Thermo-dynamic Characteristics of GASEOUS FUELS
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AN article in a recent, issue of this journal was devoted to a survey of outstanding thermochemical aspects of some of the more commonly
• used gaseous fuels. The opportunity will now be taken of reviewing such fuels from a different angle—that of their thermo-dynamic performance in high-speed internal-combustion engines.
Whilst the knowledge available on the subject is not by any means so comprehensive as that concerning liquid fuels, it is fortunate, in the present emergency, that the results of past researches provide material sufficient to enable the fundamental thermo-dynamic characteristics of the fuels to be deduced with accuracy.
Researches of considerable value, carried out by Burstall, have been described in papers read before the Institution of Automobile Engineers,* whilst other investigations, by Tizard and Pye, into an infinite number of different aspects of the problem, have been described in full in a number of journals. It is to these workers, and, of course, to Ricardo, that we owe most of our present-day knowledge of the subject.
• Experimental Equipment Employed • Much of the work, particularly that done by Burstall, was carried out on the Ricardo variable-compression engine at the engineering department of the University of Cambridge. The engine, of course, needed to be especially adapted for use with gaseous fuels, the chief modification being the provision of a gas-mixing chamber instead of the carburetter. .
The device employed is illustrated and consisted of an aluminium casting containing an annular passage through which gas was admitted, by a set of ports, into the air stream. The choke tube, which contained the ports was separately constructed so that tubes of varying sizes, and containing varying numbers of ports, could be fitted. The choke was specially designed to avoid "flat spots," whilst the disposition of the ports circumferentially around the throat ensured homogeneity of the mixture. The strength of the mixture was controlled by means of a graduated screw-down stop valve.
Pressure-time curves on all tests were obtained by means of the R.A.E. indicator, these furnishing valuable evidence as to the sequence of events within the cylinder. Due to uncertainty as to the phase-setting of the instruments, however, the I.H.P. was found by adding to the B.H.P. the lost horse-power as determined by a motoring test—a practice which has, of course, now been adopted as standard. Other components of the testing equipment were more or less conventional. Experiments were carried out with hydrogen, carbon monoxide, methane and coal gas, that is, with the three gases which feature, to varying extents, in all composite gaseous fuels, together with a fuel containing all of them. The results of the tests, therefore, furnish evidence as to the outstanding characteristics of each individual gas and the extent to.which the properties of each are modified by the presence of one or both of the others.
No governor was provided on the engine, all tests being carried out with both throttle and air intake wide open. The principle established by Ricardo, and now universally recognized, that the ignition should be timed so that the peak pressure occurs about 12 degrees after T.D.C. was adhered to, the pressure-time curves providing a reliable check on this.
The extent to which non-compliance with this principle may affect performance is illustrated in the accompanying diagram. Using carbon monoxide as fuel, it was found that, for an engine speed of approximately 1,000 r.p.n-i., the timing of the ignition at 67 degrees before T.D.C. resulted in the pressure-time curve 1, with the peak pressure occurring at 8 degrees after T.D.C. When the ignition was timed at 38 degrees, however, the thoroughly unsatisfactory 'curve 2 was obtained, denoting that the peak pressure was unduly delayed. Loss of power and efficiency due to incomplete combustion are, of course, inevitable in such a case.
• Ignition Timing a Guide to Burning Speed • This question of ignition advance, whilst not at first sight appearing to be of fundamental importance, nevertheless provides valuable evidence as to the burning characteristics of the various gases. With regard to hydrogen in particular, experiments in ignition timing reveal that the gas burns very rapidly, an ignition advance of only 2 degrees being necessary for a 15 per cent, weak mixture in an engine of compression ratio 7 : 1, running at a speed of 1,000 r.p.m.
Thus, when present in a composite fuel, hydrogen may act as an igniting constituent and assist in the more rapid propagation of the flame throughout the charge. It is sometimes pointed out that, due to its high conductivity for heat, hydrogen, when compressed, causes the heat loss to be higher than, say, with air. The rapid ignition rate of the gas will, however, minimize the heat loss at a time when the effect of such losses is greatest, that is, around the dead-centre position.
It must be pointed out, however, that the property of hydrogen which makes possible the use of very weak mixtures, militates against its use in rich mixtures, or, indeed, in mixtures approaching the, " correct " strength. Even with mixtures containing 95 per cent.
of the hydrogen in the "correct" mixture, the rapid ignition rate of the gas gives rise to pre-ignition and firing back through the carburetter. Thus it would appear that hydrogen will perform its most useful function when it is a constituent, either in the elementary or combined form, of some composite fuel.
