More and more, manufacturers are using engines that take advantage of what's called the Atkinson cycle for engine combustion, especially with hybrids. A great example is the new-for-2017 Toyota Prius Prime. But just what is the Atkinson cycle, and what does it mean if you buy a car with an Atkinson cycle engine? Simply put, it's a valve timing adjustment to the standard Otto cycle, which all modern gasoline engines use, to make the engine a bit more efficient.
The Otto cycle has four strokes (hence the term "four-stroke engine"):
- The intake stroke where the piston travels down from top dead center with the intake valves open and exhaust valves closed, letting in air and adding a bit of fuel.
- The compression stroke -- all valves are closed and the mixture of fuel and air compresses as the piston travels back to top dead center. This is where a compression ratio of, say, 10:1 happens.
- The power stroke -- all valves remained closed, the piston is at top dead center, and the spark plug ignites that hot compressed fuel/air mixture, forcing the piston back down.
- Finally, the exhaust stroke, which opens the exhaust valves as the piston once again travels back to top dead center and pushes out the burned hydrocarbons.
The Atkinson cycle modifies that process by leaving the intake valves open for the first moments of the compression stroke, as the piston travels toward top dead center, which effectively lowers the compression ratio of, say, 10:1 to something more like 8:1, or so. That adjustment reduces the force on the piston on the power stroke.
In the Otto cycle, some of the force during the power stroke goes to pushing the piston down, but more force is produced than needed. That extra force is essentially wasted energy, the Atkinson cycle adjustment reduces that wasted energy and by doing so makes the engine more efficient.
There’s a clear downside to this method: less power. An Atkinson cycle engine makes less power than the otherwise equivalent Otto cycle unit, which is why the design is more popular with hybrid models, which aim for fuel economy over acceleration and have electric motors that can compensate for the difference.
For further explanation about the Atkinson cycle, check out this video from Engineering Explained.
