BYD seems to be pretty proud of its Xiaoyun engine. It was, until very recently, the most thermal efficient mill in mass production. According to BYD, it manages to convert 43% of the chemical energy contained in gasoline into movement. Before that, Toyota had the best work in that regard with the Dynamic Force 2-liter engine (41%). BYD’s kingdom was brief: Geely announced in November the DHE1.5 engine, capable of delivering 43.32% in thermal efficiency. As remarkable as that is, that's still very far from EVs.
Considering BYD could have omitted the numbers after the comma, we contacted the company to ask if that was the case. BYD could also have rounded the number up if the Xiaoyun mill reached a thermal efficiency above 42.5%. As the Tesla supplier never got back in touch, the official numbers placed Geely in the first position.
If you are not sure why this is important, consider that thermal efficiency tells you how much money you simply burn on fuel. If you spend $100 to fill up your fuel tank, only $41 actually helps you move your Toyota with a Dynamic Force engine. A BYD equipped with the Xiaoyun will waste only $57 in fumes and heat – not $59. If this 2% difference is already small, 0.32% is even less relevant. Nonetheless, it is a superior number.
All three examples are remarkable compared to regular engines, which have an average thermal efficiency of 20%. Mazda promised to make that number reach 56% with the Skyactiv-3 mill. Whe you compare that to the efficiency of electric vehicles, you get a good picture of why governments are pushing for electric cars. The latest EV disclosed by Geely is the Polestar 6. Although the company did not mention how efficient its electric motors are, they can convert around 90% of electricity into movement. ABB said it developed one that reached a 99.05% efficiency. And people keep trying to improve them. If Mazda ever reaches 56%, it will be quite an achievement. For the time being, 43.32% already sounds pretty good.
Photo: Geely
The new Geely mill is part of a hybrid powertrain called Leishen Hi-X. The 1.5-liter, three-cylinder engine uses a supercharger, high-pressure direct injection, and the so-called Miller cycle. It is not the proper Miller cycle but rather an adaptation of the Otto cycle in which admission valves stay open a little longer so that the compression stroke is shorter than the expansion stroke. With less resistance while compressing gases, the mill gets more efficient.
Using a supercharger is an unusual choice. Most modern engines prefer turbochargers because they are not a burden for the machine. Exhaust gases move the turbine, while the supercharger depends on the mill to compress the air in the combustion chamber. This seems to be more related to the behavior Geely wanted the DHE1.5 engine to have than a choice related to efficiency.
Photo: Geely
Besides the new mill, the Leishen Hi-X system also uses a three-speed DHP Pro transmission. It comprises two electric motors and has a “maximum torque output of 4,920 Nm” (3,628.8 pound-feet). Considering it weighs 120 kilograms (264.6 pounds), it achieves an impressive torque-to-weight ratio of 41 Nm/kg (13.7 lb-ft/lb). The last component of this hybrid powertrain is the Leishen E-Drive, which can work at 800V, has a silicon carbide power module, and oil-cooling efficiency of 92%. This motor can deliver up to 475 kW (634 hp).
The first vehicle to have the most efficient combustion engine in the world was the Geely Xingyue L Hi-X, an SUV presented in December 2021, but its deliveries only started in April. The company keeps presenting new ones with the new powertrain, the latest one being the Emgrand L Hi-X. It is surprising that the Chinese company has remained so silent about the DHE1.5 mill. We would probably have heard more about it if it powered cars sold outside China.
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