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BMW 3-Series (E90 E92) Forum
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Question about ignition timing
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| 02-09-2011, 08:38 PM | #1 |
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Second Lieutenant
 
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Question about ignition timing
Excuse my lack of knowledge on this matter. I thought I would put this question out on this forum for some of the experts to answer.
When the ECU detects knock it will retard ignition timing in order to prevent detonation. With that being said as the A/F mix is being compressed the cylinder head is moving towards top-dead-center. With the compression of the A/F mix getting hotter the more the cylinder gets to top-dead-center there isa higher possibility of knock. If the ECU is retarding timing wouldn't the cylinder head be closer to top-dead-centre before the spark goes off making it more likley for knock to happen? In this case wouldn't advancing timing (having the spark go off earlier) reduce the chance of knock? Can someone explain this to me as I can't get it around my head. Last edited by oner; 02-09-2011 at 08:45 PM.. |
| 02-09-2011, 08:59 PM | #2 |
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Brigadier General
 
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Last thing you want is more force against the piston as its going up....which is what you do by advancing spark
I think you should look up some key terms before you start racking your brain. detonation knock pre ignition General concept of combustion. |
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| 02-10-2011, 08:43 AM | #3 | |
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Lieutenant
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Quote:
Harry |
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| 02-10-2011, 10:52 AM | #4 | |
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Major
 
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Quote:
the engine works on a 720degree principle -- it take two revolution of the engine to complete one combustion cycle. intake stroke -- compression stroke -- power stroke -- exhaust stroke -- in regards to cylinder pressure and timing -- as the intake stroke finishes the piston is moving upward on the compression stroke -- timing is critical for the purpose of initiating the spark at the right time to keep cylinder pressures in check. if you start the process (advance) too early, the pressures will be too high creating unchecked heat transfer to the cylinder walls and head - since the engine is designed to have a single source of ignition, and relies on a controlled burn during the partial compression/power stroke, moving the time that the spark occurs will increase/decrease the total overall pressure once the piston reaches TDC. you can advance the timing on an engine for a short period of time to increase power, but once this process has begun the propensity of the engine to knock will become greater and greater every combustion cycle. if you retard the timing, you allow the spark to occur later in the process, the overall pressures are lower and the risk of knock is decreased. if you overly retard the engine you can induce preignition -- the combustion chamber charge ignites by itself due to compressing the a/f mix and then the spark occurs -- keeping peak cylinder pressures/temps within limits will help to eliminate detonation/preignition. |
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