opposed piston engine

The Redesign of the Internal Combustion Engine: The Opposed Piston Engine

The internal combustion engine is a remarkable invention that has been powering vehicles for more than a century, but it is far from perfect; some governments have even discussed banning these engines. For all of their advantages, today’s internal combustion engines still haven’t solved the issue of harmful emissions. Improvements in efficiency have reduced the amount of emissions that vehicles create, but engineers think that our current designs may be reaching their limits even as the pressure mounts to reduce emissions further. However, one company thinks it may have discovered a solution to some of the internal combustion engine’s woes by revisiting an abandoned idea from the past: the opposed piston engine.

The Opposed Piston Engine

When the internal combustion engine was first under development in the late 1800s, several different designs were tossed around before the industry adopted today’s conventional engines. One such design was the opposed piston engine, which was first proposed by James Atkinson in 1882 as his differential engine.

By UtzOnBike (Own work) [GFDL, CC-BY-SA-3.0, via Wikimedia Commons

By UtzOnBike (Own work) [GFDL, CC-BY-SA-3.0, via Wikimedia Commons

Parts of a modern opposed piston engine:

1. Intake for fuel-air mixture
2. Supercharger (here: rotary vane pump; original: Centrix)
3. Airbox to buffer and distribute the mixture
4. Waste valve to limit the pressure level
5. Outlet crank mechanism (runs app. 20° before inlet to achieve an asymmetric control diagram)
6. Inlet crank mechanism
7. Cylinder with inlet and outlet slots
8. Exhaust
9. Water cooling jacket
10. Sparkplug

This design presented several advantages over the conventional design, but the difficulty of designing a compact opposed piston engine eventually led to the proliferation of conventional engines in automobiles. However, the ability to pull more power out of each pound of engine made the opposed piston engine a natural fit for high-performance propeller-driven airplanes, especially the fighter jets of World War II. After the war, many of the high-powered engines used to drive trains, freighters and submarines utilized opposed piston designs. As these large-format engines were refined further, more engineers began to wonder if the design could be successfully adapted to cars.

How Does an Opposed Piston Engine Work?

At the most basic level, an opposed piston engine is simpler than a regular conventional engine. Each cylinder contains two pistons, which form the roof and the floor of the cylinder. The engine works with a two-stroke cycle. When the pistons are at their furthest distance from each other, fuel and air are injected into the cylinder. As the engine rotates, the pistons are pushed close to each other. The fuel and air is then ignited, and the pistons are pushed apart.

By UtzOnBike with Autodesk Inventor (Own work) [GFDL, CC-BY-SA-3.0, via Wikimedia Commons

By UtzOnBike with Autodesk Inventor (Own work) [GFDL, CC-BY-SA-3.0, via Wikimedia Commons

Unlike a regular four-stroke engine, an opposed piston engine generates power every other stroke, making the engine theoretically more efficient. However, getting the most out of these types of engines in a compact form required precision engineering that was beyond the capabilities of the technology of the past.

Achates Power Promises Opposed Piston Automobile Engine by 2018

For the past 13 years, Achates Power has been quietly developing a functional opposed piston engine suitable for use in passenger cars. In 2014, the company pulled back the curtain on its prototype at the SAE with a presentation to MotorTrend; preliminary testing models suggested that Achates’ engine would be about 30% more efficient than comparable conventional engines. The following year, Achates began working with multiple manufacturers on a prototype opposed piston diesel engine. A $9 million grant from ARPA-E accelerated the company’s progress and allowed them to begin work on a gasoline version of their engine. As of 2016, Achates’ engine had successfully completed more than 7,000 hours of dynamometer testing, and technologies related to the engine have formed the basis of more than 100 patents.

Advantages of the Opposed Piston Engine

The opposed piston design has many compelling advantages.

First, although the engine is more difficult to engineer, it’s mechanically simpler than a comparable conventional engine; Achates’ first planned engine, a 2.7-liter 3-cylinder engine, is constructed from 60% fewer parts than a comparable V6 engine. This makes the engine simpler to manufacture and decreases manufacturing costs by about 10%.

Opposed piston engines are more power-dense than conventional engines. Each cylinder contains two pistons, allowing engineers to pull the same amount of power out of a significantly smaller and more compact engine. In turn, this reduced weight decreases the amount of work that the engine has to perform to move the vehicle,  improving the fuel economy.By adding a second piston to each cylinder, engineers can remove cylinder heads from the engine design. In conventional engines, cylinder heads serve as the lid of the cylinder, containing the combustion and allowing the engine to extract work from the fuel. However, the cylinder head also steals heat from the combustion, leading to waste heat that doesn’t directly provide power to the vehicle. Removing the cylinder head from the equation allows engineers to instead capture some of that waste heat and convert it into useful work.

Opposed piston engines are ideally suited  for compression ignition rather than spark plug ignition. Using compression to ignite the fuel allows the engine to use a leaner fuel-to-air ratio, increasing efficiency, and decreasing emissions. It also allows engineers to eliminate the complex timing and ignition systems needed to regulate the firing of spark plugs, further reducing the cost to manufacture an opposed piston engine.

Although the opposed piston engine is a new design, at its heart, it’s still an internal combustion engine, and it can take advantage of the infrastructure supporting today’s automobiles. Consumers will not need to search for a charging station or an exotic fuel station, and they can refill their tanks with a simple five-minute stop.

Both the diesel and the gasoline version of the opposed piston engine have their own advantages. Diesel fuel is naturally energy-dense, and it is easier to design a compression diesel engine. Gasoline combusts easier and with fewer emissions, and it does not require the use of a particulate filter. As a light fuel, gasoline requires less energy to pump the fuel from the tank to the engine.

Adopting opposed piston engines will allow automakers to meet the ambitious CAFE 2025 fuel standards without radically changing how their vehicles are manufactured. Relatively simple engineering changes will enable them to simply drop an opposed piston engine into their existing designs, and the reduced manufacturing costs will actually allow them to increase the efficiency of their fleets without increasing their costs drastically.


Final Thoughts

Market and regulatory pressures are combining to drive the auto industry towards greener and cheaper powertrains. Opposed piston engines are an idea from the auto industry’s past that point the way towards the future without requiring automakers to reinvent the wheel. Achates Power has spearheaded the development of modern opposed piston engines, producing several promising new engines that have major automakers intrigued. Their first models promise improved performance, increased efficiency, lower emissions, and lower costs. If they can deliver an engine that meets expectations, they may succeed at revolutionizing how our cars are powered.


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