AC/CDI. A type of Capacitor Discharge ignition system which uses a special alternator to provide both a high voltage to charge up a capacitor and a timed trigger pulse to discharge it through the coil. Mostly used on motorcycles and utility vehicles having no battery.

ACCELERATION ENRICHMENT. Extra fuel provided when the throttle is opened. May be in the form of extra injection pulses or enrichment of the basic fuel law, often arranged to decay gradually over two or three seconds. Usually variable according to rate at which the throttle is opened.

ADAPTIVE MEMORY. A capability provided in more advanced systems to 'learn' the range of signals encountered from the various sensors during normal use and make corrections for tolerance and adjustment errors.

AFTER-START ENRICHMENT. Extra fuel provided to prevent stall out after the starter has been disengaged, decaying away over 30 seconds or so.

AIRFLOW METER. A device which measures the amount of air passing through the induction tract into the engine. Ideally air mass flow is measured rather than volume which requires additional correction for temperature and for barometric pressure.

AIRFLOW METER - Cone Type. A delightfully simple device composed of a cone, suspended in a profiled air tract and pointing into the air stream, which moves laterally against spring pressure by an amount related to the airflow around it. The cone moves the slider of a linear potentiometer to produce the voltage output signal. Air temperature and barometric pressure require further correction.

AIRFLOW METER - Flap Type. A common type of airflow meter consisting of a spring loaded flap which hinges within a specially profiled air channel. A complex potentiometer converts the angular displacement of the flap, which varies according to the volume of air passing through the channel, into a voltage signal. An air temperature sensor is usually built into the device but barometric correction is not catered for.

AIRFLOW METER - Hot Film Type. A variation on the hot wire theme in which a metal foil film is heated to maintain a constant temperature relative to and above that of the passing air as sensed by a thermistor. The load signal is derived from the current required to maintain the film temperature, this current being a measure of mass air flow through the device.

AIRFLOW METER - Hot Wire Type. A widely used type of airflow meter in which the passage of air has a cooling effect on a heated wire which is part of a balanced resistor circuit, another leg of which applies correction for air temperature. An amplifier maintains the circuit in equilibrium and produces a signal voltage which rises in proportion to mass air flow according to the cooling effect on the wire. In some versions the output is a continuous train of pulses, the frequency of which is modulated by the hot wire voltage.

AIRFLOW METER - Vortex Type. A device which induces a vortex, or helical flow pattern, in the air flowing through a passage and by measuring the frequency of the vortex is able to make an accurate measurement of volumetric flow. Correction for temperature and barometric pressure is still necessary to arrive at the true mass flow.

AIR INJECTION. The introduction of air into the exhaust stream to promote oxidising reactions of CO and HC pollutants, usually, but not always, in combination with a catalyst. Usually directed into the exhaust port or manifold from an air pump but some early systems relied on low pressure pulses in the exhaust manifold at light throttle to draw air through a one way valve. The main application is from a start up, particularly from cold, when the extra air helps to promote reactions aiding catalyst light-off. The system then switches off. Later systems controlled by the ECU can be activated briefly during normal operation as part of a function check. If the extra air does not produce a corrective reaction from the Lambda feedback system a fault warning is flagged up.

AIR TEMPERATURE SENSOR. A two terminal device, situated in the induction tract, the resistance of which changes with temperature. The resistance normally falls with rising temperature hence these devices are sometimes correctly described as 'negative temperature coefficient (n.t.c.) thermistors'.

AMPLIFIER - Ignition. Switching module controlling current flow through coil primary winding.

AMPLIFIER - Injection. Normally refers to a separate injector current control amplifier such as used on D Jetronic V12 Jaguar. Also can describe the output stage within the ECU for controlling the injectors.

ANALOGUE SYSTEM. A system in which voltage levels are used to represent data. Most ECU sensor inputs are in analogue form. For example the changing resistance of a temperature sensor causes a corresponding voltage change on the signal wire to the ECU. Early ECUs in which the fuel laws are determined by selecting values for certain critical resistors and capacitors are known as analogue type.

ANALOGUE TO DIGITAL CONVERTER (ADC). A type of microchip which converts a voltage input signal to a binary digital code. Multichannel ADCs are commonly used to convert sensor information into a form which a microprocessor can make use of. Sometimes the microprocessor itself contains the ADC. See also Digital System, Binary Code, and Microprocessor.

