Heat engines

What is the Atkinson cycle? Atkinson Engine Features

The Atkinson cycle engine is a type of reciprocating internal combustion engine invented by James Atkinson in 1882. It is practically a common Otto cycle engine, but with a modified crankshaft crank for greater efficiency at the expense of a reduction in horsepower. .

The goal of the Atkinson cycle was to compete with the Otto cycle without infringing any patents. The Atkinson engine has better efficiency but is less powerful than the Otto engine.

In its original version, the Atkinson cycle requires a single rotation of the crankshaft to complete an operating cycle, keeping the phases of the Otto cycle engine unchanged: intake, compression, expansion (useful phase) and discharge.

However, thanks to a particular geometry of the crankshaft mechanism, the expansion stroke is greater than the compression stroke, which allows the engine to have a higher thermodynamic performance compared to the Otto cycle.

How does the Atkinson engine work?

The Atkinson cycle can also be applied to rotary engines with a consequent increase in power and efficiency compared to the Otto cycle.

This type of engine retains an active phase per crankshaft revolution, along with the different compression and volume expansion of the original Atkinson cycle.

During a part of the compression phase, the intake valves that regulate the amount of mixture that enters remain open. This is achieved by designing the camshaft in a special way to take advantage of the losses due to the compression of the mixture within the engine cylinder.

At the end of the expansion phase and at the beginning of the exhaust phase, the internal pressure of the cylinder is almost the same as the atmospheric pressure. In an Otto cycle, this pressure is slightly higher because part of the thrust force of the piston is used.

During the upward stroke of the piston the intake manifold fills. During this part of the stroke, the pressure increases and favors the filling of the other cylinders. In this way, energy losses due to gas reflux are reduced.

Exhaust gases are expelled from the engine through compressed air, this modification of the Atkinson cycle allows the use of alternative fuels such as hydrogen and diesel.

Why is the Atkinson cycle confused with the Miller cycle?

Today the term "Atkinson cycle" is used erroneously to describe a normal Otto cycle engine in which different compression and expansion strokes are obtained by acting on the opening / closing of the intake valves. This cycle corresponds to the Miler cycle rather than the Atkinson cycle.

In the Miller cycle, by keeping the inlet valve open for a greater angle of rotation than normal, the effect is obtained from a reflux of the suctioned air towards the suction duct, or through an early closure of the valve during the phase. suction.

In both cases, the effect is to reduce the cylinder fill, and therefore the compression ratio, without modifying the stroke. So the expansion stroke is greater than the "current" compression. This results in a lower pressure and temperature of the flue gases at the end of the expansion phase. 

Therefore, in theory, more heat is converted into mechanical work increasing the efficiency of the cycle.

Using the Atkinson cycle in hybrid engines

Very often this cycle is confused with the Miller cycle. The Miller cycle requires only a variation of the supply time, while the Atkinson cycle requires a particular crank structure.

For this reason many of the hybrid electric vehicles that combine the electric motor with a Miller cycle motor are confused with Atkinson motors. In fact, in all cases it is a common four-stroke engine with the retarded intake valve.

Some examples of models with these characteristics are the 2013 Honda Jazz, Toyota Prius, Toyota Auris hybrid, Toyota Yaris hybrid, Lexus CT200h, Lexus iS, and Ford escape.

Publication Date: June 20, 2018
Last Revision: September 19, 2021