The Top Ten* Reasons the Opposed-Piston Engine is Naturally Low in NOx – *Updated with an 11th Reason

For a Chinese language translation, please click here: 对置活塞发动机氮氧化物排放更低的十大根本原因

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.

There are two key components for controlling NOx emissions: limit their formation during combustion, and to convert the NOx after combustion into nitrogen, water, and CO2, natural components of the air we breathe. The dominant technology in diesel engines to reduce NOx is selective catalyst reduction (“SCR”). SCR is an emissions controls system that injects a liquid reductant – usually automotive grade urea, known as Diesel Emissions Fluid (DEF) – through a special catalyst into the engine exhaust stream.

1 – Rapid Catalyst Lightoff

To be effective, however, the SCR catalyst must be hot – above 200° C.  In some operating modes engines produce the necessary heat to keep the catalyst operating in its efficient zone. Unfortunatley, in too many operating modes, this isn’t the case. Low-load operation and cold starts are two key problem areas for current technology. Nearly all the NOx emissions in a diesel engine on a regulatory test cycle occurs in the first 600 seconds after a cold start.

Sharp,  Webb, et al of Southwest Research (“SwRI”) found that “a typical 2010 and later emissions system [on a heavy duty truck engine] does not…reach high rates of [NOx] reduction (i.e. better than 95%) until the half-way (600 second) point of [the cold start federal test procedure] cycle.”

The OP Engine has a distinct and significant advantage in achieving rapid catalyst lightoff.  Kalebjian, Redon, and Wahl of Achates Power describe the strategy.   During catalyst lightoff mode, the engine runs at a low delivery ratio to increase internal residuals and trapped temperature.  A sufficiently high intake manifold pressure is supplied by a crankshaft driven supercharger to achieve adequate air-fuel ratio and good combustion stability.  Split and late injections create a significant exhaust temperature rise.  Patil, Ghazi, et al of Achates Power and SwRI document test results:   peak NOx reduction occurs in less than 100 seconds on the OP Engine.

The chart to the right shows engine out temperature.  The green line is for a baseline heavy-duty truck engine from the 2017 SwRI study.  The redline is for the same engine with calibration modifications to increase cold start exhaust temperature.  The blue line is for an OP Engine tested at Achates Power.  The OP Engine achieves and maintains 200° C exhaust temperature in about 40 seconds, vs. about 400 seconds for the modified conventional engine.




2 – Low Engine Out NOx During Cold Start Catalyst Lightoff

Because the SCR is ineffective until lit off, creating low engine-out NOx before lightoff occurs is essential to reducing tailpipe NOx.  The OP Engine has a distinct and significant advantage here, too.  The high trapped residual content that helps quickly elevate exhaust gas temperature also serves as an internal, or natural, exhaust gas recirculation (more on this later) to mitigate NOx formation.   In the same paper, Patil, Ghazi, et al show cumulative engine-out NOx after a cold start.

The OP Engine (blue line) has a fraction of the cumulative NOx of the baseline (green) or modified (red) heavy duty conventional engines.





3 – Internal Exhaust Gas Recirculation

Exhaust gas recirculation (“EGR”) is a common technique to control NOx formation in diesel and an increasing number of gasoline engines.  NOx is primarily formed when a mixture of nitrogen and oxygen are subjected to high temperatures.  Mixing exhaust gas in the combustion chamber dilutes the oxygen in the incoming air stream and provides gases inert to combustion to absorb combustion heat and reduce peak in-cylinder temperatures.

In conventional engines, exhaust gas is recirculated: it is pumped out of the engine during the exhaust stroke, routed back to the inlet manifold, and inducted back into the combustion chamber to be mixed with fresh air.

OP Engines typically feature EGR, too, but they have an additional advantage:  internal, or natural, EGR[i].

The OP Engine has intake ports on one side of the cylinder (on the right, in the image below) and exhaust ports on the other for efficient uniflow scavenging.  Unlike a conventional engine, the pistons in an OP Engine do not pump air out of an engine or induct it in.  Rather, as the pistons reciprocate, they cyclically open and occlude the ports enabling gas exchange.  The exhaust piston typically leads the intake piston by a few degrees of rotation and opens first for blow down.  When the intake ports are opened, air from the pressurized intake manifold pushes exhaust gas out the exhaust ports.  After both sets of ports are closed the trapped charge is compressed.  A positive pressure differential must be maintained between intake and exhaust manifolds for proper scavenging.  The pressure differential is typically maintained by a positive displacement pump (e.g. a supercharger).  The amount of exhaust gas that is scavenged depends on the pressure differential between intake and exhaust.  High load operation requires a full charge of fresh air to mix with the full fuel charge, so the positive displacement pump pressurizes the intake manifold to fully scavenge (or even over scavenge) the cylinder.  At low loads, however, only a small amount of fuel is injected so only a relatively small amount of fresh air is required.  In this case, the positive displacement pump can minimize its work while also minimizing the pressure differential.  The results in some portion of the exhaust gas retained in the cylinder.  Depending on the operating condition and fuel, a substantial portion (>50%) of the exhaust gas can be retained, and external EGR can be added to further increase the EGR ratio.

