Mechanical Engineering Dictionary-S.I Engines

Important Terms and their Meaning of Mechanical Engineering-Spark Ignition Engines:

ACCELERATOR – Device for rapid control of speed, for quick opening and closing of the throttle. It is a foot or hand operated, spring returned, linked to the throttle valve in the carburettor. The minimum throttle opening is controlled by the setting of the throttle screw.

ACCELERATOR PUMP – In the carburettor, a small pump linked to the accelerator which momentarily injects a charge of fuel into the intake tract in addition to that supplied by the normal metering components, and thus enriches the mixture when the accelerator pedal is depressed.

ADVANCE – Setting the ignition timing so that spark occurs before the piston reaches top dead center.

AIR BLEED – An opening into a gasoline passage through which air can pass or bleed into the gasoline as it moves through the passage, to weaken the air fuel mixture.

AIR FUEL MIXTURE – Finely atomized mist of fuel and air necessary for combustion. This mixture consists of approximately 15 parts air to one part fuel (15: 1) at cruising speed.

AIR FUEL RATIO – The proportion of air to fuel in the working charge of an internal combustion engine, or in other combustible mixtures, expressed by weight for liquid fuels and by volume for gaseous fuels.

AIRGAP (spark plug) – Distance between centre and side electrodes, in a spark plug. Spark jumps across this gap.

AIR HORN – In the carburettor, the tubular passage through which the incoming air must pass.

AIR JET – A small jet in the air passage of a carburettor. This jet meters the amount of air fed to the diffuser in an air bleed type carburettor.

ANTIKNOCK SUBSTANCES – Substances added to petrol to lessen its tendency to detonate, or knock in an engine, i.e., Tetra ethyl lead.

ANTIKNOCK VALUE – The relative immunity of a volatile liquid fuel from detonation, or knocking, in a petrol engine, as compared with some standard fuel.

ANTIPERCOLATOR – Device for venting vapours from main discharge tube or well of a carburettor.

ANTI SIPHON SYSTEM – Use of a small passage designed into a carburettor to prevent fuel from siphoning from the float bowl into the engine.

ATOMIZED – Tiny particles of fuel mixed with air, making a fine mist.

AUTOMATIC CHOKE – A carburettor choke device (valve) that automatically positions itself in accordance with carburettor needs or engine temperature.

AUTOVAC – A vacuum operated mechanism for raising fuel from a tank situated below the level of the carburettor to a position from which it may be fed to the latter by gravity.

BACKFIRE – (1) Premature ignition during starting of an internal combustion engine, resulting in an explosion before the end of compression stroke, and consequent reversal in the direction of rotation. (2) An explosion of live gases accumulated in the exhaust system due to incomplete combustion in the cylinder.

BACKFIRE (intake system) – Pre-explosion of air fuel mixture so that the explosion passes the open intake valve and flashes back through the intake manifold. May be caused by faulty timing, crossed plug wires, leaky intake valve etc.

BACK KICK – Violent reversal of an internal combustion engine crankshaft rotation, during starting due to backfire.

BALANCED CARBURETTOR – Carburettor in which the float bowl is vented into the air horn, below the air cleaner, to compensate for the effects of a clogged air filter.

BATTERY COIL IGNITION – High tension supply for sparking plugs, in automobiles, in which the interruption of a primary current from a battery induces a high secondary emf in another winding on the same magnetic circuit, the high potential being distributed in synchronism with the contact breaker in the primary circuit and the engine firing order.

BERNOULLIS PRINCIPLE – Given a fluid flowing through a tube, any constriction or narrowing of the tube will create an increase in the fluid velocity and a decrease in pressure. This principle is used in the venturi tube of the carburettor.

BOOST VENTURI – also called secondary venturi is a smaller venturi or restriction, incorporated in some carburettors in the middle of the primary venturi. It increases air speed, vacuum created and hence fuel flow.

BOWL VENT – is an opening in the carburettor float chamber. This hole prevents pressure or vacuum from building up in the bowl.

