The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 217
is a heat exchanger designed to cool the air between the compressor outlet and the
induction manifold. An alternative on water-cooled Diesel engines is a charge cooler
which uses the circulating coolant to cool the air from the compressor.
The boost gauge reading is not proportional to power or engine stress in a Diesel
engine since there is no throttle. If a turbocharger is fitted, the boost gauge will indicate
that the correct turbo outlet pressure exists. A malfunctioning wastegate could cause
excessive boost and an abnormally high gauge reading. In this case it would be necessary
to fly at a lower power level to prevent engine damage.
In order to display the power level in a Diesel engine it is necessary to measure fuel
flow. However, a torque meter could also be used as in turbine practice.
6.15 The uniflow Diesel
The two-stroke engine has some advantages for aviation, not least the saving in weight
and complexity due to the elimination of some moving parts. Although power is produced
on every piston downstroke, less of the stroke is used and so the increase in power
is not as great as is commonly thought. However, the doubling in firing frequency for
a given RPM allows vibration to be halved. This may result in transmission weight
saving.
At the end of the power stroke, the exhaust gases have to be replaced by fresh charge
at one and the same time. Thus effective scavenging is inherent in two-stroke engines.
Some incoming charge goes out of the exhaust no matter what. The gasoline engine
mixes fuel and air externally and so the scavenging process passes unburned fuel into the
exhaust, doing no good to the fuel economy or the environment. A further problem with
the gasoline two-stroke is that most designs use the crankcase as part of the induction
system. When the piston comes down, the charge in the crankcase is compressed and
the pressure is used to drive it into the cylinder. The presence of charge in the crankcase
means that lubricating oil will be mixed with the charge, resulting in a smoky exhaust.
The conventional gasoline two-stroke engine will eventually be outlawed because of
these environmental concerns.
The two-stroke Diesel engine can overcome these problems because it admits air not
charge. One approach is the uniflow two-stroke Diesel shown in Figure 6.12. This is
mechanically somewhere between a four stroke and a two stroke as it still has a camshaft
and valves. However, all of the valves are exhaust valves and a huge valve area can be
used for efficient breathing.
Towards the end of the power stroke the camshaft opens the exhaust valves. Exhaust
gases exit the cylinder and their momentum causes the cylinder pressure to fall. Shortly
after, the piston uncovers ports near the base of the cylinder to admit induction air.
Effectively the cylinder is open at both ends so that induction air can sweep up the
cylinder until some of it leaves via the exhaust valves in the scavenge process. Next the
exhaust valves close but the momentum of the induction air continues to drive it into
the cylinder. This continues until the returning piston closes the induction ports. The
cylinder is now sealed and the air is compressed. Near TDC, the fuel is injected and
the power stroke begins. In an automotive engine there is one injection pump feeding
all of the cylinders. However, in aviation applications there will be one per cylinder so
that a failure will cause a power loss rather than a stoppage. The injector can run from
the camshaft.
The uniflow two-stroke Diesel has a number of advantages. The crankcase is not
involved in the induction process and remains at atmospheric pressure just as it does
in a four-stroke engine. Conventional recirculating oil lubrication can be used, without