This ability to decouple cylinder scavenging from piston motion to partially scavenge the cylinder has many benefits.  One benefit was described above – at cold start, it is used to help rapidly elevate exhaust gas temperatures because the hot exhaust gas is not fully diluted with cool fresh air.  Another benefit is it results in reduced pumping work at low loads, enabling a relatively flat fuel consumption map that leads to high cycle average efficiency.

Germane to this topic, the combination of internal and external EGR enables to OP Engine to achieve high EGR rates with little pumping work.  The high rates of EGR inhibits NOx formation enabling a naturally low NOx engine.  The Sharp, Webb, et al paper notes that once the catalyst is lit off, engine out NOx emissions can be as high as 3-4 g/bhp-hr and still be reduced to less than 0.02 g/bhp-hr tailpipe emissions because of high SCR efficiency.  As a naturally low NOx engine, the OP Engine has virtually no efficiency trade-off to achieve this level of engine-out NOx.

Mattarelli, Cantore, et al note this benefit of OP Engines: “NOx emissions are expected to be lower, since the fresh charge is always diluted by burnt gas.”

4 – High Low Load Exhaust Gas Temperature

As noted in the introduction, it is becoming increasingly clear that in many circumstances, real world NOx emissions are much higher than those measured on regulatory cycles.  One reason for this is that commercial vehicles cycles feature generally high-loads – the normal operating mode for long haul trucks.  But when those trucks are driving in urban areas or are in slow traffic they operate at lower loads, and generate less exhaust heat.  The SCR catalyst, then, can cool below effective temperatures.  The ability to partially scavenge the cylinder – describe above – also means that low-load exhaust gas temperatures are relatively high to maintain SCR effectiveness.  Moreover, using the catalyst lightoff mode, exhaust gas temperatures can be quickly elevated if necessary to maintain SCR effectiveness.

The chart to the right shows turbine-out temperature of an OP Engine across a speed load map.

5 – Low/High-Load Exhaust Gas Temperature

You will also notice in the chart to the right that high-load exhaust gas temperatures are generally lower than conventional engines, rarely exceeding 400° C.   In low-load conditions, we note that the OP Engine has an advantage in scavenging less than the full cylinder. In high-load conditions, the OP Engine can scavenge the full cylinder.  In doing so, some cool, fresh air short-circuits – it goes all the way from the intake to the exhaust, reducing the exhaust gas temperature. If desired, the OP Engine can even over-scavenge to deliberately cool the exhaust gas.  Aftertreatment system degrade with temperatures, and exhaust gas temperatures of conventional engines can exceed 700° C.  Lower peak temperature has numerous benefits:

  • Catalysts will stay effective longer, enabling lower tailpipe emissions across the life of the vehicle.
  • New catalyst formations – that do not have to withstand very high temperatures – can be formulated to be more efficient and/or lower in cost.
  • Catalyst size can be reduced (since it will stay effective longer), leading to a substantial cost savings.

6 – Reduced Volumetric Flow

Aftertreatment systems are sized to handle the exhaust flow at peak loads.  Because the exhaust gas of the OP Engine at peak load is at a substantially lower temperature compared to a conventional engine (and because cooler gas is more dense) the volumetric exhaust flow of an OP Engine is about 30% smaller than that of a comparable conventional engine.  This means that the aftertreatment system will be more effective (oversized, in effect) or the aftertreatment system can be reduced in size for substantial cost savings, or, most likely, some combination of the two benefits can be achieved.

7 – Lower BMEP

Brake mean effective pressure (BMEP) is the average pressure on the piston across an engine cycle.  OP Engines are two-stroke engines, so each cylinder has a power stroke on every revolution of the engine.  This provides an inherent power density advantage towards OP Engines compared to conventional engines.  This advantage can be used in two ways – to reduce displacement, or to reduce BMEP.  In general, both occur.  For example, to replace a 6.7L conventional medium-duty engine, Achates Power would typically design a 5.0L, two-stroke OP Engine.  Instead of reducing displacement by 50% (cutting it half), the OP Engine has 25% lower displacement.  This also means it has lower BMEP than the engine it replaces, as noted by Warey, Gopalakrishnan, et al:  “[OP] engines have leaner combustion, more optimal combustion phasing, lower BMEP and potential for lower mass.”   NOx rises with BMEP, so lower BMEP helps control NOx formation, further contributing to naturally low NOx nature of the OP Engine.

