OK, Sublime, let me see if I understand:

The perfect stoichiometric equation would be:

1 part high-octane Hydrocarbon fuel + 14 parts atmospheric air -> ignition -> X parts water + X parts carbon dioxide

(in order to make)

C(n)H(2n+2) + O(2) -> H(2)O + CO(2) (I'm not going to figure out the balancing for 93 octane right now)


However, we don't want a perfect equation occuring in the cylinders, since it would combust too quickly. The goal is to create a controlled, progressive burn so as to create a constant pressure during the downcycle of the piston, right? So we do that by running the combustion slightly rich, effectively depriving the combustion at the point of the ignition source of enough air to burn as fast as it really wants to. I've heard before that the ideal mixture for combustion in this way is a 13.7:1 mix of atmospheric air (I believe the oxygen content in air is only around 14% or so).

So if I'm understanding this correctly, lean conditions cause detonation and preignition because they're providing too much air to the normally air-deficient ideal-combustion-equation, allowing for faster than desired combustion, raising the chamber temperature greatly, and leading to the detonation and preignition problems which wreak havoc. Right?

If I'm following this correctly, then it makes sense to me. Thanks.

 

there was an article posted a few months back about why lean created heat. I had always been curious about why it was that lean made cylinders run hot and they had a really good, but complicated, answer. But of course i forgot, so i'll hunt around for it.

But i am curious as to what sublime said, how exactly can you detonate while the cylinder is going down? Detonation is, from my understanding, most often caused by compression and high heat. If you lean out you make too much heat and this allows the compression stroke of the engine to actually make the fuel spontaneously burst so to speak. I can't understand how you could detonate on the way down, the sparks already gone, which should have burnt the fuel.


ILLINI_SVT, i think you got it right, except i'm not positive that sublimes explanation as to why lean makes heat is correct. I could be but I'm not positive.

just fyi, i believe its 14.7:1 that is the perfect mixture, at all the standard pressures and temps.

 

I have another question for all of you. Detenation is more common on a motor with a super charger of some type, right? If so why? I'm asking this question because it seems like guys with high horspower cars without superchargers don't have to worry about detination as much.

Thanks Mike

 

How detonation can happen while the piston is moving down...

Fuel has a flash point (I believe thats the right term). Its a point when enough heat/energy is present to kick the reaction off. The higher the octane of fuel, the more energy thats needed to kick the reaction off. The piston has hit TDC and is moving down. The combustion kicked off a few degrees back and the flame front is moving out. Imagine the ball of fire as a solid object. The air between it and the piston and cylinder walls is being compressed. This pressure is what yeilds power.

The force to accelerate the truck is acting against the piston moving down. This causes more pressure. This pressure peaks well above what the pressure is at TDC. Pressure causes heat. A bit of carbon on the exhaust valve could heat up enough to trigger another ignition. Now you've got twice the cylinder pressure because you have two combustions happening. This causes more areas to trigger. In an instant the entire fuel mix is igniting and the pressure spikes. Basically all the HP potential of the mix is release over a very short period of time. Lots of power over a short time = lots of torque.

 

Not true... NA cars can detonate from a lean mix. You've never heard a NA car knock? SC make detonate more likely because the temp of the air going into the cylinder is higher. So SC cars get a head start towards hitting the flash point of the fuel.

 

Lean makes heat because it burns slower, not faster than a rich mixture. The EGT goes up on a lean mixture because the slow burning flame chases the piston further down the bore, and often times is still burning as the exhaust valve opens. You're correct that 14.7:1 is generally the ideal a/f, but with an efficient high turbulence combustion chamber the a/f can be much leaner without detonation. In the dyno cell I've seen 17:1 a/f mixtures with BSFC in the 300lb/min range still make power without detonation, and some really effecient 4-cylinders run in the 25:1 a/f range.

