The Top Ten Reasons the Opposed-Piston Engine is Naturally Low in NOx

NOx is a generic term for oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO2).  These gases are formed during combustion of hydrocarbon fuels and contribute to the formation of smog and acid rain, and have other adverse health and environmental impact. (

Policy makers around the world have enacted regulations to significantly reduce NOx emissions from cars, trucks, and other sources.  Nevertheless, in areas of heavy vehicle traffic – urban areas, ports, and near highways – high level of NOx impacts human health and well-being.  Moreover, many vehicles emit far more NOx under real world operating conditions than during certification testing.

OP Engine Efficiency Advantage

Many sources have cited the inherent efficiency advantages of the Opposed-Piston (“OP”) Engine.  Herold, Wahl, et al describe the source and magnitude of the “fundamental efficiency advantage of an opposed-piston two-stroke engine over a standard four-stroke engine of comparable power output and geometric size”.  Warey, Gopalakrishnan, et al of General Motors find “the opposed-piston diesel engine had about a 13-15% lower CO2 emissions compared to a four-stroke diesel engine….The efficiency advantage of the opposed-piston two-stroke engine is mainly due to lower in-cylinder heat losses due to elimination of the cylinder head and lower surface area to volume ratio “30% lower surface area to volume ratio for equivalent four-stroke engine displacement”.  Mattarelli, Cantore, et al find “the advantages [of the opposed-piston engine] in terms of scavenge and thermal efficiency are indisputable: a perfect ‘uniflow’ scavenge mode can be achieved with inexpensive and efficient piston controlled ports, while heat losses are strongly reduced by the relatively small transfer area.”

OP Engine NOx Advantage – Let Us Count the Ways

It is noteworthy that the OP Engine also has significant and unique advantages in controlling NOx emissions.  The purpose of this post is to describe these ten advantages.

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Crank Phasing and the Impact on Engine Efficiency

In an Opposed-Piston (“OP”) Engine, two pistons come together within a cylinder and the piston crowns form the combustion chamber.  The most common and widely deployed OP Engine, often referred to as the ‘Junker’s style’ has two crankshafts, one on each side of the cylinder.  Typically the exhaust crank leads the intake piston for effective gas exchange: the exhaust piston opens the exhaust ports when it is near the end of the combustion stroke, allowing blow down.  Next, the intake piston opens the intake ports when it is near the end of its combustion stroke, enabling scavenging.  The two pistons come together for compression.  The crank lead can generally range up to 12° depending on engine design and overall optimization.

Achates Power has conducted a series of test to assess the effect of crank lead on engine efficiency.  With fueling and calibration kept constant, the exhaust crank lead was varied from 3° to 12°, and brake mean effective pressure, indicated mean effective pressure, pumping mean effective pressure, and friction mean effective pressure were analyzed.  The results are depicted in the graph below: there is virtually no difference in BMEP, IMEP, PMEP, and FMEP across the entire exhaust lead sweep.

The historical record:

Martin and Pirault, in their well-researched book, “Opposed Piston Engines: Evolution, Use, and Future Application” describe many OP Engine variations in great detail.  The Jumo 205E, for example had a 9° exhaust crank lead and “set many long distance records…[it] remains the most efficient piston aero engine in aviation.”

Later in the book, Martin and Pirault describe the Fairbanks Morse 38D engine.  At the time of publication, it had 12° exhaust crank lead.  Fairbanks Morse recently announced an upgraded version of the engine which it describes has “best in class fuel efficiency”.

These two examples – perhaps the most widely deployed and best known OP Engines – demonstrate that OP Engines are the most efficient engines in their class, and support the experimental results that exhaust crank lead has little impact on efficiency.

At Achates Power we have recently reviewed two presentations that mistakenly claimed that overall mechanical efficiency of an Opposed-Piston Engine dropped off with increased exhaust crank lead.  The error in their analysis was simple.  Yes, these presentations correctly claim that the torque from the intake crank decreases as exhaust crank lead increases.  However, the torque provided by the exhaust crank increases proportionally and thereby effectively compensates for the intake torque reduction. This factor was ignored in each of those presentations.  Bottom line, total torque at the power takeoff, which is equivalent to the sum of the two crankshaft torques less any gear connection losses remains roughly constant.

