AFR debate....Kinda long. I never put time or thought when I post here. This time I did....
10-01-2006, 05:23 AM
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AFR debate....Kinda long. I never put time or thought when I post here. This time I did....
The "best" air/fuel ratio for our cars is a matter of great debate and I will do my best to avoid that debate while sharing my thoughts.
Simply put, there are a number of factors that one must consider in determining the best ratio, including power, safety, and fuel economy. Fuel economy is the easiest to understand, as a lower air/fuel ratio means more fuel and obviously lower fuel economy. As far as safety is concerned, richer is considered safer (to a point) as the extra fuel helps things run cooler. The lower temperatures help reduce the chance of autoignition and can literally keep engine components from melting. The safest air/fuel ratios are continuously being debated, but it is widely accepted that 13:1 is a good ratio for normally aspirated engines and 12:1 is good for forced induction engines. Many choose to go even richer, even 11.5:1. Autoignition (or "detonation" or "knocking") is considered a critical concern with rotary engines, and many tuners choose to go even richer than that. One must also keep in mind that these "safe" ratios are considered safe because they have been tried with many thousands of vehicles over many years by dyno operators that use the same equipment that most people are likely to encounter. Therefore, a safety margin that takes into account the accuracy of that equipment is inherently factored in. If it were common for turbocharged cars to blow up at 12:1 as measured on commonly used equipment, then the "safe" air/fuel ratio would have been lowered.
As far as power is concerned, I'll say only this: Every vehicle is different. If one wants to find the best air/fuel ratio for generating power, one should put the vehicle on a dyno and test it. Many believe that a particular ratio will result in the most power under any circumstances, and that belief is just too narrow-minded. There are far too many factors involved to make such blanket statements.
Regardless of the actual ideal air/fuel ratio number, almost everyone wishes to see a nice, flat air/fuel graph. This means that the ratio stays constant throughout the rpm range. A perfectly flat air/fuel graph is certainly not necessary for optimum engine performance or safety, but it is a nice thing to show off when tuning a vehicle. The smoother the air/fuel curve, the better the drivability will be and the smoother the power output will be. All good tuners realize that a little variation with the graph is perfectly acceptable, especially when one considers the factors involved. One must consider the accuracy of the oxygen sensor, where it is placed in the exhaust stream, the velocity of the exhaust stream at different points in the rpm band, the tools that the tuner has at his disposal to make changes, etc. Another important factor is that most air/fuel ratios are measured via a tailpipe sniffer. This method has proven to be an excellent way of measuring the ratio, but it is not perfect at low rpm. At low rpm, an engine may not be producing enough gas to displace all of the atmospheric air in the tailpipe, and this will produce a false lean reading because of the extra oxygen - as one can see in this chart. This phenomenon is going to be more pronounced in small-bore engines with large diameter exhaust piping. Two important things must be considered when one is tuning with a tailpipe sniffer because of this phenomenon. One, a flat line across the entire rpm band will mean that the actual air/fuel ratio is too rich at low rpm. Two, a real-world driver is almost never at wide-open-throttle at such a low rpm, so the air/fuel curve at that point is something that the driver will never see. One can also see from the chart that the catalytic converter has no significant effect on the air/fuel ratio in this particular vehicle.
The flatness of the air/fuel graph when one is done tuning is mainly going to depend not on the competency of the tuner but on the type of fuel management system being used and its resolution, and the patience of the customer and/or his willingness to pay for dyno time. One must also ask - is a perfectly flat air/fuel curve best? Many assume that a flat line at 12:1 or 13:1 "across the board" is best, but why is that? How could it be possible that the exact same air/fuel ratio be optimum for every rpm and load? This idea has been largely ignored in automotive enthusiast circles, as "good" tuners with adequate engine management equipment produce air/fuel curves that are flat "across the board" at the desired ratio. Thankfully, this notion has been challenged recently, and experienced racers and tuners have begun to realize that air/fuel curves should not necessarily be flat. Turbos can spool up faster if the ratio is a little lean during that time, and rich ratios are more needed in the higher rpm range where more heat is being produced. Keep in mind that wideband oxygen sensors have only been in widespread use since the late '90's, and chassis dyno testing has only become truly popular in recent years. All of us are still learning. Few people have been able to perform true scientific experiments, and therefore few people truly have the knowledge to make blanket statements concerning what is best for a particular vehicle or group of vehicles.
When performing dyno testing and tuning, one must ask oneself "what am I trying to achieve?" If maximum power is the goal, then just look at the power curve first and make adjustments accordingly. The fuel curve is only used as an aid. Many NA race car owners tune in this manner, and by the time they are done the air/fuel ratio is sometimes between 14:1 and 15:1. This is usually not considered "safe" by anyone, but most race car teams accept the fact that they usually change the engine at least once during a typical season. Most street car owners are willing to sacrifice the 3 - 5 hp that they might get by running so lean and instead opt for an air/fuel ratio that will help their engine last for many years!
