Introduction to Flying II Aircraft systems PDF

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Introduction to Flying II: Aircraft systemsEngine ClassesState the basic difference between piston engines and jet engines● Piston engine ○ Reciprocating parts ○ Intermittent internal combustion ○ Converts heat to mechanical energy● Jet engine ○ Rotating parts ○ Continuous internal combustion ○ Uses...

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Introduction to Flying II: Aircraft systems Engine Classes State the basic difference between piston engines and jet engines ●

Piston engine ○ Reciprocating parts ○ Intermittent internal combustion ○ Converts heat to mechanical energy

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Jet engine ○ Rotating parts ○ Continuous internal combustion ○ Uses action/reaction principle to provide thrust

Definitions With respect to a 4-stroke piston engine, state the meaning of the following terms: ● Cycle ● Stroke ● Top dead centre (TDC) ● Bottom dead centre (BDC) ● Bore ● Clearance volume ● Swept volume ● Compression ratio (CR) ● Firing interval ● Firing order ● Manifolds ● Manifold pressure ● Crank angle Cycle - A series of events which are repeated in a regular sequence and which constitute the principle of operation ● Induction ● Compression ● Power ● Exhaust Stroke - The distance which a piston travels up or down in a cylinder ● One cycle comprises of four strokes TDC – Position furthest away from the crankshaft BDC – Position closest to the crankshaft Swept volume – The volume in the cylinder which is swept by piston between TDC and BDC Clearance volume - The volume in the cylinder above the piston Compression ratio -

Bore - Internal diameter of a cylinder Firing interval -

Firing order - The numerical order in which the cylinder fire, eg 1,2,3,4 Manifold – ducting which leads the charge from the carb to the cylinders (induction manifold) and leads the exhaust gases from the cylinders to the main outlet (exhaust manifold) Manifold pressure – The pressure of the charge existing at any one time in the induction manifold Crank angle – same as the crank rotation from some datum, say TDC State the fundamental operating principle of the reciprocating piston engine Ideal gas law pV = nRT ● This is a combination of Charles’ LAw and Boyle’s law ● Increasing temperature restricting expansion of gas leads to increase pressure ● High pressure forces piston displacement ● Piston displacement converted to rotary motion of the crankshaft Boyle’s Law: ● For a given mass, at a constant temperature, the pressure times the volume is constant i.e pV = C (Temp is constant) Charles law ● For a given mass, at constant pressure, the volume is directly proportional to the temperature, i.e. ● V = C2T (pressure is constant) Ideal otto cycle for a 4-stroke engine

Induction Stroke ● ● ● ● ●

Point 1 corresponds to the beginning of stroke (piston at TDC) Point 2 corresponds to end of the stroke (piston at BDC) Volume, V, is the total mixture volume between the top of the cylinder and the face of the piston Intake stroke takes place at essentially constant pressure The total mass of fuel/air mix inside the cylinder increases throughout the stroke

Compression Stroke ● ● ● ● ● ● ●

The piston compresses the now constant mass of gas from a low pressure p2 to a higher pressure p3 If frictional effects are ignored, the compression takes place isentropically since no heat is added or taken away At the top of the compression stroke, combustion takes place Combustion takes place rapidly, before the piston has moved any meaningful distance (but see later slides on ignition timing) Hence, for all practical purposes, the combustion process is one of constant volume Since energy is released, the temperature increases markedly In turn, because V is constant, the Equation of State (Ideal Gas Law) dictates that pressure increases from p3 to p4

Power Stroke ● ● ● ●

The high pressure exerted over the face of the piston during combustion generates a strong force which drives the piston downwards on the power stroke Assuming frictional and heat transfer effects are negligible, the gas inside the cylinder expands isentropically to the pressure p5 At the bottom of the power stroke, the exhaust valve opens The pressure inside the cylinder instantly adjusts to the exhaust manifold pressure, p6, which is usually around the same value as p2

Exhaust Stroke ●

During the exhaust stroke, the piston pushes the burned gases out of the cylinder returning to conditions at point 1

Valve Timing Explain valve lag, valve lead, and the advantages of valve overlap. ● ● ●

Firstly, consider how the valves operate Each cylinder will have at least one inlet and one outlet valve There must open and close at the correct time in the cycle to get the desired result (valve timing)

Exactly when (in relation to piston position) the inlet and outlet valves open and close is the ‘valve timing’

