What are Fuel Injectors design and operational principles?
23 Jun '17, 01:52
A typical fuel injector can be seen to be two basic parts, the nozzle and the nozzle holder or body. The high-pressure fuel enters and travels down a passage in the body and then into a passage in the nozzle, ending finally in a chamber surrounding the needle valve. The needle valve is held closed on a mitred seat by an intermediate spindle and a spring in the injector body. The spring pressure, and hence the injector opening pressure, can be set by a compression nut which acts on the spring. The nozzle and injector body are manufactured as a matching pair and are accurately ground to give a good oil seal. The two are joined by a nozzle nut.
The needle valve will open when the fuel pressure acting on the needle valve tapered face exerts a sufficient force to overcome the spring compression. The fuel then flows into a lower chamber and is forced out through a series of tiny holes. The small holes are sized and arranged to atomize, or break into tiny drops, all of the fuel oil, which will then readily burn.
Once the injector pump or timing valve cuts off the high-pressure fuel supply the needle valve will shut quickly under the spring compression force.
All slow-speed two-stroke engines and many medium-speed four-stroke engines are now operated almost continuously on heavy fuel. A fuel circulating system is therefore necessary and this is usually arranged within the fuel injector. During injection the high-pressure fuel will shut the circulation valve for injection to take place. When the engine is stopped the fuel booster pump supplies fuel which the circulation valve directs around the injector body.
Older engine designs may have fuel injectors which are circulated with cooling water.
The figure shows a section through a hydraulically operated fuel injector as fitted to a large two-stroke diesel engine. The general design is similar for most engines and consists of a spring loaded non-return needle valve operated hydraulically by a fuel pressure wave from the fuel pump to discharge fuel at high pressure through an atomizer nozzle. A typical fuel injector will consist of a valve body or nozzle holder to which the nozzle or atomizer is secured by a retaining nut. The valve body contains the spring and its compression nut, with an intermediate spindle if required. Surfaces between the body and atomizer are ground and lapped to form an oil pressure-tight seal. A dowel ensures alignment of the oil passages.
The needle valve is lapped into the bore of the atomizer and these must be kept as a matched unit. As shown in the figure there are two chambers in the nozzle, the upper one being charged with fuel oil from the fuel pump and sealed by the needle valve when closed. The lower chamber, or sac, is sealed by the mitre seat of the needle valve and has a number of small atomizer holes of correct size and pattern to atomize and distribute the fuel spray into the combustion chamber.
Injector spring compression is adjusted under test and a compression ring fitted. It is set to allow the needle valve to lift or open at a predetermined fuel pressure. The intermediate spindle conveys the spring compression to the needle valve and may be arranged to limit its lift. The valve will open when the pressure from the fuel pump action on the shoulder of the needle valve overcomes the spring compression. As the needle valve lifts, oil flows to the lower chamber in the atomizer. The additional area of the needle mitre now subjected to pressure causes the needle to lift rapidly, allowing fuel at high pressure through the atomizer holes into the combustion chamber.
When the fuel pump cuts off pressure, the valve will close under spring compression. Since the full area of the needle is now exposed to pressure, closing will occur at a pressure lower than that at which it opened. The action of the needle valve must be rapid and positive with no oil leakage.
A find edge strainer may be fitted at the fuel inlet and a priming or venting plug is fitted to the fuel passage. Valves should be primed if the engine has been out of service or during preparation for commencement of a voyage.
Fuel injectors must be overhauled at regular intervals to ensure correct operation and combustion. The Fuel Injectors must be overhauled and tested in a separate Injection Workshop, the workshop must be maintained in a clean, sterile and tidy condition. The Testing Fluid containment vessel connected to the test pump, must be cleaned out on a regular basis with the system flushed through and the testing fluid replaced complete with new. Carbon and sulphur particles can get entrained in the testing fluid thus giving a false test result. A very fine lapping in compound must be used during the lapping in process. Carborundum type grinding paste’s particles can get impregnated in the lapping surfaces and cause problems. If the fine lapping in of injector parts is unsuccessful, the parts should be forwarded to a reputable fuel injection company for refurbishment. The injector compression spring must be screwed back before slackening the retaining nut. Parts are cleaned, inspected and renewed if necessary. Lapped surfaces must be free of damage and correctly aligned, springs inspected for distortion, atomizer holes must be clear and unworn. Defects in injectors, while in use, may be choking due to dirt in the fuel or carbon building up at the atomizer. A leaking needle valve will cause secondary burning and reduce combustion efficiency.
After assembly the injector is tested with a test pump. Operating pressure and fuel spray pattern are checked and there must be no leakages. Care should be taken as the high-pressure fuel will puncture the skin and if this happens immediate first aid must be carried out.
Fuel injectors are inserted into pockets in the cylinder cover and must form a gas-tight joint at their lower landing. They are secured by studs and nuts.
Fuel pipes between pumps and injectors are subjected to extremely high internal oil pressure with cyclic pressure fluctuations which also cause vibrations. The pipes and their connections are therefore subjected to considerable stress and fatigue. Failure may cause a very dangerous spray of high temperature, atomized oil which is both a health hazard and a fire risk. Where such pipes are exposed, they should be enclosed in a double skin tube. The outer skin will collect any oil leakage and return it to a safe place.
Medium and high-speed engines may be designed with the whole fuel pump system enclosed within protective casings. Some engines have the fuel pipe to the injector placed in a passage cast within the cylinder to the valve pocket.
When burning heavy fuel the injectors will require to be cooled. In many engines this is accomplished by circulating water or oil through additional passages in the injector assembly. Due to the risk of contamination of this coolant with fuel, an independent system is required. If atomize tip temperatures are too high, carbon may form and impede the spray; at high or low temperatures, corrosion may occur.
The design of modern two-stroke engines ensures the injectors are adequately cooled by the transfer of heat to the surrounding bore-cooled cylinder cover. Each injector is also fitted with a spring-loaded circulating valve which permits hot fuel at the circulating pump pressure to pass through passages in the valve body before returning to a buffer tank in the oil system. This maintains the valve at the correct temperature at all times, allowing the engine to be maneuvered on heavy fuels. The high-pressure wave of the engine fuel pump immediately depressed the spring, ensuring that the circulating passages are sealed off before the high pressure lifts the needle valve and injects fuel into the cylinder. A fuel valve of this type is shown below.
The ideal position or a fuel injector is in the center of the cylinder cover, allowing a symmetrical, conical spray pattern in the combustion chamber. This is achieved in most four-stroke engines. In large engines with a centrally placed exhaust valve, injectors (usually three) are placed symmetrically around the cover and charged from a common distributor connections to inject equal quantities of fuel simultaneously.
Engine fuel systems are designed for normal working conditions. If an engine is run for long periods at low power, combustion may be inefficient, leading to fouling and possible wear. When long periods at low power are anticipated a set of low-power injectors may be fitted which have reduced orifice area giving high atomization and peak pressure. It may also be necessary to adjust fuel pump timing. Engines with variable ignition timing fitted can adjust to low powers automatically. They may also be fitted with fuel injectors which maintain high efficiency over a wide range of operating conditions, including slow speeds.
23 Jun '17, 01:58