Each engine part has its own particular function to perform and in conjunction with other parts, equally as important, comprises the assembly called the internal-combustion engine or, for short, combustion engine.
An understanding of the operation or functions of the individual parts is necessary for a better understanding of the whole engine.
A person who intends to work in the diesel-engine field must know how to recognize the engine parts by sight and must learn their correct names and also their particular functions.
In Figs. A-1and A-2 are shown cross-sectional views of a diesel engine of the heavier type. In Fig. A-1 the section of the cylinder head is taken through the injector, or spray nozzle, and starting-air valve, whereas Fig. A-2 is a section one vertical plane through the center line of the engine and in the cylinder head shows the intake and exhaust valves.
In Fir. A-3 is shown the cross section of a diesel engine that is used in the automotive transportation field. Attention is called to the difference in the general construction of these two types of engines.
The following glossary of terms gives the student a preliminary understanding of the component parts used in the diesel engine and their functions as well as an explanation of some basic units and definitions of a more theoretical nature but indispensable to a person who wants to work intelligently in the chosen field and to rise from apprentice to engine operator and possibly to chief engineer of a large plant.
The way to use the drawings and the glossary is to study the drawings and to look up in the glossary every part name for explanation.
However, even a man familiar with diesel engines should have some use for the glossary looking up terms, units, and occasionally a definition to make sure that he remembers them correctly. Knowledge confirmed by an authoritative source is the best knowledge a person may have.
Absolute pressure. The pressure in
pounds per square inch, psi, above absolute zero pressure, or perfect vacuum. A
Bourdon or mercury gauge registers the difference between the pressure within a
receiver and that of the outside atmosphere. At sea level and under standard
barometric conditions, the atmospheric pressure is 14.7 psia. Tofind the
absolute pressure, add 14.7 to the gauge reading in psig.
Absolute temperature. The temperature above absolute zero. If a Fahrenheit thermometer scale is used, absolute zero is �460 deg. To find the absolute temperature, add 460 to the Fahrenheit reading.
Accelerate. To increase the speed of movement, such as increasing the speed of a piston or flywheel.
Acceleration. The rate at which the speed of an object increases.
Adiabatic. From the Greek word meaning �no pass through.� Adiabatic compression or expansion of a gas is accomplished without the loss or gain of heat through the cylinder walls.
Advance. Sometimes referred to as lead, or angle of advance, meaning the distance ahead of top or bottom (lead center of the piston as measured in degrees of crank travel.
Air cell. A small receptacle communicating with an engine cylinder into which some of the compressed air is forced, and from which air later flows back into the cylinder.
Air filter. A device for filtering the air, before it goes into the engine, to prevent particles of dust from entering the engine.
Air injection. The system of injecting fuel into the combustion chamber of a diesel engine by means of a blast of highly compressed air.
Airless injection. A general term describing all methods of injecting fuel without the use of compressed air.
Air starter. A system whereby an engine is turned over by admitting compressed air into the cylinders in order to initiate firing.
Aniline number. The lowest temperature at which equal parts of aniline and a sample of oil are completely miscible, or the temperature at which the mixture becomes turbid or cloudy.
Antechamber. Same as Precombustion chamber.
API. American Petroleum Institute.
API gravity. An arbitrary scale adopted by the American Petroleum Institute to designate the specific gravity of mineral oils. Diesel fuels range from 18 to 41 API.
Atmospheric pressure. The pressure of the atmosphere measured from absolute zero pressure. At sea level atmospheric pressure is 14.7 psi, decreasing as the altitude increases.
Atomize. To break up a liquid into extremely fine particles.
Axial. Parallel to the center line of a cylinder or shaft.
Axis. A center line. A line about which a body rotates or about which it is arranged.
Babbitt. A soft
antifriction metal used to line bearings.
Bock pressure. The resistance to the normal flow of gases and liquids.
Bedplate. The lower part of the engine resting on the foundation.
Bore. The interior diameter of an engine or compressor cylinder.
