Re: And this does not make sense to you why? - Saab General Bulletin Board - Saabnet.com
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Re: And this does not make sense to you why?
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Posted by dmz789q [Email] (more from dmz789q) on Thu, 28 Jan 2010 09:36:49 Share Post by Email
In Reply to: And this does not make sense to you why?, dmz789q, Thu, 28 Jan 2010 09:25:42
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Full link below. Here is just the important part:

When first cranking, the crankshaft position sensor indicates top dead center for each pair of partner cylinders but cannot tell which cylinder is in the compression stroke and which is in the exhaust stroke. The ECM has the ability to remember which was the last cylinder to fire as the engine was shut off and can use this information to determine which cylinders to fire when cranking. If this information is not available due to loss of battery power and memory, the DI will determine which cylinder to fire by using the spark plugs as combustion sensors. During the first few cranking revolutions of the engine the ignition will supply spark to both partner cylinders at Top Dead Center (TDC) until it can synchronize the spark to the proper cylinder. In addition to sending the actual spark, DI constantly applies a 70-volt reference signal across each spark gap. This 70-volt reference would not normally cause any current to bridge the resistance of the 1.1 mm spark plug gap. However, during the heat, pressure, and turbulence of combustion, the gases within the chamber become highly ionized. The ionized gases create a more conductive path across the spark plug gap allowing the sensing current of the synchronization circuit to flow. This current flow signal informs the ECM that combustion has occurred in this cylinder while its partner cylinder, on the exhaust stroke, does not transmit such a signal. The ECM will discontinue sparking both partner cylinders after the first 25 successful cylinder firings. The system eliminates the extra spark to prevent unnecessary spark plug erosion.


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Saab Direct Ignition Workings

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