To maximize the efficiency of an engine, one must minimize losses. Heat transfer to the cylinder and piston walls is a large source of heat loss in engines. Cooling systems keep these engine components from overheating, but all that heat being carried away by the cooling system is lost energy.
Many factors contribute to the heat transfer—and energy loss—of an engine, but a major one is the ratio of the surface area of the combustion chamber to the volume of the combustion chamber during combustion. The higher the ratio, the greater the heat loss as there is more surface area to absorb the heat from combustion. Indeed, one of the main reasons that larger engines are more efficient than smaller engines is that larger engines have a lower ratio of surface area to volume (since the denominator, or volume, increases as a cubic function and the numerator, or surface area, increases as a square function).
One of the inherent thermal efficiency advantages of opposed-piston engines, as described in this technical paper, is a favorable (i.e. low) surface area-to-volume ratio. To understand the source of this advantage, consider the following thought experiment. Compare the surface area and volume of an inline 6 (I-6) engine with a comparable three-cylinder, six-piston, opposed-piston engine (OP6). In the example below, the conventional engine has six cylinders, a displacement of 6.6 liters, and a stroke/bore of 1.16, typical figures for a medium-duty engine. From the geometry, the surface area at top dead center—primarily the piston crown and cylinder—and combustion volume are calculated.
The comparable opposed-piston engine—that generates the same power and torque—has three cylinders, a displacement of 4.5 liters, and a stroke-to-bore ratio of 2.48. Since the opposed-piston engine is a two-stroke engine, it has a power stroke in each cylinder during each engine revolution. This increased power density enables a reduced displacement without exceeding peak cylinder pressure limits. Also, because of the combined motion of the opposed pistons, a high stroke-to-bore ratio is enabled without creating excessive piston speed.
Calculating the ratio of surface area to volume is straightforward: 3.2 cm-1 for the conventional engine and 1.9 cm-1 for the comparable opposed-piston engine. Eliminating the cylinder heads in the opposed-piston engine conveys a significant advantage in the ratio of surface area to volume. A significantly lower ratio of surface area to volume leads to significantly lower heat rejection to coolant, which leads to significantly improved engine efficiency.
The graph below compares the ratio of surface area to volume across a range of engine displacements. The upper curve is a conventional I-6 four-stroke ratio of surface to volume. The lower curve is the same ratio for a three-cylinder, opposed-piston engine of the same power and torque—the OP6 has a ratio of surface area to volume about 30% lower. Another way to look at it is that an opposed-piston engine has the same ratio of surface area to volume as a much larger conventional engine. For example, an opposed-piston engine sized to match the power and torque of a 4-liter conventional engine (sized to power a large auto or pickup truck) has the same ratio of surface area to volume as a much more efficient 12-liter conventional engine (sized to power a heavy-duty truck).
The favorable surface area-to-volume ratio of opposed-piston engines is just one source of inherent thermal efficiency advantage, but it is an important one that serves to improve opposed-piston engine efficiency across all engine sizes, applications and fuel types.