Tech Tips
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Fuel System
The Diesel Engine Story
SEVENTY-FIVE YEARS OF DIESEL HISTORY
2002 marks 75 years of Bosch leadership in diesel fuel injection technology. Bosch pioneered the development of the diesel injection pump and began mass-producing the first of its early pump designs on November 30, 1927.
In October the following year, Bosch built its 1,000th pump and by March 1934, more than 100,000 diesel injection pumps had rolled off Bosch production lines. In the years that followed, Bosch continued on the leading edge of diesel advancement, introducing the first distributor pump (1962), electronic diesel control (1986), the high pressure VP44 distributor pump (1996) and common rail systems for cars (1997).
As the demand for diesel technology continues to expand, Bosch is developing new systems that deliver cleaner, quieter, more efficient performance than ever before. Though diesel engines now power more than one third of Europe's cars and light trucks (35%), light-duty diesel applications here in the States are limited to a few Mercedes-Benz and Volkswagen passenger car models and some full-size Dodge, Ford and General Motors pickup trucks. Still, there currently are about 3.5 million light-duty diesel-powered vehicles on the road - and except for Ford, Bosch diesel injection is original equipment on all of them.
WHY DRIVE A DIESEL?
Compared to gasoline engines, diesels have a number of important advantages. One is fuel economy. Diesel engines are unthrottled, so pumping losses are reduced. This improves breathing efficiency for more fuel-efficient operation. Diesel engines also run much leaner air/fuel ratios than their gasoline counterparts, and have much higher compression ratios which improves thermal efficiency and helps squeeze even more power out of every drop of fuel.
Diesel engines have no ignition system, so there are no spark plugs, plug wires or distributor to cause problems. The fuel is ignited by the heat of compression and is controlled by injector timing. For cold starting, diesels use a glow plug system as an additional heat source to aid in the combustion process until the engine is running and generating its own heat.
The injection system that delivers fuel to a diesel engine operates at much higher pressure than a gasoline: as much as 17,400 psi for traditional pump, line and injector systems and up to 23,500 psi for common rail systems, compared to 35 to 90 psi for most gasoline EFI systems. The fuel is also sprayed directly into the combustion chamber rather than into the intake port.
Most light-duty diesels use an engine-driven inline pump to distribute fuel under high pressure to each of the injectors. The injectors are mechanical and have spring-loaded poppet valves instead of solenoids, so they pop open and spray fuel when line pressure exceeds a certain limit (typically 4,500 psi). Electronic controls on late model injection pumps regulate injection timing, fuel mixture, and idle speed. On older applications, these required mechanical adjustments.
A somewhat different approach is used with Bosch common rail diesel injection systems, which are now in production on some of the newest diesel engines in Europe, 2002 Duramax engines used by GM, and the new 2003 Dodge Ram equipped with the Cummins 5.9 liter. A high-pressure supply pump delivers fuel to a common fuel rail that supplies all the injectors. The opening and timing of the injectors is controlled by individual solenoids on each injector rather than the injection pump. This allows more flexibility and control over the combustion process for improved fuel economy and emissions.
DIAGNOSING DIESEL PROBLEMS
Diesels are just as dependable as gasoline engines, and will perform reliably under all kinds of driving conditions. But like any engine, wear, neglect and environmental factors can sometimes cause problems.
Cold Weather: Hard starting during extreme cold weather conditions is one such problem you could encounter. Because of their higher compression ratios, diesel engines require a certain cranking speed to light the fire. If the oil in the crankcase is too thick (wrong viscosity), the battery is low or the starter is weak, the engine may be hard to start when temperatures drop.
Glow Plugs: If slow cranking is not the problem, hard starting may be due to one or more glow plugs that aren't working. Inside each glow plug is a high-resistance heating coil. The plugs are designed to handle high current loads (up to 300 amps in some applications!) so they will heat up quickly. On average, most standard technology glow plugs will reach 850 degrees C/1,562 degrees F (minimum vehicle start temperature) within 8 seconds, though on some older applications the wait period can be 30 to 60 seconds.
New patented Bosch Duraterm glow plug technology enables the glow plugs to reach 1000º C/1,830º F in approximately 4 seconds. The higher operating temperature combined with a faster heating rate means less waiting and less cranking to start the engine. That's why Bosch glow plugs are original equipment on virtually all European diesel applications. OE-equivalent glow plugs are available from Bosch for all domestic passenger and light duty truck diesels, as well as some domestic diesels.
Normally, current to the glow plugs is cut off when the ignition is turned to the start position. Since the late 1990's, glow plugs have been designed to remain operating while the engine is cranking and for a short period of time afterward to keep the fire lit while the engine warms up. The "post-glow" period following start up may range from 20 seconds to a couple of minutes depending on the application, ambient air temperature and engine temperature. When the glow plugs are no longer needed, the system control module or thermo time switch de-energizes the glow plug relay, cutting off power to the glow plugs so they can cool down.
To see if the glow plug module is providing power to the glow plugs, use a voltmeter to check each plug for the specified voltage when the ignition key is turned on. No voltage? Check the glow plug control module connections, ground, and wiring harness.
The glow plugs themselves can be checked by measuring their resistance or continuity. Excessive resistance or a lack of continuity would tell you the plug is bad.
Glow plugs can burn out if they get too hot. Overheating can occur if the system controller or relay fails to shut down after the normal engine start up period. A glow plug relay that is stuck on also will run down the battery, so that's another clue to look for if you suspect this kind of problem.
If one or more glow plugs have burned out, the system controller (or thermo-time switch) and relay should be checked to make sure they are working properly - otherwise the replacement glow plugs will suffer the same fate. If all the glow plugs are burned on the end, you'd better check the injection timing because it probably is over-advanced or the system controller is stuck in the on position.
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