Measurement of friction power:
-The difference between indicated power and the brake power output of an engine is the friction power.
-Almost invariably, the difference between a good engine and a bad engine is due to difference between their frictional losses.
-The frictional losses are ultimately dissipated to the cooling system (and exhaust) as they appear in the form of frictional heat and this influences the cooling capacity required. Moreover, lower friction means availability of more brake power; hence brake specific fuel consumption is lower.
-The bsfc rises with an increase in speed. Thus, the level of friction decides the maximum output of the engine which can be obtained economically.
The friction force power of an engine is determined by the following methods :
(a) Willan‟s line method.
(b) Morse test.
(c) Motoring test.
(d) Difference between ip and bp.
(a) Willan’s line method
-In this method, gross fuel consumption vs. bp at a constant speed is plotted and the graph is extrapolated back to zero fuel consumption.
-The point where this graph cuts the bp axis is an indication of the friction power of the engine at that speed. This negative work represents the combined loss due to mechanical friction, pumping and blow by.
-The main drawback of this method is the long distance to be extrapolated from data measured between 5 and 40% load towards the zero line of fuel input.
-The directional margin of error is rather wide because of the graph which may not be a straight line many times.
-The changing slope along the curve indicates part efficiencies of increments of fuel. The pronounced change in the slope of this line near full load reflects the limiting influence of the air-fuel ratio and of the quality of combustion.
-Similarly, there is a slight curvature at light loads. This is perhaps due to difficulty in injecting accurately and consistently very small quantities of fuel per cycle.
-Therefore, it is essential that great care should be taken at light loads to establish the true nature of the curve.
-The Willan‟s line for a swirl-chamber CI engine is straighter than that for a direct injection type engine.
Fig. 34. Willian’s line method
The accuracy obtained in this method is good and compares favourably with other methods if extrapolation is carefully done.
(b) Morse Test
-The Morse test is applicable only to multicylinder engines.
-In this test, the engine is first run at the required speed and the output is measured.
-Then, one cylinder is cut out by short circuiting the spark plug or by disconnecting the injector as the case may be.
-Under this condition all other cylinders „motor‟ this cut-out cylinder.
-The output is measured by keeping the speed constant at its original value.
-The difference in the outputs is a measure of the indicated horse power of the cut-out cylinder.
-Thus, for each cylinder the ip is obtained and is added together to find the total ip of the engine.
-This method though gives reasonably accurate results and is liable to errors due to changes in mixture distribution and other conditions by cutting-out one cylinder. In gasoline engines, where there is a common manifold for two or more cylinders the mixture distribution as well as the volumetric efficiency both change. Again, almost all engines have a common exhaust manifold for all cylinders and cutting out of one cylinder may greatly affect the pulsations in exhaust system which may significantly change the engine performance by imposing different back pressures.
(c) Motoring Test
-In the motoring test, the engine is first run up to the desired speed by its own power and allowed to remain at the given speed and load conditions for some time so that oil, water, and engine component temperatures reach stable conditions.
-The power of the engine during this period is absorbed by a swinging field type electric dynamometer, which is most suitable for this test.
-The fuel supply is then cut-off and by suitable electric-switching devices the dynamometer is converted to run as a motor to drive for „motor‟ the engine at the same speed at which it was previously running.
-The power supply to the motor is measured which is a measure of the fhp of the engine. During the motoring test the water supply is also cut-off so that the actual operating temperatures are maintained.
-This method, though determines the fp at temperature conditions very near to the actual operating temperatures at the test speed and load, does, not give the true losses occurring under firing conditions due to the following reasons.
(i) The temperatures in the motored engine are different from those in a firing engine because even if water circulation is stopped the incoming air cools the cylinder. This reduces the lubricating oil temperature and increases friction increasing the oil viscosity. This problem is much more severing in air-cooled engines.
(ii) The pressure on the bearings and piston rings is lower than the firing pressure. Load on main and connecting road bearings are lower.
(iii) The clearance between piston and cylinder wall is more (due to cooling). This reduces the piston friction.
(iv) The air is drawn at a temperature less than when the engine is firing because it does not get heat from the cylinder (rather loses heat to the cylinder). This makes the expansion line to be lower than the compression line on the p-v diagram. This loss is however counted in the indicator diagram.
(v) During exhaust the back pressure is more because under motoring conditions sufficient pressure difference is not available to impart gases the kinetic energy is necessary to expel them from exhaust.
Motoring method, however, gives reasonably good results and is very suitable for finding the losses due to various engine components. This insight into the losses caused by various components and other parameters is obtained by progressive stripping-off of the under progressive dismantling conditions keeping water and oil circulation intact. Then the cylinder head can be removed to evaluate, by difference, the compression loss. In this manner piston ring, piston etc. can be removed and evaluated for their effect on overall friction.
(d) Difference between ip and bp
(i) The method of finding the fp by computing the difference between ip, as obtained from an indicator diagram, and bp, as obtained by a dynamometer, is the ideal method.
(ii) In obtaining accurate indicator diagrams, especially at high engine speeds, this method is usually only used in research laboratories. Its use at commercial level is very limited.
