Multi-cylinder Engines (Automobile)

2.4.

Multi-cylinder Engines

The power developed by an engine can be increased by enlarging the size of a single cylinder
or having more cylinders of same size. A single large cylinder may be a more convenient choice
due to fewer parts to manufacture and maintain, but the advantages are over-weighed by the
disadvantages. The ratio of the piston head areas and cylinder capacities of two engines, one of
which has twice the linear dimensions of the other, are as follows.

With the same mean effective cylinder gas pressure in both engines, the piston thrust
increases in proportion to the piston head area. Therefore by doubling the cylinder diameter,
fourfold increase in the piston thrust takes place. For a given piston speed and mean effective
gas pressure, the engine power increases with the square of the cylinder diameter. Therefore
by doubling the cylinder diameter fourfold increase in the power occurs. The volume and hence
mass of the reciprocating components increases with the cube of their dimensions. Therefore
doubling the piston dimensions increases the mass by eightfold, due to which maximum piston
speed would have to be reduced. If the piston stroke for a given crankshaft speed in doubled,
the piston speed is also doubled. To maintain the same piston speed for both engines, the
crankshaft speed of the large engine is required to be halved. The torque is proportional to the
piston thrust and the crank-throw length. Hence, by doubling the piston diameter and stroke,
the piston thrust is increased fourfold and the crank-throw leverage is doubled, thereby torque
is increased by eightfold.
Therefore, by doubling the cylinder bore dimensions, the power becomes four times while
the weight becomes eight times. Hence the weight increases at a greater rate in comparison to
the power, providing a low power to weight ratio. The multi-cylinder engines can produce higher
power output due to higher rpm in comparison to single cylinder engine.

2.4,1.

The Cyclic-torque and the Flywheel Effect

The four-stroke cycle engine completes one cycle of operation in two revolutions or 720
degrees movement x>f the crankshaft ; thereby each of the four strokes corresponds to half a
‘ revolution or 180 degrees rotation of the crankshaft. Out of four strokes i.e. induction, compres-
sion, power and exhaust, only the power stroke supplies energy to drive the crankshaft against
various resisting loads, while other three remaining strokes absorb some energy in overcoming
the pumping and frictional”losses. Moreover, there are reciprocating inertia loads caused due
to the reverse effort put to change the direction of mbtion of the piston assembly every time it
reaches its TDC’or BDC pistion. As a result there is a considerable fluctation in fcrankshaft
speed in every cycle of operation due to the variation of useful cylinder pressure through out
the power stroke and the opposing friction, pumping, and inertia loads.
The flywheel fixed to the end of the crankshaft absorbs excess energy when the crankshaft
accelerates during its 180 degrees power stroke and automatically transfers this stored kinetic
energy to the crankshaft to overcome the turning resistance during the next 540 degrees
comprising of three non-working strokes. The crankshaft decelerates as the flywheel gives up
energy to drive the crankshaft over the three idle strokes, but a recovery of speed takes place
due to the expansion movement of the piston during power stroke. Thus flywheel reduces
crankshaft speed fluctuations during each cycle of operation. The energy imparted to the
flywheel and crankshaft sometimes exceeds the average resisting load in the engine and other
times it can be well below this value. This causes the flywheel to undergo corresponding speed
fluctuations (Fig. 2.10). The average height of the torque diagram represents the torque
equivalent to the steady load imposed upon the engine. The shaded area above the average-
torque line indicates the excess energy stored in the flywheel, and the energy below the average
line shows the energy drawn from the flywheel during one cycle.

Fig. 2.10. Single-cylinder constant-load flywheel effect.
At the beginning of the power impulse, the flywheel is at its minimum speed and near the
end of the power stroke it is at its maximum speed. For the cycle of event to continue, the excess
and deficiencies of energy must be equal. This means the kinetic energy of the flywheel during
its speed increase and decline must be same. Since the degree of speed variation over each cycle
depends on the size of flywheel, a large flywheel dampens the speed fluctuation to a minimum,
causing the engine to run smoothly at constant speeds. But, a large flywheel mass opposes any
rapid acceleration and deceleration of the engine, due to which the engine’s response become
torpid. On the other hand, a small flywheel definitely causes the engine to respond quickly to
rapid speed changes, but at the cost of uneven and lumpy slow-speed operation.