Such a corrected analysis can be done kinematically looking at the in-cylinder pressure traces against the position of each piston to its top dead center, confirming our measured data and the experience of others.

CAFE 2025 Regulations Are Achievable with Improved Gasoline Engines

In the recently released Corporate Average Fuel Economy (CAFE) draft Technical Assessment Review (TAR) the EPA concluded “the standards can be met largely with more efficient gasoline powered cars...” (LINK) We agree, we will meet, and exceed, future fuel economy and emissions reduction standards with advanced gasoline and diesel engines.

The Achates Power Opposed-Piston Engine has already been shown to achieve industry-best fuel economy gains of 30-50%, significant emissions reductions, and does so more cost effectively than any other solution. With engine programs in development with 12 leading engine manufacturers, we believe the Achates Power Opposed-Piston Engine will provide the foundation to meet the existing and future fuel economy and emissions regulations. As compelling as our engine’s efficiency advantage is, the Achates Power OP Engine will also be less expensive than predicted $1,000 increase from the 2021 baseline that the OEMs currently expect.

The CAFE draft TAR confirmed what many of us in the industry know; we can meet and exceed future fuel economy and emissions requirements with the advanced engine technology in development today. The regulations will help to drive further advancements in and adoption of engine technology, which will benefit everyone. Continuing towards these regulations will help protect the environment by improving fuel economy and reducing emissions, while providing consumers affordable cars and trucks that meet their needs. A no compromises solution.

CAFE 2025

CAFE 2025: Fuel Economy and Emissions Standards for 2025 (Image created by the White House)

OPGCI: An Evolution that Revolutionizes the Internal Combustion Engine

By Fabien Redon and Steve Ciatti

Automakers are working hard to meet pending fuel economy and emissions standards, like CAFE 2025 (54.5 mpg), and investing heavily in new technologies such as electric vehicles and their variants. According to a Frost & Sullivan forecast, however, 105,000,000 passenger and light commercial vehicles will be sold in 2020 – 98.6% of them with internal combustion engines.

Recently the Secretary of Energy, Dr. Ernest Moniz, announced an award to Achates Power, Argonne National Laboratory and Delphi Automotive, to develop a gasoline compression ignition (GCI) version of the Achates Power opposed-piston engine. The grant is one of the largest awarded by the Advanced Research Projects Agency – Energy (ARPA-E) in its history.

An opposed-piston, gasoline compression ignition (OPGCI) engine has the potential to be a game changer in the powertrain market, with very clean and efficient power. The combination of the two technologies could be the solution to pending emissions and fuel economy regulations and could very well be the internal combustion engine (ICE) that satisfies the challenges of ground mobility for decades to come.

The OPGCI combines proven, efficient technologies in an engine that has the potential to be about 50% more efficient than today’s gasoline engines, with comparable power; torque; noise, vibration and harshness (NVH); and, size. It does this by using the benefits of compression ignition, with a readily available fuel source – gasoline – in the highly efficient opposed-piston architecture, refined by Achates Power. Continue reading

The Reports of (Diesel’s) Death Have Been Greatly Exaggerated

Speculation about the imminent demise of diesel due to the recent VW news is frankly, overblown. Not to dismiss any aspects of the current issue, but we think that this will be a turning point for internal combustion engine efficiency and emissions, and therefore, will ultimately result in better products for consumers and cleaner air for all.

Diesel has long had a reputation as a reliable and efficient fuel. Recent advancements in emissions controls – including exhaust-gas recirculation, selective catalytic reduction, diesel particulate filters and modern computer control – have brought these to levels that, when combined with the efficiency of the fuel, make diesel engines a powerful, efficient and low emissions powertrain choice. One only has to drive the products offered by BMW, Chevrolet, Jeep, Dodge, Ford and others to see that it is possible to meet emissions standards while not sacrificing efficiency and durability.

Electric and hybrid vehicles have so far failed to be the market changer that was promised all those years ago. Costs are still too high and significant consumer behavior and infrastructure changes are still required.  Of course, cleaning up the sources of electricity on a global basis is also no small challenge and until this is done, calls into question the real cleanliness of these vehicles.  Diesel and gasoline internal combustion engines will remain the most popular option for powering vehicles for both the near and the long-term because of the high performance, low cost and the existing infrastructure.

