To answer this correctly we need to start from basics. Torque is twisting force, and is usually
measured in the U.S. as foot-pounds, or more recently pound-feet (lb-ft). A one lb-ft torque could lift
a one-pound weight at the end of a one-foot lever arm. Alternately, a two-pound weight could be lifted
but only at the end of a half-foot lever. The product of multiplying the weight times the distance will
be the same for any given torque value. It doesn't require that a weight actually be lifted, only that
there was enough force to lift it.
Torque is not the same as work. You could be trying to lift a weight up (applying torque) all day, but
until you actually raise the weight, you are not doing any work (in the Physics sense), just tiring
yourself out. To actually accomplish work, you must exert a force over a distance. Power is the
accomplishment of a certain amount of work in a given amount of time. The most common measure
of power is horsepower (hereafter referred to as HP), defined as torque times revolutions per minute
(RPM). You may see this expressed as HP = Torque x RPM, although an additional multiplier or
divisor can be used to get the exact units you'd use for precise calculations.
If you'd inspect the entire drivetrain of your vehicle at any point during operation, you would see that
the engine crankshaft is turning at a certain RPM, the transmission output shaft could be turning at a
different RPM, and the rear axle has its own RPM rate. The horsepower (torque x RPM) available at
each point, ignoring the inevitable frictional losses for now, remains the same at these measurement
points because you can't generate power for free. But the torque available would be totally different
measured at those points because the RPM rates are different. As an extreme example, you could take
a AAA battery, a small electric motor, and two gears, and generate any amount of torque you'd like
simply by changing the gear ratio. Of course the RPM would be very slow, but 1000 lb-ft of torque
(the "same" as a large diesel engine) could easily be produced. Torque is completely useless without
RPM, just as trying to lift a heavy weight, without actually moving it, accomplishes nothing. I can
generate 1000 lb-ft of torque by simply sitting on the end of a five foot bar.
Now that we've got a handle on torque vs. horsepower, you should see that what we really need in an
engine is usable power at the wheels for the following tasks:
- Providing acceleration of the vehicle mass, thus increasing its "kinetic energy".
- Climbing hills, increasing the "potential energy" of the mass.
- Providing engine cooling. You'll see why this is important later.
- Overcoming various other forces including wind resistance and tire friction.
A large vehicle like a motorhome has to wend its way on public roads alongside automobiles and mesh
intimately with traffic. Because its power-to-weight ratio is nowhere near as good as an automobile's,
you'll routinely encounter frustration from other drivers and occasionally find yourself in potentially
dangerous situations because you cannot respond as rapidly as the rest of the traffic flow. A low
power-to-weight ratio is most objectionable in two situations: not being able to accelerate fast enough
to traffic speed, and not being able to maintain adequate speed going up hills. Maintaining speed over
level ground is generally not an issue because high speeds end up being very costly fuel efficiency-
wise, and most vehicles can easily attain speeds well over the posted limits on level ground.
An engine can only deliver so much power at a certain RPM of its crankshaft (where the transmission
gets connected). When you see a single-number rating for an engine for "torque" or "HP", be aware
that they are the "peak torque at the crankshaft somewhere in its allowable RPM range", and "maximum
horsepower somewhere in the RPM range", and only for a naked engine (no external loads being
driven). What you need to look at, to fully understand things, are the engine's performance curves,
which are graphs that show the available torque and HP at various engine RPMs. Only they will tell
you how much you can actually get out of an engine when it runs through certain RPM ranges in the
course of its operation.
|Understanding Horsepower and Torque
Figures 1 and 2 show the available torque and HP at various RPMs for two typical diesel engines.
The difference between the two examples is that the engine manufacturer has de-rated the power
of the engine in Figure 1 by having the engine computer restrict fuel injection at the higher RPMs.
Both engines have the same peak torque specification (red dots at just above idle speed), but the
maximum available HP (green dot) is greater with the engine exhibiting less torque fall-off. "Torque
Rise" is an industry euphemism (weasel-word) for torque fall-off at higher RPMs. The larger the
HP "rise" from high RPMs back down to low RPMs, the larger the fall-off at higher RPM. In
Figure 1, the fall-off is great enough to actually lower HP beyond a certain RPM level (green dot),
instead of at engine "redline". Redline is the maximum allowed RPM to keep engine stress within
acceptable levels. In Figure 2, the fall-off is not great enough to cause the HP to actually decline
before reaching its maximum HP at redline, as is the case with Figure 1.
The gasoline engine curves shown in Figure 3 represent similar but slightly different processes. In
gas engines torque is largely controlled by the tuning of the intake and exhaust air passageways and
the valve timing. The engines are usually tuned for as broad a torque response as possible without
limiting the peak torque. But even with careful tuning, the torque response of a gas engine tends to
be "peakier" than for a diesel engine.
