Inertia of engine? (for car acceleration simulator)

[raymanh]

Hi,

I am writing a program (in C if you're interested) to simiulate the 0-60 mph acceleration of the mighty Rover 827. I'm having an issue with simulating the rpm drop during gear change. I could do it by using the stored rotational energy of the flywheel and engine internals, then finding the friction (that will cause the rpm drop). However I'm struggling to find rough values for the inertia of the engine, a 2.7 V6.

Could anyone give me an approximate figure, or (probably asking more reasoinably) a typical flywheel mass, crankshaft mass, and friction force?

Thanks in advance.

[Nick1point9]

Have you got access to CATIA? If you do, then model the parts and apply material properties and you'll get a pretty realistic number.

For the flywheel, you could integrate the thickness of the flywheel with respect to effective radius, (to give volume*radius) and multiply it by the density of the material.

Personally I would avoid using "typical" values.

[jeremyc]

Maybe drop a line to RichardD

I believe is the originator of clever simulation stuff like this:

[Pumaracing]

In my own vehicle performance simulation program I use a value of 25 lbs at an average radius of 3.5 inches for an average four cylinder engine and bump that up to 35 lbs for a V8. You could use 30 lbs for a V6. That covers crank, flywheel and piston inertia and generates accelerations very comparable to real car test data over many hundreds of simulations.

I don't use this for rev drop though. It generates an effective vehicle mass addition factor after taking gearing into account which is then added to the vehicle weight.

The full equation with dimensions in lbs and feet is:

(gear ratio x final drive ratio x engine mass radius / tyre radius) ^2 x engine mass factor.

I also add a factor for the rotational inertia of the wheels and tyres which for an average road car I set at 125 lbs.

[Synchromesh]

I suggest you put your 2.5 V6 on a dyno, let the engine coast down, and take a value. Remember to subtract drivetrain loss, and take into account the gear and final drive ratios, and then extrapolate the data to get the figures for a 2.7. That's if your Alfa doesn't break down in the process...

[The Wookie]

If you're looking for standing start performance, don't bother modelling rev drop OP, it's too complicated and the system isn't particularly sensitive to it. Just assume a half second (or however long you want your shifts to be) cut in Torque applied to the driveline and shift the RPM to match the gearing at the speed you end up at at the end of the half a second.

[Kozy]

I'm not really sure the inertia of the engine is relevant, the drop between gears will be dictated more by the gear ratios than it will by any energy stored in the spinning components...

[Mr2Mike]

Kozy said:

I'm not really sure the inertia of the engine is relevant, the drop between gears will be dictated more by the gear ratios than it will by any energy stored in the spinning components...

The OP is talking about speed of the RPM drop when you put you foot on the clutch and close the throttle, i.e. no engine load. This is completely independent of gear ratios.

The engine losses are not just frictional in this instance however, most of it will come from pumping losses.

[Kozy]

He said he was modelling the 0-60 acceleration? I'm not sure how the time to spin down unloaded would have any relation on this? The engine will leave one gear at X rpm and enter the next at Y rpm, I don't see what the engine does in the interim is of any significance?

Edited by Kozy on Sunday 4th November 20:11

[nsa]

I might have misunderstood the question, but if you hooked up a slow-motion camera to an accurate digital rev counter and a stopwatch would this help with your calculations?

I bought this a while ago and can record in something like 1,000 frames per second.

https://www.google.co.uk/webhp?sourceid=chrome-ins...