A6 question
Thread Starter
Racer
Joined: Sep 2004
Posts: 282
Likes: 40
From: Seattle WA
A6 question
Does anyone know if the load on the engine is less if you are stopped and in third gear instead of first? The one thing that has always bothered me about auto transmissions is they are in gear when you are stopped, which to me wastes fuel. I have noticed the idle is identical no matter if you are in neutral or in gear (1st 2nd or 3rd).
Thread Starter
Racer
Joined: Sep 2004
Posts: 282
Likes: 40
From: Seattle WA
Thanks for the answers, but it is my understanding the engine will burn less fuel if you idle in neutral instead of "D" or "S". I was curious if the same was true if you used the paddles to put it in 3rd instead of 1st when idling. Just my incessant curiosity.
Race Director
Joined: Oct 2004
Posts: 10,089
Likes: 14
In neutral, you still have the load of the pump, but the torque converter turbine free wheels. That causes less fluid heating. This heating represents energy pulled from the engine, and is equivalent to about 0.5 BTU per pound degree of temperature rise. 1 hp is equivalent to 42.41 BTU/min. If you watch transmission temperature rise while idling in neutral, and do the math, you'll find you're losing slightly less than 1 hp due to transmission heating in neutral, and slightly more than 1 hp when stopped in drive. In other words, the difference is negligible.
Now if you hold the brake and rev the engine up to torque converter stall RPM, the heating will not be negligible, and the load on the engine will be considerably greater than at idle. Don't do that for long periods.
Team Owner
Joined: Feb 2001
Posts: 27,254
Likes: 136
From: Short Hills, NJ
If anything the idle will 'jump up' slightly when going from 'D' or 'S' to neutral so in theory wouldn't the lower revs of being in gear return ever so slightly better fuel economy in the long run?
Thread Starter
Racer
Joined: Sep 2004
Posts: 282
Likes: 40
From: Seattle WA
Stopped is stopped. The engine can't tell what gear the transmission is in when stopped. The only load it sees is the front hydraulic pump and the drag on the torque converter. Those are constant at constant engine RPM with the car stopped.
In neutral, you still have the load of the pump, but the torque converter turbine free wheels. That causes less fluid heating. This heating represents energy pulled from the engine, and is equivalent to about 0.5 BTU per pound degree of temperature rise. 1 hp is equivalent to 42.41 BTU/min. If you watch transmission temperature rise while idling in neutral, and do the math, you'll find you're losing slightly less than 1 hp due to transmission heating in neutral, and slightly more than 1 hp when stopped in drive. In other words, the difference is negligible.
Now if you hold the brake and rev the engine up to torque converter stall RPM, the heating will not be negligible, and the load on the engine will be considerably greater than at idle. Don't do that for long periods.
In neutral, you still have the load of the pump, but the torque converter turbine free wheels. That causes less fluid heating. This heating represents energy pulled from the engine, and is equivalent to about 0.5 BTU per pound degree of temperature rise. 1 hp is equivalent to 42.41 BTU/min. If you watch transmission temperature rise while idling in neutral, and do the math, you'll find you're losing slightly less than 1 hp due to transmission heating in neutral, and slightly more than 1 hp when stopped in drive. In other words, the difference is negligible.
Now if you hold the brake and rev the engine up to torque converter stall RPM, the heating will not be negligible, and the load on the engine will be considerably greater than at idle. Don't do that for long periods.
