Engine Braking Gas Mileage

engines

#1

The Car Talk guys said regarding the engine braking question down the mountain, in the last episode, that engine braking would use more gas. It was my understanding that modern cars realize there is an engine overspeed condition and that the car is engine braking, and thus turn off the fuel injectors. Resulting in less or no fuel being used, even less than idling. Any thoughts?


#2

My thought is that I have never seen the totally turned off method described in a technical article, just here. I always wondered if there was a system that shut off all fuel as would this not cause a lot of drag if it was just momentum keeping the crank turning?


#3

The factory service manual description of the fuel system for my 2002 s10 states that under certain conditions no fuel will be injected. It goes on to say this was designed for fuel saving purposes. (paraphrased)


#4

It depends on the car. In my car as long as the car is in gear and the engine is turning over faster than idle speed no fuel is delivered to the car so mileage is infinity (free). Your mileage may vary.


#5

It’s hard to imagine how engine braking could use more gasoline – engine load is directly related to manifold vacuum, which itself is a product of engine speed and throttle opening. Engine braking (high engine speed, no throttle opening) creates the highest vacuum and lowest engine load. This allows the wheels to drive the transmission and engine, turning the power steering pump, alternator, etc. At idle, the engine actually has to work to make these things happen. Now, its debateable whether you’d burn less fuel in a carbureted vehicle, but with a fuel-injected system I don’t think there’s any question. Besides, any maniac that insists on flying down a mountainside in neutral is going to pay an increased cost in brakes and accidents…


#6

That is absolutely correct, at least on many modern vehicles. Engine braking uses no fuel.

On my '05 VW Jetta, if you have the transmission engaged and you are not applying pressure on the gas peddle, there is NO fuel fed into the engine. I know this as a fact, as I have an O2 sensor gauge attached to the exhaust. The reading goes to “free air” when this happens. In addition, the instant fuel mileage goes to infinity (as it is not using any fuel to move) - something anyone can see on the dash display.

Push in the clutch, or apply gas, and the engine “wakes” back up and uses fuel again.


#7

Where do highest pumping losses (from a fully closed throttle fit in with the “lowest engine load”?)


#8

You’re right that you do have highest pumping losses at lowest engine load, and that’s why it’s called engine braking – all the internal resistance, compression, and pumping losses are trying to slow the engine down. The reason why it’s efficient is that gravity (acting from the wheels and through the drivetrain) is doing all the work and the combustion process is doing little or none.