2y ago

YSK: A car takes uses about seven seconds worth of fuel to start up. If you are going to idle for a period of time longer than that, you'll save fuel by turning your engine off.

Why YSK: People seem to, on average, think that a car takes a lot of fuel to start up. In reality, it takes on the order of a few millilitres of fuel to start an engine. That means if your car isn't equipped with an automatic start/stop system to stop your engine instead of idling, it saves fuel to turn off your engine and start it back up when you need it.

Caveat: air conditioning and radio might not work with the engine turned off.

Scenarios where this might be useful include waiting for trains to pass at rail crossings, waiting for food at drive-throughs, dropping off or picking people up on the side of the road when they need to load stuff, etc. May not be a good idea to use this while waiting at a red light because starting the engine does take time which would annoy drivers behind you when the light turns green.

Some cars are equipped with systems that will automatically stop the engine when you are idling for a while (e.g. waiting for a red light). If yours is, then manually turning off your engine will probably result in reduced fuel savings compared to just relying on the car to do it for you.

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Caveat: For cars not equipped with automatic start/stop, the starter and possibly the battery might not be specced for it so it could cause additional wear. Cars with start/stop systems often assist the process with precise camshaft position measurements and the ability to squirt fuel pretty much right away so the starter doesn't need to do as much work.
Also don't do it with a cold engine - it's better to get the oil up to temp faster, it'll also reduce fuel consumption as the engine heats up.
- I walk, cycle, or longboard places whenever possible, and when these automatic cars started coming out, I thought they were manually starting and stopping their cars at each intersection. It really tripped me out.
- Cars with start/stop systems often assist the process with precise camshaft position measurements and the ability to squirt fuel pretty much right away so the starter doesn't need to do as much work.
  I always wondered why hybrids could start their engines instantaneously, when many conventional cars couldn't. This is why, isn't it?
- Thanks for reminding everyone of this. The gas savings over time will probably end up being the same, or less, than a starter on an older vehicle. Of course, if you're not planning on keeping the vehicle until it dies, this is less of an issue for you.
  
  It's the battery prematurely dying that's also an issue. Especially if the battery needs coding to the car. Could be a £300+ job. That's a lot of fuel that'd need to be saved to be close to worth it.
  It's all about emission testing anyway. Keel start stop off.

Yeah new cars usually have the ability to do that themselves and also usually do it when safe.
They auto start when power is needed or if the situation changes. Eg: touch the steering or gas pedal. Another example is my car will auto start it too many cars are around me.
Shutting your non-auto starting car off and then having an emergency happen could land your ass in trouble with insurance and the law. If you're on the road, your car should be running (unless it was designed this way). Of course people mentioned wear and tear, so that too.
Cheers -Henry

Starting and stopping a car is the worst thing you can do to it, doing it repeatedly on purpose is just asking for expensive problems, like a burnt out starter, missing or worn teeth off the flywheel, broken stater mount on the block, dead batteries, coked up and worn out turbo bearings, bearing and knock issues due to lack of lubrication, soot buildup in diesel engines, failed emissions systems, etc.
The few pennies you save in fuel is not worth it, upping the time to 5 minutes would make more sense.
- In some cities in my country (maybe all) it is regulated and fined to idle your car over 1 min, the point of it I think is to make people used to turning off their cars for train crossings and bridge openings.
  
  How does that work with stop lights? I'd say most stop lights are longer than 1 minute, so does everybody turn their car off while waiting? Does that cause a noticeable delay when the light changes?

Mild-hybrid cars with 48-volt batteries (instead of the standard 12 volt) can pretty seamlessly stop and start the engine to where you don't even feel it, and easily power the AC and radio and everything else while the engine is off.

Starting the engine puts as much wear on it as driving for 50 miles. Automatic start/stop hurts engine longevity by doing that unnecessarily and should always be turned off.
My car uses 0.5l of fuel idling for an hour. There's no way in hell that a start/stop system would even save 10$ a year, so there's no benefit to using it.
- That's not true, cold starts cause wear not warm starts.
  At most it's starter motor and battery wear, start stop cars have agm batteries for that reason.
  
  And a heavier duty starter that is specifically made to be used often.
  
  What about the fact that the oil drains to the pan in those few seconds that the engine is stopped?
  This is my real concern. Sure you can upgrade starter motors and batteries to handle the extra cycles, but you can’t do anything about increased scoring and wear on cylinders in the milliseconds before the fluids start to circulate again.

- Many new cars have start/stop features builtin. If the computer controller detects that the engine may have trouble starting (low temperature, low battery, starter motor failure, whatever), it won't stop the engine for you.
  Or that's the theory.

It's not necessarily an issue of fuel, but the overall wear on the components and engine when you start a car. A starter motor only has so many "starts" it can do before dying. The battery too.
Starter motors have gotten a lot better since the "bad old days" and engines start more smoothly thanks to fuel injection and computer control systems, so manufacturers have decided that it's ok to start/stop engines as needed, but the reason for not doing it was never a matter of fuel savings.

The whole world has weighed in and explained why more starts and stops lead to excessive wear, but I’d like to take a moment and bring a little attention to the environmental damage that having to do a bunch of work to your car because you turn it off at every opportunity does.
Let’s say you need a new starter motor early. That’s copper, aluminum, iron, steel and a handful of rare earths for the solenoid. Melt em, smelt em, form and mold em, that’s more carbon from building the replacement part than you’d have kept out of the atmosphere by shutting off the car at stoplights.
The greenest car parts are the ones already in it.

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