The Art of the Helicopter John Watkinson - Karatunov.net

216 The Art of the Helicopter
limit must be increased to keep the power the same. Using this technique carburettor
icing cannot occur, pilot workload is reduced and fuel economy is improved because
feeding the engine with hot air reduces pumping loss. Given that there have been a
number of fatalities in this machine owing to icing it is difficult to see why this solution
has not been proposed earlier. The only circumstances under which hot air would be
deselected would be where ambient air temperature is extremely high or for operation
at high altitude. These conditions can easily be deduced from the flight manual.
6.14 The aeroDiesel
The Diesel engine is a type of piston engine in which the induction process admits air
only. There is no throttle and the maximum amount of air is admitted at all times. As
only air is being compressed, a phenomenal compression ratio can be used without
danger of detonation on the compression stroke. The high compression ratio which
is maintained at all power levels, is responsible for the high efficiency of the Diesel
engine. In the gasoline engine, when the throttle is partially closed, low manifold
pressure opposes the motion of the piston on the induction stroke. The compression
pressure falls, effectively reducing the compression ratio.
The compression raises the air temperature to such an extent that if fuel is forced
into the cylinder it will burn immediately. The amount of power is controlled only by
the amount of fuel injected. As combustion takes place with excess air, all of the fuel is
burned, further raising efficiency. No ignition system is needed at all. Instead a device
called an injection pump is required. This device produces the right amount of fuel at
sufficient pressure to overcome the compression pressure and at the correct time. Fuel
is fed into the cylinder by an injector. This is effectively a non-return valve that opposes
the compression pressure in the cylinder. When the pressure from the injection pump
has risen high enough, the injector admits the fuel in a conical spray pattern.
Diesel fuel is less volatile than gasoline and represents a reduced fire hazard. On
injection, the fuel begins to burn instantly and this results in a rapid pressure rise.
The Diesel engine needs to be robustly constructed to withstand the forces generated
by the high compression ratio and the sudden pressure step. However, the Diesel has
the advantage that the materials are not subject to such high temperatures. As it is
thermally efficient, less heat is created in the engine and the exhaust is cooler, reducing
stress on the exhaust valves. Water cooling is particularly advantageous with Diesels
because the water jacket reduces noise and the reduced heat output allows the cooling
system to be light.
As the Diesel engine induces only air and has no throttle, the danger of intake icing is
insignificant and cold air will be supplied at all times for maximum power. The excess
induction air may be used to sweep all combustion products out of the cylinder ready
for the next cycle. This is called scavenging and in a four-stroke Diesel it is done by valve
overlap. Some of the induction air follows the exhaust gases out of the cylinder. The
exhaust valve is also cooled due to scavenging. This is especially easy in a turbocharged
Diesel where turbo pressure assists the scavenge flow. Clearly scavenging cannot be
used in the gasoline engine as it would result in unburned mixture entering the exhaust
system.
The turbocharger works well in gasoline engines, but the extent to which power can
be boosted is limited by the additional heat stress on the engine parts. As the Diesel
produces less heat stress the degree to which it can be boosted by a turbocharger is
higher. As the compressor in the turbocharger will increase the temperature of the
induction air, a further increase in power can be obtained using an intercooler, which