In telephone exchanges, the EA set is used for power requirement of essential loads like exchange load, pumps, lift, corridor and staircase lighting, FD and FF appliances and few essential lights on each floor. The main component of EA set are as follow:
- Diesel Engine
- Control Panel
- Fuel Tank
- Cooling System
Table of Contents
Diesel and petrol engines are combustion engines, in that combustion or burning of engine is done in a chamber. Because of the burning of the fuel a large force is exerted on the chamber walls and if one side of the chamber (i.e. piston) is allowed to move freely, this force can be used as a driving force for running of any machine.
The diesel engines differ from petrol engines in a number of ways. In case of petrol engines, the charge taken into the combustion chamber is a mixture of petrol and air and this mixture is ignited in the closed chamber by means of an electric spark. In diesel engines, air only is taken inside the combustion chambers and is compressed and diesel is injected subsequently at the right time and in right quantity. The compression ratio of the petrol engines is lower than diesel engines.
There are two types of engines: two stroke and four stroke. The working of four stroke engine is shown in figure and is explained below:
Admission of fresh air and discharge of exhaust gases are controlled by valves which are arranged in the cylinder cover and are actuated by levers. The camshaft rotates at half the speed of the crankshaft as two revolutions of the crankshaft are required for one operation cycle of four strokes (admission, compression, combustion and exhaust). The opening times of the valves do not exactly coincide with the end position of the piston e.g. the admission valve opens before the piston reaches the T.D.C. and closes when the piston is moved upwards by the crankshaft. Due to the fact that the valve is kept open for such a long time a larger quantity of air is admitted to the cylinder.
- First (admission) stroke: The piston draws fresh air into the cylinder on its downward travel through the open admission valve. With the turbo charged engines, the air is first compressed by a blower and admitted to the cylinder under pressure.
- Second (compression) stroke: On its upward travel, the piston compresses the fresh air in the cylinder with the valves closed. The temperature of the fresh air is thus increased to exceed the ignition temperature of the fuel. Shortly before the piston reaches T.D.C. fuel is injected into the combustion space under high pressure. Fuel continues to be injected after the piston reaches to the dead center.
- Third (power) stroke: The fuel injected ignites in the hot air and burns. The combustion causes a high pressure which forces the piston down, resulting into reciprocal movement of the crankshaft by the connecting rod and the crankpin.
- Fourth (exhaust) stroke: The piston moves upwards forces the exhaust gas through the open exhaust valve into the exhaust pipe. When the exhaust stroke is terminated the exhaust valve closes and the admission valve opens for a few operation cycle.
The power generated by Engine is calculated by the following method:
power = indicated power - losses.
indicated power ∝ P⋅L⋅A⋅N, where
indicated power is the power buildup inside the cylinder.
P is compression ratio.
L is length of stroke.
A is cross section of cylinder.
N is rpm.
In case more power is required, more no. of cylinders can be used. In case of petrol engines P is very high as fuel air mixture cannot be compressed too much, so they are very inefficient.
Comparison Between Two and Four Stroke Engines:
Merits of four stroke engine:
- Fuel consumption: Lesser compared to two stroke.
- Noise level: The noise level of four stroke engines is much less than two stroke engines.
- Pollution: Two stroke engines emit dense black smoke and are difficult to qualify for stringent pollution control norms.
- Availability: Two stroke engines are at present being imported, but four stroke engines are available.
- Spare parts: Readily available as compared to two stroke.
- Air induction system: Air induction system in two stroke engines is very complicated as it needs more volume of air in comparision to the four stroke engines which are turbo charged.
- Altitude: Two stroke engine efficiency decreases more as compared to four stroke on high altitude.
Merits of Two Stroke Engines:
- Life: Piston speed in case of two stroke engines if half of that of four stroke for the same RPM. Further the exhaust temperature of two stroke engine (for 1000 kVA) of about 400 °C is much less than the same capacity four stroke having exhaust temperature of 510 °C. So two stroke engines work at lower temperature and hence their life is more.
- Vibration: Torque in the case of two stroke engines is 75 % and is more uniform, and so there is lesser vibration and lesser dynamic load.
- Starting reliability: Better than four stroke.
- Power to weight ratio: It is higher than four stroke so they are easy to handle, transport and also require lesser space for installation.
Parts of the Diesel Engine
The diesel engine can be divided into the following sub-systems:
- Working System.
- Lubrication system.
- Fuel system.
- Air system.
- Exhaust system.
- Cooling system.
- Starting system.
- Control system.
Working System: The parts of the diesel engine constituting the working system are directly responsible for delivering the power. They are as below:
- Engine frame.
- Mains bearing.
- Oil sump.
- Cylinder liners.
- Cylinder heads.
- Crank shaft.
- Connecting rods.
- Cam shaft.
- Intake and exhaust valves.
Lubrication System: The moving parts of the diesel engines are lubricated for their optimum operation and to reduce friction, wear and tear. Lubricating oil is the primary cooling of engine while air/water is secondary cooling i.e. it is used to cool the lube oil.
The lube oil pump draws oil from the engine oil sump through a strainer to the heat exchanger and from the heat exchanger the lube oil goes to the lube oil filter. The safety valves are provided to protect the pump from excessive pressure due to the filter clogging or any other reason. From the filter the oil goes to the gallery pipe which runs along the entire length of the engine on its outside and supplies the crankshaft bearings and timing shaft bearing through appropriate passages. Oil from the crank shaft bearing goes through bores in the crankshaft to the connecting rod bearings and from there through center boxes in the connecting rod to the piston pin bushings. A box in the last crankshaft supplies oil to the end bearing and other friction surfaces.
