GETTING THE ” LOW-DOWN ” ON MOTOR SPIRIT
Octane numbers. The C.F.R. variable-compression engine and how it operates New Metrovic-Dodds cathode-ray testing apparatus gives further valuable information
It is a striking paradox that though England produces only a minute fraction of the millions of gallons of motor, spirit which are consumed on her roads, the quality of the No. 1 ” petrol ” to give it the popular name, sold in this country is probably higher than that of any other country in the world, including even the
United States itself. Impurities, gum, sulphur and the like have been eliminated, and while different companies claim with greater or less reason that their petrol favours easy starting, economy or what not, the ordinary touring-car is little affected by a change-over from one brand to another.
Needless to say this is not the case with a sports car, particularly one of small capacity and high performance. With an unsuitable fuel, pinking or detonation takes place with full ignition advance on full throttle, while retarding the spark causes overheating and loss of power. The petrol engine derives its power by reason of the difference of the pressures of gases in the cylinder at the beginning and at the end of the firing stroke, so the higher the compression ratio the greater the alteration in pressure. It follows therefore that the power which can be derived from any type of fuel of similar composition is proportionate to the compression ratio one can use before detona
tion begins. It is worth mentioning, incidentally that analysis of the mixture of hydrocarbons which are blended in the various motor spirits is not sufficient to determine the anti-knock value, even though the properties of the components are known. It can only be discovered by practical tests.
Compression Ratio the First Criterion
The higher the compression ratio the greater the power, as has already been stated, and so it was only natural for the pioneers of fuel research to express the ” goodness ” of the various fuels in this way. The tests were made in variablecompression engines, and the knocking point was determined by car, no easy matter in view of the assorted sounds of a large single-cylinder petrol-engine. In addition to this the tests were made on a number of engines of different design running at whatever temperature the investigator found convenient, so that it was almost impossible to compare the results of individual observers. A third disadvantage of the H.U.C.R. (Highest Useful Compression Ratio) notation was that it was linked up with engine design as well as the intrinsic quality of the fuel itself. A fuel suitable for an overhead-camshaft engine with hemispherical combustion chambers and a compression ratio of 84. to 1 might cause furious detonation on a side-valve engine with the same compression ratio, 7 to 1 being a more suitable ratio for the latter, while a push-rod o.h.v. engine with a flat head would come midway between the two. The method of comparison by permissible compression ratios proved so un satisfactory that some other scheme had to be found. Why not compare the antiknock value of the various fuels by using as a basis of comparison one of the many hydrocarbons which are blended together to form the imotor-spirit of commerce? Iso-octane, a hydrocarbon which is a true compound and therefore invariable in composition, was one of the ” yard-stick ” fuels, the other being heptane. Octane has an anti-knock value much higher than ordinary motor-spirit, while heptane is
extremely ” pro-knock.” By mixing these two fuels in varying proportions, therefore, the scientist can produce a mixture with an anti-knock value equivalent to any type of commercial motor-spirit.
Finding the Octane Number
The fuel under review is tried in one of the special variable-compression engines later to be described against octaneheptane mixtures blended in various proportions, the aim being to get one of the same anti-knock qualities as the fuel under test. The percentage of octane in the mixture then gives a simple index of the anti-knock value, and the figure sc obtained is our ever-popular friend the octane number. If a fuel has an octane number of 70, therefore, it means that its anti-knock value is equivalent to that of a mixture of 70 per cent. iso-octane and 30 per cent. heptane.
Octane and heptane are actually rareand expensive articles, and for ordinary tests ” sub-standard ” . fuels are used. These are fuels of similar composition to. the more expensive ones exactly matched as regards anti-knock value by the addition of tetra-ethyi-lead or ethyl fluid. In the octane number we have something almost tangible, a comparison with a standard fuel which can be made
up in any laboratory. After that, all the scientist requires is a testing engine with equally definite characteristics. These conditions are fulfilled by the C.F.R. (Co-operative Fuel Research) units, which. are in use all over the world, and we were interested in seeing several in operation at the Anglo-American Oil Company research laboratory.
