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.

With this established, another concept must be introduced. That is dynamic flow range (DFR) of the injector. The easiest way to understand this is to think of a valve in a cylinder head. If anyone has ever used a flow bench to test a cylinder head, you understand that as the valve is lifted the flow increases. If you do not have this experience then think of a simple faucet in a sink. At a certain point, the flow no longer increases even if the valve is opened farther. The valve needs to move away from the seat and the wall of the combustion chamber to unshroud and reach its flow potential. Once peak flow is reached the port is considered stalled and if the valve is opened farther no increase in flow is recognized. This concept applies to a fuel injector. Most injectors except the Rochester MulTec and the Lucas along with a few others use a pintle and seat as the fuel flow valve. The MulTec employed a ball and seat and the Lucas a disc. Though DFR applies to all injector designs it is most easily represented by the pintle style injectors since it is more analogous to the poppet valve in a cylinder head.

Flow Injector Flow 5/9

Just as the valve needs to unshroud to allow maximum flow to occur, so does the device that is used to control fuel in the injector. As the injector solenoid moves the flow control device (either a pintle, ball or disc), it needs to travel a certain distance before proper flow characteristics are obtained. Before the DFR is reached, the injector does a poor job at atomizing the gasoline and actually dribbles it. When this occurs the programmed versus delivered air/fuel ratio is skewed and the engine runs poorly, especially at idle and low speeds. This is the reason that an injector must be sized properly (flow rate) for the application. You cannot keep cutting back the pulse width on an oversized injector to control the mixture at idle and low speed and still maintain good atomization. If you recall from Part 1 of this series, if the atomization rate is poor then so is the vaporization (phase change) of the fuel. That is why the original ZR-1 Corvette employed two 22 lb/hr injectors in each bore instead of one 44 lb/hr unit.

When working with a calibration, the voltage correction tables are there due to the impact Ohm’s Law has on the injector response time and the need to have it enter the DFR to keep the engine drivable when charging circuit voltage is low. The goal of any calibration is to limit the rise time and have the injector reach the DFR as soon as possible while still being able to control the air/fuel ratio to the desired value. This is often the obstacle that many fall prey to. The choice of injector is based on the full power requirement of the engine and no consideration is given to how it will respond at short duty-cycle. This also becomes exasperated when a cylinder head with a large intake port is used and the velocity of the incoming air is low at idle.

Many engines have a very elastic fuel demand such as one with forced induction or nitrous. A high-powered normally aspirated engine is more linear in it’s fueling. However, a power adder forces the engine to be a Dr. Jekyl and Mr. Hyde when it comes to fueling. At idle and light load it requires very little fuel, but once things get going it needs the volume of an engine twice its displacement. This is especially difficult with forced induction since the boost pressure comes on gradually and needs to fuel along the way. The best way to accomplish this is to use an ECU that has sequential injector firings in lieu of batch-fire. While there has not been a batch-fire GM system from the factory for years, many aftermarket ECUs still offer that sequence and it could very well be on your hot rod right now.

Fuel Injector Test Bench And Ultrasonic 6/9

With batch-fire, the injector fires twice and is not sequenced to valve opening. The first firing has the fuel wait at closed intake valves while on the next firing a valve will be open. It takes two opening and closing cycles to get the full fuel load to the cylinder. There is always fuel waiting at the backside of the valve and in the port to enter the bore. This results in the possibility that at idle and low speed there will be two injector events that do not allow it to enter the DFR and are dribbling gasoline. In contrast, a sequential system (even one that is random sequential and does not use a cam sensor input) has one long injector pulse, allowing the magnetics to build and the injector to enter the DFR and properly atomize the fuel. As an aside, especially with very high flow rate injectors, a sequential firing is much more friendly to the fuel rail hydraulics. Eight large injectors opening at the same time causes the pressure in the rail to drop and then surge as the regulator tries to keep it filled. If you were to monitor the fuel pressure in the rail with a large injector batch-fire system it would swing up and down and you would be able to hear the knocking from the fuel surges.

Injector flow rate can be thought of like a carburetor jet orifice dimension. The industry uses three different standards for injector flow. These are the common pounds of fuel per hour, grams of fuel per second, and cubic centimeters (cc) of fuel per second. When dealing with lb/hr ratings, it must be noted that the SAE standard test pressure is 3 bar or approximately 43.5 psi unless otherwise stated. Even though almost every EFI system today uses more than 43.5 psi fuel pressure, the rating system is still the same as it was years back. The flow of an injector is determined at the test standard pressure and at 100 percent duty-cycle. The solenoid is energized and is not cycled opened and closed so the injector is always in the DFR. For example, if the injector is rated at 24 lb/hr it means that it will flow 24 pounds of fuel at 43.5 psi operating pressure and 100 percent duty-cycle at a test specific gravity of the fuel.

