Lesson Ten: Engines I
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Don’t read:
o Don’t have to bother with any equations in this section.
o The Two-Stroke Cycle
o Automatic Mixture Control
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Power
o Engines in C152/C172s are examples of internal combustion engines
o Take in chemical energy + heat and converts to kinetic energy to produce
motion
o Standard unit in use for measuring power is one horsepower (33000 lbs/ft.)
o Power available after considering friction and other energy losses is the brake
horsepower (BHP)
Types of Combustion Engines
o Horizontally opposed
 Two banks of cylinders working on the same crankshaft which lie directly
opposite to each other in the plane.
 Four, six or eight cylinders.
 Flat shape generates less drag.
 Commonly found in modern small-aircraft designs.
o Radial
 Cylinders arranged around a barrel-shaped crankcase
 Odd number of cylinders (five, seven, nine)
 Firing order is alternate (1-3-5-2-4…)
 Dry sump or pressure lubricated
 Easy to maintain, and acceptable weight to horsepower ratio
 Increases parasite drag, and decreases forward visibility
o In-Line
 Cylinders arranged side by side in a row along the crankcase
 Each piston works on an individual crank-throw
 Max six cylinders in a row
 Small frontal area generates little drag; although found mainly in vintage
planes
Basic parts are the crankcase, cylinders, pistons, connecting rods, valves, valve operating
mechanism, camshaft and crankshaft
o See diagram in book.
o Crankshaft is basically the long pole which turns the propeller. It is located in the
crankcase. It is forced to turn by the pistons which move up and down during
the Four-Stroke Cycle. The pistons are housed in the cylinders, which are lined
up along the crankcase depending on the type of engine. The valves in the
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cylinders control intake and exhaust; they are controlled by camshafts, which
cause the values to open and close.
Four-Stroke Cycle (crankshaft revolves through 2 complete revolutions)
o Intake/Induction
 Valve opens, piston moves down from the top to the bottom of the
cylinder, fuel/air mixture sucked into the space.
 Amount of mixture which enters depends on the throttle setting
o Compression
 Both valves closed, piston moves up the cylinder, squeezing the fuel/air
mix.
o Power/Combustion
 Spark plugs ignite, violently combusting the fuel/air mix and forcing the
piston down. Gives energy to complete the next three strokes before the
next combustion stroke.
o Exhaust
 Exhaust valve opens, piston moves up and pushes the junk out.
Read Diesel Engine
o Require no separate electric ignition source. Also doesn’t require carburetor.
Turbocharging
o Engine running at normal sea level atmospheric pressure is called a normally
aspirated engine.
o As altitude increases, air density decreases; hence we’d expect that the engine
would produce less power as you climb.
o A turbocharger is used to supply dense air into the engine, even at higher
altitudes.
o Hot exhaust gas is directed through a turbine wheel, which when turned drives a
compressor, which in turn compresses incoming air.
o Turbocharger installed between the air intake and the carburetor so that it
compresses the air before it is mixed with the metered fuel in the carburetor.
o Control is provided by either automatic or manual control.
o Allows increased performance at altitude.
o Read the rest of the section.
Supercharging
o Same concept as a turbocharger, but is internally driven i.e. powered directly by
the engine.
o Installed downstream from the carburetor and compresses the fuel/air mix after
it leaves the carburetor; this is called forced induction.
o Using a supercharger/turbocharger at low altitudes is called boost.
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Engine timing
o Valves and piston don’t move exactly as described earlier; for efficiency, there is
valve lead (open early), lag (close late) and overlap (i.e. times when both valves
are open/closed)
o Timed to open early and close late in order not to waste any of the induction or
exhaust strokes.
o Valve mechanism operated by a camshaft, which is driven by a gear that mates
with another gear attached to the crankshaft.
 Camshaft rotates at one-half the speed of the crankshaft
Valve Clearances
o Clearance necessary between the valve stem and rocker (see diagram) to
prevent valve from coming off when it gets hot and expands.
o Clearances set too wide will cause a loss of power, vibration and wear.
o Clearances set too close can warp the valves, cause serious damage.
Engine cooling
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Obviously need a way to cool down the engine.
Some heat is absorbed by engine parts, cylinder walls, etc. Excessive heat can deform
parts.
HO and radial engines are air cooled. Some in-line are air cooled, while others are water
cooled.
Air cooling
 Fins are added to cylinders to provide a greater area of metal to absorb the heat
(they’re just extra metal bits)
 Ram air passing over fins absorbs this excess heat and carries it away.
