Why has the cross-scavenged engine been superseded by the uniflow-scavenged engine?

The cross-scavenged engine cannot take advantage of an increase in thermal efficiency by increasing the stroke-bore ratio. The stroke-bore ratio of modern uniflow-scavenged engines may be between 2.4 and 2.95. This allows for a greater ratio of expansion; the increase in thermal efficiency reduces the specific fuel consumption and so reduces fuel costs. As fuel costs make up a large part of the daily running cost of a ship, engines, if they are to be commercially attractive, must have the lowest possible specific fuel consumption.
Note The ratio of expansion is governed by the compression ratio, the bore- stroke ratio and the timing of the opening of the exhaust valve. The opening point of the exhaust valve is related to the power demand of the turbocharger. An increase in the efficiency of the turbocharger allows the exhaust valve to be opened later. Opening the exhaust valve later increases the thermal efficiency of the engine and lowers the specific fuel consumption.
Note By 1981, only one of the three principal slow-speed engine builders was still building cross-scavenged engines. The other two builders had always built uniflow-scavenged engines. Today ail slow-speed engine builders and their licensees build uniflow-scavenged engines only, but large numbers of loop- and cross-scavenged engines will remain in service for some years to come.

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