Automotive
Engineering
ter 1.1
Piston-engine cycles
of operation
Heinz Heisler
1.1.1 The internal-combustion
engine
The piston engine is known as an internal-combustion
heat-engine. The concept of the piston engine is that
a supply of air-and-fuel mixture is fed to the inside of the
cylinder where it is compressed and then burnt. This
internal combustion releases heat energy which is then
converted into useful mechanical work as the high gas
pressures generated force the piston to move along its
stroke in the cylinder. It can be said, therefore, that
a heat-engine is merely an energy transformer.
To enable the piston movement to be harnessed, the
driving thrust on the piston is transmitted by means of
a connecting-rod to a crankshaft whose function is to
convert the linear piston motion in the cylinder to
a rotary crankshaft movement (Fig. 1.1-1). The piston
can thus be made to repeat its movement to and fro, due
to the constraints of the crankshaft crankpin’s circular
path and the guiding cylinder.
The backward-and-forward displacement of the
piston is generally referred to as the reciprocating motion
of the piston, so these power units are also known as
reciprocating engines.
1.1.1.1 Engine components and terms
The main problem in understanding the construction of
the reciprocating piston engine is being able to identify
and name the various parts making up the power unit. To
this end, the following briefly describes the major
components and the names given to them (Figs. 1.1-1
and 1.1-2).
Cylinder block This is a cast structure with cylin-
drical holes bored to guide and support the pistons and to
harness the working gases. It also provides a jacket to
contain a liquid coolant.
Cylinder head This casting encloses the combus-
tion end of the cylinder block and houses both the inlet
and exhaust poppet-valves and their ports to admit air–
fuel mixture and to exhaust the combustion products.
Crankcase This is a cast rigid structure which sup-
ports and houses the crankshaft and bearings. It is usually
cast as a mono-construction with the cylinder block.
Sump This is a pressed-steel or cast-aluminium-
alloy container which encloses the bottom of the crank-
case and provides a reservoir for the engine’s lubricant.
Fig. 1.1-1 Pictorial view of the basic engine.
Vehicle and Engine Technology, ISBN: 9780340691861
Copyright –
1998 Heinz Heisler. All rights of reproduction, in any form, reserved.
Piston This is a pressure-tight cylindrical plunger
which is subjected to the expanding gas pressure. Its
function is to convert the gas pressure from combustion
into a concentrated driving thrust along the connecting-
rod. It must therefore also act as a guide for the small-
end of the connecting-rod.
Piston rings These are circular rings which seal the
gaps made between the piston and the cylinder, their
object being to prevent gas escaping and to control the
amount of lubricant which is allowed to reach the top of
the cylinder.
Gudgeon-pin This pin transfers the thrust from the
piston to the connecting-rod small-end while permitting
the rod to rock to and fro as the crankshaft rotates.
Connecting-rod This acts as both a strut and a tie
link-rod. It transmits the linear pressure impulses acting
on the piston to the crankshaft big-end journal, where
they are converted into turning-effort.
Crankshaft A simple crankshaft consists of a cir-
cular-sectioned shaft which is bent or cranked to form
two perpendicular crank-arms and an offset big-end
journal. The unbent part of the shaft provides the main
journals. The crankshaft is indirectly linked by the
connecting-rod to the piston – this enables the straight-
line motion of the piston to be transformed into a rotary
motion at the crankshaft about the main-journal axis.
Crankshaft journals These are highly finished cy-
lindrical pins machined parallel on both the centre axes
and the offset axes of the crankshaft. When assembled,
these journals rotate in plain bush-type bearings mounted
in the crankcase (the main journals) and in one end of the
connecting-rod (the big-end journal).
Small-end This refers to the hinged joint made by the
gudgeon-pin between the piston and the connecting-rod
so that the connecting-rod is free to oscillate relative to the
cylinder axis as it moves to and fro in the cylinder.
Big-end This refers to the joint between the
connecting-rod and the crankshaft big-end journal which
provides the relative angular movement between the two
components as the engine rotates.
Main-ends This refers to the rubbing pairs formed
between the crankshaft main journals and their re-
spective plain bearings mounted in the crankcase.
Line of stroke The centre path the piston is forced
to follow due to the constraints of the cylinder is known
as the line of stroke.
Inner and outer dead centres When the crankarm
and the connecting-rod are aligned along the line of
stroke, the piston will be in either one of its two ex-
treme positions. If the piston is at its closest position to
the cylinder head, the crank and piston are said to be at
inner dead centre (IDC) or top dead centre (TDC).
With the piston at its furthest position from the cyl-
inder head, the crank and piston are said to be at outer
dead centre (ODC) or bottom dead centre (BDC).
These reference points are of considerable importance
for valve-to-crankshaft timing and for either ignition or
injection settings.
Clearance volume The space between the cylinder
head and the piston crown at TDC is known as the
clearance volume or the combustion-chamber space.
Crank-throw The distance from the centre of the
crankshaft main journal to the centre of the big-end
journal is known as the crank-throw. This radial length
influences the leverage the gas pressure acting on the
piston can apply in rotating the crankshaft.
Piston stroke The piston movement from IDC to
ODC is known as the piston stroke and corresponds
Fig. 1.1-2 Sectional view of the basic engine.
4
CHAPTER 1.1 Piston-engine cycles of
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