Are Diesel Truck Engines on the Verge of Becoming Even Larger? We Consulted the Experts.

      Caleb Jacobs

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      The rumors surrounding diesel trucks are constantly swirling. From speculation about new manufacturers entering the market to fanciful tales of emissions deletes becoming permissible, discussions abound. One of the most common rumors I encounter is that a larger Cummins, Duramax, or Power Stroke engine may soon appear in the next generation of Ram, GM, and Ford trucks. To ascertain the validity of these claims, I consulted the engineers responsible for those powertrains: Would this even be reasonable?

      Some believe that increasing engine displacement is the solution for boosting power while maintaining low tailpipe emissions. The idea is that larger engines, running at lower stress levels, can produce more output without depending heavily on diesel particulate filters to capture soot or diesel exhaust fluid to reduce nitrogen oxide levels. This notion has some validity, but it is not a comprehensive solution.

      Detroit's Big Three have not altered the size of their diesel engines in quite some time. Ford has relied on 6.7 liters of displacement since the 2011 model year; Ram transitioned from 5.9 liters to 6.7 liters in 2007.5; and GM has maintained a 6.6-liter engine since the introduction of the first Duramax in 2001. Nevertheless, they've dramatically increased power output. Ford's initial 6.7-liter Power Stroke generated 400 horsepower and 800 lb-ft of torque, while the latest high-output variant produces 500 hp and 1,200 lb-ft of torque. That's quite impressive!

      Even more remarkable is that all three manufacturers have achieved such power increases while adhering to stricter emissions regulations. Love it or hate it, this is feasible due to higher-performing systems utilizing selective catalyst reduction, also known as DEF, for those following along.

      Chevrolet

      GM's Duramax Assistant Chief Engineer, Rob Moran, put it this way:

      “If you compare our emissions levels now to a decade ago, we’re not just generating more power, but we're also certified at lower emissions levels. This can only be accomplished through a highly efficient aftertreatment system. In the past, before we had diesel particulate filters and SCR, what left the engine also came out of the tailpipe.”

      I inquired further with Moran about whether larger engine displacement would be advantageous, and he was largely not in favor of the idea. “There will always be trade-offs,” he stated. “Whenever emissions from the engine decrease, the aftertreatment system does not have to work as hard. But honestly, today’s aftertreatment systems are very efficient.” Moran mentioned that they can capture approximately 90% to 95% of NOx emissions.

      A crucial factor in achieving optimal emissions system performance is temperature. For systems to work best, they must operate within a "sweet spot" that maximizes NOx conversion without risking damage to the components. In this regard, larger displacement engines could actually pose more challenges for emissions, as Ford Research and Advanced Engineering Specialist Daniel Styles explained.

      “Increased displacement can actually run a bit cooler, making it more challenging to meet emissions standards,” Styles remarked.

      Ford has made adjustments in this area by relocating the SCR system closer to the engine to generate more heat. Increasing displacement could seem counterproductive based on this. Styles also pointed out that a larger engine’s negative impact on operating temperature might be worsened by drivers' behaviors.

      “We envision a Super Duty 6.7-liter pickup towing a 40,000-pound trailer over Eisenhower Pass in Colorado, but many use them for everyday errands,” Styles remarked. “Some regulatory cycles emphasize typical city driving.”

      Ford

      Styles continued, “Diesels prefer to be pushed hard, right? Thankfully, most diesel buyers purchase them for that purpose—towing or hauling heavy loads. Many are utilized in commercial applications, but the best thing for a diesel is to hit the freeway and really accelerate.”

      Of course, even if a larger diesel were run at maximum capacity 90% of the time, cooling it would still be a significant challenge. This was a main point raised by Kerk McKeon, Cummins lead engineer for the Ram pickup platform, during our discussion. In essence, this is the biggest hurdle to introducing larger engines into this segment.

      “People often ask why the front end of a Ram heavy-duty has to be so large. It’s all about thermal management,” McKeon shared. “There's only so much radiator space available. If you attach a 36,000-pound trailer to a truck in 100-degree weather, while going up a hill with the air conditioner at full power, there’s a limit to how much liquid can fit in the front of that vehicle.”

      McKeon referred to the SAE J2807 certification, which manufacturers use to assess a vehicle's maximum towing capacity. To claim that a truck can tow a specific weight, they need to