176 The Trials of Suction Gas Plants at Derby, 1906. 
The reason is that, when the gas valve is shut, only air is drawn 
in during the suction stroke ; and, owing to the throttling action 
of the air inlet valve, the compression begins at from 1 lb to 4 lb. 
per square inch below atmospheric pressure. This action is 
well shown in Fig. 25 and again in Fig. 11 where the two 
compression lines are clearly indicated ; in the latter case the 
throttling action of the air valve was considerable. 
It will be noticed in Fig. 10 that the maximum pressure 
after a miss is generally less than at other times ; this effect was 
observed in all the other engines with the exception of the 
Railway and General engine, but more especially of the Dud- 
bridge engine in which there was a great increase in maximum 
pressure (from 360 lb. to 490 lb. per square inch) after each miss. 
Fig. 12 is an ordinary indicator diagram taken with a weak 
(or “ light ”) spring ( T V) and shows the suction line previous 
to an explosion, that is, when both air and gas valves are 
open. It will be noticed that the pressure in the cylinder 
just before the beginning of compression is very little less 
than atmosphere, so that a full weight of charge is obtained. 
Fig. 13 is the Mathot diagram for the National engine at half 
load and Fig. 14 is a diagram of the same engine at no load 
after running for some considerable time in this way. Fig. 15 
is also a no load Mathot diagram of the same engine, but was 
taken a few minutes after the change from full load to no load 
was made. The very high maximum pressure observed is due 
to the richness of the gas obtained under these conditions : the 
effect, however, only lasted for a few minutes. Mathot cards 
taken on changing from full load to no load with the Crossley, 
Railway and General, Fielding, and Kynoch engines gave 
precisely the same effect. 
Fig. 16 is an ordinary indicator card from the National 
engine when working at full load with coke, and Fig. 17 is the 
corresponding Mathot card. No great difference is observable 
between Figs. 17 and 10 ; with most of the other plants, how- 
ever, considerably less pressure was obtained with coke than 
with anthracite. 
Figs. 18 to 25 are diagrams for the Crossley engine taken 
under similar conditions to those just described for the National 
engine. In Fig. 19 the drum working the Mathot indicator 
was set to run faster ; hence the closeness of the lines. The 
higher maximum pressure obtained in the Crossley engine 
is due to the higher ratio of compression. Comparing Figs. 24 
and 19 it will be seen that in the case of this engine the maxi- 
mum pressure reached with coke is less than that obtained 
with anthracite. Fig. 25 is a weak spring diagram and gives 
the suction and compression lines both for a “ hit ” and for a 
“ miss ’’ stroke ; the throttling action of the air valve when 
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