The Air/Fuel ratio (A/F) is the mixture ratio or percentage of air and fuel delivered to the engine by the fuel system. It is usually expressed by weight or mass (pounds of air to pounds of fuel). The Air/Fuel ratio is important because it affects cold starting, idle quality, driveability, fuel economy, horsepower, exhaust emissions and engine longevity. For a mixture of air and fuel to burn inside an engine, the ratio of air to fuel must be within certain minimum and maximum flammability limits otherwise it may not ignite. Too much air and not enough fuel , or too much fuel and not enough air may create a mixture that fails to burn when the spark plug fires. The result would be an ignition misfire, loss of power and increased emissions (unburned hydrocarbons or HC primarily). THE CHEMISTRY BEHIND AIR/FUEL RATIOS When the Air/Fuel mixture is perfectly balanced chemically, there is just the right amount of oxygen to burn all of the fuel. This ratio is called a STOICHIOMETRIC fuel mixture. All of the oxygen in the air and all of the hydrocarbons in the fuel will be consumed, leaving nothing but water vapor (H2O) and carbon dioxide (CO2). Under light cruise conditions and low engine load, most engines like a stoichiometric A/F mixture because it produces the lowest HC and carbon monoxide (CO) emissions and good fuel economy. This chart shows how different Air/Fuel ratios affect emissions, fuel economy and performance. The ideal or stoichiometric Air/Fuel mixture for various fuels will vary depending on the fuel and its chemical makeup. The amount of oxygen required in the A/F ratio will depend on the number and type of carbon and hydrogen bonds in the fuel, so different fuels have different optimum A/F ratios. Gasoline contains a mixture of various long chain hydrocarbons. One of its main ingredients is octane (C8H18), but it also includes many other HCs. The actual formula will vary depending on the season (winter or summer), the refining process and the emission regulations the fuel must meet in various areas. Generally speaking, gasoline will contain about 15 percent straight-chain C4 to C8 alkanes, 25 to 40 percent branched C4 to C10 alkanes, 10 percent cycloalkanes, up to 25 percent aromatics, 10 percent straight-chain and cyclic alkenes, and less than one percent benzene. Much of the gasoline sold in the U.S. is also blended with ethanol alcohol to extend the fuel supply, improve the octane rating (detonation resistance) and add oxygen for a cleaner burn. Ethanol gasoline fuel mixtures range from 10 percent ethanol (E10) to 85 percent ethanol (E85). E10 ethanol blends are EPA-approved for use in all gasoline engines, while E15 has been recently approved for use in 2001 and newer vehicles. For FLEX FUEL capable vehicles, ethanol/ gasoline mixtures containing up to 85 percent ethanol (E85) may be used. STOICHIOMETRIC AIR/FUEL RATIOS For gasoline engines, the ideal or stoichiometric A/F ratio is 14.7, which is 14.7 parts of fuel by weight to one part fuel. For E10 gasoline (90 percent gasoline with 10 percent ethanol alcohol), the stoichiometric ratio is 14.08:1. For Flex Fuel applications, the stoichiometric A/F ratio for E85 is 9.7:1. For an alternative fuel such as neat ETHANOL alcohol (E100), the stoichiometric A/F ratio is 9:1. For a racing fuel like METHANOL alcohol, the stoichiometric A/F ratio is 6.5:1. For a NATURAL GAS (METHANE or CH4), the stoichiometric ratio is 17.2:1 For a PROPANE (LP gas or C3H8), the stoichiometric ratio is 15.5:1. With diesel engines, the stoichiometric A/F ratio for #2 diesel fuel is 14.6. However, because diesel engines use the fuel mixture to control engine speed and power output, they typically run A/F ratios that can range from 18:1 up to 70:1. RICH AND LEAN AIR/FUEL RATIOS When the Air/Fuel mixture differs from the stoichiometric ratio, it burns differently and affects engine performance, emissions, fuel economy and longevity differently.

