The compound traction engine has come into use within the past few years, and I am inclined to think more for sort of a novelty or talking point rather than to produce a better engine. There is no question but that there is a great advantage in the compound engine, for stationary and marine engines.
In a compound engine the steam first enters the small or high pressure cylinder and is then exhausted into the large or low pressure cylinder, where the expansive force is all obtained.
Two cylinders are used because we can get better results from high pressure in the use of two cylinders of different areas than by using but one cylinder, or simple engine.
That there is a gain in a high pressure, can be shown very easily:
For instance, 100 pounds of coal will raise a certain amount of water from 60 degrees, to 5 pounds steam pressure, and 102.9 pounds would raise the same water to 80 pounds, and 104.4 would raise it to 160 pounds, and this 160 pounds would produce a large increase of power over the 80 pounds at a very slight increase of fuel. The compound engine will furnish the same number of horse power, with less fuel than the simple engine, but only when they are run at the full load all the time.
If, however, the load fluctuates and should the load be light for any considerable part of the day, they will waste the fuel instead of saving it over the simple engine.
No engine can be subjected to more variation of loads than the traction engine, and as the above are facts the reader can draw his own conclusions.
FRICTION CLUTCH
The friction clutch is now used almost exclusively for engaging the engine with the propelling gearing of the traction drivers, and it will most likely give you more trouble than any one thing on your engine, from the fact that to be satisfactory they require a nicety of adjustment, that is very difficult to attain, a half turn of the expansion bolt one way or the other may make your clutch work very nicely, or very unsatisfactory, and you can only learn this by carefully adjusting of friction shoes, until you learn just how much clearance they will stand when lever is out, in order to hold sufficient when lever is thrown in. If your clutch fails to hold, or sticks, it is not the fault of the clutch, it is not adjusted properly. And you may have it correct today and tomorrow it will need readjustment, caused by the wear in the shoes; you will have to learn the clutch by patience and experience.
But I want to say to you that the friction clutch is a source of abuse to many a good engineer, because the engineer uses no judgment in its use.