AUXILIARY AIR VALVE. A valve which allows the admittance of extra air for idling, controlled according to engine temperature and possibly by other parameters

BALLAST OR POWER RESISTOR. A resistor or resistor pack used to limit current flow through injectors or, in some early examples, the ignition coil, dissipating energy as heat.

BINARY CODE. A method of representing numbers by a series of digits each of which has only two possible states - either on or off (on = 1, off = 0). Each additional digit doubles the choices available thus four digits (2x2x2x2 = 16 choices) can be used to represent any number from 0 to 15. Digits are usually referred to as "bits" and eight bits equals a "byte" which can represent any number from 0 to 255 (2x2x2x2x2x2x2x2 = 256). See also Digital System.

B.M.E.P. (BRAKE MEAN EFFECTIVE PRESSURE). The mean pressure acting on the piston crown of an engine as calculated from the torque measured on a dynamometer thus taking into account all internal losses. Equal to specific torque (lbs/ft per litre) x 2.464. A good modern production engine would produce a figure of around 175 p.s.i. F3000 racing engines, with r.p.m. restricted by rules, achieve about 250 p.s.i. but higher revving F1 engines tend to be around 200-210 p.s.i.

BREAK OUT BOX. A matrix of test points which can be connected in place of an ECU to ease checking of continuity and resistance of a circuit and its components.

BROADBAND LAMBDA SENSOR. A type of oxygen sensor that can produce a variable signal output according to a wide range of air/fuel ratios. See also LAF sensor.

CAPACITIVE DISCHARGE IGNITION (CDI). A type of ignition system in which a capacitor is charged to a high voltage and discharged through the coil primary to produce the required HT voltage. Spark duration is usually very brief.

CARBON MONOXIDE (CO) ANALYSIS. Measurement of CO concentration in exhaust to determine mixture strength when richer than stoichiometric. Expressed as percentage of total gas volume. A typical full throttle mixture would produce 3-5 % CO.

CHANGEOVER VALVE. Solenoid operated valves used in combination to control fuel feed and return selection on twin tank systems.

CLOSED LOOP OPERATION. A condition in which a signal derived from an output is looped back to correct the input. Usually refers to operation where the signal from the Lambda sensor is used to make fuelling corrections. Also known as Lambda Feedback.

CONSTANT ENERGY IGNITION. An ignition system employing an amplifier module which can control current flow through the coil to a predetermined level without need for a ballast resistor. The coil energy is therefore constant over a wide range of engine speeds.

COLD START INJECTOR. A special injector sometimes used to provide a fine fuel mist in the plenum chamber to assist fire up while the starter is cranking the engine over.

COLD START INJECTION. A simple system operating independently of the ECU and main injectors, which only functions briefly at low temperatures, spraying a fine mist of fuel into the inlet tract whilst the starter is engaged. Rarely used since the improved spray patterns of later injectors made possible the simpler method of firing the main injectors at an increased rate during cranking.

COOLANT SENSOR. A two terminal device, protruding into the engine coolant system, usually near the thermostat, the resistance of which changes with temperature. The resistance normally falls with rising temperature hence these devices are sometimes correctly described as 'negative temperature coefficient (n.t.c.) thermistors'.

CRANKING ENRICHMENT. Extra fuel provided by the ECU via the normal injectors whilst the starter is engaged to assist fire up. Not the same as that provided by the cold start injector.

DARLINGTON TRANSISTOR. Actually a Darlington transistor is a package containing a pair of transistors coupled together in a configuration which multiplies the gain (or amplification) so that a very large current can be controlled by a very small one. Darlingtons are widely used as output transistors for driving coils and injectors.

DETONATION (KNOCK). Spontaneous and violent ignition of part of the charge, usually in the end gas ahead of the flame, capable of promoting pre-ignition and causing serious engine damage. Compression ratio, fuel quality, ignition advance and intake air temperature are the main factors involved in promoting detonation.

DIGITAL SYSTEM. A system in which data is represented by either a sequence of pulses or by a series of parallel channels each of which has two recognised states, on or off. Binary coded data can be stored or manipulated very rapidly often via an eight channel (eight bit) system capable of representing any number from 0 to 255. Early engine management systems employed 8 bit microprocessors but 16 and 32 bit devices are now common to provide the power necessary for complicated self-diagnostic systems. Input data is usually in analogue form which is converted to digital for processing. See also Analogue System, Analogue to Digital Converter, and Binary Code.

