|The OP4 light-duty opposed-piston diesel. Click to enlarge.|
Achates Power, which is developing a family of two-stroke, compression-ignition (CI) opposed-piston (OP) engines, has designed and is developing a light-duty diesel concept engine, the OP4. The OP4 is a two-stroke, inline two-cylinder, four-piston diesel with a swept volume of 1.5 liters.
With nominal power of 96 kW (129 hp) @ 4000 rpm and maximum torque of 325 N·m (240 lb-ft) @ 1750-2250 rpm (achieved at 14 bar BMEP), the engine will meet Euro 6 and LEV 3 emissions requirements and shows modeled best point fuel consumption of 189 g/kWh. Benchmarked against the Mercedes-Benz 1.8-liter OM651 Euro 5 engine, said Fabien Redon VP, Technology Development at Achates Power, the OP4 design shows a modeled 13.5% cycle-average fuel consumption advantage. Redon presented the basics of the OP4 engine concept at the SAE High Efficiency IC Engine Symposium in Detroit.
|Fuel consumption of the OP4 (left) and the Mercedes-Benz OM651 (right). In his presentation, Redon highlighted the “flat” fuel map of the OP4. Source: Achates Power. Click to enlarge.|
Background. Founded in 2004, Achates Power is designing and developing engines based on a two-stroke, opposed-piston, compression-ignition technology. The company has demonstrated the engine and delivered validated performance results based on more than 4,000 test hours on several engine generations.
Fundamental advantages of the two-stroke opposed-piston engine compared to conventional four-stroke engines are a 30% lower surface area-to-volume ratio; leaner combustion; optimally phased and faster combustion at equivalent pressure rise rate; and no dedicated pumping stroke.
In 2011, studies found that the Achates two-stroke opposed-piston engine could support an indicated thermal efficiency of up to 53% (earlier post).
Compared to the Ford 6.7L V8 Powerstroke diesel, a medium-duty configuration of the Achates engine showed a 15% best point and a 22% cycle-average fuel consumption advantage, along with best point BTE of 48.5%, compared to 40.9% for the Powerstroke.
Achates started with a 1.06L/cylinder engine configuration, then moved up to a 1.6L/cylinder configuration as the primary development engine. In December 2012, the US Army Tank Automotive Research, Development and Engineering Center (TARDEC) awarded Achates Power and AVL Powertrain Engineering, Inc. a $4.9-million contract for design and construction of the Next-Generation Combat Engine. (Earlier post.)
The TARDEC engine, said Redon, will use a 1L/cylinder configuration. The new OP4 configuration features a 0.75L/cylinder configuration and leverages the work the company has been doing for several years on the 1.0L and 1.6L per cylinder engines (as does the TARDEC engine.)
Light-duty OP4. In developing an engine for the light-duty market, Redon said, Achates had to take into account the more stringent emissions requirements, stringent NVH requirements, and the need for a broad speed range that will support low-speed low-load with low-emissions as well as high-speed high-power requirements.
The company picked as its benchmark target 4-stroke diesel car engines in the displacement range of 1.6L – 1.8L; it settled specifically on the Mercedes-Benz OM651—a high-volume engine with a publicly available fuel consumption map—as its baseline.
|Air handling system for the OP4. Click to enlarge.|
The bore of the OP4 cylinder is 75.7mm , with a 166.6mm stroke and a stroke/bore ratio of 2.2. Compression ratio is 16.0. The engine is supercharged and turbocharged.
Compared to the OM651, the OP4 exhibits a 15% reduction in surface area; a 35% increase in volume; and a 37% reduction in the ratio of surface area to volume.
The OP4 cylinders use dual central injectors, 180° apart. The engine uses dual pilot injections and a staggered main injection. The proprietary combustion system eliminates direct spray impingement and provides high mixing.
At 1500 rpm and 25% load, the OP4 shows an indicated thermal efficiency of 52.0%, with indicated specific NOx of 0.36 g/ikWh. Indicated soot is 0.01 g/ikWh. Throughout emissions cycle load points, the OP4 demonstrated 0.4 g/kWh engine-out NOx.
In catalyst light-off mode, the OP4 showed a NOx flow rate of 0.9 mg/s, compared to 3.9 mg/s for the baseline. Exhaust temperature of the OP4 was 410 °C, compared to 246 °C for the baseline; the rate of exhaust enthalpy for the OP4 was 9.0 kW, compared to 4.1 kW for the baseline.
Given the emissions results, Redon said, the OP4 could meet Euro 6 standard when equipped with a diesel oxidation catalyst (DOC) and particulate filter, but would not require SCR. The OP4 could meet LEV 3 requirements (also presumably US EPA Tier 3) with DOC, DPF and 85% aftertreatment NOx conversion.
The engine, which features a lower part count due to the elimination of the valvetrain and cylinder head, packages in existing vehicles. Redon said that Achates Power is hoping to find some partners further to develop and commercialize the OP4.