Air/Fuel Ratio
Explained

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Mix it up

Understanding, determining and tuning target
Air/Fuel Ratios utilizing oxygen sensors
and wideband controllers

Written and copyrighted by: Brian Barnhill
Technical Director of Tuner Tools LLC

Published in Issue #5 of Juiced

Tweaking the Recipe

If you’ve been following the past article (Volumetric Efficiency Explained in Issue #2 and Load Calculation and Control in Issue #3) you should have a grasp on the basic recipe for tuning your engine.  Similar to that secret family recipe variation of a common dish though, this mix requires personalized tweaking to take it from just a basic out of the box “heat-and-serve” style tune to something unique and optimized for your vehicle. This process requires patience, attention to detail and just a dash of artistic flare.  The good news, however, is that there are many tools that are easily accessible the helping you with this task, and some of them may even already be installed on your vehicle from the factory.

 The Ratio

We begin by first understanding exactly what we are tuning and why. The topic of interest here is the Air to Fuel Ratio (AFR) which is, as may be self-explanatory, the ratio of air to fuel in the combustion chamber.  This ratio influences the behavior of the combustion process and will make the difference between maximum (and safe) power and/or torque and potential catastrophic engine failure.  Because just a small difference in ratio can dramatically change the reaction, it is critical to understand not only the how, but the why when tuning fuel mix based on AFR.MTX-L Digital: 3844 Digital Air/Fuel Ratio Gauge

It must once again be stressed that we should treat the engine as essentially a large air pump. At this point, however, you should have a strong grasp on the how, what and whys of the air flow in an engine and have maximized these aspects before moving on to tuning air fuel ratios. At this point we will first determine the theoretical air fuel ratio based on desired performance, and then begin manipulating the amount of ratio and adjusting the mixture from our theoretical baseline to account for dynamic engine conditions, additional engine modifications, drivability and other variables which influence combustion and an engines air consumption.

A reaction in which all components are completely consumed is considered to be stoichiometric (stoich). For gasoline/petrol this mixture is approximately 14.7 parts air, to 1 part fuel (14.7:1) for E85 this ratio is approximately 9.7:1 (note these ratios are approximate based on theoretical data assuming perfect laboratory samples, these ratios may vary slightly due to variations in regional and seasonal blends of fuel.)  A ratio which has more fuel left over (ratios lower than stoich) are referred to as rich, while those higher, and thus having excess air, are lean. In all but very specific and extreme cases rich ratios should be the goal, this is due to combustion and flame behavior as well as safety reasons and avoiding accidental ignition of the mixture as leaner mixtures are easier to ignite.

AFR

Lambda (λ)

 

14.7:1

1

Stochiometric

12.8:1

0.87

Lean Best Torque (LBT)

12.2:1

0.83

Mean Best Torque (MBT)

11.76:1

0.8

Rich Best Torque (RBT)

11.01:1

0.75

Flame speed fastest in cylinder

Table 1: AFR influence on Engine Behavior (Gasoline/Petrol)

The table gives a basic overview of AFRs influence over engine behavior and dynamics and should serve as a general guide when determining air fuel ratios at full power/Wide-open-throttle. Assuming knock is not a limiting factor, Mean Best Torque should serve as a general starting point (if knock is a factor, more fuel, less spark advance, or less boot/compression will generally be required.) Note that best torque does not occur and the fastest flame speed.  Any ratios richer than 11.01:1 should be avoided, as there is a very sharp and rapid decrease in torque at ratios richer than this point. Worth mentioning is also the fact that we may often see ratios leaner than 12.8:1, which is where lean best torque occurs. While this may sacrifice a small amount of torque the fuel economy and emissions at peak power can be improve which may be desirable (and necessary) on many street/pollution controlled vehicles.

This chart should be used as a guide; there are many other factors which will also influence your tuning and target air fuel ratio. Smooth idle, throttle response, fuel economy, emissions and general drivability. Typically target ratios should increase fuel (become more rich) as load and RPMs increase and approach the peak torque power band.

Note about Lambda vs AFR

You may have noticed in the table that Air-Fuel Ratio has an equivalent value called Lambda (λ).  Lambda is representative of the stoichiometric ratio where a λ=1 will always be stoichiometric, regardless of the fuel in use.  Other ratios are simply defined as a ratio in relation to stoich. For example, a ratio of  11.76:1 would be: 11.76/14.7 = 0.8. This simplified measurement is very useful, in fact, most oxygen sensors actually read in values of lambda, as they are actually measuring the ratio of free air in the gas mixture, and thus the ratio in regards to stoich. This simple process allows vehicles to also quickly adjust to differing fuel types. Most modern flex fuel vehicles that run on both E85 and petrol/gasoline have done away with the expensive alcohol sensors and rely on the oxygen sensor to determine what the mixture of fuel is and reference the appropriate map accordingly.PLX M-300TE Tuner Edition Wideband Oxygen Sensor Controller with Display

Measuring up

Knowing the exact ratio at any given moment is critical to tweaking the recipe to achieve the desired performance and engine goals.  Thankfully there are tools which easily allow us access to this data, and at the heart of any of these tools is the oxygen sensor. An oxygen sensor basically works by reacting to unburned oxygen in the exhaust stream via a chemical reaction between this free oxygen, and the material in the tip of the sensor. This reaction causes the sensor to emit a voltage, which is then read by the controller and/or ecu to determine air/fuel ratio.  There are variations in types of controllers, sensors and settings for oxygen sensors, understanding which one is right for each situation is an important step in tuning your engine.

