Suppose to have an engine attached to a flywheel, I add power, the flywheel start to rotate and oppose a resistance to motion. More power, more resistance. I'm wondering if I can simulate this behaviour without an inertial mass, but only using a brake. This brake could be controlled by a system that measure the acceleration of the wheel and apply a braking force higher if the acceleration is higher. But, if that force is equal or higher than the force of the engine, the engine will not accelerate. So, it must be lower, but really a little bit lower. Will this system be reliable? If I set the force of the system using an engine, then I put a different engine with different power, I could measure a different acceleration time, as would be using inertial mass?
2 Answers
If you "know commercial brake dyno could do these," then you know it is possible.
The how may be a little clearer if you think of it in the other direction. When a force is applied to a mass, it results in acceleration. You need to measure that force and release your brake for some amount of movement that corresponds to the positional response you would get. Instead of sensing acceleration and setting the brake to decelerate it after the fact, think of it as trying to simulate all effects of having the mass- no movement until a minimum load due to friction from the mass and gravity, etc.
It does not change the need to measure forces and multiple aspects of position, forming a feedback loop that ties them to an output that in turn controls the brake. The brake may have its own, separate force sensor to help tune its output or detect overload, but is not the force that determines the needed acceleration; inputs are things that exist in the system that would have a mass, not the stand-in brake.
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Strictly speaking, no.
An inertial load would, sometimes, put energy back into the engine. A brake alone cannot do that.
For instance if you got the "system" revved up, and then turned the throttle to the minimum (not touching the clutch), which is a common enough situation IRL. Likewise, if the clutch was disengaged, the engine RPM was at minimum, but the load was revved up (i.e. representing the vehicle moving fast in neutral), and then you engage the clutch. (this would test disturbance response in your engine control, I suppose).
You'd have to decide whether this limitation is relevant to your testing goals. You could still test acceleration and steady load.
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