BREAKER ARM – The movable arm upon which one of the breaker points of the ignition system is affixed.

BREAKER POINTS (ignition) – Pair of points, one fixed and another movable, that are opened and closed to break and make the primary circuit. When the circuit is broken by opening the points, the spark plug fires.

BUTTERFLY VALVE – A type of valve used for choke and throttle valve in a carburettor that is so named due to its resemblance to the insect of same name. This valve controls charge flow.

CAM ANGLE (ignition) – Number of degrees breaker cam rotates from the time breaker points close until they open again. Also called DWELL ANGLE.

CAPACITOR DISCHARGE IGNITION (CDI) – An electronic ignition system designed to produce very high voltage, consisting of an exciter coil, a capacitor, diode, trigger coil, silicon controlled rectifier and ac ignition coil.

CARBURETION – The actions that take place in the carburettor, converting liquid fuel to vapour and mixing it with air to form a combustible mixture.

CARBURETTOR – The mixing device in the fuel system which meters and mixes gasoline into the air stream (vaporizing gasoline as it does so) in varying proportions to suit engine operating conditions.

CARBURETTOR ADAPTER – Adapter used to fit or place one type of carburettor on an intake manifold that may not be originally designed for it.

CARBURETTOR CIRCUITS – Series of passages and units designed to perform a specific functions—idle circuit, full power circuit etc.

CARBURETTOR ICING – Formation of ice on throttle plate or valve. As fuel nozzles feed fuel into air horn it turns to a vapour. This robs heat from air. When weather conditions are just right (fairly cold and quite humid) ice may form.

CARBURETTOR INSULATOR – A spacer, or insulator, used to prevent excess engine heat from reaching the carburettor.

CENTRIFUGAL ADVANCE (distributor) – Unit designed to advance and retard ignition timing through action of centrifugal force resulting from changes in engine speed.

CHOKE – Near the top of the carburettor, a butterfly valve that is closed when starting a cold engine. It chokes off the air flow through the air horn, producing a partial vacuum in the air horn for greater fuel delivery and a richer mixture supply to the engine.

CHOKE STOVE – Heating compartment in or on the exhaust manifold from which hot air is drawn to the automatic choke device.

COIL (ignition) – Unit used to step up the relatively low voltage supplied by the battery to the extent necessary to create a spark across the spark plug terminals.

COIL BUILDUP – Build up of a magnetic field while current is flowing through primary windings of the coil.

COLD PLUG – has a shorter heat path. Hence it runs at a much lower temperature than a hot plug.

COMBUSTION LAG TIME – A period of slow burning that occurs before the burning of the air fuel mixture, which spreads throughout the engine combustion chamber.

CONDENSER (ignition) – Unit installed between breaker points and coil to prevent arcing at breaker points. Condenser absorbs and retains momentary surge of current when the breaker points open.

CONSTANT CHOKE CARBURETTOR – is the carburettor in which the air and fuel flow passages (i.e., areas) are always maintained to be constant. But the pressure difference or depression which causes the flow of fuel is being varied as per the demand on the engine.

CONSTANT VACUUM CARBURETTOR – is the carburettor in which the air and fuel flow areas are being varied as per the demand on the engine, while the depression or vacuum is maintained to be always same.

CONTACT POINTS – In the conventional ignition system, the stationary and the movable points in the primary circuit, usually made of tungsten, platinum or silver. Also called BREAKER POINTS.

CONVENTIONAL IGNITION – Ignition system which uses breaker points.

CRITICAL COMPRESSION RATIO – The lowest compression ratio at which any particular fuel air mixture will ignite by compression under prescribed test procedure. The lower the critical compression ratio, the better ignition qualities the fuel has (Gasoline engine 4 :1, oil engine 7 :1 diesel engine 12.5 :1).

DASHPOT (carburettor) – A device in the carburettor that prevents excessively sudden closing of the throttle.

