Match it up: Near-zero two-stroke diesel and 48-volt mild hybrid

Quimby Mug Bayou Florida Headshot

Achates’ announcement of its 10.6-liter near-zero diesel engine is a serious wake-up call to the staying power of America’s workhorse.

The timing of this impressive two-stroke, opposed piston technology couldn’t have come at a better time as the industry attempts to brace fleets for 48-volt mild hybrid systems.

That was the case recently at the ATA’s Technology & Maintenance Council (TMC) Annual Meeting in Atlanta as a panel of experts told a roomful of industry insiders about the pros and cons of 48-volt mild hybrid technology.

Increased fuel economy and decreased emissions are big plusses for the new tech, while increased ground issues and various challenges inherent with a multi-voltage system remain legitimate concerns (read more about the pros and cons here).

Regarding the all-important fronts of fuel economy and emissions, fleets and government agencies—namely the National Renewable Energy Laboratory, or NREL—have long been aware of a diesel hybrid’s ability to save fuel and lower emissions.

However, the cost of hybrid technology has long hindered fleet interest. Even with incentive programs in place, adoption hasn’t been all that great. Plus, additional shop training and equipment can cast some shade on those lower fuel bills and emissions.

But that was all before Achates announced that their 10.6-liter oil burner reached the ultra-low NOx standard of .02 g/bhp-hr in the company’s San Diego lab. The engine will enter a fleet testing phase early next year with Tyson Foods and Walmart in California.

The greater fuel efficiency and lower emissions of the Achates two-stroke diesel paired up with a 48-volt mild hybrid system is a tempting match. Who wouldn’t want to be the first fleet in line to test it? (As far as we know, the Achates test mules at Walmart and Tyson Foods will not feature any hybrid capabilities.)

Unlike other hybrids, the 48-volt mild hybrid system described at TMC essentially takes the place of the engine in powering accessory-related components like air conditioning and lights. This saves fuel and lowers emissions. It’s also not as costly as a system that uses battery-powered motors to actually propel a vehicle. It’s also lighter since the battery demand is reduced for a mild hybrid.

(Eaton’s 48-volt mild hybrid system, which was on display at TMC, is currently undergoing field testing.)

Now, onto even more encouraging news. Achates told Hard Working Trucks that their two-stroke, opposed piston design reduces production costs over conventional four-stroke engines namely because there is less engine to produce—the engine does not have a valvetrain, which of course, makes it lighter as well.

In the race to go green, we can’t forget near-zero CNG and autogas engines (from Cummins Westport and Roush CleanTech, respectively). What they lack in thermal efficiency compared with diesel they can make up for in lower fuel costs, especially in more emissions-minded areas that offer attractive incentives for alt fuel use.

But diesel also has its alternatives such as biodiesel and renewable diesel, both of which have been subsidized. There’s also dimethyl ether on the horizon, a diesel substitute which burns so clean that no after-treatment system is needed.

All this to say–again–that diesel is a strong workhorse with plenty of life left to give especially when paired up with a hybrid system.