Practical thermodynamic cycles

The Carnot Cycle we saw in the first part of this series is idealistic and efficient, but is not easy to use given today’s machinery. All other thermodynamic power cycles use the Carnot cycle as a guide to achieve maximum efficiency. In this second part of the series, the author throws light on the Otto, Diesel and Rankine cycles and their efficiency. The application of these cycles in modern-day machines is also discussed.

The Otto Cycle

A Frenchman by name Beau de Rochas came up with the theory of the Otto cycle in 1862, but Nicholas A. Otto built the first successful engine in 1876. In this cycle, air is compressed from one point to another in a cylinder. Heat is added at constant volume in the form of a liquid such as gasoline. The mixture of air and gas after ignition gets expanded, and the exhaust is then discharged. After this, the cycle starts all over again. This is called a "batch" process, opposed to a "continuous" process as for a turbine.

There are two versions of the Otto Cycle used today – the two-stroke and the four-stroke. While the two-strokers are used for small engines to power lawn mowers, chain saws, weed wackers and blowers, four-strokers are used in modern automobiles, trucks, tractors and small aircraft. Besides gasoline, other Otto cycle fuels such as compressed natural gas and liquid petroleum gas (LPG) are burned to propel buses, trucks, forklifts, and large lawn mowers and farm equipment.                                                                                                                  

The Diesel Cycle

Invented by Rudolf Diesel in 1893, the Diesel Cycle uses a higher compression ratio than the Otto cycle and no spark plugs. But it is similar to the four-stoke Otto mechanically where cylinders and poppet valves are applied to control air and exhaust flow. Two-stroke diesel engines have been used, but to a lesser extent. A higher compression ratio of 12 or more is required to provide a high enough temperature to ignite the fuel as it is being injected directly into the cylinder.

In this cycle, it is important that the fuel injected ignites almost instantly and burns rapidly. The Diesel Cycle produces high torque at a low RPM, more than the Otto Cycle and is ideal for bulldozers, graders, trucks and diesel-electric train locomotives, ships and electric generators for small power plants.

The Rankine Cycle

The Rankine Steam Cycle, invented by William J. Rankine between 1845 and 1850, is known as the complete expansion cycle. His original design had all the features of a modern steam power plant: condensate pump, boiler, feed water pump, steam boiler, prime mover reciprocating engine and later the turbine, an electric generator and a condenser in one system.

The Rankine Cycle has been a successful way of producing large amounts of electrical power all around the world – both 50 Hz (four-pole generator at 3000 RPM) and 60 Hz (two-pole at 3600 RPM). But there is still some room to grow in terms of higher temperatures and pressures. This cycle is a supportive partner for the now mature gas turbine; it is working hand in glove with the gas turbine Brayton Cycle, which will be discussed in the next part of the series.

Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.