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A technical question about a jake brake.

Discussion in 'Trucks [ Eighteen Wheelers ]' started by Blackducati750, Jan 11, 2011.

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  1. Jan 11, 2011 #1
    Blackducati750

    Blackducati750 Light Load Member

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    First and foremost, I'm not a diesel mechanic. Other than checking fluids, I haven't done much under a trucks hood.
    I DO know (kind've) how a diesel engine works. There are no "throttle blades" or a throttle body to monitor or restrict incoming air. The "gas pedal" simply controls the fuel/air muxture, adding more fuel to the air/fuel mixture the further the pedal is pushed to the floor. I heard a diesel air/fuel mixture (at idle) is something like 100:1.
    Anyway, being there are no "throttle blades" to monitor/restrict incoming air, diesel engines tend not to have much engine braking like a gasoline engine does. The only engine braking felt is the "work" required to spin the crank/rods/pistons/valvetrain as the driver downshifts to a stop.

    Am I right so far?

    Okay, so to ADD engine braking forces to diesel engine, a jake brake is installed. Which is (more or less) a "flap" in the exhaust, just after the turbo. Right?

    The flap closes, restricting the otherwise "free flowing" exhaust making the engine/pistons work hard to push the exhaust out passed the restricted exhaust flap. The "work" makes the engine harder to spin, and it's felt as "engine braking" to the driver, right?

    Okay, so, here's my question.

    Is jake brake usage actually BAD for an engine?
    The way I see it, the engine is trying to push out the exhaust gases. But, the gasses hit a "brick wall" whenever the jake brake is on (flap closed). This raises exhaust pressure in the combustion chamber, exhaust manifold, and turbo. Now, the exhaust gasses try to find another way out. Leaking passed piston rings. Leaking passed valve seats, valve seals or valve guides. Leaking through turbo shaft seals and bearings. Exhaust gasses end up everywhere EXCEPT where they're supposed to go, the exhaust stack. The oil gets contaminated from exhaust gasses and that leads to engine wear in other places throughout the engine.

    So, am I 100 % wrong? Does jake brake usage not harm an engine at all?

    Oh, and I heard if you use a jake brake with your engine RPM's too high, you can blow your engine. Any truth to that?

    Thanks Everyone!




    I'm thinking about buying my first truck this coming spring and I'm sure it'll have a jake brake.
     
    Blackducati750, Jan 11, 2011
    Blackducati750, Jan 11, 2011
    #1
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  3. Jan 11, 2011 #2
    ironeagle2006

    ironeagle2006 Road Train Member

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    There are 2 kinds of Engine Brakes one is an Exhaust brake that sits behind the Turbo. The other is the Engine style were as the Engine is coming up to TDC on the Compression Stroke a Seloniod valve activates and opens teh Exhaust vales dumping the air out of the Cylinder and out the stack. Then when the Clyinder goes down on what would have been the Power stroke it is trying to pull a Vacucm and we all know those are hard to pull against. Know using a Jake is not bad for an engine if your not using it all the time. Yeah you might burn alittle more oil but the engines are built to handle it.

    As to your thrid part the Higher the engine spins the more Braking force it Generates. Max Braking energy is around 1800 RPM so coming down a mountain with the Jake on you want to be in a gear at that rpm were you not riding the Service brakes and your letting the engine brake do the work for you. Sorry this is coming from someone that had a Trainee throw his rig in Neutral on Parley and Scream we are all going to DIE by the time I threw him out of the Seat got in the seat we were at 85 MPH going Down HILL. Burned the Service Brakes to get it to a point were I could get the Jakes to hold me but I did get it to hold ME. I am STILL HERE.
     
    ironeagle2006, Jan 11, 2011
    ironeagle2006, Jan 11, 2011
    #2
    JustSonny Thanks this.
  4. Jan 11, 2011 #3
    Artbroken

    Artbroken Light Load Member

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    What you're describing is an exhaust brake.

