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.”