Reciprocatory machine

0001Title: Reciprocatory Machine

0002This invention relates to a reciprocatory machine, such as an internal combustion engine or an external combustion engine, for example, a Stirling engine or a compressor, comprising at least one piston reciprocable in a cylinder and connected by a connecting rod to a crankshaft. Such machines are inherently dynamically out of balance when the number of pistons is less than four. When such machines operate on a two-stroke cycle, eg when, in the case of an internal combustion engine, there is a combined power and induction stroke and a combined exhaust and compression stroke, the number of power strokes for each revolution of the crankshaft is, of course, doubled compared with a four-stroke cycle. Accordingly, the number of pistons can be reduced in a two-stroke engine compared with a four-stroke engine to achieve the same or similar number of power strokes per revolution. For example, an eight cylinder four-stroke engine, which has good dynamic balance, has the same number of power strokes per revolution as does a four cylinder two-stroke engine. As the cost of an engine reduces with the number of cylinders there is a tendency towards producing two- stroke engines with four or less pistons. Accordingly, there is a need to improve the dynamic balance of such engines where the number of pistons is four or less. An analogous situation applies to other machines of the kind specified.

0003An object of the invention is to overcome or reduce this problem.

0004According to the present invention we provide a two-stroke reciprocatory machine comprising at least one piston reciprocable in a cylinder and drivingly connected by a connecting rod to a crankshaft, two camshafts to operate inlet and outlet valves of the or each cylinder and balance weight means on or driven by said camshafts mutually to counter rotate and thereby eliminate or reduce any first order out of balance force of the engine.

0005The first order force may be balanced according to the present invention because in a two-stroke machine the camshafts are rotated at the same speed as the crankshaft. The drive means from the crankshaft to the camshafts may be any suitable drive means which provides a fixed angular relationship between the rotation of the crankshaft and the camshafts (hereinafter referred to as synchronisation) and the camshafts are mutually counter rotating, ie the camshafts rotate in opposite directions relative to each other.

0006The camshafts may be overhead camshafts.

0007The balance weights, whether the camshafts are overhead or not, may be fixed relative to a respective camshaft and may be integral therewith or carried thereby.

0008The balance weights may be disposed at or adjacent opposite ends of each camshaft.

0009A number 1 crank of a multi-cylinder machine is defined as that crank associated with the end piston when the piston is at top dead centre (TDC). Where the machine has a single piston, the single associated crank is the number 1 crank.

0010The angular positions of the balance weights at the end of each camshaft closest to the number 1 crank are disposed such that when the number 1 crank is at top dead centre, the centres of mass of the balance weights are between the camshafts and the pistons.

0011Where the machine has a single piston all the balance weights may have a centre of mass which is disposed at the same radial distance from the respective camshaft and at the same angular position about the axis of rotation of the respective camshaft for a predetermined angular position of the crankshaft.

0012Where the machine has two to four pistons disposed in line the balance weights at opposite ends of each camshaft may have their centres of mass disposed at the same radial position but with their angular positions 180° out of phase relative to the axis of rotation of the respective camshaft.

0013Where the machine has three pistons disposed in line the balance weights at adjacent ends of adjacent camshafts may have their centres of mass disposed at the same radial position but + 30° and -30° out of phase or antiphase relative to the number 1 crank of the crankshaft. When the engine has four pistons disposed in line the balance weights at adjacent ends of adjacent camshafts may have their centres of mass disposed at the same radial position but with their angular position + 45° and -45° out of phase or antiphase relative to the number 1 crank of the crankshaft.

0014The machine may be a diesel engine or other internal combustion engine.

0015By disposing the balance weights at or adjacent opposite ends of the mutually counter-rotating shafts a common configuration may be provided for 1, 2, 3 or 4 cylinder two-stroke engines with only a variation in the mass, size and position of the balance weights being required to adapt the balance weight arrangement of a specific number of cylinders. By disposing the weights at or adjacent opposite ends of the camshafts a couple created by the weights is optimised.

0016Embodiments of the invention will now be described with reference to the accompanying drawings wherein

0017FIGURE 1 is a diagrammatic perspective view of a single cylinder engine embodying the invention,

0018FIGURE 2 is a diagrammatic perspective view of a twin cylinder engine embodying the invention,

0019FIGURE 3 is a diagrammatic perspective view of a three cylinder engine embodying the invention,

0020FIGURE 4 is a diagrammatic perspective view of a four cylinder engine embodying the invention,

0021FIGURE 5 is a section on the line 5-5 of Figure 1,

0022FIGURE 6 is a fragmentary perspective view showing an alternative camshaft and balance weight configuration, and

0023FIGURE 7 is a fragmentary perspective view showing another alternative camshaft and balance weight configuration.

0024Referring to the Figures 1 and 5, there is illustrated a single cylinder two-stroke diesel engine comprising a single piston 10 reciprocable in a cylinder 11, shown broken away for clarity. The piston 10 is connected by a connecting rod 12 to a crank 13 of a crankshaft 14 supported in main bearings 15, supported on a crank case 16 and shown broken away for clarity.

0025In Figures 1 to 4, crank 13 is the number 1 crank, and is shown as the right-hand crank in Figures 2 to 4.

0026The cylinder is provided with two poppet type exhaust valves 17, which are operated by a first overhead camshaft 18 having exhaust cams 19, and two poppet type inlet valves 20, which are opened by a second overhead camshaft 21 having inlet cams 22. The camshafts 18, 21 are carried by a cylinder head and are driven by a gear drive, not shown, from the crankshaft so as to mutually counter rotate.

0027All the above-mentioned features are provided in conventional manner and more detailed description is not necessary.