There's a few general rules that seem to hold true. More timing increase's pumping losses and the chance for detonation. In extreme cases, more timing leads to backfires through the intake valve. Extreme low timing causes EGT's to skyrocket and the headers to glow bright red, as the charge is still burning when the exhaust valve opens. The smaller the bore, the less chance for detonation. The more turbulent combustion chamber leads to better fuel suspension/atomization, and less chance of detonation. The better the valve job that lets the exhaust valve cool quicker while seated, the less chance for detonation. The lower the static and dynamic compression, the less chance for detonation. The better the ring seal and valve seals are (less chance of oil in the CC), the less chance of detonation. Colder plugs, less chance of detonation. It goes on and on and on, you can trace factors that lead to detonation to almost every part in the engine. Thats why attention to detail with the overall combination is so important, and why certain longblocks from different vendors vary so much in price.

 

My old truck 98 f-150, 4.6 V8, denotated because of bad gas. Personally I think that is it a Ford problem. Not heard of many chevy's denotating, then again I dont know.

 

Bad gas (or using regular where premium is required) causes detonation because there's not enough fuel, just like a lean A/F mixture.

An octane rating refers to the relative amounts of long-chain hydrocarbon molecules present in the gas. A longer chain hydrocarbon has more energy potential stored inside it than a shorter one. ie:

Methane: Single-carbon hydrocarbon - One carbon atom with a bond to each of 4 hydrogen atoms.

Ethane: Double-carbon hydrocarbon - Two carbon atoms bonded, and three hydrogen atoms bonded to each of the carbons.

Octane: An eight-carbon chain, etc.

Methane and oxygen burn creating water and carbon dioxide just like every other hydrocarbon. But there is significantly more energy released by breaking the carbon to carbon bonds than just breaking the carbon to hydrogen bonds. Hence, the higher percentage of longer hydrocarbon chains, the more potential energy is stored in the fuel.

Also though, and more importantly, longer chain hydrocarbons use more oxygen for combustion than shorter ones. For example, octane consumes 25 times the amount of oxygen to combust than methane does. In the same vein, octane only weighs 7 times more than methane does.

So to sum it up, using a lower grade fuel requires less oxygen to burn, and hence, using a lower grade fuel in a car programed with an A/F table to run a higher grade fuel will result in too much air being provided to the engine, and a lean condition.

(I actually did the calculations if anyone wants to see.)

 

rscoleman, are you sure about lean mixtures burning slower? Your EGR example makes sense, but I was always of the impression that excess oxygen always leads to more intense (and faster) combustion. It's another reason that concentrated oxygen is so dangerous (flammable).

 

while bad gas i can see causing a lean problem, i thought octane issues were mostly a timing issue.

 

Timing is the next topic that I don't understand. Where's Rob on this?

 

well the objective it to make sure the flame front and the piston meet at TDC. So lets say you are timed for 91 octane, chances are your not perfect, you have it so the flame front meets the piston a little after TDC. If you stick 87 octane in a engine tuned for 91, guess what? 87 flame front reaches piston while it's still going up; detonation.

Slower the burn the more timing you can add and i'm guessing the more controllable it is so you can get it as close to that ideal spot as you can.

 

HERE we go

thanks to EZGZ to posting this link up.

http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

this should make everything better if you can hack through it.

 

The reason why the Lightning is prone to detonation/knock/ping is because our trucks do NOT have knock sensors. They do not have knock sensors due to the vibration of the supercharger. That may be the reason for other vehicles with superchargers being “more” prone to detonation/knock/ping.

The knock sensor, or detonation/ping sensor in most vehicle is to detect detonation/knock/ping and then adjust the timing and/or fuel/air mixture accordingly to a point. If you have real bad gas then you may still have a condition that the sensor (timing, and/or fuel/air mixture adjustment) can not correct.

Lightning’s are like the good ol’ days before computers and the ability of the vehicle to tune itself based on inputs/outputs as to the running condition of the motor. Use to be one way of tuning your car for pinging was to loosen the bolt on the distributor, having someone in the car holding the brake and pressing the gas pedal to put a load on the motor. Then you simply adjusted the distributor (timing) until no more pinging/detonation/knock.

That is what the knock sensor basically does and the Lightning does not have one, however a regular 5.4 does have one…

 

sure about that? I thught no ford truck engines came with em.


Also, while knock sensors may help, i'll have to jump on the band wagon that the nature of the beast makes detonation an issue. Any vehicle can knock, but boosted vehicles more so simply because of the heat issue.