How to: lean cruise with M400 and ADL3

[JulianEdgar]

I know that most people here probably aren't much interested in lean cruise but for those in the future who may do a search, here is how I have done it on my Honda Insight turbo road car.

Initially I had based lean cruise around load and speed. It worked like this: at light loads and within a certain speed band (eg 50 – 100 km/h) the AFR was leaned-out and the ignition timing advanced. This worked OK, but at times the car was in lean cruise when it should not have been, and was not in lean cruise when it should have been!

I have now developed a specific lean cruise mode - just like the factory Honda Insight has. The calculations of when to enter and exit lean cruise are handled by the maths / user conditions functions in the MoTeC ADL3 dash. An on/off output is then CAN-bused from the dash to the ECU.

When the ECU receives the ‘go’ for lean cruise, fuel is trimmed by 30 per cent a (via the Lambda trim function) and 15 degrees ignition advance is added.

Greater enleanment was trialled but it became too finicky to get good driveability. Greater advance was trialled but the jerk when switching out of lean cruise was too great.

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

- Engine coolant temp is greater than 80 degrees. This is so driveability problems don’t occur through lean mixtures when the engine is cold.

- Gear is 4 or above (ie lean cruise is available only in 4th and 5th gears).

- Speed is greater than 50 km/h. This prevents lean mixtures at low speeds – also handled in most (but not all) situations by the ‘gear’ function above.

- MAP is less than 105 kPa. In this case this signal is specially filtered to remove quick fluctuations (the normal MAP sensor signal to the ECU very little filtered). This prevents lean cruise at more than 5kPa boost.

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

Then there are the more complex ones:

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

- Driving smoothness factor less than 30 for a second. This is the most complex function. The ‘accelerator rate of change’ signal (above) is squared so that all values are positive. It is then heavily filtered with about a 30 second smoothing. This then provides a good running number showing how often and fast the throttle has 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.

To get lean cruise working as I wanted, the logic needed all these inputs (I didn't start off with all of them - I added them as they became necessary!). But now when you traverse a series of short-spaced hills at 100 km/h, the lean cruise won’t happen. Drive in on/off urban traffic and the lean cruise won’t happen. But be driving with only little accelerator movement and then as soon as the other criteria are met, lean cruise engages.

I have an indication on the SLM LEDs on the steering column of when lean cruise is operating - it’s now pretty good.

[stevieturbo]

Silly question...

But how much difference in real terms and fuel use would opting for a very lean cruise make ? Over say something running close to or slightly leaner than stioch ?

[JulianEdgar]

Depends on how lean you can go, and how often you're in that mode!

On my Insight, open road economy improves about 20-25 per cent.

Standard car runs lean cruise as lean as 24:1 AFR; I am currently using 21:1.