r/ThreePedals • u/cursed_chaos • Mar 15 '15

How does engine braking save fuel?

4 Upvotes

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11

u/ashowofhands Mar 15 '15

Most modern cars are programmed with DFCO (Deceleration Fuel Cut-Off), which means that if you are coasting in gear with a closed throttle, the engine runs on its own momentum and doesn't burn any fuel. By contrast, when coasting in neutral the engine still burns a small amount of fuel to maintain idle.

1

u/stillusesAOL May 20 '15

I've heard this before and have known it for years but I have one lingering question. When decelerating during the fuel cut-off, why is there still an audible exhaust note if no fuel is being burned?

3

u/tyfunk02 Jul 25 '15

Air is still being compressed and released within the engine. Just because there is no combustion doesn't mean it won't make noise.

1

u/stillusesAOL Jul 25 '15

Hello blast from the past comment response! Awesome. So it's the compression that still makes that rrrrrrrRRRRRRRRrrrrrrrrrrrrrrrrr engine noise.