Re: [A/S] Diesel & stick shift issues Turbos and retarders

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

Stan,
The reason turbochargers work so well on a diesel engine is the fortunate
circumstances where the laws of physics all work in your favor.  Thus, you
end up with more power, greater fuel economy and longer engine life.

The reason is that diesels operate with an excess of air to fuel when they
are running efficiently, since only the fuel necessary to produce the heat
(power) is injected into the combustion chamber.  With more than enough
oxygen present, all the fuel burns (thus a diesel can actually be cleaner
than a gasoline engine if it continues to operate in this mode).  This is
also the reason for the superior fuel economy of a diesel, since it uses all
the heat energy in the fuel [gasoline engines require a 14.5 to 1 air fuel
ratio in order for the gasoline to burn completely, whether you need the
heat energy or not. The diesel only requires the fuel necessary to produce
the power required]. In addition, a gallon of diesel fuel contains more heat
units[BTUs] than a gallon of gasoline.

Obviously, if there is not sufficient air in a diesel's combustion chamber
to burn the fuel, the system doesn't work so well.  Enter the turbocharger.
It is simply a powerful air compressor driven by the exhaust gas (via an
exhaust gas turbine).

Case number one: Diesel engine at altitude.
Here, the oxygen content and the lower barometric pressure create a
situation where a normally aspirated diesel engine simply cannot ingest
enough oxygen to burn enough fuel to produce the necessay power.  Result -
lots of heavy soot and smoke (which is simply partially burned, excess
fuel).  With the turbocharger, more air (and oxygen) is forced into the
combustion chamber so that the fuel is burned completely. More power is
produced and no soot and no wasted fuel, hence improved fuel economy.

Since a turbocharger pumps the hardest when the exhaust gas is the densest
(contains more soot and unburned fuel), it pumps additional air into the
engine when the engine needs it the most.  For safety and engine durability
sake, the maximum boost a turbocharger will provide is typically controlled
by a pressure control valve called a wastegate. The bottom line is that
whether at altitude or under a heavy load, the turbocharger helps the diesel
engine operate in its most efficient mode - running with an excess of air
and complete combustion (and heat utilization) of the fuel.

In addition, the turbocharger permits the diesel engine to produce more
torque (and power) at  lower RPMs which in turn, prolongs engine life.

At lower altitudes, the turbo maintains the oxygen supply to keep up with
the power demands placed on the engine.

Transmission braking:
I am not intimately familiar with the Allison transmission GM uses with  the
DuraMax, but I do know that it was based on an earlier heavier duty Allison
which included a "retarder" to brake the vehicle on a down grade.

A retarder is simply a rotary pump working against a flow resistance (like a
torque converter where the output side is locked).  It will resist rotation
at the expense of heat buildup in the transmission fluid. Which is solved by
adequate transmission fluid cooling.  So instead of changing the rolling
energy into heat with the vehicle's brakes, the heat is transfered to the
transmission fluid instead.
> Allison 5-speed auto and the new Ford 5-speed auto really provide
> effective engine braking in "tow-haul" mode, although I know they do
> since I've experienced it by driving both.  The Ford has a
> variable-pitch turbo, so I've been imagining that it may play a role
> in this phenomenon, but that doesn't explain the Duramax/Allison's
> behavior.

Hope this helps.
Oliver Filippi