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Camshaft is geared to and rotates at ½ the speed of the crankshaft. Camshaft operates valves via tappets, pushrod and rocker arm system During one cycle, i.e. two revs of the crankshaft, each valve will open/close once Exact timing of opening and closing is set by adjusting the tappets and valve clearances

Valve lag ●

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In practice, the inlet valve opens for the induction stroke just before the piston reaches TDC and closes after the piston goes past BDC. This has been found to increase engine efficiency. The ‘late’ closing of the inlet valve is called valve lag. This is due to the mixture having mass (therefore inertia) meaning changes in velocity take time, as well as the restriction of the inlet valve and friction from the walls of the inlet pipe The lag is typically some 60 degrees of crank angle

Valve lead ●

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The exhaust valve opens just before the piston reaches BDC on the power stroke and closes slightly after TDC on the exhaust stroke. This has been found to improve the expulsion of the burnt gases. The ‘early’ opening of the exhaust valve is called valve lead. Will result in some loss of pressure and power, however it is minimal, as while the crank angle is large, the piston has completed most of it’s downward travel. Pressure at this point has also been greatly reduced, and the angle of conn rod and crank well is too small to transmit much power to the crankshaft The lead is typically 55 degrees of crank angle

Valve overlap ● Due to the exhaust closing late on the exhaust stroke and the inlet valve opening early on the induction stroke, there is an interval during which both valves are open. This is called valve overlap.

Advantages of valve overlap ● ● ●

Increases the amount of charge induced into the cylinder by taking advantage of the decreased pressure caused by the exhaust gases rushing out Aids more complete scavenging of the waste gases by forcing them out with the incoming charge Aids internal cooling of the upper parts of the cylinder, by circulating the relatively cool incoming charge

Aircraft piston engine geometry Consider the geometry of the con rod and the crankshaft, and also the “effective” and “ineffective” crank angles

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Due to the short length of the con rod, the piston travels further in the first 90o of crank rotation (starting from TDC) than it does in the second 90o of crank rotation Piston travel is greater for a given crank rotation at the middle of the stroke than it is at the top or bottom of the stroke This has implications for the optimum timing of the opening and closing of the valves

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Con rod thrust will be most effective when the angle between con rod and crank web is at or close to 90o For geometry shown, this occurs for a crank angle of 67o Max torque will be produced if peak pressure occurs before this ‘90o angle’ is reached Note the ineffective crank angle as the crank nears BDC

Ignition timing Explain the term ignition timing and the need for spark advance. ● ● ● ● ●

There is a finite time involved in the combustion process ○ Eg. 0.003-0.004 seconds The piston is moving throughout the process e.g ○ At 2000 RPM, the crank turns 15o for every 0.001 seconds Over 0.003-0.004 second period, combustion covers 45-60o crank rotation We need to ensure that max pressure develops before the ‘90o crank/con rod angle’ is reached (e.g. 67o crank rotation) Thus, we need to ‘time’ the ignition of the mixture such that this occurs

Combustion time depends on various factors: ● Fuel-air ratio ○ CCM of 1:15 burns fastest ● Temperature of charge and cylinder ○ Higher temp gives faster burning ● Grade of fuel ○ Lower-grade than required leads to large and almost instantaneous increases in flame front speed ● CR and manifold pressure ○ Greater compression gives faster burning ● No. of ignition points ○ More points, faster burning ● Gas turbulence (swirl) ○ Increased swirl, faster burning Spark advance ● ● ● ● ●

Peak pressure needs to develop well before ‘90o crank/con rod angle’ is reached, Hence, ignition of the charge must be initiated between 15-40o TDC The exact value of this ‘spark advance angle’ will depend on the factors listed previously Most engines set at around 25o Under some conditions, this angle will be too ‘advanced’ or too ‘retarded’ with respect to the optimum

Exhaust Manifold ● ● ● ● ●

Similar construction to inlet manifold Collects combustion products, discharges to airstream via one or more exhaust outlets Should have few obstructions to allow for smooth flow, increases efficiency Proper sealing critical! Otherwise can pose fire hazard or CO2 risk (refer handout for further) Often CO2 can enter from a heat exchanger (used for cabin heat) such as an exhaust shroud if there is a leak

Carburation State the purpose of carburation ● ● ●

Carburation - the process of vaporizing liquid fuel and mixing it with air in specific proportions so that it can be burnt in an internal combustion engine The air contains the oxygen which is necessary for the fuel to burn Achieved using:Carburettor, orFuel injection system

