Project Honda Insight, Part 17 – Lean cruise and driver adjustable controls

This series is based around a 2001 model hybrid Honda Insight. The Insight remains one of the most aerodynamic and lightest cars ever made, with a Cd of 0.25 and a total mass of about 850kg from its 2-seater aluminium body. The intent of the project is to turbocharge the engine, add water/air intercooling and programmable engine management, and then provide new high voltage batteries and a new electric motor control system. The aim is to build a car with the best performance/economy compromise of any in the world. The series so far: Project Honda Insight, Part 1 – Introduction Project Honda Insight, Part 2 – Fitting an Alternator Project Honda Insight, Part 3 – Building an Airbox Project Honda Insight, Part 4 – Intercooling Requirements Project Honda Insight, Part 5 – Intercooling System #1 Project Honda Insight, Part 6 – Intercooling System #2 Project Honda Insight, Part 7 - Turbocharging Project Honda Insight, Part 8 - Building the Exhaust Project Honda Insight, Part 9 - First Electricals Project Honda Insight, Part 10 - Alternator (again!) and beginning the MoTeC wiring Project Honda Insight, Part 11 - New ignition system and cam sensing Project Honda Insight, Part 12 - The MoTeC CRIP Project Honda Insight, Part 13 - Idle Speed Control Project Honda Insight, Part 14 – First road tuning of the MoTeC Project Honda Insight, Part 15 - Refining the on-road tune Project Honda Insight, Part 16 – The digital dash This issue: making lean cruise happen, and giving the driver two dash adjustable knobs

Last issue we installed and programmed the digital dash – now it’s time to revisit the engine management programming and add two driver-adjustable controls.

Lean cruise

In standard form, the Insight runs a specific lean cruise mode. In this mode, the air/fuel ratio can be as lean as a staggering 25 or 26:1 – an almost unheard-of figure for a port-injected engine running on petrol.

In certain conditions (eg highway running at 100 km/h), this factory lean cruise mode can improve fuel economy by 30 per cent or more. It’s therefore a very significant part of the Honda’s fuel economy armoury.

The MoTeC M400 that’s been fitted to the turbo Insight does not have a specific lean cruise mode. Creating one proved to be very difficult – in fact, this was by far the most complex area of MoTeC mapping that was carried out. It took more than a month of intensive development, and road testing over more than 3000 kilometres, to develop an effective lean cruise mode that also gave good driveability.

Three different approaches to developing lean cruise were trialled before a good outcome was found.

• First approach

Initially, the following lean cruise approach was used.

Two ‘correction’ maps were developed, one for fuel and the other for ignition timing. Each map used on its axes engine load (MAP pressure in kPa absolute) and road speed (km/h derived from gearbox speed).

Whenever the car was travelling at between 50 and 110 km/h with a manifold pressure of between 40 and 80 kPa (ie in vacuum), the two maps provided substantial changes to both fuelling and ignition timing.

Specifically, as much as 40 per cent of the fuelling was removed, and ignition timing was advanced by as much as 10 degrees.

The actual amount of fuel removed, and timing advance that was used, depended on engine load.

This initial lean cruise fuel map is shown here (click on it to enlarge).

However, after driving with this system for several weeks, I decided that this approach wasn’t good enough.

The problem was that whenever the car was at light loads and was travelling at between 50 and 110 km/h, it was in lean cruise mode. The result was that at times it was in lean cruise mode when this wasn’t actually wanted, and conversely, at times it wasn’t in lean cruise when it should have been!

For example, there was nothing that stopped lean cruise occurring when the car was still warming up after a cold start. This gave poor cold engine driveability.

Another deficiency was in city traffic, where getting on and off the throttle could be jerky.

Finally, on the open road, the lean mixtures gave very doughy throttle response, especially when re-applying throttle after a full throttle lift.

• Second approach

I then developed a specific ‘lean cruise mode’ that was automatically entered only when certain conditions were met.

The calculations of when to enter and exit lean cruise were handled by the maths functions in the MoTeC ADL3 dash. (I upgraded from the CDL3 dash to the ADL3 dash in order to gain extensive maths functionality.)

When the dash decided that lean cruise mode should be engaged, a signal was sent on the CAN bus from the dash to the ECU. When the ECU received this signal, fuel was trimmed by a flat 40 per cent and 15 degrees ignition advance was added. That is, this lean cruise mode used a fixed correction in fuel and timing over the standard map values. (This is similar to the factory approach.)

