Figure 7-1.  Radial engine.  Figure 7-2.  Horizontally opposed engine.  Opposed cylinders  Most small aircraft are designed with reciprocating  engines. The name is derived from the back-and-forth, or  reciprocating, movement of the pistons that produces the  mechanical energy necessary to accomplish work.  Driven by a revitalization of the general aviation (GA)  industry and advances in both material and engine design,  reciprocating engine technology has improved dramatically  over the past two decades. The integration of computerized  engine management systems has improved fuel efficiency,  decreased emissions, and reduced pilot workload.  Reciprocating engines operate on the basic principle of  converting chemical energy (fuel) into mechanical energy.  This conversion occurs within the cylinders of the engine  through the process of combustion. The two primary  reciprocating engine designs are the spark ignition and the  compression ignition. The spark ignition reciprocating engine  has served as the powerplant of choice for many years. In  an effort to reduce operating costs, simplify design, and  improve reliability, several engine manufacturers are turning  to compression ignition as a viable alternative. Often referred  to as jet fuel piston engines, compression ignition engines  have the added advantage of utilizing readily available and  lower cost diesel or jet fuel.  In-line engines have a comparatively small frontal area, but  their power-to-weight ratios are relatively low. In addition,  the rearmost cylinders of an air-cooled, in-line engine  receive very little cooling air, so these engines are normally  limited to four or six cylinders. V-type engines provide  more horsepower than in-line engines and still retain a small  frontal area.  Continued improvements in engine design led to the  development of the horizontally-opposed engine, which  remains the most popular reciprocating engines used on  smaller aircraft. These engines always have an even number  of cylinders, since a cylinder on one side of the crankcase  “opposes” a cylinder on the other side.  [Figure 7-2]  The  majority of these engines are air cooled and usually are  mounted in a horizontal position when installed on fixed-wing  airplanes. Opposed-type engines have high power-to-weight  ratios because they have a comparatively small, lightweight  crankcase. In addition, the compact cylinder arrangement  reduces the engine’s frontal area and allows a streamlined  installation that minimizes aerodynamic drag.  The main mechanical components of the spark ignition and  the compression ignition engine are essentially the same.  Both use cylindrical combustion chambers and pistons that  travel the length of the cylinders to convert linear motion  into the rotary motion of the crankshaft. The main difference  between spark ignition and compression ignition is the  process of igniting the fuel. Spark ignition engines use a  spark plug to ignite a pre-mixed fuel-air mixture. (Fuel-air  mixture is the ratio of the “weight” of fuel to the “weight”  of air in the mixture to be burned.) A compression ignition  engine first compresses the air in the cylinder, raising its  temperature to a degree necessary for automatic ignition  when fuel is injected into the cylinder.  These two engine designs can be further classified as:  Radial engines were widely used during World War II and  many are still in service today. With these engines, a row or  rows of cylinders are arranged in a circular pattern around  the crankcase. The main advantage of a radial engine is the  favorable power-to-weight ratio.  [Figure 7-1]  7-2  1. Cylinder arrangement with respect to the crankshaft—  radial, in-line, v-type, or opposed  2.  Operating cycle—two or four  3.  Method of cooling—liquid or air  Reciprocating Engines 