6.2.4 Ignition Systems
The basic requirement of any ignition system is to deliver a high-tension spark to the spark plugs in each cylinder in the correct firing order at the correct time. This spark ignites the fuel/air mixture powering the pistons and producing work so to turn the propeller.
Magneto Ignition
A magneto uses a permanent magnet to generate an electrical current completely independent of the aircraft’s electrical system. The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder. The system begins to fire when the starter is engaged and the crankshaft begins to turn. It continues to operate whenever the crankshaft is rotating. The magneto system is mounted on the accessory drive unit located at the rear of the engine.

2502299114631500Most standard certificated aircraft incorporate a dual ignition system with two individual magnetos, separate sets of wires, and spark plugs to increase reliability of the ignition system. Each magneto operates independently to fire its own spark plug in each cylinder. The firing of two spark plugs in each cylinder improves combustion of the fuel-air mixture and results in a slightly higher power output. If one of the magnetos fails, the other is unaffected. This redundancy allows the engine to continue normal operation, although a slight decrease in engine RPM can be expected. The same concept of redundancy applies to the spark plugs. Operation of the magnetos are controlled in the flight deck through the ignition switch. The switch has five positions:
OFF
R (right)
L (left)
BOTH / START

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With RIGHT or LEFT selected, only the associated magneto is activated. The system operates on both magnetos when BOTH is selected.
A malfunctioning ignition system can be identified during the pre-takeoff check by observing the decrease in R.P.M that occurs when the ignition switch is first moved from BOTH to RIGHT and then from BOTH to LEFT. A small decrease in engine R.P.M is normal during this check. The permissible decrease is listed in the POH. If the engine stops running when switched to one magneto or if the rpm drop exceeds the allowable limit, do not fly the aircraft until the problem is corrected.
The cause could be fouled plugs, broken or shorted wires between the magneto and the plugs, or improperly timed firing of the plugs. It should be noted that “no drop” in R.P.M is not normal, the aircraft should not be flown and sent in for immediate inspection.
“No drop” means one of the magnetos is not grounding and can result in unintended start-up by turning the propeller by hand. Following engine shutdown, turn the ignition switch to the OFF position. Even with the battery and master switches OFF, the engine can fire and turn over if the ignition switch is left ON and the propeller is moved because the magneto requires no outside source of electrical power. Be aware of the potential for serious injury in this situation.
Even with the ignition switch in the OFF position, if the ground wire between the magneto and the ignition switch becomes disconnected or broken, the engine could accidentally start if the propeller is moved with residual fuel in the cylinder. If this occurs, the only way to stop the engine is to move the mixture lever to the Idle Cut-Off (I.C.O) position, then have the system checked by a qualified AMO.
Starting Aids
In order to produce a spark in the plugs, the magneto spins a magnet within a soft iron coil core. This creates an alternating current within the coil and creates as much as 20 000 volts to fire the spark plugs. Effective sparks are only produced once the magnet is rotating at speeds of about 500 R.P.M. Any speed below this results in weaker sparks and prolongs engine start-up. Low engine R.P.M on start-up require a delayed spark in order to prevent kick-back. This is a premature power stroke caused by normal magneto timing set for higher R.P.M and can lead to the crankshaft being forced in the wrong direction.
Impulse coupling is incorporated into one of the magnetos to help overcome this problem on start-up. The benefits are two-fold. Firstly, it accelerates the rotation of the magnet producing a higher voltage, and therefore better spark, and secondly to retard the spark at lower R.P.M experienced during start-up.

The magneto employs spring weights and a spring-loaded coupling which initially prevents the magneto from turning. Once the spring is fully wound it releases the magnet which now, due to the increased rotational velocity, results in both a hot and late start during start. Once the engine is running, the centrifugal force of the flyweights ensures the impulse coupling is disconnected.

Ignition leads carry the high voltage generated by the magnetos to the spark plugs. The spark plug is designed in such a way as to allow this high voltage to jump between the central insulated electrode and the grounded electrode, creating a spark of sufficient intensity to ignite the fuel/air mixture under compression.
The spark plug is a useful indicator of engine condition. At each mandatory inspection the plugs are removed for inspection and testing. Normal engine operation is indicated by a light grey coating of the end of the plugs. Excessive wear may indicate detonation. In cases where the engine has been operated with too rich a mixture, black sooty-like deposits will appear. Whereas white powdery deposits will be found on plugs from engines operated with too lean a mixture. Black oily deposits indicate excessive oil consumption. If hard brittle deposits are found in the end cavity, lead in the fuel is not being removed during combustion, and if left, the deposits can build up sufficiently causing the high voltage to ground without a spark. This often results in a “mag drop” which can be identified by a rough running engine and an excessive loss in R.P.M. If this is detected on the ground during magneto checks, the flight must be abandoned. The gap between the central and ground electrode is adjustable and should be set to manufacturer requirements. Too small, or large a gap will affect the efficiency and size of the spark produced and can lead to incomplete combustion and a rough running engine.

6.2.1 Powerplant
An aircraft powerplant, or piston engine, produces thrust to propel an aircraft. Internal combustion engines are most commonly used in light aircraft. These engines convert fuel into heat energy and then into mechanical energy through a four stroke cycle. This mechanical energy moves the propeller to produces thrust.
Design Types & Principles
Most small aircraft are designed with reciprocating engines. The name is derived from the back-and-forth, or reciprocating, motion of the pistons that produces the mechanical energy necessary to accomplish work. Engines are classified according to the arrangement of the cylinders.
IN LINE: These cylinders are arranged in a single row along the crankcase. Usually just six to allow for cooling. They take up little space in the cowl and are fairly low powered engines. Commonly used in light aircraft such as the Tiger Moth and Chipmunk.

