Page 97 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2

CHAPTER

2.1 Engine mechanical

2.1.1 Introduction and operating cycles
The modern motor vehicle engine burns a fuel to obtain power. The fuel is
Key fact
usually petrol (gasoline) or diesel, although liquid petroleum gas (LPG) and
compressed natural gas (CNG) are sometimes used. Specialist fuels have Specialist fuels have been
been developed for racing car engines. Motor vehicle engines are known as developed for racing car engines.
‘internal combustion’ engines because the energy from the combustion of
the fuel, and the resulting pressure from expansion of the heated air and fuel
charge, is applied directly to pistons inside closed cylinders in the engine. The
term ‘reciprocating piston engine’ describes the movement of the pistons,
which go up and down in the cylinders. The pistons are connected by a rod to
a crankshaft to give a rotary output ( Fig. 2.1 ).

Fuel is metered into the engine together with an air charge for most petrol
Defi nitions
engines. However, some now use injectors that inject directly into the engine
cylinder. In diesel engines, the fuel is injected into a compressed air charge TDC
in the combustion chamber. In order for the air and fuel to enter the engine Top dead centre.
and for the burnt or exhaust gases to leave the engine, a series of ports is

connected to the combustion chambers ( Figs 2.2 and 2.3 ) . The combustion BDC
chambers are formed in the space above the pistons when they are at the top Bottom dead centre.
of the cylinders. Valves in the combustion chamber at the ends of the ports
control the air charge and exhaust gas movements into and out from the
combustion chambers.

The valves are described as ‘poppet’ valves and have a circular plate at
right angles to a central stem that runs through a guide tube. The plate has Key fact
a chamfered sealing face in contact with a matching sealing face in the port. Combustion chambers are formed
The valve is opened by a rotating cam and associated linkage. It is closed and by the space above the pistons
held closed by a coil spring. The opening and closing of the valves and the when they are at the top of the
cylinders (TDC).
movement of the pistons in the cylinders follow a cycle of events called the
‘four-stroke cycle’ or the ‘Otto cycle’ after its originator ( Figs 2.4–2.7 ).

The fi rst stroke of the four-stroke cycle is the induction or intake stroke. This
occurs when the piston is moving down in the cylinder from top dead centre
(TDC) to bottom dead centre (BDC) and the inlet valve is open. The movement
of the pistons increases the volume of the cylinder so that air and fuel enter
the engine.

Automobile Mechanical and Electrical Systems.
© 2011 Tom Denton. Published by Elsevier Ltd. All rights reserved.