Injection Perfection


The last 30 years have produced a complete turnover in vehicle technology. Today's automotive components have little in common with their primitive ancestors: drum brakes have been replaced by disc brakes, solid-axle rear suspensions have given way to independent designs, and ignition systems are now electronic rather than mechanical. Yet of all these advances, none are as compelling as the shift from carburetion to fuel injection.

Certainly the use of fuel injection has been around for more than thirty years, but only since the mid-1980s has it became a widespread feature on almost every vehicle sold in America. Driven by a need for cleaner emissions, American car manufacturers were forced to give up the less-expensive carburetor for the more-sophisticated computer-controlled electronic fuel injection, or EFI. As a bonus, they also got better performance and improved fuel economy.

So what is it about EFI that makes it so much better than the simpler, less-expensive and easier-to-work-on carburetor? In a word: precision. Through the use of sensors, injectors and computer control, EFI provides a far more precise air and fuel mixture under a much broader range of operating conditions. Let's take a look at these three basic fuel-injection components and the role they play in making efficient horsepower.

The Computer

If you're reading this article, you already know that computers play a major role in just about every aspect of human existence. Besides supplying information for web surfers and entertaining games for kids (and adults), they also control most of what goes on in today's automobiles. You may have heard that the average new car possesses more computing power than the entire lunar module used to land our first astronauts on the moon. Much of this processing power is necessary in a modern fuel-injection system.

The computer at work under your hood is often referred to as the engine controller. It monitors everything from barometric pressure to ambient temperature to throttle position, in order to determine just how much fuel an engine needs. By constantly monitoring the conditions under which an engine is running, the engine controller is able to make infinite adjustments for maximum efficiency and performance. It should be noted that the engine controller performs this task not only by adjusting fuel delivery but by altering the ignition system as well. Certain forms of variable valve timing also use computer control to deliver peak performance.

The Sensors

A computer is only as capable as the information supplied to it, right? To keep the engine controller properly informed a number of sensors are utilized for monitoring purposes. The three basic sensors required by every EFI system are an rpm sensor, a manifold absolute-pressure sensor (MAP) or a mass air sensor (MAS), and a throttle-position sensor. Most modern cars use several additional sensors to monitor things like barometric pressure (for altitude changes), exhaust gasses (for emission reduction) and temperature (for a proper mixture when an engine changes from cold to hot).

If one or more of these sensors fail, engine performance will suffer or stop altogether. Sensor failure is a common cause of reduced horsepower or mileage, especially on American cars from the 1980s when domestic fuel injection was in its infancy.

The Injectors

The information gathered by the various EFI sensors determines exactly how much fuel is necessary. The engine controller uses this information to adjust for conditions ranging from idling to accelerating to highway cruising. A device, called the fuel injector, delivers this precise amount of fuel into the combustion chamber through a small plunger located in the nozzle of the injector. Since the plunger can open and shut very quickly, it makes the injector extremely accurate at modulating fuel flow and allows fuel injection to adjust for both small and large variations in the operating conditions.

EFI's accuracy is based not only on the amount of time the plunger stays open (called a pulse width) but also on the fuel pressure within a fuel-injection system. This makes the fuel pump a much more crucial element and explains why most modern fuel pumps are electric rather than mechanical.

Why EFI is Better

It's obvious that EFI makes for a great technical discussion, but is it that much better than a carburetor? You bet. Remember that while driving your car your engine is in a constant state of change. In addition to the accelerating, braking, coasting or idling that comes with stop-and-go traffic, there's plenty of variation going on when rolling down the interstate with the cruise control set. You may be going up a mountain or descending into a valley. Maybe you're on a flat plane, but the sun has just set and the temperature is dropping.

The point is that only EFI, with its computer-based control module, can effectively keep up with the many changes our vehicles experience on a typical drive. Carburetors are mechanical devices that depend on springs, rods and engine vacuum to modulate fuel delivery. They can't accommodate for variations between individual cylinders, inconsistent octane, or altitude changes. More experienced readers will remember the days when moving between Denver and Los Angeles required a trip to an automotive shop for "rejetting" to keep a car from running too rich or too lean. Today, with the power of fuel injection, engine tuning is a no-brainer when traveling from the Rockies to the beaches. And isn't that the kind of driving experience we want?

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