The Art of the Helicopter John Watkinson - Karatunov.net

218 The Art of the Helicopter
Fig. 6.12 In the uniflow two-stroke Diesel engine, induction is by ports in the cylinder wall, whereas exhaust
is via valves in the head. The short scavenging path and potentially large cross-sectional area of the intake and
exhaust ports allow high mass flow.
fear of mixing the oil with the charge. Thus the uniflow is a two-stroke technology that
can meet environmental regulations.
The uniflow system is more effective at replacing exhaust with charge than either the
conventional two stroke or the four stroke. In the conventional two stroke the fresh
charge is admitted at the bottom of the cylinder and then has to proceed to the top and
turn around to reach the exhaust port. This is called loop scavenging. The four-stroke
engine needs two complete strokes to do this. Figure 6.12 shows that the breathing
efficiency of the uniflow is high because much of the cylinder head can be occupied
by exhaust valves and the intake port can run round the perimeter of the piston save
for a few rails to stop the rings popping out. As a result the uniflow Diesel can deliver
power over a higher proportion of the cycle than can four-stroke or loop scavenged
two-stroke engines.
When a turbocharger is fitted, the efficient gas flow means that the valves can be
arranged to open for a smaller angle, lengthening the effective stroke. When this
approach is used, the engine produces its power in the form of more torque at a lower
RPM. In the helicopter this is useful as the amount of gearing to the rotor may be
reduced.
The uniflow engine is not self-starting and it needs some externally produced induction
flow during cranking. The turbocharger may be fitted with an electric motor
powered during starting.
In the two-stroke uniflow Diesel engine developed by Teledyne Continental Motors,
some additional engineering features are incorporated. As the force on the piston is
always downwards, a slipper big end can be used. Figure 6.13 shows that the big
end bearing does not encircle the crankshaft. This means that both cylinders in a
horizontally opposed engine can share the same crankpin and be exactly in line. The
thrust of a piston on a power stroke drives straight through the crankpin to push
the opposite piston up on the compression stroke. In a four-cylinder engine the two
crank pins are at 90 ◦ to give four evenly spaced power strokes per revolution. This
configuration requires balance weights on each end of the crankshaft to make the mass
centroid of the moving parts align with the shaft axis. The result is very low vibration.