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Piston Engines Ignition, Cooling and Thrust Lecture Five Introduction Faraday Law Every time a magnetic field passes through the wires of a circuit, a pulse of electricity is generated in those wires. S N Coil Windings Volt Meter Each time the magnetic field ‘washes’ through the wire an electrical current is produced called EMF – Electro Motive Force Magneto Operation Magnetic fields of force flow from the magnets’ north pole through the soft iron core to the south pole. The coil and ignition circuit consist of two sub-circuits: the primary windings, which carries low voltage; and the secondary windings, where high voltage is generated. The magnetic poles continuously change by mechanical Secondary drive from the engine which rotates the magnets. Windings For each rotation of the magnet rotor, the magnetic field in the soft iron core Primary will build and collapse four times. Windings NO Magnetic Magnetic Field Field EMP No EMF When the core is magnetised an EMF is generated in the primary windings. Soft Iron Core This generates an EMF in the Secondary windings which is used Engine Driven to the spark in the spark plugs. Rotorprovide and Magnets Magneto Operation The essential components ofsystem a magneto are: unit, The Magneto is a complete ignition in a single (except for the sparktoplugs and ignition switch). field. The Magneto System, provide the magnetic The Contact Breaker, topower break generation, the primary circuit It consists of the when the current is at its maximum. spark flow generation The Condenser, to reduce burning/pitting of the contact points. and components. The Rotor Arm to distribution direct the spark surge to each spark plug, in the correct sequence for the combustion stroke. Magneto Unit Power Generation Spark Generation Rotor Arm Condenser Coil Magneto Contact Breaker Distribution Ignition Switch Spark Plugs Dynamo/Alternator System In the Dynamo and Alternator systems the battery powers the primary windings in the coil unit. The battery is continuously re-charged by the dynamo / alternator, and these systems generally feature an engine driven distributor, containing the condenser, points and rotor arm, Coil Unit Magneto Unit Spark Generation Rotor Arm Distributor Condenser Ignition Switch Power Secondary GenerationWindings Coil Primary Windings Magneto + _ Battery Contact Breaker Distribution Ignition Switch Dynamo/ Alternator Spark Plugs The Distributor The Distributor combines the main distribution elements, directing the high tension current to each sparking plug. The Vacuum Advance Mechanism consists of a plate where the condenser, contact breaker and rotor arm are mounted. The Centrifugal Advance Mechanism consists of weights thrown outwards by centrifugal force, altering the position of the cam relative to the Rotor input Armdrive shaft. Distributor This can alter the ignition advance by up to 30 degrees. Condenser Vacuum Advance Adjustable Plate Centrifugal Weights Rotor Arm Vacuum Chamber Contact Breaker Contact Breaker Centrifugal Advance Electronic Systems Mechanically timed ignition systems suffer limitations : The contact breaker points deteriorate. Contact breaker current is limited; limiting the spark at the plugs. At higher revs the points can bounce on the cam when they close; this can weaken the spark. At higher revs the points are close for a very short time (called the ‘dwell’ angle) this produces a weaker spark. Despite these problems, mechanical systems were used until the 1970s when electronic systems started to appear. Electronic Systems Initially Electronic Systems continued to use the mechanical contact breaker points, but have now replaced them with a ‘solid’ state timing sensor, and an Electronic Control Unit (ECU). The timing disc and sensor indicate the position of the crankshaft to the ignition control unit. Coil Unit Rotor Arm Engine Speed Sensing Unit Ignition Ignition Switch Switch Timing Disc Secondary Windings Condenser Timing Sensor Electronic Control Unit Primary Windings Contact Breaker + _ Battery Dynamo/ Alternator Alternator Spark Plugs Electronic Systems Engine Management Systems (EMS) With the development of fuel injection systems, it became logical to combine fuel and ignition in one system. On most systems each spark plug now has its own coil pack, producing a current of some 30,000 volts at the plugs. Engine Speed Sensing Unit Ignition Switch Timing Disc Timing Sensor Electronic Control Unit + _ Battery Alternator Spark Plugs Coil Packs Spark Plugs The first commercially available Spark Plug was invented in 1902, and the basic construction has been the same since. When supplied with a high voltage pulse, a spark jumps across from the inner to the outer electrode. The gap needs to be set correctly; too wide and the spark won’t jump the gap, too close and the spark may not be big enough to initiate combustion. The gap can increase due to erosion and could be bridged by contaminants as a result of the combustion process. Regular cleaning and gap adjustment is necessary. Gap Cap Connector Ceramic Body Seal Outer Casing Inner Electrode Copper Sealing Gasket Securing Thread These surfaces must be kept clean Outer Electrode Introduction Of