Many of the topics we cover in Auto Academy and Engines Exposed are fairly commonplace. Most people, especially the readers of a car-based website, have heard about camshafts, nitrous, turbochargers, and the rest of the topics we’ve touched on. However, there are some topics in the automotive world that are so esoteric that even some gearheads might not know much about them. That’s where we’re going today, where the topic du jour is the distinction between the Otto, Miller, and Atkinson cycle engines.
If these names don’t mean anything to you, don’t feel bad. For most people, there isn’t a need to know the difference. But as we enter a world where the continued existence of automobiles is reliant upon maximizing efficiency, it might be worth getting to know these three chaps. Unless you’re driving a car with a Wankel (rotary) engine, a fuel cell vehicle, or a concept vehicle powered by something like a jet engine or compressed air, these three – specifically Otto and Atkinson – are powering every mile you drive.
Let’s start with some basics. For this, I’m going to draw upon combustion dynamics, which I admit doesn’t seem basic at all. If you start a fire, you’re burning something in an un-contained environment. An explosion, on the other hand, is defined as the sudden and violent release of mechanical, chemical, or nuclear energy from a confined space. The confined space part is the key to understanding how engines work. During the compression stroke of an engine’s cycle, work is done on the air-fuel mixture. This work reduces its volume and increases its temperature. Combustion occurs and causes an explosion in this confined space. The “sudden and violent release” exerts a force on the piston, which does work by driving the piston downward. This work is what drives the crankshaft and, eventually, the wheels.
You’re probably saying “I get the point!” But bear with me. The difference between the cylinder volume when the piston is at the beginning of the compression stroke (Bottom Dead Center, or BDC) and the end of the compression stroke (Top Dead Center, or TDC) is known as the “compression ratio.” It’s expressed as a ratio (obviously) with the volume of the uncompressed gas first, e.g. 9:1. The flip side of the compression ratio is the expansion ratio, which would be 1:9 in this instance. The work produced by a given engine cycle is the difference between the work that’s created by the expanding gas during combustion and the work that’s done on the gas during the compression stroke.
From this, it’s easy to see why maximizing the work generated during expansion and minimizing the work needed during compression is desirable. So how do each of these three engine types address that?
The Otto cycle engine, named for Nicolaus August Otto, was first unveiled by Otto in 1876. In short, the Otto cycle – which powers the vast majority of cars on the road today – doesn’t do much to optimize the ratio between compression work and combustion work. Additional power is generated in Otto cycle engines by increasing the compression ratio, which increases the expansion ratio, increasing the density of the air-fuel mixture through forced induction, or by attempting to capture some of the waste heat for a variety of applications. This is the traditional “suck, squeeze, bang, blow” engine: the idea is that the blow is powerful enough to offset the work of the squeeze.
The Atkinson cycle relies upon many of the same principles as the Otto cycle. It was developed by James Atkinson as a workaround for the patents that Otto had on traditional four-stroke engines. In an Atkinson-cycle engine, the intake valve remains open during the beginning of the compression stroke. This allows some of the air-fuel mixture to be pushed out of the cylinder and back into the intake manifold. Naturally, this reduces the power that is generated, as there is less fuel to combust. However, it also effectively reduces the compression ratio while leaving the expansion ratio the same. This means that less work is expended during compression, effectively increasing the amount of work that can be obtained from combustion.
This increases the efficiency of the engine at the expense of the power density per liter. The engine in the 2015 Prius, which is an Atkinson cycle engine, produces 98 horsepower from a 1.8 Liter engine (54.4 horsepower/liter). The 2015 Chevy Cruise also has a 1.8 liter engine, though with an Otto cycle, and it produces 138 horsepower (76.67 horsepower/liter). Given the lower power output, especially at lower engine speeds, the Atkinson cycle long struggled to gain popularity. However, that changed with the 1997 Prius, which introduced an Atkinson cycle engine as part of its Hybrid Synergy Drive system. The addition of an electric motor supplemented the low-end power of the vehicle, which helped to offset the weaknesses of the Atkinson cycle. Given its efficiency, the Atkinson-cycle engine has since become the engine of choice for many hybrid vehicles.
Ralph Miller brought us the Miller cycle engine in 1957. It is basically the same as an Atkinson cycle engine, but it uses a supercharger to increase the density of the air-fuel mixture in the cylinder, which helps to make up for the loss of power that is inherent to the Atkinson cycle. In addition to being used in a few limited-run models, the Mazda Millenia used a Miller-cycle V6 that was able to produce 91 horsepower per liter — quite a feat for the mid-1990s!
As engineers seek to get every drop of usable energy out of fuel, we’re likely to see many unconventional engine designs in the future, like this non-Wankel rotary design. Perhaps a day will come when these three names are relegated to the history books. For now, however, our world relies upon them and their creations.
Like classics? It’s always Throwback Thursday somewhere.