leaning ? (questin that arise when one´s temporarily bored)

[landybehr]

Hi,

now that I´ve lost my fear for lean mixtures I´ve got a satisfactory setting of the VE´s for cruising. In traffic/Town driving I manage to have around 14.1-14.5 when taking off and soon get into 15.5 or even 16.5 for keeping the pace and even small acceleration. The engine seems to be quite tolerant in these areas. Verly little surging at near 17.0 AFR. If I want to accelerate fast I have 13.0AFR (at Map above 85kPa). For time being I used convetional mixtures for the upper right part of the table.
I am really curious for the mpg-figure over the next fill-up´s, but as usual I do not want to trade sth positive in for any drawbacks that I might miss.

As I learnt that combustion temperatures are low at these lean mixtures (hottest at 14.1) there could be no danger of overheating components !?.
There is the possibility to cause damage at WOT-regions with lean mixtures - but lean then means mixtures of 14.0 where 13.0 or even 12.5 would be the save mixture (to cool components and take oxygen away). So as long as the engine manages to keep the speed I ask for with 16.5 then nothing can happen. Right ? (think this has been answered before, but it´s a good opportunity to reconfirm me ). And even if I drive 70 or 80Mph it doesn´t matter if the engine is fed with 15.5 or 16.0 AFR and it doesn´t slow down ????!

Only thing to happen is to have not enough advance and thus put stress on the exhaust valves (by not all-finished combustion when it opens), right ?

Is there something to take into consideration, like spark plugs ? They are designed/chosen by the manufacturer for the engine running 14.7 AFR all the time (excluding WOT) and when beeing mainly lean the heat range of the plugs could be out of range ?

Could knock become an issue ?

[landybehr]

sorry to put it up again ..

the point behind the "?" is not that meaningless as the title my imply.
I remember Toyota having offered a "lean-mix" vehicle in my coutry but I don´t think they were a great impact on the market. I think that´s been some 15-20 years ago. I wonder why that happened. There had been a lot of advertisement for the car.

If it were for emissions (which were much less strict that time ago) the car wouldnt have been homologized first time by the government. If it were for driveability Toyota wouldn´t have dared to sell it like that ?

EDIT - it was 1987 that a Toyota Carina II with 105bhp was been presented. And it seems that Toyota kept making lean-burning vehicles at least upto mid-90ties. So, could be that these engines/motors were successful but just without me taking notice of it.

[devastator]

So as long as the engine manages to keep the speed I ask for with 16.5 then nothing can happen. Right ?

That is correct, so long as you don't detonate obviously.

And even if I drive 70 or 80Mph it doesn´t matter if the engine is fed with 15.5 or 16.0 AFR and it doesn´t slow down ????!

Correct again, with the same above provision.

Only thing to happen is to have not enough advance and thus put stress on the exhaust valves (by not all-finished combustion when it opens), right ?

You have to have enough advance to run the lean mixtures efficiently without detonation, (which takes it's toll on the pistons and piston rings, not the valves). Too little advance can destroy the valves by burning them.

Is there something to take into consideration, like spark plugs ? They are designed/chosen by the manufacturer for the engine running 14.7 AFR all the time (excluding WOT) and when beeing mainly lean the heat range of the plugs could be out of range ?

If you run in the leaner zones a lot, you might want to consider a cooler plug.

Could knock become an issue ?

Yes, at lean mixtures, knock is a very real possibility when the engine is under load.

[landybehr]

Thanks for the input !

- by putting stress onto the piston rings you mean that the burn time is prolonged with lean mixtures and the cylinder pressure curve starts to raise earlier - even against the compression stroke ? Has anyone made any measurements how that compares to the pressures that regularly accur? BTW: Is is really an issue compared to Turbo-Diesel engines ??

- is there any guideline to the area where "load" is enough to cause knock in this context ? The areas above 80-85kPa I´ve kept conventionally rich. But I am approaching these map rows from "beneath". In the usual town traffic the engine can manage most conditions with 15.5 and leaner, even acceleration when I am in the common row of vehicles taking off from a traffic sign is possible like that - so it´s very seducing to let it work lean. All this counts for RPM below 2500 to give an idea. Is there a danger already ?

- BTW: I have put a knock(sense) sensor on. I haven´t torqued it correctly so do not trust it 100%. Would that sensor give a signal as reliable as at WOT-knock ?
(in which case MS would reduce advance enough to be felt).

[devastator]

by putting stress onto the piston rings you mean that the burn time is prolonged with lean mixtures and the cylinder pressure curve starts to raise earlier - even against the compression stroke ?

I was referring to the damage that can be caused by detonation, not lean mixture values, even though they usually compliment each other in a bad way.

BTW: Is is really an issue compared to Turbo-Diesel engines ??

That's a whole different subject.

- is there any guideline to the area where "load" is enough to cause knock in this context ?

