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).
Page from 'Automotive Industries' discussing engine compression ratios and constant-clearance aluminum pistons.
| Identifier | ExFiles\Box 50\3\ Scan040 | |
| Date | 27th January 1921 | |
| AUTOMOTIVE INDUSTRIES THE AUTOMOBILE January 27, 1921 163 SLOT THRU PISTON TO ISOLATE THE "SLIPPER" PORTIONS FROM HEAT OF HEAD BRONZE ADJUSTING SCREW—TIGHT FIT STEEL STRUT ADJUST THIS DIAMETER TO .0015 TO .0025 CLEARANCE BASED ON CYLINDER DIAMETER Fig. 3—Constant clearance aluminum piston, showing means for adjusting initial clearance Fig. 4—Type of constant clearance aluminum piston with steel strut cast integrally with slipper portion perhaps an entirely new and better way will be devised eventually to keep the load factor of the engine high to obtain better economy. The conditions revealed by the correlation of engine and car characteristics are so bad as to make it evident that both the public and engineers need a decided change in viewpoint; the public in what they demand, and the engineer in what to furnish and in what to educate the public to expect. Higher Compression Ratios As an illustration of the need for change in viewpoint, let us consider the effect of using higher compression ratios. We all know that the compression ratios used in aviation engines give us much higher fuel economy than those used ordinarily in automobile engines. Then why not use high compression ratios for automobile engines? We are told that higher compressions make the engine knock badly. The simplest and generally accepted way of getting rid of knocking is to lower the compression ratio. Shall we accept this way as final? Why not try to accomplish the same result some other way and at the same time maintain the higher economy? An engine at full load may knock badly at 500 r.p.m. a perhaps will not knock at all at 1200 r.p.m. If we study the curve of brake mean effective pressure, we will find that, at 500 r.p.m., the brake mean effective pressure is greater than at 1200 r.p.m Curve B, Fig. 2, illustrates this. By lowering the compression to eliminate the knock, we obtain curve A. {Mr Adams} Suppose we go back to the higher piston compression ratio and at the same time we delay the inlet-valve closing. Experiments show that we get a brake mean effective pressure in accordance with curve C, the peak of the curve coming at a higher speed than that given by the conventional timing. The pressures at the lower speeds are reduced, which is the desired result to overcome the knocking, while the pressures at the higher speeds are materially increased. The exact timing to use depends on the valve sizes, intake passages, carbureter characteristics and similar factors. The results obtained are more far-reaching than merely keeping the pressures within a range to eliminate the knocking at the lower speeds, and increasing the power at higher speeds. The most desirable results are obtained under car-driving conditions. The small charge of mixture required is taken into the cylinder and compressed to a smaller volume than in the case of the lower compression; also the charge is purer, due to the better scavenging of the higher compression pistons. Tests for 5 to 1 and 4.25 to 1 compression ratios at full load show increases of 13 and 24 per cent in the brake thermal efficiencies at 700 and 2100 r.p.m. respectively; while at these same speeds and at loads required by the car the increases are 22 and 41 per cent respectively. (See Figs. 23 and 24, at 20 and 60 m.p.h.. respectively.) These results are representative of only the first attempt, yet they are quite appreciable gains in economy, due solely to the change in compression ratio. Consider next the aluminum piston, which is almost universally used in aviation engines. The high thermal conductivity of aluminum allows the heat to flow from the pistons more freely than from any other metal commonly used, contributing highly to the best known results obtained from high-speed internal-combustion engines. Why are the aluminum pistons for automobiles, though largely used, condemned by some of our leading designers of national reputation? They know the sterling qualities of the aluminum aviation pistons that make the high power and economy of aviation engines possible, yet for their automobile engines they use cast iron which perhaps could not possibly be used in the aviation engines with the high compression ratios. They tell us the trouble is that the aluminum pistons expand so much when heated that they require excessive cylinder clearance and that this allows them to slap at the lower speeds, or, if fitted closer, to stick at the higher speeds. As an alternative they select cast-iron pistons, use lower compression and get lower economy and greater torsional vibration of the crankshaft due to the heavier reciprocating parts and occasionally scoring the cylinder blocks. Some engineers say, in addition, that the hotter cast-iron piston helps to vaporize the liquid fuel that comes in contact with the head of the piston. Constant-Clearance Aluminum Piston The specific heat of aluminum is greater than that of iron, but the density of iron compared with that of aluminum gives us a heat capacity per unit volume of aluminum only 68 per cent that of iron. However, the conductivity of aluminum is 2.85 times that of iron. From these figures it will be seen that even if the iron piston was 1 1/2 times as hot as the aluminum piston, the heat flow to the liquid fuel in contact with the aluminum piston-head would be about 30 per cent greater, and that, for same temperatures, the heat flow would be about 94 per cent in favor of the aluminum... Since the aluminum piston-head is usually made thicker than that of cast iron, the amount of heat available would be approximately the same in both cases. It appears that the two pistons are on a par, except that heat flow is greatly in favor of the aluminum piston. It has become a habit to think that aluminum pistons must expand. Why not design an aluminum piston that cannot expand. | ||