NOVELTY IN ENGINE DESIGN.
Page 15
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Details of the Kirke Patent Four-throw, Straight-eight Petrol Engine with Mixture Governing, Which is Claimed to Have Remarkable Characteristics, Including Selfdecarbonization and a High Brake Thermal Efficiency.
A REMARKABLE type of petrol enI1 gine has been designed by Mr. P. St. George Kirke, M.A., F.I.I)., A.M.Inst. C.E., etc., of Green Gables, Middle Road, Harrow. This engine is of the straighteight type, but has the unusual feature of embodying a crankshaft having only four throws, as each pair of pistons is carried by a forked connecting rod, the respective pistons operating in cylinders the combustion chambers of which are joined.
The Valve Arrangement.
Each pair of cylinders has two inlet valves with an exhaust valve between them, all these valves being interchangeable, the camshaft, consequently, carrying eight inlet cams and -four exhaust. There are also two carburetters, each
feeding, one cylinder of each pair. . • The area of the port, situated just above the exhaust valve and which freely connects the combustion chambers of each Pair of cylinders, is preferably made smaller than the inlet-valve area, so as completely to nullify any tendency for one cylinder Of any pair drawing its mature from the wrong inlet valve and thus spoiling the stratification which is desired.
Testshave shown that the exhaustvalve area may be half that of the inlet valve without appreciably reducing the power of an. engine. This is the reason for the adoption of the single exhaust Valve for handling the exhaust from each pair of cylinders.
The cycle of operations is of great interest. At full load, all eight cylinders work in the usual manner, but as the load
is reduced the petrol jet of one of the two carburetters is gradually closed until, finally, that carburetter delivers air only ; the other instrument, however, continues to deliver a full, charge of the usual explosive mixture, which is fired in the normal manner.
The reason for this individual controlling of the carburetters is that, when the petrol jet of one is partly closed, the instrument delivers a weak mixture which might not be fired by an ordinary spark,
but will be readily fired by the rush of flame into it coming from the explosion of the normal mixture in the adjacent cylinder, when the excess air present will ensure its complete combustion.
After the load has been reduced so far as possible by the complete closing of the petrol jet of one carburetter, further reduction is brought about by throttling both carburetters evenly in the normal manner.
As this throttling does not commence until much later than is usual, reduced compression and turbulence and increased dilution of the whole charge by the products of combustion arc all avoided between full load and one third of full load.
Another point of importance is that any carbon deposit in the four cylinders which continually receive the comparatively rich mixture, may be consumed by using these cylinders for a short periaid as the air' or weak-mixture cylinders.
Ricardo has stated that the power of an engine varies as the square of the diameter of a piston, whilst the weight of the reciprocating parts varies nearly as the cube of the diameter. It therefore
• follows that the reciprocating weight will be reduced more than 30 per cent, by substituting two smaller pistons for one of the same total area, and the inventor claims thatql,his reduction in weight will enable the engine speed to be raised nearly 20 per cent, for the same bearing stresses.
Efficiency Claims.
it is also. claimed that the system of control, by varying the mixture strength, instead of by throttling, enables a large reduction to be made in the fuel consumption when the average running load is one-third of the full load, for throttling the mixture reduces the mechanical efficiency at light loads, as it seriously, increases the losses due to fluid puniping and also reduces the indicated thermal efficiency owing to the loss of compression, increased dilution by exhaust products, slow ignition and complete combustidn.