Successful Performance from Producer-gas Oilers
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Summary of Results so far Achieved by the I.A.E. in its Investigations into Producer-gas Operation for C.I. Engines
WE have, from time to time, dealt VV with the work of the Research Committee of the Institution of Automobile Engineers in connection with the conversion of compression-ignition engines to producer-gas operation, and it is now possible to give a summary of the results so far achieved.
Before dealing with them, however, it may be as well to mention that the conversion of an oil engine to burn producer gas involves quite different problems from those connected with the petrol engine. In the latter case, results can be obtained merely by introducing a mixing valve in the induction system, but with the oiler there is the question of ignition to consider.
The injection pump can, _of course, he replaced by a magneto, or coil and battery ignition could be used, the result being a high-compression producer-gas unit, In practice, however, it would be desirable to reduce the compression ratio to about 10 to 1, as the higher ratio of the oil engine would preclude the use of petrol for starting. If a special fuel be used for this purpose it is possible to retain a compression ratio of about 14 to 1.
The alternative to using spark ignition lies in so controlling injection quantity that the minimum required to effect ignition is admitted on each. firing stroke, and as no major alterations to the engine are required, this system is by far the more attractive.
The I.A.E. points out that the only disadvantage is that the use of some liquid fuel is necessary throughout the running period and, in consequence, it is impossible to obtain 100 per cent, oilfuel economy. In the final analysis, thetefore, the value of this type of conversion is dependent upon the possible reduction. that can be obtained in liquid-fuel consumption under road conditions, and, obviously, this has been oneof the major problems with which the Research Committee has been faced.
Summary of Results so far Achieved
In the summary, it is stated that two types of direct-injection oil engine have been converted to operate on producer gas, ignition being by pilot oilfuel injection. One of these units was six-cylindered and of 7i-litre capacity, andthe other a five-cylindered engine of 7-litre capacity. Both were of the type having a combustion chamber in the crown of the piston.
Taking as a basis for comparison the power output on oil fuel when running with 40 per cent. excess air, which is common on passenger-service vehicles in order to ensure a clean exhaust, it was found that, with the minimum amount of oil injection to secure regular ignition, and with the gas to air ratio adjusted by independent control to give maximum, power output, between 80
and 90 per cent, of the basic power on oil could be obtained with gas of 118 B.Th.U./cubic ft. (net, • saturated) calorific value.
In practice, with oil-fuel injection quantities rising over the working speed range-600 to 1,800 r.p.m.—from 12 to 18 cubic pm./cycle (17 to 28 per cent. of normal), and with a gas mixer automatically giving optimum gas/air ratio, more than 90 per cent. of the basic power on oil was obtained with gas of 118 B.Th.1.1. With gas of 130 R.Th.I.J. and over, the basic power, was -exceeded.
Need for Advancing Ignition Timing
These power outputs could be obtained only with an appreciable advance of the ignition timing and, iii the case of both engines, a. fixed timing of 33 degrees before T.D.C. was adopted.
The requirements of an automatic gas mixer were formulated and were met •by one designed in the I.A.E. Research Department. Engine power was controlled by throttling only the gas supply, air being introduced through a spring-controlled flap valve. Spring rate was determined experimentally, to give optimum gas/ air ratio throughout the speed range, and the initial tension of the spring could be adjusted to allow for differences in resistance to gas flow as between one producer and another, or to suit changes in calorific values.
As to the minimum oil injection required for good combustion with a well-balanced pump, it was found to be about 8 to 10 cubic mm./cycle but, in practice, 15 cubic mm./cycle would probably be required to cover differehces between pumps • and other features.
.No advantage is gained by increasing the quantity of oil injected above about 25 cubic min./cycle, although small increases above the minimum result in an appreciable gain in power output.
With the injection systems tested, at limited fuel delivery quantities determined by fixed pump controls, it was found that fuel quantities rose with speed. The steepness of the delivery/ speed curve was smaller with, pump plungers of small diameter than with those of large diameter.
With 8 mm. elements, for instance, a 300 per cent, rise in injection quantity took place between 500 r.p.m. and maximum speed, whilst, with 6.5 mm. elements the rise was only 50 per cent.
In both cases it was found essential to allow the pump control-rod to be under governor control when idling.
It was considered impossible to replace large-diameter pump elements with others of small diameter under war-time conditions, and vaSious other methods of controlling the shape of the delivery/speed characteristics, with 8 mm. pump elements, were investigated. The methods giving the most useful results • involved modifications to the fuel-pump governor which allowed some movement of the governor flyweights to be used in resetting the pump controls at some' speed, or over a range of speeds, between idling and maximum. In this ways fuel delivery at the higher • speeds was reduced below that obtained with fixed pump-controls.
It was found, with both the engines tested, that in the synchronisation of gas throttle and putnp controls with the movement of the accelerator pedal, the employment of the same principles was desirable, Thus, the first part of the movement of the pedal was used to move the pump controls to the pilot injection position, with little opening of the gas throttle. The rest of the pedal movement was used to open only the gas throttle, thereby controlling the power output of the engine. To permit of this and to maintain the pump control in the pilot injection position, an over-riding device was used between the accelerator and pump control.
The oil-gas control system was arranged to give idling under governor control, with the gas throttle open to a sufficient extent to maintain the producer fire, or to permit of it being kindled when starting.
Finality Has Not Yet Been Reached
With the co-operation of the. M.O.W.T. and operators of public-, service vehicles, I8 converted oilengined buses have been tetted under widely varying conditions in normal service. All these machines were equipped with a gas mixer to the design of the I.A.E. Research Department. The conversion system, with modified governor adopted for pqmps of 7 mm. to 8 mm. plungers, gave reasonably satisfactory results, and the experience Rained showed the way to further possible improvements.
Even with this admittedly not perfect system, liquid-fuel consumption figures of the order of 20 m.p.g. to, 25 m.p.g. were returned, when the fuel pumps were correctly set. In the case of engines having 6.5 mm. pump elements a figure as good as 31 m.p.g. .was obtained under favourable conditions. In all cases the converted vehicles maintained the time schedules required from non-converted oilers.