# Engine braking and mpg **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2010, 11:10pm UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466 "2010-07-12T23:10:13Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [July 13, 2010, 3:35am UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/21 "2010-07-13T03:35:33Z")
[Fucking fuel injection, how does it work?](http://boards.straightdope.com/sdmb/showpost.php?p=8610085&postcount=11) ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [July 13, 2010, 4:12am UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/22 "2010-07-13T04:12:44Z")
Wow that was interesting. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [July 13, 2010, 4:34am UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/23 "2010-07-13T04:34:02Z")
> [@Throatwarbler\_Mangrove](#): > > [Fucking fuel injection, how does it work?](http://boards.straightdope.com/sdmb/showpost.php?p=8610085&postcount=11) ::: Does the happy dance::: People are finally listening! Yaaaaaaaaaaaa!!! > [@beowulff](#): > > Even so, does that indicate that the injectors are actually turned off? > I doubt that they get turned off, since that would stall the car (if it wasn’t moving). Instead, they just get set to the lowest idle injection. Yep, off and I do mean all the way off. Maybe only for a fraction of a second, but all the way off. As in no fuel. As in none, nada, zip, zilch, nuttin, bupkis. All the way off. The ECM turns them back on before the car stalls of course, but believe me they are all the way off on decel. **retterath** Yes it does cause a lean, actually zero fuel condition, and this is by design. Catalysts work on what the engineers call oxygen storage. When you take your foot off the gas, the cat gets flooded with oxygen, some of that oxygen is stored for when you step back on the gas, and it is then used to clean the exhaust (getting rid of HC and CO require additional oxygen) ---
**Author:** ![Raguleader](https://avatars.discourse-cdn.com/v4/letter/r/258eb7/32.png) [@Raguleader](https://boards.straightdope.com/u/Raguleader)\ **Post date:** [July 13, 2010, 5:18am UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/24 "2010-07-13T05:18:33Z")
OK, so using engine breaking (keeping the car in second gear when coasting down Inattentive Driver Killer Mountain Road) is a more effective long-term method of slowing my vehicle without risking overheating my breaks and turning them into glowy red discs with a consistency of popcorn butter. What are the _downsides_ to doing this? Minor or otherwise, I’m just trying to kick my ignorance while its down. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [July 13, 2010, 5:22am UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/25 "2010-07-13T05:22:29Z")
First off it is brakes. If they break, then your brakes are broken. Well as I said in the link in post #21 if it is a really really long grade, the engine can turn so long with no fuel, the coolant cools off and the heater stops blowing hot air. Other than that I can think of no practical issue with a properly running engine. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 13, 2010, 12:16pm UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/26 "2010-07-13T12:16:28Z")
I have a Camry hybrid. How does all the above information retranslate for hybrids? In some situations, like stopping at a red light, the gas engine stops, and very smoothly starts itself back up when I touch the gas pedal (not sure why standard gas engine cars can’t do the same thing). Not sure if the gas engine deliberately “stalls” when decelerating and stays off, but it must at least do what **Rick** has described regarding the fuel injection. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 13, 2010, 12:17pm UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/27 "2010-07-13T12:17:47Z")
> [@CookingWithGas](#): > > I agree that it’s better to stay in a low gear than to brake down a long downhill. And I realized on a re-read of the OP that this is really the point of the thread, so sorry for going off in the wrong direction. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [July 13, 2010, 12:46pm UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/28 "2010-07-13T12:46:37Z")
> [@CookingWithGas](#): > > I have a Camry hybrid. How does all the above information retranslate for hybrids? In some situations, like stopping at a red light, the gas engine stops, and very smoothly starts itself back up when I touch the gas pedal (not sure why standard gas engine cars can’t do the same thing). Not sure if the gas engine deliberately “stalls” when decelerating and stays off, but it must at least do what **Rick** has described regarding the fuel injection. The difference is that a hybrid has regenerative braking. When you step on the brakes, initially the electric motor in the transmission is engaged only, to slow you down while recharging the battery. Eventually the computer determines a point where the actual brake brakes are engaged to bring you to a stop. As a savvy hybrid driver, you can determine that point and control your braking (brake earlier and more lightly) so that the whole system stays in “regenerate” mode longer before the actual brakes are applied, and you get the maximum amount of regeneration to maximize your fuel economy. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [July 13, 2010, 12:51pm UTC](https://boards.straightdope.com/t/engine-braking-and-mpg/546466/29 "2010-07-13T12:51:01Z")
> [@CookingWithGas](#): > > In some situations, like stopping at a red light, the gas engine stops, and very smoothly starts itself back up when I touch the gas pedal (not sure why standard gas engine cars can’t do the same thing). They do: [Start-stop system - Wikipedia](http://en.wikipedia.org/wiki/Start-stop_system) [Previous page](https://boards.straightdope.com/t/engine-braking-and-mpg/546466.md?page=1)