In America, the average mpg of a semi truck sits firmly in single-digit territory. Efficiency improvements mean that today's trucks now average around seven mpg, but some states still see trucks achieving less than six mpg. Using less fuel isn't just environmentally friendly; there are financial incentives too. The price of diesel has risen considerably over the past twenty years, and so even a small fuel saving can result in large cost savings over time.
A key part of making engines more frugal is
maximizing their brake thermal efficiency, or BTE. Essentially, BTE is a measure of how efficiently an engine can convert fuel into power — the higher the BTE percentage, the more power an engine can make with the same amount of fuel. In 2024, Chinese diesel engine maker Weichai unveiled the Weichai WP13H, an engine with a BTE of 53.09% at the World Congress on Internal Combustion Engines.
This was a record for a production engine, and a notable improvement on the industry average of 45-46% BTE. To confirm the record, the figure was certified by the German testing organisation TÜV SÜD and the China Automotive Technology and Research Center. Weichai had rapidly improved its engine technology over the years, first unveiling an engine with 50.23% BTE in 2020, then launching improved versions every year until the 2024 announcement. The record-breaking engine was a Weichai WP13 series engine, which is available in China for semi trucks as well as dump trucks.
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Weichai's Engine Can Save A Significant Amount Of Fuel
To create its record-breaking engine, Weichai made a number of small changes from its previous engine design. It revised the design of its turbocharger to optimize its flow, improved its fuel injection, and reduced the friction between engine components wherever possible. The resulting design meant that operators could reduce their fuel costs by 14% compared to the industry average, according to the brand's estimation.
Weichai has continued to improve its engine design since the 2024 announcement, unveiling a new generation of its WP truck engines in April 2025. This latest generation offers the same 53.09% BTE as the record-breaking 2024 engine but with reduced engine weight and a focus on longevity. According to the brand, its latest WP14 and WP15 engines are designed to last around 1.37 million miles. For context, the average semi truck engine lasts around 750,000 miles before needing a rebuild.
While the Chinese manufacturer holds the BTE record for a production engine, famed American diesel engine manufacturer Cummins has developed a prototype engine with an even higher BTE. Back in 2021, Cummins announced that its SuperTruck II project had achieved 55% BTE, although it has yet to make that level of efficiency available to customers.
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