Real world driving conditions require different A/F ratios at different times, so the A/F ratio is not something that is static and unchanging,. It is dynamic and changes in response to changing operating conditions. First, we need to explain the difference between RICH and LEAN Air/Fuel mixtures. An A/F ratio that contains more air and less fuel than the stoichiometric ratio is called a LEAN fuel mixture. A lean mixture would be one with a ratio greater than 14.7:1 for gasoline. An A/F ratio that contains less air and more fuel than the stoichiometric ratio is called a RICH fuel mixture. A rich mixture would be one with a ratio less than 14.7:1 for gasoline. A LEAN A/F mixture typically burns HOTTER and uses less fuel per mile driven, which improves fuel economy. But hotter combustion temperatures also increase oxides of nitrogen (NOX) emissions and the risk of engine-damaging detonation (spark knock). DETONATION DANGERS OF A LEAN AIR/FUEL MIXTURE Detonation is an abnormal form of combustion that may occur when the combination of high temperatures and pressures inside a combustion chamber cause the fuel to spontaneously ignite before the spark plug fires. Instead of a smooth outward expanding flame balloon from the spark plug, pockets of fuel ignite and collide with one another producing an audible knocking noise. Detonation is bad because it increases combustion pressure much too rapidly. This produces hammer-like blows on the pistons that can damage pistons, rings, connecting rod bearings and head gaskets. A fuel mixture that is much too lean may even burn a hole right through the top of a piston! So one thing to always avoid is a really lean fuel mixture, especially when an engine is accelerating or working hard under load. An underlying cause of an overly lean fuel condition may be dirty fuel injectors, low fuel pressure (weak fuel pump or a restricted fuel line or filter) or insufficient fuel flow (pump or injector capacity to small for the application). On modified performance engines (especially those with a supercharger or turbocharger), a higher output fuel pump and/or higher flow fuel injectors are typically needed to keep up with the increased fuel demands of the engine. If the pump or injectors cannot keep up, the fuel mixture can go lean sending the engine into detonation and possible self-destruction! All late model original equipment engines with computerized engine controls have a KNOCK SENSOR to protect the engine against detonation. If the knock sensor detects vibrations that feel like detonation, it signals the engine control computer to momentarily retard spark timing which lessens the risk of detonation. The engine computer may also enrich the fuel mixture because adding fuel helps cool combustion temperatures and reduces the risk of detonation. RICH AIR/FUEL MIXTURES, HORSEPOWER AND EMISSIONS As for RICH A/F mixtures, adding more fuel to the mixture increases power up to a point. A richer mixture also reduces the risk of detonation, which is why engines that are supercharged or turbocharged usually have a richer A/F ratio when the engine is receiving boost pressure. But the trade-off of a richer mixture is increased fuel consumption and higher exhaust emissions (carbon monoxide primarily). The richer the A/F mixture, the higher the percentage of carbon monoxide in the exhaust. Normally, CO levels in the exhaust of a well-tuned engine running at or near its stoichiometric ratio should be zero to less than half a percent. If the vehicle is equipped with a catalytic converter, CO levels at the tailpipe should be zero or very close to zero. Carbon monoxide is a dangerous and deadly pollutant because only a tiny amount can kill! Air/Fuel ratios are constantly changing from rich to lean to suit changing operating conditions.

WHY THE AIR/FUEL RATIO IS CONSTANTLY CHANGING Although stoichiometric A/F ratios produce the best all-round results in terms of fuel economy and emissions, an engine cannot run with a stoichiometric ratio all the time. Sometimes it needs a RICH mixture and sometimes it can benefit from a LEAN mixture. Here is why: A cold engine needs a very RICH fuel mixture to start (at least initially until it warms up). The cold start period is the dirtiest time for emissions, so auto makers do a variety of things to speed engine warm up and improve fuel vaporization until the engine reaches normal operating temperature. Engines with Direct Gasoline Injection (GDI) are cleaner following a cold start because the fuel is sprayed directly into the combustion chamber under extremely high pressure. This improves fuel atomization so it will mix more readily with the air for a cleaner burn. On an older engine with a carburetor, the choke provides the initial rich mixture. Closing the choke flap restricts airflow into the carburetor to richen the mixture. As the engine warms up, the choke is gradually opened to allow more air until eventually it is no longer needed and the engine is running at its normal A/F ratio. On older fuel injected engine, a separate cold start injector provides extra fuel during a cold start. On newer EFI engines, the computer commands a richer mixture when the engine is cranking and first starts. The computer is programmed to deliver exactly the right amount of fuel based on engine temperature and air temperature. A cold engine also needs a RICH fuel mixture while it is warming up to idle smoothly. The A/F mixture will gradually get leaner as engine temperature comes up and idle speed settles down from a fast idle (around 850 to 1000 RPM) to normal idle speed (typically around 500 to 600 RPM). On a carburetor, this is handled by the choke and fast idle cam. The PCM uses a feedback control loop from the upstream O2 sensor to fine tune the A/F mixture. On a fuel injected engine, the computer maintains a rich A/F mixture until the oxygen sensor is hot enough for the feedback control system to go into CLOSED LOOP. Once this happens, the computer starts using the oxygen sensor signal to fine tune the…

Fonte: AA1Car.com