|Achates Power says that its 2-stroke OP engine offers higher best point brake thermal efficiency than diesel or gasoline engines. Source: Achates Power. Click to enlarge.|
In December 2012, the US Army Tank Automotive Research, Development and Engineering Center (TARDEC) awarded Achates Power, Inc., developer of a two-stroke, compression-ignition (CI) opposed-piston (OP) engine, and AVL Powertrain Engineering, Inc. a $4.9-million contract for design and construction of the Next-Generation Combat Engine. (Earlier post.)
TARDEC, commented David Johnson, Achates President & CEO, in general has used COTS (Commercial Off-The-Shelf) engines, but is now looking for better fuel efficiency and is funding its own development activity; the solicitation won by Achates and AVL is a big departure for them, Johnson said. “We won on our data. The proposal we submitted with AVL had a substantial set of data [for the engine].” Achates Power has accumulated more than 3,600 hours of dynamometer testing and achieved 47.5% brake thermal efficiency with its OP engine. (The Achates/AVL project was the sole award from the solicitation.)
In its spec, TARDEC was looking for a scalable engine platform that offered about 70 hp/L and less that 0.36 lbs/hp-h fuel consumption, Johnson said. Achates and AVL are basing their work for TARDEC on the Achates A48 engine gear design architecture; the task, said Johnson, is basically building a multi-cylinder engine using everything the company has developed and tested to date for its single-cylinder test cell engine. The project is due to be completed in 27 months, Johnson said, adding that “I’m convinced we’ll beat it.”
In addition to the TARDEC work, Achates has two other programs in play, Johnson said, one for commercial vehicles, and another for non-wheeled vehicles.
Opposed-piston engines. Opposed-piston engines have been around since the Junkers Jumo 205 diesel aviation engine in the 1930s. Each cylinder has two facing pistons that come together at top dead center and move outward upon combustion. Since the pistons cyclically expose or occlude the exhaust and intake ports, there is no valve train. Camshafts, pushrods, rocker arms, valves, valve springs, valve keepers, etc., are not required. Because the intake and exhaust ports are at the opposite ends of the cylinder, opposed-piston engines have efficient, uniflow scavenging.
Two-stroke engines offer fuel efficiency advantages compared to four-stroke engines—the amount of fuel per cylinder displacement injected for each combustion event can be reduced to roughly half for the same power as a four-stroke engine. This provides two thermal efficiency advantages for the two-stroke engine:
A leaner operating condition at the same boost pressure maintains a higher ratio of specific heats during combustion.
A reduced energy release density at the same power level allows for shorter combustion duration without exceeding a maximum rate of pressure rise constraints.
Compared to a conventional two-stroke engine, OP engines offer better surface area-to-volume; more efficient scavenging; a larger exhaust port area; and a better ratio of expansion to compression, all contributing to higher efficiency.
Achates Power was formed in 2004 to modernize the opposed-piston engine, and is specifically targeting the commercial and passenger vehicle markets.
Achates engines. The power cylinder system comprises pistons, cylinders, cylinder liners, port designs and fuel system. Achates used CFD (computational fluid dynamics) studies correlated with single-cylinder engine test results to develop a clean and efficient combustion system.
The design results in large stoichiometric iso-surfaces with excellent mixing and charge motion at the point of auto-ignition. These combustion system attributes are accomplished with a novel set of intake ports that provide swirl, coupled with mating converging-crown pistons that induce tumble motion at top dead center (i.e., as the two piston heads approach each other). Additionally, a proprietary nozzle design provides inter-digitated fuel plumes with the appropriate flow rates and penetration. Together, this hardware combination provides for very fast burn rates, contributing to both power and fuel efficiency, Achates says.
Achates used multiple CFD and physical testing iterations of the cylinder liner to to optimize port timing, uniflow scavenging and super-charging characteristics.
|Cutaway of power module concept, showing internal gear train. Achates says the 3-cylinder combination is optimal. Click to enlarge.|
The A48 gear architecture. There are a variety of mechanical design arrangements of crankshafts and connecting rods to articulate the pistons in an opposed piston engine. Achates’ current A48 design is a dual-crankshaft, Junkers Jumo-style arrangement.
The company made improvements to the architecture to address concerns about piston thermal management, cylinder thermal management, wrist pin durability and piston ring durability. The A48 architecture uses conventionally designed crankshafts and connecting rods.
The A48 mechanism has an internal gear train, which allows the flexibility to have the power taken off of any crankshaft or idler gear. Transmission input can be varied by altering the gear ratios of the internal gear train if power is taken off one of the idler gears. If an application requires high torque, a drive ratio can be chosen to reduce speed of the engine output shaft. Other applications might benefit from increased output speed at reduced torque to drive—e.g., a high-speed generator in hybrid drive applications. The engine can serve a wide range of applications from a core engine design by altering the gear ratio of the internal gear train.
Compared to other opposed-piston architectures, Achates says, the A48 design does not suffer from excessive width or volume, yet still maintains a thermally-efficient stroke/bore ratio.
Stroke-to-bore ratio. Achates performed extensive analysis to determine the optimal stroke/bore ratio needed to maximize engine thermal efficiency. There are three main effects to consider:
In-cylinder heat transfer decreases as the stroke/bore ratio increases due to a decreased combustion chamber surface area/volume ratio during combustion. Decreased heat transfer directly leads to higher indicated thermal efficiency and reduced heat rejection to the coolant.
The scavenging efficiency increases as the stroke/bore ratio is increased; the pumping work increases rapidly after the stroke/bore ratio decreases below 2.2.
Engine friction has a non-linear dependence on stroke/bore ratio. The net effect is that the friction increases when the stroke/bore ratio exceeds a value of about 2.3, although the magnitude of the effect is much smaller than the heat transfer and pumping effects.
Based on its analysis, Achates determined that indicated thermal efficiency and pumping work benefit from a longer stroke/bore ratio; while friction work decreases until a stroke/bore value of about 2.3 and then increases as the stroke/bore is increased further. Any opposed-piston engine with a stroke/bore below 2.0 will be compromised from an efficiency and heat rejection-to-coolant basis.
The A48 engine in Achates’ test cell was developed on an 80mm bore, Johnson said; the TARDEC application will be a bit downsized.
Three-cylinder configuration. The TARDEC engine is to be scalable to meet a variety of needs; Achates and AVL are producing a multi-cylinder unit for the contract.
Achates has determined that a 3-cylinder, opposed-piston engine is optimal from a gas exchange perspective compared to 2-cylinder or 4-cylinder versions, primarily due to gas dynamic effects.
In a three-cylinder design, the scavenging events are aligned in a way that they have minimal interference with each other and still keep enough mass flow going over the cycle to provide adequate energy to the turbocharger so it operates most efficiently to compress the intake air.
In a two-cylinder configuration, however, the gas-exchange events are too far separated in time. This separation causes the turbocharger to lose energy over the cycle, which has a negative effect on the turbine’s efficiency—especially at lower loads and engine speeds. The loss on turbocharger energy has to be compensated by the crank-driven supercharger, which causes a reduction in brake thermal efficiency.
Conversely, in a four-cylinder configuration, the gas-exchange events overlap too much. This causes cross charging to occur at a point in time when hot exhaust gases are leaving the cylinder. The interruption of exhaust gas flow causes an increase in residual gas content and, therefore, a lower scavenging efficiency—leading to a reduction in thermal efficiency.
While 2-, 4-, and 5- cylinder options are all viable as part of a family of opposed piston engines, the 3-cylinder design is optimal, Achates says.