How do modern gas cars stop and restart their engines so easily/quickly?


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?

In: 12

Cars with start-stop have starter motors designed specifically for that purpose. While your starter motor is only meant to turn over the engine once per trip, the one in a car with start-stop is more robust.

Starting an engine when warm is also considerably easier than cold because the lubricants are at the right temperature and there’s less resistance.

Such engines also have a trick where they will deliberately compress a piston or two at the top position so that when the starter turns the spark plugs will ignite and immediately get a detonation to help get the engine going.

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.

It depends on the engine.

Some have a more powerful or durable starter that is designed to handle repeated start-stop cycles.

Others have an auxiliary motor that is a part of a hybrid system.

My Ram truck has a starter motor which is powered by the 12v lead acid battery that starts the engine during ignition. Once the engine starts running under its own power, it disengages. The engine also has a much more powerful motor that’s connected to the drive shaft and is powered by a 48v Lithium ion battery. This motor adds torque when needed, recharges the lithium-ion battery when braking, and starts the engine during start-stop cycles.

The start-stop is cycle is noticeably faster than on comparable trucks from other manufacturers that use the starter alone to start the engine after a start-stop cycle

That type of stopping and starting requires there to be a starter that is designed to be used that many times. Regular starters would probably wear out if used that much.