Blow-by. Escape of gases from the engine cylinder into the crankcase because of unsatisfactory action of the piston rings.
Brake horsepower. The useful horsepower delivered by an engine that may be found by the use of a prony brake. Abbreviated bhp.
Brake mean effective pressure. The mean effective pressure, corresponding to the brake horsepower developed. Abbreviated bmep.
British thermal unit. The amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit, from 68 to 69 F. Abbreviated Btu.
Burning. Commonly substituted for COMBUSTION, as late burning, meaning late or slow combustion.
By-pass. A passage which permits a liquid or gas to take a course other than that normally used.
Cam. A disk-like piece attached to a shaft,
a portion of which is circular, the remainder (the �nose�) protruding beyond
this circle. Cams are used to impart a desired motion to poppet valves.
Camshaft. The shaft which carries the various cams required for the operation of inlet, exhaust, fuel, and starting-air valves.
Cam follower. That part of the push rod that is in contact with the cam.
Carbon. One of the chemical elements which is the main constituent of liquid and solid fuels. Also the residual substance deposited in the combustion space and exhaust system of diesel engines when combustion of fuel oil is not complete.
Carbon dioxide. Gas composed of molecules made of one atom of carbon and two atoms of oxygen.
Carbon monoxide. Gas composed of molecules made up of one atom of carbon and one of oxygen. It is formed when combustion is not complete because of the absence of sufficient air.
Carbon residue. The carbon remaining after evaporating off the volatile portion of a fuel or lubricating oil by heating it in the absence of air under controlled test conditions. It is an indication of the amount of carbon that may be deposited in a diesel engine.
Centrifugal force. The force acting on all parts of a rotating body that tends to pull thorn away from time axis of rotation.
Cetane. A hydrocarbon used in testing the ignition quality of diesel fuels.
Cetane number. A percentage indicating the ignition quality of diesel fuels.
Cetene. A hydrocarbon used formerly infesting the ignition quality of diesel fuels.
Chamfer. A beveled corner.
Charge efficiency. The ratio of the weight of the charge actually taken in to the weight of the air at standard conditions corresponding to the piston displacement.
Check valve. A valve that permits the passage of a liquid or gas in one direction only. It stops, or checks, reverse flow.
Clearance. The space between a moving and a stationary part. Clearance must be provided between two surfaces to allow for lubrication and for expansion and contraction with a change of temperature.
Clearance volume. The volume of air or liquid remaining in the cylinder of an air compressor or a pump when the piston is nearest to the cylinder head.
Coefficient. A ratio; a factor or quantity that remains constant.
Combustion. The rapid oxidation, or combination, of a combustible such as carbon, hydrogen, or sulfur, with oxygen of air.
Combustion chamber. The space above the piston in which the fuel-air mixture starts to burn.
Common rail. A pipe or header from which branch lines lead to each of the fuel valves in the different cylinder heads of a diesel engine and in which fuel is carried at high pressure, ready for delivery to each separate cylinder when the fuel valve is opened by a cam.
Compression. The act or result of pressing a substance into a smaller space. One of the events of a combustion-engine cycle.
Compression ignition. Ignition of a fuel charge by the heat of the air in a cylinder, generated by compression of the air, as in the diesel engine.
Compression pressure. The pressure of the air charge at the end of the compression stroke.
Compression ratio. The ratio of the volume of the charge in the engine cylinder at the beginning of the compression stroke to that at the end of the stroke.
Compression relief. A device to reduce the compression in a cylinder and thus to make cranking easier.
Compression rings. Piston rings placed in the upper part of a piston to seal against loss of compression pressure and against gas blowing.
Compression stroke. The stroke of the piston during which the air charge in the cylinder is compressed by the piston movement.
Compressor. The air �pump� which furnishes compressed air for starting the engine, or for time injection of the fuel in an air�injection diesel engine.
Concentric. Having a common center.
Condensation. The process by which a substance changes from vapor to the liquid state.
