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EFI System Fuel Injectors
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Now that the inputs to the computer have been covered, it’s time to look at what the outputs can do.
Not that many outputs are required to operate most engines. The advantage of EFI is the ability to
very precisely control each of these outputs. Think of jet changes in a carburetor as strokes with an
axe and injector control as using a razor blade. Obviously, creating fine details in a sculpture is
easier with a razor blade.
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Fuel Injectors
The number one priority and function of any injection system is to properly mix the right amount of
fuel with the incoming air. This must also be done in such a manner that the fuel is ready to burn as
soon as the spark event happens. In a carburetor, the suction of the low-pressure airflow through
the venturi draws liquid fuel through a fixed orifice resulting in a misting or suspension of fuel
molecule in the incoming air charge in the intake manifold. The manifold design needed to
accommodate this “wet” mix features smooth flow paths with gentle bends designed to avoid
puddling or uneven fuel distribution.
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The fuel injector is the key to precise
delivery in EFI systems. Siemens
manufactures over eight million injectors a
year in their Virginia plant, and every single
one is flow tested before shipping. (Nate
Tovey)
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Injectors from different manufacturers
shown side by side. The general
construction and design is the same
despite different lengths and internal
flow rates. (Nate Tovey)
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The fuel injector is basically an electromagnet driven stopper valve for fuel. High-pressure fuel is
fed to the top of the injector from the rails. Once the injector is actuated, the force from the
electromagnet pulls a pintle upward, opening a small hole to allow the fuel to flow into the manifold.
Injectors can use a much higher operating pressure than a carburetor because the spring-loaded
pintle design does not leak under pressure. This added pressure also provides for a more
aggressive pressure drop as the fuel enters the manifold, making a finer mist of fuel. The smaller
droplet size in turn means better ignition since each individual fuel molecule has more surface area
contact with the air charge to aid evaporation and combustion. Additionally, the relatively small size
of fuel injectors means that they can be located much closer to the intake valve. This frees up
manifold design, allowing for longer runners and tighter, more complex turns to fit a smaller
package without puddling. When each individual cylinder can have its own fuel injector, fuel
distribution can also be optimized.
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Increasing fuel pressure can be used to
provide a higher flow rate out of the same
injector. (Nate Tovey)
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The amount of time available to complete
an engine cycle decreases with speed. If
the desired injection time becomes longer
than this cycle time, the injector will be
held static. (Nate Tovey)
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Not all fuel injectors are created equal. Injectors are typically classified by the resistance of their
internal coil as either “high impedance” or “low impedance.” High-impedance injectors are the most
common and are used in almost every OEM application today. When measured with an ohmmeter,
high-impedance injectors have 10 to 16 ohms resistance. Low-impedance injectors are less
common in OEM applications, but readily available in the performance aftermarket. Low impedance
injectors typically measure 2 to 6 ohms resistance. It is important to recognize that in most OEM
applications, it is not advisable to replace high-impedance injectors with low-impedance injectors.
The lower impedance forces the circuit to draw more current through the injector controller, which is
really just a transistor. Excess current drawn through these controller circuits often literally cooks
them, leading to failure of the injector driver circuit on the board. Since replacement injector driver
circuits are so scarce, the result is usually the purchase of a new replacement PCM.
The next division is injector “size,” which is really a misnomer for flow rate rather than physical
dimensions. Most injectors are rated in either lb/hr or cc/min. These flow rates are actually the static
maximum flow through the injector if it were activated continuously. Keep in mind that if the injector
is only open for a very short period of time, a correspondingly smaller amount of fuel can be
delivered. The length of time that the injector is open during each cycle is known as “pulsewidth.”
Pulsewidth is usually measured in milliseconds. The ratio of pulsewidth to total available time for
that injection cycle is known as duty cycle.
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Duty Cycle = (Pulsewidth) / (Total Time Between Ignition Events)
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It is impossible to have duty cycles exceed 100%. If the commanded pulsewidth equals actual cycle
time at a given engine speed, the injector is not allowed any time to close. This is known as “static
flow,” as the rate of fuel delivery is no longer changing. This situation typically happens when
injectors that cannot support the necessary fuel flow rate for the engine’s power level and fuel
consumption rate (BFSC) are used. Uncorrected, this leads to an enleanment of the air/fuel mix.
Care must be taken to ensure that this enleanment does not lead to detonation and engine
component failure. If more fuel delivery is required, the only available remedies are to either
change to injectors with higher flow rate units or increase the fuel rail pressure to artificially
increase the flow of the existing injectors.
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Fuel rail pressure changes to flow rate are governed by the equation:
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(Actual Effective Rail Pressure)
-----------------------------------------
(Rated Rail Pressure)
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Actual Flow Rate = (Injector Flow Rate) x
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Keep in mind that changes in manifold pressure also have an effect on actual injector flow rate. If
manifold pressure increases from the use of a supercharger, rail pressure must increase 1:1 with
boost to ensure that the injector does not become effectively smaller. It is possible to tune around
changes in effective rail pressure and actual flow rate. This is covered later when we discuss forced
induction.
