Why the figure of over 40% thermal efficiency is so impressive

When reading catalogs or news about hybrid vehicles, you may come across the term "over 40% thermal efficiency." It is introduced as a figure representing technological progress, but honestly, many people may not grasp what 40% really means.

Is it high or low? What is that 40% being compared to? And what exactly is thermal efficiency in the first place?

In this article, I would like to explain the concept of thermal efficiency as clearly as possible while delving into the meaning behind the figure of over 40%.

【What is thermal efficiency?】
Thermal efficiency is the ratio indicating how much of the energy contained in fuel can be converted into actual power (output).

Burning gasoline generates heat. That heat is used to move pistons, rotate the engine, and drive the car. However, not all of the energy contained in the fuel is converted into power. Much of it is discarded as heat in exhaust gases or cooling water.

A thermal efficiency of 40% means that 40% of the energy in gasoline is being converted into power. Conversely, the remaining 60% is discarded as heat.

You might think, "Isn't that low?" However, in the world of internal combustion engines, this is a phenomenal figure.

【How much was it for gasoline engines in the past?】
For comparison, let's first look at the thermal efficiency of ordinary gasoline engines.

The thermal efficiency of a typical naturally aspirated gasoline engine has long been standard at around 25-30%. In other words, 70-75% of the gasoline's energy was discarded without becoming power.

For engines from the 1970s and 80s, it was not uncommon for them to be in the low 20% range. Even today, with technological advancements, the standard level for ordinary automotive gasoline engines is around 30-35%.

Into this landscape, hybrid-specific engines have brought a figure of over 40%. As a thermal efficiency for an engine alone, this is at the top level among mass-produced passenger car engines. Looking only at the numbers, it might seem like "only a 10% difference," but for internal combustion engine developers, it is a realm that took many years to finally reach.

【Comparing with thermal power plants】
Some might think, "But aren't power plants more efficient?"

In fact, some large thermal power plants reach a thermal efficiency of 40-50%. In the latest combined-cycle power generation (a system combining gas turbines and steam turbines), there are cases where efficiency close to 60% is achieved.

However, power plants use facilities the size of buildings and can always operate under optimal conditions. They keep rotation speeds and loads constant, and waste heat is recovered and reused. The more conditions are optimized, the easier it is to increase efficiency.

Automotive engines operate in a completely different environment. Loads are constantly changing with starting, accelerating, decelerating, and idling. They must fit into the limited space of an engine room and be lightweight. Moreover, costs must also be kept down.

It is under such constraints that hybrid-specific engines achieve over 40%. While they cannot be simply compared to power plants, there is no doubt that this is a considerably high level for an automotive engine.

【Where does the discarded energy go?】
A thermal efficiency of 40% means that 60% of the energy is being discarded. So, where does that 60% go?

Broadly speaking, it is divided into exhaust heat and cooling loss.

Exhaust heat is the energy carried away when high-temperature gas after combustion exits through the exhaust pipe. This is why engine exhaust pipes get hot. Cooling loss is the energy that escapes when the engine body is cooled with cooling water to prevent it from becoming too hot. This is why radiators get hot.

There is also energy lost due to friction inside the engine (friction loss) and energy used to operate pumps and other components (auxiliary loss).

Engine developers have gradually improved thermal efficiency by reducing these losses one by one. They optimize the shape of the combustion chamber, precisely control the cooling system, and apply surface treatments to reduce friction. This steady accumulation has led to the figure of over 40%.

【Why hybrid-specific engines have an advantage】
Why can hybrid-specific engines achieve high thermal efficiency?

Normal gasoline engines must operate under various conditions, from idling to high rotation. They need to handle a wide range of driving conditions, from low-load driving in the city to high-load driving on highways. However, thermal efficiency can only reach its maximum value under the rotation speed and load conditions that the engine is best at.

The hybrid system solves this problem. By combining a motor and a battery, the engine can be concentrated in its "strong area." The motor handles the low-load range where the engine is weak, and the engine is operated under efficient conditions.

Furthermore, hybrid-specific engines often adopt a combustion method called the Atkinson cycle. By taking a larger expansion ratio than the normal Otto cycle, more energy can be extracted from the combustion gas. While it is disadvantageous in terms of output, it is an advantageous combustion method in terms of efficiency. It is precisely because it is a hybrid where the motor can compensate for the output that the Atkinson cycle can be used practically.

【The meaning of achieving over 40%】
What has changed with the realization of over 40% thermal efficiency?

The most direct effect is an improvement in fuel economy. By being able to extract more power from the same amount of gasoline, fuel efficiency performance has improved dramatically. This high thermal efficiency contributes significantly to the low fuel consumption achieved by modern hybrid vehicles.

It also leads to a reduction in CO2 emissions. Being able to use energy without waste means that less gasoline is needed to travel the same distance. If fuel economy improves, CO2 emissions also decrease.

From an industry perspective, the improvement in thermal efficiency also has the aspect of extending the life of the internal combustion engine. With the shift to EVs being called for, if hybrid vehicles can achieve this level of efficiency, it serves as one of the grounds for manufacturers to judge that the internal combustion engine can still compete.

【Summary】
The meaning of the figure of over 40% thermal efficiency becomes clear through simple comparison. From the era when automotive engines were 20-25%, the fact that modern hybrid-specific engines have achieved over 40% is the crystallization of many years of development efforts. While it may seem inferior at first glance when compared to thermal power plants, it is a considerably high-level figure for an automotive engine that operates under constantly changing conditions. It is a realm that could only be reached by reducing the discarded energy one by one and combining it with a hybrid system. Even in an era where EVs become widespread, this accumulation of technology will not be wasted. The challenge toward thermal efficiency that the internal combustion engine has cultivated will continue to be the work of engineers.

I continue to write about cars and motorcycles from an industry perspective.

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