Comparing Gas, Steam and Hot Air Engines

Source: The Gas and Oil Engine
Title: The Gas Engine Method
Author: Dugald Clerk, Member of the Institution of Civil Engineers, Member of the Royal Institution
Date: First printed September 1886 - Revised Edition November 1899

The Author's point is to give an explanation of the Hot Air Engines and Steam Engines failure as modern engines, and the causes of the Gas Engine success.

Gas Engines differ from other Heat Engines

Gas engines, while differing widely in theory of action and mechanical construction, possess one feature in common which distinguishes them from other heat engines : that feature is the method of heating the working fluid.

The working fluid is atmospheric air, and the fuel required to heat it is inflammable gas. In all gas engines yet produced, the air and gas are mixed intimately with each other before introduction to the motive cylinder ; that is, the working fluid and the fuel to supply it with heat are mixed with each other before the combustion of the fuel.

The fuel, which, in the steam and in most hot-air engines, is burned in a separate furnace, is, in the gas engine, introduced directly to the motive cylinder and burned there. It is indeed part of the working fluid. This method of heating may be called the gas-engine method, and from it arises at once the great advantages and also the great difficulties of these motors.

The Steam Engine and its limitations

Compare first with the steam engine. In it there exist two great causes of loss : water is converted into steam, absorbing a great amount of heat in passing from the liquid to the gaseous state ; after it has been used in the engine it is rejected into the atmosphere or the condenser, still existing as steam.

The heat necessary to convert it from the liquid to the gas is consequently in most part rejected with it. Loss, occurring in this way, would be small if high temperatures could be used ; but this is the point where steam fails. High temperatures cannot be obtained without pressure so great as to be quite unmanageable.

The attempt to obtain high temperatures by super-heating has often been made, but without any substantial success. Although the difficulty of excessive pressure is avoided, another set of troubles are introduced. All the heat to be given to the gaseous steam must pass through the iron plates forming the boiler or super-heater, which plates will only stand a comparatively low temperature, certainly not exceeding that of a low red heat, or about 600° to 700° C.

Steam, being a gas, is much more difficult to heat than water ; it follows that even these temperatures cannot be attained without enormous addition to the heating surface. The difficulties of making a workable engine using high temperature steam are so great that even so distinguished an engineer and physicist as the late Sir C. W. Siemens failed in his attempts, which extended over many years.

It may be taken then that low temperature is the natural and unavoidable accompaniment of the steam method, arising from the necessary change of the physical state of the working fluid, and the limited temperature which iron will safely bear.

The Hot Air Engines and their troubles

The originators of the science of thermodynamics have long taught that the maximum efficiency of a heat engine is obtained when there is the maximum difference between the highest and lowest temperatures of the working fluid.

So long ago as 1854, Professor Rankine read a paper before the British Association, ' On the means of realising the advantages of the Air Engine,' in which he expresses his belief that such engines will be found to be the most economical means of developing motive power by the agency of heat.

In this opinion he stood by no means alone. Engineers so able as Stirling, Ericsson, and Siemens ; physicists so distinguished as Dr. Joule, and Sir Wm. Thomson, devoted much energy and study to their practice and theory. Notwithstanding all their efforts, aided by a host of less able inventors, the difficulties proved too formidable ; and although more than thirty years have now passed since Rankine announced his belief, the hot-air engine proper, has made no real advance.

Similar causes to those acting in the steam engine impose a limit here. It is true, the complication of changing physical state is avoided, but the limited resistance of iron to heat acts as powerfully as ever. Air is much more difficult to heat than water, and, therefore, requires a much larger surface per unit of heat absorbed. In the larger hot-air engines, accordingly, the furnaces and heating surfaces gave great trouble.

Very low maximum temperatures were attained in practice. In a Stirling engine giving out thirtyseven brake horse-power, the maximum temperature was only 343° C. ; in the engines of the ship ' Ericsson,' the maximum was only about 212° C., according to Rankine, the indicated power being about 300 horses. These figures show that the heating surfaces were insufficient, as in both cases the furnaces were pushed to heat the metal to a good red.

A method of internal firing was proposed, first by Sir George Cayley and afterwards carried out with some success by others ; the furnace was contained in a completely closed vessel, and the air to be heated was forced through it before passing to the motor cylinder.
The plan gave better results, but the temperature of 700° C. was still the limit, as the strength of the iron reservoir had to be considered, and the hot gases had to pass through valves.

Wenham's engine, described in a paper read before the Institution of Mechanical Engineers in 1873, is a good example of this class. In it the highest temperature of the working fluid, as measured by a pyrometer, was 608° C. ; higher temperatures could easily have been got but the safety of the engine did not permit it.

Professor Rankine in his work on the steam engine has very fully discussed the disadvantages arising from low maximum temperatures. He calculates that in a perfect air engine without regenerator an average pressure of 8.3 Ibs. per square inch would only be attained with a maximum of 216.6 Ibs. per square inch, thus necessitating great strength of cylinder and working parts for a very small return in effective power.

