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Pipe Fitting and Piping Handbook

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Pipe Fitting and Piping Handbook

1. Assist Head of Dewatering in test work activities of Thickeners & Filters.

2. Assist in Technical Proposals for Thickener

So, there are two main categories of pumps:

• Roto-Dynamic pumps (non positive displacement pumps).
  
• Positive displacement pumps.

Pumps duties may be range from a low pressure of a fraction of a bar and small capacities of a few litters per minute to a high pressure of several hundred bars and large capacities so, each of those categories has sub-groups consisting different types of pumps as shown in the following diagram:

Basically, pumps that discharge (pump) liquid in continuous flow are referred to non positive displacement pumps (Roto-Dynamic pumps) and those pumps that discharge liquid separated by a period of no discharge are referred to positive displacement pumps.

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Because the conventional (mechanical) system has two main problems:

1- The high voltage that is generated in the secondary winding depends on the primary current.

(You should notice that to obtain higher voltage, you can do two things: increasing the dwell angle or increasing the value of the primary current).

2- The primary current in the conventional coil doesn't exceed 5 amber to protect the contact points of the breaker point from damage when the primary current passes through it.

What about the new system?
Electronic type of ignition system provides a hotter and more uniform spark. This promotes more efficient burning of air/fuel mixture in the combustion chamber producing less exhaust emissions resulting in best engine performance.

The main difference between the electronic system and the mechanical system
In the mechanical system, the primary current is interrupted by a mechanical switch (breaker point), but in electronic type of ignition system the primary current is interrupted by an electronic switch.

• breaker point generator.
  
• hall effect generator.
  
• pick up coil generator.

Hall effect transistorized ignition system

Gap α 1 / dwell angle

http://urcar-engine.blogspot.com/2011/11/spark-ignition-timing.html

The optimum spark ignition timing is usually experimentally selected to produce the maximum cylinder pressure, max torque, max torque and minimum specific fuel consumption. The optimum spark timing varies with engine speed and loads. The ignition distributor contains mechanisms to adjust spark timing for different loads and speeds:

• Centrifugal (governor) advance mechanism.
  
• Vacuum advance mechanism.

We know well that the different modes of engine are:

• At starting: rich mixture (F/A=1/9).
  
• At part load: lean mixture (A/F=1/18).
  
• At full load: rich mixture (A/F=1/12).
  
• At no load or idling (A/F=1/9).

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• Creating a high voltage from low volt source (12 volt) where the voltage that is required to the spark plug is somewhere between (20,000 to 50,000).
  
• Controlling the timing of ignition (it must generate a spark at the right time in the right cylinder) according to rpm of the engine and engine load (engine mode).

• Battery: is a type of rechargeable batteries that supplies electric energy to automobile. This battery is made up plates of lead and separate plates of lead dioxide that is submerged in electrolyte solution of about 35% sulfuric acid and 65% water. This causes a chemical reaction that releases electrons allowing them to flow through conductors to produce electricity. The battery is usually lead acid type that is made of six galvanic cells in series to provide a 12 volt system. Each cell provides 2 volt in the group. Heavy vehicles such as highway trucks that equipped by diesel engine use two batteries in series for a 24 volt system. To measure the concentration of acid, you can use hydrometer device where;

When the concentration of sulfuric acid in the battery is 1.28,this means fully charged.

When the concentration of sulfuric acid in the battery is 1.2, this means half charged.

When the concentration of sulfuric acid in the battery is 1.1, this means discharged.

• Ignition coil (transformer): contains primary winding and secondary winding. The coil primary winding contains 100 to 150 turns of heavy copper wire. This wire is very thickness wire because the the primary winding is low voltage lead that has high current that leads to high temperature. Also, the wire of primary winding must be insulated so that the voltage doesn't jump from loop to loop. The coil secondary winding has number of turns 100 times greater than that is in primary winding for an important reason?(why). The coil secondary winding is sit inside the coil primary winding. The windings are wrapped around a soft iron core as shown in this figure.

• Distributor cover: when we remove the distributor cover from the top of distributor, we can see the condenser and breaker point. The condenser is a simple capacitor that store a small amount of current. The breaker point is attached with an eccentric cam that makes rotation motion so, the contact breaker opens and closes. The condenser and breaker point are connected in parallel. The cam takes its motion from distributor shaft. The distributor shaft is driven by a gear on the cam shaft. The cam has many loops as number of cylinders.

• Spark plug: it is the most common important things in the ignition system. It is an electrical device which located at the cylinder head of the 4-stroke engine to ignite the compressed fuel by generating a spark. The spark plug has two electrodes: insulated central electrode and ground electrode. The insulated central electrode is connecting by an insulated wire to magneto circuit. There is an air gap between the central electrode and the ground electrode. When the central electrode is supplied with high voltage, the current passes from the central electrode to ground electrode in the form of spark.

When the key of your car is turned, the ignition switch is closed and the current is going through coil primary winding (low voltage lead). We talked that the windings are wrapped around iron core. We know from electricity that electric current always produces magnetic field even if the wire itself isn't made of an magnetic material or there isn't any magnetic material sits the wire so, a magnetic flux is generated and cut the turns of primary winding and the secondary winding.

We can remember that V _primary / V _secondary α n _primary / n _secondary, but this relation is right if the rate of collapse of the magnetic flux is equaled to zero (dΦ/dt =0).

So, we need to covert DC current from the battery source to AC current to achieve dΦ/dt =0. The conversion from DC to AC is proceed by the breaker point that opens and closes due to the rotation motion of the cam. When the contact breaker is opened (contact points aren't touched), this means that the electric circuit is opened in this direction. If the current passes through the gap between contact breaker, the arcing between the two points occurs causing damage in breaker point. Also the quick rate of connecting of circuit and its open causes wear and damage in breaker point.

The condenser plays a very important task to protect the breaker point from arcing where storing the current when the circuit is opened. We can say in other words, that the condenser protects the breaker point from arcing when the rate of collapse of magnetic flux is occurred (dΦ/dt =0). According to the above relation, high voltage will be generated in the coil secondary winding due to higher turns as mentioned previously (the number of turns for secondary winding is 100-150 times greater than that is in primary winding). Any engine contains a certain number of cylinders, for example: 4-cylinder engine, 6-cylinder engine. To ensure occurring a spark in each cylinder in the right place at the right time, we need a mechanism that achieves this function. The distributor does this function where it can distribute the high voltage to all the spark plugs that are located at the top of the cylinder heads according to firing order at the right time.

At the time that the electric circuit is opened causing collapse in the rate of magnetic field, the rotor in the distributor is in the contact with high voltage lead and the spark is generated. The spark is usually produced before the piston reaches the TDC by the end of compression stroke. The spark timing may vary between 5-40 degrees before TDC.

At this point, we are realized the first task of the ignition system "creating a high voltage from a low voltage source where the voltage that is required to the spark plug is between (20,000-50,000) volt". We will resume the second task of the spark ignition system the next week.