Article discussing the power gains achievable by 'ramming' an engine's air intake.
| Identifier | ExFiles\Box 122\4\ scan0225 | |
| Date | 25th March 1939 | |
| RM {William Robotham - Chief Engineer} 1036 397 How Much Can Be Gained By “Ramming” an Engine? By P. M. {Mr Moon / Mr Moore} HELDT THE question often arises, especially in connection with the development of racing engines, as to how much power can be gained by turning the carburetor air-inlet pipe forward in the direction of motion and possibly providing it with a scoop or entrance cone. Owing to the inertia of the air, a certain ramming effect must be produced when the carburetor inlet is arranged in this manner, but very little data have been published on the subject, and little seems to be known as to the magnitude of the effect. That it is worthwhile where high speeds are attained seems to be indicated by the fact that aircraft engines generally are provided with such ramming inlet tubes. Of course, modern aircraft travel at speeds of 200 m.p.h. and over, and as the ramming effect varies as the square of the speed, it may be worthwhile on aircraft engines and still negligibly small on automobiles at normal touring speeds. Some light is being thrown on this question by a recent publication of the National Advisory Committee for Aeronautics, entitled “Wind-Tunnel Tests of Carburetor Intake Rams,” by Frank H. {Arthur M. Hanbury - Head Complaints} Highley. Mr. Highley discusses two types of rams, those located inside the cowling of radial aircraft engines and those located outside the cowling. These are again divided into two sub-types according to whether the section of the ramming tube is uniform or whether the tube is contracted at the inlet, the former being referred to as constant-area rams and the latter as expanding rams. With the constant-area rams, owing to the fact that the air velocity at the carburetor inlet is less than the slipstream velocity, upon entering the ram the velocity of the air must be reduced from that of the slip stream to that of the carburetor, and this creates a counter pressure and causes air to spill over the edges of the inlet to the ram. If the section of the ramming tube increases immediately behind the inlet, then the decrease in the velocity of the air can be taken care of by expansion and there need be no spilling of air over the edges. It was found in the experiments at the N.A.C.A. Laboratory that the expansion types of ram were the most efficient. As a result of ramming there is an increase in the total pressure head at the entrance to the carburetor, and in the discussion of the experimental results obtained by the N.A.C.A. this increase in pressure head is designated by p. The dynamic head due to the velocity of the air of the slip stream is designated by q. The increase in the pressure head would be expected to be substantially equal to the dynamic pressure of the slip stream. However, there is a certain loss due to friction in the ramming tube, and another loss due to entrance conditions. With the expansion-type of ramming tube these losses practically disappeared and the ratio of p/q had a value of about 1.01. With the constant-area ram the ratio p/q varied with the ratio Vr {E. S. Voller - Orders} /R, where Vr {E. S. Voller - Orders} is the velocity at the carburetor and V the velocity of the slip stream. Tests were made also with the inlet to the ram directly behind the propeller. Evidently the air velocity directly behind the propeller was considerably greater than the velocity of the slip stream, and in that case the ratio p/q for the expanding type of ram had a constant value of 1.175. It would thus seem that if a ramming tube were used for the air intake of the carburetor on an automobile, of the expansion type, and carried outside the hood, a ramming pressure about equal to the dynamic pressure of the air in the slip stream could be figured with. This dynamic pressure can be arrived at as follows: The kinetic energy of a mass of air of weight W is Wv²/2 g ft.-lb., where v is the velocity of the air in ft. per sec. and g the constant of gravity, viz., 32.16 ft. per sec.² Now assume a frontal area of 1 sq. ft. section Let the speed of the vehicle be V m.p.h., then v = 1.466 ft. per sec. and the volume of air whose kinetic energy is being dissipated in one second is 1.466 V cu. ft. One cu. ft. of air at normal temperature and pressure weighs 0.0764 lb. and the weight of the air whose kinetic energy is dissipated in one second therefore is 0.0764 × 1.466 V cu. ft. The kinetic energy of this air is (0.0764 × 1.466 V × 1.466² V²) / (2 × 32.16) Now, the kinetic pressure is dissipated in overcoming a certain force or pressure through a certain distance, and the energy is equal to the product of the pressure and the distance through which it is overcome. The distance is that through which the vehicle moves in one second, viz., 1.466 V, and to get the pressure we divide the energy by 1.466 V, which gives (0.0764 × 1.466 V × 1.466² V²) / (2 × 32.16 × 1.466 V) = (0.0764 × 1.466² V²) / (2 × 32.16) = V²/391 lb. per sq. ft. = V²/56,300 lb. per sq. in., where V is the car speed in m.p.h. Therefore, at a speed of 60 m.p.h. a ramming pressure of (60 × 60) / 56,300 = 0.064 lb. per sq. in. may be expected under the most favorable conditions. At 30 m.p.h. the ramming pressure would be only one-fourth as great, that is 0.016 lb. per sq. in., and at the conventional touring speed of 45 m.p.h. it would be equal to about 0.035 lb. per sq. in. As the normal atmospheric pressure is 14.7 lb. per sq. in., an increase in the pressure head at the carburetor inlet of 0.064 lb. per sq. in. amounts to a gain of only (100 × 0.064) / 14.5 = 0.44 or less than one half of one per cent. (Turn to page 407 please) Automotive Industries March 25, 1939 | ||
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).