INDUSTRIAL AXD ENGINEERING CHEMISTRY
October, 1928
1041
Some Flame Characteristics of Motor Fuels G. B. Maxwell and R. V. Wheeler DEPARTXEST OF Fr
EL
TECHSOLOCY, SHEFFIELD UNIVERSITY, EAGLAND
In order to obtain some information as to the cause of the “pink” or knock of motor fuels, a photographic study has been made of the movement of flames, simultaneously with measurements of the development of pressure, during the explosion of the charge in an engine cylinder. Explosions of mixtures of pentane and air and benzene and air, at various temperatures and pressures, and of various blended mixtures of benzene and pentane, and of ethyl ether with pentane and air, have been studied. Suggestions are made for the suppression of pinking, and on the basis of these studies the differences between pinking and non-pinking explosions are pointed out.
T PRESEKT there is a deadlock in the design of light, high-speed internal-combustion engines for automobile and aircraft use. The effect of increased compression in increasing efficiency is thoroughly realized. So also is the attendant evil, “pinking,” to which increased initial compression may lead. Designers are prepared to supply high-compression engines if the common run of motor fuels can be used in them. Ricardo,’ for example, paying due regard to foundry and machine-shop difficulties, weight and mechanical efficiency, considers that an engine having a 7.5 to 1 compression ratio can readily be put on the market. Cnfortunately, even the most carefully blended petrols pink in the average engine of 2.5- to 3-inch bore when the compression ratio exceeds 5 to 1; the amount of “dope” that would have to be added to make possible a 7.5 to 1 ratio would be excessive and would lead to secondary complications. “Pinking,” “knocking,” or “detonation,” as it is variously called, has thus come to be a serious problem to the fuel producer. Intensive research has led to improvements in the design of engines and the blending and “doping” of fuels, but the cause of the trouble is still not clear. The most prevalent view is that the “pink” is due to a sudden development of pressure on inflammation of the unburnt residue of charge which has been compressed and heated by contact with the portion already ignited. The violence of the effectit must be assumed that there is a very sudden increment of pressure-has been attributed by different investigators to the activation of the unburnt residue either by ionization, radiation, or preliminary oxidation of liquid nuclei leading to the formation of explosive organic peroxides. These various theories have led to a considerable amount of investigation into compression ignition and “igniting temperatures” of fuelair mixtures and into the ionization and radiation phenomena accompanying combustion. The writers have long considered that a solution of the problem should be obtainable through a photographic study of the movement of flame, simultaneously with measurements of the development of pressure, during the explosion of the charge in an engine cylinder. Some of their preliminary experiments have already been reported.2 This paper gives a r6sum6 of the earlier results together with more recent work. I n order to simplify photographic arrangements and to control and vary the initial conditions more readily, attention has thus far been confined to explosions within a closed cylinder, without a moving piston. The cylinder was of stainless steel, 6 inches (15 cm.) internal diameter. The internal
A
1
J . I n s l . Petroleum Tech , 14, 6 (1928).
1I
b i d , 14, 176
(1928).
