What Makes Oil Engines Smoke
Page 65
If you've noticed an error in this article please click here to report it so we can fix it.
THE degree to which vehicle engines are responsible for atmospheric pollution in towns and cities was discussed last week at a conference in London organized by the Institution of Mechanical Engineers.
During the opening session, Sir Ewart Smith presented a paper. "Some Tech-. nieal and Economic Aspects Involved in the Reduction of Atmospheric Pollu: Lion." Exhaust discharges of oily vapours were referred to as a serious source of damaging pollution. In addition, • the petrol engine discharged carbon monoxide, but concentrations seldom attained proportions prejudicial to health. The oily vapours covered street furniture and buildings with a film, and this problem had become . much more noticeable with the increase in the number of oil-engined vehicles on the roads.
Work For Engineers Opinions on damage to health were divided, but observation had shown that the vapours affected vegetation. The saving of fuel by more complete coin-. bastion would be small, but this must not deter the mechanical engineer from developing nuisance-free exhausts.
The first of four papers grouped together tinder the .heading, "Au' Pollution from Road Transport," was piepared by Mr. Arnold Fitton, principal scientific officer of the Fuel Research Station (D.S.I,R.).
He dealt with the nature, composition and toxicity of exhausts from petrol and oil enginesecomparing the total pollution with that from burning solid fuel
domestically and industrially. Whilst exhaust 'pollution was small in relation to the pollution from solid fuel burning, vehicle gases were discharged near ground level in the streets and there was tittle time for them to become . diluted before being inhaled.
Between 1938 and 1955, petrol usage had increased from 4.8m. tons to 6.2m. tons annually, whereas the consumption of dery had risen from 0.4m. tons to 1.6m. tons.
Better Combustion
The main pollutants were carbon monoxide, the aldehydes (formaldehyde, acrolein and acetaldehyde), nitrogen dioxide and sulphur dioxide. Comparison of the exhaust-gas composition of petrol and oil engines under varying conditions showed that oil-engine cornbi.stion was the better of the two, with less atmospheric pollution.
Nevertheless, it was oil-engine exhausts which occasioned public com plaint. Mr. Fitton suggested that this was because an oil-engine'exhaust was often accompanied by heavy smoke. Its sooty material gave off an unpleasant smell and which, together with the aldehydes caused irritation of the eyes, nose, throat and chest.
The Main causes of this smoke were overloading of the engine, poor maintenance of the engine, fuel-pump and injectors, and misuse of the cold-starting excess-fuel device. -Even With a properly maintained and operated vehicle, emission of visible smoke could not entirely be eliffilnated.
. The amount of smoke tended to increase with engine load, very slowly at first and then at a rapidly -growing rate when approaching the rated output of the engine. The carbon-monoxide content rose with the increase of power and it had been found that the production of a smoky exhaust might lead to the presence of carcinogenic compounds UT the smoke.
Skilled Maintenance Mr.. A. T. Wilford, director of• research, London TransPort Executive. discussed air pollution from the viewpoint of the operator of a large fleet of oil-engined vehicles. He pointed out that objectionable exhaust products could be kept within acceptable bounds by ensuring that the recommended setting of the fuel-injection pump was never exceeded, also by adopting a system of regular and skilled maintenance of the engine and its fuelinjection equipment.
With regard to fuel characteristics, Mr. Wilford mentioned the use in the U.S.A. of more volatile fuels in city passenger services to minimize smoky exhausts. Lower sulphur content in fuels could help to give better exhaust quality. Concerning catalyst and afterburner devices, Mr. Wilford thought that if an operator neglected his engine he would either not fit or else neglect such additions.
In the third paper of the group, Mr. C. L. Bailey, assistant section head (fuels) engine research section, B.P. Research Station, and Mr. E. S. Bates, technical adviser on high-speed oil engines, technical services branch of B.P. Trading, Ltd., considered the chief reasons for the deterioration of exhaust condition in the automotive type oil engine.
When exhaust colour deteriorated it was caused first by Worsened engine condition and secondly by fuel-system defects and fuel quality. In the first category,worn piston rings and cylinders, sticking piston rings or badly seating valves would cause trouble, as would increased friction or pumping losses.
As to injection equipment. the type
of injector which hadits sac remote from the needle seat reduced fuel temperature within the nozzle and so was conducive to better performance. Excessive injector temperature might be -influenced by the state of the copper washer between the cap-nut and cylinder head. If any of the four contact faces were dirty, nozzle temperature might be increased by 20° C. to 30° C., with much the same, effect on the fuel temperature.
If nozzle holes weretending to lie -blocked or the needle was sluggish. exhaust deterioration might well arise. Good filtration was important in this connection.
Fuel Value
The optimum value of fuel was probably about 50 to 60 cetane. High volatility fuels gave early . evaporation and better distribution within the cylinder, whereas low volatility gave rise to over-penetration and might lead to the production of smoke.
Symptoms of needle-sticking and hole-blockage were quite different. In the case of badly stuck needles, the exhaust became increasingly dark as the engine speed rose during acceleration. and in the worst cases might be objectionable over the whole of the speed range.
• Where light sticking of the needle had occurred, a vehicle would show visible exhaust at "the top of the engine speed • range, and then only in a sporadic manner. If hole-blockage was occurring, the deterioration in exhaust smoke slowly increased with engine life, and in most cases it would be observed as being most severe at intermediate engine speeds during acceleration under loaded conditions.
Hole Blockage
In some cases, at high engine speeds the exhaust smoke would be no worse than that obtained with clear nozzles. The hole-blockage problem was less frequently met than that of stuck needles, but where an engine was prone to blocking of the nozzles, it was to be made clear that one blocked, or even partially blocked, hole could be responsible for a great increase in exhaustsmoke intensity.
The last paper of the group described the techniques employed in exhaust-gas analysis and was prepared by Mr. B. J. Donovan, Mr. W. A. Mason, Mr. P. L. Orman and Mr.-P. H. Daniels, of Shell Research. Ltd.
During the discussion whibh followed, the testing of air pollution by London buses in connection with the health of employees working in one of the large .garages was mentiOned. Tests at Morderi had been good, and showed that there was no detrimental effect on the health of the men employed, when working among the exhaust fumes from oil engines.