 Shrouds direct airflow into the engine compartment.
 Baffles are used to force cooling air directed by the shrouds towards the engine
parts.
 Cowl flaps installed on hi-performance planes to control the sizes of openings to
expel cooling air.
 Depends on needs/attitude. i.e. During cruise, less cooling air is
required; during climbs, more is required.
 Opening sizes are set for low-performance aircraft.
 Cooling fans are sometimes mounted on the front of the engine.
Engine Oils and Lubrication
o Cooling – oils carry away excess heat generated by engine
o Sealing – provides a seal between piston rings and cylinder walls, prevents loss of
fuel/air mixture
o Lubrication – maintains an oil film between moving parts
o Flushing – cleans and flushes the interior of the engine of contaminants
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Methods of Lubrication
o Force feed method: the oil is forced under pressure from a pump through the hollow
crankshaft where it lubricates the main parts. Sprayed through tiny holes to lubricate
remaining parts of the engine by a fine mist.
o Splash method: oil is contained in a sump (tank), at the bottom of the engine; churned
by revolving crankshaft into a heavy mist where it is splashed everywhere.
 Found in older planes.
o Force feed: dry sump lubrication
 Dry sump refers to the oil being contained in a separate tank and forced under
pressure using a pump through tiny holes in the crankshaft.
 Oil is drained into a sump, and then recycled to be used again (after passing
through an oil cooler)
 Oil pressure monitored by means of an oil pressure gauge.
o Wet sump lubrication
 Improvement of conventional splash method (combo of dry sump and splash)
 Engine oil supply contained in a sump under the crank case
 Advantage of being light weight, without the exterior tank associated with the
dry sump.
 Cannot be used for aerobatic aircraft.
Requirements of good oil
o Viscosity (resistance to flow)
 Use of oil too high viscosity for existing climatic conditions will result in high oil
pressure (opposite for low viscosity)
 A high viscosity index means changes in viscosity due to changing conditions are
small
o High flash point
 High temperature of ignition
o Low carbon content
o Low pour point
 Low temperature of solidification
Additives
o Detergents – improve engine cleanliness
o Oxidation inhibitors – improve oil stability
o Anticorrosion additives – deter corrosion
o Pour point depressants – lower pour point
Oil grades given by SAE (Saybolt Universal Viscosity) Number; basically a measure of the
viscosity of the oils.
o Viscosity needs change depending on the season.
o Synthetic multi-viscosity oils exist as well.
Oil temperature
o Must keep oil temperature right; two hot, and it’ll be too runny, and may not be thick
enough to keep a good film of oil on the parts
o Too thin, and it’ll be too thick, and will not flow through passageways.
o Monitored by an oil temperature gauge.
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Read Oil Dilution
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Fuel Systems
o Tanks usually located in the wings, although extra tanks may be located in the cabin
o A line leads from each tank to a selector valve in the cockpit.
o Fuel Pump Fuel System
 Engine driven fuel pump supplies pressure which keeps fuel flowing to engine.
 Found in all low wing airplanes and any with high-performance engine.
o Gravity Feed Fuel System
 Found on high-wing, low powered aircraft.
 Gravity draws fuel down from the tanks into the engines.
o Read Fuel Tanks, Fuel Selector Valve, Fuel Lines and Filters, Fuel Quantity Gauge
o Fuel Primer
 Small hand pump located in the instrument panel, used to manually pump some
fuel into the engines to help them start
 Careful not to overprime, otherwise flooding may occur
o Fuel Types
 Fuels need to burn slowly and expand evenly rather than be explosive.
 Fuels which possess this quality have what’s called high octane ratings
 Octane is a substance which possesses minimum detonating qualities
 Heptane is a substance which possesses maximum detonating qualities
 Proportion of octane to heptane expressed as a percentage; go only as high as
100
 Beyond this number, the “antiknock” value of the fuel is expressed as a
performance number
 Fuel grades are indicated by two performance numbers
 i.e. Grade 80/87 fuel has a lean-mixture performance of 80, rich-mixture
performance of 87
 **Colours
 Grade 80/87 (Red)
 Grade 100 LL (low lead; most commonly used nowadays; Blue)
 Grade 100 (high lead; Green)
 Kerosene/Diesel (jet fuel; clear or straw)
 **Fuel Related Problems
 Detonation
o Inability of fuel to burn slowly; abnormally rapid combustion
o Puts high stress on engine parts and causes parts to deform.
o Causes
 Use of incorrect fuel
 Overheating, sometimes caused by too steep a climb
that reduces the flow of air around the cylinders
 Too lean a mixture
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Fixes
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Use mixture control full rich in an emergency
Reduce throttle
Permanent fix: use correct fuel! Use higher grade if
correct grade is not available, never use lower grade.