length wasnormally 15 inches (38.1 cm.), but this was reduced to 10 inches (25.4 cm.) in certain experiments by the use of a special bucket-shaped end plate. The moving flame was photographed through a glass window fitted into a slit in the cylinder mall and the sensitive Lumihre paper was attached to a revolving drum. On the same paper was obtained a continuous record of the development of pressure by means of an optical pressure indicator of the diaphragm type, similar in construction to the Rice manometer used by the United States Bureau of Nines. This was fitted a t the center of one end plate and the spark plug was carried centrally by the other end plate. * Combustion of a Pinking Fuel-Mixtures and Air
of Pentane
It is known that the paraffins are the most readily pinking constituents of petrol. Pentane (isopentane) was chosen to represent this class, since it is sufficiently volatile to give mixtures with air over the inflammable range a t room temperatures and can readily be obtained pure. INITIAL PRESSURE 1 ATMOSPHERE. IPiITIAL TEhlPER.4TURE 15’ C.-At this pressure and temperature the range of inflammable mixtures of pentane and air is between 1.5 and 4.5 per cent pentane by volume (26.5 to 1 and 8.5 to 1 airpentane by weight). The theoretical mixture for complete combustion contains 2.55 per cent pentane by volume (15.4 to 1 air-pentane by weight), while the mixture which gives the greatest and most rapid development of pressure contains about 3 per cent-(13 to 1 air-pentane by weight). When mixtures containing from 2 to 3.4 per cent pentane by volume are ignited under the conditions of the experiments, a flame passes through the charge with an accelerating speed until it is slightly beyond the midpoint of the cylinder. Here the flame is checked and then travels a t a slower and nearly uniform rate until it reaches the end plate. (Figure 1) Up to the point of check the flame is shaped like a blunt-nosed hollow shell; it then appears momentarily as a flat disk and, on continuing its now retarded movement, the outer edges, near the cylinder walls, lead the way. The flame appears to have been turned inside out. A faint glow appears throughout the products of combustion, behind the flame, a t the moment of check. This glow then dies away slowly from the point of check towards the igniting plug. A more actinic afterglow, somewhat diffuse, appears throughout the cylinder shortly before, and continues for some time after, the initial flame is extinguished. The pressure records show an even rise, except for a check corresponding to the flame check, up to a maximum. This is attained a t about the moment that the central, rearmost portion of the flame is extinguished at the end plate. All such explosions are inaudible. For mixtures containing more than 3.5 per cent pentane by volume the same remarks apply, except that in very rich mixtures (more than 4 per cent pentane) the flame front is very ill-defined and its speed nearly uniform throughout its travel. Mixtures containing approximately 3.5 per cent pentane (I1 to 1 air-pentane by weight) give an audible explosion and certain unusual features in the flame movement are observed. Shortly before the retarded flame reaches the distant end plate it commences to vibrate and accelerate. The pressure records show simultaneous vibrations of the
1044
ILFTDUSTRIAL A S D ElVGlNEERING CHEMISTRY
The turbulence induced by a rotating two-bladed fan in the closed cylinder has a great effect on the nature of the combustion of gaseous mixtures therein. At 2 atmospheres initial pressure and 15" C. no mixture of pentane and air gave an audible explosion when the fan was rotated a t 2600 r. p. m. a t a point about midway in the cylinder. (Figure 4) The rate of pressure development was greatly enhanced, but the records were in all cases perfectly smooth. A glow runs back throughout the cylinder at about the moment when the initial flame, which travels a t nearly uniform speed, reaches the end plate. The combustion in the initial flame must be more complete in the turbulent mixtures, however, so that there is not sufficient energy available to maintain a shock wave ANTIKNOCK CoxiPoumS-Lead tetraethyl was selected as being the most effective of these substances. The addition of this compound to pentane, in amount corresponding to 2.5 fluid ounces per gallon ( 1 6 2 ml. per liter) of pentane, greatly increases the violence of the explosion of a 3.5per cent pentane-air mixture when ignited a t 2 atmospheres and 15' C. The explosion is accelerated and the after-effects are much more pronounced than with the undoped mixture. (Figure 5 ) When, however, the lead tetraethyl vapor is decomposed before igniting the mixture, the explosion is quite silent. The method has been to heat the lead tetraethyl in a glass bulb a t about 300" C. until a puff of smoke appears. This smoke is then allowed to pass into the cylinder with the entering pentane-air mixture. When such a large amount of dope is used (2.5 fluid ounces per gallon (16.2 ml. per liter) of fuel), the flame photograph resembles that of a very rich pentaneor benzene-air mixture. There is no well-defined flame front, but combustion appears to be continuous. (Figure 6) In other experiments, the addition of very small amounts of lead tetraethyl has eliminated the shock wave and tended to produce a continuous combustion in the wake of the initial flame. These results confirm the view of Egerton and Gates3 that it is the decomposition products of lead tetraethyl, and not lead tetraethyl itself, which have an antiknock action.
* Proc. Roy. SOC.(London), 116A,516 (1927).
