Last week in Fitting & Tuning the XEDE, Part 2 we looked at tuning a Honda Insight fitted with the XEDE Processor. After unsuccessfully attempting to tune the fuelling using maps based on manifold pressure versus RPM, we altered the configuration so that the fuel map comprised axes of Throttle Position Sensor (TPS) and RPM.
Configured like this we could use the XEDE, a laptop and a MoTeC air/fuel ratio meter to set the Honda’s mixtures separately for idle, cruise and full power.
Nearly all cars on the road hold factory mixtures of about 14.7:1 – what’s called stoichiometric. These mixtures allow the cat converter to work well. On many cars, the air/fuel ratio is richened to about 12:1 at high loads. A few cars use lean cruise where the mixtures may be leaned-out to about 16:1. (In air/fuel ratio numbers expressed in this way, a bigger number = less fuel mixed with the air.)
The Honda Insight, a hybrid petrol/electric car, uses idle and ‘normal use’ mixtures of 14.7:1, rich mixtures (eg 11.8:1) at high load and, most unusually, mixtures as lean as 25:1 in short-burst cruising conditions. Most engines will not even run on mixtures as lean as 20:1 so the Honda engine is rather special. In the standard car such lean mixtures are used for only short periods. This is because with these mixtures, NOx emissions rise; the Honda uses a special cat converter to adsorb them and then, through running slightly richer, get rid of them. Presumably, the cat holding capacity dictates the length of time the car can run in lean cruise.
But with the hybrid system not working, the oxygen sensor feedback disconnected and no requirement to pass a full emissions test, we were in a position to run whatever mixtures were desired.
The best option seemed to be 14:7:1 at idle, the factory ~12:1 at high loads and as lean as possible in very light load cruising.
As recounted earlier, with the oxy sensors disconnected and the hybrid system switched off (and so the car registering fault codes), the Honda runs air/fuel ratios around idle and at low loads of 14.7:1, at moderate loads mixtures a bit richer (say 13.5:1), and at high load and full throttle reverts to mid Twelves. Therefore, with the exception of the cruise area where we desired much leaner mixtures, the air/fuel ratios were largely correct.
So how lean could we go? Using the XEDE, the load sites corresponding to light-load cruise were leaned-out. Note that the engine in the Honda is so low in power that ‘light load cruise’ involved throttle positions of up to 50 per cent and revs of up to 4000 RPM! The greatest leaning-out was made at load sites comprising around 15 – 20 per cent throttle and at less than 2000 RPM. At these sites the car happily ran on air/fuel ratios of about 21:1.
Mapping these mixtures was largely achieved on the road in less than 30 minutes; fine-tuning to cater for as many driving conditions as I could think of took a few more hours.
The interpolation of the XEDE mapping (its ability to smoothly fill-in points ‘in-between’ the mapped load sites) proved to excellent and where better mapping resolution was needed, it was as simple as inserting new load sites by adding finer spacing to the axes. (However, note that doing the latter disorganises the inputted tuning information for that map – so take care.)
Revising the ignition timing was more difficult, primarily because we didn’t have a real-time read-out of the timing being used.
As stated previously in this series, intercepting and reducing the MAP sensor signal not only makes the car run leaner, it also advances the timing. In this application that is good, as leaner mixtures require more advanced timing. But was the timing being advanced enough? That can really only be assessed by further advancing the timing and seeing what happens – too much and the engine will detonate, not enough and the performance will be sluggish.
The XEDE allows only plus/minus 10 degrees of timing change to be made. That usually enough but in our situation we found that adding 10 degrees (actually the XEDE allows 9.9 degrees; don’t ask me why) improved lean load performance and didn’t cause detonation. However, as that was the limit of what could be added, we can’t tell if that was sufficient.
Through most of the rest of the map we added 5 degrees.
In this situation it would be very good to have a readout of what the factory knock sensor was doing – for example, was timing being pulled out as the ECU detected incipient detonation? (Such a readout requires a dedicated tool that reads Honda data through the OBD port.)
However, even without being able to gain as much timing advance as may have been required, the added timing allowed the leanest of cruise mixtures to be leaned-out even further, with air/fuel ratios of 25:1 run at very light loads (eg 10 per cent throttle) at around 2000 RPM.
Initially I must say that I was a bit disappointed with the outcome. I had – rather stupidly – hoped that the retune could give me back the performance lost when the hybrid system stopped working. But that was never going to be the case – full-load mixtures from factory were fine (so no power gain was possible there) and adding fuel at full throttle and low RPM would never give the major torque boost available previously from the electric motor. The lack of electric assist is especially noticeable at low revs, where the engine feels very lethargic.
So, the post-retune performance outcome was much the same as previously. But what about fuel economy?
On my fairly demanding open-road test section, the Honda without its hybrid system working produced a fuel economy of about 4.2 litres/100km; that compared with about 3.8 litres/100km it achieved over the same stretch as a working hybrid.
With the new retune, the car achieved over this stretch of road the same fuel economy it did as a working hybrid – 3.8 litres/100km! Wow – no hybrid capability and the same fuel economy!
So was the tuned car always as economical as it was when a working hybrid?
In city cruise conditions it used to achieve fuel economy as low as 2.8 litres/100km – and after the retune the best it could do in those conditions was about 3.8. (Incidentally, what clearly killed the fuel consumption in non-hybrid form was accelerating up to speed. In those conditions the missing electric assist gave the engine great thirst.)
So the retune had achieved its aim of improving fuel economy – but without a revolutionary improvement and without a noticeable performance gain….
Next issue: But is there another string to the bow? Using the XEDE to map Exhaust Gas Recirculation…
The XEDE Processor was made available for this series at no charge.
Key XEDE Shortcuts
F8 Download and show all Maps
Ctrl-U Upload the data on-screen into the XEDE
Ctrl-B Burn (make permanent) the uploaded data into the XEDE memory – do this on regular basis
Spacebar Move cursor to current active point or cell.
Right mouse click Activate Edit Menu options
F11 Align Graphs with Maps