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The Double-acting Piston and the Rotative Engine

The double-acting piston was an important improvement for the steam engine. The steam piston for this engine still reciprocated relatively slowly. For mechanical mine water pumping with the early engines, the upward and downward piston strokes took different lengths of time, but it really didn't matter that the reciprocating was uneven for pumping water. For operating factory machinery, this was not acceptable, and the double-acting engine described here operated smoothly and identically in both directions.

Watt Rotative Engine 1788
Manufacturer: Charles Summerfield (full-scale working replica of a Boulton-Watt engine)
10 hp
50 rpm
17" bore
48" stroke

In this engine, both the upper and lower cylinder sections are connected to the condenser and boiler using valves. The engine beam is just visible at the top of the photo. The right end of the beam is connected by an arm to a sun and planet gear system that turns the large engine wheel. The engine wheel is in the rear of the photo. This large engine wheel has teeth on the outside edge to permit it to turn machinery in the factory.

This Watt Rotative Engine is housed in the Henry Ford Museum in Dearborn, MI.

This photo at left is taken looking downward along the safety railing shown above, near the engine wheel. (The spokes of the engine wheel can be seen on the left edge of the left photo.) The sun gear is connected to the shaft of the engine wheel. The planet gear is connected to the arm that runs up to the beam. The sun and planet gear is explained below.


Figure 1. Schematic of double-acting engine. (Adapted from the engraving of Stuart, 1824, p 128).

This engine uses two identical strokes. Note that the valve mechanisms of Figure 1 on the upper and lower sections of the piston are identical. There are two pipes to the left of the steam cylinder connecting the upper and lower valves, (only one pipe is drawn in the cross section of Fig 1). These two pipes enable both cylinder ends (above and below the piston) to alternately be connected with the boiler and the condenser. When one end is connected to the boiler, the other end is connected to the condenser. Thus the upward and downward stokes are identical. This also means that the beam can be balanced since the piston does equal work on both motions. Note that Figure 1 shows a cast iron beam instead of the wooden beam used in Watt's earlier pumps.

Another change that this engine incorporates is the elimination of the chain connecting the beam and the steam piston rod. This is done with a mechanism called the parallel motion mechanism. This is the invention that Watt told his son made him the most proud, even though the public remembers Watt for his other contributions. This mechanism makes it possible to match the linear motion of the piston with the arc motion of the beam. It is also rigid enough to provide pushing as well as pulling by the piston.



Figure 2. Speed Governor. (Adapted from the engraving of Stuart, 1824, p 133).

Note the speed governor in the center of Figure 1. The speed governor is connected to the engine wheel (not shown) though a chain and pulley at the base of the governor. The chain spins the governor when the engine wheel turns. When the engine starts to speed up, the governor spins faster, and centrifugal force will force the balls to rise as shown by the lighter outlines in Figure 2. The position of the balls is an indication of the rotational speed. Watt did not invent this device; the device was already in use for other rotating machinery. Watt invented the method to feed this speed signal back to the steam throttle to control the rate steam is admitted to the cylinder.

When the balls spread, the lever n-f-g at point n will be pulled down, causing point g to rise. In Figure 1, this motion will tend to close the throttle valve, decreasing steam flow into the piston, and slowing the engine. A slowing of the engine will create the opposite effect on the governor and throttle.


Figure 3. Sun and Planet Gear System. (Adapted from the engraving of Stuart, 1824, p 136).

Watt invented a sun and planet gear system to be connected to the beam. The crankshaft (like a bicycle pedal) achieves the same basic motion and is much simpler than the sun and planet system employed here. The crankshaft already was invented and patented by others but wasn't used here anyway. (Watt said that a machinist stole crankshaft idea from him after he had described it, but was too busy to worry about litigation). The reason that the sun and planet is superior for this application is that a crank will only turn a wheel once per stroke of the piston. This device, although more complex, can turn a wheel faster. Although this device isn't used very much any more because of it's complexity and because we now use pistons that reciprocate very rapidly, the device is significant for getting slow linear motion converted to rotative motion at acceptable speeds.

The large wheel in Figure 3 is the engine wheel. This wheel was used as the driving pulley for a large belt. The belt often powered a pulley system suspended from the factory ceiling. The pulley system distributed rotative power throughout the factory, mill, etc.


Updated 11/22/12 ,Copyright 2001-12, Carl T. Lira, lira@egr.msu.edu. All rights reserved.
Prepared as a supplement to Introductory Chemical Engineering Thermodynamics.