Compensating Producer-gas Power Losses
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An Introductory Article in Which a Well-known Engineer Discusses the Problems Arising From the Use of Producer Gas and Makes Interesting Suggestions
By C. SHORROCK
AA LTHOUGH it is now many years since the first .Loexperimental suction or producer-gas plant was designed and tested, and although such satisfactory and encouraging results have, during this period, been obtained with thousands of these installations in world-wide use, their field of application has been almost completely confined to large stationary engines.
With regard to the possibilities of using producer gas as a fuel for transport vehicles, development in this field has not hitherto received the attention now being given, the chief reason, no doubt, being the highly efficient operation of the transport type of engine on what might be termed " convenient " fuels, such as petrol and oil.
These fuels have, in normal circumstances, always been readily obtainable even in the countries not actually producing them, and, furthermore, their use has not necessitated the addition of the auxiliary plant required to obtain producer gas from the several alternative solid fuels more or less universally obtainable.
The present state of national affairs has, however, made it desirable, if not actually necessary, to devote somewhat belated attention to the possibilities of home-produced fuels, and the intensive interest now beintt displayed in this country in this subject may quite possibly result in the gap hitherto existing between the use of the " convenient " fuels and the alternatives being considerably closed,
• Why More Research is Required • It might be asked, in view of the fact that producer gas has been so successfully used with larger types of engine, why it is necessary to give such renewed attention to technical considerations and difficulties which might be assumed to have been already dealt with before the designs were produced for the successful installations now in use?
The answer to this question is, of course, that, whilst the large stationary installation is fundamentally the same as the transport-vehicle installation, the limitations of actual installation space make it necessary to design a producer which must be considerably smaller in proportion to the output of the engine which it will have to feed, not only as regards its capabilities of producing a sufficient quantity of gas, but of filtering and cooling the gas.
With further reference to the question of relative dimensions or bulk of the producer plant, it must be borne in mind that whilst a normal type of passenger vehicle or large lorry engine may develop about 100 b.h.p. and must he fed with a gas-producer plant more or less unobtrusively installed on the vehicle itself, or as an alternative, on a small trailer, a stationary engine used as a prime mover for industrial purposes and with the same power output would take up quite a large proportion of the available room in the engine house and the producer plant and auxiliaries would take up a somewhat similar space.
Dealing with the various types of gas producer which, more or less directly as a result of the present-day conditions, are now being designed and manufactured for use on transport vehicles: apart from the limitations imposed upon the actual power output obtainable from any engine as compared with the output obtainable on petrol or oil fuel, the chief difficulties which have to be contended with from the designers' standpoint are those associated with reasonably efficient cleaning and cooling of the gas after it has actually left the producer unit itself.
It will be obvious that the gas leaving the producer will be at a very high temperature, and will contain quite an appreciable amount of gritty foreign matter collected on its passage between the tuyeres and the outlet port of the producer, as well as certain chemical impurities, which render the gas at this stage quite unsuitable for introduction to the engine.
In order to purify the gas so far as possible, the manufacturers of the producer plant provide a series of dustcollector boxes and filters, all of which, although varying somewhat' in design, remove, so far as possible, the impurities mentioned, limitations being imposed by the space Considered available for such equipment.
' The filtering and purifying equipment is, in practically all cases, so designed that the filtering elements may be removed and cleaned or replaced without undue difficulty. With regard to the cooling of the gas before delivery to the engine, this is a very important consideration, as the higher the temperature the greater will be the power output losses in addition to the loss inseparable from the use of a fuel of comparatively law calorific value,
There appears to be a considerable difference in the final temperature of the gas after passing through the cooler on various types of producer plant now in production, resulting from the various efficiencies of the cooling systems employed, and to a certain extent on the actual layout of the plant and the consequent lengths of the connecting pipes used, which, of course, provide additional radiating surface.
Before leaving the subject of installation considerations, it is essential -to obviate another frequent loss of power output, by providing filters, coolers and connecting pipes of sufficiently unimpeded throughway to prevent undue gas resistance or depression in the system which would still further reduce the weight of useful charge available at the engine manifold.
Several manufacturers in this country have now standardised efficient gaa producers, of varying capacities, in which all the above considerations have been taken into account and dealt with satisfactorily.
• Chief Difficulty to be Overcome 0.
The chief disadvantage is still undoubtedly the rather serious drop in power output, as compared with the outputs obtained when operating on petrol or oil, and unless provision be made to rectify this deficiency, dissatisfaction with the comparative performance obtained in service is likely to be expressed.
In order to form some reasonable basis on which to work, it may be assumed that the maximum power output obtainable with an unmodified engine operating on producer gas will be approximately 60-65 per cent. of the power obtainable when operating on petrol or oil. This is assuming that care has been taken in the design and installation of the producer plant to obviate losses other than those inseparable from the use of a fuel of low calorific value, such as undue temperature and frictional losses mentioned previously.
It will be obvious that to install an ,engine of sufficient extra cylinder capacity to overcome this power loss would be a very uneconomical and inefficient proposition. Apart
from the extra cost and possible difficulties of such an installation in other than new vehicles the advantage of the extra power butput obtained due to the larger swept volume of the engine would, to a considerable extent, be nullified by the reduction in power-weight ratio due to the extra weight of the larger engine. A further point to be considered in this iespect.is that, although extra output could be obtained in this manner, it would represent the least efficient means for obtaining it.
It is, therefore, logical to consider the adoption of two other methods still available to make good, to a lesser or
greater degree, the power losses. '
These methods involve either modification and tuning of a standard engine, or the use of pressure induction or supercharging, and it will be convenient to deal with these methods individually.
