I parked my car yesterday and was straightening the tires when I saw my RPM go up from about 700 to 1000. Why does it do this?
In: Engineering
If you were at a dead stop and turning the wheels, I would chalk this one up to the power steering.
When you turn the wheels at idle, the power steering pump requires more rpms to supply the hydraulics. Much like the when the A/C compressor clutch engages, the engine will compensate for the demand in power by increasing the engine speed. It’s less noticeable at speed because the rotation of the tires aid in turning, as opposed to the friction created between the rubber and the pavement at a stop.
A similar phenomenon happens when your home A/C starts up. The initial amperage required to start the compressor from a dead stop, is about 10x more than the amperage required to run it. That is why they are equipped with start components that help aid with that, but even with the start components working properly, there is a chance you will see the lights in the house dim because of the initial spike in amp draw.
There’s a few possibilities here.
First, when you shift into Park the transmission effectively goes into Neutral. That means that the weight of the (not moving) car is taken off the (spinning) engine and the reduced load will make the RPMs increase for a moment before the engine control computer realizes and reduces fuel flow.
Second, especially in older cars, power steering is powered by a pump which runs directly off the engine from one of the belts. Turning the wheels adds requires pump pressure which is a bit of engine load, and when you finish the load on the engine is reduced causing the speed to spike back up for a split second.
There are two things happening, first, the power steering of your car needs to draw more power from the engine to work, so the engine has to turn faster. Second, turning is a kind of acceleration. You are quite literally accelerating in a sideways vector and that also requires more power from the engine. So in order to facilitate the turn, the automatic transmission of the car drops to a lower gear and spins the engine faster so it can pull more power from the engine, just as it would if it were accelerating in a forward vector.
If you were stopped and you noticed this, it’s because most vehicles have power steering now. Power steering takes power from the running engine to operate the steering pump. When you turn the wheels, you operate the pump, increasing engine rpm. Try putting it in park and turning your wheels, the same should happen. Also in park try turning the a/c off and on. The a/c pump also runs off engine power and will effect the rpm.
To tack on what other people are saying, the reason engine speed increases under load vs the engine management just maintaining that speed is because when that load is taken on at that rpm, it manifests itself in increased vibration. Raising the speed spreads the load out over more power strokes. This is more common the smaller the engine and less number of cylinders.
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