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Old 01-02-2021, 05:42 AM   #12 (permalink)
Michaelflat1
2003 PD130 Seat Ibiza
 
Join Date: Oct 2020
Location: Essex, United Kingdom
Posts: 16

Honda Jazz - '07 Honda Jazz SE
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Quote:
Originally Posted by Ecky View Post
Best I can tell, all of the Honda engines I've worked on use ignition to fine-control idle. The pre-drive-by-wire vehicles had an idle valve that let in more air when cold, whereas the post-DBW vehicles would simply crack the throttle plate, but both retard ignition significantly at idle.

Take the K24 I'm using from an Acura TSX - it's ~8 years newer but functions the same way as the Insight stock engine in this regard, or our Fit. At idle I'm generally seeing ~5-7° ignition advance. When I flip the headlights on or toggle the A/C it jumps maybe as much as 6-8 additional degrees momentarily to hold the idle perfectly steady, and then levels out at 1-2° more advance to hold the same idle speed. Touch the throttle lightly and it's instantly at ~24° advance, which is ballpark for MBT at that load and RPM.

Here's one of the timing tables my car uses, for example:



Idle timing uses separate logic, and there are different tables for every 10 degrees of intake cam advance, and separate tables for all of these in and out of VTEC.
My fit (jazz) also uses a combination of timing, throttle and EGR at idle.
Drops down to 0.12gal/hr headlights off says Torque (but i'm not sure how accurate this is).

Off idle, the torque is pretty crap, easy to stall and the engine feels rough at this speed if you use the clutch alone to get you going (low idle possibly, 650 headlights off, 750 headlights on).

I had issues with EGR at one point, I disconnected it and the quality of idle was impressive, some usable torque without using throttle pedal.

I believe that the first bit of throttle, closes the EGR (so fresh air mostly) and advances timing, even only being 100-200rpm higher, there is a lot more torque available, without the engine feeling rough.
Whether that's mainly down to EGR operation, or the advanced timing, i'm not sure. I guess if I have time and fuel to burn, I could sit the car idling, not drive it, and feel if the EGR warms up at all.. I haven't found conclusive info on the internet whether EGR is used at idle on a gasoline car

Cold start, my car runs around 1500-1700rpm (high! relatively quickly drops to 1200, and if you start moving it'll drop to 1200 immediately) with 0 deg timing advance.
As the idle speed drops, between throttle positions (as it slowly closes down) the timing fills in the gaps, to get a smooth reduction in idle speed, eg. it'll go from 13.7% throttle 0 deg to 13.2% 6 deg advance.

I think I stare too much at OBD parameters

I think I can get some more MPG adding 'trailing throttle' (what i'll call it) where RPM is too low to get fuelcut, I may aswell use the lowest throttle position to glide a bit further, rather than chucking this fuel away with retarded timing. That and a combination of engine on coasting.
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Ecky (01-02-2021)