Thermal efficiencies possible with hydrogen are high, particularly in the case of weak mixtures. In consequence, it has been found that, at a compression ratio of 7:1 and an engine speed of 1,000 r.p.m., the use of a mixture over 50 per cent, weak resulted in the attainment of an efficiency of the order of 40 per cent. The low thermal value of these weak mixtures, however, naturally results in a low power output ; for example, the I.M.E.P. developed at maximum efficiency is only about half of that obtainable with a liquid fuel at the same compression ratio.
With regard to carbon monoxide, Burstall points out that its suitability as a medium for thermo-dynamic investigations is enhanced by the fact that it does not form water vapour on combustion. Also, it is a pure gas of known chemical and physical properties, whilst its specific heat, and that of its products of combustion, are known with greater accuracy than that of water vapour. It was only to be expected, therefore, that tie: gas would exhibit good combustion characteristics, this being substantiated by the fact that it can be used over a wide range of mixture strengths.
The efficiency was found to rise steadily with increasine weakness of the mixture down to a figure of 40 per cent, weak, but in order to keep the peak pressure at its correct position, an ignition advance of 90 degrees (compression ratio 5:1) was found to be necessary.
• Slow Burning a Carbon-monoxide Feature • This high degree of ignition advance reflects the slow burning properties of carbon monoxide. It must be pointed out, however, that the gas compares very favourably with evaporated hydro-carbon fuels, in that much weaker mixtures can be uSed than are possible with the latter, if efficient flame propagation is to occur.
The highest maximum pressure and maximum power were obtained with a mixture 5 per cent. rich, but even for this mixture an advance of over 30 degrees was found to be necessary at a compression ratio of 5; 1.
Increase of the compression ratio, which can be carried out over a wide range without detonation occurring, results, however, in an increase in the rate of burning, the power and efficiency also increasing to an extent in line with fundamental thermo-dynamics.
Attention has recently been drawn to the possibilities of using methane as a fuel. In view of the paucity of
information concerning the characteristics of this gas, it is fortunate that it was included by Burstall.
Reference to the table reveals that, 'compared with other gaseous fuels, the percentage of methane in the " correct" mixture is low, this, of course, enabling higher charging efficiencies to be attained. The rate of burning of the gas is intermediate between that of hydrogen and carbon monoxide, as was to be expected.
The investigations also revealed that the combustion of methane is characterized by features which are not encountered with any other fuel. With very weak mixtures the pressure rise is steady, but as the mixture strength is increased the charge burns more quickly. Near the top of the stroke the rate of burning increases extremely rapidly, this seeming to indicate that the combustion of the gas proceeds in two stages.
• Unique Methane Characteristics • With a mixture 31 per cent, weak a spark advance of 70 degrees was found to be necessary, the I.M.E,P. being relatively high (95.7 lb. per sq. in.) and the thermal efficiency 32.5 per cent. Increase of the mixture strength to 3.7 per cent. rich raised the I.M.E.P. to 125.2 lb. per sq. in., but lowered the efficiency to 29 per cent., whilst even for this rich mixture a spark advance of 33 degrees was found to be necessary. All these results, it must be noted, were obtained at a compression ratio of 5: 1 and an engine speed of 1,000 r.p.m.
Whilst the original work referred to contains a vast quantity of data concerning the behaviour of the fuels thrcughout a wide range of operating conditions, the facts analysed above enable definite pronouncement to be made concerning the relative merits of each.
In short, it can be stated that the contribution of methane will be largely in the direction of the achievement of high power, that of hydrogen in reducing the time of burning, and that of carbon monoxide in the efficient propagation of the flame over a wide range of mixture strengths.
In the case of coal gas, containing 48 per cent, of hydrogen, 22.5 per cent. of hydrocarbons, chiefly methane, and 19 per cent. of carbon monoxide, a rising efficiency can be obtained down to a mixture strength over 50 per cent. weak. Only by the efficient exploitation of the "stratified charge" can a result in any way approaching this be achieved with liquid fuels.
When we consider that maximum power with coal gas is obtained with a mixture 20 per cent. rich, the wide range of strengths over which the fuel can be utilized becomes readily apparent. In the range of mixture strengths normally used, the achievement of a good power figure, with a reasonable degree of ignition advance and complete freedom from detonation shows bow each constituent gas contributes its quota.