Chapter
7 - Fuel Systems
Fuel System
Most stock
fuel systems are not designed to work with forced induction. The stock fuel injectors combined with the
stock fuel pump cannot provide enough fuel at a high enough pressure to maintain
a safe air fuel ratio. Upgrading the
fuel pump and injectors is a requirement for almost any forced induction.
Fuel Pump
The stock fuel
pump should be replaced with an aftermarket high flow unit. These can be found for your specific
application at any of the finer speed shops and large aftermarket retailers.
What size
fuel pump do you need? There are a few
simple calculations that will help you decide what fuel pump to use. The calculations depend on a few factors
including your Brake Specific Fuel Consumption, fuel pump flow and horsepower
output of your engine.
Here is a
calculation that finds out how much horsepower a pump will handle.
horsepower = lb/hr (fuel) / Brake Specific Fuel Consumption (BSFC)
BSFC is a
measure of an engine’s efficiency. It’s the rate of fuel consumption divided by
the rate of power production.
On
naturally aspirated engines, it is safe to use .5 for the BSFC. This of course varies depending on
modifications etc. For a forced
induction engine, the BSFC is a bit higher.
We will use .6 for our calculation, but again this will vary from engine
to engine.
Let’s use a
190lph fuel pump for our first example.
190lph =
50gph (One gallon of gasoline is roughly six pounds) Therefore: horsepower = 50gph x 6lbs / .6 = 300lb per hr / .6 =
500FWHP
Quite
frankly, I wouldn’t want to try my luck with a 190lph pump and 500 force-fed
horsepower. Some people do it, but why
take a chance spending thousands of dollars in engine repair, when a $250 fuel
pump can make it safe. An ounce of
prevention comes to mind. Let’s take a
look at a 255lph pump using the same calculation.
255lph =
67gph * 6lbs = 402 pounds per hour
Therefore: horsepower = 402 lb per hr / .6 = 670FWHP
A bit
better. I’d feel safe running up to
650FWHP with a single 255lph fuel pump.
If you are looking for extreme horsepower, you can always run two 255lph
fuel pumps. Kits are available to run
two fuel pumps inside, or outside of your fuel tank.
External Fuel
Pumps
External
fuel pumps can help provide extra fuel for high horsepower applications,
however it will be restricted by the smallest fuel pump. If you are running a stock in-tank fuel pump
with an external 255lph fuel pump, the fuel flow will be limited to the stock
fuel pump’s output.
Some of the
serious horsepower FI people choose to do away with the stock intake fuel pumps
altogether. Instead, a custom fuel tank
is used that has a sump to prevent fuel from sloshing away from the fuel
lines. External pumps are used to feed
the fuel rail. This gives unrestricted
choices for fuel pump usage since external fuel pumps tend to be universal
types.
Fuel Injectors
It is
required that the stock fuel injectors be replaced for almost any forced
induction application. The stock fuel
injectors cannot supply enough fuel to maintain a safe AFR. Your desired horsepower level will determine
what size injectors to use.
Calculating Required Fuel Injector
Rate
A simple
calculation can be used to determine injector flow requirements. Use the following formula to match fuel
injectors with your application.
Injector Flow Rate
(lb/hr) = Horsepower x BSFC / Number of Injectors x Duty Cycle
Duty Cycle
An
injector’s duty cycle is the percentage
of time that it is powered on. The
injector’s pulse width is the actual time that the injector is powered. The injector fires one time during the four
cycles of its associated cylinder or once per two crank revolutions. To calculate an injector’s duty cycle, we
need to know the time it takes for the crankshaft to turn two times at a given
RPM, and the injector pulse width (IPW) at that RPM. Let’s take for example an engine turning 6000
RPM and an IPW of 18ms.
It takes 20
milliseconds (6000/60s = 100RPS and 1/100 = .01s and .01 x 2 revolutions = .02s)
for the crankshaft to turn twice at 6000 RPM.
If the injector IPW is 18ms then we have a duty cycle of 18ms/20ms = .9
or 90%.
Most fuel
injectors should not be operated at above 85% duty cycle for extended
periods. Therefore, when calculating our
fuel injector size requirements, we will use an 85% duty cycle in our
computation.
Let’s go
back to our injector flow rate calculation and plug in our numbers. We will compute for 500HP with a BSFC of .6
and an injector duty cycle of 85%.