To enter lean cruise, all the following conditions had to be met:

• Engine coolant temp greater than 80 degrees

• Gear 4 or above

• Speed greater than 50 km/h

• MAP less than 105 kPa

• Throttle position less than 60 per cent for 12 seconds, and greater than 2 per cent

• Accelerator rate of change less than 35. This criterion used a mathematical differentiation-second function on the TPS signal. Thus lean cruise was exited if the throttle was moved quickly, giving instant throttle response even when running initially in lean cruise.

• Driving smoothness factor less than 30 for a second. This was the most complex function. The ‘accelerator rate of change’ signal (above) was mathematically squared so that all values were positive. It was then heavily filtered with about a 30 second smoothing. This then provided a good running number showing how often and fast the throttle had been moved over the last 30 seconds. The lower the number, the smaller the number and speed of throttle movements over the previous 30 seconds.

Each of these logic elements was added as it proved necessary for the control system to know when it was appropriate to be in lean cruise mode. Of course what actually is ‘appropriate’ depends very much on the road that is being driven and how it is being driven. For example, urban driving requires different strategies to rural roads, and driving hard requires different strategies to gentle pedalling. This balancing act was mostly achieved, but there was another problem.

This lean cruise approach gave a mode in which fuel and ignition were changed by a single, fixed correction. However, the Honda uses such a wide variety of throttle positions (often 100 per cent throttle in normal driving – the car is high-geared and low powered off-boost) that to be most effective, lean cruise really has to vary in lean-ness with the driving conditions – not just be a single correction value.

• Third approach

The third – and final – approach was to integrate aspects of the first two approaches. That is, to have a lean cruise mode that is entered only when set criteria are met, and then when within that mode, have air/fuel correction (and ignition correction) that vary with load.

The final enter/exit logic uses the logic steps outlined above, with the exception that speed was dropped from the list, and that gear was set at 5 rather than 4 or 5.

Two other changes were made. Firstly, rather than lean cruise fuel correction being done in a fuel map, the changes were made using a ‘Lambda aim’ correction map. In other words, the ECU was instructed to aim for a different air/fuel ratio feedback number when in lean cruise. This has the advantage that Lambda closed loop correction continues to work perfectly in lean cruise.

This is the lean cruise mode Lambda compensation map – the ‘0.1’ in the horizontal top row shows that lean cruise mode has been entered.

Where a correction of (say) 0.52 is made, this value is added to the standard Lambda figure being used in those driving conditions. So if Lambda in non lean cruise is 1, in lean cruise it becomes 1 + 0.52 = 1.52 total Lambda. (To see that as an air/fuel ratio, 1.52*14.7 = AFR of 22.3.)

Note that the lean cruise correction tapers to zero at the upper and lower ends of the map - as manifold pressure approaches 20 kPa (lots of vacuum), and as it approaches 100 kPa (full throttle off-boost). These tapers are needed so that smooth transitions in and out of lean cruise are gained. The 20 kPa end of the map is relevant as the throttle is re-applied after a complete lift, and the 100 kPa end of the map is relevant as the mode transitions from lean cruise back to normal – most often occurring with increasing throttle.

Here is the ignition timing correction chart for lean cruise. When in lean cruise, the ignition timing figures shown here are simply added to the normal ignition timing.

Massive amounts of advance were able to be used when running very lean. In fact, at one point in the lean cruise mode, the figures are 52 degrees total ignition advance at 23.5:1 AFR, this occurring at 2000 rpm and 60 kPa manifold pressure.

And what about EGR? Testing showed that running EGR in lean cruise mode resulted in poorer fuel economy than if EGR were switched off. Through the headphones, some misfiring could also be heard when running EGR in lean cruise. The EGR map previously developed was therefore altered so that EGR does not occur in lean cruise.

A final change was to filter the ‘engage lean cruise’ signal, so that the transition to lean cruise is more of a slide over a second or two rather than an abrupt step.

The result is that the car moves in and out of lean cruise, quite often (but not always) with the driver not even being aware of it. I have a green LED light on the SLM (dashboard LED indicator) that shows when the car is in lean cruise mode, and of course the read-out of air/fuel ratio very clearly indicates when this mode is engaged and loads are light.

