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Glossary of terms you will come across during your Phase 2 training.


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Air is a gas that is the main component of the earth's atmosphere. It is a mixture of many different gasses such as nitrogen (78%), oxygen (20%) and carbon dioxide (1%). There are much smaller traces of other gasses such as freon, neon, argon, helium, carbon monoxide and helium.
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This is the number of parts of air mixed with 1 part of fuel in an engine. There is an ideal, or optimum or stoichiometric ratio, which is 14.7 parts of air to one part of petrol, or 14.5 parts of air to one part of diesel.

An air fuel mixture with more air is said to be a 'lean' mixture, and a mixture with less air is said to be a 'rich' mixture.

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Do not confuse 'air gap' with 'spark gap', they are two entirely different things.

Air gap between a flywheel and an ignition coil.Air gap normally refers to the clearance between a revolving object and a stationary coil or pick-up sensor.

In the diagram on the right, we can see a magneto ignition coil and a flywheel. The correct air gap between them is 0.4 mm, with a tolerance of + or - 0.2 mm. This means that an air gap of between 0.2 mm and 0.6 mm is acceptable.

In magneto ignition systems, the air gap is the clearance between the laminated iron core of the ignition coil and the revolving flywheel which has a permanent magnet mounted on it.

In this case, the ignition coil is able to produce a HT spark because the magnetic lines of force from the permanent magnet on the flywheel move rapidly across the coil windings. This induces a current in the primary windings. When we suddenly open the circuit in the primary windings, the magnetic field associated with the current in the primary windings collapses, inducing the HT spark in the secondary windings.

The air gap between the coil and the magnet on the flywheel is vital if we are to produce a good quality HT spark.

If the air gap is too small, the flywheel can physically contact and damage the laminated iron core of the ignition coil. This is a common fault in small plant petrol engines. Always check this gap during service.

If the air gap is too big, the magnetic lines of force (magnetic flux) tend to splay out into space, and do not act on the windings of the ignition coil, thus causing the coil to produce a weak spark.

Other components where air gaps are essential are items such as inductive ABS wheel sensors. If the air gap is too large, the sensor is incapable of producing a signal of sufficient amplitude, and this will cause the control system to log a fault code and to display a warning lamp.

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This is when a diesel fuel system draws air in through a loose or badly sealed connection. The air that is drawn in can prevent the smooth running of the engine, or prevent the engine from starting at all.

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Allen keysHexagonal keys or wrenches used to loosen and tighten allen head bolts, like the ones shown below:

allen head bolts 

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An alloy is a mixture of metals. For example, the addition of nickel to steel increases strength and toughness. It also changes the rate of thermal expansion and the metal’s resistance to corrosion.
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The alt or alternative key on a computer.

Short for "Alternate" key. The alt key on the keyboard allows other keys have a third function. For example, the "a" key will produce a lower case "a" when used on its own. If it is pressed in conjuction with the shift key, it produces a capital "A". The alt key when used in conjuction with the "a" key would allow it to produce some new programmable function.

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Alternating current is the type of electricity we get from the mains in houses and workshops. On the Irish national grid, electrical flow alternates 50 times per second (50 Hz).

Mains voltage in Ireland is approximately 220V ac, and this is often written as 220V ~, with the tilde symbol (~) being used to represent ac. 

Also, position sensors on electronically managed engines (such as the crankshaft speed/position sensor) produce ac, but at much lower voltages. For example, on the Perkins 1100 series engines, under cranking, the crankshaft sensor produces about 0.4V ac.

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Electrical device used to generate electicity in plant vehicles. The device gets energy via a drive belt from the engine, and converts this mechanical energy into electrical energy to be used by the vehicle electrical systems. Once the engine is running, the alternator supplies all electrical loads in a plant vehicle.

The three things necessary to generate electicity.

Alternator, with part of the stator cut away and folded out to facilitate viewing the rotor.

As the name implies, the unit produces alternating current, which cannot be used by plant vehicles. This output must be rectified to direct current by using diodes.

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Non-ferrous metal, relatively soft, but much lighter than steel.

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