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Old 09-17-2008, 04:00 PM   #1 (permalink)
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Question --- When will gas self ignite?? --- Temp?

Hey guys, what is the equation that link at what pressure and temperature gasoline self ignites?

Like, lets say in diesel engines at 25:1 compression; what temperature must the air/fuel be to self ignite?

Also, why couldn't you just do 100:1 compression and be able to self ignite gas at much lower temperatures??



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Old 09-17-2008, 08:02 PM   #2 (permalink)
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I know this doesn't answer your question, but I will share what I do know.

Typically gasoline will self ignite on on whatever hot spot is around in the combustion chamber; typically a bit of carbon on either the head, piston, or most likely the spark plug.

One way to avoid self ignition is to do direct ignition, yes there are DI gasoline engines, many are 2 stroke outboards . . . . .

Again, not your actual answer, but I hope it helps . . .
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Old 09-17-2008, 09:48 PM   #3 (permalink)
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Quote:
Originally Posted by basslover911 View Post
Also, why couldn't you just do 100:1 compression and be able to self ignite gas at much lower temperatures??
The energy required for compression would not be recovered by any amount of gain in output.
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Old 09-17-2008, 09:53 PM   #4 (permalink)
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Quote:
Originally Posted by wikityler View Post
The energy required for compression would not be recovered by any amount of gain in output.
I'm not sure you are correct or not.

From a thermodynamis standpoint I agree that more heat would be produced, but . . . .

From a pressure standpoint fuel/ air mix is very nearly a spring. In an odd turn of events, I can coast in gear FARTHER than a 1.8t golf which was on similar rubber / pressures even though I have much higher compression (however I have less pumping losses due to no throttle plate . . . .


So I'm going with; please substantiate your claim, because I totally do not know the answer.


Granted I highly doubt that gasoline would benefit from super high CR, but maybe some other fuel would . . .
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Old 09-17-2008, 09:55 PM   #5 (permalink)
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Quote:
Originally Posted by basslover911 View Post
Also, why couldn't you just do 100:1 compression and be able to self ignite gas at much lower temperatures??
No need to worry about ignition load, typically on high compression you are worried about pre-ignition, not ignition.
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Old 09-17-2008, 09:56 PM   #6 (permalink)
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What do you think would be the max? Or let me ask, what is the engine with the highest compression that you know of?

I can say 25:1 in a diesel but I do not know if that would even be possible with gasoline... ?
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Old 09-17-2008, 10:03 PM   #7 (permalink)
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Quote:
Originally Posted by basslover911 View Post
What do you think would be the max? Or let me ask, what is the engine with the highest compression that you know of?
Very good question
From Wikipedia
Motorcycle racing engines can use compression ratios as high as 14:1, and it is not uncommon to find motorcycles with compression ratios above 12.0:1 designed for 86 or 87 octane fuel.

The highest I've dealt with was 14.5 to 1. By no means was it an economical engine to operate. It was an older 350 with Vortec heads, and some other goodies. Had to keep ignition settings conservative on pump gas, but not to bad.


Ofcourse the Dynamic compression ratio on many forced induction engines runs in to the 20to1's
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Old 09-18-2008, 12:13 AM   #8 (permalink)
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How would it not be an economical engine? I mean, I know it wasnt tuned for economy, but if it was wouldnt it be a heck of an economic engine? (higher compression can equal much better combustion and thus much better economy right?)
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Old 09-18-2008, 12:22 AM   #9 (permalink)
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Quote:
Originally Posted by basslover911 View Post
How would it not be an economical engine? I mean, I know it wasnt tuned for economy, but if it was wouldnt it be a heck of an economic engine? (higher compression can equal much better combustion and thus much better economy right?)
Well I honestly don't know how to answer this question other than to tell you about the setup.

Junkyard 350 shortblock. Burnt almost as much oil as my tdi does fuel; no, seriously in operation it used about 1 qt every 100 miles, but those were very hard miles.

Big cam required 1100+ rpm for decent idle, but sounded awesome at 900.

performer rpm airgap intake (high rpm)

kept it rich all the time.

Cold thermostat (ward off detonation)

Cold plugs that would frequently foul (more ward off detonation)

Turbo 350 with 3000 stall (no lock up)

MT ET Streets

Car was dirt cheap to build (~$1500 all inclusive) and was fun at the track, but efficient it was not.

Could that much compression be efficient? I think so. But gasoline may not be the appropriate fuel.

For some interesting reading google up the octane test engines (variable compression) and Saabs variable compression engines.
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Old 09-18-2008, 12:40 PM   #10 (permalink)
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Just to throw in my two cents...

What I think you guys are looking for is the auto-ignition temperature as compared to flash point. Below is a quote from the Wikipedia article on Flash point.

Examples of flash points
Fuel; Flash point; Autoignition temperature
Ethanol; 12.8C (55F); 365C (689F)
Gasoline (petrol); <−40C (−40F); 246C (475F)
Diesel; >62C (143F); 210C (410F)
Jet fuel; >38C (100F); 210C (410F)
Kerosene (paraffin oil); >3872C (100162F); 220C (428F)
Vegetable oil (canola); 327C (620F)[1]
Biodiesel; >130C (266F)

"Petrol (gasoline) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.

Diesel varies between 126F and 204F (52C-96C/WJ). Jet fuels also vary greatly. Both Jet A and jet A-1 have flash points between 100F and 150F (38C-66C/WJ), close to that of off the shelf kerosene. However, both Jet B and FP-4 have flash points between -10F and +30F (-23C - -1C/WJ)"

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