An important aspect of the invention relates to opposed piston diesel engines, but the invention is not restricted thereto. The use of opposed pistons in a diesel engine enables two-stroke operation to be obtained, whereby the output power of the engine is significantly increased, thereby off-setting to some extent the inherent weight disadvantage of a diesel engine.

However, certain previous proposals in relation to opposed piston two-stroke engines have suffered from significant disadvantages, notably their complexity, weight, and inability in some cases to operate at reasonably high rates of rotation. Accordingly, such engines are inherently unsuitable for application to propeller-driven aircraft, and indeed to outboard motor uses. Other prior proposals are subject to limitations in the sequence of events during use, as explained below.

Examples of prior patents disclosing the concepts discussed above include the following : GB 539231 (Goodman) GB 183501 (Enderby) GB 165861 (Vickers)

GB118135 (Penning) EP Al 0122299 (Audi) WO Al 81/03203 (Finley) US 4869212 (Sverdlin) . While these prior proposals disclose the overall

general disposition of pistons and connecting levers, it is found when a study is made of the geometries of the prior disclosures, that important operating problems arise. For example, we have discovered that as a consequence of the crankshaft axis passing through the centre line between the upper and lower cylinders, (see GB 539,231 Fig 3) , it is impossible to obtain symmetrical events between the cylinders, in terms of the cylinder charging and firing sequence. Additionally, the geometry of this prior proposal leads to the fact that the accuracy of the top dead centre position relates to the length of the connecting rods. Also, if both big-ends are on one crank pin the resulting diagonal connection arrangement leads to rocking couples applied thereto and strange angular accelerations, which are disadvantages. Likewise, we have established that you cannot obtain symmetrical events in the two cylinders with the previously proposed crankshaft disposition. Broadly speaking, this is due to the fact that the acceleration of the pistons varies between the two cylinders, due to the angularity of the connecting rods, and this is very critical to engine operation, particularly for a two-stroke engine. Arising from this, we have established that with the general engine layout and disposition of the prior proposals, it is relatively straightforward to arrange that the sequence of events for one cylinder is satisfactory, but that for the other it is not. When an attempt is made to compromise, this can only be done, we have found, at the expense of altering the angular accelerations of the relevant components, thereby resulting in unsatisfactory operating sequences. We have discovered that if the geometry of the engine layout is altered so that the centre line

of the crankshaft is located on or in the region of the centre line of the top or bottom cylinders, this greatly alleviates the shortcomings identified above. For use in aircraft engines, the centre line of the crankshaft is disposed on or close to the centre line of the top cylinder, and in line, or nearly so, with the bottom cylinder for marine applications. It is not essential that the crankshaft axis intersects the cylinder centre line, and the separation of the two may vary according to the application.

According to the invention there is provided an engine, and a method of operating the same, as defined in the accompanying claims.

In an embodiment, an opposed piston engine comprises two pairs of opposed pistons, said pistons being disposed with their cylinder axes spaced apart and extending generally in the same direction, and link means interconnecting the pistons and a rotary crankshaft. The engine is characterised by the feature that the axis of the crankshaft is spaced from a line or axis located centrally between the axes of the cylinders, and is thus nearer to one cylinder axis than to the other.

The axis of the crankshaft may be disposed so as to be closer to one of the cylinder axes than to the said central location. For example, the crankshaft axis may extend through the internal volume of said one cylinder, and in the region of the inner dead-centre position of the pistons thereof. Preferably, the crankshaft axis is located on or close to the axis of said one cylinder.

In the preferred embodiment, said link means comprises a pair of rocker pivots mounted for angular movement at spaced locations between the axes of the cylinders. Links extend between the rocker pivots and the pistons.

Further in the preferred embodiment, the link means may comprise axially-extending shaft means on each of said rocker pivots with crank means connected to said shaft means and spaced from said rocker pivot, and a connecting link being provided between each of said crank means and said crank shaft. In the embodiment, the geometry of the crankshaft disposition and power output arrangements are such as to provide the advantages of firstly symmetrical phasing of port . events, secondly symmetrical phasing of injection ignition points, thirdly symmetrical velocities of both pairs of pistons, leading to enhanced engine balancing, and fourthly symmetrical and reduced piston accelerations, or at least some of these features.

Associated advantages are provided by the preferred embodiment including late injection, reduced piston friction, arising from minimal angularity of the piston rods, improved torque characteristics due to crankshaft lead, reduced torsional vibration, a raised or lowered propeller thrust line with respect to engine bulk, without the necessity for gears on the crank, and wet sump lubrication. These features arise directly, or indirectly through the concept of providing the centre line of the crankshaft on or close to the centre line of one of the pairs of cylinders. The upper disposition is chosen for aircraft engines, and the lower one for marine engines. A notable aspect of the use of a diesel engine in relation to propeller-driven aircraft, we have discovered, concerns the need to minimise torsional vibrations introduced into the propeller by the engine itself. Quite apart from the high rate of rotation required, the torsional vibration introduced into the crankshaft of known diesel engines is, we

have found, completely unacceptable for their use in relation to driving aircraft propellers.

