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Autos Get HTML Code Word Count: 872 Mazda RX8 Air/Fuel management Autos Get HTML Code Word Count: 872 Mazda RX8 Air/Fuel management Mazda RX8 Air/Fuel management
A brief introduction to Air/Fuel ratios
Definition: the mass of air supplied to the engine divided by the mass of fuel supplied in the same period of time. The Stoichiometric, or chemically correct, air-fuel ratio (A/F ratio) is the exact ratio necessary to burn all the carbon and hydrogen in the fuel to carbon dioxide and water with no oxygen remaining. The fuel-air ratio is the reciprocal of the air-fuel ratio.
Stable combustion conditions require the right amounts of fuel and oxygen. The combustion products are heat energy, carbon dioxide, water vapor, nitrogen, and other gases (excluding oxygen). In theory there is a specific amount of oxygen needed to completely burn a given amount of fuel. In practice, burning conditions are never ideal.
When air and gasoline are mixed together and ignited, the chemical reaction requires a certain amount of air to completely burn all of the fuel. The exact amount is 14.7 lbs of air for every pound of fuel. This is called the "Stoichiometric" Air/Fuel ratio. It's also referred to the Greek letter "lambda."
When lambda equals one, you have a 14.7:1 Stoichiometric Air/Fuel ratio and ideal combustion. When the Air/Fuel ratio is greater than 14.7:1, lambda also will be greater than one and the engine will have a lean mixture.
Lean mixtures improve fuel economy but also cause a sharp rise in oxides of nitrogen (NOX). If the mixture goes too lean, it may not ignite at all causing "lean misfire" and a huge increase in unburned hydrocarbon (HC) emissions. This can cause rough idle, hard starting and stalling, and may even damage the catalytic converter. Lean mixtures also increase the risk of spark knock (detonation) when the engine is under load.
When the Air/Fuel ratio is less than 14.7:1, lambda also is less than one and the engine has a rich fuel mixture. A rich fuel mixture is necessary when a cold engine is first started, and additional fuel is needed when the engine is under load. But rich mixtures cause a sharp increase in carbon monoxide (CO) emissions. When the relative proportions of air and fuel are "just right," the mixture burns clearly and produces the fewest emissions. The trick is balancing the mixture as driving conditions, temperatures and loads are constantly changing.
Open Loop / Closed Loop briefly...
To briefly clarify two concepts, the definition of OPEN & CLOSED loop, here is what they entail to the best of my knowledge:
CLOSED LOOP: observed under light loads, partial throttle & cruising conditions. The RX8's ECU is constantly monitoring the A/F state (Rich or Lean) reported by the Narrow Band Oxygen sensor located on the exhaust and adjusting fuel injector cycle (fuel delivery) to maintain a ratio close to Stoichiometric (14.7 A/F). These adjustments are simply leaning or richening the mix by a fixed amount, until the ECU detects an opposite condition, thus it "steps" back on its last adjustment. Generally, the A/F ratio is maintained @ 14.7 to maximize catalytic converter efficiency.
OPEN LOOP: observed under wide open throttle (WOT) or high loads. The ECU will ignore the Oxygen Sensor and will receive information from Mass Air Flow (MAF), RPM, vehicle speed, load, and potentially other sources. It then determines the appropriate amount of fuel based on a lookup table stored in memory. Under this condition, the ECU no longer attempts to keep a Stoichiometric A/F ratio, but rather a richer mix. Such mix is intended to protect the engine from knock / pinging.
A word of caution...
There is one last thing I'd like to make clear before wrapping up this article.
We all know engines run @ high temperatures. The leaner the A/F ratio, the less fuel the injectors will supply, the higher the temperature of the combustion will be. On the other hand, the more fuel in the mix the cooler the combustion will be (too rich will foul plugs, etc)
Running under lean conditions all the time will raise the combustion chamber temperature. The higher the temprature, the higher the likelyhood of detonation. What is detonation? Let's just keep it at this: in a piston engine, detonation generaly means that the mix is combusted before the piston has reached maximum compression. Or even worse, even before the intake cycle is complete, having the intake valve(s) still open. This allows the flame to travel outside of the cylinder.....and... you can make out the rest.
Guess what? A way of preventing detonation is by using fuels that have higher flame thresholds. Thus being less prone to being ignited without direct exposure to spark. Now, ever wondered why 87, 89, 91, 93 & 100 octane fuels exist? You got it! The higher the octane rating, the more difficult to ignite the fuel is. Sounds counterintuitive? You'll get used to it.
Alright, by now you are wondering where am I going with this? For now, just a friendly piece of advise: DO NOT ASSUME THAT BY LEANING YOUR A/F RATIOS WITH ONE OF THOSE NICE ELECTRONIC TOYS (eManage, SAFC & SAFC-II, etc...) YOU WILL BE GAINING POWER WITHOUT ANY TRADEOFFS. It is very important that you have the necessary tools and mechanisms to detect knock, detonation or pinging on your engine before it is too late. Those three conditions are caused mainly by excessive combustion temperatures from a lean mix condition.
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