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Purpose of the nozzle: The injection nozzle is a component of the injection system that directs a metered quantity of fuel from the injection pump into the combustion chamber. The purpose of the injection nozzle is to direct the fuel into the combustion chamber in a manner that will provide optimum engine performance with a minimum of emissions. The injection nozzle accomplishes this purpose in two ways: (1) atomizing the fuel, and (2) spreading the fuel spray in a particular pattern.

Atomization is the process of breaking down the fuel into very fine droplets. This process is necessary to mix the fuel with the compressed air, while forming a vapor. For the air to mix readily with the atomized fuel, the fuel is sprayed into the combustion chamber in a particular pattern. This is called the spray pattern. The spray pattern varies, depending on the shape of the combustion chamber and type of nozzle.

Basic parts of the injection nozzle: The injection nozzle is a component that can be divided into two parts: a nozzle holder and a nozzle. The nozzle holder supports the nozzle in the cylinder head. The nozzle is in the lower half of the injection nozzle, which contains the parts necessary to allow and prevent fuel flow. The nozzle is the component that directs fuel into the combustion chamber. It also contains a valve and a seat that prevents fuel from flowing when the valve is held against the seat. When the valve is pushed off its seat, the fuel can exit the nozzle. Note that some manufactures call the nozzle and holder a fuel injector. An injection nozzle receives fuel already under high (injection) pressure and does not boost pressure on its own.

Types of Nozzles: All nozzles presently used in diesel engines are differential pressure, hydraulically operated nozzles. This hydraulic action occurs when diesel fuel sent under high pressure by the injection pumps overcomes spring pressure. When this happens, the nozzle valve opens, allowing the fuel to exit the nozzle. When the fuel pressure drops, the spring closes and the nozzle valve, cutting off fuel flow.

Nozzles can be classified into two basic groups: inward opening and

outward opening. (1) In the inward-opening nozzle, the nozzle valve moves up into the nozzle body. (2) In the outward-opening nozzle, the nozzle valve moves out, away from the nozzle body. There are several variations of these two basic types of injection nozzles.

Inward-opening nozzles: Hole-type nozzles contain holes in the top of the nozzle. The number and size of these holes dictate what shape the spray pattern will be for proper combustion. Pintle-type nozzles use a tapered valve that seats in a single orifice in the valve body. This produces a cone-shaped spray pattern.

Outward-opening nozzles: Poppet-type nozzles use a tapered valve that seats in a single orifice in the valve body. The poppet nozzle valve moves outward, producing a fine cone-shaped mist.

Nozzle testing: Nozzles are subjected to the intense heat and high pressure of the combustion chamber. Furthermore, they must provide a positive seal against any fuel leakage past the nozzle tip. Nozzles can operate for long periods of time without maintenance. However, contaminated fuel, misuse, and mechanical failure

can reduce the nozzle’s life expectancy. For this reason, nozzle servicing and testing is important.

Proper checking of a nozzle requires a nozzle tester: This tester can perform a series of tests on the nozzle. Three tests common to all nozzles are the opening pressure, spray pattern, and seat tightness tests.

Opening pressure is the point where the nozzle begins to spray fuel: Spring pressure has a direct effect on opening pressure. Distorted or binding nozzle valves adversely affects opening pressure.

Spray pattern is the shape of the fuel as it exits the nozzle: Carbon and damaged nozzle tips are two common causes of a poor spray pattern.

Seat tightness tests for fuel leakage at the nozzle tip: A fuel droplet at the nozzle tip indicates a worn nozzle valve and seat.

Two more tests performed on some nozzles are the chatter and return fuel tests.

Chatter is the noise produced when the nozzle valve opens and closes rapidly. This noise indicates that the nozzle valve is moving freely in its bore.

The return fuel test determines the amount of fuel that leaks past the nozzle valve and returns to the fuel tank. This test ensures that the nozzle is being adequately cooled and lubricated.

There are two precautions that must be strictly adhered to while using a nozzle tester.

1. CAUTION:

Test Fuel Spray is Flammable. Keep vapor away from open flames and sparks.

2. CAUTION:

When testing nozzles, do not place your hands or arms near the tip of the nozzle. The high-pressure atomized fuel spray from the nozzle has sufficient penetrating power to puncture the skin and can destroy tissue, which may result in blood poisoning. The nozzle tip should be enclosed in a receptacle, preferable transparent, to contain the spray.

II. Internal Combustion Sample Questions

Q. Excessive bearing wear in a diesel engine can cause: A. low oil pressure.

Q. A diesel engine converts:

A. heat energy to kinetic energy.

Q. Precombustion chambers used on diesel engines help with: A. lowering exhaust emissions.

Q. Oil sludge, cylinder wear and piston ring wear can be caused by: A. excessive sulfur in diesel fuel.

Q. What determines the high fuel efficiency of a diesel engine? A. Air fuel ratio, compression ratio, and heat value

Q. Fuel ratio means:

A. parts air to parts fuel ratio

Q. The four strokes of a diesel engine are: A. intake, compression, power, and exhaust. Q. Excessive sulfur in diesel fuel can cause:

A. piston ring wear, cylinder wear, and oil sludge. Q. The diesel engine exhaust valve controls:

A. the escape of burned gases.

Q. The camshaft of a diesel engine controls: A. the intake and exhaust valves.

Q. What methods are used to cool compression ignition in diesel engines? A. Water cooling and air cooling

Q. Why are precombustion chambers used on diesel engines? A. to lower emissions

Q. What are some advantages of diesel engines?

UNIT III