Explain the following in relation to fuel-air mixture: ● Rich ● Lean ● Normal workable mixture ratio limits ● Chemically correct (stoichiometric) ratio ● Approx. ratios for max power output and best economy Fuel-air mixture ratio ● ● ● ● ● ● ● ●

Mixture ratios based on weight and stated as fuel weight to air weight The chemically correct mixture (CCM) is that for which complete combustion of all the air and all the fuel will occur. The CCM is  1:15  For any other mixture ratio, there will be either surplus fuel or surplus air after the combustion process A rich mixture is one in which there is more fuel in the mixture than for the CCM, e.g. 1:10 A lean mixture is one in which there is less fuel in the mixture than for the CCM, e.g. 1:18 The workable ratio limits (for ignition by spark plug) are 1:9 to 1:18. Outside these limits successful ignition is not guaranteed Approximate ratio for max power is 1:12,i.e. relatively rich Approximate ratio for best economy is 1:16, i.e. relatively lean

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Air velocity increases at throat (venturi) and thus pressure drops Pressure in float chamber is higher than in throat and so fuel is ‘sucked’ into the throat Rate of air flow through throat, and thus the throat pressure, is controlled by the throttle valve Depending on pressure difference, a given weight of fuel is metered into the throat

Explain the need for the following in an aero-engine carburettor:  ● ● Atomisation & diffusion ● Accelerating system ● Idling system ● Power enrichment (economiser) system ● Mixture control and idle cut-off system The simple carb shown previously suffers from certain deficiencies. To make it suitable for use in an aero-engine, it requires the incorporation of the additional systems listed above Atomisation and diffusion Atomisation and diffusion improves the vaporisation of the fuel The fuel nozzle incorporates: ● An air bleed which provides a fuel-air emulsion ● A diffuser Fuel air emulsion - By incorporating an air bleed we can emulsify the fuel into a lighter, more frothy mixture of fuel

and air aiding the atomisation process. The emulsification also makes the fuel less viscous and more readily available to respond to rapid power increases. Accelerating system ● ● ●

Supplies extra fuel during demanded increases in engine power Fuel inertia means it cannot respond immediately to rapid throttle demands (hence valve lag) Without an accelerating system, rapid opening of throttle would result in a ‘lean cut’, i.e. engine cut out

Idling system ●

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At low rpm (and low velocity of flow through the venturi) there is insufficient pressure differential between float chamber and venturi to ensure sufficient amount of fuel Idling system will kick in at approx 1000 rpm and below

Power Enrichment System ● A rich mixture at high rpm and high MP is required to prevent detonation and overheating ● A second metering jet comes into operation when throttle valve is near the fully open position ● The second jet may be mechanically operated, as shown, or by high manifold pressure Mixture control & idle Cut-off ● Some form of mixture control is required so that the mixture can be adjusted to allow for the effect of altitude (decreasing air density) ● Idle cut-off can stop the engine by cutting off the fuel supply at the carb. This ensures that: ○ No unburnt mixture remains in the cylinders ○ Carb is full of fuel for the next start ● Mixture control and idle cut-off are normally both part of the same system There are three main types of mixture control systems: ● Back suction type ● Needle type ● Automatic mixture control (AMC) Back suction type ● ● ● ●

Varies the air pressure in the float chamber and thus varies the amount of fuel delivered for a given throttle setting Back-suction line exposes float chamber to venturi pressure Mixture valve is placed in the pipe which vents the float chamber to atmosphere Pilot controls the mixture valve using the mixture control lever in the cockpit

Needle Type Mixture control ● ● ●

Also known as jet restriction type The position of the needle controls the amount of fuel able to flow through the metering jet Controlled manually by pilot

Automatic mixture control (AMC)

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Usually based on back-suction or needle restriction method Replaces pilot’s hand on the mixture control in the cockpit AMC also allows for auto-rich, auto-lean and manual lean settings Manual over-ride can be employed in case of failure

Carburation Explain the consequences of operating with over-rich and over-lean mixture settings.  Over-rich: ● Loss of power, rough running, spark plug fouling, lead deposits on valves and piston head Over-Lean ● Excessive cylinder head temperatures possibly leading to detonation which can cause rapid engine damage With respect to carburettor ice, explain the process and atmospheric conditions for the formation of:  ● ● ●

Refrigeration ice Throttle ice Impact ice

State the two main disadvantages with use of a float-type carburettor in an aero-engine.  ● ●

