Operation of Reciprocating Piston IC Engines (Automobile)

2.3

Operation of Reciprocating Piston IC Engines

An assembly of a large number of parts used to do work and make transfer of energy is called
a power plant. The engine is the power plant of an automobile. The heat energy produced by
burning of the fuel is converted by the engine into mechanical power or rotary motion through
its various parts. The motion is started in the flywheel connected to the crankshaft. After
completing various cycles of operation in the engine, the flywheel transmits this motion to the
wheels through the transmission system. The IC engines are of two types; spark ignition (SI)
and compression ignition (CI). The petrol (SI) and diesel (CI) engines are further discussed in
this chapter.

2.3.1. Basic Construction

The automobile engine has a piston that moves up and down, or reciprocates in a cylinder.
The piston is attached to a crankshaft with a connecting rod. The pressure developed in the
combustion chamber pushes the piston away
and thereby, forces the crankshaft to rotate.
When the crankshaft rotates the mechanical
arrangement allows the piston to reciprocate in
the cylinder as shown in Fig. 2.1.
The engine cylinder is a cylindrically
shaped container supported in position in the
cylinder block, attached to or an integral part
of the crankcase. The volume enclosed by the
upper part of the cylinder and the top of the
piston at its lowest position is called the com-
bustion chamber.
The highest point reached by the piston is
called the top dead centre (TDC) and the
lowest point is called the bottom dead centre
(BDC). The distance of movement from TDC to
BDC is called the stroke. The operation in the
cylinder from induction of fuel mixture to its
compression, ignition, expansion and exhaust of
the burnt fuel mixture is called an engine cycle.
The combustion chamber above the piston
must be recharged after each combustion
process. The intake valve allows fresh charge
to enter the cylinder. An exhaust valve
Fig. 2.1. Piston and crank mechanism.
Piston and crank mechanism.
releases the spent gases after the piston has moved to the bottom of the expansion stroke. Valves
are opened and closed at the right time by a camshaft driven from the crankshaft.
In SI engine, a mixture of fuel and air from the carburettor enters the cylinder through
the intake manifold and intake port. A throttle in the carburettor controls the mass of
mixture entering the combustion chamber. The intake valve is located at the junction of the
intake port and the cylinder. A sparkplug, which is located near the top of the cylinder, initiates
combustion.
In CI engine, the fuel is injected directly into the combustion chamber through a fuel
injection nozzle. The quantity of fuel entering is controlled by a fuel control lever. Air enters
the cylinder from a manifold through a port.
The piston and the piston rings prevent the escape of the expanding gases from the
combustion chamber. Energy of the expanding gases is transmitted by the piston into rotational
motion of the crankshaft.
The passage through which the products of combustion leave the combustion chamber
consists of the exhaust port and the exhaust manifold. The exhaust valve is located at the
junction of the exhaust port and the cylinder.
Both intake and exhaust valves are operated by valve mechanisms. A camshaft is operated
by timing gears driven by the crankshaft. Lobed cams, integrally connected to the camshaft,
actuate pushrods and rocker arms, which open the valves. A valve spring holds the valve
closed except when the timed rotation of the cam mechanism forces the valve to open.
Every engine has a lubricating system, which provides an oil film between the bearing
surfaces to prevent contact, thus minimising wear.
A cooling system is incorporated to remove excess heat from the metal surrounding the
combustion chamber to avoid overheating, expansion and seizing of the parts.