In the ' Ericsson,' the average effective pressure was less than this, being only about 2 Ibs. per square inch ; it had four air cylinders each of 14 feet diameter, and only indicated 300 horsepower. Stirling's motor cylinder did not give a true idea of the bulk of the engine, as the real air-displacer was separate. Even with Wenham's machine the bulk was excessive, an engine of 24 inches diameter cylinder and 12 inches stroke giving 4 horse power.

Those facts sufficiently illustrate the practical difficulties which prevented the development of the hot-air engine proper. All flow from the method of heating. Low temperature is necessary to secure durability of the iron.

All hot-air engines are, therefore, very large and very heavy for the power they are capable of exerting.

The friction of the parts is so great that although the theoretical efficiency of the working fluid is higher than in the best steam engines, the practical efficiency or result per horse available for external work is not nearly so great.

The best result ever claimed for Stirling's engine is 2.7 lbs. of coal per break horse-power per hour, probably under the truth, but even allowing it, a first class steam engine of to-day will do much better.
According to Prof. Norton, the engines of the ' Ericsson ' used 1.87 Ibs. of anthracite per indicated horse-power per hour ; but the friction must have been enormous.

Compared with the steam engine, the practical disadvantages of the hot-air engine are much greater than its advantage of theory.

Owing to the great inferiority of air to boiling water as a medium for the convection of hear, the efficiency of the furnace is much lower ; owing to the high maximum and low available pressure, the friction is much greater which disadvantages in practice more than extinguish the higher theoretical efficiency.

The reasons of the Gas Engine success

The gas engine method of heating by combustion or explosion at once disposes of those troubles ; it not only widens the limits of the temperatures at command almost indefinitely, but the causes of failure with the old method become the very causes of success with the new method.

The difficulty of heating even the greatest masses of air is quite abolished. The rapidly moving flash of chemical action makes it easy to heat any mass, however great, in a minute fraction of a second ; when once heated the comparatively gradual convection makes the cooling a very slow matter.

The conductivity of air for heat is but slight, and both losing and receiving heat from enclosing walls are carried on by the process of convection, the larger the mass of air the smaller the cooling surface relatively. Therefore the larger the volumes of air used, the more economical the new method, the more difficult the old.

The low conductivity for heat, the cause of great trouble in hot-air machines, becomes the unexpected cause of economy in gas engines. If air were a rapid carrier of heat, cold cylinder gas engines would be impossible. The loss to the sides of the enclosing cylinders would be so great that but little useful effect could be obtained. Even as it is, present loss from this cause is sufficiently heavy.
In the earlier engines as much as three-fourths of the whole heat of the combustion was lost in this way ; in the best modern engines so much as one-half is still lost.

A little consideration of what is occurring in the gas engine cylinder at each explosion will show that this is not surprising. Platinum, the most infusible of metals, melts at about 1700° C; the ordinary temperature of cast iron flowing from a cupola is about 1200° C.; a temperature very usual in a gas engine cylinder is 1600° C., a dazzling white-heat.
The whole of the gases filling the cylinder are at this high temperature. If one could see the interior it would appear to be filled with a blinding glare of light.

This experiment the writer has tried by means of a small aperture covered with a heavy glass plate, carefully protected from the heat of the explosion by a long cold tube. On looking through this window while the engine is at work, a continuous glare of white light is observed. A look into the interior of a boiler furnace gives a good notion of the flame filling the cylinder of a gas engine.

At first sight it seems strange that such temperature can be used with impunity in a working cylinder ; here the convenience of the method becomes evident. The heating being quite independent of the temperature of the walls of the cylinder, by the use of a water-jacket they can be kept at any desired temperature. The same property of rapid convection of heat, so useful for generating steam from water, is essential in the gas engine to keep the rubbing surfaces at a reasonable working temperature. In this there is no difficulty, and notwithstanding the high temperature of the gases, the metal itself never exceeds the boiling point of water.

So good a result cannot of course be obtained without careful proportioning of the cooling surfaces for the amount of heat to be carried away ; in all modern engines this is carefully attended to, with the gratifying result that the cylinders take and retain a polished surface for years of work just as in a good steam engine.

The gas engine method gives the advantage of higher temperature of working fluid than is attainable in any other heat engine, at the same time the working cylinder metal may be kept as cool as in the steam engine. It also allows of any desired rate of heating the working fluid in any required volumes. In consequence of high temperatures the available pressures are high, and therefore the bulk of the engine is small for the power obtained.

It realises all the thermodynamic advantages claimed for the hot-air engine without sacrificing the high available pressures and rapid rate of the generation of power which is the characteristic of the steam engine. For rapid convection of heat existing in the steam boiler is substituted the still more rapid heating by explosion or combustion, a rapidity so superior that the power is generated for each stroke separately as required, there being no necessity to collect a great magazine of energy.

The only item to the debtor side of the gas engine account is the flow of heat through the cylinder walls, which disadvantage is far more than paid for by the advantages.