Engine Braking Torque

[anonymous]

Does anybody have any idea about engine braking torque? What is it?

I read on wiki that it is the effort engine has to put when throttle is fully closed (in case of gasoline engine). I want to find its value for our engine.

Does anybody know how to find it? Is it evaluated through Dyno testing only?

Plz help.

[mech5496]

May I ask why?

[anonymous]

May I ask why?

I want to find out fluctuating stresses on half shaft. The maximum variation of torque acting on it, is from traction limited acceleration situation to engine braking torque. Isn't it?

[mech5496]

And what if your driver applies brake and throttle at the same time? What about spikes due to sudden clutch release? Stick-slip of tires during launch? Dynamic loading due to chain slack? :)

[anonymous]

Well, I am trying to design half shafts referring to SAE technical paper. In this paper, they have used these two values as extreme limits. So that's why I am asking about it.

And I think, in all these conditions you are talking about, torque peaks would lie between these value. For fatigue failure theories, we require only extreme values.

But still I'll think about these cases too.

[murpia]

I want to find out fluctuating stresses on half shaft. The maximum variation of torque acting on it, is from traction limited acceleration situation to engine braking torque. Isn't it?
No:

http://www.fsae.com/forums/showthread.php?8870-honda-cbr-torque-values&p=84824&viewfull=1#post84824

Regards, Ian

[anonymous]

Hey Ian,

thanks a lot for directing me to that discussion. But will u plz tell me these things in detail...i am not much familiar to these words.

In fatigue failure criterion, we need to know both upper and lower values of torque that our axle is experiencing. Do u want to say that, upper limit of torque is max torque (max engine torque * primary reduction * first gear ratio * secondary reduction) minus torque which required to overcome inertia of different components that lie between engine and axle ???

So do u want to say that instead of taking max torque equal to which is limited by traction ( calculated by considering load transfer and friction between tires & road surface) , I should take it equal to above mentioned value? Plz Elaborate.

Also tell me about lower limit of torque..is it equal to braking torque produced by mechanical braking system or engine braking torque?

U may feel that I am asking silly basic questions..but its our first year..plz help me understanding these concepts..

Regards...

[murpia]

Hey Ian,
Do u want to say that, upper limit of torque is max torque (max engine torque * primary reduction * first gear ratio * secondary reduction) minus torque which required to overcome inertia of different components that lie between engine and axle ???
You haven't described your powertrain, but for a motorcycle engine / transmission you normally have: max engine torque * primary reduction * first gear ratio * final drive ratio (chain-drive reduction).

That value will give a minimum upper value for your analysis, but as mech5496 points out, you need to add transient factors on top of that.

Where's your rear brake disc(s)? If you are braking through the driveshafts then the lower value will depend on that. Otherwise I would simply assume a symmetric loading.

Regards, Ian

[VFR750R]

Does anybody have any idea about engine braking torque? What is it?

I read on wiki that it is the effort engine has to put when throttle is fully closed (in case of gasoline engine). I want to find its value for our engine.

Does anybody know how to find it? Is it evaluated through Dyno testing only?

Plz help.

Dyno testing is an accurate way to test for it yes, but only with an electric transient dyno, which I assume you do not have access to.

If you have a car, you could do a two coast downs from speed. One with the clutch in, one with the clutch out, and the difference in deceleration will be your engine braking torque. I'll let you figure out what math you'll need to convert it to axle torque and crankshaft torque and what differences you'll see in each gear. A much less safe option I probably shouldn't even mention is to put a flywheel on a waterbrake with a similar procedure.
If you do not have a dyno or a car, you can calculate approximately what the engine braking torque will be by making some assumptions about the forces on the piston on the intake stroke vs the exhaust stroke when the throttle is closed combined with an approximation of engine friction. A simple one-cylinder engine simulation model would also tell you that and so much more.

As other have alluded to, engine braking will be a small percentage of stress seen by a half-shaft, so it's importance for that is minimal, but it's not a bad thing to know from a chassis standpoint as engine braking is dynamic and can be a major force contributor on the car, with the differential being a major screw to turn in terms of where and how those forces are applied.

[Pete Marsh]

In an actual FSAE application, the plenums are large, and the breaking events are short with very fast rpm reduction, and so for most stops the engine is not providing any breaking torque at all, but rather still producing drive to the wheels.
Half shaft strain gauges will reveal all!

If not endowed with such toys, a simple observation of the rpm rate of change when free reving and on track will give you some idea.

Pete