Why/How do diesels deliver *less* power but *more* torque?

It seems to me that diesels simply have *LESS* energy content than
gasoline engines. In every comparison with a diesel engine with a
similarly sized gas-engine, I always noticed that the gas engine has
more power.

However, diesels have "a high torque characteristic". Another words,
the engines produce more torque. However, torque is easily
change-able by simply altering the gear ratios.

How and why do people claim that diesels have a "high torque
characteristic", when in fact it has a "lower power characteristic"?

I never fully understood how torque relates to power in a car,
although I do know that it is a partial function of the gear ratios.

writes:

> It seems to me that diesels simply have *LESS* energy content than
> gasoline engines. In every comparison with a diesel engine with a
> similarly sized gas-engine, I always noticed that the gas engine has
> more power.
>
> However, diesels have "a high torque characteristic". Another words,
> the engines produce more torque. However, torque is easily
> change-able by simply altering the gear ratios.
>
> How and why do people claim that diesels have a "high torque
> characteristic", when in fact it has a "lower power characteristic"?
>
> I never fully understood how torque relates to power in a car,
> although I do know that it is a partial function of the gear ratios.

Roughly speaking, power = torque x speed.

Torque is analogous to force. If you are pushing on something with
massive force, but it's not moving, you are doing no "work" by the
physical definition of work. If it's moving, however, you are doing
work. Power is the amount of work done in a unit time.

This comes down to a fundamental principle: you can convert power into
torque, but not the other way around. If you have a low-torque,
high-power engine, that means that it's running very fast. You can
attach it to gears that, within limits, will give you arbitrarily high
torque. But, even if the gears were 100% efficient, you would still
have the same power.

The way this applies to car engines is that, other things being equal,
running an engine faster results in more power, as the pressure on the
pistons remains (relatively) constant. This is why
displacement-limited racing engines tend to run at high RPM; I think
Formula 1 engines peak at about 15,000 RPM these days. For most
engines, there is a point beyond which you can't get enough air in and
out to keep the torque constant at high speeds, so peak power will be
at less than maximum RPM. There are other tradeoffs that make the
engine easier to live with, trading off peak power to give better
performance at low RPMs.

Diesels are more limited in their RPM range because of the extra mass
needed to deal with the high peak combustion pressures. I think that
the speed of combustion has something to do with it also. Anyway,
you'll nearly always find that a Diesel has a much lower redline than
the comparable spark-ignition engine. This fundamentally limits its
power output, but not its torque, which may well be higher.

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.

Here's some real technical data:

Torque (from a normal automotive engine) is measured in either
Newton-Metres (Nm) or foot-pounds (ft./lb.). This is how much twisting
force can be exerted at the end of the crankshaft (or wheels, if you
measure wheel torque).

Horsepower however, is a mathematical calculation involving torque and
RPM. The mathematical formulae is (torque in ft/lb)(rpm)/5252. So, take
the torque at a given r/min (let's say 100ft/lb and 4000rpm) and insert
it into the formula:

100(4000)/5252 = ~76.16 horsepower.

But there is an interesting thing, let's say the torque is 100ft/lb at
5252rpm :

100(5252)/5252 = 100hp !

So whenever you see a horsepower/torque graph, you will notice the two
lines intersect at exactly 5252rpm (or the point (5252,y)). Always (in a
lb/ft and hp graph).

Pretty cool...

Now diesels are built much heavier than gas motors, due to their higher
compression (as in 22:1 instead of 8:1). Therefore the diesel can make a
peak of let's say 145lb/ft, but the motor can only rev let's say
4500rpm. So, comparing to a 145lb/ft gas motor that revs 6500rpm, the
horsepower number will not be as high (this is assuming the torque peak
is at redline, which it almost always never is).

And modern diesels do give respectable horspower numbers compared to gas
motors, and crazy torque amounts too. Case in point, Volkswagen's 5.0l
V10 TDI motor (Phaeton, Touareg, etc) and Mercedes-Benz's 5.0l V8 (E
500, S 500, SL 500, etc.). The Volkswagen diesel has 313 horsepower and
delivers 28mpg on a large sedan (Phaeton). The Mercedes gas motor only
makes 298 and gets 23 on a similar car, that is slower than the diesel
Volks.


Stephen H. Westin wrote:

> writes:
>
>
>>It seems to me that diesels simply have *LESS* energy content than
>>gasoline engines. In every comparison with a diesel engine with a
>>similarly sized gas-engine, I always noticed that the gas engine has
>>more power.
>>
>>However, diesels have "a high torque characteristic". Another words,
>>the engines produce more torque. However, torque is easily
>>change-able by simply altering the gear ratios.
>>
>>How and why do people claim that diesels have a "high torque
>>characteristic", when in fact it has a "lower power characteristic"?
>>
>>I never fully understood how torque relates to power in a car,
>>although I do know that it is a partial function of the gear ratios.
>
>
> Roughly speaking, power = torque x speed.
>
> Torque is analogous to force. If you are pushing on something with
> massive force, but it's not moving, you are doing no "work" by the
> physical definition of work. If it's moving, however, you are doing
> work. Power is the amount of work done in a unit time.
>
> This comes down to a fundamental principle: you can convert power into
> torque, but not the other way around. If you have a low-torque,
> high-power engine, that means that it's running very fast. You can
> attach it to gears that, within limits, will give you arbitrarily high
> torque. But, even if the gears were 100% efficient, you would still
> have the same power.
>
> The way this applies to car engines is that, other things being equal,
> running an engine faster results in more power, as the pressure on the
> pistons remains (relatively) constant. This is why
> displacement-limited racing engines tend to run at high RPM; I think
> Formula 1 engines peak at about 15,000 RPM these days. For most
> engines, there is a point beyond which you can't get enough air in and
> out to keep the torque constant at high speeds, so peak power will be
> at less than maximum RPM. There are other tradeoffs that make the
> engine easier to live with, trading off peak power to give better
> performance at low RPMs.
>
> Diesels are more limited in their RPM range because of the extra mass
> needed to deal with the high peak combustion pressures. I think that
> the speed of combustion has something to do with it also. Anyway,
> you'll nearly always find that a Diesel has a much lower redline than
> the comparable spark-ignition engine. This fundamentally limits its
> power output, but not its torque, which may well be higher.
>