'Little' Engines, Big Power
Is it time to downsize what's under the hood? Maybe, but builders say a better idea is to
'right size' the diesel.
Tom Berg
Senior Equipment Editor
One might think that with the price of fuel well over two bucks a gallon, heavy truck operators would be looking at switching to smaller-displacement diesels to reduce fuel consumption and thus costs. By and large they're not, according to engine makers.
The 15-liter class diesel - which includes 14- and now 16-liter products and is referred to as a "big-bore" or "big-block" engine - remains the engine of choice for heavy, high-speed, over-the-road operations. At least one less-than-truckload fleet that keeps its tractors 'til they drop is reportedly going from 12 liters to 15 liters because the bigger-displacement engine will last longer than the smaller one. That fact of life has become more true in recent years as ever-stricter exhaust-emissions limits put more stresses on engines.
Diesels in the 12-liter class, which includes models with displacements of 11 to 13 liters, remain popular among many truckload fleets and specialty users.
Most builders refer to these as "medium-bore" or "medium-block" (not to be confused with medium-duty), but customers using them consider them heavy duty - and they are. Twelve-liter-class diesels often cost less and weigh less than larger products, and they perform well for operators who prefer them. They are the norm for bulk haulers - who want to cut tare weight to achieve more payload capacity - in regional tractors, and for vocational trucks, which are fully loaded half the time or less.
Why don't more OTR truckers economize with medium-bore diesels? All other things being equal, such an engine can save 0.1 to 0.5 mile per gallon of fuel over a 15-liter model, depending on the operation and who you talk to. That may or may not be worth the trade-offs of shorter life (depending on the operation), marginal performance and displeased drivers, who like the feel and bragging rights of a big engine. Right now fleet executives are struggling to hire drivers amid a labor shortage, and don't need disincentives that the "little" engines might cause.
Customers also are more concerned with simply getting trucks and hauling freight, many builders say. Truck manufacturers are selling everything they can make, and customers, who are preoccupied with hauling high volumes of freight in a strong economy, are not currently experimenting with specs changes.
Owner-operators who can afford to buy new trucks choose what's popular, and in OTR equipment that's the big-bore diesel. Besides, many fleets and O-Os are successfully charging higher freight rates and getting fuel surcharges, which relieve some of the pain of high fuel prices. And as individuals we have been conditioned to accept higher fuel prices.
However, cutting costs gives any business a competitive advantage. Would a smaller engine be wise for somebody accustomed to running a big one? Especially when modern technology has pushed output to impressive levels, even for smaller diesels? Maybe. Builders say the 12-liter-class diesel works in an OTR operation when:
• Speeds and loads are moderate, with cruising speeds of 55 to 70 mph and gross combination weights usually under 70,000 pounds. Studies over the years have shown that most operators overestimate their average GCWs, so they could be overspec'ing on power.
• Tractors are aerodynamic. An aero-style tractor requires less power to push through the air - about 50 fewer horsepower at 75 mph - than a non-aero tractor. This argues for aerodynamics even in a high-speed, high-load, big-engine scenario. (It also argues for aero aids on trailers, but hardly anybody's interested.)
The big-bore diesel works better when loads and speeds are high - close to or at 80,000 pounds GCW and sustained road speeds are 70 to 75 mph. This is common in the West where the speed limit on interstate highways in many states is 75 mph. Traveling at that speed cuts trip times over long distances and boosts any truck's weekly income potential. Also, going much slower makes a truck an impediment to traffic flow and merits its driver nasty comments on the CB. Anybody who doesn't think that's important needs to get out from behind his desk and in a truck cab for a while.
High speed requires high horsepower - this is decreed in the laws of physics - and long, stiff upgrades require hefty torque to economically overcome, because drivers don't have to downshift as early. Both power and torque are found in bigger numbers in big-bore diesels. Hills can be climbed while lugging an engine and using its torque, and this was a popular concept in the early to mid-1980s when fuel prices began climbing and stayed high. But higher horsepower at higher revs is what maintains road speed, on hills or on the level.
Torque, though, is what engine makers first talk about in discussing applications. And everyone wants to know what the truck will be used for before recommending displacement and horsepower-torque ratings. For moderate loads and speeds, 1,650 lbs.-ft. of torque is the magic number, with horsepower around 400 to 430. For high loads and high speeds, torque should be 1,750 or 1,850 lbs.-ft. and power set at 435 to 475, or higher for O-Os and some small fleets.
Emissions, Gearing
Emissions rules have made spec'ing the correct engine for the job even more important. The rules really tightened up in October '02/January '04, when diesels got exhaust-gas recirculation or other precisely engineered means to lower oxides of nitrogen. Here, airflow is among the many factors that affect fuel combustion. Too much or too little flow reduces efficiency, so working a "little" engine hard or letting a big engine loaf - practices once considered OK in many trucking circles - now costs fuel money.
In '07, rules will further limit particulate matter, and exhaust systems will be fitted with particulate filters to remove tiny particles of soot. To work properly, filters will need high heat, and that's most efficiently obtained from the exhaust itself. If the engine's not working hard, exhaust will be cool and fuel will have to be injected and burned inside the filters.
Gearing is more important now because each engine product has a "sweet spot" where economy and performance are best, and usually a narrower operating range. For some models the best cruising rpm is 1,500 or 1,600.
Sales people should have information on how to gear each truck properly; if they seem unsure, grab product literature off a rack or go online to see what the engine people themselves recommend. Call the factory's sales-engineering representatives if you have any doubts. Once ratios are built into transmissions and axles, they can't be easily altered.
Right-Sizing
Operating conditions vary and you can't change displacement to meet the moment's need - though some gasoline engines now do that by disconnecting cylinders when loads are low. But you can pick the right engine to cover most situations.
For instance, don't try using a medium-bore engine to get 500 horsepower. Some models are available for fire trucks and recreational vehicles, which occasionally need lots of power for a short time. You can't buy a commercial truck with engines rated like that because they wouldn't hold up for long and they'd use more fuel than you'd like. But you might be able to electronically uprate it to get snappy performance. In that case, you'd have to live with the consequences, which might include a higher fuel bill and broken drivetrain parts.
Beware of using a big block for small power, too, some experts warn. "Understressing" the block can add life, but fuel economy suffers because airflow isn't right. For example, a 14- or 15-liter engine set up to produce only 1,350 lbs.-ft. of torque can hinder fuel economy, because with large displacement, the flow is effectively restricted. This can even happen when torque is limited in lower gears to save wear and tear on the drivetrain. But not everyone agrees with this supposition, and we find in some models the "multi," "smart" and "intelligent" control features that limit torque output under certain circumstances.
It can also be argued that low power and torque is a waste of a big, heavy engine block in these days when smaller, lighter diesels are so powerful. So the trick is to "right-size" the engine to try to cover most of the operation. Pick a block of the best size for the power and torque needed for the job. Bear in mind, too, that the final setup must appeal to a truck's next owner, and order accordingly.
The editors thank these experts for their help in preparing this article:
• Bob Keane, customer satisfaction manager, Engine Division, Caterpillar Inc.
• Louis Wenzler, heavy duty truck sales support, Cummins Inc.
• Chuck Blake, special projects manager, Detroit Diesel Corp.
• Dave McKenna, powertrain product marketing manager, Mack Trucks.
• Ed Saxman, powertrain product manager, Volvo Trucks.
Equipment Continued...
