Piston engine technology

In its most basic form, the piston engine comprises a cylinder sealed at one end and open at the other end. A disc or piston which fits closely within the cylinder is used to seal the open end and this piston can move backwards and forwards within the cylinder. This it does in response to the expansion and contraction of the gas contained within the cylinder. The outside of the piston is attached via a hinged lever to a crankshaft. Movement of the piston in and out of the cylinder causes the crankshaft to rotate and this rotation is used to derive motive energy from the piston engine.

The manner in which the gas within the cylinder is caused to expand or contract defines the type of piston engine. In spark- or compression-ignition engines, valves are employed to admit a mixture of fuel and air into the sealed piston chamber where it is burnt to generate energy. Thus these engines are called internal combustion engines. In contrast the gas within a Stirling engine is caused to expand or contract by the application of heating and cooling from outside. This is called an external combustion engine.

Internal combustion engines form the major category of piston engines and these can be subdivided into spark- and compression-ignition engines. A further subdivision depends on whether the engine utilises a two- or a four-stroke cycle. The former is attractive in very small engines as it can provide relatively high power for low weight. For power generation, some very large engines also use a two-stroke cycle.2 However most small- and medium-sized engines for power generation employ the four-stroke cycle.

The internal combustion variety of piston engine was developed in the latter half of the nineteenth century, although some primitive engines were in existence before that. Nikolaus Otto is generally credited with building the first four-stroke internal combustion engine in 1876. In doing so he established the principle still in use.

The Otto cycle engine employs a spark to ignite a mixture of air and, traditionally, gasoline3 compressed by the piston within the engine cylinder. This causes an explosive release of heat energy which increases the gas pressure in the cylinder, forcing the piston outwards as the gas expands. This explosion is the source of power, its force on the piston turning the crankshaft to generate rotary motion.

The Otto cycle was modified by Rudolph Diesel in the 1890s. In his version, air is compressed in a cylinder by a piston to such a high pressure that its temperature rises above the ignition point of the fuel which is then introduced to the chamber and ignites spontaneously without the need for a spark.

The four-stroke cycle used by most of these engines derives its name from the four identifiable movements of the piston in the chamber - two of expansion and two of compression for each power cycle. With the piston at the top of its chamber, the first stroke in an intake stroke in which either air (diesel cycle) or a fuel and air mixture (Otto cycle) is drawn into the piston chamber (see Figure 6.1). The second stroke is the compression stroke during which the gases in the cylinder are compressed. In the case of the Otto cycle, a spark ignites the fuel-air mixture at the top of the piston movement creating an explosive expansion of the compressed mixture which forces the piston down again. This is the power cycle. In the diesel cycle fuel is introduced close to the top of the compression stroke, igniting spontaneously with the same effect. After the power stroke, the fourth stroke is the exhaust stroke during which the exhaust gases are forced out of the piston chamber. In either case a large flywheel attached to the crankshaft stores angular momentum generated by the power stroke and this provides sufficient momentum to carry the crankshaft and piston through the three other strokes required for each cycle.

As already noted, the piston is connected through a hinged lever to a crankshaft, this arrangement allowing rotary motion to be extracted from a linear movement. Normally four (or a multiple of four) pistons are attached to the crankshaft, with one of each set of four timed to produce a power stroke while the other three move through different stages of their cycles. The introduction of fuel and air, and the removal of exhaust is controlled by valves which are mechanically timed to coincide with the various stages of the cycle.

In a two-stroke engine, intake and exhaust strokes are not separate. Instead fuel is forced into the piston chamber (intake) towards the end of the power stroke, pushing out the exhaust gases through a valve at the top of the chamber. A compression stroke is then followed by ignition of the fuel and a repeat of the cycle.

Fuel drawn Fuel mixture Burning fuel Burnt gas into cylinder compressed forces piston down is pushed out

Figure 6.1 The strokes of a four-stroke cycle

Fuel drawn Fuel mixture Burning fuel Burnt gas into cylinder compressed forces piston down is pushed out

Figure 6.1 The strokes of a four-stroke cycle

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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