Wenham's air engine

Source: an article from Scientific American
Date: June 14th, 1873
Title: Wenham's Heated-air Engine

At a recent meeting of the Institution of Mechanical Engineers, in London, England, Mr. C. W. Cooke, in a paper on the subject, gave the following details regarding Wenham’s hot air engine, an engraving of which we reproduce from the pages of the Engineer. This machine belongs to that class in which the fire is inclosed and fed by air pumped in beneath the grate to maintain the combustion, the larger portion of the air entering above the fire to be heated, the whole, together with the products of combustion, being made to act on the piston.

Our illustration gives a sectional view of an engine of this description of one horse power. A special feature is the furnace shown at A, in which perfect combustion is obtained from ordinary bituminous coal, which is generally preferred for this engine. The space under the grate is separated from the upper part by a moderately air-tight diaphragm, and above the grate is an annulus of segmental fire bricks, as shown, with semi-cylindrical grooves at their joints, so that when placed together the center forms a cylindrical hopper containing a store of fuel sufficient for several hours’ work, and the grooves at the joints form a series of vertical flues through the bricks.

The column of coal descends as it is consumed on the furnace bars, and, the air coming into contact with nothing but coal in a state of intense ignition, all the products of combustion must pass through the ignited portion. The channels in the fire bricks that serve as flues being also white hot, no unconsumed fire gases can pass through.
Wenham's Heated Air Engine
The furnace has a cover, by which it can be hermetically closed in front of the ash pit, shown at Z, and there is a similar cover for filling the coal hopper at Y. The products of combustion, after leaving the channels of the bricks, are met by a baffle plate, W, lined with fire clay, which prevents the cover of the furnace from getting unduly hot.

The fire bricks are separated from the outside shell of the stove by a ring of powdered brick or ashes. There are two cold air supply inlets to the fire, the one below the fire, at T, and the other above the fire, at S; and there is a swing valve, by which more or less of the air supply is directed below or above the fire. If all the air be directed be- be comparatively cool with less increase of volume, and there will be a diminution in the power of the engine.

This difference of power serves as a very effective means of regulating the speed of the engine, and the governor was consequently attached to the lever of the swing valve at S. No other regulation for speed is required, giving this advantage that the combustion of the coal is exactly proportioned to the amount of work performed. The engine is of the steeple form, having two piston rods placed diagonally, with the main or crank shaft running between them; and in order to make the cylinder as compact as possible, the cover of it is provided with a segmental chase or depression, in which the crank passes.

The engine is single acting, the air pressure acting on the under side of the piston only; the air is admitted from the heater by means of a puppet valve, E, moved by a cam, at G, on the main shaft.
There is a similar valve, also moved by a cam, which opens from the cylinder to the exhaust. The valve is shown at F.

The chief peculiarity in this engine is the method by which the top of the cylinder serves as the air pump, and is made to convey into the heater for expansion the reduced bulk of air required for the due performance of the engine. The top of the piston does not reach the cylinder cover, but there is a clearance space left between them. The result is that the pressure in the heater should never exceed 15 lbs. on the square inch; the extent of this pressure is obtained entirely by the amount of clearance space above the piston, the action of which may be thus explained: The piston rises until it compresses the air contained in the air pump to half its volume, or to a pressure of 15 lbs. per square inch, and not till then does there exist equilibrium between the air in the air pump and that in the heater.

The pump valve, Q, then opens, and during the remainder of the stroke, air is pumped into the furnace. At the end of the stroke the valve, Q, closes, leaving still 15 lbs. pressure in the space above the piston. As there is no further escape for this, it acts upon the piston during part of the down stroke, and equalizes the action of the engine; a small fly wheel only is therefore required.

This is, of course, not any advantage in power, for whatever power is required in order to obtain this pressure of 15 lbs. above the piston must be deducted from the force of the up stroke; it is on1y transferred from the lower side of the piston to be utilized above by the subsequent expansion of the compressed air. After the expansion has ceased, the inlet air pump valve below Q opens, and admits the quantity of cold air required for the next stroke of the engine.

Mr. Cooke observed that air engine on this principle cannot be worked advantageously at a high pressure. The air is found to enter the working cylinder at 1127 F., and to leave it at 466° F. About 8 lbs. of coal per horse are found to be consumed per hour.

There can be no doubt that, in many applications requiring a few horses’ power, this engine will be found very useful, and the verdict of the discussion tended to this direction. The manufacturers do not, of course, put it forward in competition with the ordinary steam engine, but rather as a fairly economical, non-explosible, small prime motor.