DIODE. A two terminal electronic component which behaves electrically like a one way valve - it conducts current in one direction but not in the other. There is always a slight drop of about 0.6 volts across a diode when it is conducting.

DRY JOINT. A term used to describe a soldered joint which did not fully adhere at the time of manufacture and which has since developed high resistance, often of an intermittent nature. Rarely obvious from visual inspection.

ECM (Engine Control Module). A term that seems to have taken over from ECU, having much the same meaning.

ECU (Electronic Control Unit). General term for the main electronic circuit module which controls injector pulses and/or ignition timing according to various sensor inputs. See ECM.

ENGINE MANAGEMENT SYSTEM. A complex type of microprocessor based ECU which controls both fuelling and ignition.

EPROM (Erasable Programmable Read Only Memory). A type of microchip which can be programmed at manufacture with the fuel, ignition, and other functions of the ECU. A window (which must be covered over to keep out daylight) permits erasure by exposure to UV light, after which the device can be reprogrammed using special equipment. An alternative (PROM) is the same thing but cannot be erased. Electrically erasable type (EEPROM) also available from which data can be selectively erased.

EXHAUST CATALYST (CATALYTIC CONVERTER). A chamber in the exhaust system containing, in either matrix or pellet form, certain compounds, often comprised of precious metals, which promote reactions in the exhaust after it has passed out of the engine. Originally used for oxidation only in conjunction with an air pump. Becomes inactive if exposed to lead or certain other compounds like phosporous and sulphur that are now restricted to very small amounts in fuel. Some catalysts perform an effective silencing function. See also Three Way Catalyst.

EXHAUST GAS RECIRCULATION (EGR). A widely used method for reducing exhaust emission of Oxides of Nitrogen (NOx) by feeding a small proportion of spent exhaust gas back into the incoming charge. Has the effect of reducing combustion temperatures. Normally controlled by an EGR valve which limits the function to certain emission critical operating regimes. Can also be provided by extending valve overlap.

EXTRACTOR EXHAUST. An exhaust system using specific pipe dimensions to harness pulses generated as each exhaust valve opens. Positive pressure pulses travelling along the system are reflected from the end as negative pulses which return to augment the induction and scavenging processes in the cylinders. All racing systems are of this type and can contribute as much as 30% to the power output. Ram induction is a similar action taking place in the induction system.

EVAPORATIVE LOSS EMISSION SYSTEM. A system normally based on a charcoal filled canister which traps evaporative vapours from the fuel tank. A canister purge system uses engine manifold vacuum in certain operating circumstances to draw air through the charcoal so that the vapour is passed into the engine to be burnt as fuel. This is often controlled so that the slight richening effect provides a check function for the Lambda feedback system.

FUEL DAMPER. A device very much like a pressure regulator which is sometimes included in either the fuel feed or return pipe to eliminate pulsations or noise.

FUEL LAW. The fueling provided by the ECU in response to the various sensor signal inputs.

FUEL MAP. A three dimensional representation of the relationship between fueling and sensor inputs, normally considered in terms of engine speed and load, although other variables can also be used.

FUEL RAIL. The metal tube which disperses fuel to the individual injectors.

FUEL PRESSURE REGULATOR. A relief valve which controls fuel pressure to a constant value. Early versions held pressure constant on an absolute basis according to spring pressure, later systems maintain pressure constant in relation to manifold pressure sensed via an interconnecting pipe. Recent systems often use a "mini-regulator" which is much smaller than earlier types.

FULL LOAD ENRICHMENT. Extra fuel provided at full load usually activated by the throttle switch (or potentiometer) as the throttle approaches fully open or from a manifold pressure switch. Full load enrichment is generally of the order 10 to 15% added to the basic fueling.

FULL LOAD PRESSURE SWITCH. A switch sometimes used in addition or in preference to the throttle switch to detect the approach of full load to activate enrichment. Usually set to function at about 3 ins of mercury depression.

HYDROCARBON (HC) ANALYSIS. Measurement of the concentration of unburnt hydrocarbon compounds present in the raw exhaust as an indication of combustion efficiency. Expressed as parts per million (p.p.m.).

