flow fuel injection systems have been used on about every type of
engine ever made whether it runs on nitro, alcohol or gasoline for
over 50 years. It is by far the most simple of all fuel systems
in its basic design, but one of the hardest to tune. The reason
it is so hard to tune is that there is no automatic compensation
for atmospheric conditions, engine load, fuel temperature, blower
boost, cylinder-to-cylinder distribution or airflow variations due
to camshafts, cylinder heads, manifolds, blowers or headers.
|This is the inner
view of the Supercharger. In all Fuel Systems applications,the
supercharger is an intregal part.
All constant flow systems
use positive displacement fuel pumps of various designs normally
driven at 50% of engine RPMs, either direct off of the camshaft
or through a Gilmer style cog belt off of the crankshaft. The
key to a winning fuel system is matching the curve to the application,
which are all different. A Top Fuel dragster requires a different
curve than a fuel Funny Car. A Pro Modified is different than
an alcohol funny car and so on.
pump is the heart of the fuel curve. The curve is governed mainly
by the basic design of the pump itself. With so many designs and
variations of designs by different manufacturers, most people are
unaware of the differences. Vane pumps and external gear-to-gear
pumps are very sensitive to tip speed and internal leakage and internal
gear-to-gear pumps are more sensitive to clearance than tip speed.
Of course, all positive displacement pumps are sized to the application,
but size has little effect on the curve within the same design.
All drag racing applications
that use a transmission and a supercharger have the most radical
fuel curve to try and match. The engine leaves the starting line
anywhere from an idle to 6500 RPMs depending on combination and
accelerates to 8000 - 10,000 RPMs and then the shift brings it
down 1000-2000 RPMs in a couple of hundreds of a second before
having to accelerate up again to the next shift point and so on.
While all this is happening, the supercharger is pumping air at
different and inconsistent rates depending on design, and on top
of all this, the most important aspect is the changing loads on
the engine due to transmission ratios, clutch slippage, tire and
track conditions and atmospheric conditions.
can be adjusted somewhat by the use of normally open jets, normally
closed jets, high-speed valves and constant flow compensating valves.
Top Fuel dragsters and Funny Cars use up to 15 of these valves activated
by air timers, while most alcohol powered cars use only a couple.
Jets and nozzle design also have an effect on the curve and more
of an effect on the raw horsepower. Different jet designs have individual
flow characteristics at different pressures = engine RPMs.
Nozzle spray patterns
and location in the ports and manifold have an effect on raw horsepower.
Of course this is unique to each engine, cylinder head, manifold,
camshaft and blower design.
read this and wonder how anyone could make all of this work. As
stated earlier in this text, a constant flow fuel injection system
is basically very simple. What we've explained are the intricacies
and some things a person has to pay attention to if they want to
be a contender or a knowledgeable fan.
|This picture shows the primary components of your typical supercharger Fuel System.
My company, Tom Anderson
Enterprises, designs a totally computerized fuel system, with a
flow bench that samples at 100 samples per second on a sweep curve,
so we can design and test any fuel system or components that we
sell or service.
To do this article
justice, we had to separate it in two parts. The first part was
to be on the fuel system itself, and the second had to deal with
A roots type supercharger
is technically not part of the fuel system, but in reality it
is. That is due to the fact that the blower as we will call it
distributes the air and fuel differently, depending on design.
Roots type blowers have been around forever and anyone who has
knowledge of a Detroit Diesel engine has seen one.
The basic design has
not changed, but everything else has. Rotor design, boring of the
cases, location of sealing strips, inlet and outlet designs are
constantly being changed and modified in an attempt to build a better
blower. Currently there are only four companies able to produce
competitive roots type blower for nitro and alcohol classes in drag
racing and it is very difficult to build blowers that perform identical.
The blower efficiency is also constantly changing as the sealing
strips wear, changing horsepower output and requiring adjustments
to the fuel system and race car set up.
In classes without
an overdrive ratio restriction, a lot of tuners attempt to compensate
for wear by increasing the overdrive percentage. In my years of
experience with fuel Funny Cars trying to crutch a weak blower
with overdrive percentage was rarely successful. This is due in
part to the fact that it takes a tremendous amount of horsepower
just to turn the blower. And because it is a positive displacement
air pump when it turns faster it makes more boost, but it also
takes more power to make this boost and it generates more heat,
which robs horsepower and promotes detonation.
I spoke a
lot about fuel curves earlier and blowers also have different boost
curves depending on design and condition. When a blower wears it
has a dramatic effect on the boost curve, that's why it is always
best to re-strip the blower rather than speed it up because speed
will not restore the curve. The life of the sealing strips is dependent
on several factors. If the strip clearances are too tight when the
blower is built, it will wear the strips the first time it is run.
If the mating surfaces of the blower and manifold are not flat within
.001 of an inch, this will also ruin a good blower immediately.
|The new Fuel Bench at TAE uses a frequency drive to simulate the on-track curve. It displays all data in graphs and gauges in real time at 100 samples per second.
The other major factors
are rotor flex, rotor twist and the location and amount of fuel
injected to the inlet side of the blower. This varies with blower
design and speed. I feel that fuel systems and superchargers are
the two most important aspects of a drag racing engine combination,
whether it runs on nitro or alcohol. So, in addition to our computerized
fuel flow bench, T.A.E. is building a state of the art engine
and supercharger test facility in the Rochester N.Y. area. At
the heart of our facility is a modified Dynamic Test System engine
dynamometer capable of testing engines that produce over 3000
horsepower with accuracy and repeatability of 1 horsepower.
With multiple fuel flows, boost pressures in each manifold runner
and nine pitot tube air inlet sensors, this is the only facility
in the North East designed specifically for testing superchargers
and supercharged fuel injected alcohol race engines.
Now I know that some of the readers will come away from this article having learned a little more than they knew going into the deal, and for that I am grateful. That was my goal in writing it. I hope you all enjoyed the insight on fuel systems and superchargers as much as I did writing it for the best E-zine on the planet.