Tweaking the EGR, Part 1

This article was first published in 2008.

In EGR Comeback we discussed the reasons that Exhaust Gas Recirculation (EGR) is now common on late model engines, both diesel and petrol. Not only does EGR reduce emission of oxides of nitrogen (NOx), it also has the ability to improve fuel economy and, in turbo petrol engines, even remove the need for fuel enrichment at high loads.

(That’s an important list of benefits; if you need to know more detail about any of them, check the above article.)

Given that EGR is now much more than just an emissions band-aid, tuning the action of the EGR valve has potential performance benefits - in power, emissions or fuel economy. That’s especially the case since all cars – even those of the same model – are each a little different, and driver preferences also vary.

But so much for the theory – what about the reality?

Honda Insight

My car, a Hybrid Honda Insight, uses electronically-controlled EGR. That is, perched on top of the engine is a large EGR valve. This valve, normally held shut by a spring, is opened by the action of current through a coil. The amount that the valve opens is monitored by a sensor that in design is very much like a throttle position sensor. The ECU monitors this position sensor and alters the opening of the EGR valve to give the required ECU-mapped EGR flow.

This diagram shows the Honda system. Valve lift is controlled by a simple solenoid coil and the position sensor comprises a potentiometer (pot).

Apart from this diagram, the workshop manual doesn’t have a great deal more information on the valve. However, a couple of things can be surmised.

Firstly, if the flow of exhaust gas through the valve is going to be altered (eg increased), it’s likely that tweaking the feedback of valve position from the position sensor will give better results than trying to alter the signal actually governing the valve lift.

An example shows why this is the case. If the valve signal is altered so that more lift occurs, the position sensor will relay this information back to the ECU, so probably causing the ECU to reduce valve lift until it reaches the mapped requirement. In other words, the ECU will work its way around the mod.

However, if the feedback from the position sensor is altered, the ECU doesn’t have any way of directly working out that anything has changed.

For example, if this sensor signal is altered so that the ECU is told that EGR valve opening is less than it actually is, the ECU will compensate by opening the valve more – EGR will increase. Conversely, if less EGR is required, the feedback signal can be altered so that the ECU thinks the valve is more open than it really is. (However, see the ‘Self Learning?’ breakout box below.)

But before proceeding, there are some major questions that need to be answered:

• What form does the control of valve lift take?

• What form does the feedback signal take?

• When does EGR occur, and in what amounts?

All these questions can be answered by direct and simple measurement of the valve operation. I’d like to stress that on-road measurement is a million times better than theoretical supposition: in all that I have read in discussion about the Honda Insight’s EGR valve operation, no-one had their descriptions of its operation remotely correct.

Measuring EGR Valve Operation

The first step was to wire a multimeter directly across the EGR valve coil – to the pink wire and the black wire. (It is best if you have a decent multimeter than can measure frequency and duty cycle – if you pick carefully, such a multimeter can still be quite cheap.)

When the Honda was started from cold, the meter showed that the valve was doing nothing – there was no voltage across it (and therefore no frequency or duty cycle). However, after driving for a few minutes, the meter came alive. The frequency with which the valve is operated was measured at a fixed 108Hz. However, the duty cycle varies from zero (ie valve shut) to about 50 per cent (ie valve half open).

(Incidentally, at an operating frequency of 108Hz, the valve is not opening and shutting 108 times a second. Instead, because of the inertia of its moving parts, the valve hovers, with the duty cycle determining how high the pintle is in hover. This is much the same as a boost control solenoid, and quite different to fuel injectors that actually do open and shut each cycle.)

In lean cruise, the Insight’s EGR valve is closed. At idle and under full throttle, the valve is also closed. In fact, in normal driving it is closed far more often than it is open. So when is it open? The valve is open its greatest amount (about 50 per cent duty cycle) at light and moderate throttle, at both low and high revs. It also opens when the car drops out of lean cruise to consume its stored NOx emissions (see Giving the Insight a Good Driver for more on the lean cruise behaviour of this car). Finally, it is open for a short time on throttle lifts.

The multimeter was next placed to measure the feedback sensor output – ie wired between white/black wire and ground. This showed that the voltage rose as the EGR valve opened further, being around 1.2V with the valve shut and rising to about 2.5V at its peak. The values aren’t very important – what is important, is that voltage rises with greater valve opening.

Making Connections When tapping into the wires, especially for initial measurement, it’s often easiest to do it under the bonnet rather than at the ECU. And rather than baring wires to make the connections, test wires can be soldered to dressmaking pins and then the pins pushed into the back of the plug connections.

Increasing EGR Flows

As described above, to increase the flow of the EGR valve, the feedback signal can be tweaked so that the ECU thinks the EGR opening is lower than it really is. But a really important point needs to be realised. Despite this modification, the valve will still only open when it did originally– but when it is open, it will be open a greater amount.

So how do you lower the feedback signal? This is really easy to do.

If the original sensor output wire is cut, a potentiometer is wired between this and the ground wire, and the wiper of the pot connected to the original ECU input, the output of the position sensor (as seen by the ECU!) can be adjusted from being standard (with pot wiper in top position) to being decreased to nothing (wiper in bottom position).

A 10 kilo-ohm pot is suitable and, so that small changes can be easily made, it is best if it is a multi-turn design (eg Jaycar RT-4614 at $1.50). As with the initial measurements, I chose to make all the wiring connections under the bonnet – this could also have been done at the ECU.

Decreasing EGR Flows

If the flow through the EGR valve needs to be decreased, the pot can again be used. However, one end of the pot now connects to the 5V supply, rather than to ground.

Self Learning? Be aware that, in some cars, tweaking the output of the EGR valve sensor in the way shown in this article might result in a non-permanent modification. Why? Well, when the ECU is not sending any power to the EGR valve, it knows the valve must be shut. Therefore, the signal coming from the position sensor is the signal that is representative of a ‘valve shut’ condition. In some cars, the scaling and calibration of the position sensor might be set from this ‘initially shut’ position, allowing the ECU to learn its way around the altered signal. Some throttle position sensors set their own starting point in this way. In other cars, especially diesels, the airflow meter signal might be used to indirectly measure EGR flow. That is, the ECU knows that at a given manifold pressure, intake air temp and engine rpm, a certain amount of air should be being breathed by the engine. If the actual amount is less than modelled, the ECU calculates the ‘missing’ airflow as EGR. Therefore, it again may be able to learn its way around flow changes caused by modification. The easiest way to find out if either affect is taking place is to measure the duty cycle of the valve over a period of days, making sure that in the same driving behaviour, it doesn’t change from its modified value.

Vacuum-Controlled EGR Valves Not many cars have electronically controlled EGR valves of the sort used in the Honda. Instead, they are more likely to have a vacuum-controlled EGR valve actuator, and a solenoid valve that controls the vacuum being fed from the intake manifold to the actuator. If this solenoid valve is factory pulsed (to find out, just measure with a multimeter the signal going to the solenoid), its action can still be modified - but it costs a bit more. The Digital Pulse Adjuster can be used to shorten or lengthen the duty cycle of the pulses. If the desired EGR outcome requires less manifold vacuum, a bleed can be put in the manifold line, a la a turbo boost control. If the valve is simply turned on or off by the solenoid, valve control can be shifted from the ECU to a rev switch or a throttle position switch or some other switching device.

Conclusion

EGR on many modern cars is easily able to be increased or decreased in flow, with in some cases low cost fingertip adjustment possible from inside the cabin .

Next week, we’ll see what can be achieved by this adjustment.