Skip to main content Skip to navigation links

NFCRC Tutorial: Reciprocating Engine

previous link: trends.html next link: gasturbine.html

A reciprocating engine consists of (1) compression of air or pre-compressed air (or air-fuel mixture in the case of certain types of engines) within the cylinder of the engine by the action of a piston, (2) addition of heat energy into the compressed air by directly combusting the fuel in the compressed air, followed by (3) expansion of the hot pressurized combustion products in the cylinder against the piston connected to the load to produce useful work. The auto engine and the diesel engine are examples of a reciprocating engine. The compression ratio of an auto engine is lower and the combustion process is initiated by a spark plug while in a diesel engine, the compression ratio is significantly higher and the fuel is ignited by the heat of compression.

Spark Ignition Engines. Next, there are two types of spark ignition engines: the four stroke cycle and the two stroke cycle. In the four stroke engine, four strokes of the piston are required to complete a cycle: (1) intake stroke where the piston moves with the intake valve open and the exhaust valve closed such that a mixture of air with atomized and vaporized fuel is taken into the cylinder, (2) compression stroke, in which the air/fuel mixture is compressed with both valves closed followed by ignition of the air/ fuel charge by a timed spark, (3) power or expansion stroke with both valves closed, and finally (4) the exhaust stroke in which the pistons moves with the exhaust valve open and thus completing the cycle. The cylinder walls are cooled by circulating a cooling medium through the cylinder jackets or by incorporating fins cooled by air.

In the two stroke engine, the intake and exhaust strokes are eliminated by using pre-compressed intake charge to displace the exhaust gases. The two stroke engine has the advantage of a high power to weight ratio because the engine has a power stroke each revolution. This advantage, however, is offset by the loss of a portion of the intake charge with the exhaust gases, resulting in lower efficiencies. The two stroke engine has thus limited applications such as in small boat engines, lawnmower engines where low cost and weight are more important than efficiency.

Compression Ignition Engines. With a sufficiently high compression ratio and a suitable fuel, autoignition occurs in a reciprocating engine. The engine is similar to the spark ignited engines described above except that during the compression stroke, only air is taken into the piston and compressed to ignition conditions and then, the fuel is atomized directly into the combustion chamber at a controlled rate.

previous link: trends.html next link: gasturbine.html