How/why do cars with a start-stop system not use more gas or harm the engine more than regular engines?


How/why do cars with a start-stop system not use more gas or harm the engine more than regular engines?

In: Technology

I believe you mean hybrid vehicles.

The reason any car engines run at all is to create energy that will be used to accelerate the car. Traditional engines are always running even when they don’t need to, such as at a stop light of when coasting down a hill.

Gas-electric hybrid vehicles attempt to take advantage of the same thing diesel-electric engines benefit from in trains. In the case that hybrid engines might always need to be running, the transmission of power to the wheels is more efficient, delivers maximum torque without regard to engine speed, and requires no shifting between gears.

But hybrid engines, because for the most part all they do is generate energy in the electrical system, don’t need to run all the time, such as situations mentioned above. And deceleration also generates electrical energy while breaking.

They don’t use more gas because starting an engine doesn’t actually require that much energy. Let’s do some quick math.

Assuming the car has a 2kW electric starter motor and it takes 5 seconds of cranking to get the engine running it takes (5s / 60 / 60 * 2kW)=0.0027kWh or 2.7Wh of electrical energy to get the engine running.
Again, assuning we have a 60A alternator at 12V will produce 60A*12V=720W, which means in 1h it will produce 0.72kWh of energy. This means it takes the alternator 0.0027/0.72 * 60 * 60=13.5s to produce the electric energy the starter motor removed from the battery.

Of course you still get inefficiencies and your alternator will have to produce more a little more energy than that to charge the battery, but on the other hand your engine probably won’t need 5 seconds of cranking to start back up again.

And yes, a start-stop system will put more strain on the starter motor and related components. The starter systems of engines having a start-stop system are usually built more sturdy to account for that.

Your engine uses the battery to start so there isn’t any extra fuel usage when starting again at a green light. The fuel is saved by shutting off the engine when you fully stop the car so it doesn’t run the whole time at a red light.

It doesn’t harm the engine because when you take off at a green light, the car is doing everything it would do if it did not have the start-stop system; there’s just an extra step by starting the engine again. It would be comparable to stalling a manual drive car and restarting the engine quickly to take off. That doesn’t hurt the car either, it just shuts the engine off.

The vehicles that have this have heavier duty components that are made for more frequent on/off cycles. Usually a larger capacity battery and more strict tolerances on moving parts. It takes much less fuel to restart a warm engine than a cold one. In my area, these things are nearly useless because they turn off the A/C compressor, and it can reach 120 degrees in the cabin in a very short time. You can bypass chevy’s version by stopping completely, then easing forward a few inches.