Tuning for maximum fuel economy

If you have programmable engine management and want to tune for best fuel economy, what should you keep in mind?

Closed loop

Using a closed loop strategy is important. This is where a wideband oxygen sensor keeps tracking mixtures, and the ECU can correct as required to maintain these mixtures. This is important because no matter how good your tune is, it’s likely that not every fuel site will be giving you precisely the mixtures that you want.

For example, if you’re only 5 per cent out in fuel number (giving say 14.0:1 AFR instead of 14.7:1) then your fuel economy at that site with be, yes, about 5 per cent worse.

Having closed loop correction based on an air/fuel ratio ‘aim’ chart prevents this sort of minor fuelling error that, over time, can add up to much worse fuel economy.

Air/fuel ratio and load

Decide at what point you want to richen the mixtures for best performance while keeping the engine safe.

For example, in a turbo car you might decide to keep mixtures stoichiometric as high as 10 or 20kPa (around 1.5 – 3 psi) boost, gradually richening from that point to perhaps 12.5:1 at peak boost.

On the other hand, if compression ratio is high and the engine was never designed to be turbo’d, you might start going richer as soon as any boost is occurring.

From a fuel economy point of view, the later you can delay the enrichment, the better will be the fuel economy.

Then there is the other point: how rich to go at full load? Again it’s a trade-off between engine safety, performance and fuel economy. Mid-twelves will suit most engines; the very heavily stressed might need to go even richer.

Throttle enrichment

The amount of fuel that is injected with throttle movements should be kept as low as possible while still giving good throttle response. This is an area where it’s possible to put in 50 per cent more than is needed.

Therefore, it’s best to start off with no throttle enrichment and gradually introduce the fuelling until the response is as required – rather than throw in some fuel and then just see if the driveability is good.

Over-run injector cut-off

Over-run injector cut-off should be set so that it occurs as much as possible in normal driving.

For example, some tuners will set the ECU to resume fuelling as soon engine revs drop to 2000 rpm – but in a high geared car, that in turn will mean that fuel injector shut off doesn’t occur in city conditions where revs are low.

However, ensure that if the ‘resume’ revs are set very low (eg 1100 rpm) that the idle speed control system isn’t fighting with the injector cut-off function – there needs to be a rpm gap between the two functions.

Exhaust Gas Recirculation

Electronically controlled exhaust gas recirculation (EGR) is a very important part of gaining best fuel economy.

When in vacuum (ie pressures below 100 kPa absolute), as much EGR should be run as possible before bucking starts. This is because any EGR occurring in vacuum reduces pumping loads that the engine is experiencing – and less pumping loads result in more power, or less fuel injection for the same amount of power at the wheels.

Air conditioning clutch

The air-conditioning compressor clutch should be smartly controlled by the ECU.

In general it should be set to switch off as soon as a certain engine rpm and/or throttle position is exceeded. Adequate hysteresis should be provided so that revs or throttle need to drop sufficiently before the air con clutch re-engages.

If these parameters are set early enough, fuel economy will noticeably improve as the air conditioning compressor will be more frequently running on throttle over-run (so getting the power for effectively nothing) or at light loads (where the engine is poor in its specific fuel consumption and a higher load is better for the overall fuel economy outcome).

Note that a timer function should be used so that the compressor doesn’t go on and off at gear changes.

Lean cruise

The easiest way of gaining a pseudo lean cruise function is to set air/fuel ratios leaner than stoichiometric at lighter loads – for example, when in plenty of manifold vacuum (eg 50 kPa absolute) at 2000 rpm.

This is easily achieved, but the degree of leanness that can be tolerated without badly affecting driveability is fairly limited. In other words, it’s a lean cruise function that can be done with any programmable management system, but you will only be able to go a little leaner than stoichiometric before stumbles occur in normal driving.

Better is to create a specific lean cruise mode that is entered when certain criteria are met. For example, when speed is above 50 km/h, engine temp is above 80 degrees C, and manifold pressure is below 80 kPa absolute (ie a bit into vacuum), then a fuel correction chart is brought into action that leans mixtures.

Note that in all cases where fuel is being leaned out a lot, the ignition timing chart will need substantial advance. (Lean mixtures burn more slowly, and very lean mixtures at high vacuum [so giving little cylinder filling] burn very slowly.)

You will also find in most cars that it is better to switch off EGR when operating at very lean mixtures – the number of misfires will increase with EGR still operating.

Idle speed

If your car spends a lot of its time idling in city conditions, two tweaks may improve fuel economy,

The first is to simply decrease idle speed. The second is to run leaner than stoichiometric at idle. The effects of both are easy to see if you monitor fuel injector duty cycle as you change these parameters.

Driveability When tuning for best fuel economy, it’s important that you decide on the level of driveability that you are happy with. Many of the techniques discussed here will result in worse driveability, especially if taken to excess. For example, lean cruise, exhaust gas recirculation and throttle enrichment all have a major bearing on how well a car drives. In some cars, it’s easy to find another 10 per cent in fuel economy if you’re prepared to have a car that drives with occasional stutters or doughy throttle response. On the other hand, if the management is well-tuned, you can have all of the techniques described here working, but with little or any impact on driveability.

Conclusion

Tuning a car for good power is relatively easy. Tuning a car for good power and good driveability is one step harder. And tuning a car for good power, good driveability and good fuel economy is harder again!

However, if you want to save at the fuel bowser, it’s worth employing some tuning-for-economy tricks in your ECU maps.