How the Atkinson cycle engine works

Atkinson cycle engines are internal combustion engines with four-stroke cycles practically identical to those of the Otto cycle, although their disadvantage is that they produce less power. On the positive side, they make better use of fuel, which allows them to achieve better consumption figures.

Originally, and unlike how it works today, the Atkinson cycle engine completed all four strokes (intake, compression, expansion and exhaust) in one crankshaft revolution. Otto cycle engines need two crankshaft revolutions to complete the same strokes.

Today this concept has been modified, and the main difference between an Otto cycle engine and an Atkinson cycle engine is that during part of the compression stroke (roughly half of it), the intake valves remain open. This is achieved by modifying the intake cams of the camshaft, with the advantage that the losses in the engine caused by compressing the mixture are drastically reduced.

How an Atkinson cycle engine works

An Atkinson cycle engine has the same four strokes as a petrol engine, with intake, compression, combustion and exhaust. However, in this type of engine the intake valve closes late during the compression process, which reduces the effective compression, since actual compression does not effectively begin until the intake valve has closed. This can make the compression ratio as low as half of what it would be if the valve closed earlier, as occurs in a traditional four-stroke Otto cycle.

With this modification, during the compression stroke (when the air and fuel mixture previously drawn in is compressed) and because the intake valves remain open, part of the mixture is returned to the intake manifold.

The compression ratio of an engine is the volume of air in the compression stroke plus the volume of air in the combustion chamber, divided by the volume of air in the combustion chamber.

During the compression stroke this ratio is 8:1, since it starts counting from the moment the intake valve closes, whereas during the expansion stroke it is 13:1, which corresponds to the downward travel of the piston with the valves closed (the only stroke in which work is produced).

Because of this asymmetry, when the working cycle ends and the exhaust valves are about to open, the internal pressure of the cylinder is approximately atmospheric pressure, whereas in a conventional petrol engine it is slightly higher, meaning that not all of the force pushing the piston down has been used.

In turn, the upward travel of the piston fills the intake manifold (remember that the intake valves remain open until roughly halfway through the stroke), resulting in an increase in pressure inside it (since this is always lower than atmospheric pressure), which helps fill the other cylinders and minimises the losses caused by backflow of gases.

The mechanical stress of Atkinson cycle engines is lower than that of Otto cycle engines because their compression ratio is lower, which gives the mechanics a longer life.

In turn, their production cost is similar to that of a conventional petrol engine. Their weight, noise and polluting gas emissions are also lower, far from the high cost, weight and noise of diesel engines.

Disadvantages of Atkinson engines

So far we have only highlighted the strengths of these engines. It is time to explain why these engines are not fitted to non-hybrid models.

The main disadvantage is that, by compressing a smaller amount of mixture, the pushing force on the piston crown at the moment the spark jumps is lower than in a conventional engine, leading to less powerful engines. An example is the 1,800 cm§h40Fordh41§ engine of the third-generation Toyota Prius, which produces only 100 CV.

Due to the timing configuration, the maximum engine speeds of these engines are limited to around 5,000 rpm, and the specific power (power obtained relative to displacement) is low, meaning these engines deliver the same power as a smaller-displacement engine, giving the driver the feeling that the car lacks power.

However, in a hybrid vehicle, this feeling does not arise, because the sense of power comes from the electric motor and a CVT gearbox (Continuously Variable Transmission) that keeps the engine at its optimal revs.

To put it simply, the role of the combustion engine in a hybrid vehicle is only to maintain momentum once the car has stopped accelerating; the electric motor takes care of the rest.

Why the Atkinson cycle

The Atkinson cycle engine became popular after the introduction and commercialisation of hybrid vehicles, which work with an internal combustion engine combined with an electric motor.

The Atkinson cycle, based on the Otto cycle, is widely used in hybrid cars, because it increases overall efficiency and thus achieves low fuel consumption. With the arrival of more advanced technology, however, valve timing was modified so that the mechanism works in the same way as the one developed by James Atkinson in 1882.

This gives higher compression ratios, which means the mixture ignites much earlier than in the Otto cycle, increasing thermodynamic efficiency. As we mentioned earlier, this is achieved by delaying the closing of the intake valves, which allows gases to flow back into the chamber, enabling a higher compression ratio.

In the list of cars currently using this powertrain configuration, we have the Toyota Prius, Ford Escape, Lexus RX 450h, Hyundai Sonata, Chevrolet Volt, KIA Niro, Hyundai Elantra and KIA Forte, among others, where the Koreans have firmly committed to including them in an increasing number of vehicles in their range.