2010 Ford Fusion Hybrid engine (top); 2010 Toyota Prius engine. Click image to enlarge
The concept for the Atkinson Cycle engine started way back in 1882, when James Atkinson attempted to bypass patents on the Otto Cycle engine – the four-stroke engine that still powers most vehicles today.
The original Atkinson Cycle engine used a special crankshaft design that allowed for a longer intake stroke than compression and power stroke for improved efficiency. While the concept worked, it also created some challenges, such as building the complex mechanical mechanism and how to overcome the reduced low-speed engine torque. The Otto Cycle engine design has dominated since the beginning of vehicle construction but now the Atkinson Cycle, or a modified concept of it is making a comeback in today’s hybrid vehicles.
To understand the Atkinson Cycle, it is easier if we first look at the Otto Cycle and then compare differences. The Otto Cycle uses four strokes to complete one power pulse in the cylinder. It starts with the Intake stroke, where air and fuel are drawn into the cylinder as the piston moves down. The compression stroke starts as the piston moves up, squeezing the mixture into a small area where it is ignited by the spark plug. The power stroke begins as the piston is forced down by the rapid expansion of the gases as combustion takes place. The exhaust stroke completes the cycle by pushing the burned gases out of the engine and then it starts all over again. This “suck, squeeze, bang, blow” of the Otto Cycle has four nearly equally timed strokes that happen about a thousand times per cylinder for every minute you drive down the highway.
The current Atkinson Cycle engines are close in operation to the Miller Cycle engine, another design that used modified air intake stroke, but that design uses a supercharger to force the air into the cylinder while the Atkinson Cycle is a normally-aspirated engine. Like the Otto Cycle, the modern version of the Atkinson Cycle has four strokes and uses the same crank design as the Otto Cycle. The difference is in how the valves are operated.
During the intake stroke, the intake valve opens before the piston starts its downward travel, but instead of closing the valve as the compression stroke begins, the intake valve is held open even as the compression starts. The intake valve finally closes very late in the compression stroke. This delayed closing of the valve allows air in the cylinder to be pushed back into the engine intake manifold, so there is very little vacuum created in the manifold. This reduces engine pumping losses, or the restriction caused by a vacuum as the piston goes through the intake stroke. The rest of the cycle continues with compression, power and exhaust the same as the Otto Cycle design.
While the Atkinson Cycle design does enable better fuel economy because of lower pumping losses, low-speed engine torque also suffers. That is where the hybrid powertrain comes into play. Electric motors excel at low speed power but are less efficient as motor speeds go up. The Atkinson Cycle gas engine is the opposite, with less low speed power but good power at higher engine speeds. Combining the two seems obvious, so both the electric motor and the gas engine can contribute to vehicle propulsion in their most efficient ranges.
Both the 2010 Toyota Prius and Ford Fusion Hybrid use the Atkinson Cycle design engine. The inclusion of an electric motor also allows the engineers to tune the engine so it is more efficient in a narrower rpm range and let the electric motors make up the difference. In the search for improved fuel economy, there is no one big solution; the answer is found is many small improvements and the Atkinson Cycle design is one of those improvements that modern technology has enabled.