Braking Horsepower?

[Big_Daddy_T]

Alright. Here is the question. Does engine braking hp depend on the size of the engine? Compression or exhaust? Can you enlightened folks walk me through engine braking and big loads?

 

[Blakers]

Are you referring to "Brake Horsepower" or to braking power?

 

[nickleinonen]

braking HP is commonly listed in OTR truck engines. it is the amount of power their engines compression brake can deliever/convert to slow the truck down on a grade without touching the service brakes

 

[Big_Daddy_T]

Ok how much brake hp can you get from an 8. 3? How bout bigger. Where do I research?

 

[drees1]

Go to the mfgr of the brake, such as jacobs or pac brake. They will quote specific retarding HP for your engine. An engine compression brake on a unit injected engine can deliver up to nameplate HP in retarding effort at peak rpm. Pump and line engines (like our B or C have less braking effort) To my knowledge no one makes a compression brake for the B. Braking effort increases with rpm. Cummins newest dual cam engine has a cam (I believe) just for the brake, optimizing effort even more. More displacement helps.



How it works: On a unit injected engine (where the injector is powered by a rocker arm on a 3rd cam lobe) as the piston nears tdc the exhaust valve is opened and the work of compression is dumped into the exhaust. Then the piston moves down and the opening of the inlet valve creates another pop as the low pressure in the cylinder is filled with fresh air. So the engine can resist on both compression and expansion strokes creating great negative work . There is a fixture that sits on top of the rockers that has plungers and oil passages, and when solenoids are activated oil hydraulicly links the injector rocker to the exhaust rocker, so near tdc when the injector is about to inject, movement of the injector rocker moves a plunger, moves oil, moves another plunger that moves the exhaust rocker dumping the compression near tdc. This is very efficient since the compression is dumped near tdc.



Pump and line engines with only inlet and exhaust rocker in the overhead ( no fuel rocker) use exhaust rockers from other cylinders to activiate the cylinder near tdc. The timing of the compression release is poor compared to a unit injected engine so the braking effect is much less, maybe 50% to 65%. Also, these engines must pair up 2 sets of three cylinders 1,2,3 and 4,5,6 to make it work. Therefore an engine with individual valve covers makes the task that much harder. Also, the fixture that sits on top of the rockers adds about 2 - 4 inches of height to the engine.



All the time you hear of truckers saying how great jakes are on Cat 3406E, Cummins N-14, That is because they are unit injected. Cat 3406B, C and 3306 are poor jake performance because they are pump and line engines.



Doug Rees

 

[Big_Daddy_T]

Thanks doug, you rock.

 

[Blakers]

Originally posted by Big_Daddy_T

Ok how much brake hp can you get from an 8. 3? How bout bigger. Where do I research?

Brake horsepower would be what the horsepower the engine itself puts out, including any necessary accessories such as fan, water pump, etc. As opposed to rear wheel horsepower as measured on a drum dynamometer.

 

[rspinks]

Great explanation, thanks. I was surprised that exhaust brake mfr's quote 180-190 hp on the 5. 9 ISB's in our trucks. That's over 75% rated HP on an ETH, althought the brake HP number is taken at 3100 rpm.