Atkinson-Miller cycle:
Engines with a high compression ratio can deliver a lot of power. However, at low engine load (part load), the engine is inefficient: even at low load, a high pressure is built up above the piston, which causes inefficiency and is therefore undesirable in that situation. In order to still achieve high efficiency at part load with a higher compression ratio, some manufacturers apply the Atkinson-Miller principle. The terms Atkinson and Miller are sometimes confused and misused. The next chapter clarifies the differences and similarities between these inventions.
With the Atkinson-Miller principle, at part load the intake valve is kept open longer during the compression stroke (approximately 20 to 30 crankshaft degrees): some of the intake air flows back into the intake manifold. The amount of air above the piston after the intake valve closes is much lower than in engines where the intake valve closes at the end of the intake stroke. With a lower amount of air above the piston, less air needs to be compressed (less opposing force during the compression stroke). The amount of fuel to be injected is now also lower: less air also means less fuel.
The result of closing the intake valve later is a lower volumetric efficiency. This comes at the expense of engine power, but benefits overall combustion. The Atkinson-Miller cycle is ideal for hybrid vehicles, because the combustion engine is no longer the only power source, but is supported by the electric motor, or only serves to charge the battery pack (series hybrid). In addition, by changing the valve timing under operating conditions other than part load, the timing of the intake valve can be advanced.
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 images below show the indicator diagram and the PV diagram of a conventional petrol engine alongside those of an engine with the Atkinson principle. Because in the Atkinson principle the compression of the air starts later in the compression stroke, this is visible in these diagrams. The reduction in compression loss increases the thermal efficiency.
History of Atkinson-Miller technology:
In the previous paragraph we discussed the application of the Atkinson-Miller cycle. In the literature, the names of the Atkinson and Miller techniques are often combined, even though they were two separate inventions with the same goal. Below, the history of the Atkinson and Miller principles is described.
Atkinson: James Atkinson (Great Britain, 1882) worked on his invention by which he could increase the efficiency of a piston engine by enlarging the power stroke. By means of a complex system of rods and rocker mechanisms, the piston stroke during the power stroke could be greater than during the intake stroke.
The animation shows the four strokes in the well-known four-stroke process:
- intake stroke (intake, ansaugen)
- compression stroke (compression, komprimieren)
- power stroke (expansion, arbeiten)
- exhaust stroke (exhaust, ausstossen)
The Atkinson engine was not further developed at the time because this design was too complex and there was too much power loss.
Miller: Ralph Miller (United States, 1947) developed the technique in which the intake valve closes later in order to lower the final compression pressure (see the previous chapter). By changing the valve timing, the same goal is achieved as with the Atkinson principle: with less air, limiting the mechanical energy loss in the compression stroke. The difference between the Atkinson and Miller principles is that Atkinson physically creates different compression and power strokes, while Miller obtains the same thermodynamic result by altering the intake valve timing.
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