55% thermal efficiency: Two-stroke opposed-piston diesel engine

Under the global carbon neutrality trend, commercial vehicles, as a key area of fuel consumption and carbon emissions, are facing unprecedented pressure for technological upgrades. Although traditional four-stroke diesel engines have been optimized for over a century, they are still constrained by inherent structural bottlenecks. Achates Power has developed a modern opposed-piston two-stroke diesel engine (Opposed-Piston Two-Stroke, OP2S). Based on a 4.9L three-cylinder OP2S engine model (including a three-dimensional computational fluid dynamics model, a one-dimensional model, and a friction model), a correlation analysis was conducted with the measured data from dynamometer tests. The calibrated model was then extended to predict the performance of a 9.8L three-cylinder heavy-duty OP engine. In terms of intake system design, an analysis of port optimization revealed that smaller ports can lead to better scavenging performance. The combustion system was optimized using experimental design methods. Computational fluid dynamics predictions show that at a partial load condition of 1200 rpm, applying thermal barrier coatings on both sides of the piston bowl can achieve an indicated thermal efficiency of 58.4%; the brake thermal efficiency at this condition reaches 55%. At the rated condition (engine speed of 1800 rpm), the brake thermal efficiency is 48.1%; while at the peak torque condition (1200 rpm), the brake thermal efficiency can reach 50.8%. The 9.8L three-cylinder opposed-piston two-stroke engine (OP Engine) achieves a brake thermal efficiency of 55% without relying on waste heat recovery systems (WHR) or turbo-compounding technology, while meeting strict exhaust emission targets. This achievement provides a low-cost and high-efficiency solution for commercial heavy-duty vehicles through the optimization of engine structure and combustion system.

I. The Modern Rebirth of a Century-old Technology

The opposed-piston engine is not a new concept. Its history can be traced back to the Junkers Jumo 205 aviation diesel engine in the 1930s. However, Achates Power has revolutionized it by addressing the inherent flaws of traditional designs:

- No cylinder head and valve train: By eliminating the cylinder head, camshaft, valves, and over 200 other components of a conventional engine, mechanical losses are reduced by 30%, and manufacturing costs are cut by more than 15%.

- Efficient scavenging system: The traditional four-stroke engine's cylinder head is removed, and the combustion chamber is formed by two opposing pistons. The intake and exhaust ports are controlled by the ends of the pistons, achieving unidirectional and uniform scavenging. The gas exchange efficiency is increased by 40% compared to traditional two-stroke engines, while heat loss through the cylinder head is reduced.

- Thermodynamic advantages: The surface area to volume ratio of the combustion chamber is reduced by 25%, minimizing heat loss. With a high compression ratio and thermal management technology, the compression ratio is raised to 21:1. By optimizing the piston volume and combining intake pressure control, efficient combustion is achieved, with an indicated thermal efficiency (ITE) of 58.6%.

- Thermal barrier coating (TBC): A ceramic coating is applied to the piston bowl surface, reducing heat loss from the combustion chamber to the cooling system and improving thermal efficiency by approximately 2%.

II. Core Design: Perfect Balance of Efficiency and Power

1. Breakthrough in Inline Piston Architecture

With a dual-crankshaft-driven architecture, the two pistons move in opposite directions within the cylinder and are synchronized through a gear system. This design offers three major advantages:

• Compact layout: The engine width is only 60% of that of traditional models, making it suitable for existing commercial vehicle chassis.

• Low vibration characteristics: The inertia of the pistons cancels each other out, reducing the overall vibration by 50%, enhancing comfort and durability.

• Flexible power output: By adjusting the gear transmission ratio, high torque (suitable for heavy trucks) or high rotational speed (suitable for power generation equipment) can be achieved.

2. The Secret to the Efficiency of Two-Stroke Cycle

Compared to four-stroke engines, the two-stroke design enables each cylinder to complete one combustion cycle per revolution. Combined with the following optimizations, performance is significantly enhanced:

• High power density: With the same displacement, the power output is increased by 60%, and a 3-cylinder 1.6L model can reach a power of 160kW.

• Precisely controlled scavenging process: Through CFD simulation optimization of the air passage timing, the scavenging is completed within the crankshaft angle range of 145° - 230°, with scavenging efficiency reaching over 95%.

• Low pumping loss: Only partial scavenging is required to maintain combustion efficiency, and the pumping loss is reduced by 30% compared to four-stroke engines.

3. The Golden Ratio of Key Parameters

• Stroke/cylinder diameter ratio 2.4: Through multi-physics field simulation verification, this ratio achieves the optimal balance of heat loss, friction loss and pumping loss.

• Three-cylinder configuration: Avoids the turbine energy interruption problem of 2-cylinder models and the exhaust interference of 4-cylinder models, achieving efficient operation in all working conditions.

• 2000bar high-pressure common rail: Dual injector layout, combined with a proprietary nozzle design, with fuel atomization particle size less than 8μm, and the combustion duration is shortened to 10-19° crankshaft angle.

III. Application Prospects in the Commercial Vehicle Sector

The OP2S engine has demonstrated its adaptability in various scenarios:

• Heavy-duty trucks: The 4.9L three-cylinder model can output 480kW of power, reducing the weight by 300kg compared to the same power diesel engine, and saving 12,000 liters of fuel annually.

• Construction machinery: The high torque density feature enables the engine to be 40% smaller in size, enhancing the overall mobility of the machine.

• Hybrid power systems: When used as an energy booster, it covers 80% of the working conditions in the high-efficiency range, reducing the system's total fuel consumption by 25%.

The practical experience of Achates Power has proven that by deeply reconfiguring traditional internal combustion engines, significant performance potential can still be unleashed. This opposed-piston two-stroke diesel engine, which combines century-old wisdom with modern technology, not only provides a practical path for cost reduction and efficiency improvement in the commercial vehicle industry, but also builds a sustainable technological bridge during the transition to electrification. (Note: The technical data in this article is derived from the SAE International paper "Developing a 55% BTE Commercial Heavy-Duty Opposed-Piston Engine")