Look back at the engines of 25 years ago, and one notable difference is apparent: today's engines are significantly smaller. Where it was once common to find a 2.0- or 4.0-liter engine under the hood, many modern cars now rely on engines as small as 1.5 liters. But why has this change occurred? What has driven engine downsizing, and what can we expect from the next generation of car engines?
Naturally aspirated vs turbocharged engines
Up until the late 1990s and early 2000s, most cars were powered by naturally aspirated engines. For air intake, these engines only rely on the natural tendency of gas to fill all available space. As space is made available during the down stroke of a cylinder’s piston, outside air travels in through a cylinder’s intake valve. Then, as the piston begins its up stroke, the intake valve shuts and the air and fuel within are compressed and ignited.
Natural aspiration relies solely on outside atmospheric pressure, which limits the amount of air that can enter the engine. For a naturally aspirated engine, the only way to increase the amount of air available to burn is by increasing the size and number of its cylinders – that is, its total displacement. That meant that power was solely a function of engine size.
Turbocharged engines address this limitation by using a compressor powered by the engine’s exhaust gas, which forces additional air into the cylinder. To prevent this increase in air pressure from increasing the air’s temperature and reducing its density, turbochargers also use systems called intercoolers to remove heat from the air-fuel mix.
This allows an engine to burn more fuel and air during each piston stroke, meaning an increase in power and torque. This enables a lower-displacement turbocharged engine to outperform a higher-displacement naturally aspirated engine.
For an example of this principle in action, we can point to Horse Powertrain’s X-Range C15. When we announced this 1.5-liter powertrain at IAA Munich 2025, we revealed that it would be available in either a naturally aspirated or turbocharged configuration. By trading out natural aspiration for turbocharging, the X-Range C15’s power output rises from up to 70kW to 120kW.
Read more about how motor innovations can improve hybrid efficiency.
The efficiency incentive
At first, it might seem the only advantage of turbocharging is that the smaller engine can cut down the mass of the vehicle – a minor improvement on the order of tens of kilograms. After all, while it increases performance, turbocharging does this through increasing the amount of air and fuel burned in each piston stroke, which means greater emissions.
However, a turbocharged engine is significantly more efficient than a naturally aspirated engine of the same power. Why? The key to this lies in a field of science and engineering known as tribology: the study of friction.
Around 30% of the energy supplied in a combustion engine is lost to friction – and half of these friction losses occur at the points of contact between the cylinder walls and piston.
Friction between two surfaces is a direct function of their area. So, a lower-displacement engine will lose less energy to friction than a higher-displacement engine. That means that while a small, turbocharged engine may burn the same amount of air and fuel as a larger naturally aspirated engine, the former will be able to turn more of this energy into useful work.
So, the real strength of a turbocharged engine is that it allows a vehicle to run with a smaller, more efficient engine for everyday driving while still delivering the power of a larger engine when required. As a result, a downsized turbocharged engine can be 20–40% more fuel efficient than an equally powerful naturally aspirated engine; and that’s largely because the turbocharger allows the engine to be smaller, with lower friction losses.
Turbocharged engines are small and efficient for everyday driving, while still delivering the power of a larger engine when required
Read more about why does thermal efficiency matter?
The next generation of small engines
That’s not to say there is no role for naturally aspirated engines: for applications where instant peak power and torque from rest are vital, such as sudden hill climbs or towing, these engines have a real role to play. Since a turbocharger needs exhaust gas to power up its compressor, it will always face a delay between powering up and delivering peak power: turbo lag.
While there are solutions to turbo lag via hybridization or anti-lag systems, many applications that just want rapid power and torque delivery will have an identical lifetime fuel economy and emissions profile if they go with a naturally aspirated engine: with the additional benefit of reduced mechanical complexity, maintenance, or embedded emissions.
This goes to show that turbocharging is only part of the story. Advances in ignition technologies, materials, and thermal management are all playing roles in allowing smaller engines to do more work without a corresponding hike in fuel consumption or emissions.
As the global transport system continues to decarbonize and reduce its reliance on fossil fuels, demand for smaller, efficient engines will only increase. When paired with innovations in hybrid technology, we should expect ever-more efficient small engines to remain a key part of the next generation of automotive powertrains.
Find out more about our next-generation powertrain solutions here.