HYSTERESIS. A characteristic often built in to switching systems so that the switch on and switch off points are separated slightly to prevent oscillation between states. A speed sensitive function such as over-run cut off is an example in which the cut-in speed might be 1200 r.p.m. and the cut-out speed 1500 r.p.m.

IDLE SPEED CONTROL SYSTEM. An electronically controlled system which compensates for changes of temperature, load etc. to maintain a predetermined idle speed via an electrically operated valve - often using a stepper motor. May also alter ignition timing to obtain faster response to changing loads.

INDUCTANCE. A property of electricity whereby the magnetic field created by an electric current flowing through a coil of wire opposes any change in that current. The effect is of making an electric current behave as if it had inertia.

IGNITION MAP. A three dimensional representation of the relationship between ignition advance and sensor inputs, normally considered in terms of engine speed and load.

INERTIA SWITCH. A shock sensing switch which is commonly used to cut off power to the injection system in the event of the vehicle suffering an impact. Must be reset before the vehicle can be used again.

INJECTOR (ELECTRIC TYPE). Precision solenoid operated valve controlling flow of fuel admitted to engine inlet ports according to pulses from the ECU.

INTEGRATED CIRCUIT (I.C.). General term for a multi-pin microchip, composed of transistor elements, which might be equally applied to a simple voltage regulator or a complex microprocessor.

KNOCK SENSOR. A type of microphone or accelerometer which, by detecting the shock waves created when an engine suffers detonation, activates a control circuit which causes ignition to retard slightly until the condition ceases.

LEAN BURN COMBUSTION. A combustion system which enables an engine to burn very weak mixtures in pursuit of good fuel efficiency and low exhaust emission levels. Usually involves good mixture preparation and carefully controlled swirl and turbulence. See also Stratified Charge Combustion.

LAF SENSOR. A variant of the Lambda sensor developed by Honda which makes it possible to measure air/fuel ratio over a wide range, thereby making possible a Lambda feedback lean-burn system. See also Broadband Lambda Sensor.

LAMBDA. Greek symbol used to denote air/fuel ratio on basis that for no air L = 0, for stoichiometric L = 1, etc. As an example L = 1.1 would be a fairly typical weak cruise mixture.

LAMBDA SENSOR (EXHAUST OXYGEN SENSOR). A device located in the exhaust stream which produces a sharp change of voltage according to the presence or absence of oxygen, enables the ECU to determine that the mixture is weaker or richer than stoichiometric. Generally used in conjunction with a Three Way Catalyst.

LAMBDA FEEDBACK. A mode of operation in which the signal from a Lambda sensor is used to control fuelling to a predetermined air/fuel ratio. Mainly used with three-way catalyst systems requiring precise control of stoichiometric fuelling.

LIMP HOME MODE. The capability of some advanced systems to revert to a secondary simplified control of fuelling or ignition in the event of a fault developing in either the ECU or its sensors.

LOAD LAW. The way in which the fuelling or ignition advance changes according to engine load, which can be derived from throttle angle, manifold pressure or mass airflow.

MEMORY. A system for storing data or program instructions in binary form either permanently (ROM), semi-permanently (EPROM), or temporarily (RAM). Other methods also available.

MODEM (Modulator/Demodulator). An interface unit which makes it possible for a computer to communicate with other devices by serial link over long distances via a telephone line.

MICROPROCESSOR. A very complex type of microchip which can manipulate data in binary form and carry out functions at high speed according to a programmed sequence of instructions stored in a memory device such as an EPROM.

MMT (Methylcyclopentadienyl Manganese Tricarbonyl). An octane boosting additive which also gives some protection to valve seats so was adopted as a lead substitute. Now known to be a neurotoxin with suspected links to the BSE / CJD crisis and hence has been banned in many countries.

MTBE (Methyl Tertiary Butyl Ether). An oxygenated gasoline additive with a low volatility used in reformulated gasoline and also in racing fuels because of its high octane rating of 117 RON. Became a mandatory fuel component in some US States until it was found to significantly pollute ground water because of its exceptional ability to exploit any leak from storage tanks, so it has now been outlawed in California and other places. See also Oxygenated Compounds and Reformulated Gasoline.

