What is a Reciprocating Engine?
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 uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types.
A reciprocating engine is an engine that uses one or more pistons in order to convert pressure into rotational motion. They use the reciprocating (up-and-down) motion of the pistons to translate this energy.
There are many different types, including the internal combustion engine which is used in most motor vehicles, the steam engine which is a type of external combustion engine, and the Stirling engine. A rotary engine would do the same task as a reciprocating engine but in a very different manner due to its triangular rotor.
MORE: What is an Internal Combustion 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 the fuel that is injected then or earlier.
How does a reciprocating engine work?
Reciprocating engines operate by converting the heat and pressure released during the combustion of fuel mixed with air into mechanical energy.
A reciprocating engine is an engine that uses one or more pistons in order to convert pressure into rotational motion. They use the reciprocating (up-and-down) motion of the pistons to translate this energy. 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).
MORE: What is Piston?
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, driveshaft, 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.
MORE: What is Crankshaft?
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 the entry of the air and fuel mixtures while an exhaust valve lets out the 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 a 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 crankshaft, 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 shorter landing gear and greater pilot visibility.
With the 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. 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.
MORE: What are the types of engines?
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 that make up a row maybe 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, warbirds, 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 sections, 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.