Regarding the upper limit of effective thermal efficiency of internal combustion engines, "Diesel engines are about 55%, gasoline engines theoretical air-fuel ratio is about 45%, lean burn about 50%. This is determined by thermodynamic principles and is independent of human will."

On June 11, at the Automotive Evaluation Research Institute's "2026 World Power Technology Continuous Innovation and Market Trend Seminar", Professor Shuai Shijin of Tsinghua University's Aero-engine Research Institute drew a physical ceiling for the internal combustion engine thermal efficiency debate with this statement. From three dimensions: the global fuel vehicle market, the thermal efficiency limit of hybrid-specific engines, and the overseas expansion of China's oil-electric hybrid vehicles (HEV), he provided judgments driven by data and technical analysis.

Hybrid vehicles are growing globally, China has taken a unique technical path

According to statistics from the "China New Fuel Vehicle Technology White Paper" released in April 2026, global car sales in 2025 reached 95.8 million vehicles, a year-on-year increase of 4.1%, with fuel vehicles (conventional fuel vehicles + HEV) accounting for still as high as 76.6%. China, North America, and Europe are the three major global automotive markets, accounting for 73.2% of global vehicles and 67.0% of global fuel vehicles. China has the largest market size and growth rate significantly higher than North America and Europe, serving as an important support for the global automotive market.

Shuai Shijin outlined the growth trend of global hybrid vehicles with a set of data: compared to 2024, conventional fuel vehicles (ICEV) decreased by 4% in 2025, while HEV increased by 14%. Among the top 10 countries in fuel vehicle sales in 2025, China, the US, India, Germany, Canada, UK, France, and Mexico were all HEV incremental markets, with Canada growing the fastest exceeding 90%, Brazil maintaining sales volume unchanged, and only Japan decreasing by 2%, but the HEV proportion was the highest (over 45%).

China HEVs have already carved out a differentiated technical path from Toyota hybrid vehicles. First is the difference in hybrid configuration. Toyota adopts planetary gear strong mechanical coupling power split configuration, where the engine and motor switch seamlessly with smooth operation, while Chinese car manufacturers adopt series-parallel switch hybrid configuration (dominated by P1+P3), where the engine and motor can be mechanically separated and driven individually, with electrical drive as main and fuel drive as auxiliary, having wider applicable scenarios and better energy saving effects. Second is that Toyota and Honda HEV battery capacity is low, only 1 to 2 kWh, acting as an energy buffer. Chinese car manufacturers leverage years of technical accumulation in Plug-in Hybrid Vehicles (PHEV) and cost advantages of Lithium-ion power batteries, increasing HEV battery capacity to 5 kWh level, adopting electrical drive more in high-energy consumption conditions like cold start and acceleration, while solving the high energy consumption and high emission problems of traditional HEV engine frequent start-stop and NVH (Noise, Vibration, and Harshness) issues caused by engine intervention, and the increased battery cost is controllable. "China HEV technology originates from PHEV, also learned from Toyota and Honda, but has already surpassed them in development concept and technical level."

From a global policy environment perspective, oil-electric hybrid vehicles (HEV) are welcoming a more friendly regulatory space. European regulatory focus has shifted from simply compressing internal combustion engine space to providing a buffer period for hybrids — adjusting from annual compliance to three-year average compliance, incorporating HEV into the low-carbon system. The USA recalibrated CAFE (Corporate Average Fuel Economy) standards, reserving more space for HEV. Japan is also increasing the proportion of renewable fuels, supplying gasoline containing 10% bioethanol from 2030, increasing to 20% from 2040. Oil-electric hybrids have become a grasp for development in multiple countries.

Shuai Shijin judges that large capacity battery (about 5 kWh) HEVs will go from domestic to global, bringing significant impact on global HEV proportion structure. "China HEVs have already carved out a unique technical path, facing the global automotive market with a very good future."

The path for gasoline engine effective thermal efficiency breaking through 50% has been cleared

As a senior expert in the engine field, Shuai Shijin provided the thermodynamic upper limit for internal combustion engine effective thermal efficiency.

