Jake brake operation clarification, etc.

["Jason and Maria Colavecchia" <jasoncolavecchia@domain.elided>]

Just so that you folks know what a "jake" brake is and works, here is a
basic description of the modern setup:

Jacobs braking systems developed an assembly which piggybacks on top of an
OEM Diesel engine valvetrain that physically changes the valve timing in
braking situations.  There are various models which mainly differ in the
way the jake brake is activated.  The basic function is to change the valve
timing to force the diesel to become an air compressor, effectively
allowing the pistons to compress incoming air and then "blowing" it off at
TDC with the exhaust valves.  A well-tuned jake can usually reproduct about
2/3's the hp of the diesel's power in a braking situation.  Fuel during
this operation is cut off electronically (or mechanically on the older
designs) to prevent misfiring or wasting of fuel.  The most efficient
version of this is on the new Cummins Signature 600 hp engines, which can
effectively reproduce 600 BHP of braking effort, or literally 100% of the
power curve of the engine!  They are able to do this because instead of an
"add-on" Jake brake assembly, they have integrated their own version
utilizing a second overhead cam which creates compression in all phases of
the piston's position!  This is enough power to smoke the tires if you
apply it too quickly on hills...

Mack trucks used to use what is known as an "exhaust brake" or "stealth
brake".. it was simply a large valve on the exhaust system that shut during
braking conditions, effectively creating about 150hp of braking effort.  It
was very quiet, which was a plus when working heavy equipment in
subdivisions and stuff,  but suffered from reliablity and corrosion
problems, as well as being very weak compared to the Jacobs systems.

Exhaust brakes are returning onto the market, but mostly in the light to
medium duty market, where its quiet operation is a major advantage. AFIK,
jacobs (and other engine brake manufacturers, like Pacbrake) does not make
an equivalent to their "jake brake" for the smaller displacement diesels,
and nothing for gas engines.  Caterpillar also has a competing system
called the " brakesaver" which utilizes a completely different approach to
creating brake horsepower.