DETONATION – A violent, instantaneous explosion of the final portion of the burning combustion gases caused by an excessive rise of pressure and temperature, also called AUTO IGNITION.

DIESELING – A condition in which a spark ignition engine continues to run after the ignition is shut off. Also called RUNNING ON.

DISTRIBUTOR (ignition) – Unit designed to make and break the ignition primary circuit and to distribute resultant high voltage to the proper spark plug in the cylinder at the correct time. Rarely used in motor cycles.

DISTRIBUTOR CAP (ignition) – Insulated cap containing a central terminal with series (one per cylinder) of terminals that are evenly spaced in circular pattern around the central terminal. Secondary high voltage travels to central terminal where it is then channelled to one of the outer terminals by the rotor.

DOWN DRAFT CARBURETTOR – Carburettor air horn is so arranged that the air passes downward through the carburettor on its way into the intake manifold.

DUAL BREAKER POINTS (ignition) – Distributor using two sets of breaker points to increase cam angle so that even at high speeds, spark with sufficient intensity will be produced.

DUAL CARBURETTOR – An engine on which two carburettors have been mounted.

DWELL ANGLE – The number of degrees on the breaker cam during which the breaker points are kept closed.

DWELL METER – A device used to measure the number of degrees that the ignition contact points remain closed.

DYNAMIC TIMING – A test of ignition timing made with the strobe light.

ECONOMIZER VALVE – Fuel flow control device within the carburettor.

ELECTRIC ASSIST CHOKE – A choke which uses a small electric heating element to warm the choke spring, causing it to release more quickly. This reduces exhaust emissions during the start up of a cold engine.

ELECTRODE (spark plug) – Centre electrode rod passing through the insulator forms one electrode. The rod welded to the shell forms another. They are referred as centre and side electrodes.

ELECTRONIC FUEL INJECTION – A fuel injection system used for injecting gasoline into the spark ignition engines, which has an electronic control system to time and meter the fuel injected.

ELECTRONIC IGNITION SYSTEM – An ignition system using transistors, which does not have mechanical contact breaker points in the distributor, but uses the distributor for distributing the secondary voltage to the spark plugs.

ERODED PISTON (crown) – A condition caused by detonation or pre-ignition where the gas temperatures are raised so high that part of piston crown is heated and melted away.

ESC – Electronic spark control.

ENERGY TRANSFER MAGNETO – A self powered ignition system that consists of a flywheel with permanent magnets that rotate around a laminated core with coil windings, an induction coil, contact points and capacitor.

FLAME FRONT – The glowing layer of flame that separates the burned charge from the unburned charge in a SI engine during combustion process. The flame front should move in a controlled pattern across the cylinder.

FLAME VELOCITY – is the speed with which the flame front travels inside the combustion chamber. This affects combustion phenomena, development of pressure and production of power.

FLASH OVER – A condition that occurs when a spark jumps across the surface of a spark plug insulator from the terminal.

FLAT SPOT – A point during acceleration when the engine seems to lose power for an instant.

FLOAT BOWL – In the carburettor, the reservoir from which gasoline feeds into the passing air. Also called FLOAT CHAMBER.

FLOAT LEVEL – The float position at which the needle valve closes fuel inlet to the carburettor to prevent further delivery of fuel.

FLOAT SYSTEM – The system in the carburettor that controls the entry of fuel and fuel level in the float bowl.

FLOODED – A term used to indicate that the engine cylinders received an air fuel mixture too rich to burn.

FOUR BARREL CARBURETTOR – A carburettor with four throttle valves. In effect two, two barrel carburettors in a single assembly.

FUEL FILTER – A screen used to prevent contaminants in the fuel from entering the carburettor or fuel pump.

FUEL LINE – The pipe or tube through which fuel travels from the tank to the fuel pump and from the pump to the carburettor.

FUEL NOZZLE – The tube in the carburettor through which gasoline feeds from the float bowl into the passing air. In a fuel injection system, the tube that delivers the fuel into the compressed air or passing air stream.

