Sep 2012Lesson 7.1

Power Pilot

Aero Engines

Reference

From the Ground UpChapter 3:Aero EnginesPages 47 - 86

Introduction• Aero engines, in particular piston-

engines, are complex mechanical machines that create the thrust for an airplane.

• Most new pilots train on piston-engine aircraft, and therefore need to know how the they work.

Outline• Engine Types and Parts• Stroke Cycle• Turbocharging• Cooling and Lubrication• Fuel, Carburetor, and Mixture• Ignition System• Propellers

Horsepower• 1 Horsepower = Work done to raise 33,000

lbs 1 ft in 1 min

• Indicated Horsepower = Power developed in an internal combustion engine

• Brake Horsepower (BHP) = Power available after friction and other losses

Piston Engines• Radial

– Odd number of cylinders (usually 9 max) in a circle– Advantages: Easy maintenance, good air cooling– Disadvantages: Large frontal area (creating drag)

Piston Engines• In-Line

– All pistons in a single row– Advantage: Small frontal area– Disadvantages: Bad visibility (unless inverted), long aircraft nose

Piston Engines• Horizontally Opposed

– Two banks of cylinders directly opposite each other– 4, 6, or 8 cylinders– Advantages: Flat, small frontal area

Cylinder Parts

Piston

Connecting Rod

Crankshaft

Intake Valve

CamshaftSpark Plug

Camshaft

Exhaust Valve

Combustion Chamber

Four-Stroke Cycle

InductionStroke

CompressionStroke

PowerStroke

ExhaustStroke

Four-Stroke Cycle• Induction Stroke

– Intake (AKA Inlet) valve open, piston moving down

– Negative pressure sucks in fuel/air mixture

Four-Stroke Cycle• Compression Stroke

– Both valves closed, piston moving up

– Fuel/air mixture is compressed

Four-Stroke Cycle• Power Stroke

– Both valves closed, piston moving down

– Spark plugs firing ignite fuel/air mixture, combustion forces piston down to create engine energy

Four-Stroke Cycle• Exhaust Stroke

– Exhaust valve open, piston moving up

– Burned gasses blown out of cylinder

Two-Stroke Cycle• Common on small aircraft, such as ultralights

• Combines 4 strokes into 2 with different actions in the Cylinder and crankcase

Turbocharging• Turbocharging

– Hot exhaust gasses run compressor– Compressed air provides better fuel/air mixtures at

higher altitudes

• Supercharging– Same effect as turbocharging, but compressor run

off engine crankshaft instead of exhaust gas– Less efficient than turbocharging

Turbocharging

Engine Cooling• Most aero engines are air-cooled, with fins

on the engine

• Shrouds and Baffles force incoming air around engine

• Cowl Flaps can open behind engine to allow air to flow around engine quicker, thus increasing cooling

Engine Cooling

Fins

Engine Cooling

Cowl Flap(full open)

Engine Lubrication• Lubricating oil has four functions:

– Cooling– Sealing– Lubrication– Flushing

• Oil Viscosity = Resistance to flow (stickiness)

Methods of Lubrication• Force Feed (Dry Sump)

– Oil contained in separate tank and pumped throughout engine

– Used if engine size is limited (tank can be located in different locations), required for aerobatic or inverted flight

• Splash (Wet Sump)– Oil contained at bottom of crankcase, pumped throughout

engine, and splashed around by moving parts– Advantages: light weight and relative simplicity (no separate

tank and tubing)

Fuel Systems• Fuel Pump

– Engine and/or electric pump forces fuel into engine– Required on low-wing aircraft (tanks below engine)– Used on most modern and high-powered aircraft

• Gravity Feed– Fuel flows down from tanks to engine– Sometimes used on high-wing, low-power aircraft

Fuel• Octane Rating

– Octane = Substance which possesses minimum detonating qualities– Heptane = Substance which possesses maximum detonating

qualities

• Common Fuels– Grade 80 or 80/87 Red– Grade 100 (high lead) Green– Grade 100 LL (low lead) Blue– Jet Fuel Clear or Straw/Yellow

• AVGAS = Aviation Gasoline• MOGAS = Automobile Gasoline

Fuel Problems• Detonation

– Fuel burns too quickly and out-of-control– Can cause damage and severe engine malfunction– Caused by using incorrect fuel (too low octane), overheating, or too

lean a mixture

• Pre-Ignition– Premature ignition due to glowing carbon particles in cylinders– Results in backfiring and severe engine damage

• Vapour Lock– Fuel vaporizes in fuel lines, blocking flow of liquid fuel to engine– Caused by high atmospheric temperatures

Carburetor• On older engines, carburetor used to mix fuel and air

• Air flowing through venturi creates negative pressure, sucks fuel from fuel nozzle, then mixture flows into cylinders

• Throttle controls fuel/air flow with throttle valve

• Carburetor can become blocked by ice

• Newer engines use Fuel Injection, where fuel is directly injected into cylinder; No hazard of carburetor icing

Carburetor

Mixture Valve

Venturi

Fuel

Throttle Valve

Carburetor Icing• Ice can form in carburetor due to low pressure created by venturi

• Possible in moist conditions from -5°C to 30°C

• Carb Heat control switches incoming air to alternate intake

• Intake air is heated by exhaust manifold and is unfiltered air

• Hotter air melts ice, but causes slight loss of power (hotter air is less dense)

Carburetor Icing

Normal Operation Blocked by Ice

Mixture• Fuel/air mixture adjusted by mixture control

• Normal mixture is 1 part fuel to 15 parts air– Rich Mixture (more fuel) = Cooler combustion, more power,

used in high power settings– Lean Mixture (less fuel) = Hotter combustion, more

economical, used in cruise power settings

• Problems:– Too Rich = Wastes fuel, fowls spark plugs, rough engine

operation, engine failure– Too Lean = Rough engine operation, cutting-out, detonation,

engine failure

EGT• Exhaust Gas Temperature (EGT) Gauge

used to determine best fuel/air mixture

• Best mixture occurs at hottest EGT reading (Peak EGT)

Ignition System• Magnetos create high tension current from rotating crankshaft to

power spark plugs

• Usually Dual Ignition (two magnetos); each magneto powers one of two spark plugs in each cylinder

• Two spark plugs provide improved combustion in each cylinder

• If one magneto fails, other can safely run engine, although with slight loss of power

Ignition System

Propeller• Moves large mass of air backwards at a relatively low

speed (as opposed to a jet engine)

• Propeller converts engine crankshaft torque (or turning moment) into thrust

• Propeller torque is drag (of the propeller blade)

Propeller Pitch• Pitch = Distance in feet a propeller travels forward in

one revolution

• Pitch determined by the blade's angle of attack– Coarse (high) Pitch = Travels forward more in one revolution; less

power, more speed– Fine (low) Pitch = Travels forward less in one revolution; more

power, less speed

• Propellers can be:– Fixed Pitch = Blade angles cannot be adjusted by pilot; angle is

combination of decent take-off performance and cruise performance– Variable Pitch = Blade angles can be adjusted by pilot

Propeller Pitch

Variable Pitch Propellers• Adjustable Pitch = Adjustable only on ground

• Controllable Pitch = Adjustable manually by pilot during flight

• Constant Speed = Blades adjust automatically to maintain constant RPM as set by pilot; usually operates with oil pressure from engine

• Feathering = Blades go to extreme coarse position, to stop propeller wind-milling, usually when engine fails during flight

• Prop Reversing = Blades change to negative angle, pushing air forward, used to slow down after landing

Propeller Pitch