Atkinson cycle engine has been the technological de rigueur targeted at cars design for outright efficiency over balls-out performance. Chief recipients of the technology are commonly hybrid cars such as Hyundai Ioniq and Toyota Prius.
So what is Atkinson cycle? It's a variation of the four-stroke cycle where instead of closing the intake valve just before the piston reach, or is exactly at bottom dead centre, the intake valve is shut as the piston is roughly 30 percent on its upward stroke during the compression stroke.
This effectively lowers the effective compression ratio without changing the expansion ratio in contrast to the conventional Otto cycle. As a result, efficiency improves as the engine wastes less energy trying to compress the dense air-fuel mixture and ensures every drop of fuel burnt is converted into useful work (that is actual work in applying downward pressure on the piston) rather than being wasted for nothing.
Of course, this comes at a detriment to power output as leaving the intake valve open during the compression stroke pushes some air-fuel charge back to the intake port, reducing the air charge density left for combustion.
Volkswagen, however, has a different take in achieving such feat. Instead of leaving the intake valve opens during the compression stroke, the intake valve is instead closed before the piston reaches bottom dead centre.
This effectively achieves what Atkinson cycle does, which is to lower the effective compression ratio without changing the expansion ratio. If you want power, Volkswagen have you covered by employing a camshaft lobe that alters the intake valve duration, enabling a switch to Otto cycle.
That's about the lowdown I can elucidate. For a detailed rundown, do watch the video by Engineering Explained below.