V-TYPE: The cylinders are arranged in two rows and an angle of 90, 60 or 45 degrees in V form along the crankcase. Connecting rods of opposing cylinders are connected to the same crankpins. There therefore always an even number of cylinders. This reduces the weight/horsepower ratio.

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FLAT/ HORIZONTALLY OPPOSED: This is probably the most commonly used design amongst modern light aircraft. Directly opposing cylinders operate off a centrally located crankshaft resulting in a good weigh/horsepower ratio. These engines are air cooled.

RADIAL: A bank of cylinders are arranged radially about the crankshaft resulting in large, round cowls which are difficult to streamline. However this arrangement leads to a low weigh/horsepower ratio. Due to the firing order there is always an uneven number of cylinders. Usually 3, 6 or 9.

110301149531100In a four-stroke engine, the conversion of chemical energy into mechanical energy occurs over a four-stroke operating cycle. The four separate strokes of the piston occur in the following order:
The induction stroke begins as the piston starts its downward travel from the top dead centre. When this happens, the exhaust valve closes and the intake valve opens drawing the fuel-air mixture into the cylinder.
The compression stroke begins when the intake valve closes, and the piston starts moving back to the top of the cylinder. The inlet valve is timed to close shortly after bottom dead centre (B.D.C) and the exhaust valve remains closed. The fuel/air mixture is then compressed, increasing both temperature and pressure.

The power stroke begins just before top dead centre (T.D.C) when the compressed fuel-air mixture is ignited by the spark plug. This forces the piston downward away from the cylinder head, creating the power that turns the crankshaft through its first full rotation. During the down stroke, temperature and pressure decrease and the exhaust valve opens.
The exhaust stroke is used to purge the cylinder of burned gases as the piston is pushed on its second up stroke. Just before top dead centre, the inlet valve opens to take advantage of the low pressure within the cylinder and the process starts all over again.
41422546800Basic Construction & Components
Cylinders: This is the part of the engine in which power is produce through the four stroke cycle. The cylinder consists of the head which holds the inlet/ exhaust valves and the barrel manufactured from aluminium alloy and high grade steel with cooling fins on the outside.

Pistons: Are simply cast aluminium plungers that move back and forth inside the cylinders. To reduce friction between the moving piston and the cylinder wall, piston compression rings, oil control rings and oil scraper rings are mounted in groves cut into the piston.

Connecting Rods: form the link between the crankshaft and the pistons. Strength is required to withstand the force of the power stroke, while weight must be kept to a minimum to allow for the constant change in direction of the pistons.

Crankshaft: is the backbone of any piston engine. It converts the reciprocal (back and forth) motion of the pistons into rotary motion which helps turn the propeller. Much like the pedals of a bicycle. They are designed with strength and durability in mind since maximum force and wear apply to the crankshaft during operation.

Crankcase: is the housing that contains the crankshaft and serves the purpose of;
Mounting the cylinders
Support the crankshaft
Oil-tight internal lubrication
Support for attachment of accessories
Valves: A fuel/air mixture enters the cylinders through the inlet valve port and, once burned, the exhaust gas exits the cylinder through the exhaust valve port. Timing gears allow for the correct valve timing which is essential for the to the success of the stroke cycle.

Ignition Timing
Combustion of our fuel/air mixture does not occur instantaneously, it takes some time. The ignition of our spark plugs are therefore timed to occur just before T.D.C. This is known as advanced ignition. In our piston engines, since the RPM is relatively low (maximum 2700 RPM), the ignition timing is fixed. Because of this at lower RPM settings, such as start-up, the ignition needs to be delayed. One of the methods used to solve this problem is the impulse magneto. Firing off the spark plugs at the appropriate time according to the relevant RPM setting.

Detonation
Detonation occurs when the temperature and pressure of the compressed fuel/air mixture within the cylinders, or combustion chamber reaches excessive levels to cause instantaneous combustion or an explosion within the cylinder. This results in a ‘hammer-like’ blow instead of a rapid, powerful push.
CAUSES:
High manifold pressure (excessive temperatures)
High air intake temperature
Overheated engine
Low octane rated fuel (High octane fuel resists greater temperatures and pressures)
Incorrect use of mixture control (Mixture too lean)
EFFECTS:
Excessive cylinder temperature and pressure
Rough running engine (self-destruction through vibration)
Burnt valves (loss of power)
SYMPTOMS & PREVENTION:
Rough running engine and high cylinder temperatures may indicate detonation. The following action should be taken;
Mixture — Rich (assists in engine cooling)
Speed —– Increase (forward speed helps engine cooling)*Pitch nose down
Power —– Decrease (reduce cylinder pressures)
Pre-ignition
Hot spots within the cylinder cause the mixture to ignite prematurely before the spark plug fires. Hot spots can include red hot spark plug electrodes, glowing pieces of carbon or red-hot exhaust valves. Unlike detonation, pre-ignition generally occurs in only one cylinder.

CAUSES:
Fuel octane too low
Mixture too lean
Incorrect ignition timing
EFFECT:
Pre-ignition can lead to detonation, and significant engine damage. Prevention of both pre-ignition and detonation requires the engine to be operated within the correct manifold pressure settings, cylinder head temperatures and mixture settings.
Mixture settings should be slightly rich rather than too lean, as this leads to high temperatures. Ensure correct fuel rating and when in doubt, always use a higher octane rating.