I drive a slightly older (2012) truck and when I start my engine, it takes a couple seconds to for the engine to turn over. If I shut down and restarted at every red light, it would probably be annoying to drivers behind me. New cars can basically restart the instant you touch the gas. The delay is hardly noticeable. What’s different?
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In order to get start an engine, you need to ignite a cylinder full of fuel and air at the correct time. This means that you need to know the position of the pistons/valves/etc. to know which cylinder needs fueling and igniting. Conventional engine electronics often have quite crude sensors. These sensors tell the ECU how much a part has moved since the last measurement, but not where it actually is. Only at a specific position does the sensor also communicate that the part being measured has reached a certain point. For example, an engine rotation sensor may tell the ECU every 10 degrees that the engine has rotated 10 degrees. However, it is only when the engine crank reaches top-dead-centre of cylinder 1, that the sensor gives a second signal saying “TDC cylinder 1 reached”.
This means that the ECU doesn’t actually know what the engine position is until every sensor has gone past its “home” position. As some of these sensors are on the cam shafts, which rotate once for every 2 engine revolutions – this means that the ECU will have to spin the engine at least 2 whole revolutions before it even knows where the begin the fuel injection process.
With start-stop control, the engine electronics are not switched off between engine stops/starts, so the ECU remembers the position of all the engine components, and trusts them. This allows it to start the engine by fuelling the first available cylinder as soon as it is ready to receive fuel.
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