MULTIPOINT INJECTION. A system employing a separate injector (or multiple injectors) for each cylinder.

OBD (On Board Diagnostics). Originally introduced by California Air Resources Board (CARB) in 1988, OBD1 required vehicle manufacturers to provide a system which could identify emission related malfunctions in electronic engine management systems and display a warning to the driver and a fault code system to provide a technician with guidance. OBD2 was a further extension requiring manufacturers to comply with a universal protocol for self diagnostic systems using a common list of codes to identify system faults. Also to include the capability to detect and monitor embryonic faults and predict when they will cause vehicle emissions to become excessive. It also insisted that means be made available for the independent service trade to access information for diagnostic purposes.

OPEN LOOP OPERATION. Usually used to describe a situation where a system with Lambda feedback switches to operation in which the Lambda signal is ignored so that fuelling values more appropriate to the conditions can be used without feedback correction. Full load enrichment is often provided by going 'open loop'.

OPUS IGNITION. An early electronically triggered ignition named after its operating method 'oscillating pickup system'. Does not control coil current, therefore requires ballast resistor, but is capable of high spark rate. Used on racing engines (e.g. Cosworth DFV) and multi-cylinder production engines (e.g. early Jaguar V12).

OVER-RUN CUT OFF. Condition in which no fuel injection takes place if the throttle is fully closed above a predetermined engine speed - typically about 1300 r.p.m. The cutting in and cutting out speeds are separated by 2 - 300 r.p.m. to prevent oscillation between states.

OVER-RUN VALVE. A spring loaded valve which opens to admit air into the inlet manifold plenum when a predetermined vacuum (typically 20 ins mercury) is reached under trailing throttle conditions to reduce exhaust emissions and prevent 'pop back'. Essential for engines which do not have over-run cut off. Some later engine management systems use the idle air valve to perform the same function.

OXYGENATED COMPOUNDS (Gasoline Blend Components). Comprising the alcohols Methanol, Ethanol, Iso-Propyl Alcohol (IPA), Tertiary Butyl Alcohol (TBA), and the ethers DIPE (Di-Iso-Propyl Ether), ETBE (Ethyl Tertiary Butyl Ether), MTBE (Methyl Tertiary Butyl Ether), TAME (Tertiary Amyl Methyl Ether). These are compounds of high octane rating and containing oxygen used as gasoline blend components and as lead substitutes. Used in racing fuels on the one hand and low emission gasolines on the other. In the latter application the total oxygen content of the fuel is limited to a small proportion by EPA ruling.

PICKUP - Hall Effect Type. A device widely used to produce trigger or reference signals from rotating shafts, often used inside distributors. A magnetically sensitive transistor switches as it detects the passage of a magnetic pole. Usually has three wires arranged in order: 5v, signal output, 0v

PICKUP - Magnetic or Inductive Type. A very common sensor used to detect the passage of metal teeth or pegs from the changes caused in a magnetic field. Produces an alternating voltage swinging positive to negative (or vice versa) as each tooth passes, the magnitude increasing with speed. Usually has two connecting wires which are often screened against interference. Widely used to generate speed or register signals from distributors, crankshafts, camshafts and ABS wheel speed sensors.

PINKING (also Pinging). Descriptive name for relatively harmless combustion noise caused by over-advanced ignition for the fuel in use, usually occurring at low speed and high load. Not to be confused with detonation or pre-ignition.

PLENUM CHAMBER. A relatively large volume air chamber from which the inlet tracts run to the engine ports. A single throttle is usually used to control airflow into the plenum. The various vacuum signal tappings, idle control valves, over-run valves, etc. are generally connected to the plenum.

POTENTIOMETER (Throttle potentiometer). A special adaption of a resistor in which a sliding contact can be moved along the length of the resistor to tap off the voltage at any point. Widely used to sense throttle angle. If there is 5 volts across the potentiometer and the slider is one quarter travel from the lower end it would see 1.25 volts, however potentiometers can also be obtained with non linear scales, but throttle potentiometers are rarely, if ever, of that type.

PRE-IGNITION. Potentially damaging condition when ignition takes place ahead of the timed spark, often initiated by hot or incandescent deposits in the combustion chamber. The high temperatures induced by detonation can promote pre-ignition.

