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).
American passenger car engine design trends since 1925, focusing on compression ratios, anti-knock improvements, and the relationship with fuel octane numbers.
| Identifier | ExFiles\Box 150\1\ scan0011 | |
| Date | 15th April 1935 guessed | |
| 2 DIVISION OF REFINING higher brake mean effective pressure of the present automobile engine. [CHART DATA] Chart Title: Trends of American Passenger-Car Engine Design Since 1925. FIG. 1 X-Axis: Year (1925-1934) Y-Axis: Percent Increase above 1925 (0-100) Plotted Lines: Max. B.H.P, B.H.P. per Cu. In., Compression Pressure, R.P.M. at Max. B.H.P., Compression Ratio, B.M.E.P at Max. B.H.P., Displacement. Caption: Average of values listed in trade publications. Compression ratios of 1934 cars range from 4.95 to 7.18, whereas in 1925 the spread was from 3.6 to 5.1. Anti-knock Value and Engine Design Improve It is apparent, therefore, that the steady improvement in the anti-knock value of fuels which the oil refiner has made during the past nine years has contributed to progressive engine design in four ways: 1. By suppressing the detonation which normally results from an increase in volumetric efficiency. 2. By permitting higher engine speeds obtained by the higher volumetric efficiency, and by eliminating the knock which tends to increase with speed in certain types of engines. 3. By making possible improved cyclic efficiency through higher compression pressures. 4. By overcoming the greater knocking tendency of the larger displacement cylinders which have appeared in certain cars. Refiner Aids Engine-Operating Facility Increased detonating tendency has always confronted the automotive engineer, regardless of the direction in which he has turned in his search for more power; hence improved fuels have largely contributed to making his achievements possible. A gallon of gasoline is no larger today than it was in 1925, but its hidden potentialities account for the major portion of the 88-per-cent increase in average engine horsepower. The average car owner probably takes for granted the availability of gasoline suitable for the new car he purchases without realizing the part the refiner has played in advancing his motoring pleasure, convenience, and economy. As has been stated previously, there is no general relation between permissible compression ratio and fuel octane number. A glance at the compression-ratio specifications of the 1934 cars might lead to the conclusion that engine designers have varied widely in the degree to which they have utilized the fairly uniform grades of gasoline which have been made available by refiners. Actual tests of the cars do not confirm this conclusion, and one is forced to accept the fact that the problem is somewhat complicated. Ten Variables Control Compression Ratio Research and experience have shown that the compression ratio which may be used with a given fuel is influenced by the following 10 variables: 1. Engine design. 2. Cylinder size. 3. Materials of construction. 4. Carbonization, rusting, and liming. 5. Engine speed. 6. Jacket temperature. 7. Mixture ratio. 8. Mixture temperature. 9. Volumetric efficiency. 10. Ignition timing. The known facts in reference to the above-mentioned variables are voluminous, and the scope of this paper permits only a brief consideration of each. Engine Design Affects Compression Ratio Engine design, in relation to knocking tendency, has been a subject of debate since detonation began to impose limits on engine output, and the complexity of the problem is probably ample assurance that the discussion will be continued. For the purpose of illustration, engines may be roughly divided into two classes, viz.: L-head and valve-in-head. Other things being approximately equal, the valve-in-head engine permits considerably lower compression ratios on a given fuel than do L-head engines, but the lower ratios produce a power output comparable with the higher ratios of the latter. Thus an explanation is immediately offered for a substantial portion of the variation in compression ratio of the 1934 cars. THE RELATION OF FUEL OCTANE NUMBER TO ENGINE COMPRESSION RATIO 11 Ratio Not Changed During Operation The compression ratio of a given engine cannot be changed during operation, and the ratio that should be used is determined by the knocking tendency of the engine at the speed at which knocking is most pronounced. If maximum average economy over a wide range of operating conditions is desired, a compression ratio should be chosen which, under the conditions stated in the preceding sentence, requires the retardation of ignition timing below the best power setting for the elimination of knock on the fuel under consideration; but in no case should such a high ratio be chosen that the power developed with retarded spark is less than that at the highest compression ratio which permits maximum spark advance on the same fuel without knock. The latter is represented in Fig. 16 by point A, and the former by point B. At any compression ratio between these two points the thermal efficiency is higher than at A, and the heat-dissipation problem correspondingly less. Economy and Performance Gains Reported At other speeds at wide-open throttle, and at all partial throttle, the knocking tendency will be less; and the spark may be advanced as far toward the maximum-power setting as knocking will permit. The actual maximum-power setting can be used over a wide range of conditions, particularly at partial throttle. Under such conditions full advantage is taken of the increased compression ratio. The adjustment of ignition timing required for the operation described above can be obtained by the careful design of a distributor mechanism actuated by changes in engine speed and intake-manifold vacuum. The proper characteristics must be worked out by careful dynamometer tests. Substantial gains in fuel economy, and some increase in performance over that obtainable through the use of the maximum-power spark advance at wide-open throttle over the entire speed range, have been reported as a result of preliminary tests. [CHART DATA] Chart Title: The Fuel Anti-knock-Value Requirements and Torque of a Large Multi-cylinder Engine at Three Compression Ratios. FIG. 17 X-Axis: Engine Speed - R.P.M. (800-2400) Y-Axis (Left): Fuel Octane No for Incipient Knock (60-80) Y-Axis (Right): Torque - Lb. Ft. (300-500) Plotted Lines: Torque, Octane Number for Compression Ratios 5.27, 4.80, 4.40. Caption: Bore, 4 3/8 in. Stroke, 5 1/2 in. Number of Cylinders, 6. Octane No. Vs.{J. Vickers} Compression Ratio and Torque Fig. 17 shows the influence of octane number on the permissible compression ratio and torque of a large multi-cylinder engine. For the suppression of knock above 1,200 r.p.m. the 5.27 compression ratio requires a 78.7 octane fuel, and the 4.40 ratio a 71.2 octane fuel. At the same speed the torque is 445 for the low ratio, and 495 for the higher ratio, or a difference of more than 11 per cent for 7.5 octane numbers. Public, Refiner, Engine Designer Cooperate In summarizing, it is apparent that any engine having the variables of operation adjusted for minimum-knocking tendency could be run at some fraction of its potential output on almost any fuel at any reasonable compression ratio; hence a correlation of octane number and compression ratio is impossible. However, over-rich mixtures, low volumetric efficiency, and excessively-retarded ignition timing are poor substitutes for anti-knock value in the fuel. The public, the refiner, and the engine designer will all profit from a better utilization of the anti-knock value of gasoline, and greatest progress can be made through the cooperation of the three interested parties. The engine designer can make an effort to keep cylinder sizes within the efficient limits, provide for the cooling of cylinder parts whose temperatures materially affect knocking tendency, and make an effort to utilize the more volatile fuels. The refiner can strive for lower vapor pressure and better anti-knock stability of the lighter fuels. The motorist can assure himself of a full measure of the additional performance made possible through the cooperation of refiner and engine designer by operating his car on the recommended fuels, and by maintaining as nearly as possible the original engine condition and adjustments. | ||