From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical paper on the relationship between fuel octane and engine compression ratio, with handwritten notes regarding a piston seizure test.
| Identifier | ExFiles\Box 150\1\ scan0010 | |
| Date | 15th April 1935 guessed | |
| Handwritten notes: B.2. Piston Seizure Test - Drilled up W.M. Rods. Old set of Aerolite Pistons New Main Bearings Signatures: R.R.S W. Robertson Printed text: The Relation of Fuel Octane Number to Engine Compression Ratio † C. D.{John DeLooze - Company Secretary} HAWLEY * and EARL BARTHOLOMEW * INTRODUCTION Ever since the anti-knock value of fuel became of importance in the operation of automobiles, there has been a natural desire for some kind of formula that would connect engine compression ratio and the octane number of the required fuel. From time to time classifications of cars have appeared which designated certain cars as of the high-compression type and requiring fuel of high octane number, and others of the low-compression type suitable for use on lower-grade fuels. Usually the separation has been made on the basis of absolute volumetric compression ratio, all cars having engines with compression ratios in excess of, say, 5.5 to 1 appearing in the high-compression group. It would be extremely convenient if such a relation between compression ratio and fuel requirement should exist, but actually there is no necessary connection. Engine design and conditions of operation cover a wide range, and compression ratio is only one of a large number of factors which influence the required fuel octane number. A somewhat analogous problem is presented when attempts are made to measure the anti-knock value of fuels by means of chemical tests such as gravity and unsaturation measurements. Fuels consist of many series of definite chemical compounds; and, while it may be possible to develop a definite relation between anti-knock value and certain physical characteristics of a given series of compounds, such equations would be quite different for the various series, and the task would become hopeless. One definite relation exists between anti-knock value and compression ratio: the higher the compression ratio of a given engine, the higher is the octane number of the fuel required for equally satisfactory operation—other factors remaining the same. It is, therefore, interesting to note the general trend of automobile compression ratios and power output in the period during which the oil industry has been improving the anti-knock value of the available fuels. Perhaps of greater significance is a study of the engine factors which control the power-production possibilities of a given fuel. This paper attempts briefly to consider these two phases of the fuel-utilization problem. Engine Increases Its Power Efficiency Fig. 1 shows graphically the average percentage change in the displacement, speed, compression, and horsepower of passenger-car engines since 1925, based on models listed in trade publications. A brief inspection of the curves is sufficient to indicate the remarkable strides which have been made during the past nine years in the development of the automobile engine. Average power output has been increased 88 per cent. The horsepower of the cheaper, and by far the most numerous, cars is now 2 to 4½ times the 1925 figures. Increases in power must necessarily come from increases in one or more of the following: 1. Displacement. 2. Speed. 3. Brake mean effective pressure. Fig. 1 shows that the average engine now has 14 per cent greater displacement than in 1925, and there has been no change in this figure during the past two years. The automobile-engineering fraternity is convinced of the folly of a displacement race, and is concentrating its efforts on a better utilization of a given cylinder capacity and weight of metal. The average 1934 engine has a power-peaking speed 30 per cent higher than in 1925, and there is no indication that the limit has been reached. Because of the high torque requirement of the automobile engine over its entire speed range, it has not been possible to alter valve timing to favor the higher speeds; instead the better high-speed performance has resulted almost entirely from freer breathing and higher compression pressures, which also improve low-speed torque. Brake mean effective pressure at the power-peaking speed has increased 27 per cent. In other words, every cubic inch of engine delivers 27 per cent more power at a given speed than it did in 1925; and, at the same mixture ratio, there is a corresponding improvement in specific economy. Average compression ratios have gone up 30 per cent, and compression pressures at least 46 per cent. The data for the latter curve were obtained from published data on compression ratios in conjunction with average data on the relation of compression pressure to compression ratio in modern engines. It is known that restricted breathing of the earlier engines produced a lower compression pressure at a given compression ratio and, consequently, a lower volumetric efficiency. Higher compression ratios and improved volumetric efficiency are responsible for the * Ethyl Gasoline Corp., New York, N. Y. † Presented by Mr. Bartholomew. | ||