4000

Figure 9.11 Effect of manifold branch lengths on volumetric efficiency over speed range

The directed port is arranged to cause the air to enter the cylinder at a shallow angle more or less symmetrically about the inlet valve stem and roughly normal to the radius joining the inlet valve and cylinder centrelines. The directed port is more readily used for a twin inlet valve head than the helical port. Figure 9.12 shows semi-diagrammatically one form for a single inlet valve.

Although the detailed form of a helical port may vary quite widely it is usually offset from the valve axis so that a rotary air movement is superimposed on the linear velocity as it enters the cylinder. Figure 9.13 illustrates one form of helical port. Drawings for such ports are at first a little difficult to understand so a perspective view is added to show the tangential approach to that part of the port immediately above the valve, and the rapid helical descent to the back of the valve such that the air largely enters the cylinder with a rotary motion at the outer cylinder radius.

Figure 9.12 Directed port

It is necessary to assess the swirling ability of a swirl producing port prior to building the prototype engine. This is done by making a wooden or plastic full scale model of the port as designed, and blowing through the inlet port and valve in a precisely similar way to that described earlier for measuring the flow coefficients for non-swirl ports.

To measure the swirl intensity within the cylinder a swirl meter must be used. Formerly a light vane was fitted at the upper, open end, of the cylinder liner one bore diameter from the valve which was made to rotate by the swirling air and the speed of rotation measured. Since the air may have a spiralling motion down the cylinder bore, there is some doubt about the measurement of the air's angular momentum.

As a consequence, the recently developed impulse meter has now been adopted. This consists of a light aluminium foil honeycomb which is freely suspended on precision ball races with its periphery sealed by an oil filled trap. The suspended honeycomb is prevented from rotation by a light torque arm bearing on a fixed spring at its outer end. The movement of the restraining spring is measured by an electronic transducer. Thus with a swirling air charge moving up the cylinder, the axial holes in the honeycomb, through which the air must pass to escape, destroy the swirl causing the destroyed total angular momentum flux to produce a torque reaction on the suspended honeycomb. Measurement of this reaction torque is a direct measure of the swirl's angular momentum. Comparative tests indicate that the impulse swirl meter generally gives a higher swirl reading by a factor of 1.3 than the vanes swirl meter. This impulse swirl meter is fitted 1.75 bore diameters from the head face.

As for the flow testing with the simple inlet port, readings are taken at a series of valve lifts from zero to full lift, the swirl meter torque and the actual air flow through the port and valve being measured at each lift. In the light of early tests, modifications may be made to the port shape by cutting material away or adding plasticine until the best swirl and air flow compromise is obtained. Before finishing these swirl tests it is wise to check that the shape of the engine manifold to be used does not have any ill effects. Prototype heads should also be checked so that the required swirl is actually obtained.

The following is a brief derivation of the Ricardo Momentum Summation Method for predicting engine swirl. It should be noted that other workers use differing methods.

It is assumed that (i) the flow through the port is incompressible and adiabatic on both the engine and the steady state flow rig; (ii) the port retains the some characteristics (i.e. CF and AfR) under the transient conditions in an engine as it does under steady flow rig conditions; (iii) the pressure drop, Ap, across the port is assumed constant during induction; (iv) angular momentum is conserved, and skin friction does not impede swirl, (v) Volumetric efficiency is 1 00A; (vi) flow occurs between inletvalve opening and closing so that flow is dependent on valve lift.

Engine crankshaft speed

Impulse meter torque = G

corB

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