CarBuyer Tech 101: Alternative cycle engines

Alternative cycle tech explained simply: Miller, Atkinson, Budak – what do all these names mean, and how do they save you fuel?

You may have heard the terms like ‘Atkinson cycle’ and wondered what the heck it means. Don’t all cars already run on the same four-cycles? What gives?

We’re here to break it down simply.

Before we go any further, you have to know the basic Otto cycle and how it works. Thankfully, it’s quite simple.Modern gasoline engines run on the Otto cycle named after German engineer Nikolaus Otto.

It consists of four steps:

1. Intake: The piston moving down, intake valves open, sucking in air/fuel mixture (intake)

2. Compression:
All valves closed, squeezing the mixture as the piston rises upwards

3. Expansion/Combustion:
The sparkplug explodes the mixture, pushing the piston down, which creates movement, and thus power.

4. Exhaust: Exhaust valves open, the piston rises again and byproducts of combustion are pushed out.

The thing about gasoline engines is that cars don’t need to use all of the potential power all the time.  Variable valve trains, which affect how long (valve timing) or how high (valve lift) have helped improve efficiency and power in this way for decades, the most dramatic example being Honda’s early VTEC systems.

To use a human metaphor, people don’t sprint everywhere but mostly walk or jog. But early engines without variable valve trains were like people who were best at either walking/jogging or sprinting, with no in-between.

More recently, carmakers have used valve trickery to affect the actual combustion process, rather than just being facilitators for it, in a sense.

The Atkinson cycle
Used by Toyota on their hybrid, naturally-aspirated engines as well as its Lexus ‘F’ and turbo engines. It holds the intake valve momentarily during the compression stroke, so some of the air/fuel is pushed out. That means higher efficiency, since the piston has less ‘upwards’ work to do, but also less power, since not as much fuel-air is burnt.

The engines, as the video mentions, can change from Otto to Atkinson operation depending on the demands the driver makes – so it can go from jogging to sprinting much more easily.

The Miller Cycle is similar to the Atkinson cycle, but uses a supercharger, or turbocharger, to overcome the loss of power.

The B-Cycle

And finally Audi/VW’s 2.0-litre B-cycle engine uses the Budack cycle (named after the VW engineer who came up with it). Instead holding the valve open during compression, it closes it sooner during intake, so achieves the same effect of having less air/fuel taken in. It’s also seen in the VW Group’s V6 gasoline engines, the 2.9-litre biturbo V6 (Audi RS 5, Porsche Cayenne S) and 3.0-litre single turbo V6 (new A6, A7, A8, and base Porsche Cayenne).

about the author

Derryn Wong
CarBuyer's chief editor has a keen interest in all things mechanical, technological, animal and mineral. He's particularly fascinated by eco-cars and cars which make no logical sense. An avid motorcyclist and photographer, he also enjoys cats.