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The most outstanding feature of the diesel engine is its efficiency. By compressing air rather than using an air-fuel mixture, the diesel engine is not limited by the preignition issues that affect high-compression spark-ignition engines. Therefore, higher compression ratios can be achieved with diesel engines than with the spark-ignition variety; equally, higher theoretical cycle efficiencies, when compared with the latter, can often be realized.

It ought to be noted that for a given compression ratio the theoretical efficiency of the spark-ignition engine is more than that of the compression-ignition engine; but in practice it is possible to operate compression-ignition engines at compression ratios high enough to produce efficiencies greater than those attainable with spark-ignition systems. Furthermore, diesel engines do not rely on throttling the intake mixture to control power. Therefore the idling and reduced-power efficiency of the diesel is far superior to that of the spark-ignition engine.

The principal drawback of diesel engines is their emission of air pollutants. These engines typically discharge high levels of particulate matter (soot), reactive nitrogen compounds (commonly designated NOx), and smell, compared with spark-ignition engines. As a result, in the small-engine category, consumer acceptance is low.

The diesel engine is started by driving it from an external power source until conditions have been established under which the engine can run by its own power. The most basic starting method is to admit air from a high-pressure source, about 1.7 to nearly 2.4 megapascals, to each of the cylinders in turn on their normal firing stroke. The compressed air becomes heated enough to ignite the fuel.

Other starting methods involve auxiliary equipment and include releasing blasts of compressed air to an air-activated motor geared to rotate a large engine’s flywheel; supplying electric current to an electric starting motor, similarly geared to the engine flywheel; and applying a small gasoline engine geared to the engine flywheel. The selection of the most suitable starting method depends on the actual size of the engine to be started, the type of the connected load, and whether or not the load is able to be disconnected on starting.

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