While James Watt’s improvements to the steam engine may have been responsible for giving birth to the industrial revolution it would be the contribution of George Corliss that made it profitable. Watt’s contribution to engine design was one of practicality and reliability. Corliss’s contribution was one of efficiency, creating engines that took advantage of every last drop of energy available to it. Because of these improvements steam engines were finally able to surpass water power in terms of economic feasibility and surge ahead to become the de factor power source of the late 19th century.
On its surface the Corliss styled engine is very similar to its Watt’s style brethren. Steam is delivered into a large cylinder through means of a throttle valve, which is automatically controlled by the engine’s governor. Once inside the cylinder that steam expands and pushes the enclosed piston forward. As the piston moves so to does the connected piston rod, which in turns pushes the crosshead forward along its track.
Meanwhile on the engine’s business end that reciprocating motion of the crosshead is converted into circular motion by means of a connecting rod and crank. The crank rotates a disc which in turn spins the engines drive shaft and connected flywheel. This is all typical when it comes to steam engine design (see my original steam engine post HERE for more info). To see what differentiates this Corliss engine from other steam engines of the period you have to look inside the cylinder itself…
Here’s a cutaway view of a double-acting cylinder typical to an old-school steam engine. Before being allowed to act on the piston head itself, the steam is first drawn into a hallow chamber on top of the cylinder known as the steam chest. From there the steam is then sent down into the cylinder through a pair of inlets – one for each side of the piston. The movement of steam into these inlets is controlled by a slide valve, which reciprocates back and forth inside the steam chest. As the slide valve opens one inlet, it simultaneously redirects the other to an exhaust port. This way steam exhausts one side of the piston while its filling the opposite side.
Now here’s a Corliss engine. The high efficiency of the Corliss design is due to the inclusion of two major innovations – the first being the use of a rotary valve. Instead of a typical sliding valve used in previous engines, the Corliss engine utilized a series of four cylindrical valves which rocked back and forth within hollow tubes. These valves served only one specific purpose – either as an outlet or an inlet. The inlet valves sat along the top of the cylinder while the outlet valves were located down along the bottom (a design that allows for the easy expelling of condensate from the cylinder as well).
Taking a look at the 1898 engine still residing inside the Quincy Smelter engine room you can make out the housings for those four rotary valves on the outside of the cylinder – one on each corner.
Controlling when those valves would open and close – known as valve timing – was a tangle of rods and levers attached to the outside of the cylinder. At its center – literally – was a metal wheel known as the wrist plate. The wrist plate was connected to each of the engine’s valves through four short connecting rods. The rods attached to the inlet valves were known as steam rods, while those attached to the exhaust valves were known as exhaust rods. Each of these rods were connected to the valve stems themselves by means of short levers. Also connected to the steam valves were two additional rods set horizontally known as dash pot rods. These rods rested within a pair of dash pots set into the base of the engine.
The entire system of rods and levers were driven by the hook rod (A) which was pushed by the engines own motion to reciprocate back and forth in a horizontal motion. This would in turn rotate the wrist plate (B) back and forth (in much the same way as your wrist moves to turn a screwdriver – thus its name). The back and forth rotation of the wrist plate forces both the steam rods (C) and exhaust rods (D) to reciprocate back and forth as well. As they do the steam and exhaust arms rotate and turn the valves themselves. As this happens, the dash pot rods (E) are raised up and then dropped down into the dash pots over and over again. (We’ll look at the purpose of these dash pots and rods a little later.)
Here’s a look at the same linkages as visible on the 1898 steam engine found within the Quincy Smelter’s engine room. In this photo you can clearly make out the wrist plate and steam rods, as well as one of the dash pot rods.
The other steam engine in the room – the one used to drive the generator – is another Corliss design. However this particular model was known as a double eccentric engine, due to its use of two hook rods to drive the wrist plate. Besides the additional hook rod and alternate wrist plate design, the rest of the linkages work the same as they do in the 1898 model.
Another unique element of a Corliss engine is how the valves were closed. In classic engines the valves were closed gradually as the slide valve was pushed over the top of the steam inlet. Though this approach worked, it was highly inefficient. To increase efficiency those valves should be closed as quickly and immediately as possibly. Th Corliss engine accomplished this through the use of those dash pot rods we discussed earlier.
