162 The Trials of Suction Gas Plants at Derby, 1906. 
removes some of the heat due to the combustion of the fuel, 
this heat being the remainder of the sensible heat in the 
gas after vaporising the water and heating the air supply. 
This heat is wasted and goes away in the scrubbing water. It 
is essential to cool the gas before it enters the engine cylinder, 
in order to reduce its volume and thereby get a greater weight 
of gas into the cylinder, and thus as much energy per explosion 
as possible. A hot incoming charge would have further dis- 
advantages as regards the running of the engine and would 
probably increase the liability to pre-ignition (backfiring). The 
scrubber is usually provided with a water seal to prevent the 
gas in it getting back into the furnace after stopping the engine 
and possibly causing an explosion. This water seal also acts as 
an escape valve in case of an explosion. 
The scrubber was practically the same in all the producers, 
and consisted of a cylindrical vessel partially filled with coke, 
over and through which the water trickled. The water supply 
was effected by means of a rose or a circular pipe with holes. 
In the case of the Crossley producer there was a layer of 
sawdust on the top of the broken coke. 
4. An expansion box to reduce the variation of the suction 
pressure and to give a reservoir from which the engine can 
draw gas, and to lengthen the duration of the suck. 
The furnace proper, or generator, is much the same in all 
producers, the differences being principally confined to the 
shape and arrangement of the firebrick lining and of the details 
of the grate, or fuel base. 
The producers are started by means of hand fans. These 
fans were of various makes and some of them were much 
easier to turn than others. This is an important detail, and 
ease of working will appeal to the attendant. The fans produced 
a pressure in the producer, and there is an advantage in this, 
because any leaks in the producer are at once detected by 
the smoke coming out. These leaks would let air in when 
under suction, and the efficiency would be much reduced. 
To illustrate the principles on which suction gas producers 
are constructed, the producers shown by the National Gas 
Engine Co. and Messrs. Crossley Bros, will be described in detail. 
Producer of the National Gas Engine Company, Ltd. 
The producer of the National Gas Engine Company is shown 
in section in Fig. 2, and an external view is given in Fig. 3. 
In Fig. 3 the generator is on the extreme left, and the 
scrubber in the middle ; the pipes connecting the engine with 
the scrubber are under the floor, and the expansion box is 
immediately under the back end of the cylinder. The furnace 
is clearly shown in Fig. 2, and it will be noticed that it is of 
