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Why is the crankshaft position sensor used?

Isn't the camshaft position sensor enough all by itself?

Since the crankshaft and the camshafts are locked together by the timing belt, if you know the camshaft position, then you also automatically know the crankshaft position.

I don't see how the crankshaft position sensor could not be redundant and not needed, but yet there it is on the car. Why?

Accuracy, and the ability to use cheaper, low resolution sensors. Timing belt stretch or mis-installation.

The camshaft moves at half the speed of the crankshaft, so it would be tough for the ECU to know where the crank is in relation to the cam angle sensor.

The crank sensor is used for engine speed. The cam sensor is used for engine position. Theoretically you could have just one sensor, but you would need a very complex reluctor, and a much smarter ecm.

It comes down to calculation vs. sensor reading. With only a single sensor you would have to do a calculation to map the cam sensor readings into a crank speed and position. That requires quite a bit more processing speed as it would have to be done inside the inner-loop of the ignition timing routines. Reading a sensor is more accurate and doesn't require any processing.

Plus as mentioned the cam sensor would need to have higher resolution - that means it would have to respond faster, which means higher grade sensor components.

GD

The sensor itself is not the problem, as the same sensors are used to read 30-tooth reluctors. The main issue would be designing a single reluctor that would show both engine speed and position

Actually....cam shaft sensors are only needed for cars that use sequential fuel injection and non waste spark systems. For firing the injectors cars used to do batch injector firing on every other crank rotation. For firing the plugs, the advantage of the waste system is that you don't need to know where the cam is to determine if you are at TDC. When you fire the one plug, it fires the other plug and one of the two will be on the compression stroke. Camshaft sensors started showing up first when manufactures started doing SFI injection for emissions and slight performance purposes. Today with coil on plug, it is also definitely a necessity.

I think first gen legacy's were SFI, but I think they are a waste spark system. If they weren't SFI, the crankshaft sensor would be sufficient.

Good point.

The ECU can figure out just fine where the crank is with just a cam sensor. It can do the calculation. The last of the EA Subarus had crank-angle sensors in their distributors that calculated the crank angle using the camshaft.

SemperGuard is right that the sensors are reluctors, using magnetic fields to function. So it does make sense that using one magnet to do two functions would be difficult.