Diesel engines have very little compression braking compared to a gas
engine.
A gas engine has 14psi of vacuum on the piston as it goes through the
intake stroke. A diesel engine has no such vacuum. Both the gas and
diesel engine compress the air during the compression stroke, but most
of that energy is returned on the "combustion" stroke. (There probably
is no combustion during engine braking on either engine as the fuel is
generally turned off entirely.)
Engine braking is a problem on diesel trucks, thus the use of exhaust or
Jake brakes.
Diesel engines need large batteries to power the starter because the
rotating mass is generally much larger and because of the energy
required to compress the air mass to the higher compression ratio. This
has nothing to do with its braking potential. High compression gasoline
engines need a more powerful starter/battery as well. Each has a
different engine braking characteristic.
On Mon, 2008-05-12 at 23:47 -0700,
cowtown@... wrote:
> >> Diesel powered cars have very little compression braking and seem to be
> >> able to descend hills safely.
> >
> > Huh? What? Diseasel cars compression brake just fine! Hell you
> > need a GP 27 battery to crank the damn things when it's cold! Of
> > course they go down hills great in low gear, like throwing out the
> > anchor!
>
> Yeah - isn't that what a "Jake brake" on a big rig is used for? Noisy
> as hell and not legal inside city limits, but they put that 15-25:1
> compression to work as an air pump through the exhaust!
>
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