Connecting rod. The engine part which connects the piston to the crankshaft. It changes reciprocating motion of the piston into rotary motion of the crankshaft or vice versa.
Connecting rod bearing. The bearing located in the large end of the connecting rod by which it is attached to the crankshaft.
Constant. A value or figure in a formula or equation which does not change, remains constant.
Constant-pressure combustion. Combustion of fuel in a cylinder at so slow a rate that there is no rise in cylinder pressure. The slow-speed airinjection diesel engine is a constant-pressure combustion engine.
Constant-volume combustion. Combustion in a cylinder so fast that there is no change in volume. Many high-speed diesel engines have practically constant-volume combustion.
Contraction. Becoming smaller in size. In metals and fluids a result of cooling or a lowering of temperature.
Cooling water. Water which is circulated through the jacket space of cylinders and cylinder heads to prevent excessive heating of these parts.
Crank. That part of the crankshaft, which is in the form of a crank and crank pin.
Crankcase. The middle part of the engine structure surrounding the working parts.
Crankcase subbase. The lower portion of the engine structure; the BEDPLATE.
Crankpin. That part of the crank to which the connecting rod is attached.
Crankshaft. That part of the engine which transmits the reciprocating motion of the pistons to the driven unit in the form of rotary motion. That part to which the connecting rods are attached.
Crankshaft cheek. The part J the crankshaft that connects the crankpin to the main crankshaft journal.
Crankshaft journal. The part of the crankshaft which rotates in the main bearings and transmits the torque developed by the engine.
Crankshaft web. The crankshaft cheek.
Critical speed. Speed at which the natural period of vibration of a shaft or her machine part is in synchronism with the power impulses.
Crosshead. The part of an engine to which are attached the piston pin with the connecting rod and the piston rod and which is supported on guides.
Crown. The top of an engine piston.
Crush. The amount by which a precision bearing is compressed to ensure good contact between the back of the bearing and the bore holding it.
Cycle. A series of events, operations, or movements that repeat themselves in a regular sequence
Cylinder. The cylindrical part of the engine in which the piston moves, and in which combustion takes place.
Cylinder block. A number of cylinders cast in one piece.
Cylinder bore. The inside diameter of an engine cylinder. Also the surface of the cylinder in which the piston slides or moves.
Cylinder head. The part which covers and seals the end of the cylinder and usually contains the valves.
Cylinder - head stud. Threaded round steel rod, one end of which screws into the cylinder block, the other being threaded to take a nut which holds the cylinder head in correct position.
Cylinder liner. A cylindrical lining that is inserted into the cylinder jacket or cylinder block and in which the piston slides.
Delivery stroke. The stroke of a
pump during which the fluid in the pump is forced out of the cylinder.
Detonation. A violent uncontrolled burning of a fuel in the combustion chamber.
Diesel engine. A compression-ignition combustion engine first developed by Rudolf Diesel.
Distillation. Separation of the more volatile parts of a liquid from those less volatile by vaporization and subsequent condensation.
Distributor. A device which distributes and directs the flow of fuel or compressed air to the various cylinders of the engine in proper sequence.
Dribbling. Slow seeping of fuel oil from the nozzle tip after cutoff of the fuel.
Dynamometer. A device for determining the power of an engine.
Eccentric. A circle not having the same
center as another circle within it. A device mounted off-center for converting
rotary motion into reciprocating motion.
Efficiency. The ratio of output over input.
Electromotive force. The potential, or voltage, developed by a dynamo, battery, or thermocouple. Abbreviated emf.
Energy. Capacity for doing work.
Engine. A machine which produces power to do work, particularly one that converts heat into mechanical work.
Exhaust. The act of discharging gases from an engine after they have done work.
Exhaust cam. The cam that controls the operation of the exhaust valve.
Exhaust gases. Products of combustion which are discharged from the cylinder after doing work on the piston.
Exhaust manifold. The pipe that collects the burnt gases as they are expelled from the cylinders.
