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.

Clean Diesel Engine Emissions Engine Design

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