2.4.2.

Multi-cylinder Cyclic-torque

The limitations on the size of the flywheel and its inability to smooth out the torque
unevenness between the cycles has been largely resolved by using multi-cylinder engines where
the valve timing with the single crankshaft is sequenced so that the power strokes of the
cylinders occur in a phased manner, instead to take place all at the same time. When the number
of cylinders is increased, accordingly the intervals between power impulses reduce. Consequent-
ly, the torque variation throughout the four strokes of the cycle is smoothed out.
The cyclic-torque curve for a single-cylinder engine (Fig. 2.10) shows a firing stroke every
720 degrees and the variation of peak to mean torque over one cycle is around 8:1. When a second
cylinder is added, the interval between the firing impulses is halved, i.e., 360 degrees, thereby
reducing the peak to mean torque produced over a cycle to 4:1 (Fig. 2.11 A). By adding a third
cylinder, the interval between the firing impulses reduces to 240 degrees, and the peak to mean
turning-effort is further smoothed out to the order of 2.8:1 (Fig. 2.11B). The four-, five-, six-, and
eight-cylinder engines have firing intervals of 180 degrees, 144 degrees, 120 degrees, and 90
degrees respectively, with respective ratios of peak to mean torque are reduced to 2:1, 1.7:1,
1.4:1, andl.l:! (Figs. 2.11 C through F).

Fig. 2.11. Multi-cylinder engine torque diagrams.
A. Twin-cylinder engine B. Three-cylinder engine.
C. Four-cylinder engine D. Five-cylinder engine.
E. Six-cylinder engine F. Eight-cylinder engine.

2.4.3.

Merits and Limitations of Single- and Multi-cylinder Engines

The following major factors are required to be considered while comparing engines of
different cubic capacity and various numbers of cylinders.
(a) For a given maximum piston speed the shorter the piston stroke, the higher can be
the crankshaft rotation.
(b) As the cylinder becomes smaller, the piston becomes lighter in proportion to the
cylinder size, accordingly causing higher piston speeds.
(c) For the same engine cylinder capacity and maximum piston speed, a multi-cylinder
engine develops more power than a single-cylinder engine.
id) A single-cylinder engine with the same piston cross-sectional area as a multi-cylinder
engine produces a greater torque output.
(e) The smaller the cylinder size, the higher is its surface-to-volume ratio and hence higher
is the compression-ratio with an improvement in engine thermal efficiency.
if) For a given total volume, acceleration response improves with the number of cylinders,
because of lighter reciprocating components and the smaller flywheel.
(g) As the number of cylinders and the engine length increase torsional vibration becomes
a problem.
(h) As the number of cylinders increases
• the power consumed in overcoming rotational and reciprocating drag also increases,
• mixture distribution for carburetted engines becomes more difficult,
• the cost of replacement of components becomes proportionally higher, and
• the frequency of power impulses increases, due to which the power output becomes
more consistent.
Smooth operation of multi-cylinder engines is possible only when each combustion chamber
produces the same combustion chamber pressure as others in the same engine. The carburettor
should ensure the charge quality by mixing the fuel into the incoming air in the correct
proportions. The intake manifold should direct an equal quantity of the mixed charge to each
intake valve. Each intake valve must be timed same as the others to allow an equal quantity of
the charge to enter each combustion chamber. The ignition distributor must be timed to send a
spark across the spark plug gap when compression is attained to the same amount in all the
cylinders. When all these requirements are met the pressure in the combustion chambers is
equal. But practically these ideal requirements are not met under all operating conditions due ‘
to an increase in manufacturing cost. Multi-cylinder engines are preferred to single-cylinder
engines, which would give the same output due to the following reasons :