These torsional vibration loadings arise from various sources inherent in the known diesel engine designs, including loads ^' originating from firing, compression, and inertia. Firing and compression loadings arise during the firing and compression strokes. Inertial loadings arise (in four stroke diesel engines) during the exhaust gas discharge stroke of the piston.

An object of this aspect of the present invention is to provide improvements in relation to one or more of the matters discussed above, notably the provision of an opposed piston engine operating with reducing torsional vibration characteristics and/or such an engine in the form of a two-stroke diesel engine and/or the use of such an engine in relation to driving the propeller of an aircraft.

An important aspect of the present invention relates to the identification of engines comprising some or all of the above-mentioned features, and the corresponding identification of the complementary requirements for driving a propeller of an aircraft, and the matching of the one to the other. It is believed that neither of these two requirements was previously known, and therefore their simultaneous use represents a technical advance providing both novelty and inventive step.

A further aspect of the invention relates to the use of the engine for driving the propeller of an aircraft. Whereas the performance of petrol engines for this purpose is significantly affected by the air density, and therefore power output tends to decrease with aircraft height, in the case of the use of a diesel engine the requirement for a predetermined ratio of air to fuel does not apply, and therefore air

to fuel ratios from 25 to 1 to 12 to 1 can be accepted without significant variation in engine efficiency, whereby the effect of height on engine performance is relatively insignificant. A further aspect of the invention relates to fuel injection timing. With normal diesel engines, fuel injection takes place in the region of 25 degrees before top dead centre in order to achieve satisfactory fuel vaporisation and subsequent ignition, having regard to the conventional diesel engine piston speed characteristics. In the case of the present invention, fuel injection can occur significantly later due to maintenance of high or maximum compression for a longer period, whereby the well-known diesel engine knock is greatly reduced. This latter aspect of the invention arises from disposing the crank pins at less than 180 degrees, eg 160 to 175 degrees from each other, whereby, in use, the opposed pistons follow each other along for a short period, thereby maintaining a generally constant volume between them. This piston relationship results in one piston being, for example, 15 degrees past top dead centre when ignition occurs, whereby some useful power is immediately provided instead of the thrust being directed at the crankshaft axis.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which :-

Fig 1 shows a section through an opposed piston engine;

Fig 2 shows, on a somewhat larger scale, a rear view of the engine of Fig 1 showing the output arrangements whereby the rocker pivots are connected to the crankshaft; Figs 3 and 4 show, also on a larger scale, one of the rocket pivots, and a section therethrough,

respectively;

Fig 5 shows a top view of the engine, as seen in the direction indicated by arrow V in Fig 1, illustrating the drive output arrangements; Fig 6 shows a side elevation view in the direction indicated by arrow VI in Fig 1; and

Figs 7 and 8 illustrate alternative gear drive output arrangements.

As shown in the drawings, an opposed piston engine 10 comprises an upper cylinder 12, a lower cylinder 14, upper pistons 16 and 18, lower pistons 20 and 22 slidable therein, rocker pivots 24 and 26, and piston links 28, 30, 32 and 34 interconnecting the rocker pivots and the pistons. The two-stroke engine 10 has inlet ports 36 and exhaust ports 38 formed in cylinders 12 and 14.

Rocker pivots 24 and 26 are journalled on axes 40, 42 respectively. Cylinders 12 and 14 have axes 44 and 46 respectively. Fig 2 shows the drive output arrangements.

Each of the rocker pivots 24, 26 has a tubular shaft 48 which extends axially and carries a crank 50 at its end, as shown in Figs 3 and 4. The two cranks 50 are seen in Fig 2 and are connected by crankshaft links 52, 54 to crankshaft 56.

Thus, crankshaft 56 is located on an axis 58 which actually intersects the axis 44 of cylinder 12, and thus is spaced from a line 59 extending parallel to axis 44 at the mid point between the cylinders. In use, drive is transmitted from the pistons 16, 18, 20, 22 and through links 28, 30, 32, 34 to rocker pivots 24, 26 and through shafts 48 to cranks 50, and thence via crankshaft links 52, 54 to crankshaft 56. The geometry of the relative dispositions of these parts, as shown in the drawings, and as more

8 broadly described above, provides the technical advantages likewise described above in terms of symmetrical phasing of port events, and related advantages as disclosed above. These advantages can be determined from a geometric analysis of the above described engine and from an examination of its performance, but are not immediately apparent without such.

Fig 5 shows crankshaft 56, and the details of its connection to other parts of the engine, as described above, in more detail, including the bearing arrangements including races 60, 62 and an end bearing 64.

Fig 6 shows an aircraft application of the engine with a propeller mounting 66, a blower at 68, a fuel injection pump at 70, and the engine sump at 72.

Figs 7 and 8 show constant mesh gear output arrangements for providing an output drive in a chosen direction. Fig 7 shows a bevel drive 70 provided on crankshaft 56. Fig 8 shows a straight gear drive 72 to provide an output shaft 74 disposed parallel to the crankshaft axis 58.