Driveability is very good – although it should still be said, it would be even better without any lean cruise mode!

Why is this?

Well there are some negatives of being in lean cruise that cannot be altered (without adopting electronic throttle control, anyway).

For example, when moving into lean cruise, more throttle needs to be applied for a given power output. If the transition into lean cruise fortuitously occurs with a natural throttle lift, this change may not be felt. But if it occurs when the throttle is being depressed, it’s obvious to the driver that more throttle must be applied. And when actually in lean cruise mode, more throttle than normal needs to be applied to achieve a given torque increase – climbing a small rise, for example.

Honda Insight drivers wouldn’t be unduly concerned (it’s much the same in the standard car), but for non-Insight drivers, these are slightly odd driving characteristics.

However, the benefit of lean cruise is in the fuel economy – an easy 20 per cent improvement being realised in 5^th gear driving. In fact, it’s fascinating watching the ADL3’s bar graph display of instantaneous fuel economy and seeing it drop from (say) 5 litres/100km to 3.5 litres/100km as the transition into lean cruise occurs.

The variations So how does it all come together? Driving along a country road, the following scenarios are typical. Driving condition Logic MAP Air/Fuel Ratio Ignition Timing (degrees BTDC) Accelerate briskly up to speed, gears 1-2-3-4-5, up to 4000 rpm Acceleration mode up to 160 kPa as low as 12.5:1 – enriched for boost As low as minus 10.5 degrees Reach 100 km/h, fifth gear 2400 rpm Conventional stoichiometric 90 kPa 14.7:1 - stoichiometric About 4 degrees Drive along a flat road, lean cruise mode engages Lean cruise mode 60 kPa 22:1 – lean cruise About 40 degrees Change down to third gear and accelerate hard to overtake another car, lean cruise immediately drops out Max power mode 180 kPa 11.5:1 – full enrichment at max boost About 8 degrees at high revs Change down to engine brake, coming to a standstill Injector over-run cut-off 35 kPa injectors off until revs drop to 1200 rpm As much as 35 degrees When the ADL3 dash is set to show ignition advance and air/fuel ratio, it is extraordinary watching air/fuel ratio vary from 11.5:1 to 23.5:1, and ignition timing vary from minus 10.5 degrees to plus 52 degrees – all in normal driving!

Additional driver controls

Two dash-mounted pots have been added to allow greater driver control. The two pots – salvaged from an old tape recorder – have centre detents, making it easier to tell by feel when they are in their middle positions.

Each is pot fed regulated 5V by the ECU, with the pot used to provide a variable input voltage to the ECU. This input voltage is used as an axis on a look-up chart to provide the following corrections:

1. Boost

This knob controls the level of boost that occurs above the level set by the wastegate spring.

The wastegate spring provides about 30 kPa (about 4 psi) of boost, so with the knob set to minimum, 30 kPa peak boost occurs. With the knob at the other end of its rotation, 80 kPa (11.6 psi) peak boost occurs. At the centre detent, a trim of -50 per cent is applied – still plenty of performance for normal driving.

2. Ignition timing

This knob trims ignition timing over the range of -10 degrees to +4 degrees. The centre detent position is 0 degrees trim (ie the adjustment is asymmetric around that point).

Timing will be driver-retarded when fuel of a lower octane is being used.

Both the boost and ignition trims have been integrated with the digital dash.

When either knob is being turned, the bottom warning line on the dash display changes accordingly. When the boost knob is being turned, the dash displays ‘Boost Change’, and when the ignition knob is being turned, the dash displays “Ign change’.

At the same time, the dash switches from showing speed to showing boost trim (from -100 to 0, indicating percentage change), or ignition trim change (from -10 to 4 degrees).

In addition, two LEDs light up on the SLM (shift light indicator) to show that adjustment is occurring. These LEDs are green for the boost change and red for the ignition change.

These approaches were taken so that it would be hard to inadvertently alter either boost or ignition trim, and both can be accurately changed without the knobs actually being sighted by the driver.

Conclusion

That’s stage 1 of the Insight project largely complete. Next issue we’ll cover in detail the performance and fuel economy of the car in this turbo, non-hybrid and MoTeC-equipped form.

Did we meet the goals of a 0-100 km/h time in the Sevens, and open-road fuel economy in the Threes (litres/100km)?

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