158 The Trials of Suction Gas Plants at Derby , 1906. 
The suitability of the design for manufacturing on the 
interchangeable system comes under this heading. It is a 
specially important matter for agricultural engines to insure 
that spare and repair parts shall go into place without 
fitting. 
The regularity of running, when using coke as a fuel, 
depends on the amount of tar deposited on the valve seats. 
In the present state of the art no adequate provision can be 
made in the producer for preventing the formation of tar if the 
coke contains it in any material quantity. The ease with which 
the valves can be got at and cleaned is therefore of more than 
usual importance, and much stress was laid on this point. In 
this connection it is well to point out that gas coke varies 
considerably in regard to its freedom from tar, and it is to be 
hoped that when suction gas plants become more general, an 
arrangement will be made whereby some standard of permis- 
sible percentage of tar can be insured. The following points 
also come under this section : Ease of dismantling connecting- 
rod and piston, also main bearings ; the method of attaching 
the cylinder to the frame ; the various important scantlings, 
especially those of moving parts ; the material used and the 
workmanship, especially with regard to interchangeability ; the 
lubrication of the bearings, of the cylinder and of the piston. 
No comment is needed as regards the space occupied. 
3. Regularity of ivorking . — Apart from the effect tar has 
on the valves, as already pointed out, the majority of the 
engines can be relied upon for working with a minimum of 
trouble, as judged from previous long experience with town 
gas and pressure producer gas. The suction producer is there- 
fore that portion of the plant which required most attention. 
The gas is only made as required, and when working for any 
length of time at a particular load the conditions in the 
producer will adjust themselves to that load. If a sudden and 
considerable change of load then occurs, the producer will take 
a little time to adjust itself to the conditions required by the 
new load. The rapidity with which a suction producer will 
readjust itself is of considerable importance. A change from 
light load to full load is the most severe, because in this case 
the quality of the gas is poor and it is unable to give the power. 
In the reversed change of load the governor takes charge. 
Special provision has to be made for dealing with prolonged 
periods of light load, as, for instance, having a bye-pass through 
which a portion of the gas is blown to waste each stroke ; 
otherwise there might not be sufficient suction to keep the fire 
alight. A special trial was framed to test the capabilities of 
the producers to respond to changes of load based on the light 
or no load trial in the regulations (No. 9). 
