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


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Pressure is defined as Force per unit Area.

In the SI, Force is measured in Newtons, and area is the product of multiplying a distance (length) by a distance (breadth), metres x metres to give us a basic area of 1 square metre (1m2).

So, the basic unit of any pressure is 1N/m2, also known as 1 Pascal, or 1Pa.

This is a very small unit of measurement for practical use, so when we discuss much larger pressures such as atmospheric pressure, we talk in terms of thousands of Newtons of force, or kN.

Standard Atmospheric pressure is 101.3 kN/m2. We know that atmospheric pressure is constantly changing, and any figure above this is deemed to be high pressure, and any figure below this is deemed to be low pressure.

Within the earth's atmosphere, we have air. Although we are not normally aware of this, air has weight. This weight of air exerts a downward force on the surface of the earth.

As we gain altitude, the column of air above us gets shorter, and so exerts less force on us.

Force, acting on an area, is, by definition, pressure. In the SI, force is measured in Newtons, and area is measured in m2. For convenience, we measure pressure in milliBars. Check out the animation and see how the atmospheric pressure drops from the normal at sea level (1013mb) to lower and lower figures as we gain altitude.

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This is fuel that has been broken up into extremely fine particles (in a diesel engine, this is done by the injectors). The reason why fuel is atomised is to have a relatively large surface area for each atomised particle of fuelFlash video(Flash animation)

The only part of any fuel that burns is its vapour. The amount of vapour given off by a fuel is directly proportional to its surface area. If we had one large particle of fuel, this gives us a relatively small surface area. If we take the one big particle, and break it up into thousands of tiny individual particles (atomise it), then there is a huge increase in surface area.

This in turn helps the fuel vapourise, which, in turn helps the fuel to burn.

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A typical attenuatorAn attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform.

An attenuator is effectively the opposite of an amplifier, though the two work by different methods. While an amplifier provides gain, an attenuator provides loss, or gain less than 1.

The PicoScope PP198 shown is a passive 20:1 attenuator. This means that a 20V signal at its input will appear as a 1V signal on the output. As the signal is attenuated, the PP198 allows voltages of up to 300 V to be measured. This particular attenuator has been designed to allow fuel injector and primary ignition waveforms to be measured using the PicoScope automotive kit.

Attenuators are usually passive devices made from simple voltage divider networks. Switching between different resistances forms adjustable stepped attenuators and continuously adjustable ones using potentiometers. Fixed attenuators in circuits are used to lower voltage, dissipate power, and to improve impedance matching. In measuring signals, attenuator pads or adaptors are used to lower the amplitude of the signal a known amount to enable measurements, or to protect the measuring device, say, for example, an oscilloscope, from signal levels that might damage it.

More about attenuators from Wikipedia

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AutodataAutodata's core business is the research, compilation and creation of technical information for use in independent automotive workshops for the repair and service of cars, motorcycles and commercial vehicles. Although most of these workshops are non-franchised, ranging from small garages to large groups, fast-fits and a variety of specialists, a growing number of franchised workshops also need information on other manufacturers’ vehicles.

Autodata covers a number of subjects including:

Technical data, vehicle identification, service adjustments, lubricants and capacities, ignition system, fuel system, tightening torques, brake discs and brake drums, repair times, wheel alignment, timing belts, timing chains and gears, tyre sizes and pressures, service illustrations, service schedules, service interval indicators, key programming, diagnostic trouble codes, engine management systems, pin data, trouble shooter, airbags/SRS, air conditioning, ABS, electrical component locations and wiring diagrams.

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This online learning environment has a glossary, which is like a dictionary of terms that you will come across and use both during your Phase 2 training and in your work as a CP Fitter.

Each time a word that is in the glossary appears anywhere on the site, it is automatically linked back here to the glossary. To get a full explanation of the word or term, simply click on it and a small window pops up with the explanation.

You will recognise words linked in this way because they will be highlighted in grey, and when you hover over them with your cursor, the cursor changes from an arrow to a click icon, indicating that you can click on the word for more information.

For example, Phase 2, work, CP and hover are all words, phrases and abbreviations that have explanations in our course glossary. Click on any of them for more information.

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Also referred to as the 'easy-spin' system.

In order to aid cold starting, engines are often fitted with some form of decompression device.

By briefly decompressing the cylinder during its compression stroke, it allows the operator to turn the engine over faster during starting - hence the term 'easy-spin'.

Easy-spin mechanism on a typical camshaft for a single cylinder engine.

If we turn an engine over too slowly during starting, any heat we generate has time to escape into the cooling system and so is unavailable to help start the engine. The solution is to ensure the engine cranks quickly, so that any heat generated during compression does not have time to escape, and remains in the cylinder where it improves cold starting.

In addition, the speed at which we can turn an engine over when cranking has a direct bearing on the quality of the spark produced - the faster the turning, the better the spark.

The decompression button in a two-stroke engine performs a similar function.

Here is an explanation of the operation of the automatic decompression arrangement found in small plant four-stroke engines:

Contributors to this page: Shane Fagan and Declan Lynham (BnM), Phase 2, January to May, 2012.

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This stands for Automatic Voltage Regulator. This is an electronic control board fitted to generators. This circuit board ensures that if the engine speed changes, the electrical output of the generator remains constant.

Read more on wikipedia  

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Axial Loads and Radial Loads

Axial Loads acting on a shaft.Thrust, or axial loads are loads that act lengthwise along the axis of a gearbox shaft.

They are set up by helical gears that have a tendency to throw each other out of mesh.

These loads must be borne by bearings (often taper roller) or thrust washers.

Because taper roller bearings can only support axial loads from one direction, they are usually fitted in pairs.


Radial loads acting on a shaft.Radial loads act from all angles along the radius of a shaft. Again, these loads must be borne by bearings. The types of bearing used are usually ball bearings, roller bearings or needle roller bearings.

If these loads are combined with axial loads, as is the case with this shaft, then taper roller bearings, in pairs, are used.

Go here for more information on the types of bearings used to support loads.

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EMF is electro motive force.

Every motor, as well as being a motor, is also capable of generating electricity. If you examine a motor, you will see it is a series of low resistance coils of copper wire, wrapped around a soft iron core. If we connect this across a power source, this is essentially a dead short circuit. The reason why the motor does not burn out when connected is that, as soon as the motor starts to turn, it generates its own EMF, back toward the supply source that is making it turn.

This back EMF restricts the forward flow of electricity, thus preventing the motor from burning out. If we restrict the speed of the motor (overload it), we restrict its ability to generate a back EMF. As a result, too much forward current will flow, and this will cause the motor to burn.


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