A Telescopic Engine The is a massive
The engine is a massive single-cylinder affair with push-rod operated overheadvalves, coupled to a synchronous electric motor which holds the speed rigidly to. 600 r.p.m. and which is driven off the power mains. The cylinder is about threeinches in diameter, and the head may beraised or lowered by a rack and pinion mechanism to give compressions of from. 4 to 1 to 10 to 1, a range which covers commercial, touring and sports cars. The valve rockers are carried on a swinging bracket which keeps the valve clearance constant with all positions of the head.
The cylinder jacket is steam-cooled and the engine therefore runs at a temperature of 100 degrees C., while the induction pipe is electrically heated. The carburetter hasa single jet and two float chambers, one for the fuel under test and the other for the reference fuel, and the engine can be changed over instantly from one to theother by means of a quick-acting tap.
Measuring the Knock
Most important of all is the “bouncingpin ” mechanism, which indicates the amount and degree of detonation which istaking place; the tests are all carried out with the engine detonating continually. The bouncing-pin is a steel rod some six inches long insulated at its top end by means of a fibre cap and free to slide within a steel tube. At the top end of the tube is an electrical contact-breaker, at the lower end is a plug which screws into the cylinder. The base of this plug is
about inch thick, and is unaffected by the gas pressures of normal combustion, but when detonation takes place, the thin, metal is deflected and in its turn throws up the bouncing pin against the contacts, which remain closed for a time proportional to the severity of the detonation. This switches on a current from a battery which flows through a resistance, heatingup a thermo-couple set in close proximity. The thermo-couple in its turn generates a current and this is shown on a Knockmeter, which is actually, a specially-calibrated milliameter.
Thus equipped, finding the octane number of a fuel is quite a simple operation. A reference fuel with an anti-knock value higher than the fuel under test is chosen, and then ” lead ” is added to the latter until the Knock-meter reading is the same as that of the reference fuel. A table which shows the relation between the amount of lead added and the octane value then gives the required information. Improving Fuels In order to raise the anti-knock value of standard No. 1 petrols, one of three substances may be added (on the continent in fact they are often used in combination). They are benzol, alcohol, and tetra-ethyllead. The first two may be used to excellent effect in the small percentages used in commercial benzol and alcohol mixtures. In large proportions, say 50 per cent. of the fuel, benzol mixtures tend to make the car run rather hot while alcohol has the opposite effect and also calls for larger jets. The third substance, tetra-ethyl-lead, may be used at high concentration if required without affecting the qualities of the fuel
other than preventing detonation. From this it would seem that highly ethylised petrol could be used in racing cars, but in practice this is not so. So much power, and therefore heat, is liberated in the highly developed racing engines of to-day that some means of cooling the internal parts must be found. Alcohol has a high specific heat and by reason of this a powerful cooling effect and is therefore employed in spite of the small amount of power it contains and the high fuel-consumption which goes with this. Some Figures Returning to our consideration of fuel for high-compression touring cars, it is
interesting to learn that over 65 per cent. of the motor-spirit consumed in the United States contains tetra-ethyl-lead, some of it in conjunction with No. 1 fuels and selling at normal prices, while a large proportion is used for ” premium ” fuels, which are extra-high octane fuels which sell at a higher price. Pratts’ Ethyl, the first leaded petrol to be sold in this country, on its introduction in 1928 cost twopence per gallon more than standard fuels, but as everyone knows this and its successor Esso
Ethyl have for three years been brought down to the same price as regular No. 1. This is also the case with the four other brands of ethylised fuels, Redline, B.P., Glico and Power. The ethyl fluid which has such an important effect on fuels is very costly, the price being in the neighbourhood of 122 per litre. Fortunately two to three c.c. per gallon is all that is required to raise the
anti-knock value of No. 1 petrol to the high figure now ruling for ethylised fuels,. the cost being therefore a little over id. per gallon. A Tremendous Improvement One of the reasons why the ethylised
petrols are kept at the normal price in this country is the excellent quality of the standard spirits, which have improved vastly in the past seven years, largely in response to the needs of the small highrevving engine. The ethylised petrols happily enough have improved in like ratio, as is shown by the table given below.
