What is an Atkinson cycle engine and why do car manufacturers use it? – Jalopnik

Gasoline engines typically run on the Otto cycle, the standard four-stroke and two-stroke combustion process that has been powering humanity for some time. While engines typically rely on ‘suck, squeeze, pop and blow’ logic, there are subtle variations around the concept that often go unnoticed (or ignored) but provide several important benefits. The Atkinson cycle is one of those variations. You’ve probably heard it mentioned in the context of hybrid Toyotas and Hondas. There are also engines with a Miller cycle, but that is a separate discussion.

Going back to the Atkinson cycle, it all started when British engineer James Atkinson realized that the Otto four-stroke engine could be improved. More specifically, its custom cycle focused on increasing thermal efficiency. While Atkinson didn’t exactly create the greatest engine of all time, the idea of ​​achieving more with significantly less fuel was worth pursuing. Mind you, this all took place in the late 19th century. Although Atkinson’s execution – he used a multilink connecting rod for variable stroke lengths – was a royal failure, the underlying principle was not.

Simply put, on an Atkinson cycle engine, the intake valve does not close until the piston has completed 20-30% of its compression stroke. This pushes some of the intake charge back into the manifold. While Otto engines have identical compression and expansion ratios, Atkinson cycle engines have a much higher expansion ratio relative to their compression ratio.

Imagine a 500cc single-cylinder engine running the Atkinson cycle. Let’s assume the engine consumes about 300 cc of air and fuel (some of the mixture goes back to the intake) and then compresses it to 50 cc. This means that the compression ratio of the engine is approximately 6:1, but the expansion ratio becomes 10:1. Why? Because after combustion the mixture expands from 50 cc to 500 cc. If the same engine were to run the Otto cycle, the compression ratio and expansion ratio would be 10:1.

As you can see, by running the Atkinson cycle the engine saves a bit of the intake charge for the next combustion cycle, improving overall fuel economy and reducing emissions. However, this does mean that the engine will deliver mediocre performance, especially when it comes to torque. That’s why OEMs commonly combine an Atkinson cycle engine with an electric motor.

Common misconceptions surrounding hybrid engines aside, this is a combination that simply makes sense when you think about it. The electric motor(s) can intervene during low-speed situations where the engine is struggling. But when the demand for torque is relatively small, such as when coasting on a highway, the engine can drive the wheels without relying on the battery or electric motor. Modern hybrids are so good at this that you hardly notice them switching between electricity and the combustion engine. However, it’s worth pointing out that the combustion cycle isn’t limited to hybrid powertrains; some modern engines can switch between Otto and Atkinson in real time.