2.3.2. Principle and Cycles of Operation

Both spark ignition (SI) and compression ignition (CI) engines are very much alike in
construction, but differs in the type of fuel used, the way the fuel gets into the engine cylinder,
and the way the fuel is ignited. Spark-ignition engines use gasoline (petrol), a highly volatile
fuel. Before entering the cylinder, the fuel is mixed with air forming a combustible air-fuel
mixture in the right proportion for rapid and proper combustion. The mixture is compressed in
the cylinder and is ignited by electric spark. The compression-ignition engine uses diesel, light
oil, as fuel. Air is taken into the engine cylinder where it is compressed, due to which its
temperature goes up. The diesel oil is injected into the compressed air. The hot air (heat of
compression) ignites the fuel.
The various thermodynamic cycles on which the IC piston engines work are the Otto cycle,
diesel cycle and dual cycle. These cycles are non-phase change cycles. The spark-ignition engine
works on the Otto cycle, in which energy supply and rejection occur at constant volume and
compression and expansion, occur isentropically as shown in the pressure-volume (P-V) and
temperature-entropy (T-S) diagrams in Fig. 2.2. The compression-ignition engine works on the
diesel cycle, in which energy addition occurs at constant pressure and energy rejection at
constant volume. The compression and expansion take place isentropically as shown in the P-V
and T-S diagrams in Fig. 2.3.
Otto cycle on pressure-volume and temperature-entropy diagrams.
Fig. 2.2. Otto cycle on pressure-volume and temperature-entropy diagrams.
The compression ratio is much higher in diesel engines than that used in petrol engines.
Since only air is compressed in diesel cycle, there is no upper limit to the compression ratio
except the ability of the materials of the engine components to withstand the pressure. The cycle
efficiency increases with increasing compression ratio. The higher compression ratio enables
conversion of more of the energy in the fuel into work than in petrol engines.
Fig. 2.3. Diesel cycle on pressure-volume and temperature-entropy diagrams.
Usually, the high speed diesel engines do not work on the diesel cycle but on the dual cycle
in which energy addition occurs partly at constant volume and partly at constant pressure as
shown in the P-V and T-S diagrams in Fig. 2.4. The rest of the process remains the same.
Fig. 2.4. Duel cycle on pressure-volume and temperature-entropy diagrams.
All these cycles are ideal cycles and in these a hypothetical energy source and sink are
considered. The actual cycle differs from the ideal cycle because of the presence of various
Diesel cycle on pressure-volume and temperature-entropy diagrams.
Duel cycle on pressure-volume and temperature-entropy diagrams.
irreversibilities. The energy addition in an actual process is never at constant pressure or at
constant volume. The process of expansion and compression are never isentropic because of the
heat transfer to or from the system. Moreover, a finite amount of work is required for the suction
of fresh air or charge and the discharge of products of combustion. The actual pressure-volume
diagrams of four-stroke petrol and diesel engines are illustrated respectively in Fig. 2.5 A and
B. Both SI and CI engines may work on four-stroke and two-stroke cycles.
Actual pressure-volume diagrams for four stroke engines.
Fig. 2.5. Actual pressure-volume diagrams for four stroke engines.
A. Petrol engine. B. Diesel engine.
The compression ratio in diesel engines ranges from 12:1 to 20:1 whereas ordinary petrol
engine employ only 7.5:1 to 15.5:1. High compression ratio in CI engines produces pressures of
about 3 to 4 MPa at the end of compression stroke. The temperature of air at this point reaches
about 950 K at full load and full speed. At the temperature of 625 to 725 K self-ignition of the
fuel takes place. The combustion process in the-diesel engine is-not an instant explosion of charge
as in the petrol engine, but rather a process of surface combustion in the early stages that
develops later into more or less rapid explosion.

Principle of operation of a four-stroke petrol engine.
Fig. 2.6. Principle of operation of a four-stroke petrol engine.
ft 3.3 Spark Ignition (Petrol) Engines
A four-stroke petrol engine performs all the four operations; suction (induction), compres-
sion, expansion (power) and exhaust in four strokes of the piston i.e. in two revolutions of the
crankshaft. Figure 2.6 illustrates the operations of a four-stroke petrol engine. The petrol
engines take in a flammable mixture of air and petrol, which is ignited by a timed spark when
the charge is compressed.
In contrast, a two-stroke engine does all the four operations in two strokes of the piston i.e.
in each revolution of the crankshaft. The construction of two-stroke engines is similar to that
of four-stroke engines with the exception that valves are not used. The ports in the lower end
of the cylinder wall are covered or uncovered by the piston, as it travels up and down in the
cylinder. The carburettor communicates with the crankcase instead of with the cylinder directly.
A separate passage, known as transfer port, connects the crankcase with the cylinder. In Fig.
2.7 the principal of operation of a two-stroke petrol engine has been illustrated.
Principle of operation of two-stroke petrol engine.
Fig. 2.7. Principle of operation of two-stroke petrol engine.

Operation of the Four-stroke Cycle Petrol Engine.

Induction Stroke.

The piston moves downward away from the cylinder head when inlet
valve is opened and the exhaust valve is closed (Fig. 2.6A). A reduction in pressure of about 29.4
kPa below atmospheric pressure occurs at one third from the beginning of the stroke while the
average pressure in the cylinder might be 9.8 kPa or even less. The reduction in pressure, which
depends on the engine’s speed and load, induces a fresh charge of air-fuel mixture into the
cylinder. An engine which induces fresh charge by means of a depression in the cylinder is called
normally aspirated or naturally aspirated engine.

