Smoke Levels and Fuel Additives
Page 67
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By P. A. C. BROCKINGTON, AMI Mech E
TIIE availability of diesel fuels containing a smoke-depressant additive is a serious prospect that leading engine makers have been pondering for some time. Considered in conjunction with the assumption that in due course engine ratings Will of necessity be based on a specified smoke level, it is creating a number of vexed problems. It has been established that an additive can be effective to a worth-while extent, but how will additive fuels be marketed and what method or methods of smoke measurement will be approved?
Whilst the purpose of an additive is to suppress smoke. an additive-treated fuel will enable engines to be uprated if existing smoke limits continue to be accepted. What will be the notating potential of additive fuels and how much will they cost? To what extent will the potential depend on engine type and design?
If it is assumed, for example, that an additive fuel will cost Id. a gallon more than a straight fuel and that its use could enable output to be uprated by 10 per cent, would the typical operator wish to take advantage of the gain in power at this price?
In Europe. diesel fuel is substantially of uniform quality; but overseas (notably in America) quality varies considerably and. apart from California and some city areas, smoke emission currently is more readily tolerated.. Whether additive fuels are marketed as premium. grades, or additives are universally employed, their availability is therefore likely to create problems that will have repercussions internationally.
Although rating on smoke level would be performed during a type test of the engine in the laboratory. Ministry regulations regarding ,smoke emitted on the public highway, the stringency of enforcement and testing methods employed in spot checks (in this country and overseas) are all-important background considerations that can influence the policies of additive makers, oil companies and engine makers.
After confirming that additives are worth while with regard to smoke suppression, with the qualification that the advantage could vary according to the type of additive used and features of the combustion system, a technical spokesman of Perkins Ltd., Peterborough, points out that, as diesel smoke is not toxic. " objectionable smoke" can be defined only by. reference to its opacity.
For a given smoke density a small engine will produce less smoke than a large unit, and if the gas is discharged at high velocity through a pipe of reduced diameter the smoke may be . invisible. But if the size of the pipe is increased the smoke in the gas may be sufficient to obstruct visibility.
Attention is also drawn to the fallibility of smoke analysis in a free
acceleration test with an engine in neutral, which is the type of test employed in France, Belgium and Norway. In the case of France, this test is applied despite the admission in the regulations that " one must always allow temporary smoke emission during a change of engine speed ". The merit of making smoke measurements when the engine is operating under steady load is particularly noted by Perkins and the plea is made that if smoke-limit regulations are more rigidly enforced in this country, free-acceleration tests should not be used.
Apart from changes in the opacity of gases of the same smoke density, a density measurement may vary by as much as 3 to 1 without actual change of smoke density. according to the point in the exhaust pipe at which the meter reading is taken, if the sampling and measuring methods are not very carefully specified and controlled.
In a free-acceleration test, a heavy flywheel can influence the reading favourably or unfavourably, according to the type of engine (maximum acceleration in neutral from idling to governed speed normally occupies about I-5 sec., but a heavy flywheel could cause a considerable lag). After idling for a long period a warm engine may produce much more smoke than an engine that has been running under load because of nozzle fouling or temporary degradation. Moreover, smoke output may be increased very substantially if the engine is cold.
Free-acceleration testing could favour the direct-injection engine in comparison with the swirl-chamber type, it is claimed, because the smoke output of the latter is relatively high at low speeds and the smoke produced on snap opening of the throttle could have a much higher density than that emitted during steady running.
The smoke level of a typical directinjection engine increases with speed and, in a particular case in (vbich it was objectionable at high speed, a freeacceleration test could indicate a low density. Whilst the smoke produced by a swirl-chamber engine tends to decrease with an increase in speed, raising the full level marginally aboVe a critical point produces smoke of such density that further upratine is virtually impossible. In contrast, the density increase of the direct-injection engine is progressive and there is no critical point that determines the absolute maximum rating.
Perkins engineers consider that ideally a smoke test should not be made at any speed lower than the maximum-torque speed of the engine, which is normally about 30-40 per cent of the maximum speed. The Bosch smoke meter is favoured in comparison with the Hartridge type because the permanent smoke sample provided by the Bosch meter in the form of a filter paper is available for
later analysis. The meter can he more readily employed for testing a vehicle on the road and readings are not influenced by the inertia of a needle mechanism in the system. It is not, however, considerdd suitable for free-acceleration tests in its current form as the sample is taken over a period of 0.5 see.
Further to their recommendations that a smoke-type test should be taken at a steady engine speed not lower than 30-40 per cent of maximum speed and that a comparable form of test should be made if a vehicle is spot-checked on the road. Perkins engineers consider that the type test of a water-cooled engine should be 17tade without the radiator fan or other auxiliaries, apart from the water, oil and fuel pumps. The retention of the fan of an air-cooled engine is regarded as mandatory because the unit cannot operate without it, and the size of the fan, cowling layout and so on are not varied to suit installation requirements.
Comparing the duty of an air-cooled engine fan, with that of the water pump of a water-cooled unit. Perkins emphasize that the pump can absorb as much as 3-4 per cent of the power output.
Only by such elimination of nonessential • auxiliaries and by rating on a prescribed smoke level (corrections being made for temperature. barometric pressure and humidity) can accurate comparisons be made, it is claimed, between different types of naturally aspirated engine. The generator. should not be included in the auxiliaries fitted because the power absorbed by a d.c. unit is a function of brush contact with the commutator and of current output.
The maximum extent to which an engine might be uprated in practice by exploiting smoke-depressant additives is related to air utilization, which normally is about 65-70 per cent in a directinjection engine. Improvement might be possible up. to, say, 75-80 per cent, at which the influence of small variations in machining and fitting accuracies would become so great that operation commercially would be impracticable.
The case of the turbocharged engine has to be taken in isolation because there is no definable limit to the supply of air or to the increase in output that can be achieved. Smoke emission of a turbocharged engine is normally confined to the rotor-lag period at low speed during acceleration, and free-acceleration testing of smoke emission inevitably would penalize the turbocharged engine to a greater extent than the unblown.
These rating problems and recommendations. postulated by the Perkins company, highlight the difficulties of engine makers in preparing for as yet uncertain changes in engine-rating methods and for a future that is confused by the probability that additive fuels shortly will become available.