Exhaust pipe. Piping through which exhaust gases from an engine pass out to the atmosphere.
Exhaust pyrometer. An instrument used to measure the temperature of the exhaust, mostly by the small electric current developed at the junction of two dissimilar metals when exposed to heat.
Exhaust valve. The valve through which the burnt gases are allowed to pass out to the exhaust manifold.
Expansion period. The portion of the power stroke during which the combustion gases expand from the movement of the piston and thus do work.
Filter. A device to remove dirt and
other impurities from air, oil, or water.
Fit. The desired positive or negative clearance between the surfaces of two machined engine parts.
Flash point. The temperature, degrees F, to which oil must be heated before the oil vapor over the oil will ignite when a small flame is passed across the surface of the oil.
Fluctuation. Variation in value, such as of pressure or velocity.
Flywheel. The wheel on the end of the crankshaft that gives the crankshaft momentum to carry the pistons through the compression stroke.
Foot pound. Unit in which, work is measured; it is equivalent to the work of raising one pound vertically a distance of one foot or of moving an object ore foot against a resistance of one pound. Abbreviated ft-lb.
Four-stroke engine. An engine operating on a cycle which is completed in four strokes, or two revolutions of the crankshaft.
Framing. The part of an engine between the cylinders and the bedplate; the crankcase.
Friction. The resistance to relative motion between two bodies in contact.
Friction horsepower. The power consumed within the engine from friction between its parts.
Fuel injector. The device which sprays the fuel into the cylinder.
Fuel knock. A noise produced in the cylinder of a diesel engine during combustion, usually when the fuel oil has a low ignition quality.
Fuel pump. The pump that delivers the fuel to the injector.
Fulcrum. The support on which a lever turns.
placed between two surfaces that must have a leakproof joint.
Glow plug. An electrical device used to heat fuel as it is injected in the cylinder for quick ignition and starting when the engine is cold.
Governor. A mechanism used to control the speed of an engine.
Heat. A form of energy.
Heat balance. A tabulation showing the percentages of the heat developed by combustion in the engine cylinder that are (1) delivered in the form of power at the crankshaft, (2) lost in friction, (3) lost to the cooling water, and (4) lost in the exhaust gases.
Heat unit. The unit of heat, usually British thermal unit (Btu).
Heat value. The heat developed by the combustion of one pound of fuel, Btu per Pound.
Helical. Having the shape of a helix, or screw. Helical gears have teeth shaped like a helix.
Helix. A line cut on a cylindrical surface shaped like a screw thread.
Horsepower. A unit for measuring power. Rate at which work is done. One horsepower = 33,000 ft-lb. per min. Abbreviated hp.
Hunting. Erratic variation of the speed of the governor, also of the engine.
Hydraulic. Pertaining to movement of and by water, also by other liquids, such as oils.
Idling. Engine running without a load
at the lowest speed possible.
Impeller. The rotating part of a centrifugal pump or blower that imparts motion to liquid or air by forcing it outward from the center of the machine.
Indicated horsepower. The horsepower developed in the engine cylinder, as calculated from an indicator diagram.
Indicator. Instrument used to investigate the pressures inside an engine cylinder.
Indicator diagram. A diagram obtained by means of an indicator; it shows the change of pressure in the engine cylinder.
Inertia. The tendency of a body to maintain its existing velocity.
Injection. The forcing of fuel oil into the combustion chamber of a diesel engine by means of high pressure.
Injection pump. The pump used to inject fuel oil into the combustion space of a diesel engine; the fuel pump.
Inlet cam. The cam that controls the operation of the air inlet valve in a four-stroke engine.
Inlet manifold. The main pipe that lies alongside the cylinder heads and from which branch pipes take the air charge to the separate cylinders.
Inlet valve. The valve through which air or the air-fuel mixture is admitted to the cylinder of a four-stroke engine.
Intake stroke. The suction stroke.
Integral. An indivisible part of a whole, constituting a completed whole.
