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Does such a thing exist? If not, why not?
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Becaue the fuel would ignite far too soon before TDC, and the engine would try to turn backwards!
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I had an old A35 that would run for a few 100 yards after switching off the ignition.
The good old days of needing a decoke every year or two!.
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A petrol engine is suck-squeeze-controlled burn-blow, the only time a 4 stroke bangs is during pinking-which is not good for anything.
Ray
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Although I've never tried it!!, I think that if you injected high pressure petrol into a hot compressed volume of gas, it would burn in an uncontrolled way, and this would be the pinking or knocking described in other posts. The high pressure pulse so produced would cause severe engine life problems.
What you are after in an engine is a nice, controlled, progressive burn, where the flame front moves out from its inititation site, encountering neither excessive richness nor excessive weakness of mixture as it propagates. A very fast burn, but a controlled one nevertheless.
Diesel fuel is ignitable under high compression conditions, but not as prone to uncontrolled burning as petrol.
Number_Cruncher
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all you would get is a hole in the piston n/c
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Running on because of glowing carbon deposits was one thing, but there was something else called 'dieselling', not the same and didn't sound the same. Sometimes certain engines would almost stop, then fire back so that the engine spun backwards for a few revs before stopping properly.
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The point about a diesel engine (CI) is that the air is compressed alone first, then the fuel squirted in at the right moment just before TDC. It is not analogous with a pinking petrol engine, because in the latter the entire fuel mixture is compressed first, but something else (too high CR, hot spots, etc) sets it off too early.
I think the reason CI is not used with petrol is because it is not as readily ignited by pressure as diesel, despite appearing more combustible. Also, as someone has suggested, possibly the resulting explosion would be too violent.
I had an idea that dieseling was something that could happen with a two-stroke petrol engine, when it started to fire every other stroke - or was that something different again? Or is dieseling simply running-on, as opposed to pinking?
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Model diesel engines run like this, they use a fuel of equal parts of paraffin, ether & oil, run a simple carburettor & have variable compression ratio. Engines are available up to around 15cc but the bigger they get the harsher they run.
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I presume in model diesel engines the ether is the key ingredient which hasa very low ignition temperature. Doesn't it burn with what is termed a cold flame?
--
I wasna fu but just had plenty.
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I presume in model diesel engines the ether is the key ingredient which hasa very low ignition temperature. Doesn't it burn with what is termed a cold flame? -- I wasna fu but just had plenty.
Correct the paraffin is the fuel, the ether is to get it started & the oil is to lubricate, best performance comes with the max paraffin. They run considerably cooler than glow plug engines so I suspect you could be right about the ether.
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Not sure, but wouldn't it be very hard to design an engine that would compress petrol sufficiently to ignite it?
Petrol consists largely of aromatic hydrocarbons like benzene and its derivatives. These consist of rings of carbon atoms. The ring structure is actually quite stable: to burn it requires a high initial activation energy (eg the high energy spark off a spark plug).
Diesel consists of aliphatic hydrocarbons like parafins. These consist of straight & branched chains of carbon atoms. The straight structure is quite unstable: to burn it requires a relatively low initial activation energy (eg the compression in the heated cylinder).
The molecular structure of petrol can be compared to a thick rubber ring, like those dogs' toys. It's easy to bend it a little bit but quite hard to bend it completely and it quickly pops back into its original shape. Imagine if the same dogs' toy was made of identical rubber but in a straight line: this can be compared to the molecular structure of diesel. It would be quite easy to bend it completely in half and twist it into different shapes, plus the ends would quickly get frayed.
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No, ignition in a diesel engine isn't triggered by compression of the fuel. it is the high compression of the engine which heats up the air induced during the intake stroke suficiently to ignite the fuel when it is injected (think of how hot a bicycle pump gets when it's in use). The very high injection pressure produces a fine spray from the injector which is more easily ignited by the heated air.
JS
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Some versions of the mk2 Ford Escort had a compression ignition petrol engine!
Because of changes made to carburation, ignition and valve timing these engines would run on, forever, after the ignition was switched off, particularly the 1300HC - the ordinary 1300, not the Sport.
Ford's solution was to add an ignition operated solenoid valve to the inlet tract. It opened when the ignition was switched off, allowing air into the inlet manifold and destroying the inlet vacuum.
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