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This article was first published in 2008.
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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:
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What form does the control of valve lift take?
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What form does the feedback signal take?
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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.
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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.
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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.
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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.
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