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n Is there any advantage in using foui valves per
Page 78
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cylinder, as typified by Cummins diesels? It may be concluded thai a four-valve head enables more air to be introduced into the cylinders, but if so, why do not all engine makers employ four valves; Turbocharging increases the weight of ai) in the cylinders; does the bhp developed by an engine depend on the weight of the aij consumed?
A When planning the valve layout of at 1--s engine, a designer has to take int( account a number of factors including the filling or volumetric efficiency of the unit ant valve cooling, notably cooling of the exhaus valves.
While the use of two small inlet valves ir place of one larger valve should provide bette filling and in a particular case may well do so the protagonists of two-valve heads say tha a single inlet valve provides increased scopi for improving the shape of the inlet passages particularly in the case of a small-bore long stroke engine. Employing two small exhaus valves has the advantage that it aids coolini so long as the water passages can be arrangei to facilitate heat dissipation. Whether or no a valve layout enables the injector to b, located in the optimum position is also al important consideration. It is notable that , well-known engine maker employs a four valve head in the case of the industrial engine in the company's range while the automotiv. engines produced have a two-valve head There is no hard-and-fast rule that decide which type is the best. It may depend on th, rated maximum speed of the unit.
While the weight of air in the cylinders of diesel determines the power that is produce, at a given speed, output is also dependent oi the rpm of the unit. Turbocharging is normalf employed to produce a higher output. withou increasing rpm or at a slightly reduced rpm It should be emphasized that "flowing head" to get maximum filling and a directioi of air flow through the valves that promotes form of swirl in the combustion chamber that i favourable to combustion is a highly technicz development exercise.