Cumin—A Crop Valuable for the small Diversified Farm
327 test of a motor on an electric dynamometer it is best to so adjust the armature resistance that the dyna mometer will absorb the power at the lowest motor speed. At the other speeds the load can be adjusted by manipulating the field rheostat. Another advantage of the electric cradle type of dyna mometer is that it can be used to run the engine and thus the friction losses of the motor under test can be determined. The high speed dynamometer in the testing labora tory of The Automobile Club
... he Automobile Club of America is capable of handling somewhat over 100 horse-power continuously from 1,500 r. p. m. to its speed limit, which is above 3,000 r. p. m. A heavier duty machine in the same Laboratory has a continuous capacity of approximately 120 horse-power at as Iow a speed as 1,000 r. p. m. Its maximum speed is 2,000 r. p. m. This testing equip ment is especially arranged to accommodate aviation motors, inasmuch as the bed-plate upon which the motor Is erected is enclosed in a sheet metal house and a cur rent of air of approximately 40 to 50 miles per hour is blown past the motor under test. This imposes upon the motor practically operating conditions. For higher powered motors than 120 horse-power, both the dyna mometers are coupled together. The continuous ca pacity is th' en about 200 horse-power 3. Fuel Consumption-The fuel consumption can best be determined by placing the tank which contains the fuel supply for the motor upon a scale and noticing the decrease in weight. In tests of very short duration in which very small quantities of fuel are consumed, it is preferable to determine the consumption volumetrically, that is, to measure hi a pipette the quantity of fuel consumed. 4. Frictional Losses-It is not possible to obtain a reliable indicator diagram from automobile motors, chiefly due to the fact that there is no instrument made that will operate successfully at the high speed that motors are operated at. There are on the market sev eral optical indicators which are designed to draw indi cator diagrams at these high speeds, but as yet they have not been absoluteLy successful. For this reason it Ls not possible to determine the frictional losses by comparing the indicated horse-power with the brake horse-power. As noted above, however, with the electric cradle dynamometer it is perfectly possible to determine the friction losses at the different speeds. If it is de sired to obtain the indicated horse-power for use in expressing the thermal efficiency, this friction horse power at a given speed can be added to the brake horse power developed at that same speed. It is quite true that the friction determined when the engine is not operating under its own power may be slightly differ ent from the friction when the engine is operating under Lts own power. Nevertheless, the error introduced is less than would be introduced if an indicator diagram were to be used for this purpose. 5. Ignition-The spark advance, that is, the number of degrees of crank-shaft travel that the spark takes place in the cylinder before the piston arrives at top center, is of great importance. The apparatus in use for determining this in the testing laboratory of The Automobile Club of America consists of a pointer rota ting in front of a protractor which is insulated from it. The gap between the pointer and protractor is connected in series with the high tension side of the ignition sys tem, and when the spark occurs in the cylinder it will also jump the gap mentioned and the number of the degrees can be read on the protractor at which this takes place. Needless to say, the pointer is revolved at the same speed as the crank shaft of the engine and is set on zero when one of the pistons is on top-center.