Intermittent. Occurring at intervals.
Jack. A tool to lift or move a heavy
object. Also a tool to turn the flywheel to a desired position.
Jacket. The outer casing forming a space around an engine cylinder that permits circulation of cooling water.
Jerk pump. A fuel pump which injects fuel into the cylinder by action of a cam having a sharp nose.
Jet. A small orifice used to control the flow of fuel or air. Also the stream of fuel or air coming from such an orifice.
Journal. The finished part of a shaft that rotates in a bearing.
Jumper. A water-pipe connection between a cylinder head and the cylinder jacket or the water-jacketed exhaust manifold.
Keeper. A dowel or pin used to keep
piston rings from moving from an assigned position.
Key. A square or rectangular piece of steel-straight or tapering from one end to the other - used to secure a part on a shaft.
Keyway. The machined slot in a shaft or hub of a wheel to take a steel key.
Kilowatt. An electrical unite of measure equal to one thousand watts.
Kilowatt-hour. A unit of energy equal to a continuous flow of one kilowatt for one hour.
Kinetic energy. The energy of a moving body due to its mass and velocity.
Laminated. Made of thin
Laminated shim. A SHIM made up of thin metal sheets soldered together but so that each layer can be easily peeled off.
Land. The portion of the piston between two grooves carrying the piston rings.
Lanova cell. A special combustion chamber also called energy cell, for diesel engines of high rotary speeds.
Lb. Abbreviation for pound.
Lean mixture. A mixture in which the proportion of air to fuel is greater than that theoretically necessary for completes combustion.
Linear motion. Motion in a straight line.
Liner. The removable inner engine cylinder in contact with the piston.
Load. The useful output of an engine at a given moment.
Lubricant. A liquid or grease employed to separate two surfaces in relative motion to each other, in order to reduce friction.
Lubricating pump. A pump which handles lubricating oil in an engine.
Manifold. A pipe with a number of inlets
to, or outlets from, the several cylinders of an engine.
Manometer. A U-shaped glass tube, partly filled with a liquid, water or mercury, employed to measure pressure.
Mean effective pressure. The mean or average pressure which, acting on lie piston, would do the same work as does the actual variable pressure in the cylinder. Abbreviated mep.
Mechanical efficiency. The ratio of brake horsepower to indicated horsepower.
Mechanical injection. Injection with the fuel-valve operated mechanically from a cam. Sometimes, although wrongly, used to indicate airless injection in general.
Motor. A mechanism doing work by means of a ready source of energy, such as electric current, compressed air, or oil under pressure. Incorrectly applied to the combustion engine in an automobile.
Muffler. A device used to diminish noise of the intake or exhaust. Sometimes referred to as a silencer.
Needle valve. A round steel rod
with a conical or tapered point that seats against an outlet and prevents fuel
oil from entering the engine cylinder except when it is lifted by a cam or oil
Nickel. A metal which, when alloyed to steel and cast iron, improves their mechanical properties.
Nitrogen. A rather inert gas that makes up slightly more than three-fourths of the atmospheric air by volume.
Nozzle. The part of the injector or spray valve in which are located the holes through which the fuel is injected into the cylinder.
Oil-control rings. The piston ring,
usually located at the lower part of the piston, that prevents an excessive
amount of lubricating oil from being drawn up into the combustion space during
the suction stroke. Also called simply oil ring and oil scrapper
Oil grooves. The passages cut in bearings for distributing the lubricating oil.
Opposed-piston engine. An engine that has two pistons within the same cylinder, traveling in opposite directions.
Orifice. A small round opening. Usually refers to the hole in the spray
Otto cycle. An engine cycle in which combustion takes place at constant volume.
Outboard bearing. A bearing outside tine engine proper, carrying an extension of the crankshaft.
Oxygen. A gas that readily combines with other substances, such as carbon, hydrogen, sulfur, releasing heat. It makes up slightly less than one-fourth of the atmospheric air by volume.
