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Tuning programmable management on the road, Part 2

Setting acceleration enrichment, over-run fuel injector cut-off, exhaust gas recirculation (EGR) and turbo boost control.

by Julian Edgar

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Last issue in Tuning programmable management on the road, Part 1 we showed you what tools you needed to tune programmable management on the road. We then covered the tuning of idle, light loads, medium loads and high load fuel and ignition.

Now it’s time for acceleration enrichment, over-run fuel injector cut-off, exhaust gas recirculation (EGR) and turbo boost control.

Acceleration enrichment

Acceleration enrichment is more important than many people believe. Set up correctly, it makes the engine more responsive to throttle, can bring up boost faster in a turbo car, makes gear changes smoother, and avoids jerks or flat spots when throttle is re-applied at highway speeds.

1. Assuming that the acceleration enrichment is a 3D table (eg MAP pressure versus throttle), aim to set the acceleration enrichment for the mid-throttle movements first. Note that acceleration enrichment in most ECUs occurs only during the throttle movement, not when revs are rising at a constant throttle. Check to see if this is the case with the ECU you’re working with – it changes the way you think about the numbers.

2. Drive in second gear at (say) 25 per cent throttle, then quickly move the throttle to 75 per cent. Add acceleration enrichment at this point of the acceleration enrichment map until the car responds quickly without measured air/fuel ratios becoming overly rich on the transition. Depending on the speed of response of the air/fuel ratio meter, best response may come when the meter shows a little leaner in transition than you’d expect (eg 15.5:1).

3. Now do the same 25 to 75 per cent throttle movement in the other gears. These sites will typically be in other parts of the map.

4. With the mid-throttle movements sorted, pre-fill the table for the higher throttle angles. Now move the throttle from 50 to 100 per cent throttle. You may find that these figures require tweaking – it depends in part if the car runs full-throttle enrichment as part of another tuning strategy.

5. Now tune the acceleration enrichment when moving the throttle from zero per cent open to about 50 per cent. Again pre-fill these figures but note that they may end up needing to be different to other parts of the map. Again do this tuning in all gears.

6. Do a final test of the acceleration enrichment by:

  • Rapid movement of the throttle in all gears

  • Slow movement of the throttle in all gears

  • Fast gear changes (eg 1-2)

  • Slow gear changes

  • Driving at 60, 80 and 100 km/h in high gears and gently getting on and off the throttle

7. When tuning acceleration enrichment, do not change the figures in the main fuel table, or they will then be incorrect for steady state conditions. When tuning acceleration enrichment, wear the listening headphones (connected via an amplifier to a microphone in the engine bay) so you can hear if there is transient detonation occurring on fast throttle movements.

Over-run injector cut-off

Over-run fuel injector cut-off saves fuel and reduces emissions, because the injectors are switched off (or run at minimum duty cycle) when the throttle is fully lifted at higher rpm.

1. Setting this is relatively straightforward but ensure that the revs at which fuelling resumes is high enough above idle speed that the idle speed control and fuel injector resume are not fighting each other.

2. You may need to change acceleration enrichment values at around 0 per cent throttle so that re-applying throttle doesn’t give a lean spot. (It is now more likely to be lean because no fuel was being injected with the throttle lifted, whereas previously at these points it was.)

Exhaust Gas Recirculation

If your car uses electronically-controlled Exhaust Gas Recirculation (EGR), there are major driveability benefits that can be achieved by careful tuning of the valve’s operation. These benefits are most apparent in low power engines (or laggy turbo engines) at part throttle, where the use of EGR means that pumping losses are reduced. This is because the engine doesn’t have to try so hard to suck air past the nearly closed throttle… instead EGR provides that gas ‘free of charge’. In these engines, much more part-throttle torque is achieved by the use of EGR.

1. If the table axes can be configured as you wish, start with throttle position versus engine rpm. You want to be able to drive the EGR valve at varying duty cycles. You may have to configure an ECU output appropriately first – eg valve pulsing frequency, ensuring the output has enough current capability to drive the valve, etc.

