HISTORY OF DIESEL . . .
In 1892 Rudolf Diesel was issued a patent for a proposed engine that air would be compressed so much that the temperature would far exceed the ignition temperature of the fuel. Baron von Krupp and Machinenfabrik Augsburg Nurnberg Company in Germany backed Rudolf Diesel financially as well as providing engineers to work with him on the development of an engine that would burn coal dust, because there were mountains of it piled up in the Ruhr valley. The first experimental engine was built in 1893 and used high pressure air to blast the coal dust into the combustion chamber. This engine exploded and further developments of using coal dust as a fuel failed, however a compression ignition engine that used oil as fuel was successful and a number of manufacturers were licensed to build similar engines.
The original oil burning engines used very crude mechanical injection equipment so Rudolf Diesel again began using air blast to provide atomization of the fuel as well as turbulence of the mixture. This was very successful and utilized in Rudolf Diesel's third engine built in 1895. This engine was very similar to engines being used today. It was a four-stroke cycle with 450psi compression. Progress in diesel engine development has since depended on improvements in fuel injection technology.
In 1922 Robert Bosch began the development of a fuel injection system for the diesel engine. By 1927 they finally had an acceptable injection pump. The demand for this pump was so great that Bosch in Germany was unable to keep up. In 1931 agreements were made with companies in France and England to produce injection pumps. In 1934 a company in the U.S. began manufacturing under the name of American Bosch and in 1938 the Diesel Kiki company in Japan was founded. Since then licenses have been granted to numerous manufacturing companies in several countries, most of which us Robert Bosch's designs to build injection pumps.
EVOLUTION OF INJECTION SYSTEMS . . .
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 petrol 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.
By the mid 1990’s emission requirements became so strict that a fuel
system's injection lag evolved into a major issue. GM, Ford and Dodge all
went separate ways in order to meet the new standards. GM continued working
on the development of an electronic injection pump that used traditional
fuel lines and nozzles. The advance device, although still hydraulically
driven, was controlled by an electronic stepper motor rather than pump
speed. As a result, the advance operated more accurately throughout the
speed range. Since the injection pump was controlled by a computer such
things as air density, engine temperature, exhaust conditions, ambient
temperature, etc. could now be monitored. The computer then controlled fuel
delivery so that engine exhaust emissions met necessary legal requirements.
Dodge continued to use 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. More recently,
this has been replaced by an electronic injection pump. Once again the
concept of electrically controlling the injection pump remained the same.
More accurate timing and fuel metering result in lower exhaust emissions.
Most modern diesels now use 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.
DIESEL INJECTION SYSTEMS . . .
There are basically three general systems of mechanical fuel injection:
the constant pressure or common rail system, the spring pressure or
accumulator type, and the jerk pump.
In the common rail system fuel at a constant pressure is maintained in a
manifold connected to either cam actuated nozzles or with a timing and
distributor valve and pressure operated nozzles. This pressure usually from
4000 to 8000 psi, is obtained by making the fuel manifold large and
utilizing the compressibility of the fuel oil, using a pump of excess
capacity and delivering fuel between each injection, and by passing fuel
from the accumulator through a manually or governor controlled pressure
regulating valve. The amount of fuel delivered per injection is controlled
by injection pressure, total nozzle orifice area, and time that the nozzle
valve is lifted.
In order to keep the fuel quantity injected independent of pump speed a
accumulator or spring injection was developed. The basic system used upper
and lower plungers in a common bore, the lower plunger was driven by an
eccentric cam and the upper plunger was spring loaded. As the bottom plunger
is forced up the fuel between the plungers is pressurized based on the
spring force applied to the top spring. Fuel continues to pressurize until a
delivery groove in the lower plunger indexes with the outlet passage. This
pressurized fuel is then injected and continues until the upper spring
forces the plunger downward and closes the outlet passage.
The injection pump in the jerk pump system is used to time, meter and
pressurize the fuel. This is the most common and utilized system. The
plungers are driven by a camshaft that is designed to control the injection
characteristics of the engine. The spray duration in crank degrees still
increases with speed and fuel quantity but not to the extent of the common
rail system therefore the jerk system can be used on low, medium, and high
speed engines.
The jerk pump system lead to the further development of distributor style
pumps, unit injectors, the "PT" fuel system, and dual fuel pumps. New
systems continue to be developed. Utilization of electronics in the fuel
delivery system is getting more common. Some fuel injection manufactures are
developing ways for their injection pumps to charge and discharge
electronically in order to keep up with current standards for the diesel
engine. New systems such as the HEUI (Hydraulically actuated, Electronically
controlled, Unit Injector) are currently being used on several applications
in all areas especially automotive. The HEUI System develops injection
pressures as high as 18-24,000 psi by applying high pressure oil to the top
of an intensifier piston. Since the area of the head of this piston is 7
times the area of it’s plunger a 7:1 pressure increase on the fuel beneath
the plunger is achieved. By varying the oil pressure, injection rate can be
controlled independently of the crank or cam. Thus injection timing, rates,
and pressures are no longer dependent on camshaft position, speed or cam
ramp velocity. This is all controlled by a solenoid actuated valve that
determines when high pressure oil is applied to the piston.
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