Fishing Craft Technology

Unit 5 - Marine Engine and propulsion system

5.1. Introduction to marine engine
Engine: 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.

The free piston Otto – Larges Engines (1866):
This consists of a piston which was not connected to any crankshaft but was free to move vertically outwards during the explosion and expansion strokes. After expansion, the product cooled down and piston fills. Under gravity causing downward stroke. During the downward stroke, piston rod was connected by a ratchet and a rack and pinion device to the flywheel, doing useful work on fly wheel shaft on the downward stroke. The piston opened as exhaust valve of the sliding type. The inertia of the flywheel thus raised the piston from the bottom position and inducted a fresh air gas charge. The charge was ignited by a flame after about 300 of gear angle travel for the pistons from the bottom position. The connection between the frame and the combustion chamber was provided by means of an eccentric drives valve. The thermal efficiency of this was greater than of Lenoir (other) engines, the gas consumption being about 1/3 / h.p, but the operations was noisy. He got gold medal in historic Paris Exhibition.