It will be apparent that the gain in output possible with the modified standard engine will be definitely limited to a figure representing considerably less than the normal output obtainable on either petrol or oil,
• Increasing Compression and Thermal Efficiency • A considerable amount of experimental work has recently been carried out in this direction, chiefly connected with methods to increase the compression ratio and the thermal efficiency.
Alterations to compression ratio are accomplished by either fitting a new cylinder head with a reduced combustion-chamber volume, machining the existing head to. obtain the same result, where this is possible due to sufficiency of Metal at the vital points, or substituting replacement • pistons with or without the previously mentioned modifications.
By adopting the above methods, also by attention to porting and induction-track considerations, ensuring that the-volumetric efficiency of the engine is as high as possible, it is certainly possible to effect a considerable improvement when compared with the results which would be obtained with merely-a changeover from one fuel to another with a standard unmodified engine. However much attention be given to these modifications, it will still be impossible to make good a remaining loss in comparative power output, except by adopting some means for feeding to the engine a considerably heavier charge of mixture than would be possible with ordinary atmospheric induction, even assuming that 100 per cent, volumetric efficiency could be obtained with the engine and that no further losses due to excessive gas temperature had to be contended with.
The results obtained with an engine operating on producer gas may be likened to those obtained with an engine operating at a high altitude above sea level on normal fuels such as petrol or oil, that is, at a certain altitude, a similar power Toss would be experienced, due in this case, not to the lower heat value of the fuel, but to the lower density of the mixture.
• The Possibilities of -Forced Induction • Mention is made of the above at this point because the method used to rectify the loss of power in both cases will have to be similar, that is, the introduction of forced induction or supercharging.
The installation of a supercharger as an extra unit of the producer-gas plant will deal effectivelywith any remaining loss of power output, provided that this unit be efficiently installed, that is, that a supercharger be used which is of suitable volumetric displacement to force into the engine under pressure a sufficiently heavy charge of gas to compensate for the power loss experienced with any particular gas producer.
With regard to the question of supercharge or boost pressure, this will vary according to the extra power output required, but in no instance need it be sufficiently high to affect the mechanical reliability factor of the engine itself. It should be pointed out that when a supercharger is used in conjunction with a petrol engine operating on producer gas no modifications are necessary to the engine to increase the normal compression ratio; with an oil engine, however, it Wiii be desirable to employ a somewhat lower compression ratio than would be used with the engine operating on producer gas without the supercharger.
A20 At this point it would be .well to mention the fact that there are, two ways of modifying the standard oil engine to operate on producer gas, and although no really useful amount of data on the advantages of either method is as yet available, considerable experimental work is being carried out with both. One method, which involves a considerable departure from standard, is to reduce the normal compression ratio to as high a figure as possible conducive to efficient operation on producer gas, and employ electric_ ignition.
The second method, which appears to be finding considerable favour, is to retain the injector pump and, nozzles and utilize avery small oil injection sufficient to ignite the main producer-gas charge, which is fed to the engine via the normal air inlet valve.
One undoubted advantage of the latter method is that there is considerable latitude to vary the proportion of oil injection to make good to some extent certain power losses.
With 'regard to the actual installation of the supercharger, this can, in the majority of instances, be comparatively easily carried out, as it is often possible to obtain a suitable drive by means of V-belts, which have been found to be quite efficient-for this purpose.
If a convenient installation cannot be made with the supercharger mounted more or less directly on the engine, this unit may be mounted in a convenient position, say alongside the clutch. or gearbox, and a suitable drive can often be arranged in this position.
• Installation Presents Little Difficulty •
With the supercharger mounted in this manner, no trouble need be anticipated through deposition of the fuel charge, as would be the case when using petrol as a fuel. In the latter case, the length of connecting pipe between the supercharger and the engine manifold would most certainly cause serious separation and deposition troubles, but when running on producer gas, this length of pipe would provide extra radiation surface, which would, of course, help to reduce the gas temperature, with beneficial results.
Summing up on the present position with regard to the whole subject of operating on producer gas, it may be stated that there are now available several highly efficient producer plants complete with auxiliary equipment.
As might be expected, it appears that the interest taken in this subject at present is considerable: consequently, as a result of attention having been fixed thereon due to the present international situation, the difficulty would, at the moment, appear to be that the deman,d exceeds the present production capabilities of the manufacturers. It is understood, however, that suitable measures have been taken at least by certain manufacturers to overcome this.
The general efficiency and portability of plant now available enable it to be stated that there need be no hesitation in accepting such designs, and of assuming their efficiency. .Furthermore, now that it is known that no restrictions are to be imposed either directly or indirectly, such as excessive taxation of the fuel that will be used, or with regard to the additional weight of the necessary plant, the chief remaining difficulty would appear to be the question of adequate power output to meet the demands of the average potential user.
• Extra Cost Versui Higher Efficiency • As stated, this difficulty may be either considerably lessened or overcome altogether by adopting the means suggested, and it is only a consideration of the extra cost involved which will have any bearing upon the extra efficiency obtained, within comparatively wide limits, • On the assumption that alternative solid fuels may have to be used in an increasing quantity over a considerable period of time, the all-in cost of suitable producer plant, complete with every desirable auxiliary and accessory, should not be in any way prohibitive. When it is reckoned that, in all probability, considerable mileages will have to be covered on these fuels, it will be seen that the actual cost of running per mile will be considerably lower than when operating even on oil fuel, that is, assuming that the relative costs of the fuels under consideration are maintained proportionate with their present cost.