1. Piston Engines
ATC Chapter 2

2. Aim
To review principals of operation of aircraft piston engines

3. Objectives Name the main components of the piston engine
Understand the terminology used when discussing piston engines
Describe the four stroke combustion cycle
State the ways in which power and efficiency can be measured
Describe the operation of engine instrumentation

4. 1. Components Cylinder Configuration
There are two main classes of engines used in aircraft, piston or gas turbine. Gas turbine knowledge is not part of the CPL syllabus however those seeking an endorsement on a turbine aircraft must first complete a Basic gas Turbine (BGT) theory course
There are a number of different types of piston engines used in aircraft
Radial engines have their cylinders arranged in a circle around the crankshaft. They can mostly be seen in early and pre-WW2 aircraft such as the Stearman or the Harvard

5. 1. Components Cylinder Configuration
In-line engines have their cylinders arranged in a line along the crankshaft. Here we can see an inverted in-line cylinder arrangement on a Tiger Moth
During WW2 the ‘V’ configuration became popular. This can be see on the infamous Merlin V12 that powered a number of aircraft including the Spitfire and Corsair

6. 1. Components Cylinder Configuration
Horizontally opposed engines have their cylinders arranged in a flat configuration along the crankshaft. This configuration is very popular for modern piston aircraft and can be seen on the C172SP

7. 1. Components Engine Components Cylinders
Barrel in which combustion takes place and the piston moves
The exterior is normally manufactured with cooling fins
Typical arrangement on light aircraft is horizontally opposed 4 or 6 cylinder

8. 1. Components Engine Components Piston Moves within the cylinder
Three or more grooves are machined around the piston in which are set steel piston rings
The first ring prevents combustion gases from escaping
The lower rings prevent oil from entering the combustion space

9. 1. Components Engine Components Inlet and Exhaust valves
Fitted to cover ports in the cylinder heads
Inlet valve allows the fuel/air mixture into the cylinder
Exhaust valve allows spent combustion gases to escape
When not open they are held in place by springs

10. 1. Components Engine Components Spark Plugs
Ignites the fuel air mixture at the correct time
Typically arranged with one at the top and one at the bottom of the cylinder

11. 1. Components Engine Components Connecting Rod
Connects the piston to the crankshaft
Connected to the cylinder by a gudgeon pin
Connected to the crankshaft by the crankpin

12. 1. Components Engine Components Crankshaft
A shaft designed with a crank for each cylinder
Turns the up and down motion of the piston into the rotary motion required for the propeller

13. 1. Components Engine Components
Manifold – System of pipes and ducts which lead the fuel air mixture from the carburettor to the induction manifold

14. 2. Terminology
Cycle – Series of events that are repeated in a regular order
Top dead centre (TDC) – Position of the cylinder when it is maximum distance from the crankshaft
Bottom dead centre (BDC) – Position of the cylinder when it is a minimum distance from the crankshaft
Throw – Radial distance between the crankpin and the crank shaft
Stroke – Distance which the piston moves up or down a cylinder. Up is towards the cylinder head, down is toward the crankshaft regardless of engine orientation. The stroke is the actual distance between TDC and BDC and should be equal to double the throw
Charge – The fuel and air mixture
Throw

15. 2. Terminology Bore – Internal diameter of the cylinder
Clearance Volume – The space or volume above the cylinder when it is at TDC
Swept Volume – The volume of the cylinder that is swept between TDC and BDC
Compression Ratio – Ratio of the volume of the cylinder when the piston is at BDC to the volume of the cylinder when the piston is at TDC
Compression ratio = Swept volume + Clearence volume Clearence volume
Firing Interval – The interval measured in degrees of crankshaft motion between any two cylinders in the ignition sequence
Firing Interval = Number of degrees per cycle Number of cylinders
Firing Order – The numerical sequence in which the cylinders fire
Manifold pressure – Pressure of the charge in the induction manifold
BDC
TDC

