Air to fuel Ratio Blurb by Ian Staunton 

The air/fuel ratio is a common source of confusion to many people. It refers to the ratio of air to fuel exiting from the engine, and it is a good measure (in combination with an exhaust gas temperature gauge) of measuring engine tune. The air/fuel ratio is commonly represented as a single numeral as in 14.7. This actually represents the number of air particles exiting per single fuel molecule (i.e.., 14.7:1). The 14.7:1 just mentioned is actually what is called stoichiometric - or the ideal air/fuel ratio, at which temperatures are controlled and fuel economy is optimized. Typically cars make best power at about 12.5:1 air/fuel ratio, but this is harder on the emissions parts and sensors, which reduces lifetime.

As mentioned before, exhaust gas temperature, in combination with air/fuel ratio, is a good measure of the state of engine tune. This is because the air/fuel ratio affects the heat generated during the combustion phase of the engine cycle. Fuel acts as a cooling agent for the cycle, and thus having a numerically lower air/fuel ratio - which is considered to be a RICHER air/fuel ratio (i.e.., richer in the amount of gas exiting) will result in lower exhaust gas temperatures. Similarly, leaning out the mixture by injecting less gasoline causes the overall exhaust gas temperatures (commonly referred to as EGTs) to rise. Frequently a change from a lean to a rich mixture can cool the exhaust charge by as much as 400 degrees Fahrenheit, so this is a very practical matter.

That being said, it's often very difficult to determine the air/fuel ratio an engine is generating, and measuring exhaust gas temperature is problematic as well. For example, where does one place the exhaust gas temperature probe? At what distance from the exhaust ports? Should it be placed after the merge collector of the exhaust manifold or headers, or should it be placed nearer a particular cylinder? Will this result in biased readings if for any reason a single cylinder is running at higher temperatures than any other? Problems like this exist also for the air/fuel ratio. It is a constantly changing variable which your onboard computer attempts to control with some degree of accuracy.

Measuring the air/fuel ratio is problematic. Because it exists not as a physically static constant but as a ratio, the sensors that your car uses to determine it (commonly referred to as O2 sensors) are ineffectual at best. The sensors used are, in older cars, of a non-heated single-wire type, which use the amount of air and fuel in the exhaust to vary the resistance of the surface of the sensor as the hot exhaust charge passes over it (in relation to the outside air), while newer cars typically use a four-wire heated sensor. Obviously the reaction to changes in the air/fuel ratio is not immediate, and thus large lags in the detection of a change in the air/fuel ratio happen frequently. In addition, these sensors must be mass-produced cost-effectively, and as such are engineered to work only in a small temperature band. If the engine is operating in a temperature outside of the optimal band, readings can be very sluggish and delayed by several seconds, or worse, completely wrong, as the membrane on the outside of the O2 sensor is temperature-dependent. This problem is exaggerated in the single-wire type, as they rely on the engine to bring them up to operating temperatures, and during that time period the sensor is completely ineffectual. Even worse than that, the O2 sensors present on your car tend to be effective only in a very narrow air/fuel ratio band, and their responsiveness is directly proportional to their age - O2 sensors wear out and get contaminated/fouled (even by handling!), and especially when asked to monitor a continually rich condition. This isn't always true, as there are O2 sensors available which do not rely on temperature to drive the metering and have very rapid response times (these are called wideband O2 sensors, where the wideband part refers to the band of air/fuel ratios in which the sensor is accurate - stock sensors are narrowband). Unfortunately, these wideband O2 sensors typically cost several thousand dollars US!, and actually using them would cost several thousand more for custom installations, but the usage of these sensors can be purchased at dynograph facilities during tuning sessions.

As mentioned before, the O2 sensors themselves operate by the particles in the hot exhaust gas modifying the resistance of the membrane surrounding the O2 sensor in relation to the outside ambient air. The onboard computer in your car (commonly referred to as an ECU, or Electronic Control Unit) can measure this resistance variation on the fly by passing a voltage through the wire (usually between 0 and 1 volts) and measuring what comes back, though this is pretty much unimportant - all that matters is that the computer can do it.

