8 Walker, The Poly tropic Curve 



tionship between yj and the compression ratio r, it is striking 

 to notice that in each line of this series of graphs there is a 

 definite maximum efficiency value and no minimum as in the 

 series just discussed and represented by the dotted lines of 

 the same figure. These maximum efficiency points of these 

 full line curves have already been pointed out in the last article 

 to which reference has been made in this paper. It is raised 

 here again owing to the curious inverse nature of these 

 efficiency curves in relation to those of the cycle of Fig. i. It 

 should be noted, as stated before, that the only difference 

 between the two cycles is that when the fluid is in the state 

 ^ 4 ^ 4 T 4 , in either case, the state p v T is arrived at in the one 

 case represented by Fig. i, by the same type of poly tropic 

 compression throughout the whole of the stroke, while in the 

 other case, represented by Fig. 2, the same state p v T is 

 arrived at after two types of polytropic compression, namely, 

 constant pressure compression for the first part of the stroke 

 and then adiabatic compression for the remainder of the stroke. 

 The full line curves of Fig. 3 also show that the value of 

 r, the compression ratio at which maximum efficiency occurs, 

 diminishes as the value of p increases. Thus, at a value of 

 p = unity, maximum efficiency is obtained when 7=14. When 

 p = 2, maximum efficiency is derived at a compression ratio 

 7=10. For a value p = 3, maximum efficiency occurs when 

 7 = 8 and so on. This obviously falsifies the claim that the 

 Diesel cycle is theoretically the most efficient between given 

 pressure and volume limits. There is, of course, no question 

 of its relatively high efficiency as compared with contemporary 

 engines, but a perusal of Diesel's book, " The Rational Heat 

 Motor," is convincing enough evidence that Diesel himself 

 was too obsessed with the high compression ratio fetish to 

 work the problem out theoretically to its logical conclusion. 

 His primary conception was to adapt the Carnot Cycle to 

 internal combustion principles, and it appears to have required 

 actual experiment to convince him that the problem of engine 

 weight was bound to intervene and require compromise. This 

 led him from constant temperature to constant pressure com- 

 bustion. Even then he persisted in retaining the operation 

 of rejection of heat at constant temperature, and there again 

 experiment appears to have been necessary to convince him 

 of a fact which theoretical investigation would have revealed. 

 In spite of this, however, Diesel's work appears to have been 

 in large measure the direct cause of the prevailing tendency 

 to rely upon compression ratio as the determining factor in 

 engine efficiency. Many engineers, no doubt, will be seep- 



