This entry will attempt to provide the reader with a basic understanding of electronic fuel injection, air/fuel mixtures and catalytic convertors. This entry also hopes to dispel a few myths about the interrelation of the above subjects.
First, an explanation of terms and overviews of the systems involved:
Air/fuel ratio is self-explanatory, being the ratio of air to fuel entering the combustion chamber. It can also be written as A/F alpha.
Electronic fuel injection (EFI) is the method of fuel delivery used in every new gasoline-powered car sold in the United States and many of the other developed countries. The fuel delivery system keeps a constant supply of fuel at approximately 30 - 40psi. When the electronic control unit (ECU - the 'brain' of the EFI system) decides more fuel is needed, it turns on solenoids1 in the fuel system. These are called fuel injectors. The pressurized fuel then squirts through the open injectors into the intake tract where it mixes with the air before entering the combustion chamber.
The airflow meter is a section of the intake tract that measures the amount of air passing through it. The ECU reads the airflow meter to calculate the amount of air entering the combustion chamber.
Catalytic converters are used to 'scrub' pollutants out of the exhaust. Carbon monoxide, hydrocarbons and oxides of nitrogen are the three pollutants concerning emissions systems. Catalytic converters convert carbon monoxide and unburned hydrocarbons into water and carbon dioxide, both of which are common and relatively harmless. They do not get rid of oxides of nitrogen (NOx), which are monitored as a pollutant in most states in America. It is the duty of the exhaust gas recirculation system (EGR system) to minimize oxides of nitrogen.
Duty cycle is a percentage of time that a component is operated. If a fuel injector is open three-quarters of the time, it has a duty cycle of 75%.
A stoichiometric fuel ratio means that there are 14.7 parts of air for every part of fuel. At this ratio, fuel in a perfect combustion chamber would burn off completely. The products of stoichiometric combustion are water (H2O), carbon dioxide (CO2) and oxides of nitrogen (NOx). Note that NOx is a pollutant.
On-Board Diagnostics (OBD) came about in 1996 model vehicles, due to the United States setting standards for emissions systems and diagnostic capabilities. OBD II came about in 1998 model vehicles. The details of these mandates are too numerous and expansive to cover here.
A Few Side Notes
Fuel is not exploded inside an engine, it is burned at a rapid rate. Knocking (in other words, detonation) is the result of the fuel mixture exploding, and is very hard on the engine itself.
Octane rating is not a measure of the power of a fuel, the quality of a fuel or the refinement of a fuel. It is only the measure of the fuel's resistance to knocking. The only difference at the pump is that some vendors put more additives in the higher-rated fuels. Some doubt the additives are worth the expense.
Fuel Injection Operation
When your engine is running, a chemical reaction in the oxygen sensor generates a signal that is sent to the ECU. The ECU measures the voltage sent to it to determine the amount of unburned fuel. A lower voltage, of less than 0.5 volt, indicates a lean condition and that there are very few hydrocarbons present in the exhaust (combustion chambers are not perfect, there will always be some unburned fuel if the mixture is stoichiometric). A higher voltage, over 0.5 volts, indicates a rich condition and that there is an excess of hydrocarbons present.
It is a common misconception that fuel injection systems try to maintain a stoichiometric ratio2. The truth is that if you graph the oxygen sensor voltage after the vehicle is at operating temperature, the voltage will 'swing' up and down. As rpm is increased, the swings increase in frequency3. When the exhaust is rich, the excess hydrocarbons are caught in the catalytic convertor and are held temporarily. When the engine swings towards lean, nitrogen oxides and carbon monoxide chemically react with the left-over hydrocarbons to produce water, carbon dioxide and nitrogen. There is a new style of catalytic convertor, not in mass production at the time of writing, that gets rid of NOx without having to swing the fuel mixture.
The ECU recognises the hydrocarbons in the exhaust and alters the duty cycle of the fuel injectors to reduce emissions. If the mixture is rich, and has been so for the appropriate amount of time, it will shorten the fuel injector's duty cycle, leaning the mixture. When you hit the accelerator the airflow meter will sense an increase in the volume of air, will signal the ECU and the ECU will lengthen the fuel injector's duty cycle, enriching the fuel mixture for acceleration. OBD vehicles have a rear oxygen sensor that checks the function of the emission system and helps compensate for wear and ageing of the fuel injection system.
The swinging air fuel mixture is a very effective way of reducing emissions in all driving conditions, except engine warm-up, during which the oxygen sensors and catalytic convertor are not hot enough to operate. Older, fuel-injected vehicles run at a calculated stoichiometric ratio until the engine is warm enough to reduce emissions. Calculations do not account for engine wear, which is often why when cars get old they don't run well until they're warm. OBD II vehicles have oxygen sensors that warm up faster and sense mixture ratios from start-up, reducing cold-start emissions and improving driveability.