Internal combustion engine effective efficiency equals combustion efficiency multiplied by cycle thermal efficiency multiplied by mechanical efficiency. Modern gasoline engine combustion efficiency approaches 100%, mechanical efficiency is about 90%, the improvement space for these two types of efficiency is limited. What truly determines the upper limit of effective thermal efficiency is cycle thermal efficiency. The ideal cycle of an internal combustion engine is not the Carnot cycle, but the constant volume cycle. The two core thermodynamic parameters affecting constant volume cycle thermal efficiency are compression ratio and specific heat capacity ratio. Compression ratio to a certain extent (e.g., 17) thermal efficiency improvement slows down, marginal returns diminish, internal combustion engine constant volume cycle theoretical efficiency is about 60%~70%, but internal combustion engines are hard to achieve constant volume cycle, working fluid is not ideal air, actual working fluid specific heat capacity ratio is far less than 1.40.

Current hybrid-specific gasoline engine effective thermal efficiency has far exceeded 40%, theoretical air-fuel ratio can approach 45%, moving towards 50% for lean burn is feasible. Compared to traditional gasoline engine 15 percentage points effective thermal efficiency improvement, the core driving force comes from hybrid-specific — not pursuing extreme power (with motor assist), instead pursuing extreme economy, more efficient technologies such as lean burn, Miller cycle, high swirl ratio, high compression ratio, high EGR rate, high energy ignition, efficient turbocharging and low pumping loss, low friction technologies are applied. The technical path for exceeding 50% effective thermal efficiency is clear, walked through in the lab, but whether these technologies are fully adopted on actual engines requires comprehensive consideration, balancing cost, power, economy, and emissions."

Shuai Shijin also points out, hybrid-specific engines must have high thermal efficiency, and also have a wide efficient zone, this can simplify the transmission system, no need for multi-gear transmission to match structurally.

Chinese car manufacturers going overseas need to join hands, carbon-neutral fuel is the long-term support for engines

Shuai Shijin provided a judgment on the overseas topic different from the technical perspective — policy environment and industrial ecosystem are equally key.

In recent years, trade protectionism against the automotive industry by major global economies has significantly intensified, showing systemic, continuous upgrade trend, trade barriers have expanded from single tariff measures to a composite combination of tariffs and non-tariff measures. This policy tightening driven by geopolitical game is fundamentally reshaping the underlying logic of global automotive trade. At the national level, need to handle global especially major power diplomatic relations well, actively create a good, more fair and reasonable international trade environment, let the concept of Community with a Shared Future advocated by China receive global recognition.

He specially calls on the automotive industry to face squarely the contribution of joint venture brands to China's automotive industry. "For joint venture car brands, give them more friendly policies and survival space. Chinese car manufacturers going overseas to gain foreign customer recognition, handling the cooperation upgrade of domestic joint venture brands well, is also a required course." He takes Thailand as an example to explain the correct mode of automotive industry going overseas together: not one company fighting alone, but Chinese car manufacturer legion layout, full-chain localization, this is the mode welcomed by overseas markets.

From data perspective, China's fuel vehicles including HEV are new growth points for exports. Looking at 2025 China HEV export regional distribution and year-on-year growth rate, except Japan and Korea, South America, Africa, Southeast Asia and Central America etc. regions growth trend is very obvious, Europe, Middle East, North America and CIS are also growing. "From single product export to supply chain collaborative overseas expansion, is a required answer Chinese car manufacturers must complete."

He also points out, carbon-neutral fuel is the key support for the long-term existence of hybrid-specific engines. If engines superimpose carbon-neutral fuel or zero-carbon fuel — burn hydrogen, methanol, ethanol and other flexible fuels — has important significance for HEV energy power sustainable development strategy, it is also an important layout for China's automotive energy diversification strategy.

From internal combustion engine thermal efficiency thermodynamic upper limit to global oil-electric hybrid vehicles (HEV) growth data, from large capacity battery HEV technical breakthrough to industry joining hands going overseas strategic thinking, Shuai Shijin's core viewpoint runs through a logic: engines will not disappear, but will continue to develop in new forms — high-efficiency hybrid-specific, carbon-neutral fuels. China HEVs have already walked out a unique path from learning to surpassing, the key of the next step is Chinese automotive industry chain collaborative overseas expansion.