Figure 5 shows the effect of the fan's parasitic load in determining where the peak rear-wheel HP
of the engine will actually occur. When a direct-drive cooling fan is in operation, the RPM point of
maximum HP output will be substantially lower than for the engine operating without the fan (as it
is delivered to the chassis manufacturer). Many diesel owners' Operator Manuals recommend
keeping the RPMs down closer to the point of peak engine torque while hill-climbing for "best
performance". What they are letting you know is that you will get the best mileage at the point of
maximum torque (which is true of any engine), and that redline RPM will not yield the highest HP
to the wheels because of both the "torque rise" of your particular engine and the high fan load at
that point. Reading between the lines, they're also saying they'll be more comfortable if you don't
operate at 100% for an extended time period. That recommendation not withstanding, the best hill-
climbing speed will be achieved somewhere above the maximum torque RPM point but less than
the redline RPM, exactly at the point of maximum rear-wheel HP. Obviously, torque is required to
generate the HP, but the engine's peak torque specification at some arbitrary RPM is essentially a
moot point. Deliverable rear-wheel HP is the real issue.
On a related note though, if your engine is overheating due to extended operation at peak power
(and an under-designed cooling system), you may want to down-shift the transmission to a lower
gear to allow a higher fan RPM. This will push more air through the radiator, although your
vehicle speed will decrease somewhat.
So, getting back to the question asked at the beginning of this discussion: If you had to choose
between two different engines, and the largest vehicle performance concern you had was getting
the best hill-climbing speed, would you rather have an engine with a higher torque spec or a higher
HP spec? Think of an answer before reading further.
If you answer "higher torque", you might need to review the material again. The peak torque spec
is just an arbitrary data value that you could change easily with a simple gear, without affecting
usable engine output. The HP spec cannot be similarly manipulated. If you answered "higher HP",
you've realized that only usable HP is going to keep you going up the hill without slowly coming to
a halt. And you've probably realized that engines and transmissions are designed to work together
to select the gear that will give you the most usable HP when you floor the accelerator pedal,
regardless of vehicle speed. In this case, your engine RPM should be at the point yielding
maximum rear-wheel HP, which has nothing to do with the RPM point yielding maximum engine
torque at the crankshaft. A good 6-speed transmission will allow enough adjustment of gear ratio
to keep the engine within a hundred RPM or so of the point of peak engine HP.
Some engine manufacturers confuse the issue by trumpeting "torque" as the ultimate gauge of
power. This is largely a marketing gimmick for diesel manufacturers to claim an edge over gas
engines, and for gas engine manufacturers who can coax a higher torque value out of their curve
somewhere than a competitor can. Remember that torque without RPM accomplishes nothing, but
when multiplied by RPM to do work it's called horsepower – and that's what really gets you
somewhere. Admittedly though, diesel engines with their flatter torque curves tend to produce
more horsepower at lower RPMs, which helps when you are accelerating up to speed going
through the gears, and is one of the reasons diesels are so popular for powering heavy vehicles, in
addition to generally being more cost-effective overall for that purpose for other reasons.
If you try to explain all the above to someone who doesn't have a decent grasp on physics and
math and who is enamored with the concept of "torque", you're probably just wasting your time
and his. Just pull a "Clinton" and say "It depends on how you define torque", and you're off the
hook and technically correct.
For further reference, see Caterpillar's excellent discussion "Understanding Coach/RV
Performance" by searching on the Internet for their publication "LEGT5364".
Back to the Discovery Main Page
Figure 1. Diesel Engine
with large Torque Rise
Figure 2. Diesel Engine
with small Torque Rise
Figure 3. Typical
Figure 5. Effect of Fan on Maximum HP RPMs
Most people don't understand what either torque or horsepower really is, let alone what the difference
between them is. Many who say they do understand probably don't, but will gladly misinform you
based on their personal anecdotes involving their 1976 Ford truck. Misconceptions can stem from a
problem in term definition, but there are real-world issues at stake:
In buying a large vehicle like a motorhome, if you had to choose between two
different engines (maybe gas vs. diesel), and the largest performance concern
you had was hill-climbing speed, would you be better off having an engine with a
higher torque spec or a higher horsepower spec?
Figure 4 shows the horsepower curves from Figure 1 (diesel) and Figure 3 (gas) superimposed on
each other. If you look at the RPM range used in accelerating from a stop, the diesel can be seen to
deliver higher horsepower, in this particular case during the entire range of an average transmission
shift (shaded red). This translates to faster acceleration "off-the-line". However, in the range of
engine RPMs used when the accelerator pedal is fully depressed to climb a hill, the higher HP of
the gas engine will provide greater power for climbing (shaded blue), which means greater speed.
If that was all there was to it, the performance curves in Figures 1 through 3 would determine the
exact engine RPM to achieve the greatest power at the contact points of the tires. Naturally, it's not
that simple. A multitude of forces limit the usable (net) HP of a drivetrain, but the one major factor
that must be considered is the load of the cooling fan, because it gets very large very fast as RPMs
increase. On a 500 HP engine, the cooling load of the fan alone at RPM redline can exceed 100
HP. This is because the load increases roughly to the third power relative to RPM. So, for example,
if you double the RPM, you increase the fan load eight times (2 to the 3rd power).
Figure 4. Diesel vs. Gas Engine Horsepower Availability