FUEL PUMP – The electrical or mechanical device in the fuel system which transfers fuel from the fuel tank to the carburettor.

FUEL SCREW – A fine point screw that projects into the slow jet outlet passage, used to adjust the fuel mixture at slow speeds, located at the engine side of the carburettor.

FUEL SYSTEM – In an automobile, the system that delivers to the engine cylinders, the combustible mixture of vaporized fuel and air. It consists of fuel tank, lines, gauge, carburettor, fuel pump and intake manifold.

FULL ADVANCE – The point at which the advance unit will no longer continue advancing ignition timing.

FULL THROTTLE – A wide open throttle position with the accelerator pressed all the way down to the floor board.

HEAT RANGE – A term used to describe the ability of a spark plug to carry away heat. Plugs with longer nosed insulators take longer path and time to carry heat off effectively.

HEMISPHERICAL COMBUSTION CHAMBER – A combustion chamber shaped like a round dome, allowing use of large valves placed opposite each other in the chamber.

HIGH SPEED CIRCUIT – The circuit in the carburettor that supplies fuel into the air passing through the air horn during, medium and high speed, part to full throttle operation.

HIGHEST USEFUL COMPRESSION RATIO – is the compression ratio at which a fuel test engine can be operated without detonation with any mixture strength or with any ignition timing, at a speed of 1500 rpm.

HOLED PISTON – A condition caused by severe detonation or pre-ignition, where a hole is eaten or burned through the piston crown, as a result of extreme heat and pressure.

HOT PLUG – has a longer heat path, hence it runs at a much higher temperature than a cold plug.

IDLE MIXTURE – The air fuel mixture supplied to an engine during idle. The mixture is usually rich. The idle mixture screw(s) are sometimes adjusted as a part of tune up.

IDLE MIXTURE ADJUSTMENT SCREW – The adjustment screw in some carburettors, that can be turned in or out to vary the quality of the idle mixture.

IDLE SYSTEM – In the carburettor, the passages through which fuel is fed when the engine is idling.

IGNITION (spark) – In an engine, the act of spark in starting the combustion process in the cylinder.

IGNITION ADVANCE – To set the ignition timing, so that a spark occurs earlier or more degrees before TDC.

IGNITION COIL – That part of the ignition system which acts as a transformer to step up the battery voltage to many thousand volts, the high voltage surge then produces a spark at the spark plug gap.

IGNITION DISTRIBUTOR – That part of the ignition system, which opens and closes the circuit to the ignition coil with correct timing and distributes to the proper spark plugs the resulting high voltage surges from the ignition coil.

IGNITION SWITCH – The switch in the ignition system which is operated with a key to open and close the ignition primary circuit.

IGNITION SYSTEM – The part of the electrical system that provides high voltage sparks to the engine cylinder to fire the compressed air fuel mixture, consisting of a voltage source, timing device, capacitor, ignition coil, secondary wiring and spark plugs.

IGNITION TIMING – is the crank angle at which spark occurs relative to the top dead center, during compression stroke.

INTAKE MANIFOLD (SI engine) – The part of the engine that provides a series of passages from the carburettor to the engine cylinders through which air fuel mixture can flow.

INTAKE STROKE – The piston movement from TDC to BDC that occurs as the intake valve opens. This movement causes entry of fresh charge.

JET – A calibrated passage in the carburettor through which fuel flows.

KNOCKING COMBUSTION – is the auto ignition or instantaneous ignition of the end charge due to the compression of the same by the expansion and radiation heat of the burning charge.

LEAN MIXTURE – An air fuel mixture that has a relatively high proportion of air and a low proportion of fuel.

LOW SPEED CIRCUIT – The circuit in the carburettor that supplies fuel to the air passing through the air horn during low speed, part throttle operation.

MAIN FUEL NOZZLE – The fuel nozzle in the carburettor venturi that supplies fuel when the throttle is opened partially to fully opened position.

