Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(32) Semi-Diesel Type Engine.

In the “Semi-Diesel” Type Engine the oil is injected into the cylinder at the point of greatest compression in the same manner as in the Diesel engine, and like the Diesel it compresses only pure air. In regard to the compression pressure, however, it stands midway between the pressure of the Diesel engine and that of the ordinary “aspirating” type oil engine, as the compression averages about 150 pounds per square inch. While this is a much higher pressure than that carried by the ordinary kerosene engine which compresses a mixture of kerosene vapor and air, it is not sufficiently high to ignite the oil spray by the increase in temperature due to the compression, but ignites the charge by means of a red hot bulb or plate placed in the combustion chamber.

This type of engine is built both in the two stroke and four stroke cycle types, the events occurring in the same order as in the two stroke and four stroke Diesel types, that is, pure air is drawn into the cylinder on the suction stroke (four stroke cycle) or is forced in at the beginning of the compression stroke (two stroke cycle), and is compressed in the combustion chamber. At the end of the compression stroke, the fuel is injected against the red hot bulb or plate by which the charge is ignited. Expansion follows on the working stroke after the fuel is cut off, and release occurs at the end of the stroke.

Fuel oil is supplied to the spray nozzles by a governor controlled pump having a variable stroke or by compressed air as in the Diesel engine, making the supply of fire proportional to the load. A separate pump is generally supplied for each cylinder, which is capable of developing a pressure of about 400 pounds per square inch. Several of the Semi-Diesel type engines have water sprayed into the cylinder for the purpose of cooling the cylinder and piston, and as an aid in the combustion. This water spray increases the output of a given size cylinder by the amount of the steam formed by the heat of the cylinder and piston walls, and by the increased rate of combustion. The amount of water supplied to the cylinder is equal, approximately to the amount of fuel oil. The water connection is made in the air intake pipe so that the water spray and the intake air are drawn into the cylinder at the same time.

There is very little difference in the efficiency of the Diesel and Semi-Diesel in favor of the true Diesel type for both have accomplished records of a brake horse-power hour on .45 pound of crude oil in units of the same capacity. Neglecting the question of efficiency the Semi-Diesel has many advantages which are due principally to the differences in compression pressures. Valve and piston perfection in regard to leakage is not as essential with the semi-type as with the Diesel, as the former is not dependent on compression for its ignition. This means that the Semi-Diesel has a lower first cost and a lower maintenance expense. Its low compression pressure makes starting possible without the use of compressed air with engines of a considerable horse-power. As the explosion pressure is much lower than with the Diesel type there is less strain on the working parts and lubrication is much more easily performed.

Compared with the ordinary type of kerosene engine the Semi-Diesel is much more positive in its action as the oil is sure to ignite when sprayed on the hot surface of the bulb or plate when under the comparatively high compression. In the engine where the air is mixed with the vaporized fuel before it is drawn into the cylinder, it is difficult to obtain perfect combustion because of the uncertain mixtures obtained on varying loads by the throttling method of governing. At light loads the only difficulty encountered with the Semi-Diesel type is that of keeping the igniting surface hot enough to fire all of the charges.

In the majority of cases the two stroke cycle type of Semi-Diesel engines compress the scavenging air in the crank chamber in the same way that a two stroke cycle gasoline motor performs the initial compression, although there are several makes that compress the air in an enlarged portion of the cylinder bore by what is known as a “trunk” piston. This initial compression determines the speed of the engine, the pressure limiting the time in which the air traverses the cylinder bore and sweeps out the burnt gases of the previous explosion.