A pressure control valve at the end of the gallery monitors the lube oil pressure up to 4.5 kg/cm² and provides an overflow to the oil sump when the pressure increases. An oil stream for lubrication of the rocker shafts and the valve mechanism in the cylinder head is branched off from the feed passage of the last timing shaft bearing. Another branch pipe at the end of gallery supplies oil to the bearing of the intermediate gears and governors.
Oil emerging from the bearing drip into the sump. Part of it is caught by the rotating crankshaft and splashed against the cylinder liners for lubrication of the piston runaway. A dip stick in the oil sump serves to check the oil level. The lube oil level and the pressure of lube oil has to be checked for satisfactory performance and long life of the engine.
The grades of lube oil are as below:
- Grade 20 up to 30 °C room temperature.
- Grade 40 up to 40 °C room temperature.
- Multi-grade more than 40 °C room temperature.
Fuel System: Fuel system is used:
- To control the quantity of fuel to each cylinder.
- To ensure that the fuel is delivered at correct time.
- To deliver the fuel in correct condition.
- To govern the engine speed.
Depending upon the position of the fuel, the fuel is supplied to the distribution pipe through a fuel filter either by a natural head from an elevated tank or by a supply pump. From the distributor pipe the fuel is led to the injection pumps. The injection pumps are positioned in the engine frame above the camshaft between the pushrods of the valves. The fuel injector pumps supply a quality of fuel which meets the actual requirements of the engine under pressure to the injection nozzles through which the atomized fuel is injected into the combustion chamber. In the combustion chamber the fuel ignites in the high temperature of the compressed air.
Air System: For combustion of fuel, air is required. The air system should:
- Ensure sufficient air. 1 liter of of fuel needs 12 500 liters of air for complete burning.
- Ensure Clean air. Dust in air is harmful for engine. Oil bath chamber and air filter is provided for this purpose.
- Provide cool air. Air at high temp will ignite fuel early & air at high temp is thin and has less oxygen.
- Provide dry air. Air with moisture will have less oxygen & moisture in combustion chamber is harmful to engine
After the combustion the exhaust gases are taken away from the engine through suitable ducting or piping. Considering the high temperature of the exhaust gases, the exhaust ducting or piping is normally insulated. In the case of turbo-charged engines, the exhaust gases are used to drive a turbine wheel and a centrifugal blower. The level of the gas changes so the engine is supplied with enough air to burn the fuel.
The back pressure should be kept within limits by smooth bends in exhaust pipe, giving proper support to avoid load on engine and flexible exhaust connection. Excessive bac pressure results in high combustion temperature & early burning of fuel. This causes low power delivery, overheating, poor fuel economy, high engine wear rate, early bearing failure and carbon deposition on valves & pistons
Cooling System: Cooling system is essential for cooling the engine body and to act as heat exchanger for lubricating oil. This can be either water-cooled or air-cooled.
Tremendous heat is generated during engine operation - up to 3 500 °C to 4 000 °C as roughly only 1/3rd of fuel energy gets converted into useful work and 1/3rd passes out as exhaust. Rest 1/3rd has to be removed by cooling system.
Starting System: The diesel engine is equipped with the various starting system viz either with an electric starter with a pinion which engages with fly wheel of the engine or with a compressed air starting system. In cold weather starting aids like glow plugs are also provided into the air intake. The power to electric starting is provided by means of a battery which is kept in charged condition by means of a dynamo or electric rectifier.
Engine Control: Important engine controls are:
- Engine start/stop: The start stop controls operates the excess fuel for starting and is also used to shut down the engine.
- Speed control.
- Engine shutoff solenoid valve: The purpose of the solenoid valve in conjuction with an electric monitoring device is to set the fuel control linkage to zero admission and thereby stop the engine.
- Lube oil control: Whenever lube oil in the system reduces below a preset level, this control gives a signal for fuel shutoff to stop the engine.
- High temperature control: Whenever cooling system is not able to cool the engine, this control stops the engine.
Types of Engines
The Engines are classified into following types based on aspiration (breathing):
- Naturally aspired: (Denoted by N or without symbol). They inhale air at ambient pressure without any aid.
- Super charged: They have separate externally driven blower/pump for getting increased amount of air. Not used in DG set applications.
Turbo Charged: (Denoted by TC). They are fitted with a turbine driven compressor / blower at the engine exhaust system and uses the otherwise wasted energy in running the intake air pressure aid. This is normally used above 100 kVA capacities.
- TC and After Cooler: Here cooler derives energy from exhaust itself, but thereafter cooled air normally not designed to have overload capacity.
Based on cooling system employed, Engines can be air-cooled, water –cooled: radiator or heat exchanger type.
The alternators standars are given in IS 4722 : 1992 for Industrial Generators and IS 10242 for Marine Generators. The frequency and speed of an alternator are related by the formula:
N = 120 f / P, where N is speed in rpm and P is number of poles.
Following points should be considered for tank:
- Storage capacity is normally selected as 8, 12, 24 hours.
- No engine should consume more than 230 g/(kW⋅h) or 150 g/(hp⋅h) fuel.
- Fuel storage more than 1000 l not allowed except with permission from Deptt. of Explosives.
- Fuel line is made of MS only and is of 6 to 10 mm diameter.
- If tank is at lower level than engine, then fuel feed pump is required.
To reduce vibration, the EA set should be normally kept in outhouse. Anti vibration mountings of suitable type and suitable foundation is required to reduce the vibration.
Depth of foundation: 0.6 to 1.5 m
The ventilation requirement of EA set is 75 (m³/hp)/h. Room temperature should not exceed 5 °C above ambient.
Up to 60 kVA normally air cooled engine is used and for capacities more than 60 kVA water cooled (radiator type) is used. For basements water cooled (cooling tower/heat exchanger type) EA set is used (not economical up to 250 kVA). The water temperature difference is around 32 °C and required flow is 1.5 (l/hp)/min.