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Engine Braking?


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Diesels need an exhaust brake because there is no throttle body to close off the continuous air supply that the motor has, therefore the exhaust brake causes a restriction in exhaust flow, slowing the engines RPM's because of the backpressure.

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I have done Multiple tests. I have driven fully loaded trucks. By fully loaded, I mean 15,000 lbs. on the trailer. With a STOCK, unmodified pickup, (like your stroker) the diesel will run away downhill. With a gasser, backing out of the throttle on the same grade will produce the braking effect. I am by noe means saying that a diesel engine won't slow down when you back out of the throttle, it just doesn't happen as well as a gas motor. With the same load behind my Ram with a 360, (15,000 pounds) I can slow the load with backing out of the throttle and/or downshifting. Downshifting in a diesel can be felt, but it will not be as good as a gas engine...

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:blink::huh: I think my brain is overloaded from reading all the posts.

But thanks for all of the replys. Reason I asked was cause I was riding a blaster with no brakes and was trying to slow down with out going through the fence so I down shifted and swerved all over the place, didn't know if it was engine braking or swerving that slowed me down.

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Not in a diesel engine.  Diesel engines, by their inherent design have an unrestricted air intake.  Meaning, there is no butterfly, slide, or other device to close off the air intake.  The engine can suck in as much air as it wants, no matter where your foot is in the throttle.  When you back off the throttle on a diesel, you are just shutting the fuel flow off.

Arent there valves in a diesel engine? Humm wouldn't it seem that perhaps the closed valves in the compression stroke would slow or "brake" the engine if verry little fuel were added?

 

Damm your stupid...

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i know little about diesels but my friend owns a big garage for them and i have walked around plenty of times to see engines all apart 1/4, 1/2 and full together. i am pretty interested in them so im going to respond to this.

 

if a diesel has valves for the intake, then what does it matter how much unrestricted air is behind that valve?

 

i am on no ones side just trying to learn myself

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just thought of something.

 

on gas engines, air is coming into the carb, or etc, and mixing with the gas and going through the intake valves into the cyl. when you let off the gas both of these will not be there so no combustion. no power out, power coming in from driveline and powering the engine over.

 

on diesel i read the air and fuel are separate. how does this work?

 

 

either way i dont get why there would be lil to no engine braking on a diesel. as long as you are shutting down the fuel intake there is going to be no combustion and no power going out???

 

is this a decent why of thinking?

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well i dont know much but here are my thoughts.....air and fuel (the mixture for idle) are still being sucked in on a closed throttle just not as much, so combustion is still happening. however at the speed the piston is traveling it is trying to draw in more air that the carb allows because of the slide or whatever being nearly closed. ever take one of those syringe deals (not a needle kind the kind that can draw fluids) and put your finger over the end and try to pull the plunger back? it is difficult, there is quite a bit of resistance. this is the same kind of resistance that causes the engine to slow with the throttle blocked off. The piston is trying to draw air, but its pulling againts a basically blocked passage way. since a diesel has nothing to block the air this cannot happen. So thats what makes sence to me...although maybe boonman has just polluted my mind with his lies :lol::P

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i always thought it was compression braking, but that does sort of make sense.

it just seems wierd to me that if that were true, on a carb like a holley 2 or 4bbl,

if there were that much pressure differential on the intake that was sucking on

a closed throttle plate, it would just suck it right open because it is only closed

lightly with spring pressure, and you know how easy it is to move the throttle

plate. what do you guys think about that?

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May I add an opinion?

I understand what I have read from everyones posts. But What about centrifugal force and all of the laws of physics and crap in the transmission and the rest of the vehicle? Here is my point/ thought: When you downshift the trans (in any combustion vehicle), the engine is force to turn faster as long as the clutch is engaged. (this is also saying you are not accelerating) Now that the engine is turning faster, natural laws of physics will cause the engine to eventually slow down, even if you are headed down hill (maybe not a 90 deg incline :) . But with all of the resistance in the mechanical parts of the vehicle from the clutch to the compression of the pistons, to the axle bearings you are gathering resistance, therefore slowing down the vehicle. eventually. I have driven diesel vehicles and have used the "retarder", I couldnt say how it works but I was under the impression that it used the exhaust gasses in some way to help slow the engine. Either way it makes a nice noise to blast at people working under a bridge :<.

 

So Basically my opinion is that the engine compression (Without fuel), transmission and the resistance of wheels and terrain etc are what slows the vehicle down (besides brakes of course).

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