Designing an Opposed-Piston Engine for Light-Duty Applications

Regulatory agencies and consumers are demanding a reduction in CO2 emissions—putting greater pressure on auto manufacturers to enhance overall vehicle efficiency. What some don’t realize, however, is that the opposed-piston, two-stroke (OP2S) engine can provide reduced fuel consumption and low emissions without added cost and complexity. In fact, Achates Power has already demonstrated a 21% cycle-average and 15% best-point advantage versus the leading medium-duty diesel engines. But, do these same efficiency benefits extend to light-duty applications?
While heavy-duty vehicles are expected to achieve million-mile durability over the most highly loaded duty cycles, light-duty cars and trucks have to meet cold-start emissions and operate under a broad range that includes low-speed, low-load conditions. They also have to account for stringent noise, vibration and harshness (NVH) requirements.


The Achates Power OP4 light-duty engine features a four-piston (two-cylinders), two-stroke design with a swept volume of 1.5 liters.

At this year’s SAE High Efficiency IC Engine Symposium, Achates Power presented the results of an in-depth study on the OP4™: a four-piston (two-cylinders), two-stroke diesel engine with a swept volume of 1.5 liters. This powertrain—which has a 75.7 mm bore, 166.6 mm stroke and a 2.2 stroke-to-bore ratio—meets LEV III emissions with typical aftertreatment and Euro 6 without NOx aftertreatment. Its best point fuel consumption is 189 g/kW-hr and it delivers nominal power of 96 kW (129 hp) at 4000 RPM, maximum torque of 325 N•m (240 lb-ft) at 1750-2250 RPM (achieved at 14 bar brake mean effective pressure, BMEP), and a nominal compression ratio of 16.0.
To see how the OP4 matches up to today’s leading four-stroke, light-duty diesel engines, we selected the Mercedes-Benz 1.8 liter OM651 Euro 5 as the performance benchmark. Compared with this engine, the OP4 delivers a significant fuel economy advantage—in this case, a 13% cycle-average reduction in fuel consumption—while meeting Euro 6 requirements without selective catalytic reduction (SCR). As we have described in previous posts, such as the Heat Transfer Advantage of Opposed-Piston Engines and Why a Two-Stroke Engine?, there are several reasons for this. One that is easy to see is that the OP2S has a 37% lower surface area-to-volume ratio of the combustion chamber, leading to less heat transfer. And, throughout the emissions-sensitive operating range, the OP4 achieved sub 0.4 ikW-hr and met NOx emissions levels without NOx aftertreatment for Euro 6 standards.
Light-Duty Fuel Consumption Comparison

The Achates Power OP4 has a much flatter fuel map as compared with the Mercedes-Benz OM651. When modeled, it shows a 13% cycle-average fuel consumption advantage.

In a separate blog post and technical paper, we have demonstrated how our engine can achieve rapid catalyst light off after a cold start. In catalyst light-off mode, the OP4 engine produced exhaust gas temperatures of 410° C at idle—120% more than the baseline condition—while generating low NOx, low soot, low combustion noise and good combustion stability.
In addition to low combustion noise, the OP4 showed very low vibration, better in some respects than the OM651.
The results of this study demonstrate that the Achates Power OP2S efficiency advantages easily translate to light-duty applications. And, knowing that this engine—like our medium- and heavy-duty powertrains—can respond to current, and future, fuel efficiency and emissions regulations, gives auto manufacturers a low-cost, high-performance solution to regulatory and consumer demands.

12 thoughts on “Designing an Opposed-Piston Engine for Light-Duty Applications

  1. To develop smaller OP engines is very promising because of the high fuel saving potential in this market segment.
    Do You think about adding cylinder shutdown capability to Your engine? I understand it could make problems with the turbocharger being not fed enough or too intermittently. What is Your line of thought as of yet?

    • Dominik:
      Thank you for your question. In order to answer it, we have to study at least two things:
      1) What is the motive; what benefit can be realized?
      2) What are the issues with cylinder deactivation; is there anything specific to the OP2S?
      Cylinder deactivation on a diesel engine increases the specific load; therefore, the operating point shifts into more fuel efficient islands on the brake-specific fuel consumption (BSFC) map. That is the benefit. If you study the BSFC map above, you will see the OP4 engine has broader minimum BSFC islands and starts at lower loads. The OM651 Mercedes-Benz engine’s minimum fuel consumption areas are toward the upper end. More specifically, if an 1800 rpm 100Nm load would represent steady-state road load, cylinder deactivation would increase the load to 200Nm (assuming half of the cylinders were deactivated). The Achates Power OP4 engine BSFC map does not show a significant difference while the OM651 BFSC map shows a 35g/kWh (230->200) deduction.
      Now let’s examine any OP2S-specific issues from the turbocharger perspective. The reference engine in the article is the Mercedes-Benz OM651 four-cylinder engine. If two cylinders were deactivated, the remaining two will produce a firing sequence the same as an OP2S with one cylinder (a two stroke has twice the firing frequency as a four-stroke engine); therefore, the pulses will be similarly spaced. The same is true if you assume a six-cylinder, four stroke with two cylinders deactivated will produce the same firing frequency as a three-cylinder OP2S with one cylinder deactivated. This is not different from a four stroke, which means the OP2S would suffer the same level of turbine efficiency drop as the four stroke because of highly pulsed intermittent flow.
      One major advantage of the OP2S is that every cylinder is naturally balanced—noise, vibration and harshness (NVH) concerns because of engine configuration or crankshaft geometry will not influence cylinder deactivation.
      Fabien Redon

