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Saturday, June 22, 2013

Nearby Corliss Steam Engine

After putting together a series of blog posts about the history of steam engines, I ran across one in my own backyard!  Being new to the area, we were playing tourist, and poking around in the nearby town of Newport, Washington.  Imagine my surprise when we rolled up on this monster!  Yep, that's a standard bus stop bench in front of it. 

According to the sign, it put out 478 horsepower and the big wheel turned 100 RPM. 

This got me thinking about what it must have been at the turn of the 20th century to see one of these beasts in operation... So I checked Youtube, and guess what?  They have a few videos of Corliss steam engines running.  Check them out!

Below is a 500 Horsepower Corliss engine that was once used to power looms in a textile factory.  It's a little freaky watching that massive connecting rod move back and forth so quickly.  Most impressive in fullscreen mode!

Unfortunately the above video doesn't clearly show the steam valve train.   But happily, I found another video that does.  You can easily see how the eccentric wheel operates the wrist-plate, which in turn drives the entire valve train.  I spent a little time trying to understand the purpose of the vertical rods dangling from the upper steam inlet valves and learned something. 

The inlet valves do not shut when the wrist-plate rocks back.  Instead they are closed by the governor tripping them each cycle.  The vertical rods, which are attached to dashpots, allow the steam valves to close more slowly than if they were shut by the wrist-plate, admitting more steam to the cylinder.  An early (and succesful!) type of variable valve timing.

Once again I will turn the valve train explanation over to Wiki, who does a far better explanation than I ever could:

"The inlet valves are pulled open with an eccentric-driven pawl; when the pawl trips, the rapid closure is damped using a dashpot. In many engines, the same dashpot acts as a vacuum spring to pull the valves closed, but Corliss's early engines were slow enough that it was the weight of the dashpot piston and rod that closed the valve.

The speed of a Corliss engine is controlled by varying the cutoff of steam during each power stroke, while leaving the throttle wide open at all times. To accomplish this, the centriugal governor is linked to a pair of cams, one for each admission valve. These cams determine the point during the piston stroke that the pawl will release, allowing that valve to close.

As with all steam engines where the cutoff can be regulated, the virtue of doing so lies in the fact that most of the power stroke is powered by the expansion of steam in the cylinder after the admission valve has closed. This comes far closer to the ideal Carnot cycle than is possible with an engine where the admission valve is open for the length of the power stroke and speed is regulated by a throttle valve."

And lastly, a drawing of this complex arrangement, showing the pawls that open the inlets, and the vertical rods with attached weights.

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