Budack cycle: A cycle that saves fuel
Combustion engines can work in different working cycles, from the Otto cycle to the Atkinson cycle to the Miller cycle. However, Volkswagen came up with the Budack cycle after the engineer who invented it.
The Budack cycle is a solution to achieve higher engine efficiency and reduce fuel consumption. But how does this cycle work?
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Alternatives to the Budack cycle:
The Otto, Atkinson, and Miller cycle concepts are already very well known to some of us, but it will not hurt to say a few words about them. The Otto cycle is the standard cycle of four-stroke internal combustion engines.
The Atkinson cycle is a cycle with prolonged expansion and shortened compression. The expansion ratio is greater than the compression ratio. The Miller cycle is the same as the Atkinson cycle, but with the difference that it concerns supercharged engines.
Simply summarized, the Otto cycle is the standard four-stroke internal combustion engine cycle, and the other two mentioned are just variations of it where the length of compression is less than the length of the expansion.
How does the Budack cycle work?
The Budack cycle is an evolution of the Atkinson cycle, and its goal is to reduce fuel consumption and, thus, the amount of emissions. There are only two differences between the two cycles.
The first is that the Budack cycle uses variable intake valve timing, which switches between the Budack and Otto cycles, respectively between the Atkinson and Otto cycles, and the second is that the compression phase is not shortened by delayed closing of the intake valve, but rather by earlier closing of the intake valve.
Thus, the Atkinson cycle engine leaves the intake valves open at the beginning of compression, which means that part of the cylinder's contents (the mixture) is returned to the intake manifold. However, this will cause the compression ratio to decrease, but the expansion ratio will remain at the maximum possible. But what is it good for?
Shortening the compression phase in favor of the expansion phase will cause the burning mixture to transfer more energy to the piston during expansion because it has more time to do so.
Although the engine power is slightly less with this cycle than with the Otto cycle, the advantage is that we can inject less fuel into the cylinder, which means that the engine consumption will be less and the emissions will also be lower. This is the point of the Atkinson cycle.
The Budack cycle works on the same principle, but with the difference that the intake valves are closed even before the piston reaches the bottom dead center. So less air gets into the cylinder again, reducing the compression ratio compared to the expansion ratio and increasing the combustion efficiency.
The Budack cycle and its benefits
As with the Atkinson cycle, engine power is lower, but combustion efficiency increases. Thus fuel consumption decreases. However, the advantage of the Budack cycle is that it is equipped with variable valve timing, which allows the engine to switch to the Otto cycle.
Switching the engine to the Otto cycle requires maximum engine power. The camshaft of the intake valves will thus enable a larger and longer opening of the valves, and the engine thus runs in the Otto cycle with maximum power. In the Budack cycle, the engine runs only at a low load.
Application of the Budack cycle in practice
Volkswagen applied the Budack cycle in the TSI engine of the EA888 series in the Tiguan, which is intended for the US market. According to the US EPA, the Tiguan with this engine consumes an average of 8.7 liters of gasoline per 100 kilometers, which is nothing amazing.
It is a step forward, because the results with this engine are better than in the past, and every improvement counts.