GB2062747A - Inlet ports for internal combustion engines - Google Patents

Abstract

A cylinder head of an I.C. engine has at least one inlet port 13 provided with an inlet valve of mushroom- headed poppet type, and an inlet duct 11 formed or provided with means for causing swirling of incoming air about the axis of the cylinder, the inlet duct being formed with a step 30 facing upstream extending transversely to the direction of flow of the air across the part of the duct periphery remote from the cylinder. <IMAGE>

Description

SPECIFICATION Inlet ports for internal combustion engines This invention relates to inlet ports for combustion engines, and is particularly, although not exclusively, applicable to the inlet ports of both compression ignition and spark-ignition engines of the liquid fuel injection type.

An object of the invention is to provide a method of producing inlet ports shaped to induce a high degree of swirl in the inlet gases entering the combustion chamber of the engine.

The general principles of generating a rotary air movement within the cylinder of an internal combustion engine are well known. The moving air charge, which retains its angular momentum during compression, and is, in fact, usually caused to rotate faster at T.D.C. due to being compressed into an open combustion chamber in the piston crown having about half the cylinder bore diameter sweeps by the atomised fuel jets injected into a direct-injection diesel engine combustion chamber, thereby assisting the burning of the available oxygen in the air. This rotary air movement is generally known as 'swirl' and is used to assist combustion in both the direct-injection or open chamber diesel engine, and in certain spark-ignited petrol or gas burning engines.For satisfactory operation, the speed of angular rotation of the swirling air charge must be matched to the form of combustion chamber and the number of fuel jets used. For typical modern diesel truck engines, four-hole nozzles are usually fitted to give four fuel sprays, but if it is decided to use a smaller number of fuel jets, it will be found necessary to generate a higher degree of swirl for satisfactory combustion and hence power output. The problem is to obtain a high swirl and at the same time maintain a low pressure drop across the inlet valve, in order to achieve the highest practical engine volumetric efficiency, and hence power.

Two kinds of inlet ports which generate swirl are known, the first being the so-called helical inlet port; in which swirl is created in the gas on the upstream side of the valve, and which is defined as being an inlet port in which the inlet gases are led through an inlet duct to enter a substantially annular portion of the port, (referred to as the 'bowl') above the valve seat, and around the valve stem. The inlet duct intersects the bowl asymmetrically (with respect to the axis of the inlet valve and the valve seat), so that the outer side of the inlet duct merges smoothly with the internal surface of the wall of the bowl.

The other type of inlet port is so-called 'directed' inlet port, in which a swirl is produced on the downstream side of the valve in the cylinder and in which the axis of the straight downwardly-inclined inlet duct intersects, or almost intersects, the axis of the valve stem in an arrangement which is symmetrical in plan, so that the inlet gases are biassed to enter the combustion space in one direction or in other words the inlet gas has a linear momentum which is converted to the necessary angular momentum within the combustion chamber.

According to the present invention, a cylinder head of an internal combustion engine having at least one inlet port provided with an inlet valve of mushroom-headed poppet type, the inlet duct being formed or provided with means for causing swirling of the incoming air about the axis of the cylinder, is characterised in that the inlet duct is formed with a step facing upstream and extending transverse to the direction of flow of the air across at least a portion of the periphery of the duct. The step may be formed in the roof of the duct, i.e. the part of the periphery remote from the cylinder. In this connection it will be appreciated that the cylinder may be in any orientation but for convenience of description herein, it will be assumed that it is vertical with the valve having its axis vertical and opening into the upper end of the cylinder.

In one form of the invention the duct is of the so-called helical type producing swirling in a socalled bowl upstream of the valve, and the step is located on the side of the valve axis remote from the cylinder axis, slightly downstream from the plane containing those axes.

The step may be straight, but preferably it so departs from a straight line, that on a line transverse to the direction of flow, the air encounters its ends before encountering an intermediate part of it. Thus the step may be generally of chevron of 'V' form preferably having an included angle between about 1000 and 1200.

