FUEL CONSUMPTION :
Specific fuel consumption (gm/BHP/Hr) has to be considered from two different angles; they are -
(1) the engine related environments.
(2) ship's hull resistance & such other allied environments.
Engine related environments -
To consume fuel efficiently, the following factors are to be considered,
a) Influence of speed :-
Curves obtained from test results of compression ignition engine with speed Vs BHR fuel consumption, and B.m.e.p., shows that fuel consumption is minimum for a particular speed. Initially as the speed increases, fuel consumption decreases and power increases as also efficiency increases. At a certain speed, when the fuel consumption becomes minimum, efficiency becomes maximum. Thereafter as the speed increases, fuel consumption increases rapidly and though power generation increases, efficiency starts falling.
In another curve, between fuel consumption and power output, it is observed that for a particular range of power output, fuel consumption is minimum and most economical condition occurs.
SPEED (RPM)
b) Influence of compression ratio :-
It will be noticed from the equation for the ideal efficiency of the Otto, Diesel and Dual combustion cycle that in each case the ideal efficiency depends on the ratio i of compression and increases as the compression is raised. The highest possible value for the compression ratio will depend on practical consideration.
However, the tendecy of an engine to detonate increases with the increase in compression-ratio. The compression ratio at which detonation occurs is kown as highest useful compression ratio (HUCR), after which power of the engine commence to decrease due to overheating.
c) Influence of air-fuel ratio :
For perfect combustion in a compression ignition engine, air-fuel ratio is a guiding factor. For more power, more fuel need to be burnt for which additional air is required which is achieved by turbo-charging.
From -the curve, it will be seen that minimum specific fuel consumption reaches at a air-fuel ration of 23:1 and this corresponds to the maximum efficiency. As the air-fuel ratio increases further, fuel consumption increases, b.m.e.p. falls and consequently output power.is reduced and efficiency decreases.
In another curve of b.m.e.p. against specific fuel consumption, it will be seen that efficiency at first increases with the increase in b.m.e.p due to decrease in fuel consumption. Efficiency continues to increase with the increase in b.e.m.p and consequently in the b.h.p. Then the curve shows that at maximum power, there is a rapid falling - off in the efficiency due to exessive fuel consumption.
d) Influence of dissociation on temperture & power :
Dissociation of the burnt gases in the cylinder of an internal combustion engine occurs at high temperature and is mainly due to dissociation of CO2 into CO and O2: The dissociation of CO2 commences at about 1000°c and at 1500°c, it amounts to one percent and it increases with temperature. The presence of O2 in the gases tend to prevent dissociation ; this is noticable in rich fuel mixture, which by producing more CO, suppresses dissociation of the CO2. On the other hand, there is no dissociation in the burnt gases of a weak fuel mixture as the temperature produced is too low for this phenomenon to occur. Hence, maximum dissociation occurs in the burnt gases of the chemically correct fuel mixture but decreases with the weaker & richer mixtures. As the dissociation is the reverse process of combustion, dissociation absorbs some amount of heat, thus causing a reduction in the maximum temperature and pressure as well as power. The heat absorbed by dissociation is later released in the expansion stroke.
(e) Effect of turbocharging :-
By utilising the energy contained in the exhaust gases(arround 35%, a turbo-blower can be driven to provide charge air inside the cylinder, thereby increased power can be obtained from the engine.
(f) Effect of combustion of fuel :-
Calorific value of the fuel and efficient combustion are also responsible for obtaining higher output with optimum fuel consumption.
2) Fuel consumption based on Sea Conditions :
A ship moving at the free surface between two fluids, water & air will experience two kinds of resistance to its motion - viscous and wave making. By far the greater part of the viscous resistance will be due to water flow around the hull and a small component of viscous drag will be due to air flow above the water part of the ship.
Wave making resistance of a ship can be minimised by attention to the basic shape of hull form and by fitting a bulbous bow.
On the other hand viscous drag or skin fiction depends on i) Speed of the ship, (ii) Wetted surface area, (iii) Length of the ship, (iv) Roughness of the hull and (v) the density of water.
Since power has to be developed by the engine to propel the ship against this , resistances under a certain speed, the fuel consumption will depend upon the resistance of the ship and its velocity.
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