What exactly is the Budack Cycle? It’s a question you may have asked since news of an engine thus configured has surfaced. Named after one of its powertrain engineers, the Budack Cycle – developed by Volkswagen – is a variation of the Atkinson combustion cycle which is already commonplace in the market.

To recap, the Atkinson Cycle lowers the cylinder’s effective compression ratio by leaving the intake valve (or valves) open for a brief moment after the piston has started making its way back upwards from the bottom dead centre position, which means some of the intake charge is pushed back out from the combustion chamber before all valves are shut and the effective compression stroke begins.

This is relative to the cylinder’s expansion ratio (the piston’s downward stroke), which doesn’t change. In the case of the Budack cycle, effectively the opposite is done, which is to shut the intake valve before the piston reaches bottom dead centre, effectively reducing the compression ratio by reducing the amount of air-fuel mixture that is being pulled in before the compression stroke begins.

The goal in both approaches remains the same, which is to attain atmospheric pressure when the piston is at bottom dead centre. This of course is also in comparison to the traditional Otto combustion cycle, where the intake valves close when the piston is at or much closer towards bottom dead centre. In effect, this is variable valve timing of sorts, which trades outright power and torque figures for improved effiency.

In the case of the Volkswagen engine, it can also switch from the Budack cycle back to the conventional Otto cycle for maximum power and torque by a camshaft lobe which alters the intake valve duration. Watch the video above as Engineering Explained gives a rundown on the engine innovation.