      • Thank You for Your answer.
        I find this comparatively large efficiency plateau intriguing. It is shifted to lower speed. This is probably because of scavenging issues coming at higher rpms. But the shift to lower torque is astonishing. I would naively expect lower peak temperature there and therefore lower thermodynamical efficiency. You got even a decrease in efficiency at the high torque end.
        Could You drop a line about how this large efficiency plateau – and its low-torque-shift – is connected to the specifics of the OP principle or Achates-specific design decisions?

        • Dominik:
          There are several factors that shape the BSFC iso-lines, including OP2S design-related and calibration-specific factors. Everything starts with indicated thermal efficiency. After accounting for pumping and friction losses, the brake thermal efficiency (or BSFC) map is formed.
          Let’s start with the easy one: there is no characteristic difference in the friction map between a four-stoke and OP2S engine.
          The OP2S engine has a different pumping loss characteristic than conventional four-strokes, which affects the BSFC iso-line shapes. Pumping loss depends on intake and exhaust port restriction (the intake and exhaust areas are significantly larger on the OP2S than on a conventional, four-stroke engine, which has valves), fresh air and EGR requirements, trapped cylinder conditions, and turbocharger and supercharger sizing, which defines the work split between those two. The pumping loss is really a mechanical loss, equivalent to the work required to drive the supercharger.
          Obviously, the indicated thermal efficiency is also influenced by these factors, which show the highest values in lower speeds and loads. The higher indicated efficiency is really coming from lower heat transfer (higher surface-to-volume ratio) and leaner and shorter combustion.
          When all of these are added together, it forms BTE or BSFC iso-lines peaking at lower speeds and loads compared to conventional four-stroke engines.
          David Johnson, CEO of Achates Power, will present at the SAE Heavy Duty Vehicles Symposium (September 30 – Exceeding SuperTruck Efficiency Goals with Opposed-Piston, Two-Stroke Engines) where you can see the BTE, friction and pumping loss map.
          Fabien Redon

  2. Your brake horsepower output projections are fantastic. If I read correctly and translate to common terms and extrapolate, you are postulating (extrapolating your predictions) 150 BHP (112 Kw) at approximately 3 (US) Gph? Also I notice your mass through put seems to indicate an approximate 400 percent airflow in excess of stoiciometric. please indicate whether or not I have understood the situation correctly and correct any misunderstandings, Thank you, John B

    • John:
      To answer your question:
      1) Fuel Volume Flow
      3 gph fuel volume flow while the engine is producing 150 hp (112 kW) would mean 84.8 g/kWh brake-specific fuel consumption (BSFC), which is slightly above 100% break thermal efficiency. The BSFC map we presented shows approximately 250 g/kWh at rated output. If you assume the same BSFC for 150 hp (112 kW) power, the fuel flow is 28 kg/h, which equates to 8.83 gph volume flow (for your reference: If you use best-point fuel efficiency, which is 189 g/kWh at approximately 1100 rpm and 100 N•m, this volume flow becomes 0.68 gph.
      2) Air Mass flow
      On page 10 of the presentation that I made to the SAE High Efficiency IC Engine Symposium (, AFR is stated as 27.1 at 1500 rpm and 25% load, which is 1.84 times more than stoichiometric combustion would require. The light-duty concept described in the presentation has an air system very flexible to optimize the calibration under a wide range of operating conditions. This allows us to run a more balanced AFR between part load and full load.
      Fabien Redon

  3. You should look at the spark assisted version. It has many advantages.

      • Andrew:
        Thank you for your interest. Once you subscribe to the blog, and reply to the confirmation email (which it looks like you have done), you will automatically receive an email once new postings are made.
        Kendra DeWitt

    • Green energy is deefnitily the best solution in most cases. Technology like solar energy, wind power, fuel cells, zaps electric vehicles, EV hybrids, etc have come so far recently. Green energy even costs way less than oil and gas in many cases.

  4. Just interested in keeping up to date . Would appreciate comments regarding Univ of Wisconsin double fuel approach ,but applied to opposed piston.

Leave a Reply

Your email address will not be published. Required fields are marked *


This site uses Akismet to reduce spam. Learn how your comment data is processed.