Engine fumigation could eliminate combustion lag
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IS ULTRASONIC ATOMIZATION THE KEY TO SOLVING THE PARTIAL-ASPIRATION PROBLEM? GOING back 12 or more years, two combustion systems promised to provide a solution to the combustion-lag problem of diesel engines. In fact, both systems met with success, but at the cost of higher fuel consumption, and little has been heard of them in recent years.
Pilot injection, followed by injection of the main charge, virtually eliminated combustion lag and therefore diesel knock by initiating combustion with such a small quantity of fuel that relatively little heat was required to warm the fuel to ignition point, and plenty of heat was later available to burn the main charge derived from the heat produced by the burning pilot fuel. Why the system reduced combustion efficiency is rather obscure but might be explained by undesirable pressure-wave effects.
Concurrently with pilot-injection experiments a number of engine makers were exploring the possible benefits of employing partial aspiration with the help of a carburetter or vaporizer. A fuel-mixture was aspirated in the manner of a petrol engine but it was too weak to be ignited by the heat of compression and burning was impossible until an extra charge was injected in the normal way. In this case not only was the weight of the initial charge (and of the total charge) reduced and less heat absorbed by the fuel, but the fuel in the cylinder was ready to burn with a small increase in the fuel-air ratio. The partiallyaspirated engine ran as smoothly as a petrol engine and jts performance was satisfactory; its poor fuel consumption was probably a function of distribution and condensation problems.
According to news from Russia and other sources, designers are taking a renewed interest in pilot injection and, of more immediate interest, fumigating the engine by ultrasonic atomization promises high rewards by improving the efficiency of partial aspiration and by eliminating lag.
I visited Acton to discuss technicalities with Mr. C. H. Bradbury of Simms Group Research and Development Ltd., and Mr. Bradbury showed me ultrasonic atomizers in operation on the test bed and gave me a copy of a paper he read in June of this year at the 1966 API research conference on distillate fuel combustion in Chicago, in which SGRD applications to diesel engines are mentioned.
Shortly, the device causes the surface of the fuel to vibrate at ultrasonic frequencies which produces a capillary-wave action that atomizes the fuel. In practice atomization is so fine that the fuel "drifts" out of the horn of the atomizer rather like smoke from a contemplative smoker's mouth.
In diesel applications, the fumigator is placed in the intake, and supplies a small proportion only of the total fuel required to the cylinder through the inlet valve. It has been proved that the system enables the engine to run more economically at high loads, that it can increase the maximum power output and that it gives quieter running and reduces smoke.
Mr. Bradbury says in his paper that the fuel aspirated into the cylinder "undergoes certain pre-flame reactions which materially assist the smooth and prompt ignition of the main fuel when it is injected into the cylinder-. An indicator diagram, published in the paper, shows that in a particular application fumigation virtually eliminated the combustion-delay period. In a test carried out by an engine manufacturer, the noise of a single-cylinder research unit operating at a b.m.e.p. of 250 p.s.i. was reduced considerably at the two highest peaks--325 cycles /sec. and 2,000 cycles / sec.
It is unfortunate that (at the present stage of development) the system only provides an improvement in fuel consumption at high loads, but an increasing number of vehicles are operating at sustained high loads on the motorways and the users would be willing to "pay for economy". The cause of higher consumption at reduced loads has not been revealed but in the author's opinion it is probably a function of maldistribution. Distribution is obviously the key factor as indicated by the quoted view of Simms technicians that the value of the system is derived from its ability to provide good air-fuel mixing without manifold wall wetting.
It was Mr. Bradbury who, at the symposium on diesel engine breathing at Cranfield, mentioned a method that facilitated the starting of diesel engines evolved before the war by a Hungarian engineer, and it is pertinent that the subject of easy starting has been revived as an important issue because of the current interest in the potential of the lowcompression diesel.
As mentioned in the report of the symposium, Mr. West of Rolls-Royce stated that a 6.5-litre opposed-piston engine had produced 500 b.h.p. when operating on a compression ratio of 10 to 1. The only snag was that special means had to be provided to get it started, the object of the exercise being to investigate the potential of a variable-compression-ratio piston, which if fitted would have given a high ratio for starting and automatically adjusted the ratio when running to match the load.
Is it possible that fumigation could be employed with advantage in a lowcompression diesel? At least the possibility should be considered.
The Hungarian engineer mentioned by Mr. Bradbury, Mr. George Jendrassik of Budapest, presented a paper in that country in 1929 in which he described an easystarting method based on the principle that the temperature of a gas is increased when it flows into a cylinder in which a vacuum has been created. The method was successfully applied by the Ganz Co. of Budapest to anti-chamber diesels by using a sliding inlet-valve camshaft that could be adjusted to delay valve closing until late on the intake stroke.
It will be recalled by many engineers and operators that until comparatively recently Gardner provided an easy starting device which closed the inlet valves at bottom dead centre on the compression stroke, thereby raising the effective compression ratio.
Apparently all the leading authorities in this country are now agreed that a compression ratio in excess of around 10 to 1 is more wasteful in terms of friction and pumping losses than it is beneficial in terms of improved thermal efficiency. At one time, the highest useful compression ratio (HUCR) of an engine was taken as the yardstick of its performance potential, but now LUCR (lowest useful compression ratio) is frequently mentioned as the measure of a unit's performance potential.