Compression Stroke.

Both the inlet and exhaust valves are closed, and the piston moves
towards the cylinder head (Fig. 2.6B), due to which the volume decreases to one eighth to one
tenth of the original. The charge in the cylinder is compressed so that the air and atomised-petrol
molecules are squeezed closer together. As a result, the pressure and temperature of the charge
in the cylinder increase. Typical maximum cylinder compression pressure ranges from 785 to
1373 kPa with throttle open and the engine running under load.

Power Stroke.

A spark plug ignites the dense combustible charge just before the piston
approaches the top of its stroke during compression (Fig. 2.6C). The charge mixture burns,
generates heat, and rapidly raises the pressure in the cylinder. The burning gases expand and
push the piston away from the cylinder head to its outermost position producing useful power.
The cylinder pressure then drops from a peak value of about 5890 kPa under full load down to
around 390 kPa.

Exhaust Stroke.

At the end of the power stroke the inlet valve remains closed but the
exhaust valve is opened. The piston now moves towards cylinder head (Fig. 2.6D). Most of the
burnt gases are expelled by the existing pressure of the gas through the exhaust port to the
atmosphere, and the returning piston pushes the remaining burnt gases out of cylinder. The
gas pressure in the cylinder falls to atmospheric pressure or even less as the piston nears the
inner most position.

Operation of the Two-stroke Cycle Petrol (SI) Engine.

Induction and Exhaust Phase.

As piston moves away from cylinder head during rotation
of crankshaft, first it uncovers the exhaust port (E), releasing the burnt exhaust gases to the
atmosphere. Simultaneously the underside of the piston compresses the previously filled air-fuel
mixture in the crankshaft (Fig. 2.7A). Further outward movement of the piston uncovers the
transfer port (T), and the compressed mixture in the crankcase is transferred to the combustion
chamber. The fresh charge entering the cylinder also pushes out any remaining exhaust gases.
This process is generally referred to as cross-flow scavenging.

Compression Phase.

As piston moves towards the cylinder head, it first seals off the
transfer port, and then closes the exhaust port after a short time. The fresh charge in the
combustion chamber is compressed to about one seventh to one eighth of its original volume
(Fig. 2.7B). At about half way up the movement of the piston, it uncovers the inlet port (I) and
fresh mixture is induced into the crankcase.

Power Phase.

A spark plug situated in the centre of the cylinder head ignites the dense
mixture just before the piston reaches the top of its stroke. The burning of the charge rapidly
raises the gas pressure to a maximum of about 4905 kPa under full load, which expands forcing
the piston back along its stroke with a corresponding reduction in cylinder pressure (Fig. 2.7C).
As the piston moves down producing useful power, the piston skirt covers the piston port.
Subsequently piston compresses the mixture in the crankcase in preparation for the next charge
transfer into the combustion chamber.
To improve scavenging efficiency, a loop-scavenging system or reverse flow system
known as the Schnuerle scavenging system was developed. This design has a transfer port on
each side of the exhaust port. The transfer port direct the scavenging charge mixture practically
in a tangential direction towards the opposite cylinder wall. This form of porting provides
minimum turbulence and intermixing of fresh fuel mixture with residual burnt gases over wide
range of piston speeds.
It is evident that the power produced by two-stroke engine is theoretically double that of
same engine operating on the four-stroke cycle. Practically, a small amount of burnt gas always
remains in the cylinder along with the fresh charge. Also some of the incoming fresh charge
escapes with the exhaust since both the inlet and exhaust ports open almost simultaneously.
As a result, the efficiency of a two-stroke engine is lower and it does not develop double the
power to that of a four-stroke engine of the same bore, stroke and speed. However, two-stroke
engine is simpler to manufacture than four-stroke engine as it has no valves and camshafts.
Since it has a power stroke in each rotation of the crankshaft, the turning moment on the
crankshaft is more uniform requiring a smaller flywheel than a four-stroke engine.

Comparison of Two- and Four-stroke-cycle Petrol engines.