8 – Exhaust cut off

Another reason real-world NOx emissions in many circumstances is much higher than measured on regulatory cycles is that the SCR catalyst is being actively cooled when a vehicle with a conventional engine is decelerating.  Fuel flow is cut off, and the engine is motored by the vehicle inertia.  Nevertheless, a full charge of cool fresh air is inducted into the cylinder and pumped into the exhaust, cooling the catalyst.  In an OP Engine, however, since cylinder scavenging is independent of piston motion, air flow through the engine can cease during decelerations, substantially improving thermal inertia of the SCR catalyst.

9 – Low fuel consumption

There is a well known, inverse relationship between fuel consumption and NOx formation.  NOx can be reduced…but at the expense of increased fuel consumption (and CO2 emission).   Numerous design and operating variables affect this tradeoff, including compression ratio, EGR rate, and injection timing.  Indeed, Sharp, Webb, et al find that reducing engine-out NOx invariably comes at the cost of increased CO2 emissions.

In addition to the numerous NOx advantages outlined here, the OP Engine has significant and inherent efficiency advantages – summarized above – that provides a larger trade space in controlling NOx emissions.

10 – Low particulate matter

There is also a well-known, inverse relationship between NOx and particulate matter (“PM”) formation – one increases at the expense of the other.

Once again, the OP Engine has a significant advantage in having an inherently low PM engine, broadening the NOx-PM trade space.  Two primary features of the OP Engine enable low PM operation.  First, an OP Engine typically has two diametrically opposed injectors, spraying across the bore.

The dual sprays provide a large surface area between air and fuel.  Little fuel is sprayed into the crevice volumes between piston & skirt.  Momentum of the two sprays largely cancel to avoid wall wetting.

Second, the two piston crowns of the OP Engine can be shaped to provide a favorable set of air dynamics.  Vane angles on the intake ports create swirl of the intake air, to help the scavenging and mixing.  The shapes of the two piston crowns convert the swirl into tumble at the moment of autoignition, creating highly turbulent kinetic energy that shears any agglomerated fuel particles, and mixes the air and fuel to provide locally lean mixtures for low PM formation.

11 – BONUS – Close Coupled Light-Off SCR

At the 2018 Opposed Piston Engine Symposium, Chris Sharp of Southwest Research pointed out an 11th and, as he noted, often understated low NOx advantage of the OP engine: the ability to closely couple a light-off SCR.   A light-off SCR is placed in front of the rest of the aftertreatment system to provide NOx reduction before the main SCR is lit off.  In order for the light-off SCR to reach effective temperature quickly, it is placed as close to the engine exhaust (or ‘closely coupled’) as possible.  As Chris noted, the OP Engine profile allows for easy packaging of a close-coupled light-of-SCR.






Countries around the world are struggling with the desire to reduce smog and other adverse health and environmental effects of transportation while at the same time aiming to reduce energy consumption to reduce climate change impact and enhance energy security.  Because of the inherent trade-off in NOx vs CO2, these two goals once seemed incompatible.

Remarkably, and fortunately, the Opposed-Piston Engine has significant, distinct, and demonstrated advantages in reducing NOx and CO2 simultaneously.  The California Air Resources Board recently issued a grant for an industry project team to optimize an Achates Power Opposed-Piston Engine design for a heavy-duty truck in order to achieve a 90% reduction in NOx and a 15% reduction in CO2 at the same time.

In addition to Achates Power, aftertreatment experts SwRI, Corning, BASF, and Faurecia are part of the project team.  Testing on prototype OP Engines combined with sophisticated aftertreatment models using aged catalysts suggest the heavy-duty OP Engine may achieve tailpipe NOx emissions of 0.01 g/bhp-hr on the FTP cycle, 95% below the current U.S. standard (which is already the toughest in the world) and below even the toughest optional California standard for ultralow NOx.  While achieving this unprecedented low level of NOx, the engine is also expected to be well below the EPA 2027 CO2 limit of 432 g/bhp-hr.  Detailed engine and vehicle tests and demonstrations will take place in 2019.


[i] It seems odd to call it exhaust gas recirculation because the exhaust gas is not recirculated…it is simply retained in the cylinder.  Nevertheless, EGR is so commonly used that the term ‘internal EGR’ is readily recognizable.  Bicycle riders might recognize a similar phenomena in their sport – pedals with clips are called ‘clipless’.

10 thoughts on “The Top Ten* Reasons the Opposed-Piston Engine is Naturally Low in NOx – *Updated with an 11th Reason

  1. Hello,
    First hx for all the great work at Achates even though I work for a major 4 stroke diesel Mfg company in Illinois I am a big fan of all you guys are doing in IC engines.

