Exhaust brake for gasoline engines

  • Automotive
  • Thread starter HeyJoe
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  • #1
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Main Question or Discussion Point

Everything I've read about them has said they would be ineffective or have very minimal benefit due to lower compression on non-diesel engines and other factors. Yet I am aware of one that sold well in the '80's and '90's and was tremendously effective as a "mountain tamer" in the RV industry with no notable drawbacks. There's much more to the story, and this being my first post, I'd rather keep it brief and have a discussion as to the viability of a gasoline engine exhaust brake system. Thank you.
 

Answers and Replies

  • #2
.Scott
Homework Helper
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First we need to note the difference between an exhaust brake (where the exhaust is simply obstructed) and a Jake Brake (where the engine is used to compress air).

Even if you don't have an exhaust brake, keeping your engine in gear while descending a steep hill will spare your brakes.
Mt Washington (6289 feet) is a few hours drive from where I live. I have seen the results of drivers trying to descend that road without using low gear and without stopping to rest their brakes. The result is a lot of heat, a lot of smoke, and a car that will cannot fully come to a stop without hitting something.

So given that a non-diesel engine is already capable of providing some braking. Any incremental improvement will be that much more beneficial.

The key advantage in engine braking (low gear, exhaust braking, or Jake braking) is that the engine is bigger that the brake with a more effective cooling system. So it is in a better adapted to slowing the vehicle without damage to itself. This braking isn't a replacement to use of the braking system - but is supplemental. It takes enough of the load away from the brakes to keep the brakes from overheating in heavy (but not urgent) braking situations.
 
  • #3
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https://en.m.wikipedia.org/wiki/Compression_release_engine_brake said:
The name is derived from the manufacturer, Jacobs (of drill chuck fame), and was patented 1962–1965 by Clessie Cummins.[1] When the driver releases the accelerator on a moving vehicle powered by an internal combustion engine, the vehicle's forward momentum continues to turn the engine's crankshaft. With Mercedes diesel engines being a notable exception, diesels by design have no throttle butterfly in the intake so regardless of throttle setting a full charge of air is always drawn into the cylinder. As such, even with fuel supply cut off and thus new detonation prevented, each time air is compressed in a cylinder virtually 100% of that passively generated energy is returned to the crankshaft, providing very little in the way of engine braking to the vehicle.

The typical compression release engine brake, as originally developed, uses an add-on hydraulic system using engine oil. When activated, the motion of the fuel injector rocker arm is transferred to the engine exhaust valve(s). This occurs very near the engine's "top dead center" (or TDC) and releases the compressed air in the cylinder so that the energy is not returned to the crankshaft. Energy is instead expelled out the exhaust system rather than being retained in the drivetrain. If used properly, a compression release brake can assist a vehicle to maintain speed or even slow it with little or no use of the service brakes. The power of this type can be around the same as the engine power.[2]

Contrast a petrol engine under deceleration, where a closed throttle prevents free flow of air into the cylinders, resulting in little pressure to release at the top of the compression stroke. The closed throttle provides engine braking by forcing the engine to generate a vacuum between the throttle and the cylinders.
The key difference is that when you cut the throttle in a gas engine, you cut both fuel and air flows, but in a diesel you cut only fuel.
 

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