Classes of Atomizers or Fuel Valves.—The many fuel valves employed on the various makes of Diesel engines fall into two classes—the closed-nozzle and the open-nozzle valves. The former is the one adopted on the pioneer Diesels and is found on all vertical engines of the present day; in fact, this is necessary for structural reasons. The open nozzle, known in Europe as the Lietzenmayer nozzle, is largely used on the horizontal engines, both of the domestic and foreign manufacture.
Open-nozzle Fuel Valve.—In the design of this nozzle, or fuel valve, the needle valve controls the flow of air to the atomizer tip, this needle valve being opened at the proper time by a cam actuated rocker. Between the valve and the cylinder is inter posed a small cavity, or enlargement of the passage to the cylin der; into this cavity is deposited the fuel charge. Since the fuel is pumped during the suction or exhaust stroke of the piston, the fuel pump works against only a few pounds pipe-resistance pressure. This enables the governor to be very sensitive in action since the reaction of the pump is at a minimum. At the proper time the needle valve is opened, and the air from the compressor, as it sweeps along the passage to the cylinder, picks up the oil charge and carries it into the cylinder. The swirling of the air serves to thoroughly break up the mass of the fuel; this is further increased by perforated disks or other devices at the nozzle tip. Figure 111 outlines this class of injection valve. It would appear that the duration of the fuel injection would be in exact ratio to the amount of oil deposited in the atomizer and that the first particles of oil entering the cylinder should be as completely nebulized as are the last few oil droplets. In actual practice the air, when it strikes the fuel charge, produces slugs of oil that.enter the cylinder in an unatomized condition., Where perforated disks intercept the slugs, as in some designs, the entire charge is satisfactorily broken up.- The only real ob jection the operator can well offer is the rapid carbonization of the fuel passage and atomizer disks; this can be attributed to the absorption of heat from the combustion chamber since the temperature in the nozzle tip fairly approaches that ex isting in the cylinder on the compression stroke. This neces sitates a more frequent cleaning of the atomizer than is re quired by a closed type of nozzle.
The open-type valve is of advantage where dirty oil is burned. Since nothing but pure air passes the needle valve, the scoring of the valve seat, so prevalent with the closed-nozzle fuel valve, is entirely absent. Consequently, this valve requires less than
half as much regrinding as does the closed type.
Closed-nozzle Fuel Valve.--The earlier Diesels employed the closed ,nozzle, and it was exclusively used on all Diesels until five or so years ago. Figure 110 outlines the basic principle of this type of nozzle. The fuel valve has a body in which a cavity is formed, enclosing the atomization device. The fuel needle valve is seated below this device and is actuated by a cam controlled lever. The interior of the fuel valve is in connection with air line through the passage and is at all times under an air pressure of 900 or more lbs. per sq. inch. The fuel pump forces the oil charge through the line A, the oil settling around the valve stem at E above the valve seat. When the needle valve opens, the compression pressure of the engine is around 500-550 lbs., while the air pressure in the valve body is about 900 lbs. This pressure difference results in a rapid flow of the air into the cylinder. The air charge forces the oil along with it, and, in passing through the tortuous passages of the atomizer disks, the oil is completely nebulized.
This form of fuel valve has the advantage of depositing the oil in a receptacle entirely isolated from the influence of the, hot compressed air in the cylinder. Furthermore part of the oil, being immediately around the valve tip, enters the cylinder ahead of the air and ignites, even though it is not thoroughly atomized. This primary ignition provides a flame to fire the remainder of the oil, which enters the cylinder at a somewhat low temperature due to the expansion of the air charge at the valve tip. Unfortunately, with many closed-nozzle designs, an entirely too great a percentage of the fuel enters the cylinder ahead of the air; in some it appears that all the oil is forced ahead of the air. If the disks are designed with perforations of small diameter to enable the air to mix with the oil, the "braking" or resistance of.the atomizer is increased since the disks are entirely filled with oil at full load. This compels the employment of a. higher injection pressure, or the time interval of fuel injection is prolonged. On low loads, with small perforations, the fuel charge is of small weight and does not flow down around the valve seat. The consequence is that the first part of the injec tion consists of air only, which, in expanding, chills the nozzle tip and housing. This delays the combustion, producing a smoky exhaust.