## Re: [Algorithms] Car engine behaviour

 Re: [Algorithms] Car engine behaviour From: Madoc Evans - 2006-04-27 12:35:15 ```This is built on top of my rigid body dynamics and so things like aerodynamic drag are implicitly present (and necessary). My vehicles are articulated bodies, wheels are rigid bodies constrained to the chassis in more or less complex ways depending on the vehicle. Wheel traction is based on collision with other surfaces (and here I think I might want to use a different friction model from what I normally use with rigid bodies). My model is based RPM * gear ratio * (differential * tire circumference) = tire tread velocity. This velocity (at this point in "world-space) is then compared to the velocity of the surface it makes contact with and through friction results in forces being applied to the wheel and the other body involved. This all works though my rigid body model, the basic difference being that I don't rely on the standard path for computing the tangential velocity and friction coefficient because I don't expect it to be accurate with the velocities involved. So, not only are these factors considered but they are also localised with notable precision. Gears are present (including reverse and neutral), there is a kind of clutch and RPMs do adjust themselves to the wheel RPMs when switching gears, partly as a function of the clutch behaviour and feedback from the wheels. Within this model, engine power is a function of how much the engine is able to resist the feedback from the wheels, again, much as Jon described. However, the implicit result of this is quite unlike what I observe in real-world vehicles, low gears are more effective up-hill and perhaps do accellerate more effectively than high gears but high gears are perfectly capable of accellerating at low speeds also. Feedback acts by limiting accelleration at high velocities more than at low velocities. Mathematically this make sense to me as a result of increased aerodynamic drag: an increasing difference between what the engine is trying to output and what it is actually achieving. This is not really noticeable unless I grossly underpower the engine, though. Now, if it is just as easy to accellerare from 4 to 5 kph as 40 to 41, then why can high gears do the latter and not the former? Even if it's not from a stand-still, accellerating in 5th gear from 20mph is not very effective, to the point that I don't think I've ever even attempted it, I expect that commonly the RPM would drop to the point of stalling. Basically, that's the question I'm trying to answer. If the momentum of the vehicle is not pertinent to the force required to accellerate, then why can I not accellerate from a stand-still in 4th gear unless I have a 1000hp or so car? Or how can a car accellerate at all in 4th gear if it can't at low speeds and there is no difference there? Thanks, Madoc ----Messaggio originale---- Da: algorithms@... Data: 27- apr-2006 12.48 PM A: Ogg: Re: [Algorithms] Car engine behaviour Madoc, > The unfortunate result of it however is > that high gears are not really much worse at accellerating from a stand- > still than low gears, and in fact the feed-back from the wheels acts > more as break limiting top-speed rather than accelleration. I can see > that this might be a result of drag at higher velocities resulting in a > greater shift in velocity at each integration step. You might be missing one important point. It's not the torque that accelerates the car, its the LINEAR force that is applied at the contact between the tyre and the road. These are related through the *effective* radius of the wheel (which is affected by the current gear ratio). Therefore, if you are in a high gear that has twice the gear ratio of first gear, then at 2000rpm (for example) you will be applying half the force at the wheel. This is why it is much easier to accelerate in low gears. Of course, as the speed of the car increases, aerodynamic drag *will* become important, but you probably don't need that in your code immediately. > Out of my > ignorance, I'm thinking that this could be the lack of consideration of > the vehicle's mass and momentum. Something along the lines of: > accellerating a mass from 4 to 5 kph takes far more torque than > accellerating it from 40 to 41 (bare with me for the simplication). I don't think this is true :-| In terms of pure dynamics, not considering aerodynamic drag, it is just as easy to accelerate a car from 40 to 41 kph as it is from 4 to 5 kph!! The laws of physics are the same in any inertial frame after all !! :-) The difference you want is related to the gears I think. Another thing you might need is that forces applied to the car wheels also cause a feedback to the engine (again through the gearbox). You might need that, but I've done plenty of simulations that just fudge that. > So, to narrow my > most immediate problem down: How should I describe the fact that higher > gears will have a harder time at accellerating at lower velocities > while they fair pretty well at higher velocities? I get the idea that > this is simple dynamics and I'm just being dumb by not just working it > out but I don't feel confident with basing it on my own assumptions. HTH, G ```