Automotive Technical Terms: I-Beam Axle to Iso-Octane
Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.
An axle, especially the front axle of much older vehicles, not having independent suspension, consisting of a forged beam having a cross-section of "I" shape.
A small orifice above the fuel level allowing air entry into the duct of the idling system to form an air fuel mixture of appropriate strength to allow the engine to idle. Sometimes this is known as the "idling air jet"
Gear wheels which are currently rotating but not transferring torque. The reverse idler is the gear wheel in mesh between the driving and the driven wheels to reverse the direction of the latter and which, irrespective of size, has no effect on the gear ratio.
The speed of the engine automatically attained when the engine, at normal temperature, is running out of gear, with the throttle control at its minimum setting, ranging from 600 to 800rpm on various engine types (see Tick-over).
On fixed-choke carburettors, as the throttle butterfly is progressively closed and engine speed falls, air flow through the venturi (choke tube) reduces to a point where normal fuel delivery rapidly decreases, giving a mixture too weak to support combustion and the engine stops. To allow satisfactory running at low speeds and particularly when idling, a separate system is built into the carburettor which supplies fuel through an idling jet to one or more outlet holes in the barrel adjacent to the edge of the throttle butterfly in the closed position. The high air velocity past the butterfly in this position causes sufficient depression to draw off fuel/air mixture from the idling system. One orifice on the engine side of the closed butterfly will be under depression in the idling position, while one or two holes just on the other side will then allow air to bleed into the idling supply duct. These latter are known as "progression orifices" or "transfer ports" because as the butterfly is open they, too, come into the area of depression and become fuel outlets so tending to enrich the mixture as the engine speed is accelerated. On later constant vacuum carburettors a separate idling system, previously considered unnecessary with a variable choke, may now be found.
The initiation of combustion of the fuel or air/fuel mixture in the engine. Usually, however, refers to the electrical system which produces a spark in the combustion chamber to ignite the air fuel mixture. Thus ignition switch, ignition coil and ignition timing.
In the days before electronic ignition, a lot had been written about ignition system faults, but one common source of trouble was frequently overlooked. This was the rather insignificant-looking condenser. Since it was about the cheapest item and the simplest to replace, it is well worth having a spare condenser if you are driving an older car. It's a safeguard against one kind of roadside trouble, and it's worth trying in any case of baffling engine trouble which could be due to ignition.
The prevention of interference on radio or television sets by the car's ignition system by fitment of resistors in the centre electrode of the sparking plug, or externally in the high-tension (HT) leads which change the ignition secondary oscillating frequency at the instant of arcing at the plug points to a frequency outside those of television or radio sets.
The key-operated switch at the driver's controls which connects the battery supply to the low-tension (LT) side of the ignition system and to those other electrical items only operable when the ignition is "ON", such as instrument gauges and those other additional switches such as direction indicators and windscreen wipers.
The position in crankshaft angle relative to TDC on the datum piston at which the ignition spark occurs. The correct position is usually marked on the fly-wheel or crankshaft pulley of the engine. Adjustment may be effected to advance or retard the ignition timing. All modern vehicles have automatic advance mechanisms which re-set the timing automatically while the engine is running to suit speed and load. See Centrifugal advance and Vacuum advance mechanisms.
A modern ignition system designed to overcome the limitations of the traditional system, which depended on contact breaker control of the primary circuit, such as falling high-tension (HT) voltage at high speeds, irregular timing due to cam and contact breaker wear; erosion of points and the need for frequent servicing and timing checks. The transistor system may still utilize the contact breaker to provide controlled timing of the transistor base current, or this may be effected by a magnetic pick-up, a light pulse, or other means, superseding the contact breaker.
A dual-purpose warning light - usually red - on the instrument panel which illuminates immediately as the ignition is switched on but extinguishes as revs increase from idling to the point where the dynamo or alternator is providing sufficient voltage to close the points of the automatic cut-out so charging the battery. The light thus reminds the driver to switch off the ignition before leaving the vehicle and warns him when the system is faulty and producing no charge, by failing to extinguish at normal speed.
A rotating, finned disc used to propel or increase the speed of a fluid, such as the rotating part of the coolant pump. Impellers are also used in the majority of fluid couplings. For example, they are found in torque converters and power steering pumps.
A type of vehicle suspension where the movement of one wheel does not necessarily result in the movement of the other wheel, because the wheels do not share a common axle. It is often abbreviated to IFS for “Independent Front Suspension”, and IRS for “Independent Rear Suspension”. It is known as a system in which two opposite wheels, which formerly would have been mounted on one transverse axle, are mounted on a system of arms and a spring, so that the wheel on one side may be deflected up or down relative to the vehicle without directly causing movement of the other. This permits the vehicle to ride on a more even keel when the wheels are deflected by road irregularities and cuts down unsprung weight. Independent front suspension has been universally adopted for many years on private cars but independent rear suspension has failed to completely supersede the "live" axle.
Power calculated from indicated mean effective pressure (IMEP) and thus the rate of useful work of the pistons of all cylinders. Such power is reduced by that required to overcome friction in the engine and to operate essential parts, such as valve actuation and lubrication pump, and the residual power is Brake Horse Power (BHP).
A graphical representation of the changing pressures in the cylinder of an engine on test, plotted on a base line, giving the piston position at any point between TDC and BDC. This can be produced on paper by an indicator mechanism in the case of very slow running engines or traced by an oscilloscope and photographed. The diagram permits assessment of the useful work done on the piston in one cycle and from this the mean effective pressure during the power stroke.
