Introduction

Diesel engine fuel injection systems have many different designs. Some technicians think there are only one or two methods of delivering fuel to the diesel engine. In fact, there are several diesel fuel injection system designs, such as the unit injector, plunger pump, distributor pump, and common rail fuel injection systems, to name only a few. Historically, manufacturers like Detroit Diesel, Cummins, and Mack have had completely different fuel injection systems. Considering the various designs mentioned, today’s diesel engines extensively use electronic fuel controls with increased fuel delivery pressures designed to meet today’s strict emission control standards.

Because of their durability and fuel efficiency, diesel engine passenger vehicles have been growing in popularity, particularly in Europe, where diesel vehicles outsold petrol/gasoline vehicles for the first time in 2006, and where the trend toward diesel is still growing. Diesel engines are more robust in construction because they operate with much higher compression ratios—high enough to make the compressed air in the cylinder hot enough to spontaneously ignite the fuel when it is introduced into the combustion chamber. Once the fuel is injected, ignition lag creates the familiar diesel knock, a loud clattering sound that has generally been tolerated by drivers of heavy goods vehicles, but is less acceptable to the discerning owners of more luxurious passenger vehicles. This lag time is called the ignition delay period, which is the time it takes for the fuel to ignite after being injected into the engine.

 
 
You are the Automotive Technician
A customer comes into the Mercedes-Benz dealership with his diesel ML350. He explains that he is having difficulty starting his vehicle in cold weather. He informs you of the cetane rating in the fuel he has been using, and you determine that the low rating might be contributing to his problem. You advise him that you will need to complete a more thorough inspection to determine what needs to be repaired. You ask him to wait in the customer lounge during the inspection and you tell him you will keep him informed. After using several different time-measured tools and techniques, you determine that the system’s timing is not in conformance with the manufacturer’s specifications. You return to the lounge and tell the customer that your recommendation is retiming the system to the manufacturer’s specifications for a more consistent start-up in cold weather.
  1. What is the difference between a low and high cetane number?
  2. Why are water traps or separators important in diesel fuel systems?
  3. What are three types of lift pumps that are common on light-duty diesel cars and trucks?
  4. What are some of the benefits with the latest “clean diesels”?