Basic Engine Performance Concepts

Chapter 1

Basic Engine Performance Concepts

In this chapter the concepts fundamental to engine performance will be introduced. Various types of internal combustion engines are described and compared, and the advanatages of each in meeting particular application needs are discussed.  The two- and four-stroke operating cycles are described, and an initial comparison is made between Diesel and spark ignition engines and combustion processes.  A description is then given of various engine air handling configurations, introducing terminology and concepts which will be covered in later chapters. The chapter concludes with a review of the parameters important in measuring the performance of internal combustion engines.

1.1  Types of Internal Combustion Engines and Their Operating Cycles

The purpose of any engine is to harness the energy liberated in an exothermic, or energy releasing chemical reaction to produce as much useful mechanical work as possible. The vast majority of engines rely on a combustion process to provide the energy.  Combustion processes are chemical reactions most commonly of hydrocarbon fuel molecules with oxygen, to form primarily carbon dioxide and water.  They may take place in an external combustion chamber, and the energy used to heat a working fluid, such as the steam in a Rankine cycle powerplant, or within the work producing device, such as an internal combustion engine.

The internal combustion engine is defined as any engine where the combustion reaction of fuel and oxidant takes place within the operating cycle of the engine, thus directly affecting the chemical composition of the working fluid.  The engine does not rely on a heat source external to the cycle transferring energy to the working fluid.  The energy released during the combustion process is manifested as an increase in pressure and temperature.  The function of the engine is to provide a device which effectively harnesses this chemical energy to produce useful mechanical work.  The Diesel, spark-ignition and gas turbine engines are all considered internal combustion engines.  The gas turbine is distinguished from the Diesel and spark ignition engines by the fact that each process within the gas turbine cycle is continuous.  The working fluid passes through different parts of the engine as a steady flow, and each process takes place simultaneously.  Both the Diesel and spark-ignition engines operate with intermittent processes, where the working fluid is batched through the engine in unsteady processes (processes changing with time).

Figure 1-1 Slider crank components

Gas turbine engines offer the advantages of few moving parts, and a very high power-to-weight ratio.  The light weight makes them ideally suited for aircraft applications.  Another advantage of the gas turbine is that its torque output is at its maximum at zero rpm.  As will be seen later this would result in excellent driveability in heavy duty applications.  For this reason the gas turbine received very serious attention as a potential heavy truck engine in the late 1960s.  However, a fundamental disadvantage of the gas turbine is its inherently low thermal efficiency.  This makes the engine quite non-competitive in markets where fuel costs are important.