Internal Combustion Engines Continue to Get Greener

An aerial shot of trucks driving on a curvy road with lots of green trees.

Alternative fuel vehicles hold significant promise for reducing emissions, but new advancements in diesel-powered internal combustion engines are also helping fleets improve fuel economy and reach their sustainability goals.


“Decarbonizing trucking has become a very hot topic, but what we find in our work is far too often decarbonizing is only thought of in zero-emission trucks, like you have to have an electric truck or a hydrogen fuel cell truck to decarbonize, which is wrong,” said Mike Roeth, executive director of the North American Council for Freight Efficiency. “Anytime fleets burn less fuel to ship the same amount of goods, they are decarbonizing.”

Allen Schaeffer, executive director of the Diesel Technology Forum, said diesel will remain the prime technology of America’s trucking industry for decades, which makes advancements in engine technology critical. “We are seeing the next generation of advanced diesels that are more efficient and lower in emissions than any previous generation,” he said.

Greenhouse gas regulations have required manufacturers to improve base engine and truck MPG, particularly in 2017 and 2021 when compliance steps took effect.

Schaeffer said the industry saw significant gains with the introduction in 2010 of Selective Catalytic Reduction (SCR) systems, which enabled manufacturers to tune engines more toward fuel efficiency allowing the new SCR exhaust aftertreatment system to take on most of the burden of reducing emissions.

“Before this, the approach was all ‘in cylinder,’ meaning that engineers were tweaking the combustion process and trying to manage temperatures and formation of emissions mostly inside the cylinder with some exhaust gas recirculation (EGR) so the exhaust would meet the requirements. That strategy really limited how much more fuel efficiency you could obtain because everything that would increase fuel efficiency would also increase emissions,” Schaeffer said.

NACFE’s 2022 Annual Fleet Fuel Study found that the national average of the 1.7 million tractors in over-the-road use averaged 6.24 MPG in 2020 from less than 5.9 MPG in 2010, which it said demonstrates “the fact that the MPG increases over the last ten years are starting to be reflected in the overall population’s efficiency.” The average fuel economy of the fleets participating in NACFE’s study was 7.23 MPG.

In the report, Jennifer Rumsey, CEO of Cummins Inc., said that improving fuel economy doesn't always grab the headlines but adds up to significant results. Rumsey estimates that 100,000 internal combustion engines that are each 10% more efficient are equivalent to the improvement gained by putting 10,000 zero-emission vehicles on the road.

GHG Phase 2 standards will continue to increase, with model year (MY) 2027 requiring at least 25% lower carbon dioxide emissions and fuel consumption than a 2017 model year Phase 1 tractor. EPA said the reduction could be achieved through improvements in the engine, transmission, driveline, aerodynamic design, tire rolling resistance, idle performance or other aspects of the tractor.

Phase 3 standards will take effect beginning with MY 2030. EPA is also proposing new GHG standards for heavy-duty highway vehicles starting in MY 2028 through MY 2032. The proposed standards are the most stringent federal standards to date for GHG emissions from heavy-duty vehicles and engines.

Penske Truck Leasing offers customers late-model equipment that meets the latest emissions and fuel efficiency requirements. Customers can also spec onboard technology to drive additional fuel economy improvements, such as adaptive cruise control, and equip vehicles with auxiliary power for heating, air conditioning and electrical power to the truck cab to reduce idling and save fuel.