Packing. A material used to seal a joint
Packing rings. Rubber rings used to form a watertight joint at the bottom of the cylinder liner.
Pintle. A small extension of the needle-valve tip projecting through the discharge nozzle. When the needle lifts, the oil passes through the opening between the circumference of the orifice and that of the pintle.
Piston. A cylindrical part which reciprocates in the cylinder bore of an engine and transmits the force of the gas pressure through the connecting rod to tine crankshaft.
Piston crown. The top of the piston; the piston head.
Piston head. The top of the piston or that part of the piston against which tire gas pressure acts.
Piston pin. A pin that rests in two bored holes in the piston and passes through the eye of the connecting rod, to join the two together flexibly.
Piston-pin bearing. The bearing either in the eye of the connecting rod or in the bored bosses of the piston, in which the piston pin rocks.
Piston-pin boss. That part of the piston on the inside, through which the hole is nude to take the piston pin.
Piston-pin lock. The device used to hold or lock the piston pin in the piston.
Piston ring. A split ring placed in a groove of the piston to form a leakproof joint between the piston and the cylinder wall.
Piston-ring gap. The space between the ends of the piston ring when it is in the cylinder bore.
Piston-ring land. The part of the piston on the outside surface located between the piston-ring grooves.
Piston skirt. The part of the piston below the piston-ring grooves.
Piston stroke. The movement of the piston from one end to the other of the piston travel in the cylinder bore. The piston stroke is equal to twice the throw of the crankshaft.
Plunger. The long piston of a single-acting pump, such as a fuel-injection pump.
Poppet valve. A valve opened by the action of a cam and closed by a spring.
Port. An opening hole, or passage.
Pound per square inch. The unit used to measure the pressure exerted by one body upon another. It is found by dividing the total force, pounds, acting normally upon a surface by the area of the surface, square inches. Abbreviated in psi.
Pour point. The lowest temperature at which fuel oil will just flow under test conditions. It is an indication as to how suitable a fuel is for cold-weather operation.
Power. Rate at which work is performed.
Power factor. The proportion (expressed as a decimal) which the actual power an a-c electrical circuit bears to the apparent power indicated by instruments measuring the electrical pressure (volts) and current (amperes). Standard abbreviation pf.
Power stroke. The working stroke of a piston.
Precombustion chamber. A chamber in the cylinder head of some engines into which the fuel is injected, ignited, and partly burned, the rest of the fuel being thrown out into the main combustion space where combustion is completed. Sometimes also called antechamber.
Preiqnition. Ignition taking place before the desired time in the operating cycle in spark-ignition engines. In a diesel engine can occur only if the fuel-injection timing is deranged.
Pressure. The force due to the action of a gas or liquid in a closed vessel. Usually measured in pounds per square inch. Small pressures are measured in inches of a column of mercury or water. Also force applied to an area.
Psi. Standard abbreviation for pounds per square inch.
Punk. A slow-burning material inserted by means of a steel plug into the combustion chamber to provide the additional heat necessary to ignite the first fuel charge in starting some engines, especially in extremely cold weather.
Push rod. The rod that transmits the action of a cam to the cam-operated valve
Pyrometer. An instrument for measuring high temperatures, as of the exhaust gases of a diesel engine.
Radial. Extending from a center to the
circumference, having the direction of a radius.
Reciprocating. Having a back-and-forth or up-and-down linear motion, such as an engine piston.
Reclaimer. An apparatus in which dirty lubricating oil, which often is discarded is treated and made usable, reclaimed.
Relief valve. A valve held closed by a spring and forced open when the pressure in the system rises above the desired height.
Resistance. Mechanically, a force opposing the motion of a body measured in pounds. Electrically, that which opposes the flow of an electric current measured in ohms.
Rheostat. A device to regulate the flow of electric current by transforming part or all of it into heat.
Ring grooves. Grooves cut in the piston barrel to hold the piston rings.
