Can you tell what this is ?

[Gemaeden]

callmedave said:

Simpo Two said:

Fair point. Does it hold true for cars? In other words if you want to go from 0 to 60, do you use less fuel acclerating hard for a shorter time, or vice versa?

Yes. Imagine driving in top gear but at idle revs, how much fuel would you use to get 100 miles.

Now same gear but do 60 mph. you would have used less fuel after 100 miles. - its not rocket science!

The faster car will be subject to greater drag and as long as the idling car can move without problems the slower car will use less fuel.

[Zad]

Engines aren't very efficient at idle. Effectively 100% of the energy you put in is wasted just keeping the engine rotating. Actively driving the car forwards, it will also only have a low maximum power (wide open throttle) at low rotational speeds. So, if you are accelerating only very gently, a large % of the power being generated is just overcoming those internal friction losses. You will quickly meet the point where the rolling friction and air drag equal the extra power the engine is generating. At the other end of the scale, those friction losses, both in the engine and the car itself, increase approximately as the square of the speed (although in the engine at higher speeds third and higher order losses gradually appear).

Exactly where the optimal sweet spot depends on many many things, petrol vs diesel, normally aspirated, supercharged, turbocharged, engine friction, blah blah... Don't even consider going up a hill, that really messes the maths up even more.

[callmedave]

Zad said:

Engines aren't very efficient at idle. Effectively 100% of the energy you put in is wasted just keeping the engine rotating. Actively driving the car forwards, it will also only have a low maximum power (wide open throttle) at low rotational speeds. So, if you are accelerating only very gently, a large % of the power being generated is just overcoming those internal friction losses. You will quickly meet the point where the rolling friction and air drag equal the extra power the engine is generating. At the other end of the scale, those friction losses, both in the engine and the car itself, increase approximately as the square of the speed (although in the engine at higher speeds third and higher order losses gradually appear).

Exactly where the optimal sweet spot depends on many many things, petrol vs diesel, normally aspirated, supercharged, turbocharged, engine friction, blah blah... Don't even consider going up a hill, that really messes the maths up even more.

Thanks for this Zad, I was wanting to say this but could not put it into words.

[bearman68]

Drag is proportional to speed ^3 (?) so the faster you go the more energy you use for the same distance. There may be some optimisation of engine design and speed, but for the same engine conditions, faster = more energy.