the total heat produced in a piston engine, about 50% is used as energy, 40% passes into the atmosphere, and the remaining 10% is absorbed by the pistons, cylinder heads, valves etc. and by the lubricating oil that splashes against them. Unless these components are cooled by some means, they become over-heated, causing distortion, increased wear, and a reduction in strength. Excessive temperatures can also result in engine seizure. Two methods of cooling are in common use; Air Cooling and Liquid Cooling. Air Cooling The advantages of Air Cooling include :- Simple and less costly to produce. Lighter engines. But there are disadvantages :Difficult to cool all cylinders evenly, especially inline engines. Difficult to get engine to run at most efficient temperature on ground and at altitude. Except for small amount of ‘prop-wash’, early systems only effective when aircraft or vehicle is moving forward. Air cooled engines tend to be noisier than liquid cooled engines. Not so easy to streamline. Air Cooling Air Cooling Flows In air-cooled engines the outer surfaces of the cylinder head is extensively finned and a flow of air is directed over the exposed surfaces. The fins are kept thin and close together to obtain a maximum surface in contact with the air Cylinders to allow the air to take the heat away. Air Intake Air Outlet Cross section of typical light aircraft A typical cooling airflow goes through the forward facing intakes, one each side of the spinner, directly inline with the cylinders. An outlet at the bottom is provided to allow the cooling flow to escape. Liquid Cooling The advantages to Liquid Cooling include :- A steady engine operating temperature is more easily maintained and can be kept within a narrower range, making the engine more efficient. The cooling ‘jacket’ makes the engine quieter. The engine can be more easily streamlined and thus produce less drag. But there are also disadvantages :Tend to be heavier. More complex, more weight, costlier to produce. The system needs more maintenance compared to air cooling. Liquid Cooling The Thermostatic Valve prevents cooling in the radiator until the engine has warmed up. It is usually housed in an easy to access bell housing on the engine block, generally where the top hose is connected to the engine. Thermostatic Valve Consisting of Typical a temperature sensitive waxThermostatic pellet, within a rubber diaphragm,Valve it is no more than a heat switch. Rubber Diaphragm Wax Pellet Liquid Cooling The Thermostatic Valve With a cold engine, the valve is closed by spring force; isolating the radiator flow, and forcing coolant around the engine block only. As the coolant heats up, it heats the wax pellet, pushing the valve open. CLOSED OPEN No Flow with Flow To Radiator ‘Jiggle’ pin Rubber Diaphragm Wax Pellet When the valve opens, coolant is directed to flow through the radiator. A ‘Jiggle-pin’ in the valve plate allows air to escape, and a small coolant flow to the radiator so it starts to heat up before the hot coolant is allowed full flow. Liquid Cooling Coolant / Engine COLD:HOT:Pressure Relief Valve Thermostatic Valve CLOSED OPEN Cooling Fan Radiator Coolant Coolant circulates circulates around around engine engine block block and radiator only Thrust Augmentation What is it? A method of extracting more power from Internal Combustion Engines. It’s a form of ‘Power Boosting’ There are two types of booster: - Supercharging and Turbocharging both working on air compressors The Air Compressor Works with a rotor using centrifugal force, and is driven by the engine crankshaft. The rotor spins the air between the rotor vanes, forcing it from the centre out to the rim, then pushing the air through the stator vanes, and finally to the engine via the outlet ducts. OUTLET DUCTS STATOR VANES CROSS SECTION ROTOR VANES TAKE A CROSS SECTION The Super Charger Supercharging is an air compressor driven by the engine via a gear train or belt drive. Its main function is to increase the amount of air and fuel delivered to the cylinders and increase the pressure in the induction manifold It can increase overall engine power up to 50% or more. GEAR OR BELT DRIVE COMPRESSED AIR TO ENGINE The Turbo Charger Turbocharging is driven by the engine exhaust gases, passing through a turbine. Turbo’s operate best at high engine speeds taking very little power from the engine to drive them, therefore the net gain in power is relatively greater, although they can suffer from what is called ‘turbo-lag’. COMPRESSED AIR TO ENGINE THROUGH EXHAUST PIPE TURBINE BLADES Other Cooling Methods Intercooling Intercoolers are fitted in the induction system between the booster and fuel controller and work exactly the same as engine cooling radiators. Water Injection Water injection systems were introduced primarily to cool the boosted air to the engine. Adding fluid reduces the heat of the inlet air to the engine. This protects against detonation and allows more fuel to be added. More air and fuel means more power is developed, and more thrust produced ! Check of Understanding A pulse of electricity is called an EMF what does EMF stand for? Electro Magnetic Force Energy Motion Force Electro Motive Force Energy Magnet Force Check of Understanding Which