Downshift or exhaust brake?
I was just thinking after reading a post about gas vs diesel trucks where somebody mentioned they liked diesel because of the exhaust brake. I've never had a diesel, always gas. If I come up to a long...
Forum Discussion
wnjj wrote:4x4ord wrote:Turtle n Peeps wrote:
The reason an exhaust brake works better on a diesel is because of the compression ratio. It takes a lot more energy to compress an 18 to 1 cylinder with air in it than a 10 to 1 cylinder with air in it.
The high compression ratio of a diesel contributes much to engine braking when a "Jake Brake" is used because the exhaust valve is opened at the top of the compression cycle. With these little pick ups and the exhaust brakes they have the compressed air is allowed to push the piston down on the powerstroke there by largely offsetting any braking that was accomplished on the compression stroke. The one thing that does make the diesel more effective at engine braking is the fact that the turbo is pushing more air through the engine than with a naturally aspirated engine. On the other hand the higher revving gasoline engine can pump more air than the diesel because it is comfortable revving higher.
The diesel with an exhaust brake is like an air compressor that leaks. It can build significant pressure against the piston when intake air is drawn in, compressed, re-expanded (cancelling the compression) then finally forced out through the restricted turbo in the exhaust. That final part is where it creates many psi against the top of the piston. Constrast that with a throttled gas engine where the most psi it can have acting against the piston is a pure vacuum at 14.7psi. Put an exhaust restriction on the gas engine and it too can have similar braking performance, although you’d actually want to open the throttle to force more air into that restriction.
I agree although a gasoline engine with an exhaust brake would be much less effective at the same rpm as a diesel because it lacks a turbo charger. The braking work being done is a function of the volume of air being pumped past the exhaust brake x the pressure created at the exhaust brake. Because the turbo charged engine pumps more air per rev it can do more braking at a given rpm then a similar size non turbo engine.
