Precise control… of what?

Posted on March 29, 2008

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This is something that has been in the back of my mind for quite a while. When you step on the gas pedal, what exactly are you expecting to happen?

It’s one of those questions that is so simple, there has to be a trick in it. And in fact, there is.

If you think your car’s pedal controls speed, think again. It’s called an “accelerator”, but it’s wrong. It was made to control the acceleration of the vehicle (change of speed per time unit), but it doesn’t really do that. Why? Because that’s difficult to do with a combustion engine.

What it actually does is control power. And power is the rate of delivery of raw energy (fresh combustible mixture) to the engine (and only after that, to the wheels). And that has a very complicated relationship with speed, and in a real car it’s definitely not a linear one, no matter how smart the engine controller unit is.

To keep things simple, let’s think about examples.

  1. You’re cruising along the highway at constant speed, so your foot is pressing steady on the gas, about mid way. Now comes a climb, and you’d like to maintain your present speed, so you press on the pedal as much as you have to in order to avoid speed loss. So your gas pedal is controlling torque, in order to maintain speed.
  2. You’re now back on the flat road, and you want to overtake a tourist bus. To make sure you pass safely within the window of opportunity, you slam on the gas for a little bit until you get a nice overtaking speed. Once you are side-by-side with the bus, your foot almost goes back to the middle position (there’s no need for acceleration any more), but not quite. So you used it to increase the speed, but also to increase torque (higher steady speeds demand higher torque because of aerodynamic drag).

In my opinion, we don’t expect anything precise when we step on the gas pedal. We expect something to happen, and that something is hard-linked to the car’s personality (the final systemic result of the engine and transmission, inertia distribution, aerodynamics, etc.) versus the current road conditions, speed, load, and even the weather. So it’s always subjective, never quite deterministic. And we adjust accordingly: our foot instinctively adapts its weight on the pedal so that we can do what we want with the car.

In replacing the engine-linked accelerator with another one that controls an electric motor, I’m pressed into choosing exactly which variables this analog input should influence, in order to give the car’s driver the same familiar, comfortable, and safe feeling.

Initially I thought “Speed, of course.”, but that was obviously wrong. Then for a long time I settled for “Torque, most probably! It covers all situations.” Now I’m inclined to think “Power!… it is definitely the most realistic copy of today’s cars.”

What do you think?

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Posted in: Controllers