> > simulating car behaviour I got the impression that the flywheel is the
> > thing driving the wheels
> The flywheel is attached to the crankshaft, the main driven part of
> the motor, so it can be considered as driving the wheels through
> the drive train.
> >> You can make the simple assumption that response to throttle
> >> inputs are linear (assume that the car has a real smart ECU).
> > What would be a better assumption for response to throttle? I can easily
> > throw a graph lookup at the problem if data is available, but if not is
> > there some general shape that is more realistic?
> I'm not sure of the shape of the curve, but a classic throttle opens up
> valve(s) controlling airflow into the engine. At small throttle openings,
> rpms will be low, even in a no load situation. At slightly larger opening,
> rpms will be high, but below the limit set by the rev-limiter, under a
> no-load situation, and under load, the rpms will be less, depending on
> the load versus the torque from the engine. At larger still openings,
> but less than full throttle, the engine will run at max rpms set by the
> rev-limiter in a no load situation, and lower rpms depending on the
> load. As throttle is increased from this point, torque from the engine
> increased until the airflow versus rpm is at or beyond the optimal
> point. Assuming an ECU with fuel injection, the ECU controls the
> amount of fuel going into the engine to correspond with air flow
> and throttle position.
> The curve shown for torque versus rpm is almost alway the curve that
> occurs at full throttle. At lower throttle settings, the torque curve
> is lowered, with more of this lowering occurring at higher rpms.
> As far as actual data, hopefully Todd can help here.
> In some cars, the throttle is not mechanically connected to anything
> other than a sensor. Intake opening, fuel flow, ... are all computer
> controlled in this case. Such a setup could produce a situation where
> torque output was relative to throttle position, up to the maximum
> torque for the rpm the engine is running at.
commonly found online, I've found. In general there's a sort of
quadratic relationship there between throttle position and what the
torque curve looks like. The most common approach in sims is to have
two torque curves. One for full throttle and one for zero throttle.
The full throttle torque curve looks like the dyno graph I showed. The
zero throttle one should have 0 torque at whatever the idle rpm is,
positive torque at rpm's below that, and progressively more negative
torque (below 0) as rpm increases. This simulates the engine braking
effect. What folks generally do then is extrapolate between the two by
the throttle opening percentage.
As Jeff pointed out in the last paragraph, there are systems that
control the throttle position electronically depending on how far
you've pushed the gas pedal down, engine speed, and so forth. On such
systems you could pretty much make the part throttle curves look
however you want.
Yes, the component of real wheel torque you're referring to here is
indeed force * effective radius.
By
"Effective radius" generally means the distance from the center of the
wheel to the ground, including vertical tire deflection. And yes,
under any kind of load this will be smaller than the relaxed radius.