(a) The two-stroke engine completes one cycle of events in each revolution of the
crankshaft, whereas the four-stroke engine in two revolutions.
(b) Theoretically, the two-stroke engine should produce double the useful power to that
of a four-stroke engine of the same cylinder capacity.
(c) In practice, removal of the exhaust gases and charging of the fresh mixture through
the crankcase in the two-stroke engine is less effective than having separate exhaust
and induction strokes. This causes lower mean effective cylinder pressures in two-
stroke units than in equivalent four-stroke engines.
(d) The two-stroke engine runs smoother than the four-stroke engine for the same size of
the flywheel.
(e) Due to absence of the separate exhaust and induction strokes in the two-stroke engine,
the piston and small-end of the connecting rod have a tendency to overheat under
heavy driving conditions.
(/) Comparatively less maintenance is expected with the two-stroke engine compared
with the four-stroke engine.
(g) Lubrication of the two-stroke engine is achieved by mixing small quantities of oil with
petrol in proportions anywhere between 1:16 and 1:24.
(h) Two-stroke engines are generally cheaper to manufacture, as there are fewer working
parts in a two-stroke engine than in a four four-stroke engine.
(i) In the two-stroke engine with inferior scavenging, there is a tendency for
• insufficient filling of fresh mixture into the cylinder,
• retention of large amounts of residual exhaust gas in the cylinder, and
• direct release of fresh charge through the exhaust port.
These undesirable performances greatly affect both power and fuel consumption and may
take place under different speed and load conditions.


2.3.4. Compression Ignition (Diesel) Engines

Compression-ignition (CI) engines use fuel oil, which is injected into the combustion
chamber when the air charge is fully compressed. Burning of fuel takes place due to self-
generated heat of compression. These types of engines are also known as ‘oil engines’. Same as
four-stroke SI engines, the CI engines also completes all the four phases of the cycle in two
crankshaft revolutions or four piston strokes. Figure 2.8 illustrates the principle of operation of
four-stroke diesel engines.
The problem of escaping of certain amount of fresh mixture along with outgoing exhaust
gases in a two-stroke petrol engine is avoided in a two-stroke CI engine as in this case only air
enters the cylinder during induction cycle. Fuel is not injected until all the ports and valves are
closed. Figure 2.9 illustrates the operation of two-stroke diesel engines. A typical 12 litre
four-stroke engine and a 7 litre two-stroke engine having the same speed range develop similar
torque and power ratings. The ratio of engine capacities for equivalent performance for these
two engines is 1.7:1.
Principle of operation of four-stroke 'diesel engine.
Fig. 2.8. Principle of operation of four-stroke ‘diesel engine.
Fig. 2.9. Principle of operation of two-stroke diesel engines.
Principle of operation of two-stroke diesel engines.

Operation of the Four-stroke Cycle Diesel Engine.

Induction-Stroke.

With the inlet valve open and the exhaust valve closed, the piston moves
down away from the cylinder head (Fig. 2.8A). This causes depression in the cylinder, the
magnitude of which depends on the ratio of the cross-sectional areas of the cylinder and the inlet
port, and on the speed of the piston. A maximum depression of the order of 14.7 kPa below
atmospheric pressure occurs at about one third of the distance of the stroke, while the overall
average pressure in the cylinder might be 9.8 kPa or less. The reduction in pressure in the
cylinder induces air into the combustion chamber.

Compression Stroke.

With both the inlet and exhaust valves closed, the piston moves
towards the cylinder head (Fig. 2.8B) and air enclosed in the cylinder is compressed from one
twelvth to one twentieth of its original volume. The pressure of compressed air becomes 2945
to 4905 kPa with increase in temperature to at least 873 K under normal operating conditions.

Power Stroke.

With both the inlet and the exhaust valves closed and when the piston has
reached almost the end of compression stroke (Fig. 2.8C), diesel fuel is injected into the heated
and dense air as a high-pressure spray of fine particles. The heat of compression quickly
vaporises and ignites the tiny droplets of liquid fuel. Extensive burning of fuel then releases the
heat energy, which is rapidly converted into the pressure energy causing expansion of the
products of combustion. As a result the piston is pushed away from the cylinder head producing
useful work.

Exhaust Stroke.

When the piston has reached the outermost position, the exhaust valve
is opened. The piston now moves in the reverse direction towards the cylinder head (Fig. 2.8D).
The sudden opening of exhaust valve towards the end of the power stroke releases the burning
products of combustion to the atmosphere. The pressure energy of the gases at this point helps
their expulsion from the cylinder. Only towards the end of the exhaust stroke, the piston actually
catches up with the tail-end of the outgoing gases.

