For instance, for r =13, lc =1.41, L =1.5, we get 'it 61.5 per cent and for r=-• 13, K 1.41, L='3 ; qt.= 54 per cent.
This shows that the thermal efficiency of the Diesel cycle decreases with the increase of the ratio of .
maximum combustion pressures, ps, are about the same.
2. Transmission of Power from Cylinder to Shaft. (A) Indicated Efficiency. Indicated Horse Power.— The thermal efficiency as com puted above can never be realized in practice since many factors upon which it depends can not be mathematically considered. Thus the assumption . that compression and expansion curves are adiabatic in character is not true. There must be heat losses due to, cooling, radia tion, incomplete combustion and several other. reaspns. The degree of variation of the spe cific heats with a change in temperature or pressure or both is still an open question. The combustion lines are never true constant-vol ume or constant-pressure lines. Fig. 16 shows n 'ig., e ciencies of e • tto as we as of the Diesel cycle are graphically sented for equal ratios of compression, r, heats, and loads, L. From these diagrams it is evi dent that at equal compression ratios, the Ott. cycle is superior to the Diesel. In practice, how ever, the Otto cycle is limited in regard to the compression ratio (r=8), in order to pre vent pre-ignition, while the Diesel engine must essentially use a high compression, r being in this case approximately 16. At a low ratio of load, L =1.5, the Diesel engine has the highe fficiency. At full load however, L being ap proximately 3, the thermal efficiency of both Iles is about the same in which case thei the theoretical diagram representing the heat Q, and the actual diagram representing the in dicated heat, actually converted into work. The difference of the two areas gives the ac tual loss due to the factors cited above. The ratio has a value from .5 to .8 and is some times called the card factor.
The theoretical efficiency, vt, is thus re duced to: ('it the Indicated Efficiency.
The Indicated Horse Power, denoted by the symbol Ni. is the work actually developed within the cylinder.
It is in direct proportion to the area of the pressure-volume or indicator diagram as actu ally obtained by means of an indicator, and can be computed by means of the formula pi No= indicated horse power, in which Pi is the mean indicated pressure from the indicator diagram in pounds per square inch, S the length of the engine stroke in feet, A the net area of the piston in square inches, X= the number of power strokes per minute, and 33,000 the number of foot-pounds per min ute in one horse power.
(B) Brake Horse Power. Mechanical Effi ciency.— The Brake Horse Power or Effective Horse Power, denoted by the symbol N., is the actual work delivered from the main shaft of the engine to the power consumer. Ns is less than Ni to the amount of the friction horse power, N,, lost in the engine due to friction. It is calculated by use of the formula N. 2 r I n Brake Horse Power, in which 33,000 I is the length in feet of the lever arm of the absorption brake used in measuring the power of the engine, n the number of revolutions per minute, and W the net weight in pounds on the scales used in weighing the load on the brake.
The Mechanical Efficiency, rim, is the ratio of the brake horse-power, and is expressed as nut = N--J' Test results show an increase in gm with an increase of the ratio of loads, but a decrease of the with an increase of the ratio of compression if pi does not change materially.
(C) Over-all Efficiency.—The product of ei >OM X VIC = Over-all Efficiency, commonly called °Brake Efficiency)) or °Economical Ef ficiency?' It furnishes the means by which the value of any internal combustion engine with regard to fuel economy may be measured.
This economical efficiency, eio, varies with the load of the engine, as shown in Fig. 17, which is the heat-balance diagram of a 15 HPe (effective) Diesel Oil Engine. At nor mal output, 15 H.P. of the engine, 30 per cent of the available heat is converted into useful work, while the remainder is lost in friction, cooling water, exhaust gases and radiation, as illustrated in the heat-balance diagram. Be tween half and maximum load, the economy of the Diesel Oil Engine varies only slightly, in which respect it is superior to the Otto engine which shows a more rapid decrease in economy.