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Engine braking

  1. Dec 11, 2009 #1
    please explain engine braking? the definition i found was " braking effect caused by throttle position induced vacuum in petrol engines".In what way is this better than external braking?
  2. jcsd
  3. Dec 11, 2009 #2
    When you let off the gas the engine gets no fuel. Since it's still getting spun by the wheels, but yet produces no power, it then becomes a simple air pump. Since the throttle is closed, it takes alot of work to pump the air as the restriction to the air flow is massive. This results in slowing down of the car.

    This can be useful when you have crappy/ small brakes and you're going down a long hill. Instead of riding the brake for many miles until the brakes overheat and stop working and you fly off the cliff, just throw the car into a low gear and you can go down as long a hill as you want.

    Some people also use them in normal driving situations to "save the brakes", but I wouldn't recommend it. It also doesn't "increase your braking power", as your brakes can usually more than lock up all 4 tires. If anything, it would unbalance your brake-bias.

    I believe tractor trailers use engine braking much more often in normal situations, but they are obviously a different situation than a everyday personal car. I think they also work slightly differently because they are diesel engines.

    Thanks for asking, by the way! I actually didn't even know how engine braking works until I read the question and gave it some thought...
    Last edited: Dec 11, 2009
  4. Dec 11, 2009 #3

    Ranger Mike

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    that is a pretty good explanation of "Jake braking"
  5. Dec 11, 2009 #4
    but Lsos explains engine braking only in petrol engines and jake brakes
    are solely intended for diesel engines.
    but wont engine braking affect the fuel efficiency?
  6. Dec 11, 2009 #5
    Almost any braking is pretty much the worst thing you can do for fuel economy. The whole point of a car is to get mechanical energy, so that you can move around. By braking, you're taking your car's mechanical energy and dumping it away as heat. Not the most efficient way of getting from point A to point B.

    Other than that, the actual act of engine braking should consume no fuel. Your computer should turn the injectors off.

    Diesels have no throttle so engine braking doesn't do much of anything. The engine compresses air, and then the air acts like a spring and returns the favor. A jake brake opens an exhaust valve so that the air can't push back on the piston...
  7. Dec 11, 2009 #6
    Engine braking is ususally used on overrun, the injectors are shut off so it uses no fuel.

    It's better than normal braking, in the fact that it won't wear your brakes out.
    It's worse than normal braking in that it won't slow you down as much.

    For example when you come to a huge hill, or are going down a mountain, you may see a sign saying stay in low gear.

    This is becuase if you are in a high gear, you are using the brakes to slow that car. At the bottom of the hill its very likely you would have boiled off the brake fluid (depending on the steepness) and have no brakes left.

    Using the engine a low gear allows a large amount of engine braking, meaning you have to use the pedal less, meaning you still have braking ability when you reach the bottom of the hill.

    EDIT: Just read the responces to the thread, and Lsos's post is just about perfect. I didn't copy i swear ;P
  8. Dec 12, 2009 #7
    Additionally, acting so may reduce the brake wear but increase clutch wear. Especially when you downshift continuously. Comparing the service costs of replacing the brake pads and clutch, dealing with the brakes is more economical.
  9. Dec 12, 2009 #8


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    As a general rule, with modern vehicles, you are correct. I, on the other hand, have never driven anything 'normal'. There are pictures somewhere in PF of my El Camino. It's a '76, frame-stacked on top of a '74 Jimmy chassis, with a '76 455 Olds Rocket engine and 33" Trail Handler tires. The tranny is a Chev 350 TurboHydro. (I hate automatics, but it had the damned thing when I bought it.) Luckily, the trans has a full manual valve body, so I can downshift any time that I want to. If I leave the thing in 'drive', the brakes are squealing within 2 blocks of home. As when driving a standard, I always downshift for deceleration and use the brakes only when necessary.

    edit: Holy sheepgarbage! There have been a lot of posts put up whilst I was composing this one!
    I also note that a couple of posts mention computer crap that real cars don't have.
  10. Dec 12, 2009 #9

    Ranger Mike

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    danger, glad to see you are with me on the " small " cubic inch IC to save the planet!
    save yer old engine oil and old tires for earth day..thats when i have a huge bonfire to celebrate!
  11. Dec 12, 2009 #10


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    You, me, and Uncle Hemi until the day that we croak, Mike. :biggrin:
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