-The difference between indicated power and the brake power output of an engine is the friction power.
-Almost invariably, the difference between a good engine and a bad engine is due to difference between their frictional losses.
-The frictional losses are ultimately dissipated to the cooling system (and exhaust) as they appear in the form of frictional heat and this influences the cooling capacity required. Moreover, lower friction means availability of more brake power; hence brake specific fuel consumption is lower.
-The bsfc rises with an increase in speed. Thus, the level of friction decides the maximum output of the engine which can be obtained economically.
The friction force power of an engine is determined by the following methods :
(a) Willan‟s line method.
(b) Morse test.
(c) Motoring test.
(d) Difference between ip and bp.
(a) Willan’s line method
-In this method, gross fuel consumption vs. bp at a constant speed is plotted and the graph is extrapolated back to zero fuel consumption.
-The point where this graph cuts the bp axis is an indication of the friction power of the engine at that speed. This negative work represents the combined loss due to mechanical friction, pumping and blow by.
-The main drawback of this method is the long distance to be extrapolated from data measured between 5 and 40% load towards the zero line of fuel input.
-The directional margin of error is rather wide because of the graph which may not be a straight line many times.
-The changing slope along the curve indicates part efficiencies of increments of fuel. The pronounced change in the slope of this line near full load reflects the limiting influence of the air-fuel ratio and of the quality of combustion.
-Similarly, there is a slight curvature at light loads. This is perhaps due to difficulty in injecting accurately and consistently very small quantities of fuel per cycle.
-Therefore, it is essential that great care should be taken at light loads to establish the true nature of the curve.
-The Willan‟s line for a swirl-chamber CI engine is straighter than that for a direct injection type engine.
Fig. 34. Willian’s line method
The accuracy obtained in this method is good and compares favourably with other methods if extrapolation is carefully done.
(b) Morse Test
-The Morse test is applicable only to multicylinder engines.
-In this test, the engine is first run at the required speed and the output is measured.
-Then, one cylinder is cut out by short circuiting the spark plug or by disconnecting the injector as the case may be.
-Under this condition all other cylinders „motor‟ this cut-out cylinder.
-The output is measured by keeping the speed constant at its original value.
-The difference in the outputs is a measure of the indicated horse power of the cut-out cylinder.
-Thus, for each cylinder the ip is obtained and is added together to find the total ip of the engine.
-This method though gives reasonably accurate results and is liable to errors due to changes in mixture distribution and other conditions by cutting-out one cylinder. In gasoline engines, where there is a common manifold for two or more cylinders the mixture distribution as well as the volumetric efficiency both change. Again, almost all engines have a common exhaust manifold for all cylinders and cutting out of one cylinder may greatly affect the pulsations in exhaust system which may significantly change the engine performance by imposing different back pressures.
(c) Motoring Test
-In the motoring test, the engine is first run up to the desired speed by its own power and allowed to remain at the given speed and load conditions for some time so that oil, water, and engine component temperatures reach stable conditions.
-The power of the engine during this period is absorbed by a swinging field type electric dynamometer, which is most suitable for this test.
-The fuel supply is then cut-off and by suitable electric-switching devices the dynamometer is converted to run as a motor to drive for „motor‟ the engine at the same speed at which it was previously running.
-The power supply to the motor is measured which is a measure of the fhp of the engine. During the motoring test the water supply is also cut-off so that the actual operating temperatures are maintained.
-This method, though determines the fp at temperature conditions very near to the actual operating temperatures at the test speed and load, does, not give the true losses occurring under firing conditions due to the following reasons.
(i) The temperatures in the motored engine are different from those in a firing engine because even if water circulation is stopped the incoming air cools the cylinder. This reduces the lubricating oil temperature and increases friction increasing the oil viscosity. This problem is much more severing in air-cooled engines.
(ii) The pressure on the bearings and piston rings is lower than the firing pressure. Load on main and connecting road bearings are lower.
(iii) The clearance between piston and cylinder wall is more (due to cooling). This reduces the piston friction.
(iv) The air is drawn at a temperature less than when the engine is firing because it does not get heat from the cylinder (rather loses heat to the cylinder). This makes the expansion line to be lower than the compression line on the p-v diagram. This loss is however counted in the indicator diagram.
(v) During exhaust the back pressure is more because under motoring conditions sufficient pressure difference is not available to impart gases the kinetic energy is necessary to expel them from exhaust.
Motoring method, however, gives reasonably good results and is very suitable for finding the losses due to various engine components. This insight into the losses caused by various components and other parameters is obtained by progressive stripping-off of the under progressive dismantling conditions keeping water and oil circulation intact. Then the cylinder head can be removed to evaluate, by difference, the compression loss. In this manner piston ring, piston etc. can be removed and evaluated for their effect on overall friction.
(d) Difference between ip and bp
(i) The method of finding the fp by computing the difference between ip, as obtained from an indicator diagram, and bp, as obtained by a dynamometer, is the ideal method.
(ii) In obtaining accurate indicator diagrams, especially at high engine speeds, this method is usually only used in research laboratories. Its use at commercial level is very limited.
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