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This subject may be of interest to others on the list so here's a
non-technical description of the circuit in a diesel engine that has a
separate fuel injection pump. The circuit is a little different, but the
results are the same in diesel engines that use each injector as the
injection pump.
The injection pump is the most complicated part of a diesel engine with the
smallest clearances of all the parts. It is the very brain and brawn that
allows a diesel engine to be a diesel engine. In passing, it is interesting
to note that Robert Bosch invented the injection pump before Rudolph Diesel
invented the diesel engine, and that those two and Otto (the inventor of the
spark-ignition engine - gasoline) were contemporaries in Germany who lived
within 50 miles of each other.
The injection pump delivers fuel under high pressure and very precise
amounts to the injectors. The injectors inject the fuel into the cylinders
either directly or indirectly through a pre-combustion chamber. The
injectors have springs that maintain the fuel pressure from the injection
pump before it is delivered into the cylinders. The pressure needed to
overcome the injector spring pressure can be as high as 2000-psi. Diesel
engine specifications can be inspected to see just what the pressure is in a
particular engine. This pressure must be higher than the pressure in the
cylinder as the piston approaches top-dead-center (the point where maximum
pressure is obtained).
The job of the injection pump is so important that some means is necessary
to keep the fuel supply to the injection pump constant. If is not constant,
the injection pump will have a very difficult time of delivering the precise
amount of fuel required by each cylinder. To allow the injection pump to do
this critical work, most diesel engines include a "lift-pump." The job of
the lift-pump is just to make sure the injection pump always has enough fuel
to do its job correctly. To make all this as fool-proof as possible, the
lift-pump is designed to supply more fuel than the injection pump can ever
use. The excess is returned to the fuel tank from the injection pump
through a spring-loaded relief valve in the injection pump. Most injection
pump designs have a chamber to hold the incoming fuel and the injection pump
draws its fuel from that chamber. It is the injection pump's "day-tank," if
you will.
The lift-pump on most engines is a very crude, diaphragm operated pump with
spring-loaded valves on the inlet and outlet. These valves work is such a
way so that any positive pressure from the suction side of the pump will
allow the fuel to pass right through the pump. They also work to keep fuel
from returning from the lift-pump to the suction side of the pump -- check
valves of a sort. These pumps are very low pressure pumps in comparison to
the pressure generated by the injection pump. There is no way that the
lift-pump can put any fuel into the cylinder. It can only provide fuel for
the injection pump.
Then we have the little Walbro pump used in CaptnWil's diesel polishing
systems. Its maximum outlet pressure is 7-psi. All it can do is circulate
fuel. About all it can do is keep a little positive pressure on the suction
side of the lift pump. I doubt if much, in any, fuel passes through the
lift pump, the injection pump, and back to the fuel return when the engine
isn't running. The pressure drop through the lift-pump and the relief valve
in the injection-pump will make up much, if not all, of the discharge
pressure of the Walbro pump. The bypass line around the engine in the
polishing system is there for that very reason.
In addition, the lift-pump-injection pump part of the circuit is in parallel
with the bypass around the engine. Since the pressure drop through the
bypass is very much less than the pressure drop through the other part of
the circuit, almost no fuel will flow through the injection pump part of the
circuit while the engine is not running.
CaptnWil
[SNIP]
> I am not an expert on diesel engines, so please bear with what might be a
> very basic question: Looking at your diagram I see that the fuel is
pumped
> by the Walbor into the lift pump on the engine, whether the engine is
> running or not. If the engine is NOT running (and the lift pump is not
> turning) how does the fuel get through? Is it able to flow through the
> lift pump anyway? Also, what happens to the fuel when it goes into the
> engine? It would seem to me that most fuel would return via the return
> line, but also some would collect in the cylinders since they are not
> firing. Clearly your system works just fine, so I'm obviously confused
and
> ignorant about just what's going on in there.
>
> Could you enlighten me [us] as to why this doesn't flood the engine with
> fuel?