MANIFOLD VACUUM – The vacuum in the intake manifold that develops as a result of the vacuum in the cylinders on their intake strokes.

METERING ROD AND JET – A device, consisting of a small movable rod, which has a varied diameter, and a jet (which accommodates the movable rod) that increases or decreases fuel passage and hence the flow of fuel according to engine throttle opening, engine load or a combination of both.

MISSING – In the engine, the failure of the air fuel mixture in a cylinder to ignite when it should and thus causing the engine to run roughly.

MULTIPOINT FUEL INJECTION (MPFI) – has one injector for each cylinder. Fuel is injected in more than one location. This is often called port injection.

NORMAL COMBUSTION – refers to the combustion of the entire air fuel mixture in the SI engine combustion chamber, layer by layer by the moving flame.

OCTANE NUMBER OF A FUEL – is the percentage by volume of isooctane in a mixture of iso-octane and n-heptane which will exhibit the same antiknock characteristic of the fuel under test, when tested in a standard (CFR) engine, under a set of standard test conditions.

OCTANE RATING – The number indicating the quality of gasoline based on its ability to resist knock. The higher the number, the better the quality. Higher compression engines require higher octane fuel.

OPTIMUM SPARK TIMING – is the ignition timing which will cause that half of the pressure rise occurs at dead center. This happens in practice when 75 percent of the charge burns after ignition.

PERCOLATION – A condition in which the fuel actually “boils” due to excess heat. Percolation prevents proper atomization of the fuel causing rough running.

PERFORMANCE NUMBER – is the ratio of knock limited imep with the fuel in question to the knock limited imep with iso-octane when the inlet pressure is used as the dependent variable.

PING – A metallic rattling sound produced by the engine under acceleration. It is usually due to incorrect ignition timing or poor grade of gasoline.

POST COMBUSTION ACTIVITY – The last phase of combustion, during which the piston descends, the volume inside the cylinder increases, and the cylinder eliminates spent gases.

POWER PISTON – In some carburettors, a vacuum operated piston that allows additional fuel to flow at wide open throttle to permit delivery of a richer air fuel mixture to the engine.

PREIGNITION – Abnormal combustion that occurs when the air fuel mixture ignites before the spark plug fires and this is due to some hot spot.

PRIMARY IGNITION CIRCUIT – Section of the ignition circuit including contact points, condenser and primary winding of ignition coil, power supply (battery or ignition generating coil), ignition switch and related wiring.

PRIMARY WINDING – Part of an ignition coil, a separate winding of heavy wire wound a few hundred turns around a laminated steel core.

QUENCH – The space in some combustion chambers which absorbs enough heat to quench or extinguish the combustion flame front as it approaches a relatively cold cylinder wall. This prevents detonation of the end gas but results in hydrocarbon emission.

REED VALVE – A type of valve used in the crankcase of some two cycle engines. Air fuel mixture enters the crankcase through the reed valve, which then closes as pressure builds up in the crankcase.

RESISTOR SPARK PLUG – A spark plug incorporating a resistor to shorten the spark duration. This suppresses radio interference and lengthens plug life.

RICH MIXTURE – An air fuel mixture with high proportion of fuel.

RISE TIME – The length of time between the begining of the voltage at the plug and the begining of the spark.

RUN ON – Condition when a SI engine continues to run, even when the ignition key is turned off. Also called DIESELING.

SECONDARY IGNITION CIRCUIT – Part of the ignition circuit consisting of secondary windings of ignition coil, spark plug wire, spark plug terminal and spark plug.

SECONDARY WINDING – Part of an ignition coil, a winding of fine wire wound many thousands of turns around a laminated steel core.

SIDE DRAUGHT CARBURETTOR – also called horizontal carburettor, consists of a horizontal mixing tube, with the float chamber on the side of it.

SINGLE POINT INJECTION – has one or two injectors mounted inside the throttle body assembly. Fuel is sprayed into one point or location at the centre of the engine intake manifold. Single point injection (SPI) is also called throttle body injection (TBI).