Further features and details of the invention will be apparent from the following description of one specific embodiment, that will be described by way of example with reference to the accompanying drawings; in which FIGURE 1 is a diagrammatic sectional elevation of an inlet duct for an internal combustion engine; FIGURE 2 is a diagrammatic sectional plan view of the inlet duct; FIGURE 3 is a graph showing the variation of swirl and flow coefficient with valve lift, with and without a step in accordance with the invention; and FIGURE 4 is a diagram similar to FIGURE 2 identifying a number of possible positions in which a step was placed in other experiments described below.

It is usual to evaluate a proposed swirl-producing port during the early design stages of a cylinder head by making a wooden or plastic block incorporating a model of the inlet port. This is then attached to a cylinder liner of the same bore as the proposed engine and with the inlet valve set at a series of fixed lifts from its seat, up to a maximum designed lift, air at a low pressure -- about 250mm water - is blown through the port and valve. Depending on the details of the inlet port shape the air has a swirling motion within the cylinder after passing through the valve. The degree of swirl produced at each valve lift is measured by means of either a rotating vane, whose speed of angular movement is measured, or by an impulse swirl meter. In the present case an impulse swirl meter was used.Essentially this is a matrix of parallel tubes whose length is large, circa 1 5 times, compared with the equivalent diameter of the individual passages rotatably mounted at the exit from the cylinder but restrained from free rotation by a torque measuring device. The swirling air flow entering the matrix has its angular momentum destroyed and, as a consequence, a torque proportional to the swirl is imposed on the rotatably mounted matrix. At each inlet valve lift the swirl momentum is measured, and, at the same time, the volume of air flowing through the inlet valve in unit time is measured.

Numerical processing of these measurements enables a 'non'dimensional swirl number' and a flow coefficient to be obtained for the test inlet port and valve lift to be derived. It can be stated that the higher the swirl number the greater the within-cylinder swirl intensity. The flow coefficient is a known technical term representing the resistance to air flow of the port/valve combination. The higher the flow coefficient or performance factor the lower the resistance.

The 'swirl number' is compared with similar experimentally determined values known to result in a good engine performance with the form of combustion chamber being considered.

FIGURES 1 and 2 show details of an inlet port shape actually incorporated in a wooden model used for experimental purposes in tests as referred to above. The general shape of the port is typical of helical-type swirl ports, already used by many engines in production. Thus, as shown in FIGURE 1 the cylinder head 10 has in it an inlet duct 11 to which air enters as shown by the arrow 12, and which curves down to the seating 13 of the valve. This comprises the usual poppet valve, (not shown) with a stem extending up through a guide 15 and as indicated above assume to be vertical.

As shown in FIGURE 2, the duct as seen in plan view is somewhat in the form of a question mark approaching the axis of the valve asymmetrically in a generally spiral form.

Numerous efforts have been made in the past to enhance the swirl produced by an inlet duct of this general form but it has now been found that an appreciable improvement can be effected by providing a step in the roof of the duct. Accordingly, a step 30 is provided extending across the roof of the duct as shown in FIGURE 1 and facing upstream, so that the gases encountering it will be sharply deflected downwards by the sudden drop in the port roof height.

As indicated in FIGURE 2, the step is located on the side of the valve axis 31 remote from the cylinder axis 32 in the region of a plane containing both those axes. FIGURE 3 shows the measured swirl at a series of valve lifts with the step incorporated as shown in dotted lines in FIGURE 2. The augmented swirl particularly at low lift and high lift can be seen. The flow coefficient is barely affected by the intrusion of the step.

Perhaps the meaning of 'Non-dimensional Rig Swirl' and 'Flow Coefficient' should be defined: If; w = Measured swirl angular velocity G = Torque measured by impulse swirl meter B = Engine cylinder bore M = Mass flow rate through inlet valve V0 = Theoretical velocity through port

AP = Pressure drop across inlet port e = Air density at test conditions.