PRESSURE SENSOR (MAP Sensor). A sensor (transducer) producing an output signal according to inlet manifold pressure for use as a measurement of engine load by the ECU. May be contained within the ECU or may be a separate unit but in either case will have a pipe connection to the engine manifold. Term may also refer to a device providing the ECU with barometric pressure correction.

PULSE DURATION (PULSE WIDTH). The electrical pulse output produced by the ECU to open the injectors to admit a quantity of fuel according to the width or duration of the pulse. Pulse widths typically range from 2 or 3 milliseconds (thousandths of a second) at idle up to 8 or 9 milliseconds at full load.

PURGE VALVE. A valve which controls the purging action of evaporative fuel vapour from the evaporative loss charcoal canister. See also Evaporative Loss Emission System.

RAM INDUCTION. A technique for harnessing resonant pulses created by the induction process to improve volumetric efficiency, dependent on the length, cross section, and sometimes the interconnection, of passageways. A good ram system can boost torque by as much as 20%. Not to be confused with a forward facing air intake.

RAW EMISSIONS. Emissions direct from the engine before treatment by a catalyst or similar.

REFORMULATED GASOLINE. A new 'low emissions' type of unleaded gasoline fuel introduced by Atlantic Richfield Oil in the US in 1989 with the ability to reduce emissions from older vehicles without any emission control systems. It had low volatility to reduce evaporative emissions, low benzene and aromatic content to reduce toxic emissions, and contained oxygenates to reduce HC and CO emissions. Now widely used and mandatory in some parts of the world. See Oxygenated Compounds and MTBE.

RS232 (Recommended Standard number 232). A widely used standard for sending serial data between computers and other devices. The full RS232 specifies 22 interconnecting wires, but for many purposes 3 or 4 wires will suffice. See also Serial Link.

SELF DIAGNOSIS. The ability of advanced systems to record certain failure modes which can then be displayed, usually in a simple code or as a fault number, to assist diagnosis.

SEQUENTIAL INJECTION. System in which each injector is fired independently at the optimum point for its particular cylinder. Requires separate wiring to each injector. Has advantages for emissions and drivability in certain circumstances.

SERIAL LINK. A means of communication between computers and accessories in which data is transferred as a series of timed pulses along one wire in a connection conforming to a recognised standard such as RS232. A common method is for each byte (8 bits) of data to be preceded and followed by register pulses to maintain synchronisation. Serial data transfer, although slower than the parallel system within the computer, is much more reliable and convenient over any distance. A serial link is widely used for connecting interrogation instruments to vehicles for diagnostic or tune up purposes. Such a link can be made with just three wires, one for data in, one for data out, and ground.

SINGLE POINT OR THROTTLE BODY INJECTION. System where injection takes place at the throttle body only and mixture is distributed to the individual cylinders by the inlet manifold, resembling a carburetter system.

SPEED LAW. The way in which fueling or ignition advance changes according to engine speed, often influenced by harmonic effects in the induction and exhaust processes or by the need to control exhaust emissions.

SQUISH. Turbulence induced into the air/fuel charge within the combustion chamber as the piston comes close to some part of the combustion chamber surface as it approaches TDC. Any gas contained in this zone (squish zone) is squeezed out into the main combustion chamber and promotes vigorous charge movement therein. Mainly useful for the later stages of combustion.

STEPPER MOTOR. A type of electric motor having multiple windings energised by pulses causing the armature to rotate in small increments. Often used to control idle air supply.

STOICHIOMETRIC. Term used to describe a chemically correct mixture containing excess of neither air nor fuel (Lambda = 1). Stoichiometric fueling provides the ideal conditions for operation of a three-way catalyst, the fueling being controlled by Lambda feedback. Otherwise the condition has little merit because best economy from gasoline is obtained with fueling that is weaker by about 15% and best power is obtained when about 15% richer. Some examples of stoichiometric air/fuel ratios for different fuels are:- Gasoline 14.7:1, MTBE 11.7:1, Methanol 6.45:1, Nitromethane 1.7:1.

STRATIFIED CHARGE COMBUSTION. A type of Lean Burn combustion system in which the air/fuel ratio is deliberately divided into a relatively rich zone in which combustion can easily become established, with the extremities containing much weaker mixture. The overall air/fuel ratio can therefore be considerably weaker than would normally be possible. Stratified combustion can be achieved by careful control of swirl and fuel admission or by using a divided combustion chamber with the richer mixture confined to a pre-chamber containing the spark plug.

