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Auto ignition

Auto ignition

(OP)
Hello everybody,
While I was browsing the web, I came across something that makes me wonder. the auto ingnition tempreture of gasoline is higher than diesel, then why is compression ratio in a gasoline engine less than in diesel engines? what am I missing?

RE: Auto ignition

Diesel uses compression alone to ignite... gas engines use a spark (too much compression there will prevent the spark from igniting the fuel/air mixture).

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Auto ignition

Diesel engine relies on auto-ignition in order to function, so the temperature at the end of compression must exceed auto-ignition by a wide enough margin to ensure ignition. "Pre-ignition" - premature auto-ignition prior to nearing the end of the compression stroke - cannot occur because the fuel is not mixed with the air until just before the end of the compression stroke, when it is injected at high pressure.

Petrol engine relies on a spark for ignition and has pre-mixed air and fuel. "Pre-ignition" - auto-ignition prior to the end of the compression stroke - is extremely destructive because the air and fuel are already mixed and ready to go bang, and must therefore be avoided, so the temperature at the end of the compression stroke must be BELOW the auto-ignition temperature by a wide enough margin to ensure that it does NOT occur.

RE: Auto ignition

Long Before auto ignition occurs, "too high" compression would cause "detonation" in spark ignition engines. As would "too advanced" ignition timing.

Gasoline Combustion is a pretty complicated multi-stage process involving radical formation and other wild things in the "end gas" in the far reaches of the combustion chamber. The common explanation that high octane gas "burns slower" can be useful to understand the basics of detonation, but doesn't really reflect reality.

I believe this guy.
http://www.factorypipe.com/t_deto.php

RE: Auto ignition

SI (gasoline) recip IC engines mix the air and fuel prior to compression and then initiate combustion using a precisely timed spark. CIDI recip IC engines compress just the air charge and then inject a precisely regulated spray of fuel into the compressed air, which has been heated sufficiently by compression to initiate combustion of the injected fuel.

An interesting example you might look at is a dual-fuel engine. It uses a compression ratio similar to CI diesel, a pre-mixed intake charge of air/NG, and a direct injection of a tiny quantity of diesel fuel to initiate combustion.

RE: Auto ignition

The simple explanation is, gasoline is ignited by a spark. And a diesel is throttled by the amount of fuel and does not have a consistent air fuel ratio in the combustion chamber, at idle it has way more air than fuel. A gas engine uses a pretty consistent air fuel ratio for all rpm ranges.

RE: Auto ignition

A diesel wants auto-ignition.
A spark ignited engine (gasoline engine) Does not want auto-ignition.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Auto ignition

Quote (waross)

A diesel wants auto-ignition. A spark ignited engine (gasoline engine) does not want auto-ignition.

Technically, neither a conventional CIDI diesel or SI Otto cycle engine wants auto-ignition (detonation) to occur. Both engines want a combustion process controlled by precisely timed injection or ignition events. There is no way to consistently and accurately control the auto-ignition/detonation process in a combustion engine.

RE: Auto ignition

Sorry. I was under the understanding that auto ignition referred to a substance igniting when the temperature of the substance exceeded the ignition temperature. Detonation is one type of auto ignition.
I was under the impression that precisely timed fuel injection started the auto ignition process in a diesel engine.

Quote (Dictionary.com)


autoignition
[aw-toh-ig-nish-uh n]

Word Origin

noun
1.
Automotive. the spontaneous ignition of fuel when introduced into the combustion chamber of an internal-combustion engine, as a result either of glowing carbon in the chamber or of the heat of compression.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Auto ignition

In order for combustion to occur there must be the correct combination of oxygen, fuel and heat. With CIDI diesel engines there is always some amount of delay between the start of injection and the start of combustion. The spray produced by a diesel injector consists of tiny droplets of liquid fuel. And combustion only occurs at the surface of these fuel droplets where there is the correct mixture of fuel and oxygen. This is why a well dispersed and finely atomized spray of fuel from the injector nozzle, along with lots of mixing from air turbulence, is critical for efficient and rapid diesel combustion.

Auto ignition is by definition a largely uncontrolled process. A CIDI diesel uses precisely timed and regulated fuel injection to control the combustion process, so I would not consider it to be auto ignition. A modern high-pressure, high-frequency CR digital piezo fuel injector might use a half-dozen discreet injections per cycle, with pre-injections, main injections, and post-injections. The pre-injections reduce combustion noise, improve mixing of the main injection sprays, and reduce ignition delay. The post-injections help with exhaust emissions.

RE: Auto ignition

Are we talking about "auto ignition temperature" or "auto ignition".

The former is a key property of every fuel. Every IC engine must somehow elevate the fuel beyond this temperature for combustion to occur.

The latter is an undesirable phenomenon where a portion of air-fuel mix gets hot enough to ignite at some undesired moment.

je suis charlie

RE: Auto ignition

Maybe the latter (undesirable) should be spelled as one word?

RE: Auto ignition

HCCI combustion is probably the closest thing to auto-ignition. But HCCI combustion is regulated by both A/F ratio and charge temperature.

RE: Auto ignition

The 'burn rate' of gasoline is so much higher than diesel, the engine RPM would have to be at such an impractical level of operation to replicate the end of combustion points that provide the best efficiency. Resulting in a static compression ratio that would not fire at reasonable RPM levels.

RE: Auto ignition

Quote (ICEaddict)

The 'burn rate' of gasoline is so much higher than diesel, the engine RPM would have to be at such an impractical level of operation to replicate the end of combustion points that provide the best efficiency. Resulting in a static compression ratio that would not fire at reasonable RPM levels.
To me this is incoherent. Perhaps you could elaborate.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Auto ignition

With IC engines heat release rate is of concern. And a true constant volume combustion process can be very efficient since it occurs almost instantaneously. In theory almost any fuel (diesel, gasoline, alcohol, hydrogen, natural gas, ammonia, etc.) can be made to combust at constant volume if conditions are right.

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