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).
The relationship between engine parameters like ignition timing, compression ratio, cylinder size, and materials on fuel octane requirements and performance.
| Identifier | ExFiles\Box 150\1\ scan0012 | |
| Date | 15th April 1935 guessed | |
| [PAGE 10] 10 DIVISION OF REFINING and torque are reduced as manifold vacuum increases. When the former is reduced from 75 to 64, engine torque is reduced by 3 per cent. It is obvious that throttling is a most inefficient method of reducing knocking tendency. A reduction in compression ratio is much to be preferred. It should be pointed out that because of the surging of the intake-manifold pressure of a single-cylinder engine and the consequent difficulty of measurement, the manifold vacuum shown in Fig. 14 is only relative. Ignition Timing Affects Fuel Consumption Ignition timing is a well-known factor which influences fuel requirement. Fig. 15 shows the effect of spark advance on octane number and torque for a single-cylinder engine. Multi-cylinder engines may be more critical. It should be borne in mind that the spark advance for best power at any engine speed or throttle opening is influenced by engine temperature, atmospheric conditions, carbon accumulation, and other factors; hence engines must be sent out with a spark setting which represents a compromise for average conditions. It cannot be too far advanced because of excessive knocking tendency during the summer or too far retarded because of lack of performance during the winter. Problem Is One for Research Fig. 16 shows the relation for one engine of brake mean effective pressure, compression ratio, spark advance, and knocking tendency at one speed and wide-open throttle; and indicates an interesting field for research on performance and economy. It will be noted that for a fuel of a given octane number there is a combination of compression ratio and spark advance which produces the maximum brake mean effective pressure, but that the spark advance is retarded from the maximum-power setting determined for the same compression ratio when a non-knocking fuel was used. At other speeds and compression ratios different relations between brake mean effective pressure and spark advance, determined on the same fuel, would exist. The problem, therefore, becomes one of determining for a range of speed and throttle opening the combination of the above-mentioned variables which will produce the highest brake mean effective pressure. [GRAPH: FIG. 15] Left Y-Axis: Fuel Octane No. for Incipient Knock Right Y-Axis: Percent of Maximum Torque X-Axis: Spark Advance - Deg. F.{Mr Friese} Curves: Torque, Octane No. Caption: The Effect of Variation in Ignition Timing on the Torque and Fuel Anti-Knock-Value Requirements of a Single-Cylinder Engine. FIG. 15 Graph Data: Bore, 3 1/4 in. Stroke, 3 7/8 in. Speed, 800 r.p.m. Air intake temperature, 85 deg. F.{Mr Friese} Throttle opening, full. Cylinder-head material, cast iron. Compression ratio, 5.75. Jacket temperature, 190 deg. F.{Mr Friese} Air-fuel ratio, 13.5. Spark advance, varied. [GRAPH: FIG. 16] Y-Axis: Brake Mean Effective Pressure X-Axis: Compression Ratio Graph Labels: IGNITION TIMED FOR MAX. POWER, Required Octane No. Increasing, IGNITION TIMED FOR INCIPIENT KNOCK. CONSTANT OCTANE NUMBER. Caption: Relation Between B.M.E.P., Octane No., Ignition Timing, and Compression Ratio at Constant Speed. FIG. 16. [PAGE 3] THE RELATION OF FUEL OCTANE NUMBER TO ENGINE COMPRESSION RATIO 3 tional brake mean effective pressure that was obtained by increased throttle opening of a high-compression single-cylinder engine made possible on the same fuel by abnormal cooling of the exhaust valve and piston. The piston was cooled by a spray of soap suds against the under side, the same liquid having been used also for crankcase lubricant. The exhaust valve was hollow, as shown in Fig. 3, and was cooled by the circulation of cold water through the stem and head. The percentage gains in power output would not have been so large had the throttle opening been kept constant and the increased compression pressure gained by increases in compression ratio, but would have been of approximately the same magnitude had the engine been operating at full throttle been super-charged. Both of the cooling methods described above are limited to laboratory use, but the results obtained should serve as a goal for engine builders in designing within the limits of practicable possibilities. It may be pointed out that the economical mass production of automobile engines seems to preclude the use of many of the types of construction which have contributed to the superior cooling of aircraft engines. Cylinder size bears a definite relation to permissible compression pressure on a given fuel. Any increase in cylinder dimensions tends to increase detonation tendency; hence the larger cylinders are limited to lower brake mean effective pressures than are possible with the smaller cylinders of the same general design. Fig. 4 shows a series of single-cylinder heads and variable-bore blocks used for a study of this variable. Two crankshafts, giving different strokes, were also used. A single-cylinder engine was chosen for a study of this and the other variables covered by the paper because of the better control that could be exercised over the variables not under investigation. Experience has indicated the good correlation of the data with those obtained on multi-cylinder engines, and a few data for the latter engines are included in this paper. Fig. 5 shows the variation in permissible compression ratio on a given fuel for the several combinations of bore and stroke, and Fig. 6 shows the variation of indicated mean effective pressure. It is apparent that the smaller cylinders, because of higher permissible compression pressures on a given fuel, develop more power per cubic inch at a higher thermal efficiency than do the larger cylinders. Construction Materials Affect Ratios It is definitely known that the thermal conductivity of metals is of much importance in aircraft-engine design. The possible advantages of the more highly-conductive metals in automobile-engine design have not yet been very well evaluated. Fig. 7 shows a comparison of iron and aluminum cylinder heads over a range of compression ratios when used on the single-cylinder engine previously mentioned. The data indicate a slightly lower brake mean effective pressure and octane-number requirement for the aluminum heads at the same compression ratio. The brake mean effective pressure at any given octane number is practically identical for the two types of heads which, incidentally, were cast from the same patterns. Fig. 8 shows octane-number requirement and brake mean effective pressure over a wide speed range for an eight-cylinder automobile engine equipped with cast-iron heads of 5.3, 6.3, and 7.0 to 1 compression ratio, and with an aluminum head of 7.0 to 1 compression ratio. In this case the aluminum head shows an advantage of approximately 2 octane numbers for the same brake mean effective pressure. Both types of heads were cast from the same patterns. There has been some progress in combustion-chamber design during the past few years, and it is possible that the combustion-chamber shape which is best for cast iron is not the most efficient for aluminum. Comparisons should be made of designs which represent the best achievements to date in the use of the two metals. The aluminum head has possibly not appeared to best advantage, because it has been made interchangeable with iron heads. Conventional steel cylinder studs have an expansion coefficient entirely different from that of aluminum, and a tendency has been noted for aluminum heads to be crushed under the stud nuts. Under certain conditions of operation the cylinder head may not subsequently be in too good contact with the cylinder-head gasket. It would seem that more attention should be given to higher-expansion cylinder studs and to any [GRAPH: FIG. 2] Y-Axis: Increase in B.M.E.P.—Per cent X-Axis: Air-Fuel Ratio Curves: Valve and Piston Cooling, Piston Cooling, Valve Cooling. Caption: The Effect of Abnormal Cooling of Piston and Exhaust Valve on B.M.E.P. of Throttled Engine for Equal Knocking Tendency. FIG. 2 Graph Data: L-Head, Single-Cylinder Engine. Bore, 3 1/4 in. Stroke, 5 in. Jacket temperature, 212 deg. F.{Mr Friese} Spark advance, maximum power. Compression ratio, 7.0. Throttle opening, varied. Cylinder-head material, cast iron. Speed, 600 r.p.m. Air-fuel ratio, varied. Piston material, aluminum. | ||