It is almost
certain that your training aircraft will be fitted with a four
cylinder Lycoming or Continental engine. These are air-cooled
flat fours, that is four horizontally opposed cylinders. The
old VW Beatle car had an engine of the same configuration and
indeed some homebuilt aircraft have used a modified version of
this powerplant
with some success.
Aircraft
engines are built to very different criteria to those in
automobiles. As you cannot pull over to the side of the sky in
the event of breakdown, reliability has been the first and
foremost priority. These designs have been around for decades,
and just about every possible bug has been designed out. They
are also designed to give maximum power output continuously
for 2000 hours. If you tried that with an automobile engine,
it would be in the scrap yard very fast indeed! As a student
pilot, you will be expected to understand the basic principles
of operation, and you will be taught how to check oil levels
and check for integrity of the alternator belt.
Engine Operation
Flat four aero engine - The cylinders are arranged in two banks on opposite
sides of the engine. This arrangement is by far the most common.
The Intake
Stroke
During the
intake or admission stroke, the piston moves downward as a
charge of combustible fuel and air is admitted into the
cylinder through the open intake valve. At the completion of
this stroke the intake valve closes.
Compression Stroke
During the
compression stroke, the crankshaft continues to rotate, the
piston is forced upward in the cylinder, and both intake and
exhaust valves are closed. The movement of the piston upward
compresses the fuel-air mixture.
Power Stroke
As the piston
approaches the top of its stroke within the cylinder, an
electric spark jumps across the points of the spark plugs and
ignites the compressed fuel-air mixture. This is the ignition
event. The intake and exhaust valves are closed.
Having been ignited, the fuel-air mixture burns. It expands as
it burns and drives the piston downward. This causes the
crankshaft to revolve. Since it is the only stroke and event
that furnishes power to the crankshaft, it is usually called
the power stroke, although it is sometimes called the
expansion stroke for purposes of instruction. This is event
power stroke. The intake and exhaust valves are closed.
Exhaust
Stroke
During the
power or expansion stroke, the hot gases obtained by
combustion exert tremendous pressure on the piston to force it
to move downward, but near the end of the stroke this pressure
is greatly reduced because of the expansion of the gases. At
this stage, the exhaust valve opens as the crankshaft
continues to revolve and the piston is again moved upward in
the cylinder by the connecting rod. The burning gases
remaining in the cylinder are forced out through the exhaust
valve, hence this stroke is usually called the exhaust stroke,
although it may be called the scavenging stroke for purposes
of instruction. One engine cycle has been completed.
Reciprocating-Engine Horsepower
According to the most common
definition of horsepower, one horsepower is defined as
exactly:
1 hp = 745.69987158227022 W
Most persons
are acquainted with the term horsepower as applied to
automobile and aircraft reciprocating engines.
The horsepower was first used by
James Watt during a business venture where his steam engines
substituted horses. It was defined that a horse can lift
33,000 pounds force (the weight of a 15,000 pound mass on
Earth) with a speed of 1 foot per minute: 33,000 ft·lbf·min-1.
This is roughly equivalent to lifting 147,000 Newtons (the
weight of a 15,000 kg mass on Earth) at a speed of 0.005 metre
per second.
If an aircraft reciprocating engine is rated at 200
horsepower, it means the engine is capable of producing this
much power. However, the engine has to be running at a certain
speed before that much power is produced. The same is true for
all other types of reciprocating engines. Unlike car engines,
aircraft engines are designed to operate at their full rated
power output for a considerable time (usually 2000 hours)
Engine
power output
Engine power
is the product of force by time: torque is the force and
engine speed, measured in crankshaft revolutions per minute [rpm],
is the time. Torque is the rotational force produced by
a force acting about the engine crankshaft i.e. it is the
product of the firing stroke in the cylinder and the radius of
the crank to which the connecting rod is attached; and the
bigger the cylinder the bigger the force the bang.
Normally aspirated aero engines
The maximum
power which can be developed, in the cylinders of a particular
piston engine, increases or decreases directly with the intake
manifold air density, and air density decreases as altitude
increases or temperature increases. Thus the full throttle
power output of a normally aspirated engine ( i.e. one
that relies solely on the ambient atmospheric density)
decreases as operating altitude increases. The diagram in
below shows how maximum brake horse power, delivered at full
throttle in a normally aspirated engine, decreases with
altitude. A 100 hp engine operating at 65% power will be
delivering 65 hp.
