2 Stroke 4 Stroke Refrigeration.docx

2 stroke and 4 stroke The stroke in an engine is the distance covered by the piston from top dead center to the bottom dead center. In simple words, stroke is the distance of cylinder between piston moves. If a piston moves 2 times in the cylinder, that means, engine is known as two stroke engine and if it moves 4 times in a four stroke engine. The crankshaft rotates one time between 2 strokes. 4 stroke Intake, Compression,Power,Exhaust 2 stroke Intake/Ignition Compression/exhaust The basic and main difference between two stroke and four stroke engine is that the crankshaft complete one revolution in one power stroke in 2 stroke engine and complete 2 revolution in one power stroke in four stroke engine. So the 2 stroke engine give high power compare to 4 stroke engine but the 4 stroke engine is more fuel efficient. Here I like to list many other differences,It 1.               

Two stroke engine (2$) has one revolution of crankshaft within one power stroke. Four stroke engine (4$) has two revolution of crankshaft between one power strokes. 2$ can generate high torque compare to 4 strokes engine. 4$ generates less torque due to 2 revolution of crankshaft between one power strokes. 2$ used port to inlet and outlet of fuel. 4$ used valve to inlet and outlet. 2$ engines require lighter flywheel compare to other engines because it generates more balanced force due to one revolution for one power stroke. 4$ requires heavy flywheel because it generates unbalance force due to two revolutions for one power stroke. 2$ has high power to weight ratio compare to others. 4$ engines have less power to weight ratio. 2$ creates more noise. 4$ is less noisy Two stroke engines are less efficient and generate more smoke. Four stroke engines are more efficient and generate less smoke 2$ engines are generally lighter. 4$ engine comparatively heavier than 2$.

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2$ are mostly used in ships, scooters etc. 4$ engines mostly used in car, truck, and other automobiles. In 2$ Due to poor lubrication more wear and tear occurs In 4$ Less wear and tear occurs.

A four stroke engine is an internal combustion engine in which the piston completes four separate strokes which comprise a single thermodynamic cycle. A stroke refer to full travel of piston along the cylinder in either direction. The four separate strokes are termed as INTAKE, COMPRESSION, POWER and EXHAUST. A two-stroke (or two-cycle) engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaf revolution. At the top of the stroke, the spark plug ignites the fuel mixture. ... (At the same time, another crankcase compression stroke is happening beneath the piston.) Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size.

Ships refrigeration plant may vary from the small domestic refrigerating unit for provisions to large plant for reefer vessels. The Chief Engineer is responsible for the correct temperatures being maintained, delegating the good operations and maintenance of the plant to the 2/E. Larger plants may have a Refrigeration Officer. Machinery under ship's engineer responsibility may include:  Domestic ref. plant.  Cargo ref. plants  Air conditioning plants  Ventilation and heating plants  Cargo refrigerated containers All maintenance recommendations from the makers have to be carried out regularly and according to instructions, entered in the refrigeration maintenance log, together with the test of all cut outs,

i.e. HP, LP, LO, HT, that have to be carried out at regular intervals, generally one month. All adjustment must be made according to standard good practice and records of the same entered in the log. Filter separators and driers should be regularly cleaned in order to have always the circuit moisture, dirty and oil free. When shutting down a plant all refrigerant gas must be pumped in the liquid receiver or condenser. Four basic components of a refrigeration system The four main components of a refrigeration system working on the vapour compression cycle are:  The compressor  the condenser  the expansion valve  the evaporator. Compressor :The function of the compressor in a refrigeration system is to raise the pressure of the vapourised refrigerant, causing its saturation temperature to rise so that it is higher than that of seawater or an air cooled condenser. The compressor also promotes circulation of the refrigerant by pumping it around the system. Condenser :The function of the condenser is to liquefy the refrigerant and sub cool it to below the saturation temperature by circulating seawater or air. Latent heat originally from the evaporator is transferred to the cooling medium. The liquid refrigerant still at pressure produced by the compressor passes on to the expansion valve. Expansion valve : The function of the expansion valve in a refrigeration system is to regulate the flow of refrigerant from the

HP side of the system to the LP side of the system. The drop in pressure causes the saturation temperature of the refrigerant to fall so that it will boil at the low temperature of the evaporator. The expansion valve controls the flow of refrigerant to the evaporator thermostatically. Evaporator : The function of the evaporator in the refrigeration system is to cool the air in the fridge space. It does this because the temperature of the refrigerant entering the evaporator is lower than that of the air in the space and this causes the refrigerant to receive latent heat and evaporate. The evaporator normally has a fan to circulate the air around it. Three desirable properties of a refrigerant are: 1. Low boiling point 2. low condensing pressure 3. high specific enthalpy of vaporisation. (This reduces the quantity of refrigerant in circulation and lower machine speeds, sizes, etc). The effects of insufficient refrigerant in the system are a low reading on the LP pressure gauge and a lack of frost on the suction pipe. The high pressure (HP) cut out is fitted on the discharge side of the compressor in a refrigeration system. This will shut down the compressor in the event of an over pressure and can only be manually reset.

Details of refrigeration cycle -How the system works ? Refrigeration of cargo spaces and storerooms employs a system of components to remove heat from the space being cooled. This heat is transferred to another body at a lower temperature. The cooling of air for air conditioning entails a similar process.

The transfer of heat takes place in a simple system: firstly, in the evaporator where the lower temperature of the refrigerant cools the body of the space being cooled; and secondly, in the condenser where the refrigerant is cooled by air or water. The usual system employed for marine refrigeration plants is the vapour compression cycle as shown in diagram here.

Fig: Vapour compression cycle

The pressure of the refrigerant gas is increased in the compressor and it thereby becomes hot. This hot, high-pressure gas is passed through into a condenser. Depending on the particular application, the refrigerant gas will be cooled either by air or water, and because it is still at a high pressure it will condense. The liquid refrigerant is then distributed through a pipe network until it reaches a control valve alongside an evaporator where the cooling is required. This regulating valve meters the flow of liquid refrigerant into the evaporator, which is at a lower pressure. Air from the cooled space or air conditioning system is passed over the evaporator and boils off the liquid refrigerant, at the same time cooling the air. The design of the system and evaporator should be such that all the

liquid refrigerant is boiled off and the gas slightly superheated before it returns to the compressor at a low pressure to be recompressed. Thus it will be seen that heat that is transferred from the air to the evaporator is then pumped round the system until it reaches the condenser where it is transferred or rejected to the ambient air or water. It should be noted that where an air-cooled condenser is employed in very small plants, such as provision storerooms, adequate ventilation is required to help remove the heat being rejected by the condenser. Also, in the case of water-cooled condensers, fresh water or sea water may be employed. Fresh water is usual when a central fresh-water/sea-water heat exchanger is employed for all engine room requirements. Where this is the case, because of the higher cooling-water temperature to the condenser, delivery temperatures from condensers will be higher than that on a sea water cooling system.

Temperature Records Temperatures of domestic refrigerated rooms have to be corrected daily by the 2nd Engineer or delegated Officer, passed to the Chief Engineer and to the Master. On larger plant suitable logs will be supplied in order to enter temperature of the cargo and all other relevant details.