In the case of the inlet valves, the motion of the steam rod did not directly turn the steam valve itself. Instead the steam rod was attached to a double arm which sat loosely on the valve stem. Attached to this double arm was a large hook, known as the steam hook. As the steam rod moved back and forth, its movement was transferred through the double arm to raise and lower the steam hook. As the steam hook raised upward it would pull up along with it the actual steam arm which would turn the valve – an arm also known as the dash pot arm. Connected to this arm was the vertical dash pot rod, its other end sitting inside a dash pot at the base of the engine. It was this rod which enabled the valve to close quickly and immediately, using nothing more than gravity to assist it.
Sitting between the dash pot arm and the double arm was a knock-off cam. A metal spring on the steam hook held the hook tight against this cam as it travelled upward with the dash pot / steam arm in tow. At the top of the cam is a short protrusion which forces the steam hook to open and let go of the dash pot / steam arm. The dash pot / steam arm then drops down suddenly into the dash pot due to gravity and the steam valve is closed. The steam hook then travels back down to pick up the dash pot / steam arm once again to repeat the process all over again.
To help better understand how this process worked, take a look at this video of a corliss engine in action. It becomes much more clearer.
Here’s a closer look at this entire contraption on one of the steam valves from the steam engine from the smelter. You can clearly make out the steam hook, double arm, and dash pot arm all attached to the valve stem. In this photo the steam hook has picked up the dash pot arm and lifted it halfway up the knock off cam before the engine was stopped. The knock off cam is hidden under a pile of dirt and debris in the center of the photo. If the engine was still running the steam hook would pull the dash pot arm up an inch or two more before letting it go and allowing it to drop.
The timing of the valves and motion of the attached rods and wrist plate are all powered by the engine itself, compliments of an eccentric mechanism attached to the engine’s drive shaft. An eccentric is a mechanical device which converts rotary movement into reciprocating motion. That motion is transferred along an eccentric rod to a lever known as a carrier arm. The carrier arm then transfers the reciprocating motion onwards to the hook arm.
Taking a closer look at the business end of the old 1898 engine we can clearly see the eccentric mechanism attached to the engine’s drive shaft. Connected to eccentric is the eccentric rod, which would have been pushed back and forth with the motion of the drive shaft.
Further up the line we come across the carrier arm and hook rods. This time we’re taking a look at the newer engine at the smelter, the one which runs the generator. This particular engine is a double eccentric, so there are two hook arms present in the photo (one for the steam valves and one for the exhaust valves).
Also visible in this photo is the engine’s governor. The governor utilizes a pair of weighted balls which spin at the same speed as the engine itself. As the engine’s speed increases, the balls spin faster as well. Due to centrifugal force those balls began to move outward which in turn pulls up on a lever at the governor’s base. In classic engine design this lever was attached to the throttle and controlled the amount of steam entering the cylinder. As the engine’s speed increased the amount of steam entering the cylinder was decreased.
But in a Corliss engine steam always enters the cylinder at full boiler pressure, which meant that the throttle was left in a full open position. In order to control the amount of steam entering the cylinder a new and improved approach was used instead, an approach which was the Corliss engine’s second innovation: variable valve timing.
Variable valve timing meant that the length of time each valve was left in the open position could be decreased or increased as needed. In classic engine design the amount of time its slide valve was in the open position was always constant, and thus the throttle had to be used to control the amount of steam entering the cylinder. With variable valve timing the throttle could be taken out of the equation completely. At higher speeds the valves were simply opened for shorter periods of time.
In much the same was as the governor once controlled the throttle in old engine design, a Corliss engine uses the governor to control the length of time each valve is open. This was done by attaching a pair of rods from the bell crank on the governor to each of the steam valves knock off levers. As the engine ran faster and the governor spun faster, the rods would turn the knock off cams closer to the steam hook. This way the hook would disengage the dash pot rods – and in turn close the valves – sooner then they would have previously. As the engine slowed back down those rods would push the knock off cams further away from the steam hook and allow the valves to stay open longer before the hook disengages.
With a better understanding of how a Corliss engine worked and how it was different then other steam engines now in hand (hopefully), it was time to move on to the part of the smelter complex that provided these engines with their steam. The boiler complex.