A reciprocating engine, also often known as a piston engine, is typically a heat engine although there are also pneumatic and hydraulic reciprocating engines that use one or more reciprocating pistons to convert pressure into a rotating motion.
This article describes the common features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the niche application Stirling engine.
Internal combustion engines are further classified in two ways: either a spark-ignition (SI) engine, where the spark plug initiates the combustion; or a compression-ignition (CI) engine, where the air within the cylinder is compressed, thus heating it, so that the heated air ignites fuel that is injected then or earlier.
What is a Reciprocating Engine?
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.
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.
How does a reciprocating engine work?
All types have one or more pistons. Common engine block configurations include a single row of cylinders (in-line), two rows converging to a point (V-engine), a double zigzag (W-engine), and two horizontal rows (opposed engine). The engines mentioned above (internal combustion, steam, Stirling) all use somewhat different processes to complete the cycle, so the general case will be explored.
- Intake: To begin the cycle, a fuel mixture is introduced inside the cylinder through the intake port, expanding the piston to the bottom of the cylinder.
- Compression: The piston then gets pushed to the top, compressing the fuel mixture and igniting it via the spark plug.
- Ignition: The ignition pushes the piston downwards providing useful work to the engine.
- Exhaust: The waste chemicals get output through the exhaust port and the cycle repeats.
The four-stroke cycle is what gives the engine its energy, but now it must translate this energy into rotational energy for the transmission, drive shaft and wheels. This is done by the crankshaft. The crankshaft converts this up-and-down motion into rotational motion, which is often combined with a flywheel to retain the discontinuous reciprocating energy as rotational energy.
Reciprocating Engine Parts
Major parts of a reciprocating engine include the cylinders, pistons, connecting rods, crankshaft, valves, spark plugs and a valve operating mechanism. These are all used to power conventional vehicles.
A cylinder in a reciprocating engine refers to the confined space in which combustion takes place. Cylinders are arranged in several ways. These include: a single row arrangement, a V-shape arrangement, a W-shape arrangement and a horizontal or flat arrangement.
Pistons in a reciprocating engine are usually attached to each cylinder. In a reciprocating engine, a piston slides up and down to create a rotary motion. A piston’s wall is usually grooved to hold rings that fit tightly against a cylinder wall, preventing gases from escaping the combustion chamber.
3) Connecting Rod
A connecting rod in a reciprocating engine links a piston and the crankcase held by a crankshaft. The connecting rod in a reciprocating engine, while connected to a rotary motion piston, is used to turn a propeller. This results in the rotary motion of the crankshaft.
A crankshaft in a reciprocating engine transforms the up and down movement of a piston into rotary motion. While connected to a piston with a connecting rod, a crankshaft yields a rotary motion as the piston moves up and down. During an intake stroke in a piston engine, a piston is pulled downward, creating a vacuum in the cylinder chamber. During a compression stroke in a reciprocating engine, a crankshaft drives a piston upward in the cylinder.
A reciprocating engine has an intake and an exhaust valve. These are located adjacent to the fuel-air mixture inlet and exhaust outlet at the top of a cylinder, respectively. An intake valve in a reciprocating engine regulates entry of the air and fuel mixtures while an exhaust valve lets out exhaust and burned gases from the combustion chamber.
6) Spark Plugs
Spark plugs in a reciprocating engine are usually located on top of a cylinder above the valves. They serve to ignite the compressed air and fuel mixture during the compression and ignition strokes in a reciprocating engine. Ignition takes place just before a piston reaches its top position. This results in very hot gases expanding rapidly to drive a piston down while turning the crankshaft to yield rotary motion.
Types of Reciprocating Engine
These are types of reciprocating engine includes:
- Inline Engines
- Opposed or O-Type Engines
- V-Type Engines
- Radial Engines
An inline engine generally has an even number of cylinders, although some three-cylinder engines have been constructed. This engine may be either liquid cooled or air cooled and has only one crank shaft, which is located either above or below the cylinders. If the engine is designed to operate with the cylinders below the crankshaft, it is called an inverted engine.
The inline engine has a small frontal area and is better adapted to streamlining. When mounted with the cylinders in an inverted position, it offers the added advantages of a shorter landing gear and greater pilot visibility. With increase in engine size, the air cooled, inline type offers additional problems to provide proper cooling; therefore, this type of engine is confined to low- and medium-horsepower engines used in very old light aircraft.
Opposed or O-Type Engines
The opposed-type engine has two banks of cylinders directly opposite each other with a crankshaft in the center Figure 1. The pistons of both cylinder banks are connected to the single crankshaft. Although the engine can be either liquid cooled or air cooled, the air-cooled version is used predominantly in aviation. It is generally mounted with the cylinders in a horizontal position. The opposed-type engine has a low weight-to-horsepower ratio, and its narrow silhouette makes it ideal for horizontal installation on the aircraft wings (twin engine applications). Another advantage is its low vibration characteristics.
In V-type engines, the cylinders are arranged in two in-line banks generally set 60° apart. Most of the engines have 12 cylinders, which are either liquid cooled or air cooled. The engines are designated by a V followed by a dash and the piston displacement in cubic inches. For example, V- 1710. This type of engine was used mostly during the second World War and its use is mostly limited to older aircraft.
The radial engine consists of a row, or rows, of cylinders arranged radially about a central crankcase. This type of engine has proven to be very rugged and dependable. The number of cylinders which make up a row may be three, five, seven, or nine. Some radial engines have two rows of seven or nine cylinders arranged radially about the crankcase, one in front of the other. These are called double-row radials.
One type of radial engine has four rows of cylinders with seven cylinders in each row for a total of 28 cylinders. Radial engines are still used in some older cargo planes, war birds, and crop spray planes. Although many of these engines still exist, their use is limited. The single-row, nine-cylinder radial engine is of relatively simple construction, having a one-piece nose and a two-section main crankcase.
The larger twin-row engines are of slightly more complex construction than the single row engines. For example, the crankcase of the Wright R-3350 engine is composed of the crankcase front section, four crankcase main section, rear cam and tappet housing, supercharger front housing, supercharger rear housing, and supercharger rear housing cover. Pratt and Whitney engines of comparable size incorporate the same basic sections, although the construction and the nomenclature differ considerably.