EFI Tuning - The Art Of Calibration

Understanding the dynamics of a fuel injector

Ray T. Bohacz Jan 16, 2014 0 Comment(s)
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The fuel injector is the least understood component of the EFI system, but it is a critical part for a successful calibration. The job of fueling the cylinders comes with many design and engineering obstacles that must be recognized if a proper calibration is to be performed. The conflict is rooted in the need for an injector to be very elastic in its fuel delivery. At idle and light load it must provide only a small amount of fuel and then at full power a large volume. This task is very difficult since in contrast to a carburetor that adds circuits (pathways), the fuel injector does not have that luxury. The only tuning tool the calibrator has is to alter the opening time (pulse width). For this reason, the selection of the proper injector design, and not just its flow rate, is paramount.

Injector Basics

An EFI calibration is a synergy of different elements—some in software or computer code while others are rooted in mechanical operation such as the cam profile, cylinder head port design, or our subject matter this month, the design characteristics of the injector. Understanding injector design is where it all begins.

GM Fuel Injector 2/9

It is customary to identify an injector by a number of different characteristics: its flow rate, fuel feed point, attachment to the fuel rail, electrical resistance, and tip design. In GM OE applications another criteria is the placement of the injector. General Motors employed different injector designs in the throttle-body, port, and early Vortec-style systems often referred to as Central Port Injection. Since the other variations in injector design and location are no longer used and were not a component in a performance application, this primer will stay focused on the common individual port injector.

Most enthusiasts usually classify injectors by the flow rate and electrical resistance, but other elements of the design become critical based upon the intended use. If these were the only design factors that are important the industry would only have a few different injector types and not the myriad options that are on the market today in both OE applications and aftermarket performance uses.

All GM performance oriented applications use a double O-ring top feed injector that is patterned from the Bosch EV1 design. This describes an injector that takes in fuel through the top from the fuel rail and is sealed there with a rubber-based O-ring. The bottom of the injector is sealed to the intake manifold with another O-ring and fuel is discharged below that seal.

All EFI injectors, regardless of brand, share the same basic core components. They are a solenoid that is attached to a fixture that opens and closes a fuel flow orifice. When the ground circuit is completed by the ECU (battery positive is kept constant) a magnetic field is created and the solenoid is energized, lifting the fuel flow closing device and exposing an orifice that allows gasoline to pass. When the ground circuit is removed, the magnetic field collapses and the solenoid is moved via an internal spring and fuel flow stops. Thus, the injector pulse width that is part of a calibration table is the amount of time the ground circuit is evoked and a signal to open the injector applied.

This concept at first glance seems very straightforward, but there are several crucial factors that need to be understood. The need to open the injector quickly and allow it to accurately atomize (break into small particles) the fuel is where the problems lies while still responding to the elastic fuel demand of an engine. Most readers of GMHTP are familiar with the term injector pulse width, but do we truly understand what it means?

GM LS Fuel 3/9

The length of time the injector ground circuit is applied is measured in milliseconds or thousands of a second (1/1,000). This is referred to as injector pulse width. The length of time it takes for the magnetics to build and the solenoid to move and uncover the fuel flow orifice is called the rise time. Even though to the human eye the opening of the injector is quicker then can be captured, once broken down to the finite measurement of 1/1000 of a second the slowness of the injector opening becomes apparent.

As with any electrical device, an injector obeys Ohm’s Law. A review of this concept shows that if the voltage in a circuit remains constant but the resistance (opposition to electrical flow) is changed, then the amount of current (amperage) will respond in an opposite way. If the resistance in a circuit is reduced (less opposition to electron flow) then the circuit will pass more current. Conversely, if the resistance is increased less current will flow. As an aside, this is why if there is a wire touching ground, the fuse and/or the circuit burns up—there is no resistance to current flow and the full battery potential tries to pass through the corrupted wire. And thus, a high impedance fuel injector allows less current to pass through it than a low impedance version. The higher the current flow the quicker the magnetic field builds and the injector becomes more responsive from a closed position. But increased current flow through the injector means higher heat in the circuit and special components in the ECU to control it.

Most if not all early GM port EFI systems employed high impedance (12 to 16 ohms) injectors due to their lower cost and the ability to employ saturated drivers in the ECU. Many applications today still are high resistance. The driver in the ECU is what does the work of sending the signal to open the injector. For our purposes there are two driver styles: saturated and peak and hold. Either design can be thought of as a relay. The purpose of a relay is to control a high current load circuit from a remote location such as a set of fog lights from a switch on the dashboard. The main difference between a driver and a relay is that the former has no moving parts. The task is performed with circuitry.

Fuel Injector Flow 4/9

Low impedance injectors (2 to 4 ohms) respond quicker (shorter rise time), but necessitate the use of peak and hold drivers. These are not only more complicated but costly to manufacture. By design a saturated driver will keep current draw constant during the entire duty-cycle. Conversely a peak and hold driver will initially surge the current up and then step it down to a lower value and maintain that setting throughout the event. If a peak and hold driver is rated at 4/1 amps that translates as 4 amps to open the injector and 1 amp to keep it open. Industry data states that a low impedance injector has a rise time of around 1.2 to 1.5 ms while a high impedance style would need around 2 ms.

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