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THE CONTINUING EVOLUTION OF DIESEL FUEL INJECTION Stricter emission laws are constantly forcing the automobile manufacturers to keep their engine exhaust emissions at acceptable limits, and do so has necessitated the application of some increasingly advanced electronic technology. This is true for gasoline as well as diesel engines. While light and medium duty applications usually have to meet stricter emissions a few years before heavy duty and off-road, eventually all engines come under these more stringent mandates. Through the years, the majority of light duty automotive engine manufacturers chose to utilize a mechanical injection pump with separate nozzle holder assemblies (NHA) to atomize the fuel into each cylinder. As engines began turning higher rpms, it became more difficult to maintain proper pump-to-engine timing. This resulted from a condition known as injection lag. The injection pump builds pressure, and as the delivery ports open, fuel is forced through the injection lines to the nozzle holder assembly or NHA. The length of clock time it takes to get the fuel from the injection pump to the NHA remains fairly constant throughout the speed range, but this creates a problem at higher rpms because the same amount of clock time results in a greater amount of crank angle. This of course causes a retarded timing condition. Manufacturers of distributor style injection pumps have used an automatic speed advance device in most applications in order to start the fuel delivery from the injection pump earlier, so it will arrive at the nozzles at the proper crank angle throughout the speed range. This allowed the engine manufacturers to meet emission requirements for several years, but most automatic advance devices were hydraulically driven and totally dependent on transfer pressure generated by the injection pump itself. Pressure was controlled by pump speed, so the pump had to reach a specific speed before it could change the advance. This resulted in an imperfect, but still acceptable condition. In 1994 the emission requirements became so strict that a fuel system's injection lag evolved into a major issue. GM, Ford (Navistar), and Dodge (Cummins) all went separate ways in order to meet the new standards. GM continued working with Stanadyne on the development of an electronic injection pump that used traditional fuel lines and nozzles. The advance device, although still hydraulically driven, is controlled by an electronic stepper motor rather than pump speed. As a result, the advance operates more accurately throughout the speed range. Since the injection pump is controlled by a computer and not directly by the operator, such things as air density, engine temperature, exhaust conditions, ambient temperature, etc. could now be monitored. The computer then controls fuel delivery so that engine exhaust emissions meet necessary legal requirements. This enabled GM to use their engine in 1/2 ton trucks as well as heavier duty applications because emission levels were under computer control. Dodge continued to use the Cummins engine which utilized a Bosch mechanical injection pump, fuel lines, and NHAs. Bosch increased injection pressure and raised the opening pressure of the NHA so that the fuel was broken down, or atomized, into smaller droplets, thus insuring a cleaner and more complete burn. The injection pump required to generate this much peak injection pressure was a very large, expensive device. More recently, Cummins has started using an electronic Bosch injection pump on this engine. Although the Bosch pump is somewhat different from the Stanadyne injection pump, the concept of electrically controlling the injection pump remains the same. More accurate timing and fuel metering result in lower exhaust emissions. Ford has been utilizing Navistar engines for some time and they continue to do so. The new engines, however, utilize what is known as a HEUI system. This stands for Hydraulically actuated, Electronically controlled, Unit Injection. This system uses an engine lubricating oil pump to actuate an intensifier piston in the injector. An electronically controlled solenoid opens and closes the oil passageway to the intensifier which builds injection pressure. When the solenoid is de-energized the piston is moved back to its original position by a spring. This also replenishes the fuel in the plunger chamber across a ball check valve. Fuel is supplied to the nozzle via internal passages, and as pressure increases, the nozzle lifts off of its seat and injects fuel into the combustion chamber. Injection continues until the solenoid closes and pressure drops, allowing a spring to return the nozzle valve to its seat. Utilizing this system does away with injection lines, and allows more precise fuel delivery and timing. The vehicle electronic control module (ECM) is used to store operational maps which allows it to identify the optimum rail pressure for best engine performance based on the various information fed to it by a multiple of sensors. Since this engine is a V-8, eight of these injectors are used, and replacing them can be expensive. Virtually all of the
engines now utilize a turbocharger. This allows more air to be
pumped into the cylinder, and more air will allow the fuel to
burn more completely. This in turn cuts down on the amount of
dangerous exhaust emissions.
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