16. 3. Combustion Cycle Otto Cycle Intake
Engines typically found on light aircraft are of the four stroke type, as the name suggest it takes four strokes of the cylinder to complete one cycle
Intake
The piston is moving in a downwards direction, reducing the pressure in the cylinder, sucking in the fuel/air mixture (also known as the charge) through the open inlet valve

17. 3. Combustion Cycle Otto Cycle Compression
The inlet valve closes and piston moves back towards the cylinder head compressing the charge.
As the charge is compressed the temperature rises considerably, diesel engines will use this increase in pressure and temperature to ignite the charge

18. 3. Combustion Cycle Otto Cycle Power
The charge is ignited by the spark plugs resulting in rapid expansion of the gases in the cylinder, increasing pressure n the top of the piston forcing it to travel down

19. 3. Combustion Cycle Otto Cycle Exhaust
Just prior to completion of the power stroke the exhaust valve opens and as the piston moves back up towards the cylinder head the exhaust gases are forced out of the cylinder via the exhaust manifold

20. 3. Combustion Cycle The Valves
The inlet and exhaust valves must open and close at the correct time. This is achieved using a gear driven camshaft
The camshaft operates the pushrods and rocker arms which in turn open the valves
The valves are typically closed by utilising a spring mechanism.

21. 3. Combustion Cycle The Valves
Timing is achieved by setting the cam at a specific angle
The camshaft rotates at half the speed of the crankshaft, if we have a power setting of 2400RPM the camshaft will rotate at 1200RPM meaning each valve must open and close 20 times per second, This means there is very little time for the fuel/air mixture to enter the cylinder and the exhaust gas to exit the cylinder

22. 3. Combustion Cycle Ignition Timing
To increase efficiency the intake valve will open slightly before the piston reaches TDC and close just after it reaches BDC, this is called valve lag and allows more time for the induction process to take place
Similarly the exhaust valve opens slightly before the piston reaches BDC and is close slightly after the piston reaches TDC, this is called valve lead
There is a short period of time when both valves will be open, this is called valve overlap

23. 4. Power and Efficiency Definitions Power - Rate of ding work
Horsepower - Is imperial measurement of power, aircraft engine manufacturers still use this measurement to rate aircraft engines
Kilowatts - Is the measurement of power under the international system of units. 1 HP = Kw
Indicated Power – Power produced within the engine when fuel is burnt
Friction Power – Power lost through engine friction and driving of auxiliary systems (fuel pump, oil pump, alternator, etc)
Brake Power – Actual power output of an engine measured from the crankshaft; Brake power = Indicated power – Friction Power

24. 4. Power and Efficiency Thermal Efficiency
When the charge is ignited the heat, and corresponding pressure increase, is converted to mechanical work
It can be seen that:
25% of the heat is lost through convection and conduction
40% through exhaust gases
5% through friction losses
The brake thermal efficiency can be calculated by:
Brake thermal efficiency = Brake power Fuel consumption
The overall efficiency can also be expressed in terms of brake specific fuel consumption (the ratio of fuel flow to power produced)
Brake specific fuel consumption = Fuel Flow Brake power

25. 4. Power and Efficiency Mechanical Efficiencies
Mechanical efficiency is part of the overall thermal efficiency
It is the relationship between mechanical power input and power output. It can be calculated by:
Mechanical efficiency = Brake Power Indicated power %
At the most efficient RPM mechanical efficiency will approach 90%

26. 4. Power and Efficiency Volumetric Efficiencies
Volumetric efficiency is the ratio of the actual weight of the mixture in the cylinder just prior to ignition compared to the weight of the mixture if there were no resistance to flow. It is measured by:
Volumetric Efficiency = Actual weight of charge Maximum amount of charge
ISA standard temperatures and pressures are used for comparison
Normally aspirated engines can achieve an efficiency of 70%-80%
Turbocharged can achieve efficiency of 100%+

27. 5. Engine Instrumentation
Tachometer
The tachometer gives engine crankshaft RPM
There are three main types
Mechanical
Electrical
Electronic

