Atkinson, Miller, or Otto: Internal Combustion Cycles Explained

Atkinson, Miller, or Otto: Internal Combustion Cycles Explained

Despite its expansive usage as a means of propulsion, the four-stroke engine is by no means a modern invention. In fact, the core functional components within this form of internal combustion border on being archaic when placed into context.

Trace its lengthy history back far enough, and you will find the first “Otto” engine emerging on the scene during an era when horsepower quite literally meant what the name entails. The year was 1876, and a German engineer by the name of Nicolaus Otto was concocting a revolutionary piece of machinery that would grace everything from assembly lines and personal forms of motorized transit to diesel-powered battle tanks and the trucks that deliver our groceries.

It was a device that was decades ahead of its time, relied upon air induction and internal combustion to create power via compression, and did not require the use of a whip to get up to speed. An invention that earned the German scientist notoriety nearly overnight, setting the stage for the birth of “das automobil” and the evolution of every four-stroke internal combustion engine to date.

But before getting too deep into all of the dirty details that make the more modern Miller and Atkinson engines superior to the original Otto design, we must first pay tribute to the brilliant powerplant that got us to this point.

A crudely hewn contraption by today’s standards, the original Otto four-stroke had a fuel efficiency rating that was only rivaled by its abysmal power figures, and an exhaust output that fired off more lead than a Dirty Harry flick. Nevertheless, if it weren’t for this dated design the modern automobile would have likely been quite a bit different…

Original Otto Four-Stroke Design

The original Otto four-stroke design was about as cumbersome and inefficient as it got… but it worked. By closing the intake valve before the compression stroke started, the entire length of the cylinder was used to capture and compress the air-fuel mixture and produce power.  Photo Credit: driving 4 answers/YouTube

Das Otto!

For those of you who are just now joining us, and are somewhat new to the whole four-stroke internal combustion schematics side of things, here are a few basics and some fun history to help get you up to speed.

First of all, it’s called the four-stroke engine for a reason. Nicolaus Otto’s formula involved four key stages in the power production process, all of which are still seen in piston-powered petrol powerplants today. A fantastic foursome that focuses on intake, compression, combustion, and exhaust strokes.

For over a century, these core components have given this form of engine life, and even today much of Otto’s original four-stroke design is still being utilized in more modern Atkinson and Miller cycle engines. The crankshaft, connecting rods, pistons, valves, combustion chambers, intake plenum, camshaft, fuel atomizers, exhaust manifold… they are all still there. They just are lighter, stronger, and more efficient than ever.

In the ideal Otto cycle the fuel-air mixture is introduced during the induction stroke and compressed to a much lower compression ratio (around 8:1) and is then ignited by a spark. The combustion results in a sudden jump in pressure while the volume remains essentially constant. The continuation of the cycle including the expansion and exhaust processes are essentially identical to that of the ideal Diesel cycle. — The First Law of Thermodynamics for Closed Systems, Ohio University

But whereas Old Man Otto’s four-stroke engine was revolutionary in so many ways, his patented design was quite antiquated and insanely inefficient when compared to more modern designs of the day. The compression and expansion strokes, valve intake timing, spark timing, and inferior internal components all made for just as much (if not more) parasitic loss as they did power.

Furthermore, old man Otto was later stripped of his patent, when it was discovered that another inventor came up with the design prior to his invention of the four-stroke engine. 

Original Atkinson Four-Stroke Engine Patent

A schematic blueprint image of the original Atkinson Four-Stroke Engine, first patented in 1882. Photo Credit: Toyota Canada/YouTube

Getting Lean on the “Atkinson Diet”

Naturally, it did not take long before every scientist and engineer on the planet was looking for ways to improve on Otto’s four-stroke engine. While he may never have become as well renowned (or wealthy) as the four-stroke originator, British engineer James Atkinson was able to assist in the advancement of the modern internal combustion engine. The issue was that Otto had already patented his engine’s design, so it would take some time before Atkinson could come up with a redesign that didn’t infringe on the original.

Patented in 1882, Atkinson’s four-stroke redesign relied on variable stroke lengths to boost efficiency and power over the traditional Otto engine. The design utilizes a shorter, more efficient compression stroke, while allowing combustion to act on the piston longer via a physically longer power stroke. This was made possible via the use of multi-link connecting rods, making for a far more efficient thermodynamic cycle within the piston and flywheel portion of the engine block. This also allowed all four cycles of the engine to occur within 360-degree of crankshaft rotation, as opposed to the 720 degrees of crank rotation needed in the Otto cycle. 

Atkinson cycle

Animation of the Atkinson Cycle, by MichaelFrey

A failure at first, the Atkinson engine received the love it deserved well over a century after its release, largely thanks to the advent of hybrids and a modification of the Atkinson cycle to work with a less complex Otto-style engine. The problem with Atkinson’s design was that its higher efficiency ratings came with the tradeoff of less pep at slower speeds. With the introduction of an assist motor or battery pack, the Atkinson cycle can get up to its optimum operating speed at a far more speedy rate, in turn taking over where the hybrid’s electrical system tends to taper off.