The computer acts to vary the air/fuel ratio by taking into account engine RPM, throttle position, engine temperature, timing advancement, and a number of other factors. As the engine cycles/turns, the computer calculates the optimal period for the fuel injectors to open and spray fuel into the engine (remember that fuel is pressurized, so that the time the injector is open is directly proportional to how much fuel gets sprayed in - i.e.., fuel spray happens at an effectly constant rate, so that having an injector open for 0.05sec will spray less fuel than 0.10sec). Obviously, the air/fuel ratio is proportional to how much fuel is injected, and therefore the computer uses the determined air/fuel ratio continually to tune and optimize its fuel injection maps. This is what causes the 'bouncing' or 'scrolling' commonly observed in air/fuel gauges at part throttle - the computer seeks to keep the engine operating at the optimal air/fuel ratio by richening and leaning the fuel mixture, attempting to get the mix *just right*. However, there will always be a certain set amount of 'bounce' no matter how long the computer seeks to tune the mixture, as it is also used to test that the O2 sensor is still functioning.

While we're at it, we might say a thing or two about the ECU. It usually has three modes: closed loop, open loop, and WOT. The 'open loop' mode is used when the car is warming up and sensors are below normal operating temperatures; during this time, the car runs off a preset (usually tuned for safety) set of fuel injector mappings and ignition timing tables, among other things. It is called thus because single-wire sensors are ignored, so the ECU is running without accepting outside input. The 'closed loop' mode is when the ECU is accepting input from outside sensors, and this is where it attempts to tune the air/fuel ratio. With an air/fuel ratio gauge, you can actually see when the computer switches over from one mode to the other when the computer begins to tune the air/fuel ratio. The third mode, WOT, is when you put the pedal to the floor (WOT = Wide Open Throttle). In this mode, everything about the computer switches over to making maximum power (kind of a generalization, but usually true). Typically air/fuel ratios go to the rich side for more power at WOT, and the computer switches back into open loop mode and stops accepting input from extraneous sensors.

Now, this is where your standard air/fuel ratio gauge comes in. Air/fuel gauges are, 99% of the time, piggyback devices which have three wires - one for power, one for ground, and one for tapping into the stock O2 sensor wire to the computer. This introduces lag at the air/fuel ratio meter as well, as the electronics present in the meter must process the signal being received. While this lag is minimal, it is present, although it is lessened in the case of needle gauges. Thus, the standard gauges you can buy are simply readouts of what the computer is seeing and do not actually represent the car's state of tune. Even when used in combination with devices which can alter the air/fuel ratio, such as the A'pex-i S-AFC or V-AFC, the PMS piggyback fuel system, adjustable fuel pressure regulators, etc, the actual air/fuel ratio being displayed is frequently inaccurate, being hampered by lag and bad sensing equipment.

However, when used in conjunction with an EGT gauge, the actual exhaust gas temperature may be recorded, and from this it may be determined whether a richening of the mixture or a leaning out, towards stoichiometric for more power, would be optimal. In addition, the air/fuel ratio gauge can be used to determine trends and characteristics - while it's not entirely accurate, tests are usually repeatable, and that's what matters.

Some common air/fuel ratio misconceptions:

1) Running an extremely rich air/fuel ratio creates more power.
This is a good one for novices, who think that you need to burn every last bit of air going into an engine. Actually, an engine spits out most of the air it ingests, as the ignition cycle for a particular cylinder is far too small and short to effectively utilize the entire air mixture. When high-power turbo cars or N/A (naturally aspirated) cars run extremely rich mixtures, it is to keep cylinder temperatures down. Frequently, cars gain power as they run more optimized air/fuel ratios, typically slightly richer than stoichiometric. In addition, rich mixtures can wear out sensors, waste gas, and clog your catalytic converter. If it were true that more gas equalled more power in all cases, race cars would simply dump fuel directly into the ports instead of worrying about metering. Tuning is key. Dumping more fuel in just results in the need for more spark.

2) Going lean creates an 'instant detonation' condition.
Again, in the case of street engines, this just isn't true. Manufacturers try to keep engines running as lean as possible as often as they can in order to improve emissions, mileage and power. Thus they build in significant safety margins. Detonation isn't actually caused by a lean condition; it is a result of a lean condition. Gasoline actually acts as a suppressor to the ignition event (ever hear people say "With that much fuel you need more spark"?). Leaning out under hard throttle causes temperatures to rise, which causes lean mixtures to pre-ignite, but they don't cause preignition. The temperatures do.