How do you get engine braking with fuel use? Timing Retard?

[Isaac Zachary]

Originally Posted by Tahoe_Hybrid

engine braking is transmission is related...

I was pretty sure that was a given.

Also every vehicle is different. The 1996 Buick LeSabre I had for a short time felt like it was going into neutral every time I let off the gas pedal even at 65mph unless I downshifted. My 1985 VW Golf diesel felt like I was hitting the brakes if I let off the accelerator pedal at 40mph in top gear. I rarely ever used the actual brakes in that car.

But in the end, if you're in gear, RPM's above idle and the engine is taking in fuel, timing WILL affect engine braking. If you have a way of controlling the timing yourself (like with a distributor) and you advance the timing at idle, idle speed will increase. It's kind of hard to engine brake if your engine wants to idle at a high speed.

[Michaelflat1]

Originally Posted by Ecky

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.

[Ecky]

The lowest RPM fuel is allowed to be cut is editablec with the Hondata software I use.

[Michaelflat1]

Originally Posted by Ecky

The lowest RPM fuel is allowed to be cut is editablec with the Hondata software I use.

No engine tuning options for my engine, there are for the VTEC ones, but mine is the 8 valve, 8 spark one. (i-dsi).
Even the flashpro thing for the USDM fit (which probably won't work with the i-dsi anyway) costs about the same as the car is worth here, no tuning around unfortunately.

At least I can tune my driving have got very good at rev match downshifts in traffic to slow the car down, can get into DFCO in 2nd gear above 1500rpm.. 1st gear DFCO I don't try, too much braking considering no brake lights.

[serialk11r]

@Ecky ah okay that's interesting, and actually kind of makes sense. AC uses quite a lot of power so I would expect the engine to open the throttle or IAC for that, but electrical loads are small enough they might just use a timing adjustment. The thinking is probably some combination of keeping the catalyst hot, smooth idle, and fast control, at the cost of a very small amount of fuel.

Timing retard is also used to intentionally limit torque by Porsche (if you look up one of their advertised hp/tq/rpm graphs, often there's a dead straight line for half the rev range, and the timing map shows decreasing advance), most likely to encourage buyers to spend more money on a higher end model On DBW throttle cars, the throttle is also used to cut torque. Pretty weird stuff.