Automatic car at rest with the engine on

[dpopiz]

When you're stopped at a light in a car with an automatic transmission, does the transmission shift out of gear? It seems like it would have to because otherwise the engine would not be able to turn and wouldn't that cause it to stall?

But on the other hand, the second you take your foot off the brake, it jumps forward which would indicate that it was in gear.

 

[goodoptics]

Try this link:

http://auto.howstuffworks.com/torque-converter.htm

 

[Varun]

It stays in gear, and like the previous poster pointed out, the torque converter allows the engine to turn but not the transmission. There actually is no physical connection between the engine and wheels in an automatic transmission (well except when cruising and the torque converter clutch kicks in). The connection is through the transmission fluid.

 

[Peter]

Purely mechanical "legacy" auto gearboxes stay in "gear", the torque convertor spins (almost) freely. Foot off brake, you'll notice that powerful and/or lightweight cars tend to creep forward in idle.

Newer, computer controlled auto gearboxes indeed actively shift into idle when the car has been standing still for a few seconds and the foot is on the brake - and shift back into gear when the brake foot is lifted. This saves a bit of fuel in town traffic, but puts some extra wear on the gearshifting mechanism inside the box as well as the motor suspension.

 

[Cattlegod]

Originally posted by: Peter
Purely mechanical "legacy" auto gearboxes stay in "gear", the torque convertor spins (almost) freely. Foot off brake, you'll notice that powerful and/or lightweight cars tend to creep forward in idle.

Newer, computer controlled auto gearboxes indeed actively shift into idle when the car has been standing still for a few seconds and the foot is on the brake - and shift back into gear when the brake foot is lifted. This saves a bit of fuel in town traffic, but puts some extra wear on the gearshifting mechanism inside the box as well as the motor suspension.

Also newer cars shift out of gear when you are slowing down at highway speeds.

 

[Calin]

If you have an older car with automatic transmission, you will see that at idle, stopped and in gear the rpm of the engine is lower than at idle, stopped and in park or neutral. So, on older cars it stays in gear

 

[Peter]

Yes it does, we said that - but the engine rpm need not be lower. Many cars have idle regulation that keeps it at a steady rpm, regardless of gear engaged, air condition running or not, etc. This has been done because a lowered idle rpm causes a drop in oil pressure, particularly in worn engines.

 

[Calin]

I've seen 3 auto cars running in my life - one I haven't seen almost at all, the other I don't know about rpm at idle in D or N, but the third I know for sure drops rpm from 1200 to 800 when stopped, idle from N to D

 

[Peter]

That's the primitive ones ... and manufacturers or garages who care about oil pressure in idle-D set those to have the idle-N a lot higher - exactly like you observed. That goes at the expense of gearbox wear when switching from N to D, plus motor suspension wear in the same moment.