2 14’7 or 8.45 which gives some idea of the possibilities of the new fuels with efficient engines. Fuels with an octane value approaching 100 may soon be in commonuse in air-craft, while another possibility of the future is the use of ethyl fluid with commercial petrols though the disadvantages, such as difficult starting and impurities, which prevent the touring-car owner from making use of them at present, will not be removed by raising the anti-knock The latest ethylised fuels with an octane number of 80 can be used to advantage in any engine with a compression ratio. as high as 7 to 1 where 51 to 6 would have been the limit without the addition of benzol to the petrols of five years ago. We have even used one of these fuels in a supercharged engine with a compression ratio of 61to 1 and a blower pressure of 10 bs. to the square inch, giving an effective compression ratio of approximately 65 x /041+10
value of the fuel. Incidentally it is worth noticing that ethylised fuels can be used in .low-compression touring engines in place of straight fuels without any loss of power, though naturally there is no gain from doing so. To get the maximum results of course the compression should be just short of pinking point when accelerating hard on full throttle.
Seeing inside the Cylinders
The C.F.R. engine has given the fuel research engineer a thorough knowledge of the properties of the various fuels, but until a short time ago his fellow-investigators working on the internal-combustion -engine had very little real data as to what actually happened within the cylinders of an engine, particularly when detonation was taking place. The type of pressure indicator employed on steam engines are -of little use on high-revving, i.e. power plants, in which the ten degrees of crankangle movement during which detonation takes place may only occupy a of a second. A new piece of apparatus, the Metrovic-Dodds Indicator, has however recently been developed and has achieved
the seemingly impossible. Mr. Dodds is one of the research staff at the AngloAmerican Oil Company laboratories and we actually saw it working on one of the test engines already described.
The Indicator consists of two units, a pressure element which screws into the cylinder of the engine, and a cathode-ray tube unit connected to it by wires, which may be installed at any convenient distance. The pressure element is rather like a sparking plug body, made of stainless steel, bored’ away until the end which projects into the cylinder is about 35 thousandths -of an inch thick. In the centre of this is a pile of carbon discs highly compressed with copper wires connected to top and bottom. The outer part of the ” plugbody ” is water-cooled, with rubber insulation to keep the carbon discs isolated from the water jacket. Pressure variations in the cylinder are communicated to the car
bon pile in much the same way as in the case of the ” bouncing-pin ” but in the case we are considering now, every variation of pressure takes effect on the carbon, causing a change in its resistance and a varying current through it. The vacuum-tube has the usual filament which shoots out minute particles or rays, and these are directed on to the ” screen,” a thin internal coating at the conical end of the tube, which here measures some
6 inches in diameter. The rays as they arrive are seen as a small point of light.
A Glowing Indicator Diagram
This point of light can be moved about the screen by means of plates set close to the path of the rays. One pair move it horizontally and the second vertically. The first pair are therefore connected up to an electrical circuit which moves the rays in accordance with the crank-shaft movement, returning them instantaneously to the start at the end of the cycle. The other pair of plates are connected to the pressureelement and move the spot of light vertically in accordance with the pressure. The combination of the two movements causes the spot of light to trace an indicator diagram on the screen of the tube, and owing to the phenomenon of persistance of vision the image remains practically stationary, uncanny with its blue glow, and can be traced or photographed. Furthermore the traversing controls can be adjusted so that only a small portion, such as the 10 degrees during which detonation takes place is visible. This portion of the diagram can then be expanded until it occupies the whole width of the screen. This feature is probably the most valuable part of the indicator and pre-conceived ideas of detonating pressures which are now found to rise only some 50 per cent. above normal explosion pressures, are completely changed. The MetrovicDodds equipment plugs into the mains like a wireless set, but for the moment research on existing engines is limited to those which have two sparking-plug holes per cylinder. Later on it is hoped to be able to combine the pressure element with a conventional sparking-plug, in which case the experimental department of every factory will be able to tell exactly what is going on inside the various engines which pass through its hands and we may even see the fully equipped service-man bringing along his Indicator to diagnose a fault in a client’s car. There will be no excuse for ignorance then I