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


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V

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Space void of air.

Atmospheric pressure will always seek to fill this space with air.

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Valve with parts namedThis is the angled area on the valve head, which forms the gas-tight seal with the cylinder head. It is normally cut at an angle slightly different to that of the valve seat. For example, if the land is cut at an angle of 450, then the valve seat would be cut at an angle of, say, 45.50. This ensures narrow valve seats.

Valve land and valve face

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Valve land and valve faceThis is the vertical part of the valve head, immediately below the angled part of the valve that forms the gas-tight seal with the valve seat when the valve is closed.

During valve overhaul and repair, it may become necessary to re-grind the valve face in order to produce a surface suitable for lapping and subsequently producing a gas-tight seal with the valve seat.

You must be careful not to remove too much metal from the valve face, as this reduces the amount of metal in the valve land. There is a danger that the small amount of remaining metal can become incandescent in the combustion chamber and cause pre-ignition, which can cause catastrophic damage to the engine.

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Check valve guide for wearThe valve seat is in the cylinder head of a four stroke engine. There are at least two seats (often, there are more), one for the inlet valve, and one for the exhaust valve. The inlet valve and seat tend to be the bigger of the two. It is vital that the valve face and the valve seat form a gas-tight seal.

Valve seat

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Gear timing marksThe process whereby the camshaft is timed to the crankshaft, so that the inlet and exhaust valves open at the right times. Camshafts are driven by the crankshaft by means of a belt, chain or gears. With each of these systems, there is some sort of marking system on the gears, pulleys or sprockets to assist you in correctly timing the two shafts.

As a general rule of thumb, in a four cylinder engine, you can get the crankshaft at tdc by having the keyway pointing directly upwards, at the '12 o'clock' position. With the crankshaft at this position, if you get number 4 cylinder rocking, that means number 1 cylinder is firing. With both shafts in these positions, they are timed relative to each other.

Note: This is only a general rule of thumb. Many modern engines use specific methods and specific markings both on gears, sprockets and pulleys, as well as on the engine block, to set up valve timing. Always consult the manufacturers specifications, because if you get it wrong, it can result in serious, expensive, damage to the engine.

Once you time an engine, always turn it over two full turns by hand. This is to ensure that pistons and valves are not colliding due to incorrect timing. Doing this will not prove your valve timing is correct, but it will prove the engine can turn over freely. The reason why we turn it twice is that, in a four stroke engine, each stroke and each opening of a valve is going to happen at some stage during those two turns.

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Electronics supplier of components and instruments such as the PPS10 and HPS10 hand held oscilloscopes.

Velleman website

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In physics, velocity is defined as the rate of change of position. In order to properly define it, both speed (the rate at which a body undergoes displacement) and direction are required.
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Venturi

A fitting or device that consists of a tube constricted in the middle. A fluid's velocity will increase and a fluid's pressure will decrease while passing through the constriction. (Bernouli's Principle)

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Giovanni Battista VenturiGiovanni Battista Venturi discovered the venturi effect - when air is made to flow through a pipe, if a restriction is placed in that pipe, the air must speed up to get through the restriction.

This increase in air speed produces a corresponding reduction in air pressure.

This is the effect used in carburettors to get petrol into the air flow going into an engine.

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CarburettorThis refers to the restriction put into the choke tube of a carburettor.

The reason why this restriction or venturi is put in a carburettor is to cause the incoming air to speed up as it passes through this restriction.

When the air speeds up, there is a corresponding drop in pressure. This localised drop in pressure is in the middle of the venturi.

If we have a float chamber full of fuel, and we allow atmospheric pressure to act on the surface of the fuel, then the relatively low pressure in the venturi will draw fuel from the float chamber.

As we draw this fuel into the air flow, we can pass it through a small hole or holes. This will cause the fuel to break up into tiny particles (to atomise). This greatly aids the combustion process, because the only part of any fuel that burns is its vapour. The amount of vapour we can get off any fuel depends on the surface area of that fuel. If the fuel is broken up into many tiny particles, then the surface area of the fuel and the amount of vapour we get from it is hugely increased. This leads to very efficient combustion.


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