Probably the last commercial atmospheric engine was built in the 1880’s by Hathorn, Davey and Co. of Leeds / England. The engine, a double acting atmospheric engine without steam expansion, was intended for small businesses, and the advantages listed then were very similar to those we see today:
- low working temperature,
- operation at atmospheric and negative pressures (i.e. safety regulations are much less stringent than for high pressure engines, and the machine can be installed practically anywhere)
Fig. 1 shows the engine itself, Fig. 2 some technical details. Unfortunately, no dimensions are given in the drawings.
a. Overall view b. Cross section
Fig. 1: Davey’s engine
It was however possible to determine the approximate piston area and stroke length from the power, speed and pressure information given in the two articles. The cylinder had an inner diameter of 190 mm (7.5”?), and a stroke length of 150 (6”?) mm (approximate values). With a pressure difference of 85 kPa, the power output was just about 1.1 bhp (or Cheveaux-Vapeur – CV as the French say) or 0.8 kW. A 600 mm diameter flywheel smoothened the power output. Davey’s engine had an overall space requirement of approximately 1.25 × 1.25 m area, and a height of around 2m. The total weight of the machine was an incredible 900 kg.
a. Side elevation b. Cylinder cross section
Fig. 2: Details
In the article in Revue Industrielle, test results are given (see table below). The engine ran with a speed between 118 and 125.6 rpm, and delivered between 0.99 and 1.11 bhp (0.728 to 0.816 kW). Unfortunately, no efficiency figures are mentioned in the article. However, the information given in the article also allowed to calculate the efficiency of the engine from the steam demand of the cylinder, as well as from the water demand for the boiler (as cross check). The analysis first showed, that the engine did not employ steam expansion as was already suggested by the drawings in Fig. 3 (cylinder details). The efficiency determined were:
(a) Cylinder efficiency: 5.2% using the steam demand of the cylinder (cylinder volume times speed).
(b) Total efficiency: 3.6%, calculated with the overall energy demand (water volume evaporated).
The second value includes dead space as well as thermal losses, and gives us a benchmark figure of what we can expect from a real engine. The first value really is a check (it must be higher than the total, and lower than the theoretical efficiency) and an indicator of the power conversion efficiency. The difference between the two values gives us an idea of the total system losses, which amount to 44% of the power output or 31% of the total power production.
In a modern engine, the insulation of the cylinder etc. would be much better so that we could expect a better performance.
So, what do we learn from the engine?
The most important thing is probably that here we have reliable information about the efficiency of a condensing engine. With 3.6%, the engine achieves about 60% of the theoretical efficiency. The efficiency determined from the cylinder is 5.2%, so clearly there are some gains possible e.g. through a better heat insulation (or any heat insulation at all, Davey’s engine does not seem to have such a thing).
There are some interesting technical details, regarding the volume of water required for condensation (heat rejection really is an important topic here), the steam valves etc.
And lastly, and more generally, it shows us how simple things are today. A 1 kW electric motor weighs around 10 kg, it starts immediately, and requires only air cooling…
Test results as given in Revue Industrielle 1885 (1 bhp = 1 Cheveaux-Vapeur CV = 0.735 kW):