SPARK ADVANCE – The adjustment in ignition timing made by the timing device for changes in load, speed or other conditions.

SPARK DURATION – The length of time spark occurs at a spark plug.

SPARK PLUG – The assembly, which includes a pair of electrodes and an insulator, that has the purpose of providing a spark gap in the engine cylinder.

SPARK PLUG HEAT RANGE – The distance heat must travel from the centre electrode to reach the outer shell of the plug and enter the cylinder head.

SPARK PLUG REACH – is the length of the threaded portion of the spark plug. This is the distance between the end of the plug threads and the seat or sealing surface of the plug.

SQUISH – is the radial inward or transverse gas motion that occurs towards the end of the compression stroke, when a portion of the piston head approaches the cylinder head closely.

STATIC TIMING – The test of ignition timing made with a buzz box or a continuity light while the engine is at rest.

STRATIFIED CHARGE ENGINE – In a gasoline fuel, spark ignition engine, a type of combustion chamber in which the flame starts in a very rich pocket or layer of fuel air mixture and after ignition, spreads to the leaner mixture filling the rest of the combustion chamber. The diesel engine is a stratified charge engine.

SURFACE IGNITION – is the ignition of the air fuel mixture by any hot spot in the combustion chamber.

S/V RATIO – The ratio of surface area of the combustion chamber to its volume, with the piston at the top dead centre. Often used as a comparative indicator of hydrocarbon (HC) emission levels from an engine.

THROTTLE (valve) – A round disc valve in the throttle body of the carburettor that can be turned by the driver to admit more or less air fuel mixture, thereby control the engine speed.

THROTTLE CRACKER – Linkage from the starting motor switch to the throttle, which opens the throttle slightly when the engine is being cranked.

THROTTLE RETURN CHECK – A device in the carburettor which prevents excessively sudden closing of the throttle, also called dashpot.

TIMING LIGHT – A light that is connected to the ignition system to flash each time the number one spark plug fires, used for adjusting the timing of the ignition spark.

TIMING UNIT – A device that determines, when the ignition system fires the spark plugs.

TRANSISTOR ASSISTED CONTACT IGNITION (TAC) – An ignition system similar to battery point ignition but which uses a transistor to eliminate heavy current flow at the contact points.

TRANSISTOR POINTLESS IGNITION – A battery assisted ignition system using a pulse generator to trigger a transistor to fire the spark plug.

TURBULENCE – The state of being violently disturbed. In the engine, the rapid swirling motion imparted to the air fuel mixture entering the cylinder.

TURNS RATIO – The ratio of the number of coils in the primary and secondary windings of an ignition coil.

TWO BARREL CARBURETTOR – A dual carburettor in which there are two throttle valves.

UNIT DISTRIBUTOR – An ignition distributor, used by General Motors, that uses a magnetic pick up coil, and timer core instead of points and condenser. It has the ignition coil assembled in the distributor as a unit.

UPDRAFT CARBURETTOR – is the carburettor in which the air fuel mixture flows upward. This unit can be placed on the side of the engine.

VACUUM ADVANCE – A method of advancing the ignition timing by applying engine vacuum to a diaphragm mounted on the distributor.

VACUUM GAUGE – In automotive engine service, a device that measures intake manifold vacuum and thereby indicates action of engine components.

VAPOUR LOCK – A condition in the fuel system in which gasoline has vaporized and turned to bubbles in the fuel line, or fuel pump so that fuel delivery to the carburettor is prevented or retarded.

VAPOURIZATION – is the process of changing the liquid or atomized fuel into a vapour.

VENTURI – In the carburettor, the restriction in the air horn that produces the vacuum responsible for the movement of gasoline into the passing air.

VENTURI PRINCIPLE – The physical law which states that air moved through a constriction increases in speed and decreases in pressure at the point of constriction.

WEDGE COMBUSTION CHAMBER – A combustion chamber resembling in shape, a wedge.

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