Some engines feature horizontally opposed cylinders, which are an extension of the V-engine, with the angle widened to 180 degrees. This design offers several advantages, including reduced height and improved balance in certain aspects. The cylinders, where the pistons operate, are cast directly into the engine block. Additionally, the block includes mountings for ancillary equipment, such as an oil filter that lubricates the engine and a fuel pump. An oil reservoir, known as the sump, is bolted underneath the crankcase.
Opposed-Piston Engines
Compared to contemporary two-stroke engines, which utilize a conventional design of one piston per cylinder, the advantages of the opposed-piston engine have become widely recognized. Notably, this design eliminates the cylinder head and valve-train, thereby reducing weight, complexity, cost, heat loss, and friction loss. This also creates a uniflow-scavenged movement of gas through the combustion chamber, avoiding the drawbacks associated with contemporary crossflow-scavenged designs. Despite later advancements providing methods for achieving uniflow scavenging in conventional piston engine designs, the opposed-piston engine remains distinctive in its reduced height.
Drawbacks and Design Complexity
However, the main drawback of the opposed-piston engine is the need to gear together the power from the two opposing pistons. This requirement adds weight and complexity compared to conventional piston engines, which use a single crankshaft as the power output. The most common layout for the opposed-piston engine includes two crankshafts, with these crankshafts geared together either in the same direction or in opposing directions.
Superior Scavenging and Piston Design
Several notable engines, such as the Koreyvo, Jumo, and Napier Deltic engines, use one piston per cylinder to expose an intake port and another to expose an exhaust port. Each piston in these designs is referred to as either an intake piston or an exhaust piston, depending on its function. This layout provides superior scavenging since the gas flow through the cylinder is axial rather than radial, and it simplifies the design of the piston crowns. For example, in the Jumo 205 and its variants, the upper crankshaft serves the exhaust pistons, while the lower crankshaft serves the intake pistons. In designs with multiple cylinder banks, each big end bearing serves one inlet and one exhaust piston, utilizing a forked connecting rod for the exhaust piston.
Modern Applications of Horizontally Opposed Engines
In the context of modern applications, horizontally opposed engines remain popular, particularly in specific niches.
Automobiles
The 4-cylinder opposed engine and the horizontally opposed 6-cylinder engine are commonly found in cars with horizontally opposed engines, prized for their balance and compact design. Horizontally opposed engine cars, such as certain models from Subaru and Porsche, are well-regarded for their low center of gravity and handling characteristics.
Diesel Engines
Horizontally opposed diesel engines are notable, especially in marine and industrial settings, where their robust design and efficiency are highly valued.
Aircraft
Furthermore, the horizontally opposed engine is widely used in aircraft, providing reliability and balance critical for flight. Horizontally opposed piston engines are a staple in opposed engine aircraft, where the reduction in vibration and improved balance are highly beneficial.
