When it comes to truck engines, bigger may not always be better, as once universally thought. But bigger may still be better depending on the vehicle application and duty cycle. Bear in mind that diesel engines for GVW Class 6-8 trucks and tractors have gone through tremendous technological change in the past several years to meet ever stricter rounds of emissions mandates and also to operate more efficiently and to live longer in all sorts of fleet service.
Put all this together and one thing is clear: There is now no one-size-fits-all heavy- or medium-duty truck engine. That means both the in-house engines and those supplied by an independent vendor that power the trucks and tractors of Class 6-8 OEMs must be selected and spec’d carefully to perform efficiently in the given job at hand and for their later resale value.
“Detroit’s engine lineup of the DD13, DD15 and DD16 offers the customer a choice, enabling the customer to pick the best displacement for their application,” says Brad Williamson, manager-engine and component marketing for Daimler Trucks North America, parent of Detroit.
According to Williamson, the DD13 (12.8L displacement) is designed for in-town, construction and LTL applications while the DD15 (14.8L displacement) is a heavyduty engine that meets performance demands for power, fuel economy and emissions. The larger DD16 (15.6L) is designed for extreme duty or heavy on-highway trucks.
“A lower displacement is ideal in some applications, but there are many factors that contribute to this decision,” he advises. “By allowing the customer to choose the best powertrain for their application, Detroit provides optimal performance, fuel economy and reliability for the customers’ specific application.
“Many other engine manufacturers are using single-displacement or lower-displacement options to offer benefits to certain customers,” he contends, “but not allowing them to choose the best engine for their application.”
Williamson relates that Detroit is not making significant changes to its DD13, DD15 and DD16 diesels to produce power. Rather, he says the engine maker is “doing extensive research on what is the optimal powertrain combination to partner with our engines to provide the best fuel economy in the industry.
According to David McKenna, director of powertrain sales & marketing for Mack Trucks, truck engines have in the past several years evolved rapidly with new technologies to deliver greater efficiencies to meet customer expectations.
He says there now is a trend to smaller-displacement diesels for certain applications.
“At one time,” he relates, “it was a given that going to a smaller engine that produced higher horsepower would result in the trade-off of shorter longevity. And that was acceptable to some customers running ultra-light operations. But fast forward 20 or 25 years to today, and now we have the technology—including the materials the engines are built from—to make them lighter without sacrificing performance or longevity.”
McKenna adds that by one mark engines have changed markedly in the past 30 or more years. “Back in the ’70s,” he points out, “the average power density for highway diesels was 25 hp. per liter. In the ’80s and ’90s, that measure rose to between 29 and 31 and now it ranges from 36 up to 39 hp. per liter. So the size and weight once needed to get 350 hp. from an engine can now deliver 400 hp.”
Power Density
McKenna says key factors that have contributed to this power-density advance start with the metallurgy of today’s engines and include new designs for such internal parts as crankshafts and connector rods, and the much higher quality of today’s lube oils. Mack introduced its MP engine line in 2007. It consists of the MP 7 (11L), MP8 (13L) and MP10 (16L).
The MP7 comes in horsepower ratings from 325-405 and boasts power density of 36 hp./liter. Mack regards it as ideal for applications that are very weight-sensitive as well as in refuse operations.
The MP8’s horsepower range is 415-505 and with its power density ranging up to 38.8 hp./liter, it is aimed at heavy-duty vocational and highway applications. According to Mack, thanks to its power density, the 505-hp. version weighs several hundred pounds less than a 15L engine at an equivalent horsepower.
“Power density is key,” points out McKenna, “as it determines the overall efficiency of an engine. We’re finding even bulk haulers with sleeper cabs are requesting the 405- hp. MP7 Econodyne engine because it is 350 lbs. lighter than a 13L engine. As for fuel, an MP7 will be 1-2% better than the closest horsepower rating of a 13L.
“Depending on application and duty cycle,” he continues, “moving from a 13L down to an 11L engine can lower weight and increase fuel efficiency while providing comparable life and performance.”
He emphasizes that before making such a decision it’s crucial to study the expected duty cycle for the truck. “If the truck is going to be loaded close to maximum or operate out West or through Eastern mountain ranges on a consistent basis, we would recommend the customer spec an MP8 with the lowest horsepower to get the job done,” McKenna explains. “Bear in mind that the horsepower can be raised in the field via an electronic programming change if more performance is needed or at the point of resale.”
Denny Mooney, Navistar’s vice president of global product development, relates that the OEM’s data going back two to three years indicates that 15L diesels had dominated all heavy-duty segments. But he reports that last year was the first in which 13L engines outsold 15L ones industry-wide.
Navistar offers both displacements within its full line of MaxxForce diesels that power its medium- and heavy-duty trucks. MaxxForce engine models range from 4.5 to 15L displacement.
