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Atkinson-Miller Cycle


  • Atkinson-Miller Cycle
  • The origin of the Atkinson-Miller cycle

Atkinson-Miller Cycle:
Engines with a high compression ratio can deliver a lot of power. However, at low motor load (part load) the motor is inefficient: even at low load a high pressure is built up above the piston, which causes inefficiency and is therefore undesirable in that case. In order to obtain a high efficiency in part load with a higher compression ratio, the Atkinson-Miller principle is applied by some manufacturers. The designations Atkinson and Miller are sometimes confused and misplaced. The following chapter explains the differences and similarities in these inventions.

With the Atkinson-miller principle, in partial load, the intake valve is held open longer during the compression stroke (approximately 20 to 30 crankshaft degrees): the intake air partly flows back to the intake manifold. The amount of air above the piston after the intake valve has been closed is a lot lower than in engines where the intake valve closes already at the end of the intake stroke. With a lower air volume above the piston, less air needs to be compressed (less counterforce during the compression stroke). The amount of fuel to be injected is now also less: less air also means less fuel.

The consequence of closing the inlet valve later is a lower degree of filling. This comes at the expense of engine power, but benefits the overall combustion. The Atkinson-Miller cycle is ideal for hybrid vehicles, because the combustion engine is no longer the sole power source, but is supported by the electric motor, or only serves to charge the battery pack (series hybrid). In addition, changing the valve timing in operating conditions other than part load can advance the timing of the intake valve.

Compression stroke normal (left) and Atkinson (right)

A number of manufacturers apply the Atkinson-Miller principle to the combustion engines of their hybrid cars. These are mainly Korean and Japanese manufacturers: Hyundai, Honda and Kia.

The figures below show the indicator diagram and PV diagram of a normal gasoline engine in addition to that of the Atkinson principle engine. Because with the Atkinson principle the compression of the air only starts later in the compression stroke, this can be seen in these diagrams. The reduction in compression loss increases thermal efficiency.

Indicator diagram Atkinson (left) and normal gasoline engine (right)
PV diagram Atkinson and Ottomotor

The origin of the Atkinson-Miller cycle:
In the previous section we discussed the application of the Atkinson-Miller cycle. In the literature, the names of Atkinson's and Miller's techniques are often combined, when they were two separate inventions with the same goal. The history of the Atkinson and Miller principles is described below.

Atkinson: James Atkinson (Great Britain, 1882) worked on his invention in which he could increase the efficiency of a piston engine by increasing the power stroke. By means of a complex system with rods and rocker mechanisms, the piston stroke of the power stroke could be higher than that of the intake stroke.

The animation shows the four strokes in the well-known four-stroke process:

  • intake stroke (intake, ansaugen)
  • compression stroke (compression, compressing)
  • labor stroke (expansion, work)
  • exhaust stroke (exhaust, ausstossen)

The Atkinson engine was not further developed at the time because this design was too complex at the time and there was too much power loss.

Miller: Ralph Miller (United States, 1947) developed the technique in which the inlet valve closes later to lower the compression final pressure (see the previous chapter). By changing the valve timing, the same goal as with the Atkinson principle is achieved: to limit mechanical energy loss in the compression stroke with less air. The difference between the Atkinson and the Miller principles is that the Atkinson physically makes different compression and power strokes and the Miller obtains the same thermodynamic result with the exit of the intake valve timing.