Condensed from a report to the Third International Automobile Congress, held at Milan, 24th-29th May 1906. Supplied by Thos. Hindle.
By Leon Serpollet (Courtesy of Light Steam Power, March-April, 1971).
Few Changes have been made in automobile steam motors since 1903. All the motors at present employed are very compact, run at high speed and employ high pressure. A compound motor is evidently more economical, but is also heavier, and in small sizes the complication of its parts and their maintenance wipes out any advantages which might be derived from it. Nevertheless, in cases where there is sufficient space available, and where the weight is not limited, such as commercial vehicles operated and cared for by good mechanics, we believe that the compound motor presents considerable interest.
Touring vehicles are generally equipped with one or two types of motor. Those which employ saturated steam are generally fitted with a double-acting double-cylinder motor, with cranks at 90 degrees, and with steam distribution by slide valves, with link motion for the reverse; these motors are generally of the vertical type. Motors running on superheated steam are nearly always single acting, and have a distribution by poppet valves; they are generally of the four-cylinder type, horizontal opposed, with cranks at 90 degrees. Freidman, and later Weyher and Richmond, placed the four cylinders on the same side of the shaft. This entails some complication of the crankshaft and the distributing system, but offers the advantages of a greater accessibility of the connecting-rod bearings and a more compact structure of the engine.
Since 1905 the Serpollet motors are double-acting, but the system of steam distribution by poppet valves has been retained. This arrangement necessitates stuffing boxes on the piston rods, but experience, confirming logical reasoning, has shown that the maintenance pf these stuffing boxes is almost negligible, as they are subjected only to the pressure in the cylinders; moreover, they are of special construction, and the rods are of relatively large diameter and well guided, the cross heads being cylindrical and separating completely the crank chamber with the moving parts from the cylinders. Experience has shown that these stuffing boxes may not require to be replaced in a period of six months of operation.
This new arrangement offers numerous advantages. The oil, being no longer projected against the hot cylinder walls and pistons, remains cool and may be used almost indefinitely: any leakage past the pistons rings (although still objectionable as far as efficiency is concerned) does not result in heating of the crank-chamber and of the parts is contains.
All motors are reversible and work with variable expansion, at least for forward running; almost all driver the road wheels directly, without the intermediary of a change gear, owing property possessed, particularly by flash boilers, of permitting of unusual momentary effort when necessary, as, for instance, in starting.
Lubrication and condensation
Lubrication is generally automatic. Oil is generally fed under pressure to the steam. More than fifteen years of experience has shown beyond the shadow of doubt that it is possible to perfectly lubricate a motor supplied with steam superheated to 500-550 degrees centigrade-932-1022 degrees Fahrenheit-normally, and even more for short periods, without this high temperature in any way prejudicially affecting the life and operation of the pistons and cylinders.
The condensation is generally effected in the plain tube condenser beneath the car supplemented by a flanged tube radiator at the front. The multi-tubular radiators ensure very good cooling, owing to the division of the fluid; but it is necessary to expand the steam in an apparatus presenting a quite large volume before it can be passed into the radiator, particularly if the latter is multi-tubular, in order to avoid back pressure. On this year’s Serpollet model, is used an apparatus called a “recuperator” (feed-water heater), which consists of a nest of small tubes through which is passed the water moved by the feed pump, the tubes being located in a chamber through which circulates the steam exhausted from the motor.
The use of this device on automobiles offers very important advantages. The water passing through the small tubes becomes gradually heated, abstracting heat from the exhaust steam, and arrives in the generator at a temperature sometimes as high as 140 degrees centrigrade-284 degrees Fahrenheit. In this way the fuel economy is appreciably increased, and at the same time the conditions of condensation are improved. We may here give some interesting figures which will complete the remarkable report of M. Turgan at the preceding congress, and his observations on condensers on automobiles.
Let us take as an example a vehicle with a total weight of 3,256lbs., divided as follows: Chassis 1,870lb, body, 550lbs.; supplies (average) 220lbs.; four passengers 616lbs.
This vehicle is fitted, for condensing purposes, with a “recuperator” formed of three parts, which together weigh 46.2lbs., and the radiator at the front weighs 72.6lbs., making together 118.8lbs. The water capacity is 20 gallons. The maximum speed of the vehicle on the level is 40 miles per hour or more. On a trip in undulating country this vehicle regularly accomplishes stages at 125 miles refilling.
Another example is furnished by the 36-passenger omnibus, which never travels at a speed beyond 12.5 miles per hour, has a total loaded weight of 13,000lbs., and mounts the steepest grades without a trace of steam being visible at the end of the condenser. It should be stated that the condensing apparatus of this omnibus is composed of two nests of three recuperating elements, which receive the exhaust steam from the motor, a plain tube condenser, beneath the frame, and finally a flanged tube radiator at the front. The total weight of this condensing apparatus is 440lbs.
In conclusion, we desire to quote results of experiments made to determine the economy of superheated steam. M. Lachaussée, an engineer of Liège, Belgium, has made some very interesting and precise tests along the line. These experiments were made in a 5HP Serpollet car, which had already been in regular use for six months. After all the data necessary for the calculations had been obtained, brake tests were made. The temperature of the steam was 422 degrees centigrade (791 degrees Fahrenheit.), and the pressure was 100PSI. The effective horse-power that absorbed by internal resistances were determined separately. The effective power was found by measurement to be 2.62HP. The mechanical efficiency was found to be 80.7 per cent. During the tests the motor drove the pumps and the automatic lubricator. When the motor operates at its normal power, its mechanical efficiency rises to 88.7 per cent. The consumption of steam was found to be 19.34lbs. Per horse power hour., and the consumption of kerosene 1.90lbs. Per horse-power hour. One pound of kerosene produced 10.17lbs. Of steam superheated 422 degrees centigrade (791 degrees Fahrenheit.) the temperature of the feed-water being 11 degrees centigrade (52 degrees Fahrenheit). M. Lachaussée concludes his report as follows:-
“The motor is a four-cylinder simple-acting (single acting) one. Each of the cylinders operates at 50 per cent cut off, has a clearance of 10 per cent., and during the test developed about 0.65HP. All the conditions are therefore unfavourable to high economy, and yet the consumption (performance?) is that of a good slide-valve engine of 80 to 100HP. This results is an important argument in favour of superheated steam, which reduces very materially the wasteful influences of the dead surfaces.”
The above text is contributed by Mr. Thos. Hindle, who points out that much better economy is obtainable by the use of higher pressure steam with a greater expansion ratio.
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