ENGINE SPEED: HOW LOW SHOULD YOU GO?
As soon as it hit the street, a road-test article in our October 1999 issue had owner-operators asking truck dealers for a similar spec — one with potential trouble built in. The test was of Caterpillar’s great new 600-hp 15.8-litre C16 engine (‘What a Blast!’) and it begs for a little clarification.
The issue, in essence, is the engine speed at which it makes sense to cruise a loaded truck on the open road. And the challenge is to combine engine, transmission, axle, and tire size to come up with a speed that promotes decent fuel efficiency while also creating real driveability. It’s easy to sacrifice one for the other, and for most drivers just such a sacrifice was built into that test truck.
The big Cat powered a Kenworth T2000 with, especially for Canada, a somewhat bizarre spec — a double-overdrive Eaton Fuller 18-speed with an 0.73 top gear, 3.36:1 Dana rears, and 22.5-in. Michelins that created a 1260-rpm engine speed at 60 mph. That’s ‘aggressive’ gearing, or even ‘over-gearing’, though it may make sense on the face of it for an 80,000-lb Interstate cruiser. The object, after all, is to keep average engine revs per mile low, right?
Well, there’s low and then there’s low. In fact, 1260 is some 200 rpm or more lower than what engine makers routinely recommend for cruising. And just 60 rpm above peak torque in that case.
Yes, modern engines can be ‘lugged’, but you don’t want to cruise them that way — you could run into driveline-damaging vibrations if you go too low and stay there. By all means, if you have the right spec, let the revs fall to 1200 and a bit lower to battle a hill. Torque’s going to get you over the top, so let the motor run briefly where torque is king. No problem.
But if you’re geared to cruise close to peak torque, there’s nothing in reserve as you hit that grade or battle a big wind. And the heavier your gross, the more inadequate your engine will feel because the revs are going to fall off faster. Driveability is gone because top-gear gradeability has been compromised.
This is where it pays to check out an engine’s performance curves — not just for spec’ing, but for deciding how to drive it well, too. Almost every engine’s torque curve will show a dramatic fall-off below the peak at 1200 or so. You can easily lose 500 lb ft between there and 900 rpm. That’s a sharp drop, and as you start up that grade it means your engine will feel progressively weaker as the revs fall. Result? You shift down.
If you’d been cruising at 1450 rpm — likely near the engine’s fuel ‘sweet spot’, don’t forget — you’d fall neatly into big torque and probably wouldn’t need another gear.
Worse yet, run too low on the tach and even that big 600 may feel so anemic that you’ll start cruising one or even two gears down from top. It’ll feel right because then you’ll feel power in reserve. Trouble is, especially with a double-overdrive box, now you’re running through four gear meshes instead of just two, so you bring extra drivetrain friction into the mix and your fuel economy suffers.
If gear-down protection isn’t programmed into the engine, and it won’t be in most owner-op or even fleet rigs, you might be allowed a highway cruise as many as four gears down.
Steve Bellinger, assistant chief engineer, vehicle engineering, at Cummins, felt compelled to comment on all of this. He’s the guy who designs the performance curves for all the company’s engines, and there may be no engineer who understands a driver’s real world better. He’s worth listening to.
“In my experience,” he told me, “95% of low-power and fuel-economy complaints involve trucks that were over-geared. You may put the engine on a dyno and find that it’s producing advertised power, but I guarantee you that the engine will feel weak [if it’s over-geared]. So drivers don’t leave it in top gear. If the truck can’t carry top gear, they shift down and leave it there.”
Bellinger says gearing for less than 1450 rpm, maybe 1400, at 65 mph will create a low-power complaint. And he’s talking about an 80,000-lb gross. Raise the ante to heavier Canadian weights and you want to cruise between 1500 and 1600 rpm, he says. And if you were somehow up there in the 150,000-lb-plus range, you’d spec to cruise between 1600 and 1700 rpm. As the weight rises, you simply need more reserve, and luckily the fuel-efficiency penalty of high engine speeds diminishes.
My old friend Steve Sturgess, editor of Newport’s RoadStar, our U.S. ‘cousin’, was driving the test truck along with Cat’s fuel economy guru Jim Booth. They actually cruised at 65 mph, which is — significantly – about 10 mph faster than Jim usually likes to travel, but even at that the engine was only turning 1370 rpm. That’s patented Booth territory, but not everyone is as good or patient as he is, nor as focused so completely on fuel economy. Also, don’t forget it was an 80,000-pounder running high-speed-limit western states.
We weren’t recommending that spec, I should point out. We take our test trucks as we get ’em. You, on the other hand, can choose.
They can be tough to find, but an engine’s power curve chart is a crucial part of your fuel-saving arsenal. Get the one for your specific engine ASAP. In this case, a Volvo engine, the green line shows the engine’s torque output at a given rpm. Note how torque falls off rapidly below 1100 rpm. The red line is the horsepower curve. The bottom line indicates fuel use, in terms of ‘brake specific fuel consumption’, not miles per gallon. The lower the BSFC, the less fuel you’re using. The ‘sweet spot’, where fuel consumption is lowest, is shown by the big asterisk, and the shaded section shows where this engine should be cruised. If you were cruising it at 1500 rpm you’d have 450 hp to use, and if you hit a grade, you’d have a rising torque curve to back you up. If the hill looked substantial, a downshift would put you pretty much at peak torque and chances are it would pull you over the top. If you were cruising at just 1300 rpm, you can see that a downshift would put you on the losing part of the torque curve.
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