Injector Flow Rate
(lb/hr) = Horsepower x BSFC / Number of Injectors x Duty Cycle
Injector Flow Rate
(lb/hr) = 500 x .6 / 8 x .85 = 300/6.8 = 44lb/hr
In the
example above, we may be able to get by with 42lb/hr injectors as they are
rated at 42lb/hr at 40psi fuel pressure.
This example would need to run around 60psi fuel pressure with forced
induction. The 42lb/hr injectors will
actually flow around 50lb/hr at 60psi fuel pressure.
Other common
flow rates for turbocharged engines are 60lb/hr and 96lb/hr. Most engines use high-impedance (high-Z) type
injectors. Drivers can be purchased to
run Low-Z injectors on high-Z computers, however Motron and Siemens as well as
others manufacture high-Z injectors for some engines. It is also wise to have your injectors
flow-matched to insure that there is not any variation in fuel supply between
cylinders.
Fuel Pressure and Forced Induction
If your
fuel system is set up correctly, with the correct pump and injectors, your fuel
pressure should not drop at the fuel rail during wide open runs. A drop in fuel pressure can bring the AFR
dangerously high, and is an indication that the fuel system is not adequate for
the required f
uel flow.
Return Type Fuel System and Boost
Referenced Fuel Regulator
In a forced
induction application, the intake manifold becomes pressurized. Therefore, the injectors need to overcome
this positive pressure to provide the correct amount of fuel.
Say for
example, you are seeing 10psi of boost and 60psi fuel pressure at the rail. The effective pressure of the fuel injectors
now becomes 50psi and they are no longer flowing at 60psi. To overcome this problem, a boost referenced
fuel pressure regulator can be used.
A boost
referenced fuel pressure regulator (BRFPR) senses boost and raises the fuel
pressure at a ratio of 1:1 with boost pressure.
So in our example of 10psi boost and 60psi fuel pressure, the BRFPR
raises the fuel pressure 1psi for every 1psi boost to make 70psi of fuel
pressure at the fuel rail, allowing the injectors to flow 60psi (70psi fuel
pressure – 10psi boost pressure).
In a return
type system, fuel is pumped into the fuel rail and an on-rail regulator
restricts the amount of fuel that exits the rail and returns to the fuel tank,
keeping the fuel pressure at the rail constant. A BRFPR actually controls the
fuel pressure in the fuel rail by restricting more fuel as boost pressure
rises, actually raising the rail pressure rather than keeping it constant.
Do not
confuse the BRFPR with a rising rate regulator or FMU. These units are designed to increase fuel
pressures at ratios higher than 1:1 for use with undersized fuel
injectors. An FMU is not an advisable way
to go with forced induction. They were
common a few years ago before EFI programming and standalone fuel injection
solutions became available at reasonable costs.
The return
type fuel system combined with a BRFPR is the ideal setup for high powered
forced induction systems.
Air/Fuel Ratio
Air/fuel
ratio or AFR, is the ratio of air mass to fuel mass in a combustion chamber
during combustion. In gasoline engines,
the air and fuel are chemically balanced in the combustion chamber to a stoichiometric mixture of 14.7:1 by the engine management system. A lower ratio is a rich mixture, and a higher
ratio is a lean mixture.
In forced induction applications, a richer fuel mixture
is needed under boost conditions to stave off detonation. Most tuning pros agree that an AFR of 11.5:1
is ideal for a turbocharged engine under full boost. Anything lower is too rich, and anything
higher could cause detonation or fuel starvation leading to broken pistons.
Wideband O2
Sensor
A wideband oxygen sensor is used to monitor the AFR. The stock narrowband O2 sensors can only read
the stoichiometric ratio of 14.7:1 for
gasoline. A wideband O2 sensor can read
a much wider band of ratios, from 9.65:1 to 20:1. They can also sense changes in AFR much more
accuratley.
A wideband O2 sensor and gauge are a requirement for both
tuning and monitoring your turbocharged engine.
Without one, there is no way to be sure that your tune is correct. A wideband gauge should be mounted on the
dashboard and closely monitored. It will
likely be the first indication of fuel system problems.
The installation of a wideband system is relatively
easy. A bung is welded into the exhaust
before the catalytic converters, and the wideband sensor is screwed in. The wiring harness is plugged in and is
attached to the gauge, and or tuning computer.
A tuner uses the values read from the wideband to adjust parameters in
the PCM, adjusting the AFR at a safe value.