916 Opposedpiston engine

The Lucas and Trojan engines, with parallel cylinder barrels, may be described as uniflow opposed-piston engines since the scavenge air flows continuously from the inlet ports uncovered by one piston to the exhaust ports uncovered by the other, the two pistons moving towards each other for compression and away from each other for expansion.

More usually, however, the description is confined to the construction

Fig. 9.15 This GM 92 Series V6 two-stroke diesel unit has a charge cooler between its banks of cylinders

incorporating a single straight cylinder barrel in which the two pistons move in opposite phase towards and away from each other.

This involves either two crankshafts phased by gearing or equivalent means, or a single crankshaft to which the power of one piston is transmitted directly, and that of the other by means of a pair of side connecting rods, or alternatively by means of a symmetrical reversing rocker system for each piston.

Side return rods involve either a three-throw crankshaft, a crank and two eccentrics with smaller throws (Harland & Wolff), or the special arrangement used in the successful Fullagar marine diesel, in which, in each unit pair of cylinders, oblique tie rods connect pairs of pistons which move in phase, suitable crossheads and slides being provided to take the side thrusts. Thus two cranks and two connecting rods serve four pistons. In the return rocker arrangement a two-throw shaft with offset rockers is sufficient for each cylinder.

These have been exemplified in (1) the Junkers Jumo aircraft engine, (2) the Oechelhauser horizontal gas unit even earlier and the more recent Peugeot-Lilloise single-line stationary type made under Junkers licence and (3) a number of recent designs and proposals of which the Commer TS.3 is the best known. The return connecting-rod method, if applied to a multi-line design, would clearly involve a crankshaft of prohibitive complexity and flexibility, with a lack of symmetry leading to balancing difficulties.

When first installed for electric generating purposes in Johannesburg, the Oechelhauser gas engines gave considerable trouble through the distortion of crankshafts and side rods arising from pre-ignition due to dirty gas.

The large gas engine, though important in its day in the utilisation of blast-furnace gas, is obsolete as a prime mover.

The third of the constructions listed above, namely the return rocker arrangement with a single crankshaft, has been the subject of many patents and designs.

Two of these, which are of special importance and interest, are illustrated in Figs 9.16 and 9.17.

Figure 9.16 shows the Sultzer engine produced in about 1936 which exhibited features of historic interest as representing a design by an experienced firm of repute, in which the opposed-piston rocker arrangement is combined with a phased reciprocating scavenge pump, in a manner that will be clear from the illustration.

The admission and discharge of air of the scavenge pump, the capacity of which is considerably in excess of the displaced volume of the main cylinders, is by automatic valves of the reed type.

Fig. 9.16 Sultzer opposed-piston engine

Fig. 9.17 Commer TS.3 ci engine

The bore and combined stroke are 90 and 240 mm respectively, giving a volume of 1527 cm3 per cylinder, the maximum rating under intermittent full load conditions being 22.4 kW per cylinder at 1500 rev/min. This corresponds to a bmep of 586 kN/m2.

Phasing of the pistons, to give a lead to the exhaust, is accomplished by the disposition of the rocker pins and their arc of movement. With clockwise rotation of the crankshaft, it will be noticed that the right-hand rocker has passed its dead point while the left-hand one is just on it, the 180° disposition of the cranks being maintained for balancing purposes.

The Commer TS.3 compression-ignition engine was a post-Second World War example of the opposed-piston and rocker construction which was in production and successful use for about 20 years.

Instead of the phased pump, a Roots blower is used for scavenging, this being more in line with later two-stroke practice. The engine has three cylinders of 82.55 mm bore and a combined stroke of 203.2 mm giving a swept volume of 3262 cm3.

At 2400 rev/min the maximum output of 78.3 kW is obtained, corresponding to a bmep of 414 kN/m2.

The maximum value of the bmep is 724 kN/m2 attained at 1350 rev/min. The minimum specific fuel consumption claimed is 0.232 g/kWh, an excellent result.

The forged crankshaft has four main bearings and six crankpins in pairs opposed at 180°, and for each crankpin there is a normal connecting rod, a rocker, and a short swinging piston link, and piston. Except for the couple arising from the distance between the planes of the two connecting rods of each pair, and the corresponding offset of the two arms of the rocker, the two sets of reciprocating masses are in dynamic balance (see Section 2.4), but heavy pin loading in each set of reciprocating parts is involved in all examples of this construction because of the large masses. Reversal of loading due to inertia forces is less likely to occur than in a four-stroke engine, as compression is met on each inward stroke. Thus running should be quiet at some sacrifice of the benefit to lubrication of thrust reversal. It will be noticed that the rockers in the Commer design are longer than the centre distance between cylinder and crankshaft, but owing to the reversal of the porting relative to the direction of rotation, lead of the exhaust piston is again provided. Figs 9.17 and 9.18 show the general layout of this engine arranged in such a manner as to make it very suitable for under-floor installation.

Full details will be found in Automobile Engineer, Vol. 44, No. 8.

There is also an interesting article by R. Waring-Brown in Vol. 47 of the same journal.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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