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.
• 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.
In 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:
a) 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.
b) 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.
c) 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.
d) 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.
Basic Construction & Components
a) 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.
b) 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.
c) 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.
d) 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.
e) Crankcase: is the housing that contains the crankshaft and serves the purpose of;
i. Mounting the cylinders
ii. Support the crankshaft
iii. Oil-tight internal lubrication
iv. Support for attachment of accessories
f) 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.
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 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.
a) High manifold pressure (excessive temperatures)
b) High air intake temperature
c) Overheated engine
d) Low octane rated fuel (High octane fuel resists greater temperatures and pressures)
e) Incorrect use of mixture control (Mixture too lean)
a) Excessive cylinder temperature and pressure
b) Rough running engine (self-destruction through vibration)
c) 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)
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.
a) Fuel octane too low
b) Mixture too lean
c) Incorrect ignition timing
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.
Power Output as a Function of RPM
Opening the throttle results in an increase in RPM. This means faster firing pistons, or greater power being produced. Power is defined as the amount of work done over time. It is also responsible and directly related to the amount of thrust being produced by the propeller in a constant speed, or fixed pitch propeller. RPM readings are a valuable indication as to the quality of power being produced by the engine. Abnormal RPM readings (too high or too low) on the take-off roll should be good reason to abort, and further inspect the problem.