28. 5. Engine Instrumentation
Mechanical Tachometers
Most light aircraft utilise mechanical tachometers which utilize a flexible drive between the engine and the instrument
Centrifugal tachometers utilise rotating flyweights to convert the rotational movement of the drive into movement of the needle
As the engines RPM increases the flyweights are forced outwards and an increase in RPM is displayed
This system is typically only found on early aircraft

29. 5. Engine Instrumentation
Mechanical Tachometers
Drag cup tachometers use a rotating permanent magnet and a drag cup to convert the rotational movement of the drive into movement of the engine
The magnet rotates in the metallic drag cup creating a magnetic field
This field interacts with the magnet creating a torque (or drag) on the cup which rotates until the torque is balanced by the attached spring
This system is used on modern light aircraft

30. 5. Engine Instrumentation
Electrical Tachometers
Electrical tachometers use small engine driven electrical generators to turn the rotation energy of the driveshaft into an electrical signal
Some older aircraft may use DC generators however most aircraft utilise AC generators
The 172SP utilises this system coupled with an electronic indication on the G1000
Most systems still utilise a drag cup to drive the pointer

31. 5. Engine Instrumentation
Electronic Tachometers
The electronic tachometer utilises a connection to the magneto to determine the frequency of the spark plug firing
This is then electronically displayed in the aircraft

32. 5. Engine Instrumentation
Manifold Pressure Gauges
Manifold Pressure (MAP) gauges are required on aircraft fitted with constant speed units of superchargers
Manifold pressure is the pressure in the manifold downstream of the carburettor, fuel injection system or supercharger
Pressure is measured against a vacuum, therefore without the engine running on a standard ISA day the gauge will read 29.92” Hg (atmospheric pressure)
When idling on the ground the gauge will read much lower than atmospheric
For naturally aspirated engines at full power the gauge will read just below atmospheric
At high power supercharged engines will read higher than atmospheric
The system uses an aneroid capsule, as this expands or contracts the pointer moves

33. 5. Engine Instrumentation
Burdon Tube
The Burdon tube type system can be used to read pressure and is typically used for measuring oil pressure
The system uses an anchored Burdon tube attached via a mechanical linage to the pointer
As pressure builds in the tube it expands resulting in movement of the mechanical linkage
The tube is often made of a malleable metal, typically brass

34. 5. Engine Instrumentation
Thermocouple
The thermocouple system is used to measure temperature in relatively high temperature areas of the engine, for example cylinder head temperature or exhaust gas temperature
A thermocouple is a circuit made of two dissimilar metals, when the hot junction is heated a current will be produced
In piston aircraft the metals are either copper-constantan or iron-constantan which are capable of measuring temperature up to 400⁰C
Turbine engines use chrome-alumel types which can measure up to 1100⁰C

35. 5. Engine Instrumentation
Mechanical Bulb
The mechanical bulb is used to measure temperature in lower relative temperature areas of the engine (up to 150⁰C), for example oil temperature or outside air temperature gauges
The temperature probe (or bulb) contains a volatile chemical which at low temperature is liquid, as it heats up and turns into a gas
The resulting pressure increase can be measured and displayed using a Burdon tube

36. 5. Engine Instrumentation
Bi-metallic strip
Bi-metallic strips are used to measure temperature in vey low relative temperature areas of the aircraft, for example outside air temperature
Two different types of metal with different co-efficients of expansion are joined together, as temperature increase the strip will bend
The strip is often bonded together in a coil

37. 5. Engine Instrumentation
Synchro system
In a large number of modern aircraft (such as the 172SP) use syncro systems to transmit data from the sensor in the engine to the display in the cockpit
A.C. or D.C. systems can be used and all work on a similar principal
The input shaft is connected to the rotor. When the rotor moves it induces a current in the stators
This current passes through the stator on the receiver causing the receiver rotor to rotate the same distance as the transmitters rotor until a null position is reached

38. 5. Engine Instrumentation
Variable Resistance/Ratiometer
Some remote indicating systems will utilize a rheostat to transmit data between the engine and cockpit instrumentation
Here a rheostat has been used to remotely display oil pressure

39. Questions?