Then torque measured by impulse swirl meter = G=lo (1) Since assuming forced vortex 1 =M (B)2 = M.B2 (2) 22 8 Non-dimensional swirl number for vaned swirl meter defined as: NR = linear swirl velocity x 2 =w.B (3) theoretical port velocity Vo Substituting, when impulse swirl meter used,(1) and (2) into (3) gives: NR = 8.G = Non dimensional swirl number (4) M.B.Vo The flow Coefficient, CF, is the ratio of the volume of air (0) actually flowing through the inlet valve and port combination at the experimental lift being measured, to the ideai flow through a hole having an area, A, corresponding to the inlet valve inner seat diameter.Expressed mathematically, 'CF Q (5) A.Vo The higher the flow coefficient the lower is the resistance to air flow for the valve/port combination.

Tests were initially made where the step was of straight radial form, a simple vertical step 3.2 mm deep in the roofline in a port suitable for an inlet valve having a diameter of 33.5 mm.

A series of swirl readings were taken at 4 valve lifts, with the discontinuity or step-down at a series of angular positions around the vertical axis of the inlet valve as described in FIGURE 4.

The following table summarised the non-dimensional swirl readings obtained.

Vee-Step Radially straight step (in plan)

Non-Dimensional Valve Lift L/D Position 0.270 0.330 0.390 0.450 No-step 0.684 0.773 0.773 0.678 A 0.658 0.767 0.771 0.687 B 0.693 0.777 0.764 0.684 C 0.699 0.764 0.780 0.687 D 0.696 0.764 0.757 0.667 E 0.699 0.761 0.764 0.697 F 0.703 0.767 0.754 0.707 G 0.703 0.777 0.728 0.680 Dx 0.735 0.831 0.815 0.720 Gy 0.713 0.777 0.748 0.707 Gz 0.693 0.754 0.705 0.671 Non-Dimensional Swirl (NR) It will be seen that position 'D' or 'C' represents the optimum position for the step. Next the radially straight step at positions 'G' and 'D' was modified, the plan shape being an arrow head or chevron with an included angle of 1100. This was found to give an improved swirl 'G"' and 'Dx' compared with the straight step at the corresponding radial positions 'G' and 'D'. At the position 'G' only, a trial of a reversed arrow head shape was tried but this resulted in a lowering of the swirl produced.

As a result of this series of tests it is concluded that the best position for the step-down in the port roof height is at, or close to, position 'D'. Radially the step-down should be broadly as an obtuse angled arrow head in plan, i.e. the air flowing through the port impacts, and is deflected downwards at the radially outward edges of the vertical step first. In practice a forwardly facing arcuate shape in plan would do as well and has been adopted in the port shown in chain lines in FIGURE 2.

Whilst the specific embodiment described employed an essentially rectangular shaped port which conformed to the cylinder head stud pattern, etc, in the engine for which it was designed, there is no basic reason why the step-down in port roof height cannot be incorporated in a fundamentally elliptical or round section inlet swirl port.

Claims (7)

1. A cylinder head of an l.C. engine having at least one inlet port provided with an inlet valve of mushroom-headed poppet type, and an inlet duct formed or provided with means for causing swirling of incoming air about the axis of the cylinder, in which the inlet duct is formed with a step facing upstream extending transversely to the direction of flow of the air across at least a portion of its periphery.

2. A head as claimed in Claim 1 in which the step is formed in the roof of the duct, i.e. the part of the periphery remote from the cylinder.

3. A head as claimed in Claim 1 or Claim 2 in which the duct is of so-called helical type producing swirling in a so-called bowl upstream of the valve, and the step is located on the side of the valve axis remote from the cylinder axis slightly downstream from the plane contacting those axes.

4. A head as claimed in any one of the preceding claims in which the shape of the step so departs from a straight line that a line transverse to the direction of flow encounters its ends before an intermediate part of it.

5. A head as claimed in Claim 4 in which the step is of a chevron or "V" form having an inclined 1 angle between 1000 and 1200.

6. A head as claimed in Claim 4 in which the step has an arcuate form in plan approximating to the Vee form as claimed in Claim 5.

7. A cylinder head as specifically described herein with reference to the accompanying drawings.