VOl. 20, No. 10
SHAPEOF COMBUSTION SpacE-Parallel experiments have been carried out on the explosion of mixtures of pentane and air in cubical and spherical vessels. In a cube or a sphere with central ignition no mixture a t 1 atmosphere initial pressure produces an audible explosion. When the initial pressure is 2 atmospheres there is a slight sound on explosion of a 3.5 per cent pentane-air mixture in a cube. The pressure record shows slight vibrations, but either the flame is not sufficiently rapid or the initial combustion more complete, because in no instance is there any indication of a shock wave. These results are in accord with the prevalent view that pinking in an engine, for a given compression ratio, is greatly reduced by having central ignition in a compact combustion-head. Comparison of Pinking and Non-Pinking Explosions
A study of the records on which this paper is based reveals the following differences between a pinking and a non-pinking explosion : In a pinking explosion, such as those of pentane-air mixtures a t high initial pressures, combustion is not completed in the flame front and is not continuous behind the flame. It would seem as though some additional impetus were required to cause the completion of the reactions. When this impetus is given-for example, by the production of a shock wave when the accelerating, vibrating flame is arrested at the end of the cylinder-the combustion is completed almost instantaneously throughout the cylinder, with a consequent increase in pressure. This subsequent energy release may maintain the shock wave, thereby producing effects similar to those of a "detonation wave." I n a non-pinking explosion, such as those of benzene-air mixtures, the combustion reactions are continuous and longcontinued behind the flame front. Even when conditiods are such that the initial flame can commence to vibrate and accelerate, its subsequent arrest does not lead to any violent after-effects as the energy available is not sufficient.
Colored Motion Pictures On July 30, 1928, the Kodacolor Process of colored motion pictures for amateurs was first demonstrated a t the home of George Eastman by C. E. K. Mees, director of the Eastman Kodak Research Laboratories, Rochester, N. Y. The motion picture camera for amateurs has now been developed to the point where the photographer has only to insert a color filter into the lens and thread the Kodacolor film in the camera. After exposure the film is processed and comes back as a roll of black and white film which can be projected in an ordinary projector as a black and white picture. But if the projector is fitted with a special color filter, like that used in the camera, a colored picture will appear on the screen. A great deal of research was required to achieve this result. The color filter is composed of three separate areas-red, green, and blue. The secret of the Kodacolor process is in the film. The film surface is embossed by running it through steel rollers with tiny cylindrical lenses composed of the film base material and extending lengthwise on the film. These lenses are about seven times narrower than the dots making up the illustrations in a newspaper, and are invisible except under the microscope. They completely cover the surface of the side of the film away from the sensitive emulsion. That surface faces the camera lens, and the emulsion is away from the lens. When the trigger of the camera is pressed, light reflected from the subject passes selectively through the three-color filter, through the camera lens, and thence through the tiny embossed lenses on the film t o the sensitive emulsion coating on the opposite side, where it is recorded. The function OF the lenses is to guide the rays of light falling upon each tiny area and lay them on the sensitive emulsion as three distinct impressions corresponding t o the three filter areas, so that the three colors covering the lens are imaged behind each tiny cylindrical lens as three parallel vertical strips, because the tiny cylindrical lenses are parallel to the stripes of
color on the filter. Thus the width of each of the minute areas of emulsion is subdivided into three parts related t o the three filter areas and affected by light that is able to pass through the different colors. The sum of these invisibly small affected areas of film constitutes the whole photographic image. I n order to project the pictures, the developed film is put in the projector, which contains exactly the same optical system reversed. Behind the film is the condenser and the source of light. The color filter consists of the same three primary colorsred, green, and blue. When a picture is on the film, the opaque areas of the film cover up certain of the filter areas and prevent the light from penetrating where it is not needed. For any point on the scene, the only colors projected are those which, on the screen, blend into the corresponding original colors of the scene photographed, each ray contributing its speck of light t o the color or blend of colors a t once point. An important point which required much research is the processing of the exposed film, making the original picture available as a positive for projection on the screen. For this, a most complicated machine has been designed, which works quite automatically. The film is fed in as it comes from the photographer, and is taken out of the drying cupboard as a positive ready for projection. During its travel through the machine it is first developed t o a negative, the developed silver is removed in a bleaching bath, the film cleared of the bleach, resensitized, and then exposed t o an extent dependent upon the original exposure and controlled by the optical density of the li!m itself. After this second exposure, the new image is developed as a positive, the film fixed, washed, and dried, all these operations going on as the film travels forward through the machine. I n a little more than a n hour from the time it entered the machine, the film is ready for projection.
