Piston Engines ATC Chapter 2

Aim To review principals of operation of aircraft piston engines

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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 = 0.746 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

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

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%

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%+

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

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

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

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

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

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

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

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

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

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

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

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

Questions?