Most internal combustion engines are incredibly inefficient at turning fuel burned into usable energy.

The efficiency by which they do so is measured in terms of "thermal efficiency", and most gasoline combustion engines average around 20 percent thermal efficiency. Diesels are typically higher--approaching 40 percent in some cases.

Toyota has now developed a new gasoline engine which it claims has a maximum thermal efficiency of 38 percent--greater than any other mass-produced combustion engine.

The new units, 1.0 and 1.3-liters in capacity, should enable 10-15 percent greater economy than their existing equivalents.

Toyota has applied several familiar technologies to its engines to achieve these levels of efficiency.

One of these is the same combustion cycle used in the firm's hybrid models--the Atkinson cycle.

Used on the 1.3-liter unit, Atkinson-cycle engines typically feature variable valve timing, allowing inlet valves to remain open as the compression stroke begins. The lower air density leads to a more efficient fuel burn and higher thermal efficiency.

Typically, the engines lack power compared to conventional Otto-cycle engines--offset in hybrids by additional power from the electric motor.

MORE: 2015 Toyota Prius: Next Hybrid Aims For 55 MPG, More Room, Better Handling

In the 1.3-liter engine, a 13.5 compression ratio makes up for some of the lost compression through the engine's cycle--in theory, the engine should perform similarly to a regular 1.3-liter unit.

Redesigned intake ports, variable valve timing and cooled exhaust gas recirculation are also employed to improve the engine's efficiency.

On the 1.0-liter unit, co-developed with Toyota's Japanese partner Daihatsu, similar engine technologies (without the Atkinson cycle, this time) contribute to 37 percent thermal efficiency.

Through the use of stop-start technology though, the new engine is 30 percent more efficient than equivalent 1.0-liter units on the city-biased Japanese JC08 test cycle.

Toyota hasn't confirmed which vehicles the new engines will be used in, nor whether either powerplant will come to the U.S. It's likely several Japanese-market vehicles and selected models like the Yaris and Aygo sold overseas will eventually benefit from the units.

What it does show is that there's still plenty of room for improvement in conventional gasoline engines.

Regular internal combustion engines will remain dominant in road transportation for the next few decades at least--so any effort to improve them in the meantime should be applauded.


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