Simply put, there are a number of factors that one must consider in determining the best ratio, including power, safety, and fuel economy. Fuel economy is the easiest to understand, as a lower air/fuel ratio means more fuel and obviously lower fuel economy. As far as safety is concerned, richer is considered safer (to a point) as the extra fuel helps things run cooler. The lower temperatures help reduce the chance of autoignition and can literally keep engine components from melting. The safest air/fuel ratios are continuously being debated, but it is widely accepted that 13:1 is a good ratio for normally aspirated engines and 12:1 is good for forced induction engines. Many choose to go even richer, even 11.5:1. Autoignition (or "detonation" or "knocking") is considered a critical concern with rotary engines, and many tuners choose to go even richer than that. One must also keep in mind that these "safe" ratios are considered safe because they have been tried with many thousands of vehicles over many years by dyno operators that use the same equipment that most people are likely to encounter. Therefore, a safety margin that takes into account the accuracy of that equipment is inherently factored in. If it were common for turbocharged cars to blow up at 12:1 as measured on commonly used equipment, then the "safe" air/fuel ratio would have been lowered.
As far as power is concerned, I'll say only this: Every vehicle is different. If one wants to find the best air/fuel ratio for generating power, one should put the vehicle on a dyno and test it. Many believe that a particular ratio will result in the most power under any circumstances, and that belief is just too narrow-minded. There are far too many factors involved to make such blanket statements.
Regardless of the actual ideal air/fuel ratio number, almost everyone wishes to see a nice, flat air/fuel graph. This means that the ratio stays constant throughout the rpm range. A perfectly flat air/fuel graph is certainly not necessary for optimum engine performance or safety, but it is a nice thing to show off when tuning a vehicle. The smoother the air/fuel curve, the better the drivability will be and the smoother the power output will be. All good tuners realize that a little variation with the graph is perfectly acceptable, especially when one considers the factors involved. One must consider the accuracy of the oxygen sensor, where it is placed in the exhaust stream, the velocity of the exhaust stream at different points in the rpm band, the tools that the tuner has at his disposal to make changes, etc. Another important factor is that most air/fuel ratios are measured via a tailpipe sniffer. This method has proven to be an excellent way of measuring the ratio, but it is not perfect at low rpm. At low rpm, an engine may not be producing enough gas to displace all of the atmospheric air in the tailpipe, and this will produce a false lean reading because of the extra oxygen - as one can see in this chart. This phenomenon is going to be more pronounced in small-bore engines with large diameter exhaust piping. Two important things must be considered when one is tuning with a tailpipe sniffer because of this phenomenon. One, a flat line across the entire rpm band will mean that the actual air/fuel ratio is too rich at low rpm. Two, a real-world driver is almost never at wide-open-throttle at such a low rpm, so the air/fuel curve at that point is something that the driver will never see. One can also see from the chart that the catalytic converter has no significant effect on the air/fuel ratio in this particular vehicle.
The flatness of the air/fuel graph when one is done tuning is mainly going to depend not on the competency of the tuner but on the type of fuel management system being used and its resolution, and the patience of the customer and/or his willingness to pay for dyno time. One must also ask - is a perfectly flat air/fuel curve best? Many assume that a flat line at 12:1 or 13:1 "across the board" is best, but why is that? How could it be possible that the exact same air/fuel ratio be optimum for every rpm and load? This idea has been largely ignored in automotive enthusiast circles, as "good" tuners with adequate engine management equipment produce air/fuel curves that are flat "across the board" at the desired ratio. Thankfully, this notion has been challenged recently, and experienced racers and tuners have begun to realize that air/fuel curves should not necessarily be flat. Turbos can spool up faster if the ratio is a little lean during that time, and rich ratios are more needed in the higher rpm range where more heat is being produced. Keep in mind that wideband oxygen sensors have only been in widespread use since the late '90's, and chassis dyno testing has only become truly popular in recent years. All of us are still learning. Few people have been able to perform true scientific experiments, and therefore few people truly have the knowledge to make blanket statements concerning what is best for a particular vehicle or group of vehicles.
When performing dyno testing and tuning, one must ask oneself "what am I trying to achieve?" If maximum power is the goal, then just look at the power curve first and make adjustments accordingly. The fuel curve is only used as an aid. Many NA race car owners tune in this manner, and by the time they are done the air/fuel ratio is sometimes between 14:1 and 15:1. This is usually not considered "safe" by anyone, but most race car teams accept the fact that they usually change the engine at least once during a typical season. Most street car owners are willing to sacrifice the 3 - 5 hp that they might get by running so lean and instead opt for an air/fuel ratio that will help their engine last for many years!
10-01-2006, 10:08 AM
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in our cars, more gains are to be had by optimizing timing than fueling.
the fact that our cars DONT USE the o2 sensors during open loop should tell you something. close is good enough. we have EGR to tell us if we are in the danger zone, and knock sensors to tell us whether we are on the onset of detonation or preignition.
The question isn't "whats the best AFR at load X" but rather, "what AFR at load X will allow me to optimize timing at load X".
The question isn't "whats the best AFR at load X" but rather, "what AFR at load X will allow me to optimize timing at load X".
10-01-2006, 11:54 AM
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I think you have it backwards, A/F ratio drives the timing. It also factors into how much
energy you are getting out of the combustion process. And a narrowband O2 sensor is not to be trusted away from stoich, so not using it during WOT conditions makes sense.
Obsessing over timing, without giving equal regard to A/F ratio's is overlooking the direct correlation the two have and a recipe for a less than ideal setup.
Obsessing over timing, without giving equal regard to A/F ratio's is overlooking the direct correlation the two have and a recipe for a less than ideal setup.
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