Operation of the Two-stroke Cycle Diesel Engines.

Induction and Exhaust Phase.

The piston moves away from the cylinder head and, when
it is about halfway down its stroke, the exhaust valves open causing escape of the burnt gases
into the atmosphere. Towards the end of the power stroke, a horizontal row of inlet air ports is
uncovered by the piston lands (Fig. 2.9A) so that pressurised air from the blower is admitted
into the cylinder. Simultaneously exhaust gases are forced out of the cylinder into the exhaust
system. This process of fresh air coming into the cylinder and pushing out burnt gases is known
as scavenging.

Compression Stroke.

The piston moves towards the cylinder head. The inlet air ports are
closed, and just a little later the exhaust valves are closed (Fig. 2.9B). Subsequently, the fresh
trapped air is compressed and its original volume reduces to about one fifteenth to one
eighteenth as the piston reaches the top position. This change in volume corresponds to a
maximum cylinder pressure of about 2945 to 3925 kPa.

Power Stroke.

Shortly before the piston reaches the innermost position on its compression
stroke, highly pressurised liquid fuel is injected into the dense and heated air (Fig. 2.9C).
Consequently the injected fuel droplets vaporise and ignite leading to rapid burning. The heat
liberated in the process of burning is converted into gas-pressure energy, which expands and
produces useful power.

Comparison of Two-and Four-stroke-cycle Diesel Engines.

(a) Theoretically, a two-stroke engine can develop almost twice the power compared with
a four-stroke engine.
(b) With the two-stroke engine, the emptying and filling can be carried out by light rotary
components, unlike a four-stroke engine where same parts perform all jobs.
(c) With a two-stroke engine 40 to 50% more air consumption is necessary for the same
power output.
(d) In a two-stroke engine, about 10 to 20% of the Upward stroke is sacrificed in emptying
and filling the cylinder.
(e) The time available for emptying and filling a cylinder is respectively about 33% and
50% of the completed cycle in a two-stroke engine and a four-stroke engine. More power
is needed to force a mass of air into the cylinder in a shorter time.
if) At low engine speeds, more power is needed by the piston for emptying and filling the
cylinder in a four-stroke engine compared with a two-stroke engine due to pumping
and friction losses. At higher engine speeds the two-stroke’s Rootes blower consumes
more engine power up to 15% of the power developed at maximum speed.
(g) With reduced engine load for a given speed, a two-stroke engine blower consumes
proportionally more engine power developed by the engine.
(h) A two-stroke engine runs smoother and relatively quieter as compared with a four-
stroke engine. .

Comparison of SI and CI Engines.

Fuel Economy.

Thermal efficiency is defined as the ratio of the useful work produced to
the total energy supplied and is considerably influenced by the selected compression-ratio and
design. Petrol engines can produce thermal efficiencies ranging between 20 and 30%, whereas
the corresponding diesel engines generally develop improved efficiencies, between 30 and 40%.

Power and Torque.

The petrol engine is usually have a shorter stroke and operates over
a high speed range of the crankshaft than the diesel engine. Therefore, more power is obtained
in petrol engine towards the upper speed range, which matches with the requirement for high
road speeds. On the other hand a long-stroke diesel engine has improved pulling torque over a
relatively narrow speed range, which is ideal for heavy commercial vehicles. A.diesel engine,
therefore, provides higher torque at low speeds, but with a rough running.
, Reliability. Diesel engines are built sturdier, tend to run cooler, and ha”ve only half the
speed range of most petrol engines. Therefore, the diesel engine is more reliable and has
extended engine life relative to the petrol engine. Diesel engines also require relatively less
maintenance.

Pollution.

Diesel engines are noisy and vibrate on their mountings at the part load
operation. The combustion process is quieter in the petrol.engine and it runs smoother than the
diesel engine. * • • ‘ •
The diesel engine exhaust is more noticeable, particularly if any of the injection equipment
components are out of tune. The visible smoky diesel exhaust contains more NOx and the
relatively invisible exhaust gases from the petrol engine contain HC and CO.

Safety.

Unlike petrol, diesel fuels are not flammable at normal operating temperature, so
they are not handling hazard and have no fire risks.

Cost.

Diesel engines are more expensive than petrol engines due to their heavy construction
and injection equipment.

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