Page 355 - Advanced Thermodynamics for Engineers, Second Edition

CHAPTER

RECIPROCATING INTERNAL

COMBUSTION ENGINES 16

16.1 INTRODUCTION

The most common types of engine, or prime mover, met by many people are reciprocating internal
combustion engines. The idealised operating cycles of these engines were considered in Chapter 3, and
these were designated the Otto, diesel or dual combustion cycles. The use of these idealised heat
engine cycles enabled some characteristics of the engines to be considered, but they fall short of
defining the operating parameters of actual engines – partly because these engines are not ‘heat
engines’. The assumptions made in Chapter 3 to define the ‘air-standard cycles’ were the following:

• the combustion process was replaced by a heat transfer process in which an amount of energy
equivalent to the energy released by combustion was added to the air;
• the gas exchange process was replaced by a heat transfer process to a cold reservoir, so that the hot
gases after expansion were returned to the state of the air after induction; and
• the compression and expansion processes were isentropic.

The first part of this chapter will extend the consideration of engine cycles in Chapter 3, and
introduce some of the features that differentiate actual cycles from air-standard cycles.
It will then describe engine combustion systems in some detail, and will further discuss more
realistic cycles, in which friction, and finite rates of combustion will be considered. Different com-
bustion chamber shapes will be introduced, and the reasons for them will be discussed. It will be shown
that each combustion chamber design is arrived at in an attempt to modify the intrinsic characteristics
of the combustible mixture being used. The combustion rates can also be modified by changing the
basic parameters of the engine cylinder, e.g. compression ratio, and also the operating conditions, e.g.
air–fuel ratio. A more detailed review of engine combustion systems, including mixture preparation,
and gas flow generation can be obtained from Stone (2012).
The processes of liquid atomisation, evaporation and ignition are of fundamental importance to the
behaviour of all engine combustion systems: these processes are extremely dependent on the particular
fuels being used. A significant effort has been expended by a range of manufacturers, particularly for
the military, to make engines which are independent of the fuel – success has been limited. The early
aspirations of Rudolf Diesel were to operate his engine on pulverised coal, and this approach was
considered again in the 1980s, but wear was an overriding problem. Attempts have also been made to
operate gas turbines on pulverised fuel, again with little success. So the majority of prime movers
today operate on liquid or gaseous fuels containing a broad range of H–C ratios.

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