of the following describes a magneto as used in a piston engine? A dynamo to produce a spark. A dynamo to turn the engine A fuel pumping system A fuel metering system Check of Understanding In a magneto, one purpose of the condenser (capacitor) is to Make the secondary current flow evenly Reduce pitting of the contact breaker points Make the primary current flow evenly Reduce erosion at the spark plug points Check of Understanding How many times will the magnetic field build and collapse with each rotation of the magnet rotor? Twelve Eight Six Four Check of Understanding In a magneto system, where is the power for the spark generated? In the contact breaker points In the coil secondary windings In the condenser In the coil primary windings Check of Understanding In a piston engine, the purpose of the distributor is to distribute - The correct mix of fuel and air to the cylinders Power to the primary windings in the coil Electrical impulses to the cylinders Power to the secondary windings in the coil Check of Understanding In a distributor, where are the condenser, contact breaker and rotor arm mounted? The Vacuum Advance Mechanism The Electronic Control Unit The Centrifugal Advance Mechanism The Spark Plug Coil Packs Check of Understanding In an electronic ignition system, what creates the spark and ensures the correct firing sequence? The spark plug coil packs The electronic control unit The solid state timing sensor The input drive shaft Check of Understanding On most electronic systems, each spark plug has its own coil pack, producing what amount of voltage? 300 volts 3,000 volts 30,000 volts 300,000 volts Check of Understanding The distributor includes two mechanisms, centrifugal weights and a pressure diaphragm, to advance the spark timing. What are these mechanisms for respectively? To advance the timing for engine temperature and air pressure To advance the timing for engine speed and air pressure To advance the timing for engine pressure and exhaust temperature To advance the timing for engine speed and engine intake manifold pressure Check of Understanding In a breakerless electronic ignition system, what components indicate the position of the crankshaft to the ignition control unit? The coil packs The distributor timing disc and sensor The altenator The valve timing belt Check of Understanding The 2 main methods of cooling piston aero engines are liquid cooling and air cooling. Which of the following is an advantage that air cooling has over liquid cooling? Can hold the engine at a more even temperature Much cheaper to make Engine can be more easily streamlined Promotes longer life between engine overhauls Check of Understanding What is the purpose of the fins arranged about the cylinder and cylinder head of an air-cooled engine? To reduce the engine weight To allow air to move freely through the engine compartment To allow air to take the heat away To prevent the engine coolant from boiling Check of Understanding What is the main benefit of using liquid, as opposed to air, to cool an engine? The engine will be lighter A steady temperature is easier to maintain The engine will be simpler The engine will be less costly to maintain Check of Understanding In a liquid cooled engine, what is the purpose of the thermostatic valve? It prevents cooling in the radiator until the engine has warmed up It prevents cooling in the radiator until the driver/pilot selects it open It prevents heating in the vehicle interior until the engine has warmed up It allows the coolant to be cooled until the engine is hot, then it closes Check of Understanding How is the thermostatic valve operated? Manually closed, opened by a spring Closed by heat, opened by a spring Opened by heat, closed manually Opened by heat, closed by a spring Check of Understanding How is the air compressor driven in a piston engine supercharger? By air pressure By the exhaust gases By an electric motor By the engine crankshaft Check of Understanding What is the main function of a supercharger when fitted to a piston engine? It increases the amount of air and fuel delivered to the cylinders It delivers a stronger spark to the sparking plugs It increases the amount of lubricating oil delivered to the bearings It ensures that the batteries are always fully charged Check of Understanding Which of these statements, about an exhaust-driven turbocharger, is true? It operates best at low engine speeds It operates as soon as the throttle is opened It operates best at high engine speeds It is more effective than an engine-driven supercharger at increasing the power output Check of Understanding What is the primary function of a supercharger on a piston engine? To ensure that the batteries are always fully charged To make use of unburnt fuel in the exhaust gases To speed up the extraction of exhaust gases To increase the pressure in the induction manifold Check of Understanding Water injection can be used with, or instead of, intercoolers. How does water injection help to increases engine power? It cools the air flow allowing more fuel to be added It makes the combustion burn slower and longer It heats the air flow allowing more fuel to be added It allows less fuel to be used Piston Engines End of Presentation