The physicist and science communicator Johannes Kückens reignites the energy debate in Germany by questioning the narrative of the so-called “efficient combustion engines”.
His statements arise in a political context marked by proposals to delay the end of thermal engines beyond 2035.
The physical limits of efficiency
For Kückens, the term “efficient engine” is misleading: it does not correspond to any real physical magnitude. Thermal engines are, by definition, heat machines subject to unchangeable limits.
The second law of thermodynamics states that the conversion of heat into motion can never be total. There will always be a significant part lost as residual heat.
- Even in the most advanced engines, the theoretical ceiling is around 65%.
- In real conditions, current diesel and gasoline engines rarely exceed 25% useful efficiency.
- The rest dissipates as heat that does not propel the vehicle.
For decades, the industry perfected valves, sensors, and injection, but that path has reached its limit. “Today we are around 45% efficiency and we hit physical limits. It will never be possible to reach 80% or 90%,” says Kückens.
The comparison is clear: electric motors already exceed 90% efficiency under ideal conditions.
The mirage of e-fuels
Kückens views with skepticism the political hope that synthetic fuels (e-fuels) can save the combustion engine. He describes their production as an extremely energy-intensive three-stage process:
- Electrolysis to obtain hydrogen.
- CO₂ capture from the air.
- Hydrocarbon synthesis.
The result is discouraging:
- E-fuels contain only half of the renewable energy invested in their production.
- When burned in an inefficient engine, barely 10% of the initial energy reaches the road.
- With the same amount of electricity, an electric car travels six times more than a combustion engine powered by e-fuels.
Renewable electricity: a valuable resource
Kückens’ argument is not limited to energy calculations but to its everyday impact. If renewable electricity is limited, does it make sense to spend it on fuels that reduce the available energy to a fraction?
Countries like Germany and Spain are expanding solar and wind energy, but supply remains a strategic resource. The difference between 10% and 70% efficiency completely changes the energy landscape.
Economic and climate risks
While China advances with cheaper electric models and consolidated supply chains, Europe manages a slower and more contradictory transition. For Kückens, prolonging the life of the thermal engine is a climatic and economic mistake:
- More emissions and pressure on already stressed ecosystems.
- As the price of CO₂ rises, maintaining a combustion vehicle will be more expensive than operating an electric one.
- Companies that do not adapt their technology could fall behind in a market that no longer waits.
Advantages of the electric motor
The electric motor makes better use of each kilowatt:
- On real roads, efficiency is around 70%, even considering losses in charging and transmission.
- Its mechanical simplicity reduces maintenance.
- The critical materials of the batteries (lithium, nickel, cobalt) are recycled and returned to the production chain.
Europe is already promoting a battery recycling network, key to reducing external dependence.
Social reluctance and cultural change
Resistance persists among drivers: doubts about autonomy, price, and charging points. Many perceptions come from an early stage of electric vehicles when they were expensive and had little infrastructure.
Today the landscape is changing:
- More affordable models.
- Ranges exceeding 400 km.
- Expanding charging network on highways and urban areas.
For Johannes Kückens, delaying the elimination of the thermal engine would be a profound mistake. From physics, economics, and environmental logic, the electric motor is the superior technology. Each additional year of combustion means more emissions, more heat trapped in the atmosphere, and more pressure on vulnerable ecosystems.
The energy transition is not just a technical challenge but also a cultural and political one. Europe has the opportunity to lead, but it needs to accelerate the pace to avoid falling behind in a global market that is already betting on electrification.
