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rojo's avatar

What is happening when the engine cuts off on a newer vehicle?

Asked by rojo (24176points) May 8th, 2017

I am wanting to know about the ones that do it as a fuel saving measure when you stop and apply the brake. Does it really stop completely? If so, how does it start up again as soon as the foot is removed from the brake? My son says he thinks they just drop to an idle with only two cylinders firing but it doesn’t feel like any parts are still active?

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LuckyGuy's avatar

The engine does indeed stop and your accessories are powered by the battery. When you step on the throttle the battery sends high current to the starter which is strong enough to spin the engine up to speed.
Each manufacturer has taken a different approach and they call them different names. ISG used to be called Integral Starter Generator. One device is used for both generating/charging the battery and starting. Some call it Intelligent Stop and Go. Some call it Idle Stop and Go. There are other names.

Rather than having a starter motor bolted to the side of the engine some manufacturers have the starter armature as part of the flywheel. That way they can use it to both spin the engine on start up, and slow it down using the excess power to charge the batteries. It is a clever idea that saves energy but is very difficult to produce in production. Magnets need to be powerful and clearances need to be incredibly small for the system to be efficient.

Here is a nice write-up in the GM system. They were one of the first to try it. Stop Start System

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