602 The Trials of Light Portable Motors at Plymouth. 
Most 
Messrs. 
Otto gas 
economical 
Turner's 
engine 
Otto gas 
Priestman 
- - 
large con- 
non-con- 
using 
engine 
oil engine 
densing 
deiising 
using Dow- 
11 «i n tr Ttrnx 
ii mil i>rui" 
steam 
steam 
gas 
son gas 
bourne oil 
engine 
engine 
(Kennedy) 
Lb. of fuel used per in-~| 
dicated horse-power I 
1-5 
8 461 
20-70' 
1-3 
1066 
Heat value of fuel per \ 
lb. Th. TT J 
U.SO 
l ( rAA 
Heat value of fuel used 1 
Th. U. per hour . . J 
Ol AAA 
in'j KAA 
i ^ aaa 
1 Q OQA 
Equivalent in heat value "l 
of fuel used per hour 1 
per indicated horse- ( 
1-5 
8-461 
0-890 
1-3 
1*387 
power in lb. of coal J 
Cost of fuel used per"! 
hour per indicated \ 
01929 
1-088 
0-7474 
01672 
0 8548 
horse-power in pence J 
cated horse-power per hour, the result is probably unprecedented, 
except in the case of gas engines using coal gas, where the 
actual weight of fuel used is a little less, in at least the trial 
quoted, which represents the highest result as yet obtained with 
the Otto gas engine. Taking, however, the more scientific com- 
parison of the heat value of the fuel, the Priestman engine i3 
better than the best large condensing steam engines, six times 
better than Messrs. Turner's steam engine, and very slightly 
worse than an Otto engine using Dowson gas. Taking the 
market value of the fuel, which of course is a matter varying 
greatly in different localities, so that the comparison can only 
be a rough one, the Priestman engine is very distinctly less costly 
in fuel than the Turner steam engine, and only very slightly 
more costly than a gas engine using coal gas. It is probable 
that the Priestman engine could be used with a less costly oil 
than that employed in the trial. Then the economy of the 
engine would be still more striking. 
Indicator diagram of the Priestman engine. — An indicator 
diagram made from an average of three or four diagrams is 
shown in Fig. 6. It is very similar to an Otto engine 
diagram. 
'Thermal calcidations. — The following short statement rests 
on an assumed value of the heating power of the oil used. It is 
probable, nowever, that if there is any error, the heating value 
of the oil is rathor over-estimated than under-estimated : — 
i These quantities are in c, ft. and Th. U. per <:. ft 
