Operation EFI: Injection Methods, Part I

Fuel Injectors are precisely manufactured and electronically controlled valves.	I've touted the advantages of a fuel injection system in previous entries, and I wanted to specifically address some key systems and components to give the reader a good idea of how an EFI system works and how it compares to a carbuerator. In this entry I'll address the fuel injector, a key component in precisely controlling fuel flow into the engine.

A fuel injector is a device designed to shoot an easily-atomizable spray of fuel droplets into an engine. Depending on the engine type, the injector can be placed in the combustion chamber in a configuration known as direct injection (utilized by Diesel and some newer gasoline engines,) directly above the intake valve, known as port injection (or tuned port injection, TPI,) or directly after the throttle body, called throttle body injection (TBI.)

These injection methods vary in their implementation by manufacturer, but the basic operation is the same - as the engine revolves, the fuel injectors fire at a programmed moment. The timing of these events is important, especially in port and direct injection setups - in port injection systems, open the injector too early and the fuel will drop out of suspension, open the injector too late and the intake valve will close before the fuel reaches the combustion chamber. In direct injection, inject the fuel too early or too late and the engine will have reduced power or possibly suffer from destructive detonation.

While the timing of all these events is controlled by the ECU, the fuel injector itself must be able to operate on the same time scale as an engine rotating at several thousand RPM. It must be able to open and close in milliseconds, disperse fuel in a fine mist, and have accurate, repeatable response. To achieve this, the fuel injector functions with solenoid-controlled needle valve.

As indicated in the diagram, fuel enters into the injector, is filtered to prevent debris from entering, and pushes against the backside of the control valve. The solenoid acts against the fuel pressure, so the position of the injector is 'normally closed.' With no current applied, the fuel pressure itself holds the valve closed. Once the ECU applies current to the coil of the solenoid, the valve is opened against the fuel pressure, allowing the fuel to exit through a nozzle and into the engine. These components must be light and precisely manufactured so the fuel injector can operate without sticking or any excess lag of operation. The less mass the solenoid must move the faster it can move it (remember F=ma!)

A fuel injector is simply a valve operated by a solenoid. Image courtesy of Wikimedia Commons

For Operation EFI, I will be using a set of eight Bosch EV6 fuel injectors. Diameters of most gasoline injectors are standardized, however there are exceptions in some cases, and installed height of injectors can vary with manufacturer design. In custom applications this is less important, but in OEM replacement situations it is important to ensure the injectors have correct dimensions.

A Word about Impedence: The solenoids that operate a fuel injector vary in design. These variations can change the electrical resistance of the injector, and thus the resulting current required to activate them. While I will not get into injector triggering methods in this post, it is important to know if the injectors are high or low-impedence, and what type the ECU can handle. The PE-3 utilizes high impedence injectors, and using low-impedence without some form of resistor in-line will burn out the injector control circuit.

With your ECU, injection method and required impedence known, it's time to think about flow rate. To feed an engine that is producing some number of horsepower, a certain amount of fuel must be provided. The amount of fuel the injector sprays is dependent on open time. Ignoring the time it takes for the injector to open and close (which is a parameter called dead time the ECU takes into account) if the time the injector is open is doubled, the resulting total fuel flow will double. Once the injector is open 100% of the time, it flows some maximum amount. This is known as the static fuel flow rating. In the case of my Bosch injectors, that amount is 30 lb/hr (or 315 cc/min.)

The injectors should never operate at 100%, but should be open for some fraction of that time. The percentage of time spent open is called the duty cycle of the injector, and most injectors should not operate above 80% for risk of damage or overheating of the solenoid coils. Additionally, if they do reach 100% duty cycle, more fuel cannot be injected and the engine can run lean, potentially causing detonation and certainly reducing power output. [See Operation EFI: A Not So Quick Primer for more details]

On the other end of the operating range, injectors should not be too big. Since the injector requires a certain amount of time to open and close, if your injector flows a lot of fuel the tuner may not be able to reduce the open time enough to control flow and the engine will run rich at low engine speeds and loads, reducing economy, idle quality, and decreasing tip-in and off-idle throttle response. Because the valve requires some amount of time for the valve to open and close once current is applied (engineers call this hysteresis,) there is a point where the ECU cannot reduce the open time any more without the injector failing to open at all. This is the mininum open time.

So, knowing this, how do you determine required flow? With known or estimated values for horsepower, volumetric efficiency, and brake specific fuel consumption of your engine, you can use stoichiometry to determine an appropriate flow rate. Or, to avoid math, you can simply search online for a fuel injector flow calculator. It is important to be honest with your power numbers in these estimations. You can lie to your friends, but you can't lie to physics. Overestimating a little, or going a little bigger to prepare for future mild upgrades is okay, but pretending your stock small-block Chevy with a 3/4 cam makes 500 horsepower, or that your Civic with a cold-air intake and chrome lug nuts makes more than my blender will bite you when it comes time to tune.

Image from www.dune-buggy.com

Note on Fuel Pressure and Flow: The flow of any fuel injector (or orifice for that matter) is dependent on the pressure drop across the injector. Injectors are rated by the manufacturer at a specific fuel pressure, and changing this pressure will change the fuel flow. This is also why many fuel pressure regulators are referenced to manifold pressure (called boost-referenced in forced induction and vacuum referenced in naturally aspirated engines,) because it is the ratio of inlet and outlet pressures that defines flow. For example, if manifold pressure is raised to 6psi by a turbocharger, you must raise your fuel system pressure 6 psi to inject the same flow as at atmospheric pressure. Pressure referenced regulators do this by design, and many are adjustable to allow you to tweak fuel flow. Be sure your injectors operate at an appropriate pressure for needed flow.

With flow rate determined and system pressure known, the engine can be tuned by the methods described in my Not So Quick Primer. On non-programmable OEM systems, fuel pressure can be tweaked with an adjustable fuel pressure regulator, adding a little extra fuel when the system is operating in open loop. Ultimately, the system offers a huge amount of adjustability, and within the sizing limitations of the injector, there is no need to swap any parts to change fuel tuning, a considerable step up from carburetors that require tweaking of separate circuits for idle, acceleration, cruise, and WOT. In the next entry I will talk about carburetors, and attempt to explain their basic operating method, despite my poor grasp of the intricacies of carburetor tuning.