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Written by: Brandon Franzke
Written on: March 1st, 2002
Tags: chemical engineering, transportation
Thumbnail by: Buschtrommler/Wikipedia Commons
About the Author
In March 2002, Brandon Franzke was a Biomedical/Electrical engineering student, minoring in Neuroscience. After graduation, his intents are to go to graduate school and pursue a PhD in EE for work in neural prosthetics.
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Volume II Issue IV > Fuel Injection
The introduction of fuel injection to the automobile has been a major factor in increasing the power available to engines in recent years. However, its introduction was initially slow due to the inherent complexities of the system. Computer integration revolutionized the design of this automotive subsystem and has become the onboard controller of the fuel injection system itself. Modern automobiles are forced to meet stringent emissions and fuel efficiency standards since the institution of the clean air mandates of the 1970's, and these have for the most part been achieved through improvements in the engine fuel entry systems. We begin with an examination of the basic concepts of the automotive combustion engine and discuss the function and the principles behind the injection system. We then see how intelligent computer controls for fuel-injection system have effectively forced the carburetor into obsolescence.

Introduction

The desire to create powerful automobiles is a driving force in the advancement of engine technology. Until the early 1970's additional horsepower came from larger and more costly engines [1], which came from the belief that burning more gasoline was the best way to increase available power. However, by the outset of the 1970's cities and countrysides were beginning to be blanketed by thick black smog, a byproduct of the combustion [2]. In an attempt to reverse this, US governmental regulations were enacted that raised minimum fuel efficiency requirements of automobiles. To meet these new standards, car manufacturers were forced to reduce the chassis and engine size in an attempt to decrease overall weight, ultimately increasing fuel efficiency. This resulted in a significant loss of power and "pep."
However, with the emergence of computer-automated controls, auto manufacturers are finding that it is possible to build smaller engines with power to spare by increasing the efficiency of the combustion process [1]. Most of these advancements have come from precise timing of the fuel entering the combustion chamber, as well as timing of the ignition itself [3].

The Otto Cycle

The principles underlying the combustion engine focus on one concept: burning a chemical to obtain energy and then using this energy to do work. This energy is obtained by burning the gasoline that is put into the automobile. In an automobile, a controlled process called the 4-stroke cycle, or Otto cycle, is utilized to extract the work from the gasoline. The Otto cycle can be viewed as four individual steps:
1. Intake: The piston moves from top to bottom, creating a slight vacuum. This pulls fuel and air into the chamber. When the piston reaches the bottom of its course, the fuel intake valve is closed. This position is normally called bottom dead center.
2. Compression: The intake valve is then closed, and the piston moves upward and pressurizes the air/fuel mixture. An electrical spark is ignited to burn the gasoline. The mixture burns very quickly and the expansion of the exhaust gases causes a rapid rise in the pressure of the system
3. Power: As the pressure increases, the piston is forced downward. This is the only point during the cycle that usable power is actually generated by the engine.
4. Exhaust: The piston then begins to move upward. Simultaneously, a valve is opened to let the exhaust gases leave [3].
Armchoir/Wikimedia Commons
Figure 1: Depiction of the Otto cycle.
When the engine goes through steps 1-4 once it is called a cycle (Fig. 1), and by repeating cycle after cycle the engine can use the energy gained from many small ignitions to move the vehicle. The movement of the actual vehicle is the result of transferring the cyclic movement of the piston into the rotary movement of the wheels. This distribution is facilitated through the use of a crankshaft, which is connected directly to the pistons. As the pistons go up and down, the crankshaft is forced to rotate. This rotation is then indirectly coupled to the wheels by the front and rear differentials after being geared by the transmission. Ultimately, the result is that as the engine cycles the wheels can be made to turn by putting the car "in gear."

Driver Control of the Cycle

We can now look at this process with regards to the familiar responses of the car while driving. By pressing the gas you are telling the car to allow a larger volume of gasoline and air into the piston. Then, as you release the pedal, less gas is allowed in. Through this mechanism the driver is then able to control the amount of energy acquired by the engine and thus the energy converted into movement. Until recently, the device that controlled the fuel intake mechanism was the carburetor. However, the fuel injection system has since proven to be more efficient and reliable and has made the carburetor obsolete. These devices each control fuel intake by opening or closing a device called the throttle valve, which regulates the precise volume of gasoline and air admitted into the piston.