ULTILEARN PISTON ENGINE PROPULSION REVISION
PISTON ENGINES
Engine layouts
Inline 4
Inline 3
Vee 8
Flat 4
V Twin
Operation
In a two stroke piston engine the crankshaft rotates once to complete one cycle
In a Wankel engine for every rotation of the triangular piston the crankshaft rotates 3 times
In a four stroke engine the nominal crankshaft angle to initiate combustion is 30 degress before TDC
The firing order of a four cylinder piston engine is 1, 3, 4, 2
In a four stroke piston engine the cam shaft rotates a half the crankshaft speed
In a 4-stroke piston engine a spring closes the exhaust and inlet valves
In a piston engine, ineffective crank angle occurs in the region of both BDC and TDC
The sequence for the operating cycle of a four stroke piston engine is Induction, Compression, Power and
Exhaust
In a four stroke piston engine the crankshaft rotates twice during one complete cycle
In the modified 4-stroke cycle valve lead occurs when the exhaust valve opens before the piston reaches BDC.
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In order for compression to occur in a piston engine all inlet and exhaust valves must be closed.
Components
W = gudgeon pin
Z = crankshaft main bearing
Y = connecting rod
X = Valve
Z = Camshaft
X = Cam
The gudgeon pin attaches the piston to the connecting rod
On the camshaft the difference between the low diameter and the high part of the lobe is called the valve lift
In a 4 cylinder engine there are 5 mainline bearings supporting the crankshaft
Supercharging
The air compressor in a piston engine turbocharger is driven by the exhaust gases
The air compressor in a piston engine supercharger is driven by the engine crankshaft
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The primary functions of a supercharger on a piston engine are to:
- increase the pressure in the induction manifold
- increases the amount of air and fuel delivered to the cylinders
An exhaust-driven turbocharger operates best at high engine speeds
Ignition
In a distributor the purpose of the centrifugal weights mechanism is to advance the timing and the purpose of
the pressure diaphragm is to increase the intake manifold pressure
EMF = Electro Motive Force
A magneto is a dynamo to produce the spark for the spark plugs
The purpose of the condenser (or capacitor) in a magneto reduce pitting of the contact breaker points
The purpose of a magneto is to produce the energy for the spark at the spark plugs
In the modified 4-stroke cycle, ignition takes place just before TDC on the compression stroke
In a breakerless electronic ignition system the position of the crankshaft is indicated by a distributor timing
disc and sensor
In a petrol engine, the basic maintenance required to maintain healthy operation of the spark plugs is to clean
the electrodes and body, set the electrode gap and check the plug ceramic insulator for cracks
In a magneto system the power for the spark is generated in the coil secondary windings
In a piston aero engine the purpose of the distributor is to distribute high voltage electrical impulses to the
cylinders
In a dynamo/alternator system the primary and secondary windings are incorporated in a separate coil unit
Fuel System
In a simple 4-stroke piston engine the air-to-fuel ratio (by weight) is 15:01
In a simple 4-stroke piston engine the fuel -to-air ratio (by weight) is 01:15
In a fuel injection system the fuel is atomised by forcing it under pressure through small spray nozzle
In a two stroke piston engine the fuel air mixture is supplied to the combustion chamber by the piston moving
down the cylinder
In a freight or passenger type aircraft the fuel is stored in the wings
Lubrication
Dry sump lubrication systems are used in aircraft and high performance cars
The higher the viscosity of oil the greater the thicker the consistency
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The purpose of the scavenge pump in the dry-sump lubrication system of a piston engine is to transfer oil from
the crankcase to the oil tank
In a piston engine far more oil is circulated than is needed to reduce friction in order to help cool the engine
internally by removing friction generated heat
Oil is delivered to the shell type mainline bearings in a piston engine by pressure feed through internal drillings
and pipes
Cooling
The advantage that air cooling has over liquid cooling is that it is much cheaper to make
The purpose of the thermostatic valve is to prevent cooling in the radiator until the engine has warmed up
The main benefit of using liquid (as opposed to air) to cool an aircraft piston engine is that a steady operating
temperature will be more easily maintained
The purpose of the fins which are arranged about the cylinder and cylinder head of an air-cooled engine is to
allow heat to dissipate rapidly
A thermostatic valve is opened by heat and closed by a spring
Carburation
The purpose of an accelerator pump when fitted to the carburettor is when accelerating it prevents the
mixture from becoming weak
In normal running conditions the throttle valve in the carburettor controls the engine speed
The arrow points to the choke tube
X = Main Jet
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X points to a device which prevents the fuel/air mixture from
becoming richer as the aircraft climbs
In a direct fuel injection system there is no choke on the intake
In an electronic common rail fuel injection system the fuel flow to the engine is controlled by varying the time
the fuel injector valves are open
PROPELLORS
Propeller braking involves turning the blades beyond fine pitch
Another term for coarse pitch is feathered
The variation of the blade angle on a propeller is called Blade Twist
Constant speeding variable pitch propellers were developed to reduce the work load of a pilot
In an aircraft propeller system the function of the constant speed unit (CSU) is to maintain a selected engine
speed within the power available
When a propeller rotates the two motions that produce the helix angle are the forward speed and engine RPM
A variable-pitch propeller is said to be feathered when the leading edges face forward in the direction of flight
In a propeller control unit the engine speed and propeller pitch is controlled by a spring balanced with bob
weight centrifugal force
Blade twist in a propeller helps to even out the thrust along the length of the blade
A propeller governor know the rotational speed of the propeller from a received mechanical speed signal
On a variable-pitch propeller, the largest obtainable pitch angle is known as Coarse Pitch
B = Blade Angle
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