A Reciprocating Engine is also called as Piston Engine. In this engine pressure is converted into rotating motion with the help of one or more reciprocating pistons. There are various types of Reciprocating Engines, the most common type is the Internal combustion engine which is used in motor vehicles, the steam engine which is used in the industries and the Stirling engine. The compression ignition engine and the Spark-ignition engine are the two types of internal combustion engine. In the compression ignition engine, fuel is injected into the cylinder and the air inside the cylinder is compressed so that the heat energy produced is then used to ignite the fuel. In the Spark-ignition engine, the combustion is initiated by the spark plug. In the reciprocating engines, the piston (one or more than one) is always inside the cylinder into which the fuel is also injected.
Engine Combustion types
In the Reciprocating Engines, the potential energy of the chemical fuel is converted into mechanical kinetic energy. In may happen in two stroke cycle or four stroke cycle.
Two Stroke Engine: In two-stroke cycle, the combustion cycle is completed in two strokes i.e. one intake and one exhaust of the piston which is achieved with one revolution of the crankshaft.
Four Stroke Engine: In four-stroke cycle engine, the combustion cycle is completed in four strokes i.e. intake, compression, power and exhaust of the piston which is achieved with two revolutions of the crankshaft.
Diesel cycle engine: The Diesel cycle engine is also four stroke cycle engine but unlike two and four stroke cycle engines, diesel engines do not have spark plugs. The combustion is achieved through high compression ratio and the heat released is then used to ignite the diesel/fuel.
Combustion engines are very well used in trucks, automobiles, marine propulsion, equipment used in construction and other applications used for power back-up. The steam engine used externally produced steam in its operation while the modern internal combustion engine uses a fuel-air mixture which is compressed in the cylinder by the piston and then ignited by the heat produced. The spark ignited combustion engine is based on Otto cycle, in which fuel is not burnt through the spark from a plug while in the diesel engine, air is compressed to the auto-ignition temperature of the fuel. The fuel is then injected into the cylinder and gets ignited.
Engine Operation
In all the reciprocating engines, the piston is inside the cylinder in which gas is there and the movement of the piston is called the stroke. In the intake stroke, in Spark ignition engine the air and fuel are premixed and put into the cylinder. In the diesel engine, air is drawn into the cylinder. In both the Spark ignited and Internal combustion engine (diesel engine), the piston moves down to "bottom dead centre" position. Then, in the compression stroke, in the spark-ignition engine, the fuel and air mixture after compression from the piston is ignited from a plug's spark. The compression ratio is controlled in the SG engine so that auto-ignition of the fuel is prevented. In the Internal combustion engine (diesel engine), air is compressed to auto-ignition temperature and the fuel is then injected which is ignited by the heat produced by air compression.
In the power stroke, combustion of fuels in both the engines leads to expansion of high-pressure gases, which moves the piston to the bottom of the cylinder and imparts rotation to the crankshaft. In the exhaust stroke, the piston comes back to the top of the cylinder and the exhaust gases are pushed out of the exhaust valve. Thus, the crankshaft has multiple cylinders connected to it so that some piston may impart rotation to the crankshaft while others may go back to the top of the cylinder during the exhaust stroke.
In the steam engine, the gas is introduced under pressure into the cylinder, the hot gas then expands and pushes the piston the bottom of the cylinder. The Stirling engine is a bit different, in which same sealed amount of gas is cooled and heated again and again. The stroke is defined by the maximum distance that the piston can move in one direction.
Figure 1 Spark ignited engine during Compression stroke
Advantages of Reciprocating Engines
The Reciprocating engine has the advantage that the gap between the cylinder and the piston can be sealed which results in low fuel consumption and high compression ratio. When the travelling direction changes, then at dead-centre points, the piston loses speed which gives enough time for intake, combustion and exhaust as well. The reciprocating engines have high durability and less friction as the oil film is used between the cylinder bore wall and the piston ring. This hydrodynamic lubrication produces less friction.
Thus, Reciprocating engines are used widely in automobiles, power back-up, trucks, marine propulsion, equipment used in construction, industries etc. as they have high efficiency and high compression ratio whereas the fuel consumption is less.
References
Arcoumanis, C., & Kamimoto, T. (2009). Flow and Combustion in Reciprocating Engines. London: Springer.
infoplease.com. (2012). internal-combustion engine. Retrieved from http://www.infoplease.com/: http://www.infoplease.com/encyclopedia/science/internal-combustion-engine-reciprocating-engines.html
Lopez, J. (2016). Combustion Engine for Power Generation: Introduction. Retrieved from http://www.wartsila.com/: http://www.wartsila.com/energy/learning-center/technical-comparisons/combustion-engine-for-power-generation-introduction