0028However, Figure 5 shows one way in which the cylinder head and piston may be configured. Detailed description is not necessary but a diesel injector is shown at I, inlet and exhaust passages at N and X, hydraulic tappets at H, valve springs at S and valve guides at G.

0029If desired, the camshafts may be driven by means other than a gear drive, such as a toothed belt or chain drive, which maintain the camshafts in a synchronous relationship with the crankshaft.

0030The exhaust camshaft, in accordance with the invention, is provided with a sector shaped balance weight, 24a, 24b, at opposite ends. Similarly, the inlet camshaft is provided with balance weights, 25a, 25b, at opposite ends. The balance weights are fixed relative to the respective camshafts either by virtue of being integral therewith or by virtue of being formed separately form and being fixed to, the camshaft by any suitable means.

0031The mass of the balance weights is determined as set out below:

0032(F) = Mrec * R * W² where:-

0033F = Force due to reciprocating masses

0034R = Crank radius Mrec Reciprocating mass (generally complete piston + 7₃ of conrod)

0035W = Angular velocity

0036The following forces are to be balanced: No. of Cyl 1 2 3

FI F

0038Ml - a*F 1.73*a*F 1.414*a*F

0039Requ. Fweight F/4 a/b*F/2 a/b*0.865*F a/b*0.707*F where:-

0040Fl = First order Force

0041Fweight = Centrifugal force, each balance weight

0042Ml = First order moment (couple) a = Cylinder spacing b = Balance weight spacing x = Distance of balance weight from mass centre of engine

0043Secondary out of balance forces are not considered here, as on all four engines, these will be less than those generally occurring on four cylinder in line four-stroke engines.

0044As shown in Figure 1, the arrangement of the balance weights is symmetrical in that all the balance weights are of the same mass, are centred at the same radial distance for the axis of rotation of the respective camshaft and at the same angular position about the axis of rotation of the respective camshaft for a predetermined angular position of the crankshaft. As shown, when the piston is at top dead centre (TDC) the balance weights are at bottom dead centre (BDC).

0045Referring now to Figure 2, there is shown a two cylinder engine which is of essentially the same configuration and construction as the single cylinder engine described hereinbefore with reference to Figures 1 and 5, except for the extra cylinder. The same reference numerals have been used to refer to corresponding parts with the addition of a preceding 2 for the parts associated with the additional cylinder.

0046The mass of the balance weights in this engine is set out in Table 1 hereof. In this case the balance weight, 24a, 25a, is out of phase by 180° compared with the balance weight, 24b, 25b, at the opposite end of the respective camshaft in order to counteract the couple created by the pistons, the crank 213 of the second cylinder 211 being 180° out of phase with the crank 13 of the first cylinder.

0047Referring now to Figure 3, there is shown a three cylinder engine which, like the engine of Figure 2, is essentially similar to that of Figure 1 and, again, the same reference numerals have been used for corresponding parts as were used in Figures 1 and 2 but with a preceding 3 for the parts associated with the third cylinder. Again, the mass of the balance weights are set out in Table 1. In this case not only are the balance weights at opposite ends of each camshaft 180° out of phase, but the phase relative to the crankshaft is different to the examples of Figures 1 and 2. In particular, the crank 213 is disposed at 120° from TDC and the crank 313 is disposed at 240° from TDC when the crank 13 is at TDC. The balance weights are disposed as follows, when crank 13 is at TDC:-

0048Balance Weight 25a 30°

0049" " 24a 330°

0050" " 25b 210°

0051" " 24b 150°

0052Referring now to Figure 4, there is shown a four cylinder engine; again, the only difference being a further additional cylinder and the same reference numerals as were used in Figures 1 to 3 are used for corresponding parts but with a preceding 4 for parts associated with the fourth cylinder.

0053In this embodiment when the first crank 13 is at TDC the other cranks and the balance weights are disposed as follows:-

0054Crank 213 180°

0055Crank 313 90° Crank 413 270°

0056Balance Weight 25a 45°

0057Balance Weight 24a 315°

0058Balance Weight 25b 225°

0059Balance Weight 24b 135°

0060For the four cylinder engine, the crank position (firing order) of 1324/1423 has been assumed in Table 1 as this results in the lowest initial primary out of balance. Other crank positions, ie 1342/1243 or 1234/1432, are also possible, but require larger balance weights for primary balance. (Crank positions 1342/1243 in fact have no secondary out of balance forces and with large balance weights, this arrangement can be in perfect balance on primaries and secondaries.) The firing order will also define the angular disposition of the balance weights.

0061If the weight distance (x) from the longitudinal mass centre of the engine is not symmetrical, the size of the weights can be adjusted proportional to this lever length.

0062Should there be a problem in accommodating the weights, due to a relative small centre distance between the camshafts, these can be overlapped, as shown in Figures 6 and 7.

0063Table 1 shows that the largest weights will generally be required on the three cylinder engine, as its basic out of balance couple is relatively large. For the design of a range of engines, therefore, the weight size of the three cylinder engine will determine the camshaft drive layout.

0064The inclination angle of the inlet valves may not be the same as that of the exhaust valves, so that the camshafts will not have exact equal centres from the cross sectional centre line of the cylinder(s). This will create a small offset out of balance couple, which can be neglected in practical terms.

0065Although the previously described embodiments have overhead camshafts, if desired the camshafts may be disposed at any height position and the valves operated by, for example, push rods. It is necessary, however, that the axis of rotation of the camshafts be at the same height. Slight deviations in relative centre position will only produce negligible amounts of out of balance forces.

0066Analogous balance weight arrangements may be provided for other reciprocating machines such as a spark ignition internal combustion engine, a Stirling engine or a compressor, all of which have at least one piston reciprocable in a cylinder and connected by a connecting rod to a crankshaft.

0067The features disclosed in the foregoing description, or the following claims, QΓ the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.