“What’s happened is that engines are now being engineered for higher power density, which is measured in hp./ liter, by such improvements as more precise injection and better air control via the turbo. As a result, a smaller displacement [13L] diesel can provide the necessary power and torque yet weigh less and be more fuel-efficient [than a 15L engine].
“There is also more confidence building in making the smaller engine choice as driver acceptance of 13L power is growing,” he continues. “For them, it’s all about response from the throttle and power density affects that. Once drivers say they like the engine’s performance, it helps fleets make the decision to go to a smaller displacement.”
Mooney points out that emissions mandates helped move the power-density bar. “Emissions rules drove everyone to work on fuel controls, air controls, etc. All those changes yielded cleaner exhaust, but also more efficient engines.”
Although 13L power is proving itself in a wide array of applications, Mooney does state that fleets with severe-duty cycles, such as heavy vocational uses or those running over mountains, may spec 15L engines to gain more torque.
According to Ed Saxman, product manager-powertrain for Volvo Trucks North America, the OEM powers its trucks with four engines: Volvo’s own D11 (10.8L), D13 (13L) and D16 (16L) as well as the Cummins ISX15 (15L). “From our standpoint,” he relates, “each of our Volvo diesels is a distinct engine. Horsepower ratings do overlap for each.
“The average horsepower of the D13 when sold in one of our trucks is a little higher than that of the 15L vendor engines (Cummins ISX15) we also sell,” he continues. “Horsepower level is not necessarily reflective of displacement.”
Saxman sees a trend of customers moving to lower-displacement engines, noting that “our D13 is by far our most popular engine; and it can be had in horsepower ratings from 375 up to 500. And it is an extremely beefy engine for its displacement.”
Upping hp
A big advantage that today’s engines have when the point of resale is reached is that their horsepower can be upped via reprogramming at a dealership, Saxman points out.
While he says the D13 is engineered for linehaul, regional and vocational operations, the lighter and smaller D11 is aimed at regional operations and the heavier D16 is targeted at both high-payload linehaul and heavy-haul operations. “Because it is lighter in weight, the D11 holds appeal especially for gasoline bulk haulers and other operations that are very weight-sensitive and which run lower miles and typically have their trucks back home every night,” he advises. “The advantage of the D11 for these operations is they can carry more payload on each run and—theoretically at least—the fuel efficiency should be higher due to less parasitic losses on the engine.”
Sister OEMs Kenworth and Peterbilt both go to market with proprietary diesel engines manufactured by their parent company, Paccar. The heavy-duty MX engine has a displacement of 12.9L. It is offered in horsepower ratings from 380 to 485 by both Pete and KW.
Peterbilt chief engineer Landon Sproull says the OEM aims the MX at both over-the-road and vocational applications while it targets the smaller Cummins ISX12 (11.9L) engine it also offers to vocational operations and the larger Cummins ISX15 (14.9L) it provides to heavyhaul customers.
“Our 13L MX is integrated into all of our models except for refuse trucks,” Sproull relates. “It can handle GCWs up to 130,000 lbs.; get beyond that, and the ISX15 would come into play.”
As to why the truck market has seemingly overnight largely downsized from 15 to 13L power, Sproull says to keep in mind the impact of global marketing and costing. “Truck OEMs are becoming more vertically integrated and doing more leveraging of product from their worldwide portfolios,” he explains. “Globally, the 13L powerplant is very popular. And it’s not the power or displacement per se that makes it so. It’s the lower weight provided without any loss in performance or efficiency that puts these engines in demand.”
“The commercial vehicle industry—the truckers—are working very hard at matching vehicles to the job,” says Jeff Jones, vice president of sales & market communications for independent engine builder Cummins Inc. “That’s why we will keep on offering a full range of engines for all duty cycles—from 15L to 12L to even 9L power for Class 8 trucks.”
Starting from the top, Jones says that Cummins regards the 15L market as very stable “and one that we might argue will grow. There’s simply no replacement for [higher] displacement when it comes to fuel economy, reliability and longevity in a 65,000- to 80,000-lb.-GCW operation running mostly on highway pulling loads.”
He explains that with such duty cycles, “higher displacement is an advantage when it comes to fuel economy. The larger engine size enables broader power and torque curves so the engine can be operated at lower rpms than smaller engines for improved fuel economy. Given that as well as the proven longevity of these engines, we simply do not see the 15L market declining.”
On the other hand, Jones says that in stop-and-go and urban duty cycles, the smaller displacement of a 12L or 13L engine, such as Cummins’ ISX12, does offer an efficiency advantage since high load factors are not an issue. “They also have their place in many vocational applications where weight is a significant factor,” he notes.
“There is a place for both lower and higher displacement engines in this marketplace,” Jones sums up. “We’ve built large bore 14 to 15L engines throughout our history and been building medium-bore 12 to 13L engines since 1981. The industry needs and wants both.”
