Why does a car engine idle at a lower RPM while in drive than when in neutral?

Question

Why does a car engine idle at a lower RPM while in drive than when in neutral?

In: Engineering

Answer 1

Imagine an axle connected to a small toy motor, like a Lego motor or something. It spins freely at a constant speed.

Now imagine taking your hand and gripping it lightly around the axle. Just enough to let it barely touch your hand. The axle is still spinning more or less completely freely.

Now imagine clenching your fist around the axle, not very strongly, but strong enough to get a bit of a grip on it. The axle, rubbing against your tightly-gripped hand, will slow down a bit. The whine of the motor will lower a bit as it struggles to keep the axle going. Your hand goes nowhere, though, because the friction between the hand and the axle isn’t enough to move you.

If you kept gripping that axle tighter and tighter, one of two things will eventually happen. Either you will stop the motor completely, or the motor will be able to transfer enough torque into your hand to twist it off your arm. Depends on the strength of the motor.

In automatic transmission cars, there is a component that behaves much like your hand gripping the axle. It’s called a torque converter. Its primary job is to help get the car moving when it’s been standing still and it’s ready to start going. But one of the consequences of how it works is that when you’re idling in Drive gear, it acts like that hand partially squeezing the axle. It’s gripping hard enough to slow down the engine a bit, but not enough torque is getting transferred through the connection to actually get the wheels moving.

This is on purpose. It allows you to shift the car all the way into Drive gear without killing the engine. You can’t do that in a manual. Granted, manual transmission cars don’t _need_ to be able to do this, they have their own similar method of getting moving, but it demands slightly more finesse from the driver.

When you’re in Neutral, both in an automatic or a manual transmission car, it’s like the hand isn’t gripping the axle at all. The engine spins more or less freely.