Rocker arm. A lever that transmits the action of the cam, usually by means of a push rod, to the stem of the intake or exhaust valve, sometimes also to the starting-air valve and fuel valve.
Rocker-arm shaft. The shaft, usually at the top of the cylinder, that serves as a fulcrum for the rocker arms.
Rotary. Turning on an axis.
Rotative. Pertaining to rotation.
Rpm. Abbreviation for revolutions per minute.
Saybolt viscosimeter. The standard merican
instrument used to measure the viscosity of oils.
Scavenging. The removing from the engine cylinder, by a stream of slightly compressed air, of the products of combustion of the preceding cycle.
Screen. A wire cloth with a fine mesh used to remove dirt from oil or water.
Seal. Any device to prevent leakage of gas or liquid, oil or water.
Semidiesel engine. A term applied to oil engines using rather low compression pressures and requiring a hot surface for ignition of the injected fuel.
Sensitivity. Change in engine speed before the governor begins to act.
Servomotor. A motor operated by oil or air pressure and used for operating heavy control mechanisms.
Shaft. A round bar of steel or other strong metal that is used to transmit rotary action.
Shaft horsepower. The power rating of a diesel engine used for turning a propeller shaft in marine installations. Abbreviated shp.
Shell. The steel or bronze backing to which the babbitt of a shaft bearing is bonded. Also the whole removable bearing.
Shim. A thin sheet of metal or other material which is inserted between two machine parts to obtain their correct relative location.
Silencer. A device to deaden the sound of the intake or exhaust of an engine; a muffler.
Silent chain. A chain made up of small pins and steel plates that engage the teeth on sprockets resembling spur gears, and that is used to transmit power from one shaft to another and by its construction is less noisy than the ordinary roller chain.
Skirt. The lower part of the piston. Also the lower part of a liner if it protrudes below the cylinder jacket.
Sludge. A tar-like formation in oil resulting from the oxidation of a portion of the oil.
Solid injection. A rather misleading term applied to airless injection.
Specific fuel consumption. The fuel consumption per hour divided by the brake, or shaft, horsepower developed, expressed in lb. per blip or lb. per shp.
Specific gravity. (1) Weight of a liquid or solid compared with the weight of an equal volume of water at 60 F. (2) For a gas, its weight as compared with the weight of an equal volume of air at the same temperature and pressure.
Specific heat. The amount of heat needed to raise the temperature of one pound of the substance one degree Fahrenheit.
Speed droop. The difference in speed between no-load and full-load engine speed.
Spray valve. The fuel injector.
Spring. A coiled piece of round or square steel wire which, when compressed, exerts a force that may be used to do some work.
Stability. (1) Ability of lubricating oil to withstand physical change under severe operating conditions. (2) Ability of a governor to maintain the required engine speed without fluctuations or hunting.
Stress. The internal forces set up in a body when it is subjected to forces tending to deform it by tension, compression, shear, bending, or torsion.
Stroke. The distance a piston travels up or down inside the cylinder.
Suction stroke. The stroke of the piston of a four-stroke engine during which a fresh charge is sucked in or forced by atmospheric pressure into the space vacated by the piston.
Supercharging. Supplying of combustion air to an engine at higher than atmospheric pressure, usually 2 to 4 psig, in some engines up to 30 psig.
Surface-ignition engine. The semidiesel engine.
Synchronous. Occurring at the same time or in phase.
Tachometer. An instrument
indicating instantaneous rotary speed of a shaft in rpm.
Tangent. A straight line touching a circle at one point and forming a right angle with the radius connecting this point with the center of the circle.
Tangential. Having the characteristics of a tangent.
Tangential force. The component of the force applied to the piston acting at a right angle to the crank arm.
Tappet. The part of the valve-actuating mechanism in contact with the cam; the cam follower.
Template. A pattern used as a guide for shaping something. In engine-foundation work a wooden frame used to locate the foundation bolts.
Temperature. The intensity or degree of heat.
