Combustion Engine for Power Generation: Introduction

Combustion engines are a well-known technology used in automobiles, trucks, construction equipment, marine propulsion, and backup power applications. Combustion engines employ the expansion of hot gases to push a piston within a cylinder, converting the linear movement of the piston into the rotating movement of a crankshaft to generate power. While the steam engines that powered the industrial revolution were driven by externally-produced steam, modern combustion engines used for electric power generation are internal combustion engines in which an air-fuel mixture is compressed by a piston and ignited within a cylinder. Reciprocating internal combustion (IC) engines are characterized by the type of combustion: spark-ignited (SG) or compression-ignited, also known as diesel.

The SG engine is based on the Otto cycle, and uses a spark plug to ignite an air-fuel mixture injected at the top of a cylinder. In the Otto cycle, the fuel mixture does not get hot enough to burn without a spark, which differentiates it from the Diesel cycle. In diesel engines, air is compressed until the temperature rises to the auto-ignition temperature of the fuel. As the fuel is injected into the cylinder, it immediately combusts with the hot compressed air and expanding combustion gases push the piston to the bottom of the cylinder.

Engine Operation
Each movement of the piston within a cylinder is called a stroke. Reciprocating IC engines are described by the number of strokes to complete one power cycle and the speed of crankshaft (expressed in revolutions per minute, rpm). For electric power generation, four-stroke engines are predominately used. During the intake stroke, the premixed air and fuel (SG engines) or air (diesel engines) is drawn into the cylinder as the piston moves down to “bottom dead center” position. During the compression stroke in SG engines, the air-fuel mixture is compressed by the piston and ignited by a spark from a plug. Auto-ignition in SG engines is prevented with proper limits on the compression ratio.

Reciprocating Engine for Power Generation Introduction

Spark-ignited combustion engine during compression stroke

In diesel engines, the fuel is injected into the cylinder near the end of the compression stroke when the air has been compressed enough to reach the auto-ignition temperature. Combustion of the air-fuel mixture causes an accelerated expansion of high pressure gases, which push the piston to the bottom of the cylinder during the power stroke, imparting rotation to the crankshaft. Combustion occurs intermittently – only during the power stroke – whereas in gas turbines combustion occurs continuously. As the piston is returned to the top of the cylinder during the exhaust stroke, the products of combustion (exhaust gases) are pushed out an exhaust valve. Multiple cylinders are connected to the crankshaft, oriented so that while some pistons are imparting rotation to the crankshaft during their power strokes, other pistons are being pushed back to the top of the cylinders during their exhaust strokes.

The size and power of a combustion engine is a function of the volume of fuel and air combusted. Thus, the size of the cylinder, the number of cylinders and the engine speed determine the amount of power the engine generates. By boosting the engine’s intake of air using a blower or compressor – called supercharging – the power output of the engine can be increased. A commonly used supercharger is a turbocharger, which uses a small turbine in the exhaust gas path to extract energy for driving a centrifugal compressor.

Fuel Flexibility
Combustion engines can burn a variety of fuels, including natural gas, light fuel oil, heavy fuel oil, biodiesel, biofuels, and crude oil. Diesel engines are generally more efficient than SG engines, but also produce more nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM). SO2 and PM formation is a function of the fuel, with natural gas producing low emissions. The formation of NOx is coupled with combustion temperature. In SG engines, premixing of air with the fuel to produce “lean” conditions (more air than is needed for combustion) has the effect of lowering the combustion temperature and impeding NOx formation. New engine designs have been developed to take advantage of the diesel process while maintaining the benefits of lean burning. Dual-fuel (DF) engines are designed with the ability to burn both liquid and gaseous fuels. When operating in gas mode, the gaseous fuel is premixed with air, injected just after the compression stroke and ignited by a pilot fuel flame. In this process, the pilot fuel flame acts a “spark plug” to ignite the lean gas-air mixture. DF engines retain the ability to use a backup liquid fuel when gas supply is interrupted. Gas-diesel (GD) engines utilize highly compressed gas which is injected after a liquid pilot fuel is ignited. This process allows the use of lower quality gas.

In a power plant, many SG or diesel ICEs are grouped into blocks called generating sets. Every engine is connected to a shaft which is connected to its electric generator. These generating sets provide modular electric generating capacity and come in standardized sizes, ranging from 4 to 20 MW.

Reciprocating Engine for Power Generation Introduction 2
Engine Hall at Goodman Energy Center in Kansas, USA

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