IMEP is the average net pressure on the piston which is effective in doing useful work. The product of IMEP and piston area is the work done per cycle; that is, the work done by the gas on the piston less the work required by the other three strokes of the four-stroke cycle.
Current flowing in a conductor causes a magnetic field around it and this can cause a current to be set up in an adjacent conductor, particularly if the conductors are in the form of concentric coils. This effect is known as "mutual inductance" and is the principle on which ignition coils operate.
The process of ingesting air or fuel/air mixture into an engine cylinder during the stroke in the cycle of operation that is known as the "induction stroke".
The pressure in the induction manifold while the engine is running. Always below atmospheric pressure, it can be recorded on a gauge in mm, lb/ins2 or mercury (Hg).
The resistance of a body to acceleration. A body naturally tends to continue in its present state of rest or constant-velocity (Newton's 1st Law) and a force is necessary to effect a change, such as accelerating the body. This inertia force depends on the mass of the body and the degree of acceleration. Reciprocating items require an inertia force to decelerate in one direction and then accelerate in the reverse direction - in the case of an engine's pistons, this is provided by the con-rods. Reduction in reciprocating mass reduces the inertia force and permits better acceleration, hence the inherent superiority of the OHC design which dispenses with the reciprocating mass of the OHV pushrods and rockers. Similarly, rotating masses, such as fly-wheels, have inertia which requires torque to produce accelerations or decelerations and thus a spinning fly-wheel stores energy from the power stroke to carry out the work in the succeeding three non-productive strokes in the four-stroke engine.
A long-established method of automatic but temporary engagement of the starter motor to an engine's fly-wheel, consisting of a pinion carried on helical splines on the starter shaft such that, when the shaft starts to spin, the inertia of the pinion causes it to lag behind and thus move along the shaft into engagement with the ring-gear on the fly-wheel. When the engine starts the pinion is forced to rotate faster than the starter shaft, causing it to be thrown back to its former position and out of mesh.
A seat belt location method that allows the belt to pull in and out by a spring-loaded reel set-up as the occupant wishes, but on sudden forward movement (ie: in a collision), the reel locks, thus securing the occupant.
The engine-driven pump on a fuel-injected engine which supplies to each cylinder, at the requisite time, an exact quantity of fuel as required by the prevailing operating conditions determined by load, throttle position and engine speed. It may have a separate pumping element for each engine cylinder or one pumping element may feed all cylinders in turn (see Distributor type pump).
The duct in the cylinder head casting which connects the induction manifold to a combustion chamber. The similar passage on a side-valve engine is in the cylinder block casting.
The spring-loaded poppet type valve which is opened by a camshaft to allow ingestion of air or air fuel mixture on the induction stroke, closing at the end of the stroke to contain the charge in the cylinder during the succeeding strokes. Its head is given a ground seating to the mouth of the inlet port and, for volumetric efficiency, it is frequently made larger than its mating exhaust valve.
An air-tight rubber ring that may be fitted inside the tyre of a road wheel to contain air at sufficient pressure to inflate the tyre and support the weight of the vehicle.
Annular seatings, shrunk or peened into recesses machined in the cylinder head at the valve ports, of special steel (basically nickel chrome) to give better life for the ground valve seats than would be obtained from the material of the casting. Essential in the case of light alloy heads but frequently used in cast-iron heads also.
The dial type gauges or other devices which indicate the current values of variables such as oil pressure and coolant temperature. Such gauges may be mounted conveniently on an instrument panel which is readily observable by the driver.
Part of, or contained in a larger major item (a) The type of construction of private cars and other light vehicles which superseded the previous concept of a separate foundational chassis and used sheet metal pressings instead of thicker chassis members, these being integrated with the floor panels and other body pressings, or (b) The casting of crankcase, cylinder bores, or water jackets in one-piece, together with supporting webs and stiffeners for the crankshaft.
This exists when two parts fitting one inside the other have the external dimension of the inner item slightly larger than the internal dimension of its host, requiring fitting together by a press. Dry cylinder liners are fitted this way.
The means of heating the interior of a car, depending on the diversion of engine coolant to a small radiator through which air is driven into the passenger compartment by an electric fan. The flow of hot coolant is controlled by a valve and the air is ducted from outside the vehicle, so that fresh air is constantly supplied and, when the coolant flow is shut off, the air flow gives some cooling effect.
Devices fitted to the selector rods of a gearbox which prevent any rod moving out of its neutral position unless the other selector rods are in a neutral position, thereby preventing any possibility of two gears being simultaneously engaged.
The traditional design of drum brake with friction-lined shoes inside the drum being forced against the inner surface of the drum. On earlier cars a friction band was applied on the outside of the drum giving external contracting brakes.
The resistance to electrical current through a car's battery which causes the on-load voltage to be less than the open circuit voltage. Sulphating of plates increases this resistance with age.
A type of lighting unit, also known as "tungsten halogen" or "quartz halogen" bulbs, in which a tungsten filament contained in a very strong quartz envelope is heated in the presence of one of the halogens, causing a continuous chemical cycle which prolongs the filament life and gives intense light.
One of the petrol family of hydro-carbon fuels, having a high resistance to detonation and therefore used for comparative testing of anti-knock rating or other fuels (see also Heptane).
