The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 6.5 A typical horizontally opposed aircraft engine in section. See text for details.
Engines and transmissions 199
is constructed so that the pistons all move in and out simultaneously. The reaction due
to accelerating the piston on one side is then balanced by that from the other side and
vibration is reduced. The cylinders are not precisely opposite one another to give room
for the crank web.
The large bearing in the connecting rod is called the big end. The aluminium pistons
have slots fitted with rings which are springy metal strips. These press outwards against
the cylinder wall and ensure a good pressure seal. In a piston engine four distinct stages
are needed in the complete cycle, and in the four stroke or Otto cycle engine, the piston
traverses the cylinder four times, which requires two revolutions of the crankshaft.
The cylinder head is fitted with valves allowing fresh charge to enter the cylinder and
burned charge, or exhaust, to leave. The valves are operated by a camshaft fitted above
the crankshaft. This carries a series of rounded bumps called lobes. As the camshaft
turns, each lobe presses against the flat end of a small piston-like object known as a
tappet. In the tappet there is a swivel joint that connects to the push rod. This travels
in a tube up the outside of the cylinder to a rocker in the cylinder head that pushes the
valve open. When the cam lobe retreats, the valve is closed by the valve springs, which
also push back the rocker, the pushrod and the tappet. When the valve is closed there
must be a little slack in the pushrod, known as valve clearance, so that the whole spring
pressure is keeping the valve shut.
Since the full four strokes require two crankshaft revolutions, the camshaft is driven
through a 2 : 1 reduction gear so the sequence of valve openings resulting from one
camshaft rotation is spread over the two crankshaft revolutions. On the intake stroke,
the inlet valve is opened and charge is drawn into the cylinder. On the compression
stroke both valves are closed and the charge is squeezed into the remaining space above
the piston. On the power stroke, pairs of spark plugs ignite the charge, and it burns and
expands, driving down the piston and turning the crankshaft. On the exhaust stroke, the
exhaust valve opens and the piston pushes out the spent charge. The cycle then repeats.
In practice momentum of the gases means that the valves operate at a slightly different
time than this simple explanation would indicate. Figure 6.6 shows a typical valvetiming
diagram. During the induction stroke the charge entering through the inlet
manifold is travelling at high speed and cannot stop easily. The inlet valve is left open
until after bottom dead centre (BDC). Momentum will continue to force charge into
the cylinder so that extra charge can be admitted allowing more power to be generated.