Thermal efficiency. The percentage of the total chemical energy in the fuel consumed that is converted into useful work.
Thermocouple. Two strips or wires of dissimilar metals joined at one end used to measure temperature differences.
Thermodynamics. The science of changing heat into mechanical work.
Thermostat. A mechanism to convert the expansion of a heated metal or fluid into movement and having power sufficient to operate small devices, control electric circuits or small valves, etc. Can be set to operate at definite temperatures.
Throw of crankshaft. The distance between the center of the crankpins and the center of the journals of the crankshaft. It is equal to half the stroke of the engine.
Thrust. An axial force acting on a shaft.
Thumbscrew. A screw or bolt whose head is in the shape of a flattened, vertical fin, so that the bolt can be turned by the fingers.
Timing. The angle made by the crank with its top or bottom dead-center position at which some valve opens or closes.
Timing chain. A chain that is used to connect the crankshaft and camshaft by which the camshaft is made to rotate.
Timing gears. Gears keyed to the crankshaft and camshaft, by which the camshaft is made to rotate.
Tolerance. An allowable variation in dimensions. For example: a dimension of 0.753 in. with a tolerance of +0.000 and �0.003 indicates that any dimension from 0.750 to 0.753 in. is acceptable.
Top dead center. The position of the crank when the piston is in its farthest position from the crankshaft, in its nearest position to the cylinder head. Abbreviated tdc.
Torque. The effect which rotates or tends to rotate a body. Torque is the product of force multiplied by the arm, or normal distance from the center of rotation to the force. Torque is measured in lb.-ft or lb.-in.
Torsion. The deformation of a body caused by a torque or twisting effort.
Torsional vibration. Oscillatory twisting vibration in a rotating shaft which tends to make a gear mounted on one end of the shaft whip back and forth with respect to a gear on the other end.
Trammel. A metal rod having pointed ends, used to mark off a span equal to its length. Also called a tram.
Transfer pump. A pump employed to force fuel oil from storage to the engine fuel tank.
Turbulence. A high-velocity swirling of air, fuel vapor, or a mixture of both within the combustion chamber or cylinder.
Two-stroke engine. An engine operating on a cycle that is completed in two strokes, or one revolution of the crankshaft.
U.S. gallon contains 231 cu in.; 1 U.S. gal of water weighs 8.33 lb.
Vacuum. A pressure below atmospheric,
referring to a vessel filled with gas.
Valve. In a combustion engine, an intake or exhaust valve usually consists of a disk with a stem, which is opened by a cam and closed by a spring.
Valve seat. That part of the valve mechanism upon which the valve face rests to close the port.
Valve spring. The spring which is used to close a valve.
Valve-spring retainer. The part which is held against a groove or grooves on the valve stem and in turn holds the valve spring in a state of compression.
Vanes. Baffles employed to deflect the flow of a fluid, gas or liquid. Vanes may be stationary or, as in a centrifugal-pump impeller, moving.
Velocity. The rate of motion or the speed of a body at any instant. Measured in feet per minute (fpm) or revolutions per minute (rpm).
Venture. A tube with a narrowing throat or constriction to increase the velocity of the gas or liquid flowing through it.
Viscosity. Internal resistance to flow in a liquid or gas. In practice, for oils it is measured by the number of seconds required for a definite quantity to flow through a standard orifice under stated test conditions.
Viscosity index. A number given a lubricating oil to indicate its performance, particularly its change of viscosity with the temperature.
Volatility. Ability of a liquid to turn into vapor.
Volumetric efficiency. Ratio of the volume discharged from a pump to the piston displacement of the pump. In diesel engines a term often used instead of the correct term charge efficiency.
Water jacket. The outer casing forming a space around an engine
cylinder to permit circulation of cooling water.
Work. The transference of energy by a process involving the motion of the point of application of a force. Work is done when a force moves a body through a certain distance.
Working stroke. The piston stroke during which the combustion gases exert a pressure on the moving piston.
Wrist pin. Piston pin