Rudolph Diesel made tremendous progress in improving fuel efficiency by developing his diesel engine in the 1890s. Gasoline engines had been in development and commercial use for some years but were limited in fuel efficiency to somewhere between 15% to 25%.
Fuel efficiency is how well an engine converts the chemical or electrical energy stored in the “fuel” into actual movement down the road. Gasoline engines are inherently less efficient than diesel ones. Diesels can readily get above 40% efficiency, and with some extensive engineering demonstrated on the DOE SuperTruck 1 and 2 programs, can get to 55%.
There have been many refinements of Diesel’s groundbreaking design over the decades. Inventors have come up with a number of different approaches. One example is the opposed piston diesel engine developed by Fairbanks-Morse (FM) and used extensively in U.S. submarines to power generators for running both batteries and electric motors that moved these boats. A typical diesel-powered submarine had four big FM engines, each linked directly to generators that supplied electrical power to both the batteries and the electric motors powering the ship’s screws. Think of it as a hybrid-electric powertrain.
In 1938, the U.S. Navy arrived at the conclusion that this arrangement was best for their submarines. They powered every submarine built between 1938 and the last diesel-electric boat in 1959. They continued to be used on nuclear-powered submarines as back-up power systems well into the 1990s. These were ideally suited for submarines, providing high efficiency and high horsepower in a compact layout needed to fit into the subs. The chief innovation on these was an opposed-piston arrangement that removed much of the complexity of valves, while also reducing weight, vibration and noise.
Opposed-piston engines use two pistons per cylinder, each working in tandem off of two crankshafts. The engine is a two-stroke diesel. The pistons move toward each other compressing the air in the cylinder until the high pressure is nearly reached at which point fuel is injected and combustion occurs. The two pistons then move away from each other opening up exhaust ports and then drawing in new air.
Servicing the engine inside the submarine also was easier than other diesel configurations — a critical factor in surviving thousands of miles from safe ports during war. The reduction in complexity of the engine also meant it was more reliable with fewer failure points.
Other configurations of the opposed-piston diesel were developed by other manufacturers, but Fairbanks-Morse’s version set the bar.
The design was tried as power for railroads but with limited success. A key difference between a submarine and a train is that a sub is surrounded by water. Cooling an engine inside a submarine is much less complicated than cooling a train that has to rely on air. Picture a sub idling in an ocean versus a train idling in a train yard. Heat is easily dispersed in the ocean, but it is very challenging to keep an engine cooling in a railyard. A Russian manufacturer used opposed-piston diesels in rail services and tanks. A British company tried a configuration in a three-cylinder truck in the 1950s. Another British group tried the Napier Deltic configuration that used three banks of opposed-pistons in the shape of a triangle in military tanks, ships and trains.
In 2014, Achates Power issued a technical paper claiming significant improvements over a standard diesel. The company received funding from California Air Resources Board and demonstrated an opposed-piston, 2-stroke diesel installed in a Peterbilt 379 tractor run by Walmart. In 2025, after testing, they published an SAE paper citing results that showed the engine could exceed 2027 NOx standards and also provide efficiency gains over a standard benchmarked diesel. Testing indicated also that the engine could exceed an 800,000 mile life requirement.
Diesels are the standard powertrain for heavy-duty trucks, and in concert with electric motors and generators for trains. There always seems to be different ways to do things. The opposed-piston system in light of evolving emission requirements may be a new addition to what NACFE calls the Messy Middle — that period where fleets face an abundance of technology choices to power their trucks.
History has seen a lot of diesel engine manufacturers. Technology never rests. Fleets always want lower costs and better reliability. Companies like Fairbanks-Morse, Massey-Ferguson with their diesel manufacturer Perkins Engines, Allis-Chalmers with Buda Engine Company, Continental and other diesel-based companies have done significant research over the decades for innovators to dig through.
I never bet against the engineers to come up with improvements, even in technology as old as Rudolph Diesel created over a century ago. A great summary of his development of the diesel can be found in the book The Mysterious Case of Rudolph Diesel by Douglas Brunt.
Diesel engines have been refined and improved for more than 130 years. In 2009 when I was working on the proposal for the first DOE SuperTruck, the baseline MPG was optimistically about 6 MPG for on-highway trucks. The goal of the SuperTruck program was to improve freight efficiency by more than 50%. By 2016, four OEMs had done it. In 2017, NACFE Run On Less showed production Class 8 trucks exceeding 10 MPG. In 2026, stories of fleet drivers regularly exceeding 10 MPG are common.
New ideas in diesel are possible, even after more than a century of innovation.
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