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).
Article from an automotive journal discussing the design and testing of aluminum pistons and fuel vaporizers.
| Identifier | ExFiles\Box 50\3\ Scan041 | |
| Date | 27th January 1921 | |
| 164 AUTOMOTIVE INDUSTRIES THE AUTOMOBILE January 27, 1921 EXHAUST GAS INLET Fig. 5—Type of aluminum piston in which clearance increases as the piston is heated Fig. 6—Transverse section of intake pipe, showing exhaust heated ribs for vaporizing fuel EXHAUST GAS OUTLET 2-7/8 DIA. expand as far as cylinder clearance is concerned? This viewpoint changed the complexion of the whole problem and led to the development of an aluminum automobile piston design that gives results believed to be in advance of the combined merits of the aviation piston and the castiron piston. This piston is illustrated in Fig. 3. In this design the adjustable steel strut controls the cylinder clearance in the direction which prevents piston slap. It will be noted that the aluminum has nothing to do with the cylinder clearance. The strut is subject to almost the same temperature range as the cylinder; hence, the clearance remains constant through the range from a stone-cold to a steaming-hot engine. This type of piston proved very successful from the outset. It is the only type of piston, regardless of material or design, that we have not been able to make stick under abnormal conditions. It has absolutely no slapping tendencies even in a stone-cold engine. Maximum car speed can be maintained indefinitely without causing the pistons to stick. For a more severe test the pistons were run at full load for 30 min. at 3000 r.p.m., with the radiator cooling water shut off so that the engine steamed continuously during the run. This run was made at the end of a full day of high-speed testing with no provision for cooling the oil. The cylinder clearances of the pistons were less than those of any cast-iron pistons that are so far as we know used in quantity production. Two of the pistons had cylinder clearances of 0.0015 in., based on the diameter. The other four pistons had clearances from 0.0020 to 0.0025 in. In spite of all the abuse we have been able to impose on this type of piston, the pistons have always come out of the tests entirely free from any scoring marks and show a decided general tendency to polish-up smoother than the conventional aluminum piston. This undoubtedly follows from the maintenance of the proper clearance at all times, thus avoiding excessive bearing loading. Another striking characteristic of the pistons is their smoothness of operation, indicating that even when the slap in the conventional piston is not audible there is a rumbling sound which becomes noticeable when compared with the operation of the constant-clearance piston. This difference is very marked at both high and low speeds. When the pistons are used with the conventional timing, the knock at full load and low speeds is very materially subdued compared to the conventional type of piston. This clearly shows that cylinder piston clearance has much to do with the degree of audibility of the knock. Fig. 4 illustrates a design which has the strut cast integrally with the “slipper” portion of the piston, the latter being well insulated from the heat of the pistonhead by being separated from it. An alternative of this design is a steel strut cast in place or fastened in some suitable manner. Fig. 5 illustrates an aluminum piston that contracts when heated, so far as cylinder clearance is concerned. The steel struts in this case are shown cast in place. The reason the cylinder clearance increases with the heat on the piston is that the ends of the steel struts attached to the piston-ring-groove portion of the piston-head are carried outward, drawing the “slipper” portion inward since they are attached to the opposite ends of the struts. A large variety of designs can be made embodying the strut idea to accomplish variations of cylinder clearance adjustment and the like, as may be desired for particular cases. It is hoped that these piston illustrations will fix the idea firmly that, so far as cylinder clearance is concerned, we have nothing to fear from the highly expansive aluminum as a piston material. As for the practical merits of the constant-clearance type of piston, they must be tried to be appreciated, because the results they give are so far in advance of one’s highest expectations. The results are indeed a striking illustration of what can be accomplished by a mere change in viewpoint. Fuel Vaporizer Consider now the general experience with exhaust-heated intake-manifolds. It is generally agreed that the results are fairly good, at the expense of a loss of maximum power due to heating the air. On heating the fuel by “hot-spots,” the air is also heated. The experience has been so general that it has practically fixed in many minds as an irrevocable fact that using exhaust heat must necessarily and unduly heat the air. To show that this is not the fact, first let some suppositions be given which can be agreed to readily. Suppose we run all the hot exhaust gases through a jacketed intake pipe, say some 10 in. long, to get ample surface for the “hot-spot”; that is, ample surface to transmit the exhaust heat required to vaporize the fuel, which, it has been observed, goes to the walls of the intake pipe or points of lowest air velocity. Such an intake pipe works well but it also heats the air, coming into contact with the large highly heated surface with the result that the maximum power cannot be obtained. Suppose we now corrugate the intake-pipe so that the air passage is say 2½ in. long, without reducing the area of the inner or outer surface. Fig. 6 shows a cross-section of such an intake pipe designed for the 295.2-cu. in. six-cylinder engine used in the tests. Note the relatively small amount of exterior exposed surface of the heating chamber, an important item for starting out with a cold engine, and the efficient heating of the pipe at low car speeds. The heated portion of the pipe is set at an angle above the carbureter so that the inertia of the fuel globules will throw them directly into the large highly heated surface. The flow of liquid following the wall is toward the inner or smaller radius of the bend. Gravity helps to make the liquid flow over the heated surface. It cannot get out again into the air-stream before being highly vaporized. The highly heated corrugated surface effectively traps the fuel and quickly vaporizes and super-heats the vapor. Tests indicate that the air is very slightly heated while the fuel is highly vaporized. Kerosene is vaporized as readily as gasoline, even at speeds as low as 200 r.p.m. with wide-open throttle. The air is slightly heated because only a very small portion of the air comes in contact with the edges of the ribs at the inner diameter. Tests in which the intake-pipe was abnormally heated, have been run with a loss of only 1.2 per cent of power at 1200 r.p.m. and 2 per cent loss of power at 2400 r.p.m., compared with the best results that could be obtained from the most favorable degree of heat or from unheated plain manifolds. The design as shown | ||