Marine Engines
Engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work.
Heat engines are divided into two broad classes.
 External Combustion Engine
 Internal Combustion Engine

a) External Combustion Engine:

In an external combustion engine the product of combustion of air and fuel transfer heat to a second fluid which is the working fluid of the cycle as in case of steam engine or a steam turbine.
Plant where the heat of combustion is employed to generate steam which is used in a piston engine or turbine. In all steam engine, the steam is used as the working substance. (Steam is a vapour of water e.g. it is invisible when pure and dry when dry vapour is further heated in superheated or so it will become superheated vapour which behaves more or less like a perfect gas.) These steam engines operated on the principle of first law of thermodynamics. i.e., heat and work are mutually convertible. In a reciprocating steam engine, as the heat energy in the steam in converted into mechanical work by the reciprocating (to and fro) motion of the piston it is called reciprocating steam engine. turbine moreover as the combustion of fuel taken place outside. The engine cylinder, it is so called as external combustion engines.
External combustion engines “advantages or external combustion plants advantages are – use of cheaper fuels including solid fuels, and high starting torque (Internal combustion are not self-starting). The steam turbine plant which is the most important external combustion engine is
mainly used for large electric power generation.

b) Internal Combustion Engine:

In an Internal combustion engine, the products of combustion are directly the motive fluid, petrol, gas and diesel engines, wankel engine and open cycle gas turbines are examples of internal combustion engines. Jet engines and rockets are also internal combustion engines.
Historical Development: Hygiene Gunpowder Engine (1680) It consists of a vertical cylinder having a sliding fit type of piston. Explosion of a charge of
gunpowder in the cylinder drawn the piston on its upward stroke and the returns stroke was censed by atmospheric pressure acting as the piston and arising from the drys in pressure in the cylinder as the gas products of the explosion began to cool. The useful pone products on the downward stroke and by an arrangement g-ropes and pullys applied to arise loaded platform. He finds difficult to produce means for delivering the sequences of charges necessary for continuous operation and controlling the rate of explosion.
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Components of Engines

Cylinder: It is the heart of engine. Here only fuel is burnt and power is developed. The inner side of cylinder is formed by liner or sleeve, made of nickel  chromium alloy steel. The inner diameter of cylinder is called bore of cylinder.

Cylinder head: It is usually a single piece of casting made of alloy iron, bolted to the top of the engine. It covers the top end of cylinder and has provision for fitting inlet and exhaust value arrangement.

Piston: It is fast moving part inside the cylinder which converts thermal energy into useful mechanical energy.
 
Piston ring: These are rings fitted to the body of the piston. Piston rings are classified into compression ring and oil ring according to their purpose. Compression rings are fitted to the top of the body provides a seal to maintain power developed. Oil rings are used for circulating lubrication oil over the entire body of the piston and cylinder to reduce wear and tear.

Connecting rod: This is tapering rod with small end connected to the piston and big end connected to the crankshaft. The small end is connected to the piston by means of piston pin. The big end is split in two halves to facilitate fixing with the crankshaft. This connecting rod converts reciprocating
motion of piston into rotary motion of crankshaft.

Cam shaft: It consists of cams arranged on it with suitable phase difference in order to control the opening of the valves at different times as and when required. It gets its drive from the crankshaft by suitable gear and chain mechanism.

Cam: These are the parabolic projections machined on a cam shaft or ellipsoidal metal pieces attached to the cylindrical shaft in such way that there is parabolic projection. This projection lifts the cam follower thereby opening the corresponding valve.

Cam follower: This is flat piece attached to end of push rod which transmits the motion from cam to push rod.

Push rod: This is rod which transmits motion to the rocker arm assembly by a suitable ball and socket arrangement.

Rocker arm: This is actuated by the suitable lifting of push rod which in turn opens the valve.

Valve spring: When the cam is released, the opened valve is closed back into position by the tension in the spring.

Crank shaft: This is output shaft, which delivers the useful work to the driven shaft. It is supported by two bearings fixed in the crank case. To one end of the shaft a fly wheel is attached.

Fly wheel: It takes care of fluctuation of the cyclonic vibration in speed. It stores energy during power stroke and release during other stroke, thus giving fairly constant output torque.

Cooling water jackets: To cool the cylinders head and wall a jacket is provided to circulate the cooling water.

Fuel nozzle: Fuel is delivered by this nozzle in a fine spray under pressure in diesel engine. This is an optional part of diesel engine.

Spark plug: At the end of the compression stroke, the air fuel-mixture supplied to the engine cylinder is ignited by an electric spark, produced by spark plug. This is an optional part in petrol engine.

Inlet Valve: To supply air at the suction stroke.

Exhaust Valve: To remove the exhaust gas at the exhaust stroke.
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Systems of diesel Engines
The desired result, i.e., the development of useful power by the engine is achieved only when these parts function as a group only. Some parts have similar functions to perform or some parts assist other parts in carrying out their respective functions. In other words a diesel engine’s performance or operation can be easily understood if the different functions are grouped together into “Systems of operation”. These operating systems of a diesel engine can be broadly classified as:

- Frame System
- Energy Generating System
- Power Transmission System
- Intake Exhaust System
- Valve Mechanism System
- Fuel System
- Lubrication System
- Cooling System
- Starting System

Frame System: The heaviest system of the diesel engine which supports all the vulnerable part is the frame system. Cylinder, crankcase, bed plate, main bearing etc., are some fo the important parts of this system. One of the main function of the frame system is to hold the dead load of all the
components, the inertia load of the moving parts, the forces arising due to the combustion pressure etc., So utmost care has to be taken not only in designing but also in the material used for manufacturing these components which should have sufficient strength and stiffness to bear all the above said loads.

Energy generating system: This system develops the energy. In a diesel energy is developed by the burning of fuel inside the combustion chamber. The lower part of the cylinder head, top part of the piston and the cylinder liner form parts of this system.

Power Transmission System: The generated energy has to be transmitted outside the engine where it can be utilized. The power transmission system consists of piston pin, connecting rod, crankshaft, flywheel which performs this task. The parts of this system are all movable and also subjected to torsional and transverse forces. Hence they have to be abrasive resistant and able to withstand high torsional forces.

Intake and Exhaust System: Suction and exhaust ports, manifolds, silencers etc., form the main parts of this system which is responsible for giving oxygen for combustion inside the cylinder as well as scavenge the unwanted gases outside the cylinder.

Valve Mechanism System: Valve mechanism system works in conjunction with the intake and exhaust system. Controlling the timing, quantity of air admitted and expelling burnt gases out are performed by the intel and exhaust valves, cam, push rod, rocker arms and these parts constitutes this system.

Fuel system: Like the heart of a human being fuel system can be termed as the heart of S for combustion. It has various parts which are explained in the detailed study. 

Lubricating and Cooling and Starting System: These are auxiliary systems but for which the smooth starting and operation of the engine cannot be affected. Like the fuel system all these systems have a series of accessories as well as various methods of operation which are elaborately explained in the specific chapters.
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