11 replies to this topic

[Paolo]

Has anyone data about drivetrain (that is, engine+ gearbox, without wheels) inertia ?
I need them for some calculations about F1, but would also appreciate to have data from other cars, even street ones.
Thanks

[clSD139]

External combustion engine efficiency: 10%
Internal combustion engine efficiency: 30%

Drivetrain efficiency: 80%
Electric engine efficiency: 90%

The only friction an electrical engine has are bearings.
It doesn't heats, explodes, links, and the only intertia
is on the central axle itself (if speed is changed).

[Greg Locock]

? So what ?

Paolo, WAG is about 0.3 kg m^2 for the crank/conrods/referred inertia of the pistons, and much less than that for the gearbox. Driveshafts you can work out yourself.

Remember the n^2 rule for adding them together!

[Paolo]

Thanks, Greg, that was the data I was looking for.
By " much less for the gearbox " I take a tenth ( 0.03) could be OK ?

[Greg Locock]

I doubt it is that much. Hmm, that 0.3 may be a bit high, thinking about it. If the crank weighs 20 kg then .05 might be nearer the mark (ie 20*.05^2)

[Yelnats]

I have wondered about the contribution of reciprocating masses like pistons and conrods to inertia when compared to purely rotating masses. Since these objects must be acelerated and stopped during every revolution I assume their contribution to inertia is different than rotating objects like crank/clutch which carry all their inertia until it is released on deceleration.

But of course during the acceleration phase the release of energy by the deceleration of one reciprocating mass on reaching TDC/BDC no longer sufficient to accelerate the accelerating mass to speed as it would on average be travelling slightly faster than to opposing decelerating mass of a moment before (the very definition of acceleration). As this energy differential must come from the combustion forces like everything else, the two types of motion are not different after all as far as stored inertia is concerned.

No?

[Greg Locock]

A more complex way to work it out is to analyse the average speed, whether angular or translational, over one revolution, of each part, then add mass*average speed+polar moment of inertia*rotational speed for every part, then divide that by the rotational speed of the engine.

This gives you the total referred rotational inertia of the engine.

You also might need to do the camshafts etc.

In practice you'll probably find that taking the roational inertia of the crank and big ends is sufficiently accurate

[Yelnats]

So pistons and con rods contribute insignificantly to the force/energy required to accelerate an engine? Seems counter-intuitive to me. The masses of the reciprocating components should contribute significanty to the inertia of the moving masses.

[Greg Locock]

Well, do the sums and let me know. I am interested in the result, but I'm not gong to sit down and do the work!

Alternatively we can have a 4 page thread about it.

[Paolo]

I thought about making the sums, but, alas, don't know weights and inertias of components.
Anyway I found in several references that a car's apparent mass can be 50% higher in first gear than its real weight.
This makes the 0.05 Kgm^2 inertia realistic for a 620 Kg car.
One interesting consequence is that there is a limit to the reduction ratio one can apply in search for better acceleration (wheelspin apart).
After a certain limit, say around 10-12, the increase in apparent mass beats the increase in torque !!!

[Greg Locock]

That is interesting isn't it? I suppose it is merely common sense in some respect. If you floor the throttle in neutral the spin up time for a typical production engine is 1 or 2 seconds (WAG)

[wegmann]

You can estimate this value also by determining how fast the engine revs in neutral. Of course that's a bit difficult with F1 engines because they rev almost instantly (ever heard that clip of an engine playing 'oh when the saints come marching in'?).

Take the average engine torque and the time it takes to rev between two speeds, all in the appropriate units, and voila!

Anyway, I agree that for an F1 engine we're talking about 0.05 kgm^2. That's everything up to the 'flywheel'.

The clutches in F1 cars are around 5 inches diameter, IIRC, but I don't know how thick they are or how much they weigh.

EDIT: Oops, didn't read Greg's last post, which has the same idea.