Grumpy

The spark doesn't start the combustion process. The reaction is already in progress, and the spark just cascades the event.

Compressing air, heats it. Look at any shop air compressor, and you'll probably see some cooling fins on it. Aim a IR temp gun at it and you'll see the compressor's discharge port gets toasty warm, if your in any doubt.

This heat (of compression), starts driving the disassociation process of the fuel. First the ingredients of the reaction have to disassociate, and it's then as they reassociate, they give of more heat/energy.

You can get to where you don't have ENOUGH timing, and the motor will then detonate, or suffer from preigniton.

Also being too rich or too lean can cause detonation, or preigntion. Too lean, and there's not enough fuel to disassociate, and that drives the reaction temp up prematurely, and too rich can drive the CR up, and cause it.

There are many things that can happen when you miss the tune. not always is more timing to more fuel better, finding out what makes the engine happy as you raise the HP level, takes time and logic to work out.

Especially at lower RPM when you have so much time for the injector PW's. Then the over fueling can lead to needing more timing, and while the engine feels real crisp, you've actually missed the tune, and then are on the slippery slope of possibly missing the WOT tune since you actual non-PE mode AFR isn't really Stoich (if your running closed loop).

The *numbers* all need to make sense. If your running alot more PW at 1,500 RPM then at 6,000 RPM or alot of timing (for your combustion chamber size/ CR), then you need to sit down and look at your notes, or start afresh with rebuilding your calibration.

No, I'm not trying to educate anyone in chemistry, as I certainly don't have the learnings to do, but rather am trying to explain it in a way that the average guy here, can get what's going on. This is just a few short paragraphs to cover a very complex SERIES of events.

For a good read search the archives at DIY-EFI for a posting by Robert Harris about water Injection, and in one of them he explains some of what happens in the dis/association mode when water's added to the reaction. It's an interesting read, as I recall.

JohnL

The combustion process may be in progress before the reaction is initiated by the spark, but if the chemical reaction (ie oxidising the hydrocarbons) starts before the spark, this is preignition at least.

"Will" is a very strong word to use here- implying it is always the case that you can get to this point. It's hard to see how having retarded timing would ever cause detonation. It may "cause" preignition because a hot spot plus excess compression stroke time produced auto-ignition, but this doesn't seem like a real world performance car issue (given that few tuners are likely to retard factory timing that far).

My $0.02 worth.

John

Grumpy

Care to define the difference between process and reaction?.
I thought using the terms disassociate, and reassociation were easy enough for the average guy to grasp. And with all the various chemicals present I would find you statement rather misleading as some of the chemicals used, start to *change* long before the some of the higher end aromatics (and other *stuff*).

Might be hard to see it, but if you want to drop your car off, I can tune it to do it. This is a real world issue. Your also making an assumption about how far one might change the timing from stock, whereas I've received enough emails off list that I just thought it was appropriate to devote some time to explain that this can happen. You can also cruise over to the TB.com board and search for similiar topics, and see that it is somewhat common, and the 89 TTA ties it directly to being to 3rd gens. related.

I will stick with what I've said as being universally true, for the average 3rd gen tuner, as per my original disclaimer.

I thought I was saving some folks some time, but googling for gasoline and it's component chemicals and the MDS's for the component chemicals would all serve to verify what I've said, and point out the why's and wherefores for Toulene, Zylene and Benzine as additives, as well as a host of other chemicals. Boiling temp differences are one of the points to look at, as far as how they may respond at the disassoication stage of the reaction. Depending on the *brew* of the fuel some are hardly higher then ambient temps on a real hot day, and compared to some underhood temps, they're actually lower the underhood temps on some days.
Might even try for Honda F1 and heated fuels for even more giggles. The heated fuel material, and how they did what they did was extremely interesting, IMO.
Also, if your an SAE member a search for waxy and fuel can generate another paper about gasoline, and making it reactionable, er, rather about how temps can change things early in the disassociation stage, as a function of droplet size.

Having even a slight understnading about these matters, IMO, WILL make the average tuner, much better at tuning.

Like I said earlier, was just trying to make things easily understandable for the average guy.

onebinky

iTrader: (1)

My forearm was reminded of this today when I was reaching for the pressure regulator. Left me a nice .15"x1" blister

Isn't the compression ratio something like 55:1 in a compressor though?

JohnL

Moi? Misleading? Quelle horreur . I did define "reaction" as "chemical reaction" because I wasn't quite sure about what you were getting at with that term. By "process", I mean the physical changes that are going on up to the point of ignition (piston movement, adiabatic heat rise, turbulence/ mixing, etc). And I'm not sure what you're getting at by "associate" and "disassociate"- I guess I wasn't paying enough attention in the combustion class I took as part of my master of engineering science degree.

AIUI, the key to spark timing is to initiate the chemical reaction in a controlled manner, so that the maximum power is extracted from burning the fuel. You don't want the timing too soon, or the other "process" elements may create a pressure rise that results in self-ignition ahead of the flame front (detonation). And you don't want "process" failures (eg hotspots) that can cause ignition before the spark (preignition).

Now $0.04 worth (and about my value limit). And no, I don't want to take my car all the way over to the US so you can show me how to make it detonate. I'' save the transport cost, because I have demonstrated several times that with my level of tuning "skill", detonation is a surprisingly frequent unintended outcome .

John