Torque has to be the most misunderstood term in trucking — aside from ‘backhaul’. Just listen to anyone talking about an engine: always, horsepower gets the glory even though torque does most of the work. It gets little of the credit, but more importantly, few of us reap the full benefit of the modern diesel’s ample torque output.
So what is torque, and why should we concern ourselves with it?
Torque is pure twisting force — not a measure of how fast the engine can do work, which is horsepower — but just the bare potential for work arising out of that twisting motion. Like twisting the lid from a tightly sealed jar, for example. Getting a little more technical, a Cummins document says “the torque output of an engine is a measure of the amount of turning force it produces which will move a load.”
In an engine, torque is generated by the pressure load of the expanding gases on the top of the piston multiplied by the stroke, meaning how far the piston moves. Two basic principles apply: 1) torque is stronger at the lower end of an engine’s operating range, while horsepower is higher at the upper end; and 2) bigger displacement engines produce more power than smaller ones, simply because there’s more area for combustion to force down those pistons.
But before you turn the page fearing some long and convoluted technical explanation, there’s just one thing you need to understand about torque: using it wisely can save you money.
All engine makers produce visual representations of their engines’ power output — known as torque charts or power maps — but as you’ll see, there’s quite a difference in the way various makes and models of engines behave. Their torque and horsepower ‘personalities’ can be quite different, which begs the question: why would you not operate the engine according to the way it was designed to perform?
Torque charts are the key to spec’ing the right engine for the job, and to driving the engine as efficiently as possible.
Horsepower, Torque, and Fuel
One thing you need to know about engines is that fuel consumption increases along with rpm — not necessarily in a linear fashion, but in general. That’s why going fast costs money. Going fast requires horsepower, so it stands to reason that running the engine at a higher horsepower output (higher rpm) is going to consume more fuel than running it at a lower rpm. Now, look at the chart — any chart really, all engines function the same way — and you notice that the horsepower starts to drop off as you work down in the rpm range at about the same point that the torque starts to increase.
That’s where the engine transitions from running to pulling — not a very technical explanation, but one that you can feel in the seat of your pants. The opposite is true as you accelerate: at low rpm, the engine is pulling; at higher revs, it’s working to keep the truck running at speed.
So, knowing that less fuel is consumed at lower rpm is hardly rocket science, but knowing where the torque is on the engine’s power band is useful, and that’s what the torque charts can show. Remember, each make and model of engine is different, so don’t use these charts unless they apply to your engine.
Assume you’re running relatively flat ground with rolling hills. You start climbing a short grade and the rpm begins to drop. At what rpm do you decide to downshift?
Many drivers accustomed to driving mechanical engines will drop a gear at the first sign of a loss of rpm. That means they’re using horsepower rather than torque to pull the hill. Sure, they’ll get over the top faster, but that speed comes with a fuel economy penalty. Letting the engine drift down into the high torque range of the engine, you’ll pull the hill more economically. For example, referring to the ISX chart, the peak torque band is from 1550 rpm down to 1125 rpm. That’s where you need to keep the engine in a pull, and frankly, the lower the rpm the better.
Experience will dictate when to make the next downshift — if necessary. You don’t want to drop down out of the torque range (below 1125) because it will feel as if someone has turned off the key. If a downshift is necessary, make it as close to 1125 as you can without sacrificing road speed. Depending on the transmission (the close-ratio 13- and 18-speeds require less of a bump up in rpm), drop a gear but stay within the peak torque range for best pull and best economy. If you’re pulling a long grade, get down to a gear where you can maintain the minimum engine rpm for the duration of the climb and let the engine do what it was designed to do.
And keep your foot into it: forget about ‘lugging’ or ‘over-fueling’ the engine. The ECM is running the show, not you. Trust it, and let the engine work. The ECM will only feed enough fuel to maintain engine speed. It’s not like the old days where 100% throttle pedal position meant 100% fuel poured into the engine. You simply can’t over-fuel an electronic engine.
Of course the opposite is true when going up through the gears, where you definitely can over-fuel the engine, and many drivers do. We’re talking about revving too high as we upshift. A technique called progressive shifting encourages drivers to upshift as soon as sufficient rpm has been attained to permit operation in the next gear. Really, that means using the bare minimum engine speed to keep the truck moving. Revving beyond 1200 rpm in low range is unnecessary in most circumstances — shifts can often be made a little above idle in the first three gears. Going much higher than 1400 or 1500 rpm in any gear but top gear just wastes fuel. Try it yourself if you doubt the claim. Torque is the key here.
Gearing and Drivability
The torque charts are also useful when it comes to spec’ing the rest of the powertrain around the engine, or matching an engine to the application and/or powertrain.
As we have already stated, torque is the product of the energy of combustion multiplied by the distance the piston travels, so on a 15-litre engine like Cat’s C15 or Cummins’ ISX, the stroke of the piston won’t change, but the engine programming can adjust torque and horsepower output to suit the user’s needs.
A Cat C15 rated at 500 hp can produce 1650 lb ft of torque at 1200 rpm, as does a C15 programmed for 435 hp. The ISX can be programmed similarly. Both brands can also be programmed to produce as much as 1850 lb ft of torque at various horsepower ratings, giving the user the benefit of loads of torque in the low end of the power band, while limiting the available horsepower.
Why would one want to limit horsepower but retain a high torque output?
Good driving habits dictate moderate road speeds. Horsepower is required for higher-speed operations, whereas high torque is required for better pulling. It’s logical then, that operators who don’t intend to drive fast can spec a powertrain that puts the optimum engine speed at the point where the gearing allows the truck to run at the selected road speed. You pay upfront for high-horsepower engines, and you pay again at the pumps, so cutting back on the horsepower and buying all the torque you can get for your money means a more economical truck, beginning to end.
Higher horsepower is required to keep the truck moving at high speeds. This graph illustrates how high speed requires higher horsepower — but high horsepower means high fuel consumption. If you want to run at 60 mph and you want good fuel economy, gear the engine to run at 1400 rpm. If you want to run at 70 mph, you’ll need 100 rpm more, which hurts fuel economy. Canadian weights demand gearing that keeps the engine about 100-200 rpm higher at cruise speed than most U.S. specs (consult your dealer for exact demands of Canadian specs).