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.”