csimo
Veteran Member
There was a recent thread regarding diesel engine braking. The subject caused me to do some further investigation as to why diesel engines have much lower engine braking than a comparible gasoline engine. I asked both VW and Cummins to give me an answer. VW didn't respond, but below is the response by Cummins:
Diesel engines are 'compression ignition' engines, which means that there are no
external means to ignite the fuel and air mixture, except by compressing the
fuel/air mixture to cause enough heat to promote combustion of the mixture. The
compression ratios for Cummins diesel engines are currently around 14:1 for
larger Cummins engines and 16.5:1 or 17.2:1 for some of the B series smaller
engines and gasoline engines, for example are about 8 to 1. Since gasoline
engines are spark ignited, they have less compression ratio, where the diesel
must 'compress' the fuel/air mixture into a smaller space to create enough heat,
to initiate ignition.
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. Most all diesels today are turbocharged to
allow more air to be forced into the engine, thereby allowing more fuel to be
added for increased horsepower and torque output of the engine. Diesel engines
are 'heavy duty' engines, expected to run nearly one million miles to overhaul.
There is some difference in driveability; Cummins Diesels have maximum torque in
the 1200 to 1600 rpm range and maximum horsepower in the 1700 to 2700 rpm range,
which is considerably lower than that of a gasoline engine. This lower rpm
means longer life for the engine and less fuel consumed for the same amount of
work.
Typically diesels last about twice as long as gasoline engines and use about
half as much fuel. Choosing between a diesel or gasoline powered pickup truck
really depends on the customers needs and wants and intended use for the
vehicle. There are several advantages and disadvantages to both and the
end-user is the ultimate decision maker on which power plant best meets each
individual's particular requirements.
Most Dodge Ram trucks with the diesel engine have either the 3.54 or 4.10 rear axle
ratio; the 3.54 would give slightly better mileage and the 4.10 has slightly
better pulling power in the mountains with a load, therefore, knowing what you
intend to do with the truck will help decide which ratio to buy. Transmission
selection is another choice that comes down to preference and the manual may
provide slightly better mileage and shifting options but also more work for the
operator.
Thank you for your interest in Cummins. We hope this information helps.
Thank you for choosing Cummins power. Let us know if you need assistance in
locating the nearest Cummins-authorized distributor or dealer.
Please feel free to use our International Dealer Locator:
(http://www.cummins.com/partserv/intro.html).
found on our Customer Assistance page of Cummins website
(www.cummins.com)
Please let us know if you have other questions, and if away from your computer,
feel free to call us toll-free at 1-800-DIESELS (343-7357).
Regards,
PowerMaster
Cummins Engine Company, Inc
Customer Assistance Center
E-Mail Address: powermaster@cummins.com
Diesel engines are 'compression ignition' engines, which means that there are no
external means to ignite the fuel and air mixture, except by compressing the
fuel/air mixture to cause enough heat to promote combustion of the mixture. The
compression ratios for Cummins diesel engines are currently around 14:1 for
larger Cummins engines and 16.5:1 or 17.2:1 for some of the B series smaller
engines and gasoline engines, for example are about 8 to 1. Since gasoline
engines are spark ignited, they have less compression ratio, where the diesel
must 'compress' the fuel/air mixture into a smaller space to create enough heat,
to initiate ignition.
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. Most all diesels today are turbocharged to
allow more air to be forced into the engine, thereby allowing more fuel to be
added for increased horsepower and torque output of the engine. Diesel engines
are 'heavy duty' engines, expected to run nearly one million miles to overhaul.
There is some difference in driveability; Cummins Diesels have maximum torque in
the 1200 to 1600 rpm range and maximum horsepower in the 1700 to 2700 rpm range,
which is considerably lower than that of a gasoline engine. This lower rpm
means longer life for the engine and less fuel consumed for the same amount of
work.
Typically diesels last about twice as long as gasoline engines and use about
half as much fuel. Choosing between a diesel or gasoline powered pickup truck
really depends on the customers needs and wants and intended use for the
vehicle. There are several advantages and disadvantages to both and the
end-user is the ultimate decision maker on which power plant best meets each
individual's particular requirements.
Most Dodge Ram trucks with the diesel engine have either the 3.54 or 4.10 rear axle
ratio; the 3.54 would give slightly better mileage and the 4.10 has slightly
better pulling power in the mountains with a load, therefore, knowing what you
intend to do with the truck will help decide which ratio to buy. Transmission
selection is another choice that comes down to preference and the manual may
provide slightly better mileage and shifting options but also more work for the
operator.
Thank you for your interest in Cummins. We hope this information helps.
Thank you for choosing Cummins power. Let us know if you need assistance in
locating the nearest Cummins-authorized distributor or dealer.
Please feel free to use our International Dealer Locator:
(http://www.cummins.com/partserv/intro.html).
found on our Customer Assistance page of Cummins website
(www.cummins.com)
Please let us know if you have other questions, and if away from your computer,
feel free to call us toll-free at 1-800-DIESELS (343-7357).
Regards,
PowerMaster
Cummins Engine Company, Inc
Customer Assistance Center
E-Mail Address: powermaster@cummins.com
