167 
Description of the Gas Engines used. 
Other Producers. 
It is not proposed to describe any of the other producers, 
since the differences between them and the two already dealt 
with consist in detailed matters of arrangement. 
Description op the Gas Engines, used. 
The gas engines used do not in reality require special 
description since, with the exception of two, they were of well- 
known types. They were in fact just the ordinary type of 
horizontal gas engines used with town gas or with pressure 
producer gas. The method of ignition was the only variant 
as regards the former ; because producer gas is not so easily 
exploded as town gas, and therefore the hot tube method of 
ignition is not so suitable and the electric spark method is 
adopted. Certain of the proportions have also to be slightly 
modified, such as the sizes of the valves and the clearance 
space ; the former owing to the difference in the proportion 
of gas to air (namely, 1 to 9 for town gas and 1 to 1J- for 
producer gas) ; the clearance must also be reduced because a 
greater compression pressure is possible without risk of pre- 
ignition, and is also desirable in order to increase the mean 
pressure. Moreover, the incoming charge is at a lower pressure 
in the case of a suction plant, and therefore a greater ratio of 
compression is necessary in order to obtain the desirable 
compression pressure which is from 120 lb. to 150 lb. per 
square inch or even more. The compression ratio was 
measured for some of the engines, and was found to be 
considerably more than that usual for town gas. 
The National engine. — -Fig. 3, page 164, gives an outside view 
of this engine. The cylinder jacket is inserted into a circular 
bored seating in the bedplate and is bolted to a vertical circular 
facing, also on the bedplate. The cylinder liner is a fit in the 
jacket at the front end and is held up by a faced joint at the 
rear end, so that it is free to expand, and the jacket communi- 
cates the explosion thrust to the bed ; there is a rubber ring 
at the front end of the jacket to prevent water leakage. The 
spindles of the exhaust and air valves are vertical and that of 
the gas valve is horizontal. All these valves can be easily 
dismantled by breaking three small joints. The shape of the 
cams driving the valves has been carefully worked out to 
avoid noise. The magneto is driven in a simple way by a 
striker on the 2 to 1 shaft, and there is a freewheel arrange- 
ment to prevent catching if the engine should reverse ; this 
freewheel also contains a device for retarding the spark by 
a definite amount for starting, which device is released by a 
trigger so soon as the engine is running. The connecting rod 
