Being more efficient than powerful, the Atkinson modified engine cycle has become popular among hybrid models. Manufacturers are using engines that take advantage of what is called the Atkinson cycle for engine combustion, especially with hybrids.
For example, the new-for-2017 Toyota Prius Prime.
However, a lot of people are wondering just what the Atkinson cycle is, and what it means if you buy a car with an Atkinson cycle engine. And we have the answer: basically, it is a valve timing adjustment to the standard Otto cycle, which all modern gasoline engines use, to make the engine more efficient.
The Otto cycle has four strokes:
1. 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.
2. 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.
3. The power stroke – all valves remain closed, the piston is at top dead center, and the spark plug ignites that hot compressed fuel-air mixture, forcing the piston back down.
4. The exhaust stroke – this opens the exhaust valves as the piston, once again, travels back to top dead center and pushes out the burned hydrocarbons.
On the other hand, the Atkinson cycle modifies this process. It leaves 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. This adjustment reduces the force on the piston on the power stroke.
In the Otto cycle, a part of the force during the power stroke goes to pushing the piston down, but more force is produced than what is necessary – this extra force is, basically, wasted energy.
For this reason, the Atkinson cycle adjustment reduces that wasted energy and, by doing so, makes the engine more efficient.
Of course, there is a downside to this method: less power. The Atkinson cycle engine produces less power than the Otto cycle unit, which is why the design is more popular with hybrid models which aim for fuel economy over acceleration and contain electric motors that can compensate for the difference.
Watch the video for further explanation about the Atkinson cycle!