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Tuning Air/Fuel Ratios

What numbers do you use?

by Julian Edgar

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Programmable engine management allows the selection of pretty well any air/fuel ratio and any ignition timing at any load and rev point. But what are the "right" settings that should be used? It's unlikely that you'll be doing a full programmable management tune yourself (you really need a dyno and a wide-band air/fuel ratio meter) but here's a guide to the settings that will give good fuel results.

Measuring Air/Fuel Ratios

Before a programmable management system can be effectively tuned, the air/fuel ratio needs to be measured. As described below, the air/fuel ratio will need to vary in different conditions, and so the meter needs to be accurate across a wide range of ratios. While the oxygen sensor found in the factory management systems of all cars can determine rich/lean scenarios, it is not accurate enough to be used in the tuning of programmable management.

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Air/fuel ratios are typically measured using a so-called "wideband' air/fuel ratio sensor. This is usually just a normal oxygen sensor that is a little more linear in its behaviour away from the 14.7:1 'switchover' point (where the sensor output voltage suddenly changes from high to low) than a typical oxy sensor. More sophisticated sensors use UEGO or oxygen pump designs, but in tuning workshops these are still almost unheard of.

In addition to this high speed measurement, some workshops use a slower speed gas analyser, logging its results during dyno power runs so that they can compare those readings with the oxygen sensor system. The disadvantage of gas analysers is that they are too slow to get the instant results which are needed when tuning real-time. But for setting the steady-state light-load cruise mixtures, for example, a gas analyser is fine.

Most workshops have high-speed air/fuel ratio metres than read too rich at the rich end. All meters will be able to read around 14.7:1 mixtures in light-load, closed loop cruise - but that same meter may read a full ratio too rich at 10:1 air/fuel ratios. Meters typically read too rich because the exhaust gas temperature compensation is poor. Mixtures around 9-10:1 (ie ultra rich) will cause the car to blow black smoke, but even when workshop meters are displaying that figure, smoke is rarely seen. However, a meter reading richer than reality is in many ways a safe meter - the tuner won't set up the car dangerously lean. But a key question to ask of tuners is: how long since you replaced your air/fuel ratio sensor?

Ratio Requirements

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A well-tuned engine used in normal road conditions has an air/fuel ratio that is constantly varying. At light loads, lean air/fuel ratios are used, while when the engine is required to develop substantial power, richer (ie lower number) air/fuel ratios are used.

Bosch state that most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.

There is no one air/fuel ratio where all emissions are minimised. At an air/fuel ratio of 14.7:1 oxides of nitrogen peak, while hydrocarbons and carbon monoxide (CO) increase substantially as the air/fuel ratio richens.

1. Cranking and Idle

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The amount of fuel that needs to be added during cranking can best be determined by experimentation. This enrichment may be configured by just a one-dimensional variable based on engine coolant temperature, or it may be able to be controlled in a more sophisticated manner. Examples of the latter include post-start enrichment and enrichment decay time. Cold start is one of the dirtiest times in regard to emissions, and so if emissions requirements are to be met, a sophisticated ECU with multiple starting enrichment and decay maps should be used. Reducing the cold start enrichment but increasing cold acceleration enrichment will reduce the total amount of emissions. Some factory systems open the idle air bypass during cold deceleration, presumably to act as a form of exhaust air injection.

The air/fuel ratio required for a smooth idle will depend on the engine's combustion efficiency and the camshaft(s) used. Some engines with hot cams will require an air/fuel ratio as rich as 12-12.5:1 for a smooth idle, while others will run happily at 13-13.5:1. Engines with hot cams that are fitted with sequential injection management systems can run leaner idle mixtures than systems using bank or group fire. Those engines that can be configured to run in closed loop at idle will use an air/fuel ratio of about 14.7:1 when fully warmed, although they will still usually idle better at a slightly richer air/fuel ratio. However, keeping the engine air/fuel ratio as close to stoichiometric as possible will benefit emissions because the cat converter works most efficiently at this ratio.

2. Cruise

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Light-load cruise conditions permit the use of lean air/fuel ratios. Ratios of 15-16:1 can be used in engines with standard cams, while engines with hot cams will require a richer 14:1 air/fuel. If a specific lean cruise function is available, air/fuel ratios of 17 or 17.5:1 can be used, normally at the standard light-load ignition advance. However, running too lean a cruise mixture will cause the cat converter to overheat. If a dyno and exhaust gas temperature probe is available, the cruise air/fuel ratio can be leaned out until exhaust gas temperature becomes excessive for these load conditions (eg 600 degrees C+), or torque starts to significantly decrease. Remember, an engine in a road car will spend more time at light-load cruise than in any other operating condition. The air/fuel ratio used in these conditions will therefore determine to a significant degree the average fuel economy gained, especially on the open road.

3. High Load

A naturally aspirated engine should run an air/fuel ratio of around 12 - 13:1 at peak torque. The exact air/fuel ratio can be determined by dyno testing, with the ratio selected on the basis of the one that gives best torque. Rich air/fuel ratios can be used to control detonation, and this is a strategy normally employed in forced induction engines. Thus, on a forced induction engine, the mixture should be substantially richer: 11.6 - 12.3:1 on a boosted turbo car and as rich as 11:1 on an engine converted to forced aspiration without being decompressed. As is also the case for ignition timing, the air/fuel ratio should vary with torque, rather than with power.

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Most factory forced induction cars run very rich full load mixtures, with 10:1 being common. This is done for engine and cat converter safety reasons - in case an injector becomes slightly blocked, or the intake air temperature rises to very high levels. These cars will normally develop more power if leaned out. Note that emissions testing does not normally take place at full throttle, so full load emissions can be high without legal problems.

In the engine operating range from peak torque to peak power, a naturally aspirated engine should be slightly leaner at about 13:1, with the forced induction factory engine about 12:1 and an aftermarket supercharged engine staying at about 11:1.

4. Acceleration

During acceleration the engine requires a richer mixture than during steady-state running, with the extra fuel provided by acceleration enrichment. Under strong acceleration, the air/fuel ratio will typically drop 1 - 1.5 ratios from its static level. The amount of acceleration enrichment that is required is normally found by trial and error, and this is best done on the road rather than the dyno. The acceleration enrichment should be leaned out until a flat spot occurs, then just enough fuel to get rid of the flat spot should be added. This approach usually gives the sharpest response. Note that both over-rich or over-lean acceleration enrichment will result in flat spots, and that a greater amount of acceleration enrichment is needed at lower engine speeds than higher speeds.

5. Over-run

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In road-going vehicles, deceleration enleanment is used to reduce emissions and improve fuel economy. This normally takes the form of injector shut-off, with the shut-off often occurring at mid-rpm (such as 3000-4000 rpm) and the injector operation re-starting at 1200-1800 rpm. High rpm injector shut-off can, in some cases, have the potential to cause a momentary lean condition.

Conclusion

How a car drives on the road is a pretty damned important part of owning a modified car - and in both the power that is developed and the driveability, air/fuel ratios are a vitally important ingredient.

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