Much like any other spring-mass system being driven by an electric current, opening of the injector
is not an exact step change. It takes a small amount of time to build up enough energy in the coil to
begin to move the pintle of the injector off the seat and allow fuel to flow into the manifold. The
initial delay is known as “dead-time,” but is also followed by a period of exponential movement of
the pintle until it hits the fully open position. (Figure 5-1) Likewise, closing the valve takes time as
well. Once the current to the coil is removed, the pintle is pushed back to its seat by internal spring
pressure as well as fuel pressure behind it. The difference between the opening delay and closing
delays is called “injector offset.”
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Figure 5-1 A small delay
occurs between when
injectors are energized and
when fuel actually begins to
flow. Likewise, there is a
similar lag between shutoff
and closing. (Nate Tovey)
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Remembering that fuel injectors are actuated by electromagnets, it is important to further
understand how their performance can change. The strength of the electromagnet in the injector
varies relative to voltage. (Figure 5-2) Having more voltage across the field of the coil increases the
strength and allows the injector to open quicker. This in turn means that fuel begins to flow into the
manifold slightly sooner if voltage is higher. Knowing that cars almost never have constant voltage,
the PCM needs to be able to adjust. A failed alternator, dead battery, or even normal cranking can
send voltage to 11.5 or lower. Normal charging usually keeps voltage around 14 volts, and a failed
voltage regulator can send output above 17 volts. The bottom line here is that the same injector
under these varying conditions can change its actual output by 40% or more. All modern PCMs
have tables built into their software code to model this change, even if they aren’t overtly visible to
the calibrator. Various injectors exhibit different voltage compensation curves. While all injectors
change relative to voltage in a similar manner, the exact offsets at a given voltage are slightly
different as internal construction of the injector changes. To best model the actual fuel delivery to
the engine, it is ideal to accurately input the voltage compensation for the injector used. A quick
Internet search can often yield the exact voltage offsets for most injectors.
To add more complexity, the actual flow rate changes based on pulsewidth. As the injector first
opens, more fuel flows for the split-second that pressure differences are the highest. (Figure 5-3)
Additionally, when the PCM commands an injector-opening event for a short duration, there is a
tendency for the injector’s spring-mass system to overshoot the desired duration. The net result is
that at small pulsewidths, the injector tends to deliver fuel at a rate slightly higher than the static
flow rate of the injector. This often leads to modeling the injector with two different flow rates, one
for the normal pulsewidths of cruising and power delivery and another for the shorter pulsewidths of
idle and starting. This in turn leads to the need to determine where this change, known as the
“break point,” occurs. This break point is usually relatively small, on the order of 1 to 3 ms, so the
effect is often only seen at idle and very low loads. Again, entering this break point into the PCM
routine allows for more accurate modeling of exactly how much fuel can be expected to enter the
engine for a given commanded pulsewidth.
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Previous | Next
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This has been a sample page from
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Engine Management: Advanced Tuning
by Greg Banish
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As tools for tuning modern engines have become more powerful
and sophisticated in recent years, the need for in-depth
knowledge of engine management systems and tuning techniques
has grown. Tuning engines can be a mysterious art, as all
engines need a precise balance of fuel, air, and timing in order to
reach their true performance potential.
Engine Management: Advanced Tuning explains how the EFI
system determines engine operation and how the calibrator can
change the controlling parameters to optimize actual engine
performance. This book takes engine-tuning techniques to the
next level. It is a must-have for tuners and calibrators and a
valuable resource for anyone who wants to make horsepower with
a fuel-injected, electronically controlled engine.
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Click below to view sample
pages from each chapter
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Author Greg Banish is a calibration engineer with extensive
aftermarket performance calibration experience. With over a
thousand unique calibrations performed over five years, he has
worked with enthusiasts and OEMs alike to improve the
performance and driving behavior of a wide range of vehicles.
The book contains detailed equations, graphs, and illustrations.
Also included are valuable and practical examples, including real-
world examples based upon the author’s experience that will help
more advanced readers apply this new information to situations
that are commonly seen during calibration.
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1 - Introduction to EFI
2 - Basics of Fuel Injection
3 - Carbureted Engines
4 - EFI System Inputs
5 - Fuel Injectors
6 - EFI System Fuel Control
7 - Ignition Systems with EFI
8 - Data Logging
9 - EFI System Calibration
10 - Idle Calibration
11 - Tuning for More Power
12 - Fine Tuning EFI
13 - Tuning EFI with Blowers
14 - Tuning Ford EFI Systems
15 - Aftermarket EFI Systems
16 - INCA OEM Calibration
17 - External EFI Controllers
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8-1/2 x 11"
Soft bound
128 pages
200 color photos
Item # SA135
Price: $22.95
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Click here to buy now!
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Other items you might be interested in
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Performance Ignition Systems
Performance Ignition Systems is a comprehensive guide to
significant increases in power, mileage and overall engine
performance by custom tuning electronic or breaker point
ignition systems. Sections include increasing engine power,
efficiency, mileage and longevity using upgraded ignition
equipment, judging and troubleshooting ignition components,
including diagnosis and reading spark plugs, electrical wiring
problems and solutions, tech tips and custom wiring for the
ultimate performance ignition and much, much more.
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Price:
$18.95
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Building & Tuning High-Performance Electronic Fuel Injection
Provides specific, detailed info on what fuel injection is & how it works.
Covers buying and installing the proper system for your performance
application. After a description of what programmable EFI offers its
users, author Ben Strader (founder and senior instructor of EFI
University) gives a detailed account of what you want to accomplish with
your EFI system, then shows you how to get there.
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Price:
$18.95
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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MRE
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