The actual performance of the engine is the most important factor to the user of the engine. Indicator cards may be taken that, together with the fuel consumption, reveal much more about the power and efficiency of the engine.
Tests on the engines are generally carried out either at constant speed or at variable speed. Tests at variable speed are generally carried on automotive and marine engines while that at constant speed are generally meant for the engines used for generators and pumps.
These tests are basically meant for determining the maximum power the engine can develop and the minimum specific fuel consumption and secondly to get the information in respect of the variation of all parameters with respect to each other.
1. Test at Constant Speed – SI Engine:
Constant speed tests is run with variable throttle from no load to full load in suitable steps of load—generally zero load, 25 % of the full load, 50 % of the full load, 75 % of the full load and full load so that we will get the smooth curves. Starting at zero loads the throttle is opened to given the desired speed.
The engine is run for period of time until water, lubricating oil have been brought to definite operating temperature. Again when the temperature of the exhaust remains stable, the readings of different parameters are recorded. Then the same procedure is followed for other loads and the test is completed. Recorded items include all data necessary to calculate the required results.
The performance curves are plotted as shown in Fig. 25.14 on X-axis, different loads are plotted while on Y- axis, other quantities such as Mechanical efficiency, Brake thermal efficiency, fuel consumption, Brake power and Brake specific fuel consumption are plotted with different scales. Generally the last run will be at wide open throttle.
Note the following:
(1) Brake power, being directly proportional to load, variation is a straight line passing through origin.
(2) Fuel consumption is also a more or less straight line. At zero load, certain quantity of fuel is required to run the engine and account for the mechanical losses.
(3) Mechanical efficiency varies with load and is zero at zero load.
(4) Brake thermal efficiency again varies and is maximum at 75 % of maximum load approximately. This load is the economic load for the given engine.
(5) Brake specific fuel consumption decreases as the load increases and is minimum at the load where Brake thermal efficiency is maximum. This is the load at which the performance is best and is for general interest.
2. Test at Constant Speed – CI Engine:
Constant speed test on CI Engine will be similar as that for SI Engine. In land engines, the speed is kept constant by the action of the governor which varies the quantity of fuel required for the load.
In this engine, the last run would show smoke in the exhaust gases.
The performance curves are plotted in the same manner as in SI Engine.
3. Test at Variable Speed – SI Engine:
For a maximum power test on an SI Engine, the throttle is fully opened and the lowest desired speed is maintained by the brake or external load adjustment. The sprat is adjusted (if manual) to give maximum power at this speed. The engine is run for a period of time until the water and lubricating oil have been brought to definite operating temperature.
When the engine is operating in approximate temperature equilibrium, the test is started and the average speed, brake load, temperatures, fuel weight etc. are recorded. Recorded items include all data necessary to calculate the required results.
After the completion of this run the brake load is adjusted until the speed has changed by the desired amount while the spark is adjusted for maximum torque. Equilibrium conditions of temperature are again obtained, and the procedure of the preceding paragraph is repeated.
Typical performance curves for automobile engine are shown in Fig. 25.15:
(i) Brake Power (BP):
The power output increases to a peak after which the net output decreases. It is significant to note that although the friction power, which includes fluid frictional losses in pumping the gases into and out of the cylinder, is relatively small at low speeds, it increases rapidly at the high speeds.
(ii) Specific Fuel Consumption:
For this type of the engine, the specific fuel consumption if a minimum and the thermal efficiency is a maximum at about 2200 to 2400 rpm. Over a fairly wide range, this fuel consumption and the efficiency vary little, a desirable characteristic, but the curve of fuel consumption increases rapidly at speeds, from about 3200 rpm for this engine. Thus the power necessary to drive the car at high speeds increases rapidly, about as the cube of the speed, and the efficiency of the engine falls off.
The brake torque curve is significant for an automobile engine because the acceleration (pick – up) of the car depends upon the available torque, which is the maximum possible torque at a given speed minus the torque necessary to maintain the speed constant.
(iv) Mean Effective Pressure (MEP):
Since the MEP indicates the relative power of an engine of a given size and speed, high values are desirable. In a particular type of engine, Otto or Diesel, an increase in the MEP is the result of burning more fuel for cycle, a change that may be brought about by improving the volumetric efficiency.
The curve for the brake thermal efficiency characteristically rises to a peak and turns down, the curve for brake engine efficiency follows the same pattern.
(vi) Mainfold Vacuum:
From the mainfold vacuum curve, we may check on the proper size of carburetor, the proper setting for the carburetor, and the proper timing of the values. Too high a vacuum (100 low pressures) suggests too much resistance to flow in the carburetor and other intake passages.
4. Variable Speed Test with CI Engine:
In a full power test of a CI engine at variable speed, the problem is more difficult than for the SI engine because there is no sharp limit of output at any speed. Following the procedure of the SI engine test, the brake is adjusted until the lowest operating speed is obtained with the fuel pump injecting a quantity of fuel sufficient to make the exhaust gas of the engine slightly coloured. This indicates that the engine is near full load because some of the fuel is being wasted is smoke.
Since the CI engine inducts a constant amount of air on the intake stroke, a small amount of fuel injected into the engine will not use all of the air in the cylinder. This occurs at part load. As the load is increased, greater amounts of fuel are injected and more and more of the air is required for combustion.
At some stage, further injection of fuel leads to part of the fuel not being fully oxidized and to the production of smoke. Even at this condition, part of the air in the engine may not be used because of failure of the injected fuel to find the air.
In SI engine, the throttle was opened to the limit; in the CI engine, there is no sharp limit and the colour of the exhaust smoke is used to indicate that limit. It must be realised that smoke—colour observations are not an absolute index of degree of loading, because smoke may be the result of other conditions, such as poor atomization, very late injection, inadequate compression and unbalanced fuel feed to different cylinders. However, with an engine in good condition, the smoke indication may be considered a relatively satisfactory index of degree of loading,
Variable – speed tests of a CI engine at part load are run in the same manner as for SI engine. Figure 25.16 gives the performance curves in different forms.