A copy of a letter sent to me by Cummins on the operation of the CBRAKE.
Cummins Engine Company founder, Clessie Cummins, originally designed an
engine
retarder (brake), some 40 years ago. Jacobs Brake Company used the
design and
began manufacturing an engine brake, which was named the
?Jake Brake?. Cummins
now markets its own brake named the ?CBRAKE?, which ironically is now
manufactured by Jacobs Brake Company, for Cummins. Since there is no 'engine
brake' made for Cummins B & C series engines, an exhaust brake is used as a
vehicle retarder.
The principal theory and actual operation of an engine retarder brake is that it
turns the diesel engine into an air compressor. Rather than store the energy of
the pressurized air in the cylinder, created by the piston coming up on the
compression stroke, the engine brake retarder mechanism hydraulically opens the
exhaust valve near the end of the upward piston stroke. It also restricts fuel
to the injector at the same time.
The stored energy in the cylinder is released to the atmosphere so that when the
piston descends on what would normally be the power stroke, no pressure remains
in the cylinder to act on the piston. Thus, the energy transferred from the
driving wheels, into the compressed and highly heated air (through gravity or
forward momentum of the vehicle) goes mostly out the exhaust pipe, with a
portion entering the cooling water from heat transfer through the cylinder
walls.
The engine brake on a diesel engine is activated with the drivers foot off the
accelerator and the clutch engaged. When activated, the brake hydraulically
opens the exhaust valves near top dead center of the compression stroke.
With the brake active, the piston does work compressing the air in the cylinder,
and then this compressed air is released through the exhaust system. It
basically turns a power producing engine into a power absorbing air compressor.
For more specific details on engine brake operation and applications, you may
contact the Jacobs Brake Company website:
http://www.jakebrake.com/products/theory/jaketheo.htm
Also a diesel, being a free-breathing engine by virtue of having no valving or
venturi restrictions (carburetor) offers inherently less braking effort than a
gasoline (spark ignition) engine even though the diesel engine has about twice
the compression ratio of the spark ignition engine (which obviously means that
it requires more effort to force up the piston on the compression stroke), that
work is given almost entirely back (less friction and heat transfer losses) when
the air is allowed to expand on the next outward stroke.
Conversly, the spark ignition engine becomes a vacuum pump when being motored
with the throttle closed. This is a basic reason why the spark ignition engine
is so much less efficient than the diesel or compression ignition engine, when
operating at idle or light loading.
An exhaust brake also retards the engine, but in a different way. The exhaust
brake is a butterfly valve that closes off the exhaust pipe to create ?back
pressure? on the diesel engine, during downhill operation. When an exhaust brake
is used with an automatic transmission, the braking effort will be about 80%
compared to a manual direct drive transmission. We hope this helps to explain
the two types of engine retarders.
Thank you for your interest in Cummins.
Regards,
Powermaster
Customer Assistance Center
Cummins Engine Company, Inc.