    A jake brake works by opening the exhaust valve at the top of the compression stroke - the engine literally "farts". :)
    Since there's no air/fuel mixture to ignite, the piston "sucks" against the closed valves, rather than being forced down as normal. This vacuum is what generates the braking effect.

    Wikipedia has a decent article about all types of engine braking:

    http://en.wikipedia.org/wiki/Engine_braking

    Edit: IronEagle beat me to it, but, yeah... what he said. :)
     
    Artbroken, Jan 11, 2011
    Artbroken, Jan 11, 2011
    #3
    zinita17601 Thanks this.
  5. Jan 11, 2011 #4
    Blackducati750

    Blackducati750 Light Load Member

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    The engine brake the holds the exhaust valve open, that's what Mack uses, right? Or, do other companies use that too?


    What style is used in a Detroit Diesel Series 60?
     
    Blackducati750, Jan 11, 2011
    Blackducati750, Jan 11, 2011
    #4
  7. Jan 11, 2011 #5
    ironeagle2006

    ironeagle2006 Road Train Member

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    On an Jake Style were the Exhaust valves Dump some but Not all the Compressed air out of the Cylinder that way you still get the cushion of air on the way back up. Diesel motors are Interferance fit were if the Valves are open when the Piston is at TDC or close to it your going to Smack a valve bend it and your in a world of HURT there. The Other Style of vlave were it is in the Exhaust system aft of the Turbo works sorry had to read up on them been a while since I used that style. How they work is they basically force the motor to become ahuge airpump and try and force all the air out a restriucted passage.
     
    ironeagle2006, Jan 11, 2011
    ironeagle2006, Jan 11, 2011
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    JustSonny Thanks this.
  8. Jan 12, 2011 #6
    ratherbtruckin

    ratherbtruckin Light Load Member

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    My freightliner classic uses an engine brake, not an exhaust brake...its a detroit 60 series. I also have dual switches for mine, I can use 2 cylinders, 4 or all 6 by hitting both switches. (its not "mine", just the one I'm learning to drive in)
     
    ratherbtruckin, Jan 12, 2011
    ratherbtruckin, Jan 12, 2011
    #6
  9. Jan 12, 2011 #7
    heyns57

    heyns57 Road Train Member

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    The Jacobs Engine Brake turns the diesel into an air compressor by opening the exhaust valves when the intake air has been compressed. I was not aware the exhaust valves are closed quickly to produce a vacuum effect. I thought robbing the engine of its power stroke and ignition heat was enough. "Jakes" are much more effective than the butterfly valves in the exhaust pipe. Williams Exhaust Brake is one of those cheaper alternatives.
     
    heyns57, Jan 12, 2011
    heyns57, Jan 12, 2011
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    JustSonny Thanks this.
  10. Jan 12, 2011 #8
    Blackducati750

    Blackducati750 Light Load Member

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    Are valve adjustments more expensive when a trucks engine has a jake brake?

    I could only assume yes, because of the "thingy" that holds the valve open...
     
    Blackducati750, Jan 12, 2011
    Blackducati750, Jan 12, 2011
    #8
  11. Jan 14, 2011 #9
    Krooser

    Krooser Road Train Member

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    No........
     
    Krooser, Jan 14, 2011
    Krooser, Jan 14, 2011
    #9
  12. Jan 14, 2011 #10
    Volvo92906

    Volvo92906 Light Load Member

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    Ohio
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    They pretty much covered the Jake issue.. and as Krooser said, no, it doesnt cost more. Actually, Jakes are very easy to adjust. A jake can not and will not hurt an engine. Maybe if its not adjusted properly??? But, its a great thing to have for mountainous driving which is the reason it was ever used. You probably dont need it if youre not OTR.

    And, A diesel engines ratio is more like 16:1 as opposed to a gasoline engines 8:1. (at idle). Your 100:1 spec is a little high...
     
    Volvo92906, Jan 14, 2011
    Volvo92906, Jan 14, 2011
    #10
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