But being that no one really gave a damn about efficiency ratings or fuel prices back then, and only cared about having as much power as possible, the Atkinson cycle was shelved as a nonsensical solution.

Atkinson Four-Stroke Engine

Today, engines labeled as “Atkinson Cycle” largely use a modified Atkinson cycle, where the compression stroke is shortened by keeping an intake valve open during the start of combustion to bleed off pressure, as opposed to having a mechanically shorter stroke.  However, it’s not a true Atkinson cycle, as all four stages only occur in 720 degrees of crankshaft rotation.  Photo Credit: Toyota Canada/YouTube

It’s Miller Time!

The next big change came in 1957 when an American by the name of Ralph Miller came out with his own engine patent. It was called the Miller Cycle engine, and it was equal parts simple and smart.

By capitalizing upon the Atkinson engine’s compression stroke improvements, and utilizing a far simpler connecting rod design, Miller was able to take the four-stroke motor’s efficiency even further. The trick was allowing the intake valve to remain open for even longer, which typically translates to the first 20- to 30-percent of the compression stroke, and making up for the compression losses by using forced induction.

By allowing the rising piston’s compression stroke to push air back into the intake manifold, the cylinder itself never reaches maximum capacity, but also reduces pumping losses. While this design does prove to be detrimental to performance figures when at lower speeds, ignition efficiency ratings spike significantly when that piston starts to drop. The result? A shorter intake stroke with less power, and a complete downward “power stroke” for a more optimum compression process.

Miller Cycle Four-Stroke Mazda Engine

Mazda is one of the few automakers to produce engines that rely solely on a Miller Cycle four-stroke engine design. Photo Credit: Mazda

Since Miller’s engine keeps the intake valve open when the compression stroke starts, the piston’s return to the top (a.k.a. “expansion process”) forces any remnants leftover from the combustion process back into the intake manifold. This explains in part why things like EGR valves and oil catch cans became so commonplace over the years.

Furthermore, Miller’s design came affixed with a supercharger to offset the performance losses encountered due to the intake valve release. But as Nissan discovered during the development of the HR12DDR concept, the addition of a supercharger also made the motor more efficient.

With time, certain automakers began to find ways to make Miller Cycle motors work without a blower, with companies like Mazda producing both supercharged variants and pint-sized naturally aspirated Miller Cycle motors. But neither of these car manufacturers would see the success that the largest of all Japanese automakers would encounter when it decided to toy with both Miller and Atkinson Cycle designs.

Nissan Miller Four-Stroke Illustration

An illustrated look at how the Miller Cycle four-stroke functions compared to a traditional Otto Cycle system. Photo Credit: Nissan Motor Corporation

With Our Four-Stroke Powers Combined, We Shall Be… Boring 

Like peanut butter, jelly, and toast, it was the combination of Miller and Atkinson Cycles within old man Otto’s four-stroke engine idea that made for the most notable performance and efficiency gains.

It was the later 1990s, and Toyota had just unveiled its 1NZ-FXE engine. A design that relied upon both forms of four-stroke genius for maximum performance and petrol consumption, but substituted the supplemental supercharger for a hybrid gas-electric motor combo. The car was called the Toyota Prius, and it single-handedly made the modified-Atkinson/Miller cycle a household name. The design proved to be so reliable, that it eventually was slapped on every engine bearing a Toyota hybrid vehicle badge.

Combined with things like direct injection, variable valve time (VVT), and computerized exhaust gas recirculation (EGR) monitoring, the modified Atkinson cycle in particular quickly became a widely utilized “on-the-fly” engine design, as it only gets implemented when needed. This translates to more power down low, better efficiency ratings across the board, and less wear and tear on engine internals over time.

Prius Atkinson/Miller Engine

The first generation Toyota Prius was the first of its kind to utilize both Atkinson and Miller four-stroke cycle designs, with the hybrid’s battery serving as the supplemental power source for low-speed acceleration. Photo Credit: Toyota UK

Now as for the Miller-style four-stroke engine cycle, this design has recently seen a resurgence thanks to Mazda… again. While the Japanese automaker first gained notoriety for its utilization of the Wankel rotary engine, slapping a Miller-style motor into the supercharged (and hideous-looking) Millenia was another one of its revolutionary innovations. And now, Mazda has brought the design back from the dead (again…) in its latest and greatest Skyactiv engine program.

Referred to as the “Skyactiv-X” engine, this motor combines a pipsqueak of a roots supercharger and a Miller-style four-stroke power cycle — along with Spark Controlled Compression Ignition — to generate pep. A design that much like its naturally aspirated Atkinson alternative, relies upon an augmented four-stroke design originally engineered by the dethroned father of the modern motor, Mr. Nicolaus Otto.

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About the author

Micah Wright

Raised on LEGOs by grandfathers who insisted on fixing everything themselves, Micah has been a petrolhead in training since age four. His favorite past times include craft beer, strong cigars, fast cars, and culinary creativity in all of its forms.
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