Fuel flow can be disturbed by manoeuvres which upset the float mechanism, especially ‘negative-g’ manoeuvres The susceptibility to ice formation

Float-type carburettor failures ● ● ● ●

Float becoming waterlogged leading to over pressurisation of the float chamber, leading to over-rich operation and even total flooding Fuel contamination leading to blocked jets (fuel starvation) Fuel nozzles breaking off and jamming butterfly valve In any case, failures at best will lead to rough running with possible power loss, while at worst engine cut-out 

Pressure injection Carburettor ● ● ●

The float chamber is replaced with a pressure regulator unit This unit controls the metering of the fuel into the engine The PI carburettor still uses a venturi and throttle valve for controlling the amount of fuel-air mixture entering the cylinders

Basic fuel injection system ● Fuel delivery pump system ● Fuel distribution system ● Fuel injectors – introduce fuel my means other than air through a venturi Pressure injection carburettor The four main components are: ● Throttle unit ● Fuel pressure regulating unit ● Fuel control unit ● Discharge nozzle For operation under varying conditions, the pressure injection carburettor also requires additional systems: ● Mixture control/idle cut-off ● Accelerator pump ● Power enrichment ● Idling system Advantages of pressure injection carburettor: ● ● ● ● ●

Less tendency for ice to form due to the positioning of the discharge nozzle Gravity and inertia have little effect since there is no float and the regulator unit is fully enclosed More accurate fuel metering is achieved at all engine speeds, throttle settings and atmospheric conditions Better atomisation/mixing  Some protection against vapour lock in the fuel lines ○ Poor man’s fuel injection system

Engine cooling and lubrication Function of engine oil ● Lubrication ● Cooling ● Cleaning ● Sealing

Lubrication ● Reducing friction between moving parts ○ Replacing mechanical friction with shear stresses (friction) in the oil film ○ Thus relieving the very high temperatures and associated wear and failure that would occur with bare metal-to-metal contact ● Provides cushioning against mechanical shock (shock absorption) for highly loaded parts, e.g. big-end bearings Cooling ● Oil assists with removing heat from vulnerable engine parts ○ e.g. Pistons, bearings ● Oil is pumped around, splashed and sprayed onto parts as required by the oil system ● Hot oil passes through a cooler and re-circulates Cleaning ● Removes contaminants (more later) which would otherwise cause abrasive wear on the engine ● Circulating oil is ‘cleaned’ by passing through a filter ● Filter must be properly maintained and replaced at periodic intervals Sealing: ● Helps piston rings in sealing crankcase from combustion gases & pressures Aviation oil vs Automotive oil ● Aviation oils have different characteristics when compared to oils used in cars. ● The main reasons for this is due to: ○ Higher operating loads on bearings and gears ○ Increased RPM of bearings ○ Higher temperatures of air cooled engines Oil system configuration ● Sump ○ A reservoir attached to engine lower casing ○ After circulating around the engine, oil accumulates in sump Two types of oil system design based on sump tyoe ● Wet sump ● Dry sump Wet sump ● Sump also acts as a storage tank for oil ● Most aircraft will have wet sump (DA40) ● Not suitable for radial engines, aerobatic aircraft Dry sump ● Oil is stored in a separate tank ● Scavenge pumps feed oil from the sump to the oil tank ● Aerobatic aircraft typically have a dry sump which allows for continuous lubrication in extreme attitudes

Oil system components Oil pump ● Engine driven, usually gear type ● Receives oil from sump or tank and pumps at pressure through oil lines, passages, ect to moving parts. Oil pressure relief valve ● If oil pressure exceeds a predetermined level, it is returned to pump inlet and not to engine Filters and screens ● These remove contamination, e.g. dirt, carbon deposits ● Must be inspected & replaced as specified in maintenance schedule ● Filter has a bypass valve in case of clogging ● Engine debris can indicate condition of engine Oil cooler ● Insufficient heat is removed from oil as it passes through the sump ● Further cooled by passing oil through a radiator ● Thermal valve allows oil to bypass cooler if it is already sufficiently cool ● Includes a pressure bypass in case of cooler blockage ● Pre-flight check required to ensure no blockages or leaks Oil pressure gauge ● Situated in cockpit ● Indicates pressure of oil being delivered by pump to engine Oil temperature gauge ● Measures oil temperature after it has passed through cooler ● Also located in cockpit Oil types ● Mineral oil ○ Derived from crude oil ○ Typically used in an aero pistion engines ●

Synthetic oil ○ Created by polymerisation of hy...