    However I strongly disagree with your assertions regarding BMEP in point 7. I do not believe you can have a truely similar 2 stroke if BMEP is not similar. BMEP is essentially torque to the dyno divided by bore area times pistons . It is calculated over the ful engine cycle be it 360 or 720 deg crank angle. Assertions linking this to NOx are poorly founded. NOx is driven by combustion temperature, 3/4 of an engine cycle have little to do with it. Point 7 would make much better if you substitute PCP (strong link to combustion temp) for BMEP.

    If you disagree please humor me with an explanation if you could. I am always trying to learn.

    • Tim,

      Apologies for the delay in responding, will have an answer for you asap.

    • Hello and thank you for your question and for your support! It’s always good to hear from an engine expert such as yourself. Yes, you are right – if everything is equal between a two-stroke and a four-stroke (compression ratio, EGR, trapped mass, trapped temperature, etc.), peak cylinder pressure would be indicative of NOx emissions. However, as the two engines are different, there are many contributing factors to why lower BMEP results in lower NOx signature of the OP Engine – the primary reason being the firing frequency of the two-stroke vs four-stroke. First, it’s important to keep the comparison between a two-stroke and a four-stroke on equal footing. Mean effective pressure is (torque / volume) x number of crank revolutions for each power stroke (2 for 4S, 1 for 2 S – Heywood, J.B., “Internal Combustion Engine Fundamentals”, McGraw-Hill, Inc., New York, ISBN 0-07-028637-X, 1988). Torque or power is really what we need to keep constant between the two engine comparisons as that is what is required to move a vehicle or generate electricity. Everything else being equal, for the same torque output, BMEP is 2x higher for 4S due to the number of crank revolutions required for each power stroke. For a given compression ratio, displacement, EGR rate, and boost level, 2S PCP will be lower since reduced fuel mass is required to achieve lower BMEP (but same engine torque due to a combustion event occurring every revolution). This reduces combustion temperatures through lower flame temperatures and thus creates lower NOx formation. This is also suggested in SAE International’s “Opposed Piston Engines” book on page 531 – 532 (Pirault, J.-P. and Flint, M., “Opposed Piston Engines: Evolution, Use, and Future Applications”, SAE International, Warrendale, PA, ISBN 978-0-7680-1800-4, 2009, doi: 10.4271/R-378). We are effectively referring to PCP when we discuss BMEP, however the lower BMEP is the cause of the lower PCP. Per your suggestion, if BMEP between a 2S and 4S was kept constant, the torque of the 2S will be higher – this is not an equal comparison as the engine power output is different. But now things start getting complicated since compression ratio, boost, EGR, and displacement are all different between our 2S and 4S engines. High rates of EGR, for example, can cause higher PCP and simultaneously lower NOx. Other factors (e.g. compression ratio) increase both PCP & NOx at the same time. We stand by the general principle that when comparing two engines, the one with the lower (normalized) BMEP has an advantage is achieving lower NOx operation. Thank you for your thoughtful question and for your interest!

  2. Hi,
    When will you next be able to make public progress announcements on the Peterbult low NOx program? I think class 8 trucks appear to be a wonderful application for OP mass & ITE advantages with or without low NOx. Packaging constraints, market size and fuel cost savings could be very compelling. How will you demonstrate durability?

    • We had an event on November 8 with details on the heavy-duty engine (press release) These demonstration vehicles will run in fleet service for Walmart and Tyson Foods in late-2019 and early-2020. We’ll continue to share progress and updates on the demonstration program over the course of the program.

  3. Andrew ; Any consideration to differing fuel types,as new fuel specs are coming on board,considering renewable bio-fuel specs,differing from fossil (diesel) fuels?

    • Al, we’ve done work with and/or studied a variety of fuel options (bio-fuel, DME, Natural Gas, JP8) in addition to “pump” diesel and now gasoline. In general, there are no issues with using other fuel sources in our engine – there is engineering work that would need to be done to make the switch. We can and do work with our engine manufacturer customers to meet their fuel specifications.

  4. Thank You , Andrew.
    Was aware that multi-fuel capability is a reality in OP engine specifications. My remarks are focused on the fact that some renewable fuels,based on production technologies,can increase cetane ratings on the same Testing fuel requirements, at prox,56, could be possible to increase to as much as a 76 rating, and extremely low NOx emissions at the pump. Imagine how that would dramatically reduce these issues, and potentially allow increased beneficial design upgrade criteria in the 2S current design. There are such Bio-fuels in Beta testing,ready for production.
    Just some thoughts for consideration from an old H D Sales Guy,who replaced many an Electric motor .with an I C Diesel ,variable speed , engine, pumping Ag Irrigation systems, in CAlif. Central Valley.
    Thank you . Big Al B

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