Closed or Open loop?

These terms refer to the control method being used by the engine management system and determine how any information received from the oxygen monitoring sensors is used and applied.  The loop refers to the path of data. In a closed loop system, data from the oxygen sensor is relayed to the engine management system. The control system will then use this information to determine if the engine is operating at the desired ratio, based on the programmed tables in the tune, and then adjusts fueling as necessary. This allows the engine to more accurately maintain the requested air to fuel ratio. Note, however, that if the base tune in inaccurate this method will cause the system to “seek” and constantly add or subtract fuel as it tries to maintain control. This will appear as the AFR fluctuating around the desired value. This can cause degraded performance, hinder tuning efforts, and even cause harmful engine damage.

Additional closed loop control is possible in systems using fuel trims. Fuel trims are a representation of the amount the control system is altering the fueling tables to achieve the requested AFR. Short term fuel trims are the instantaneous adjustments made by the control system and can be used by a tuner to monitor, log and apply the changes the system is making during closed loop to help tune the engine. In more complex control system long term fuel trims are used to apply changes to the base tune. The long term trim will average changes made in each site over a pre-determined time period. Once the time period or number of data points have been met, it will change the base tune by a calculated value to help dial in the base tune and correct for changes in the engine or permanent operating conditions.

The equation to determine % change of the injected value of V.E. based on AFR is:

% Change = Actual AFR/Desired AFR *100%

Open loop systems do not use this data to make on the fly changes. This mode is desired in operations where rapidly changing engine conditions may make closed loop control difficult or dangerous. Wide open throttle and very heavy loads are examples of such conditions. Since conditions change faster than the sensor is capable of reading and the changing the mixture, closed loop control may allow for very rich or lean conditions at precisely the moment they would be the most dangerous. Again, having an accurate and complete tune is critical in these conditions to maximize performance and engine longevity.

Wideband vs Narrowband

There are two main differences when speaking about types of controllers and methods of measuring air fuel ratios: Narrowband and Wideband. Both are very useful tools in tuning and deserve a thorough discussion and understanding. Understanding how they work and when to use each method will simplify your tuning efforts and increase the quality and accuracy of the tuning and control strategy.

Narrowband

The oxygen sensors traditionally used by most OEM manufacturers are a Narrowband type sensor. These sensors are used to measure AFR in a very narrow range (thus the name) and are only accurate within this narrow area. The sensor will normally have a 0-1 voltage output and will be most accurate around a lambda of 1 (stoichiometric). The intent of these sensors, as equipped by the factory, is to control the vehicle in controlled loop operations, such as cruising on the highway as well as monitoring to pollution control systems of the vehicle. These operations are critical for maintain proper emissions and maximizing fuel economy and performance.

The drawback of these sensors comes precisely from this narrow accuracy band. Outside of this range, which is approximately 14.2 to 15.0, the sensor cannot be accurately relied upon for any changes, and it thus ignored. This prevents their use in applications such as wide open throttle or heavier load, where the conditions are too fast or ratios out of these ranges are desired. Recall from the table earlier in this article that for maximum power and torque the ratios are far outside this range. This limits the use of a Narrowband sensor to cruising and light load use only. It is, however, much more accurate than a wideband sensor in this range and is still a vital tuning in properly tuning a well rounded street vehicle.

WidebandInnovate Motorsports Sensor (Bosch LSU 4.2)

Wideband sensors, on the other hand, have a 0-5v output and a much wider accuracy range (The Innovate Motorsports sensor is accurate from ratios of 7.35 to 22.39 for example.) This increased range allows the sensor to measure the ratio accurately in all engine conditions. This information is critical when tuning your engine, as most of your tuning will focus on areas other than light load and cruising. Additionally, depending on the speed of the sensor and ability of the engine management system, can be used to create a closed loop control system in conditions other than just cruising. This control allows the engine to automatically adapt to changing conditions and correct for inaccuracies in the tune (as previously mentioned, and worth repeating however - closed loop, should NEVER be used as a “band-aid” for an incomplete tune.)

The final product

The air to fuel ratio is simply a representation of the most basic ingredients of combustion. Understanding how this ratio influences engine behavior is critical to controlling and tuning your engine and will be where most tuning efforts will begin. The main tool for monitoring this ratio is the oxygen sensors and controller and they are an integral part of modern vehicle control systems. In controls systems both open and closed loop controls will be necessary to ensure safe and controlled operation – and through the use of narrowband and wideband controllers it is possible to tune your vehicle for maximum performance, while also maximizing fuel economy, emissions and drivability – all of which are critical for a modern street vehicle.

 

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