Power produced
is proportional to the air density at the air intake manifold,
the cylinder displacement and compression ratio, the number of
cylinders and the rpm. Of those items only the air density at
the air intake manifold and the engine rpm alter, or are
alterable, during flight. A traditional four stroke light
aircraft engine, such as the Lycoming O-235, has an individual
cylinder displacement of 950 cc, a compression ratio of 7:1
and a maximum design speed of 2600 rpm at which its rated 110
brake horsepower [bhp] is produced in sea level ISA
conditions. The Rotax 912, the most common light weight four
cylinder aero engine, utilises an individual cylinder
displacement of only 300 cc, a compression ratio of 9:1 but
doubles the maximum design speed to 5500 rpm to achieve its
rated 100 bhp. The very light weight Jabiru 2200 has an
individual cylinder displacement of 550 cc, a compression
ratio around 8:1 and a maximum design speed of 3300 rpm to
achieve its rated 80 hp.
The three engines mentioned are all horizontally opposed, four
stroke and four cylinder; a popular configuration providing a
fully balanced engine that doesn't require crankshaft balance
weights. Engines are often described in terms of 'total
capacity' [cylinder displacement by number of cylinders] in
litres or cubic centimetres. Thus the Lycoming O-235 is 3800
cc or 3.8 litres [235 cubic inches], the Rotax 912 is 1200 cc
or 1.2 litres and the Jabiru 2200 is 2.2 litres. Most engines
used in ultralights tend to be around 30% lighter (in terms of
weight per rated hp) than the ubiquitous Lycoming and
Continental piston engines used in general aviation aircraft:
thus they are cheaper to manufacture but less robust, with a
consequent shorter time between overhaul [TBO].
The term 'brake horsepower' is a measure of the power
delivered at the engine output shaft measured by means of a
dynamometer or similar braking device. The term 'shaft
horsepower' [shp] is a measure of the engine power available
at the propeller shaft. Generally it is the same as bhp but if
the coupling is not direct drive reduction gearing is
interposed between the crankshaft output and the propeller
shaft as in the Rotax 912 the shp will be a little less than
bhp because of the power loss in driving the belt, or gear,
driven reduction device.
Although aero engines can quite happily operate continually at
their rated power, doing so is not good practise, because it
is uneconomical in terms of fuel efficiency but, more
importantly, it may shorten engine life if engine operating
temperatures and pressures are exceeded. Normally the maximum
and optimum power setting for continuous cruise operation
is 75% of rated power.
Turbocharging
The volumetric
efficiency (i.e. the cylinder filling capability) of an engine
can be improved by increasing the density of the fuel/air
charge delivered to the cylinders through compressing the
air in the atmospheric air intake manifold; this process is
supercharging and develops more torque at all engine
speeds. The compressor is usually a lightweight centrifugal
impeller driven by a gas turbine that utilises the otherwise
wasted engine exhaust gas energy. Such a system is a turbine
powered supercharger, otherwise known as a turbocharger.
An oil pressure driven butterfly valve or waste gate is
incorporated within the exhaust manifold system, automatically
adjusting according to the pressure within the intake
manifold to allow all, or a portion, of the exhaust gases to
bypass the turbine; thus continually maintaining the system
within the designed operating limits. There is a slight
penalty in that turbocharging also increases the temperature
of the charge, and consequently decreases the achievable
density, unless a charge cooling device an intercooler
is incorporated between the compressor and the cylinders.
Turbocharging
may be used to increase the sea level rated power of the
engine, but the use of that full throttle power at low
altitudes would normally be limited to short periods because
of engine temperature limitations. The big advantage is the
increase in power available at altitude. The diagram plots the
power achieved (percentage of rated power) at full throttle in
ISA standard conditions for a normally aspirated engine and
the turbocharged version. The turbocharged engine can maintain
its rated power from sea level up to the 'critical altitude',
probably around 6000 or 7000 feet, after which it will
decrease. The waste gate would probably be fully open at sea
level and then start closing with altitude increase so that it
would be fully closed at, and above, the critical altitude.
Turbocharging raises the service
ceiling of the aircraft. The service ceiling is the
ISA altitude at which the aircraft's best rate of climb (from
an extended climb starting at MTOW and unassisted by any
atmospheric phenomena) drops below 100 feet per minute which
is recognised as the minimum useful climb rate. This should be
the aircraft's ceiling quoted by the manufacturer.
Two stroke aero engines
The lower
power (say 70 hp and below) engines used in ultralight
aircraft are usually two stroke engines. Two strokes don't
have very good volumetric efficiency and the engine is
efficient only in a narrow rpm and throttle opening range
occurring at very high rpm; in fact ultralight two strokes
tend to run very roughly at speeds below 2500 rpm. The three
most common two strokes are two cylinder models with
individual cylinder displacements around 250 cc and achieving
their rated power at 6800 rpm with power dropping off very
quickly as rpm is reduced below that figure. Gearing or belt
reduction is used to improve the torque delivered to the
propeller shaft whilst at the same time reducing the rpm to
something more suitable for the propeller. The torque
increases because of the larger rotational radius of the
driven gear.
Reliability of
two stroke engines has improved in recent times, however they
are still not favoured in light aircraft.
