Powertrains
Higher fuel prices make efficiency more important, while behavior of new engines requires special attention to gearing.
Tom Berg
Equipment Editor
Ever-tightening exhaust emissions regulations have greatly changed engine configurations and resulted in higher prices, lower fuel economy and sometimes spotty reliability. As a result, the October 2002/January 2004 diesels were shunned by most heavy truck operators. But they're now being ordered in large numbers by fleets wanting to beat the next round of emissions-reduction regulations, due in January 2007. Most diesels then will need exhaust aftertreatment devices that will be somewhat bulky, even more expensive, and require more maintenance.
Even the U.S. Postal Service - a semi-autonomous arm of the same government that promulgated the emissions regulations - announced that it's replacing its entire heavy truck and tractor fleet between now and the end of 2006 so it can avoid the '07 engines. You get the feeling that now might be a good time to buy - and many truckers are.
Builders have geared up to produce higher numbers of vehicles as orders have jumped, and production schedules are stretched out toward year's end. Even with the rush, haste makes waste and careful spec'ing can be done with the help of knowledgeable sales people.
Time To Downsize?
Times have changed, so the spec'ing recommendations now made by engine and truck builders may vary from what you've been doing. Among them:
• Consider a smaller engine model. In Class 8 over-the-road tractors, many fleets and most owner-operators have stuck with diesels displacing 14 and 15 liters. These so-called Big Bore engines are usually rated at 425 to 475 horsepower. The strongest ones from Cummins and Caterpillar now go to 565 and 625 hp - definitely very satisfying to drive, but rather high in price, costly to feed and hard on tires, if drivers are on the hammer a lot.
To be sure, 14- and 15-liter diesels generally last longer than smaller engines, and bring better money at selling time because most buyers of certain used trucks want big engines, too. If you buy trucks at the right price in the first place and drivers operate them sensibly, the higher resale value will pay for a lot of fuel. And a big engine might be wiser in heavy, high-speed operations, such as cruising at 70 and 75 mph in states where it's legal.
However, today's 11-, 12- and 13-liter diesels can now develop really healthy power and torque. All are available with ratings of 425 hp or more, and torque as high as 1,650 lbs.-ft. Like bigger diesels, some of the smaller-block models have variable-geometry turbochargers or double turbos that respond well at low revs, so they're pleasant to drive.
They accelerate and climb hills well - major concerns for any self-respecting driver - and they are a legitimate operational and safety consideration. Experience shows that the lower displacement will deliver about a half-a-mile per gallon better fuel economy, and maybe more, all other things being equal. That can save thousands of dollars per truck per year.
An 11-, 12- or 13-liter diesel also weighs 400 to 600 pounds less than a Big Bore engine, which could allow you to carry extra payload, or at least worry less about a truck being overloaded. And the smaller engine will cost several thousand dollars less.
Engine-Truck Matching
Sometimes the smaller engine proposition works and sometimes it doesn't. If you're going "aero" - buying an aerodynamic tractor - you're after better fuel economy anyway, so the smaller diesel is appropriate, and especially in medium-nose models. Also, it will bring a decent resale price because the next owner wants the same thing - economy.
But get the big-bore diesel if you're going to order a "large car" tractor with traditional styling, a long nose and a long-and-tall sleeper. Used-truck people will say that kind of truck should have a Big Bore diesel because that's what the potential second owner thinks it should have.
Will that attitude change if high fuel prices linger? We'll see.
• Consider variable-power, multi-torque engines. High power and torque are usually needed more at highway speeds than around town, and many engine models are set to deliver the extra oomph when it's called for and cut back when it's not. This preserves performance but saves fuel; however, drivers have to know about the feature so they can let it work.
Various operating parameters should also be set to encourage drivers to upshift as soon as possible. Revs in lower gears can be limited to require drivers to use "progressive shifting" - revving just a little in the first few gears where little actual power is needed, and gradually raising RPMs and power as road speed increases.
Idle limiting - shutting off the engine after a few minutes - is another setting available in the engine's electronic control module. Ask sales and service people about all the settings possible in the ECM.
• Gearing is more important than ever. Until recently, the "sweet spot" of many engines was about 1,600 RPM, but the most efficient speed of most October '02/January '04 diesels is down to 1,450 or so. Cummins testing shows that an engine geared to run 1,450 RPM at 65 mph gets approximately 4% better miles per gallon than one geared to cruise at 1,600 rpm. Other engine builders report similar results.
Obviously, gearing of the transmission and rear axles has to be matched to get the correct engine speed. The axle ratio needs to be a bit "faster" - lower-numbered - than before, especially if it's paired with a direct-drive transmission. For high-speed cruising, a double-overdrive transmission would be wise. Kenworth recommends close-ratio transmissions be used to keep revs as close as possible to their most efficient point, and that transmissions that automatically shift between the top two gears be considered.
• Consider automatic and automated transmissions. Skilled drivers enjoy and take pride in shifting properly, but keeping an engine in its most efficient mode takes extra concentration and work. It also implies that drivers know exactly how to drive the engines assigned to them, and this is not always true.
And many drivers simply are not skilled enough, or care enough, to do a good job.
Time To Automate?
Mating an engine to a self-shifting transmission is an increasingly cost-effective solution to the problem of improper operation. Allison, the original builder of automatics for medium- and heavy duty trucks, has lowered its prices to make them more affordable. More active competition from makers of automated mechanical transmissions helped force the move by Allison, because AMTs can do many jobs for less money. This is primarily true in over-the-road trucks, where full automation is not needed when the transmission spends most of its time in the top few gears.
Also, fully automated transmissions are now more reliable and easier to operate. Eaton says glitches in early generation products have been worked out, and the "three-pedal" AutoShift with a manual clutch is giving way to "two-pedal" UltraShift designs with automatically operating clutches. UltraShifts are now available in medium-duty models and a heavy duty product is due out soon. ZF Meritor's European-sourced two-pedal FreedomLine models have developed a following, and like the latest Eatons, work well in highway trucks.
AMTs also work well in dump and other vocational trucks, while Allison automatics become a wiser choice as off-road running becomes a bigger part of an operating day. Eaton, meanwhile, is said to be working on an automated LL transmission for on/off-road use.
• Watch the details. Gearing must take into account startability and gradeability - the ability of a truck to start out from a dead stop on an uphill grade and climb a grade at higher speeds with minimal downshifting. These are expressed as percentages, and sales people using computer software from truck and engine builders can quickly and easily calculate the right gearing to achieve the proper specs. Of course, one of the factors in gearing is wheel and tire diameter, so once the truck's in service, do not change tire sizes without considering any effect on road and engine speed.
Cooling systems are under more stress because today's engines need to shrug off more heat. Be sure the radiator and other parts of the system are big enough to carry the heat-transfer load. Components of exhaust-gas recirculation systems, which send the heat to the cooling systems, are claimed to need no extra maintenance. Time may prove this true, but it's a good idea to watch them closely.
And it's mandatory that coolant be treated correctly and oil changed religiously according to manufacturers' specifications. Extended drain intervals are still possible, but only with regular oil analysis and use of motor oil that's approved by the engine builder.
Related Story: Automatics
