The diagrams in Fig. 168 are for cylinders set at 90° and firing
alternately.
The interval between consecutive explosions is not quite regular,




180 360 640 720

Crank Angle.
Fig. 167. — The Two-Cylinder Opposed Engine Torque Diagram.
the intervals being alternately | and J of a revolution, or expressed
in strokes, three strokes and one stroke respectively of idle
working.
The ratio of maximum to mean torque is 4-0.
It wiU be apparent that the smaller the angle between the
cylinders, the nearer does the total or combined torque curve
approach that of a two-cylinder engine with one crank only, and
the more regular do the firing periods become, that is, the more
uniform the periods of maximum crank effort.
Case V. — Four-cylinder with cranks iti pairs at 180°, as shown
in Figs. 169 and 169a, the order of firing being 1, 2, 4, 3 — ^this
being the ordinary motor-car engine arrangement.
V



290 HIGH SPEED INTERNAL COMBUSTION ENGINES

The crank effort diagram indicates that the firing periods are
quite regular and follow on consecutively with no interval be-
tween.
There are four periods of firing or useful crank effort during




Fig. le



-The 90° Twin Engine.
250 -1
















200 -




Max._^Q




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f




150 -


Mean "^ "


100 -
50 -


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_ —


\\
\ \
\ \

. 4- V —


i- _


\




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^--\ — ,




\ \
\ \


i—/!^


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^^


^^:>X-




\ \ J


/




50 -

















180 360 540 720

Crank Angle.
Fig. 168a. — The 90° Twin Engine Tokqtje Diagkam.
every two revolutions, and consequently four peaks to the com-
biaed torque curve.
The ratio of the maximum to the mean torque in this case is
2-0 approximately.
