Re: [A/S] Stick shift for towing?

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

Jim,

A gasoline engine typically has a throttle which controls the amount of air
that enters the engine. Prior to port fuel injection, the flow of air into
the engine determined how much fuel is sucked in by the venturi action
created by the air flow through the carburetor - (14.5 to 1 air to fuel
ratio in order to burn).  Thus, when you let off the gas pedal, the throttle
closes and does not let very much air into the cylinders.  Since the engine
is like a piston pump, (air compressor) by closing off the intake, the
engine has a hard time turning which can be used to retard a vehicle going
down hill.

Diesels do not use a throttle to control their speed and power output.
Rather, a diesel intakes as much air as it can and compresses it, which
generates heat in the combustion chamber.  Then, fuel is injected into the
hot combustion chamber and ignites.  Power output from a diesel is
controlled by how much fuel is injected (assuming there is adequate air to
burn it) so there is no need to put a restrictor on the air intake.

Automotive diesels (and perhaps others) often have a throttle on the air
intake which is activated when you shut the engine off, so it will stop.
This device is not active when the engine is in run mode.

Yes, there is some resistance to rotation in a diesel engine when no fuel is
injected, but not as much as a comparable size gasoline engine with a closed
throttle.

Note that the diesel exhaust brake is simply a way of restricting the flow
of gases through the engine (pump) thereby creating resistance to engine
rotation.  Since the efficiency of a diesel engine is partially determined
by how easily, and how much air it can ingest (the rationale for a
turbocharger), restricting the air intake is not a smart idea, hence the
restriction of the exhaust as a means of engine braking.

Hope this explanation helps

Oliver Filippi