Hi,

I came across an engine performance graph as attached:

I have basically two questions looking at that:

• To do any engine testing we keep some parameter constant - May be throttle position or applied load (torque) or speed or power. What is kept constant for this test?
• In the torque Vs. engine speed curve, till 1750 rpm (approx.) the speed appears to be rising. I am little confused here. If torque rises, it means the load on the engine climbs up. Thus, should it not result in drop of the speed instead of rising?

I have these questions unresolved for long. Pl help. Thanks.

 

Throttle position is usually a constant at full open.
The applied load is varied by the dynamometer and the resultant RPM is measured.

Some dynamometers work on an acceleration principal.
It measures the speed of rotation of a large mass. The acceleration at any rpm is proportional to engine torque.
In one acceleration run you can calculate out the entire Torque vs rpm curve.

 

Thank you very much Erich.

I was comparing this graph with the tests I have conducted on engines as a college student. There, I used to measure torque using loading the engine. There used to be a drum connected to the crank shaft. I used to apply load on it. Thus the product of load and drum radius becomes the torque. In that case, torque in the graph is directly proportional to the load applied on the engine. That was the basis of beginning this thread.

In this particular test we are discussing on, can you please explain me how torque is meausred? I mean the working principle of the accelaration principal dyno used here. Also, looking at the Power Vs Speed curve, you can identify easily that power is found out by multiplying the speed and torque of all the instances from the Torque Vs Speed curve. Is it applicable for all type of dyno test? Thanks.

 

Regarding your second point in your initial post,

your reasoning cannot be quite right, though I'll admit that I can't quite figure out why. Just consider the fact that an IC engine cannot produce torque at 0 RPM (unlike an electric motor), which means that the torque curve must initially climb up from zero as RPM rises.

As far as your second post

If a mass with a known moment of inertia was fixed to the crank shaft of the motor and the motor was allowed to accelerate the mass at full-throttle, the torque produced by the motor could be found by the measured angular acceleration of the mass by T = I*a (T= torque, I = moment of inertia, a = angular acceleration). Maybe this is how the rising half of the torque speed curve is measured.

Yes, the power is always equal to the torque times the speed (adjusting for units).

 

Thank you, Matthew. I feel this forum is really helpful for mechanical engineers for people are really interested to share their knowledge! Thanks, again.

 

Yes I agree. I've been promoting this forum at work because I think this is a great resource, and the more people on it, the better the resource it will be.