Post by minetymenace » Fri Dec 14, 2018 12:08 am

Think you should build a combo and challenge beat!
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Re: The science of engine combustion.

Post by beat » Fri Dec 14, 2018 9:06 pm

skippy wrote:
Wed Dec 12, 2018 10:23 pm
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.
Hmmmmm <201

1. lean mix is faster
skippy wrote:
Wed Dec 12, 2018 10:23 pm
Think big single cylinder engine
What is " BIG " ? --the one in the picture ( self ignition ) --ore 500 cc B50 ??

2. ...............

3. ..............

beat <017

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Re: The science of engine combustion.

Post by skippy » Tue Dec 18, 2018 12:10 pm

Here’s the information on the size of the engine
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Re: The science of engine combustion.

Post by minetymenace » Tue Dec 18, 2018 2:08 pm

A year ago, I attended the the Christmas meal for the volunteers who run Heckington Mill (an 8 sailed windmill in Lincolnshire). When there is no wind, they use one of two old oil engine to turn the stones, one of these engines is called "Maud". At the meal, MAUD Awards were dished out, MAUD standing for "Mill Award For Undying Devotion", and the guy who tended the engines got one. The next day, while working on the engine, he got the toggle of his fleece caught in the peg that pins the flywheel. "Undying" is now considered inappropriate. I know of two of these engines, both have killed their custodians. Beware of rotating machines!
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Re: The science of engine combustion.

Post by beat » Tue Dec 18, 2018 11:02 pm

Hmmmmm <201

this Hornsby 16", dos it really have a Ignition ?
if so, is this ignition not used only for starting prozess ? ( and shortly after starting, it runs on self ignition ??

after my experience, backfire on a big self ignitor is the result of a ( partly ) clogged exhaust, so glowing particles from the combustion bringing the new incoming fuel to early to ignition.

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Re: The science of engine combustion.

Post by minetymenace » Tue Dec 18, 2018 11:30 pm

Think they have a bulb into which the fuel is sprayed, connected to the cylinder by a narrow tube. To start you need to heat the bulb with a torch and then glows proportionally with load. Huge flywheel, lots or rotating mass and low compression.
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Re: The science of engine combustion.

Post by skippy » Wed Dec 19, 2018 9:34 am

They are low tension ignition. It has a trip generator and ignitor in the head, which is the same as points without a condenser. As the ignition is tripped the generator generates power and then an arm flicks open the ignitor creating a spark.
There is a lot of carbon in the head and the mixture can be very rich.
When the governor comes in and cut off the fuel valve there can be some petroleum still in the inlet manifold which gets sucked in as a very lean mixture and the engine pings
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Re: The science of engine combustion.

Post by skippy » Wed Dec 19, 2018 9:42 am

I have a video of one engine starting and running but I don’t know how to post it.
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Re: The science of engine combustion.

Post by stew79 » Wed Dec 19, 2018 10:01 am

skippy, 2 weeks ago i made some carb and regulator parts for a friends American engine which is very similar to this one (but half the size) its governor stops the points from opening, but is not linked to the carb. he runs his engines occasionally at shows but not with a proper load on them. this causes all sorts of problems as you would expect. its usual to remove one spring from the governor (if it has two) to help slow them down. to regulate the speed with the fuel will always be inaccurate, (as you describe) but with the greater size of this engine over my friends version, to cut the ignition may cause other problems ? if the big engine was worked a bit harder it would not be on the regulator at all and run very sweet just as intended.
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Re: The science of engine combustion.

Post by beat » Wed Dec 19, 2018 9:45 pm

skippy wrote:
Wed Dec 19, 2018 9:34 am
gets sucked in as a very lean mixture and the engine pings
IMO, - lean mix burns faster. AND pings !
and it ignites on glowing carbon very easily to cause backfire.
to much carbon is often the result of feeding a bad fuel such as a very heavy crude oil with to low pre-heating of it ore feeding some mix of diesel with vegetable oil and sorts of.

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Re: The science of engine combustion.

Post by skippy » Wed Dec 19, 2018 9:55 pm

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Re: The science of engine combustion.

Post by minetymenace » Wed Dec 19, 2018 10:23 pm

Ah, you can see the ignition coil...and there's four of the beasties!
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