Every engine is different, and without a dyno to take it to detonation and then back it off a little, you will just have to take an educated guess IMHO.

In the usual town traffic the engine can manage most conditions with 15.5 and leaner, even acceleration when I am in the common row of vehicles taking off from a traffic sign is possible like that - so it´s very seducing to let it work lean.

If your engine likes this go for it, you'll get better mileage.

Is there a danger already ?

Not if you are not detonating and your cyl head temp's are not outrageously high, (which will cause detonation too). Usually an engine will either start pinging or misfiring when run too lean, the latter being more common. If neither of these is taking place, you should be fine.

- BTW: I have put a knock(sense) sensor on. I haven´t torqued it correctly so do not trust it 100%.
Would that sensor give a signal as reliable as at WOT-knock ?

I think you should remedy this sensor soon. Sometimes you can detect detonation as speckles on the spark plugs. I run E85, so I just have to listen for it as I can't see much of anything on my plugs. Yes, the sensor should work at lower RPM also.

(in which case MS would reduce advance enough to be felt).

Depends on how you have it set in MS.

[landybehr]

Thanks again !!

[landybehr]

when the normally burning fuel after a spark compresses and heats the unburned fuel, causing the remaining fuel to spontaneously ignite.

Hi Lance, you´ll soon put me right But isn´t that what happens each time after a spark plug fired. The ignited fuel burns and expands, same time releasing heat. So the not-yet burnt fuel will be heated and compressed until it starts to burn itself.


1.BTW: is there any guideline how much advance an engine needs for how much change in AFR ? The burn time of fuel is relatively constant IIRC, so hope arises there will be a rough guide which then could be subject to fine tuning.

2.BTW: I am seduced to make mixtures at 1000-2000RPM and below 80kPa as lean as 15.5 (even lower with decreasing MAP), what happens if I drive up a mountain. The higher I get the lower ambient kPa will get, so I will loose my "safe" AFR of 13.0 at 2000RPM and 95kPa. Is there a danger to put undue load to the engine with labouring that lean then ??

[landybehr]

thanks again!
now that´s very concrete and detailed stuff in that link. And challenging to me (math´s never been my strong point).
I do not get to a valid result with the degree of crank-revolution @ given time. Well, I do but it´s frightening:


For example:

-at 14.7AFR flame speed is 50feet/s (equals to 15240 mm/s)
--With my engine bore is 94.0mm so halved that´s 47mm (spark plug is offset by I neglect that).
---so I get 47/15240 = 0.00308s which is about 3.1ms of burn time from spark to cylinder wall

-to turn that 3.1ms into degrees of crank revolution I know that 3000RPM equals to 50 rounds per second
--in 3.1ms I therefore get: 50 * 0.00308 = 0.154 (so the crank did the 0.154 part of a whole revolution in 3ms)
---to turn that into degree > 360° * 0.154 = 55.8°

so the answer is at a mixture of 14.7 in my engine at 3000Rpm the fuel is burnt within 56° of crank revolution.
If I subtract the 17° which I want the burn to happen past TDC then I get 39° of advance.
This is quite high and literature advised for that open wedge combustion chamber 34-36° of max. total advance.
But it is not too far off and may show "where to go".


NOW I want to do that for 16.0 AFR @ 3000RPM.
-16.0AFR flame speed is 25feet/s >> 76200mm/s.
-- 47mm / 76200mm/s = 0.00617s of time for a whole burn.
-in 6.1ms I do get 50 * 0.00617 = 0.3085 rounds per second.
-- 360° * 0.3085 = 111°
Even if I subtract 17° there´s still 94° of advance left :O
The piston hasn´t even gone half way up the compression stroke

Ok, I´ll try it for 2000RPM shortly - rough estimate tells me it is 2/3 of the 3000RPM figure, but here we go:
-2000RPM = 33.3 rounds per second
--at 16.0 AFR >> 33.3 * 0.00617 = the 0.205 part of a round in a second
--- 360° * 0.205 = 74°
---- 74° - 17° = 57°

I ain´t quite sure what to make out of that.
Currently I have somewhat half of the advance that I just calculated.

really

[landybehr]

if you know the general principles, the tuning process should be more straightforward.

that´s what forces me into asking and asking and ..

So then I was caught by the whole calculation being valid for WOT only. That explains something. Meanwhile I wondered why big AFR-changes do not make a big difference for advance at idle (aside from the lower piston speeds giving more time for a burn). Well, I haven´t got the whole thing, but am getting closer all the time.

(I think tuning advance is most demanding. On a dyno it will be straightforward. On the road it´s only easy with WOT and idle - both giving the most steady parameters like MAP and TPS etc.; the cruise areas will ask for someone else to drive and me changing the advance with the laptop and watching for knocksensor and raising<>lowering revs - while I don´t think of myself being a very sensitive diagnostik tool. Gradient of roads f.i. will influence that a lot. Anyway, I´ll have to try in lack of alternatives )