4. Lower air compression in the cylinders for ignition, and economy in air for starting the engine. 5. Reduction in first costs; and 6. Reduction in upkeep expenses. [Illustration: 900 B.H.P. HIGH DUTY REVERSING ENGINE FOR LIGHT CRAFT] [Illustration: 1,250 B.H.P. LOW DUTY VICKERS MARINE ENGINE FOR OIL TANK VESSEL] With regard to (4), the low compression claimed is rather interesting, as Messrs. Vickers have successfully demonstrated that, although a high compression temperature is necessary in the ordinary Diesel engine with the usual air spraying compressor, a much lower degree suffices for their mechanical injection system, whilst there is a greater certainty of ignition of the fuel on its first introduction, even with the existence of lower compression in the cylinder. The reason of this is that the spraying air used in the ordinary Diesel is usually compressed to about 60 atmospheres (900 lb. per square inch). What happens when air spraying is practised is this. When the cold air carrying the very high pressure above mentioned enters the cylinder, it necessarily expands, owing to the lower temperature already existent there, and such expansion chills the whole mixture, frequently preventing ignition on the first introduction of the fuel. Under the Vickers system of mechanical fuel injection, there is, of course, no introduction of very highly compressed air, and, consequently, first ignition is rendered easier. From this, it will at once be seen that an oil possessing a high flash point can be more easily burned in the Vickers engine than in the ordinary Diesel, with the necessary adjunct of an air compressor. Provision is also made (should the type of fuel used require it) for a higher temperature of compression, and such oils are, therefore, much more easily dealt with than in the ordinary Diesel engine. From what I have already said, it will be evident to the reader that it is only a question of time for air spraying, with its attendant use of the compressor, to become a thing of the past. The mechanism involved by the adoption of the new system of fuel injection developed by Messrs. Vickers is exceptionally simple. It consists of a small fuel pump, such as is ordinarily used for pumping fuel, a reservoir or accumulator of novel form to retain the charge, and a valve with a special nozzle to admit the fuel in the form of a fine spray into the cylinder. The accumulator, I may here mention, is merely a tube, flattened slightly on the sides, and of sufficient length, when the oil is forced into it, to enable it to yield and store up a charge of fuel at the required high pressure, as explained in the next paragraph.