Pre-Ignition
o Premature ignition of the mixture due to glowing carbon
particles, or “local hot spots”
o Causes
 Attempting to start a hot engine and usually results in a
backfire through the intake manifold
o Can lead to warped pistons, cracked cylinder heads, etc.
Vapour lock
o Caused by high atmospheric temperatures, resulting in the fuel
to vaporize and block the flow of liquid fuel in the line
Fuel/Air Mixtures
o Heat energy in an internal combustion engine developed from the burning of fuel/air
mix.
o Ratio of fuel/air mix regulated by the pilot with the mixture control.
 Running mixture: 1 to 8
 Best power mixture: 1 to 14
 Leanest running mixture: 1 to 20
o The throttle is what regulates the flow of air into the engine
o Engine temperatures depend heavily on the fuel/air ratio used when entering the
combustion chambers
 An engine will run hotter with a lean mixture than with a rich mixture because
the lean mixture is slower burning and therefore the cylinder walls will be
exposed to high temperatures for a longer period of time
 Richer mixtures burn more quickly, thus exposing the cylinder walls to heat for a
shorter period; additional fuel in the fuel/air mix also helps cool the engine.
o Too rich
 Lowers combustion temperatures; fuel/air mixes may not burn completely
(leads to wasted fuel and poor efficiency)
 Can foul spark plugs, and result in combustion chamber deposits
 Can cause rough engine operation and loss of power
o Too lean
 Rough engine operation, sudden “pop-outs”
 High combustion temperatures, can lead to detonation and overheating
 Engine failure
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Carburetors
o The Float Carburetor
 Fuel flows through the fuel supply lines, past the fuel strainer and enters the
carburetor at the float/needle valve
 Fuel flows into the float where its level is controlled by a float which opens or
closes the float valve as it rises or falls
 When the float rises to a predetermined level, it shuts off the float valve.
 Fuel flows through fuel metering orifices which let in a certain amount of fuel
(as set by the mixture setting) into the venturi
 Outside air rushes through the carb. Air filter, which is then drawn through the
venturi.
 A venturi is constriction which squeezes the air as it flows through it.
What happens to the speed of the air as it passes through it? What
about its pressure?
 Fuel and air then pass by the throttle valve, which is controlled by the throttle,
and enters the intake manifold to be then distributed to the cylinders.
o Read Pressure Carburetor and Acceleration Pump
o Idling
 When an engine is idling, throttle valve is closed and there is insufficient
movement of air through the venture to draw fuel from main nozzle.
 Idle jet provided near the throttle valve to supply a tiny amount of fuel to the
moving air, reducing pressure enough to send some of the fuel through.
Mixture Control
o As altitude increases, air density decreases
o Carburetors normally calibrated for sea level operations, so correct mix of fuel/air will
be obtained at sea level when the control is in the full rich position
o Since air density decreases with height, leaving the mixture control in full rich will result
in over-richness; so you need to lean to get the right amount of mixture
o Idle Cut-off
 Some engines fitted with an idle cut-off control to stop the engine; essentially
the extreme “lean” position (C152/C172 airplanes have this)
 Will stop the engine immediately with no tendency towards pre-ignition or
after-firing
o Manual Mixture Controls
 Follow the POH!
 In general, use rich mixtures during high power settings, leaner mixtures during
cruise power settings.
 In case of doubt, richer is better.
 Mixture control has the greatest effect on engine temperatures.
 Read the tachometer when deciding how much to lean
 When going from full rich to lean, it will be observed that an increase in
RPM occurs; when max rpm is first reached it is called “rich best
power”. Further leaning will hold the rpm at max value until “lean best
power is reached”, upon which the rpm will drop.
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 Read the rest of this section (not too important for our purposes)
When to Lean the Engine
 At cruise power, and above 5000’, it’s permissible to use the mixture control as
an economy device
 At high manifold pressures, the mixture control should be set at full rich.
 Special considerations must be made in leaning turbocharged engines
 Why lean the engine
 Economy of fuel
 Smoother running engine
 More efficient engine (higher IAS and better airplane performance)
 Extended range of the airplane at cruise
 Less spark plug fouling and longer life for spark plugs.
 More desirable engine temperatures
 Cleaner combustion chambers, less chance of pre-ignition.