Langley Flying School Private Pilot Groundschool Manual Sample ...

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Private and Recreational Pilot Groundschool Manual Page 38Langley Flying School, Inc.Parts of an EngineThe shell of an engine is referred to as the crankcase; the crankcase has at its centre the crankshaft, which runslongitudinally through the crankcase and is connected to the propeller at the front end of the crankcase; affixed tothe crankcase are the cylinders which contain the pistons; the pistons move up and down in the cylinders and thismovement, caused by the timed explosion of an air/gas mixture, is the source of engine power; the explosions in thecylinders are caused by the timed spark on the ends of the spark plugs; the sparking of the spark plugs, in turn, iscaused by the magnetos which are connected to the rear of the engine in what is referred to as the accessories box;as the pistons move up and down, the introduction and exhaust of the air/gas mixture is controlled by the timedopening and closing of valves located at the top or “head” of the cylinders; the timed movement of these valves iscontrolled by the camshaft which runs parallel to the crankshaft inside the crankcase; between the valves and thecamshaft is a valve-operating mechanism; the pistons are connected to the crankcase by connecting rods; and themixture of the air/gas mixture prior to entry into the cylinders is an induction system—usually a carburettor or fuelinjection system; between the carburettor and the cylinders in the intake manifold, and as spent gases exit thecylinders they pass into the exhaust manifold and out the exhaust pipe.IntakeCompressionPowerExhaustcrankshaft.Engine Power CycleThe bore of a cylinder is its diameter, while the stroke of apiston is its repetitive movement from upper-most position—referred to as top-dead-centre—to its lower-most position—referred to as bottom dead centre.Four Strokes of an EngineThe production of power in an engine is based on themovement of the pistons which can be broken down into thefollow strokes:InductionstrokeCompressionstrokePower strokeExhauststrokeTimingPiston moves down with intake valveopened and exhaust valve closed.Piston moves up with both valvesclosed.Spark plugs fire forcing pistondownward, with both valves stillclosedPiston moves up with intake valveclosed and exhaust valve open.Timing of an engine is associated with the firing of themagnetos and the opening and closing of valves; the timing iscontrolled by the camshaft, which rotates ½ the speed ofAn AME (Aircraft Maintenance Engineer) can control the timing of an engine by adjusting the timing of the valves;the term valve lead is the opening of valves early (with valve lead set on the exhaust valve, for example, pressurefrom the end of the power stroke can be used to push-out spent gases; valve lag is the closing of valves late (with thevalve lag of the exhaust valve, the end of the exhaust stroke can be used to facilitate the induction of fresh air/gasmixture; valve overlap is where both valves open simultaneously—for, example the exhaust and intake valve areboth opened just prior to induction stroke to facilitate engine cooling.© 2012 David L. Parry
Private and Recreational Pilot Groundschool Manual Page 39Langley Flying School, Inc.CoolingMost general aviation engines are air-cooled, whereby ram air entersthe engine cowling from the front, and is circulated around the cylindersusing baffles; the baffles surround the cylinders in the form of a lightmetal box structure that controls and maximizes the movement of coolair.With higher performance aircraft that are more tightly cowled, thequantity of air moving through the baffles can be controlled by the pilotvia cowl flaps, which are like reverse air scoops; the cowl flaps areopened during taxiing or during climbs to increase the volume of airmovement, and are closed to reduce drag during the cruise phase.Engine heat from cylinders is dissipated by fins that form part of thecylinder outer wall.EngineCowlingCooling AirIntakecylinderheadBafflescylinderheadBafflesEngineCowling(open during taxiing,takeoff, and climbs)OilThere are four functions of engine oil: cooling, sealing (pistons), lubrication, flushing (cleaning).Pilot rule: Oil types cannot be mixed; for example, when oils contain additives (detergents and dispersants, etc.)only oil of the same type should be added.Oil is graded by viscosity (resistance to flow, stickiness); the higher the viscosity, the smaller are the changes in theviscosity of the oil owing to temperature changes.Three systems for grading viscosity are used:1. S.A.E. Number (Society of Automotive Engineers).2. Saybolt Universal Viscosity.3. US GradeThe formula to translate the oil grades: Saybolt Viscosity (100, for example) equals 2 times the S.A.E. number (50).The U.S. Grade is the Saybolt Viscosity plus 1000 (1100).The type of oil to be added to an aircraft can be determined by the pilot by reviewing preceding maintenance entriesin the aircraft Journey Log.Oil temperature should be regularly monitored in the cockpit, as an indication of engine health. The oil temperaturegauge probe is located where the oil enters the engine. If the oil gets too hot, viscosity is lost and engine parts losetheir protective film of oil; if the oil is too cold, the thick oil cannot flow through engine passageways, and oilfunctions are impaired.Oil pressure monitors pressure of oil supplied by the oil pump. It must be checked right after start, 22 and duringoperations. 23Oil pressure and temperature gauges provide redundancy and a means of verifying engine trouble; a loss of oilpressure on the gauge may indicate engine trouble but it may also indicate an erred reading on the instrument itself;but a loss of pressure indication, combined with an increased temperature indication means a forced landing isimminent.There are two types of oil systems: Wet Sump—oil supply kept in sump or pan under crank case—and Dry Sump—oil supply contained in separate tank (aerobatic aircraft).22 Oil pressure should be indicated in the green arch of the gauge within 30 seconds of engine start, and if not the engine should be shut down andthe problem investigated.23 There are several causes that could be associated with low oil pressure during flight, but the most common are insufficient oil quantity,excessive blow-by of oil past the piston rings, oil leaks, clogged oil-pressure relief valve, or excessively high oil temperature; high oil pressureindications during flight are likely caused by inaccurate instrument indications. Generally, aircraft engines can continue to develop oil pressurewith as little as two or three quarts.© 2012 David L. Parry
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