If you recall in Part 1 of this series it was mentioned that the weight of the fuel would impact the delivered air/fuel ratio for a given injector opening time. This is important when dealing with race gasoline or a calibration that runs on both race and street gas. If the fuel pressure is increased above the test value, the injector flow capability increases. Conversely if the pressure degrades the flow suffers. But it cannot be forgotten that the injector is designed to atomize the gasoline with a minimum fuel pressure and will evoke poor performance much below that. Atomization in theory does improve at higher pressures above 43.5psi. The higher pressure now impacts the injector response time since the magnetics needs to work against a greater head. This now brings us to another area that is related to injectors. That is duty-cycle.

Fuel Injector Internal 7/9

The easiest way to understand injector duty-cycle is to think of it as the length of ignition event time that the ECU is grounding the injector winding for. Since the ECU triggers the firing of the injectors from ignition events it only has a given amount of time to get the fuel into the cylinder before the next event occurs. The length of time between ignition events is much shorter at 6,000 rpm than it is at 3,000 rpm. If the injector is staying open longer than the ignition event time it is considered to be static and not able to control fuel flow. That is why injectors are offered in different resistance values and flow rates. If it cannot pass enough fuel during one event to satisfy the engine you either need to raise the fuel pressure or the flow capacity of the injector. Decreased injector resistance allows for quicker response time, and in turn the ability to increase the amount of fuel delivery time between ignition events. If the injector is quicker to lift the pintle, it affords the opportunity to be able to fuel the engine in a more timely fashion.

A good rule for the enthusiast in regard to duty-cycle on an engine that will see short excursions of high RPM, such as a drag race use, the injector can run near static with no problem as long as the air/fuel ratio is maintained. When sizing an injector for an endurance engine, always allow for a maximum of 80 percent duty-cycle to allow for the injector windings and the drivers in the ECU to cool off. In simple terms a road race engine of the same power and operating speed will need a larger flow rate injector than one in a drag car.

Fuel Injector O Ring 8/9

To properly choose the size of an injector not only do you need to understand the design criteria, but also the brake specific fuel consumption values (BSFC) and accurate flywheel horsepower. This will reveal how much fuel in pounds it takes to produce one horsepower. This is where injector suppliers have led the enthusiast astray.

The industry has used a textbook BSFC value that is not valid for a modern engine, especially a performance variant. They use 0.50 BSFC. This means that the engine is taking a half pound of fuel to produce one horsepower. Using a 0.50 value for BSFC, a 30-lb/hr injector at 100 percent duty-cycle can support about 60 horsepower per cylinder or 480 horsepower on a V-8. The problem being that most normally aspirated engines a GMHTP reader would have enjoys a BSFC of around 0.42 or lower (numerically). When that same injector is placed in the more thermally efficient engine and is exposed to the same duty-cycle and fuel pressure, it now has the ability to support about 581 horsepower, a big difference. This is where tuners get into trouble. They over estimate the horsepower and under estimate the thermal efficiency, and buy too large an injector. They then have problems trying to get the mixture under control at idle due to the pulse width not being long enough for the DFR to be entered. The engine is then run too rich at idle and light load, so that it will see some vaporization of the gasoline and in a short while the rings are worn from the oil being washed from the cylinder walls.

The next area of concern is injector stroke-to-stroke repeatability and its effect on engine performance. Most take the approach of out of sight, out of mind and feel that as long as the injector is pulsing that all is well. When dealing with a port fuel-injected engine it needs to be thought of as one with a carburetor for each cylinder. Anyone who has worked with multiple carburetors, specifically of the Webber type, on an individual runner (IR) manifold has a full understanding of fuel distribution and the problems it can cause. Since each injector is responsible for all of the fuel requirements for that cylinder, any flow variation from injector to injector will cause differences in mixture for each bore. When injectors become dirty they lose their ability to properly atomize the fuel and cause high emissions, poor idle quality, and weak overall performance. With today’s gasoline, injectors tend to stay cleaner longer than they did ten years ago, but it is still a concern.

Fuel Injector Tip 9/9

Even if the tip of the injector does remain clean, the constant charging and discharging of the injector solenoid will eventually lead to a lazy injector (electrically) with an increase in rise time. The only way to quantify injector performance before you make any calibration changes is to remove them and have each confirmed on a test bench. This will give you the ability to view atomization patterns and measure flow accurately. It is not uncommon to see huge flow variations of 15 percent or greater, in even new injectors, that are of less quality let alone those that have been in use for years. That is why I suggest flow testing both new and in-service injectors prior to performing a calibration.

The key to having the fuel injector work for, instead of against, you during a calibration is rooted in employing a sequential firing ECU, keeping the fuel pressure near the rated value and sizing the flow rate properly. In addition, a charging system that is not fitted with an under drive pulley and has strong alternator output will limit the skewing created by low voltage, even if the correction tables are not yet evoked.

Sources

Marren Fuel Injection
203-267-3835
https://www.injector.com/

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Understanding the dynamics of a fuel injector
Ray T. Bohacz Jan 16, 2014

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