374 Diesel Engine Technology Copyright Goodheart-Willcox Co., Inc. Spray Patterns Figure 18-9 illustrates the spray patterns of the various nozzle types discussed thus far. To the naked eye, the spray cone generated appears to be solid, but as shown in Figure 18-10, the cone actually consists of different regions of air-fuel mixtures. Spray cones are precisely calculated by design engineers to produce maximum fuel efficiency, complete combustion, and minimal emissions. These objectives are achieved through the manipulation of nozzle hole size, nozzle hole length, spray angle, and injection pressure. Servicing Fuel Injector Holders and Nozzles When servicing injectors and nozzles, it is very important to follow the test specifications listed by the injector manufacturer. For example, if the injector opening pressure were mistakenly increased due to poor servicing or adjustment, the start of the injector spray would be delayed and fuel spray velocity would increase. As a result, the length of the spray cone would increase and the spray angle would decrease. Fuel droplet size would decrease and the fuel penetration distance into the combustion chamber would increase. Fuel would now spray to areas in the chamber out of reach of the air stream. This would cause an increase in the delay time, and the engine would operate with increased emissions, a loss of power, high fuel consumption, and noticeable fuel knock. Injection nozzles are precision units that will operate for very long periods if given proper maintenance. This involves changing all fuel filters at required intervals and using clean, quality diesel fuel. General maintenance also includes a complete cleaning and inspection at manufac- turer recommended intervals—usually every 1000 hours of operation. Factors That Can Reduce Injector Life Several factors can reduce the service life of diesel fuel injectors. Abrasives (dirt) in the fuel can wear down fuel-carrying components, enlarge the tolerances of the needle valve and bore, damage the valve seat, and enlarge the spray openings. Excessive heat will change the expan- sion rate of the nozzle and accelerate wear. This acceler- ated wear causes higher leak-by and reduces the amount of fuel available for injection. Water in the fuel will cause corrosion and can dilute the lubricating oil. Water will also not pass through the spray holes on multi-hole nozzles, and may damage or break off the nozzle tip. If the sulfur content of the fuel is too high, deposits may form that block passages and affect performance. Sulfur is also corrosive. Finally, improper installation can result in insufficient cooling, seizure of the needle valve, or poor valve seating. Procedures for Injection System Service When servicing a fuel injection system, always follow a logical step-by-step method. Always perform the fol- lowing checks and service before turning your attention to the injectors. ❑ Operate the engine to check overall condition. ❑ Check for fuel supply up to the fuel filter(s). Hole spray angle Airflow Main spray stream Fuel spray angle More air than fuel Total spray length (penetration) Initial region Air-fuel mixing region Figure 18-10. Parts of a typical fuel injection spray cone. Multi-hole nozzle Delay nozzle Slightly open (pintle spray) Delay nozzle Fully open (main spray) Pentaux-type nozzle Auxiliary spray hole Pintle nozzle (conical pintle) Pintle nozzle (cylindrical pintle) Figure 18-9. Spray patterns produced by various nozzle designs.