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Re: Knocking Engine Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 5 Apr 2002 10:01:05
In Reply to: Knocking Engine, Saabster [Profile/Gallery]
, Fri, 5 Apr 2002 04:21:48
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As Eric points out, the engine control indeed does knock detection. Most cars use a piezolectric sensor that 'listens' for knock. When it hears knock, it adjust ignition timing.
Saab uses the spark plug as a sensor. When the engine knocks, the combustion charge burns differently than normal (non-knock), and the system actually pushes a small current through the plug after setting off the spark, and it measures the ionization of the combustion charge in that cylinder. Pretty cool.
As Eric points out, you MUST use the exact spark plug specified, gapped as specified. You can't use some other plug that the autoparts guy's book said was cross-matched.
The engine control can adjust the timing for each cylinder individually, and also reduce boost to control knock. But the control can only do so much. If you are running the wrong plug, or have a carbon buildup problem in one cylinder (increases compression - more knock), or something else is wrong in the system, then the engine can keep knocking.
When you run lower octane fuel and the engine knocks, do you get full boost? The boost gauge should go to the top of the yellow/beginning of red under full boost. If you can only get to mid-yellow, then you know the engine control is dialing back boost to compensate for knocking.
If it isn't dialing back boost - you're getting full boost and hearing knock - then something is amiss. Either the spark plugs are wrong, or the BPC (boost controller) has failed in a way that it can't reduce boost.
You want to get this addressed. Yes, some engines are designed for a little knock. Turbo engines AREN'T. That's why there's a boost controller based on knock. Knock is a little explosion, and under the increased pressure of boost, can damage the pistons and head.
Ideally, you should be able to run your car on 87 octane (US rating) Since you use terms like petrol, I assume you arent from the US, so ratings differ. But you also have an 8 year old car. Some engines need higher octane fuel as they age. The engine control may not be able to adjust boost and timing enough to prevent knock on low octane fuel.
Run higher octane fuel for now, and check your plugs.
posted by 140.157.4...
, Fri, 5 Apr 2002 04:21:48, Fri, 5 Apr 2002 10:01:05 <-- Viewing This Message
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