Manufacturers are facing increased pressure to meet stringent and ever increasing global environmental standards without adding additional costs and complexity to their vehicles or manufacturing facilities. Efficiencies such as light-weighting, fuel management, exhaust treatment and others have led to significant improvements, but to meet future demanding regulations a step change in powertrain technology is necessary.

The industry needs to continue looking at ways to reduce emissions without compromising power, utility, and performance. With the Achates Power opposed-piston, two-stroke engine, we can meet the EPA 2010, Euro 6 and Tier 3/LEV 3 regulations, with an engine that is 30 percent more efficient than a comparable diesel engines and 80 percent more efficient than gasoline fueled engines.

While the VW issue is a setback for both clean air and the perception of diesel, the underlying issue is not with the diesel engine technology nor diesel fuel.  Regulators in Europe and the U.S. will increase their vigilance, VW will modify their engine controls and, most customers will not notice a significant decrease in efficiency or performance.

Change in regulations and testing will affect the industry but ultimately be a boon for the end consumer with cleaner, more efficient engines…like the Achates Engine!

The History of The Achates Engine

Meet Dr. James Lemke – a serial entrepreneur with a passion for science, engineering and education and a track record of using scientific and engineering breakthroughs to create new products and companies.
Dr. Lemke founded Achates Power in 2004 with an idea that will revolutionize the automotive industry.  His stroke of genius was recognition that advances in computer aided engineering and engine component technology enable the revitalization of the efficient opposed-piston engine.
Lemke realized that more efficient transportation solutions are required if we are to maximize the utility of the world’s finite petroleum supply, particularly in light of carbon dioxide and climate change concerns. Dr. Lemke started Achates Power with a group of engineers and scientists, and a focus on scientific and engineering fundamentals. As each technical challenge was overcome, skeptics turned into advocates.  Current measurements show a 30% fuel efficiency improvement compared to the industry’s best diesel engines and nearly double the fuel efficiency of a standard gasoline engine with everything else (power, torque, emissions, noise, cost…) held the same.


Here is the story of how “light reading” on a Mexican vacation turned into the concept and development of one of the most revolutionary engines of today – The Achates Power Engine.

Defining Success

Recently, Green Car Reports ran an article, Fortune Puts Volt in ‘Dustbin of History’ Despite New Model Coming, which discusses the “success” (or lack thereof) of two different plug-in electric vehicles – the Chevy Volt and the Tesla Model S.  Strangely, it doesn’t even count the Nissan Leaf or the Prius plug-in models.  But I digress.

Maybe from a consumer or environmental standpoint, the most common measure of success is the number of units sold.  But from a business perspective (and I’ll argue that is the key perspective – especially with respect to the environment), the only measure of success is profitability Continue reading

Achates Power Opens New Office in the Metro Detroit Area

Achates Power is pleased to announce our latest expansion into the Detroit metro region. While San Diego will still be headquarters for us, rapid growth and new customers have led to our expansion into the Detroit area.

Leading the expansion is our newly appointed mid-west regional manager, John S. Major Jr. John, who has been with us since March 2015, brings a mix of small company leadership and big company engineering and program experience to our leadership team. John will be responsible for initiating, building and growing our customer application engineering capability in southeast Michigan. Continue reading

AUSA 2015

Heading into the Association of the United States Army’s (AUSA) Global Force Symposium and Exposition last week in Huntsville, Alabama, I had no idea what to really expect from the event this year. This was my seventh AUSA, spread amongst three different locales, and I still have several stacks of business cards in my office from these events from years past, and the days of MRAP.

While normally at conferences such as this, I am prepared to offer published data to answer questions regarding Achates Power technology, but this time we were able to offer additional validation – the announcement of our $14 million dollar military engine project. Continue reading

Achates Power Wins $14 Million Military Engine Project

Achates Power is pleased to announce today that we have been awarded a $14 million project by the National Advanced Mobility Consortium to support research and development work of the U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC). The project, the Single Cylinder Advanced Combat Engine Technology Demonstrator, is part of the Army’s 30 year strategy to modernize tactical and combat vehicles. This new project builds upon the technological advancements and proven results we delivered during our previous TARDEC project in 2012 for the design and construction of the Next-Generation Combat Engine.

We are also excited to announce that we have partnered with Cummins Inc. for this new chapter of modernizing the Army’s combat vehicles. The end goal is to deliver a product that improves the mobility and performance of combat vehicles. Continue reading