SURFACE MOUNT TECHNOLOGY. A method of achieving miniaturisation in which electronic components are glued to a special printed circuit board which is then flow soldered to make the connections. The placement of the components may be performed by robot machines. Many recent ECUs have a mixture of surface mount and conventional through board components.

SWIRL. An orderly rotating motion of the air/fuel charge in the cylinder induced by the incoming flow through the inlet valve and port which may be specially shaped to promote the condition. Useful in the early stages of combustion but usually allowed to then degenerate into random turbulence to spread flame through the charge.

THERMO-TIME SWITCH. A two terminal bimetallic switch and heating coil device threaded into the coolant system, and using the engine block as a ground terminal, which controls the cold start injection system.

THREE-WAY CATALYST. A type of catalytic converter used with stoichiometric mixture precisely controlled by a Lambda sensor. Nitrogen oxide compounds are broken down into the separate constituents of nitrogen and oxygen, the latter then being used to oxidise carbon monoxide and unburnt hydrocarbons.

THROTTLE SWITCH. Device attached to throttle spindle or linkage and used by the ECU to detect certain conditions - usually closed throttle and approach of full throttle. D Jetronic also has sequential contacts to produce extra fuel pulses as throttle opens.

THROTTLE POTENTIOMETER. Device attached to throttle spindle or linkage and used by the ECU to detect closed throttle, approach of full throttle, rate of throttle movement, and throttle angle. Racing systems often use the throttle potentiometer to provide the load signal. Produces a voltage output proportional to throttle angle. May also provide a similar signal for use by a transmission controller.

THYRISTOR. A type of high voltage current switching device which, once triggered into conduction by a brief pulse, will remain 'on' until current ceases to flow through it. Widely used as current choppers in a.c. mains power control circuits, (lamp dimmers, speed controllers etc.) but also used in Capacitor Discharge ignition systems.

TOLUENE. A high octane aromatic compound used as a gasoline blend component, particularly for racing gasolines.

TRANSDUCER. General term for a device which converts some physical dimension such as temperature, pressure, or movement, into an electrical signal.

TRANSISTOR. There are actually a great variety of transistor types, Bipolar and Mosfet, being the two main families within each of which there are two choices of polarity. Bipolar transistors are current operated devices (a small current governs a larger current) which have been around for years. Mosfets came along later and are voltage operated (a voltage input controls current conduction) and are becoming favoured as output devices because they can be driven directly from microcircuits and are very durable.

TRIGGER SWITCH. A switch, usually paired, which, by briefly completing a circuit to ground, triggers the injection functions of the Bosch D Jetronic system. May take the form of simple contact breakers or sealed magnetically operated switches. A pair are moulded into the trigger board inside the distributor to trigger the D Jetronic system applied to the Jaguar V12

VARIABLE VALVE EVENT SYSTEM. A general term applied to cover any method of changing some feature of the valve operating cycle whilst the engine is running. Cam advance or retard, changes of lift and duration, and alteration to the number of valves in use, all fall in this category. Can be used to control exhaust emissions, widen torque band, or improve idle quality.

VOLATILE MEMORY. Memory which requires continuous power to retain data, often also known as RAM (Random Access Memory). Some RAM chips (known as Dynamic Ram) require repetitive scanning to refresh data which would otherwise be lost. Adaptive data and logged fault data is normally stored in RAM and is therefore lost if the battery supply is disconnected.

WARM UP ENRICHMENT. Extra fuel provided by the ECU according to engine temperature as sensed by the coolant temperature sensor. The overall fueling is proportionately enriched and acceleration, cranking, and other additional enrichment factors are usually enhanced as necessary.

WASTED SPARK IGNITION. A distributorless arrangement where spark plugs fire twice per cycle, the second spark being 'wasted' because it occurs while exhaust is being scavenged. Has the advantage of simplified triggering arrangements usually used with double ended coils firing two cylinders simultaneously.

ZENER DIODE. A type of diode which, at a certain voltage (called the Zener voltage), conducts in the reverse direction. Zener diodes rated from 2.4 volts to hundreds of volts are available and can be used to set or limit the maximum voltage in an electrical system. Accuracy varies with current and temperature.