2. Ensure that the system is working by setting the EGR valve duty cycle to 100 at idle speed. The engine should stall, or at least stagger very badly. If there is no change in engine behaviour, the system is not working.

3. Add EGR at low and medium throttle angles and revs. Increase values until the car starts to get staggers and other bad driving characteristics, then pull the numbers back until the car drives well. You will probably not be able to use EGR at idle or very low loads / low rpm – eg where the engine is on the map when driven away from a standstill.

4. High rpm, zero throttle can have lots of EGR added. This will decrease engine braking – adjust values to give you the throttle-closed coasting behaviour that you want.

5. After EGR addition, other maps may need to be revised. For example, acceleration enrichment may need to be increased if you have a large amount of EGR set for zero throttle at higher rpm. Decreased pumping losses may need small revisions to main fuel and ignition tables.

6. Check that EGR doesn’t intrude negatively during a cold start, where cold idle revs may be higher than for normal idle. If EGR can be easily configured to start operating only when the engine is up to temp, that may be a solution to EGR problems at cold start.

Turbo boost

Turbo boost in most programmable ECUs can be set up in either open loop (the wastegate valve pulsed with predetermined map of values) or closed loop (the wastegate pulsing values modified by the actual measured boost value). Closed loop boost control is similar to closed loop idle in that PID values may need to be set. Most people use open loop boost control, at least initially. That’s what will be covered here.

1. If the ECU has a fuel or ignition cut based on peak boost, set this before doing any boost tuning. Set the cut value a fraction above the maximum boost value you intend using.

2. Drive the car with the electronic boost control valve not operating. That is, boost will be set by the spring pressure in the wastegate vacuum canister. This is normally a low value, like 4 psi.

3. Set up a map with throttle position and rpm on the axes, with the output valve duty cycle. Taking this approach rather than using just a map of duty cycle output versus rpm will give you the potential to have much better part-throttle control.

4. Put some valve duty cycle numbers in the tables (starting off at low values) and see what effect these values have on boost. You will then quickly learn what sort of duty cycle numbers correspond to what sort of boost levels.

5. Progressively set the full throttle numbers, creeping up slowly to the boost level you want. Test in the tallest gear you can safely drive on the street and yet still go through most of the rev range, eg third gear. Most cars will develop higher boost levels in higher gears when controlled in open loop. Therefore, setting the boost level in first gear may give much higher boost levels in third gear.

6. At full throttle, typically you will set the valve duty cycles at 100 per cent (full open) until boost starts to rise and then taper the duty cycle back to give the required boost. This will bring up boost the quickest.

7. At throttle positions of less than full throttle, aim to set the values so that when a constant part-throttle is held (eg 50 per cent) boost is developed progressively and is then held at a lower maximum level through the rest of the rev range. Doing this means you will have much better throttle control of engine torque output – you can still get full boost by planting it, but driveability will be much better. Test by selecting (say) second gear at low revs, and holding your foot at a constant 50 per cent throttle. As revs rise, boost should come up progressively and then hold at a lower level than at full throttle. Then do the same at 25 per cent throttle and 75 per cent throttle. Adjust map accordingly.

8. Note that there is no requirement to hold full-throttle boost at a flat level through the engine rev range. You may choose to run higher boost in the midrange and lower at the redline, or lower boost in the midrange and higher at the redline. For example, if a turbo has been added to an engine that develops peak torque at high revs, the increase in max con rod loading will be reduced if boost is tapered back at high revs.

Conclusion

Patience and concentration are needed when mapping programmable management on the road. Take it slowly, watching how the engine is responding to the changes that are being made. Always investigate (eg by checking the spark plugs) if there are stutters or changes in driving behaviour. Work with an assistant to either drive or operate the laptop, and remember that for a road car, gaining good driveability is much harder than getting maximum power.

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