| "1. He labels pre ignition and detonation as an unknown phenomenon?? Whaaa? What really had me scratching my head is how he claims detonation only happens atdc which if that was the case I don't see how it could even happen if the cylinder has already passed it's peak compression point and the a/f mixture has already been ignited and the exhaust valve open." Detonation only happens after the spark plug fires. I'll get into details later, but it shouldn't read ATDC, it should read after plug firing. "2. Octane is nothing magical? What is this guy smoking? Leaded, unleaded, alcohol, ethanol all burn different. I was taught octane is anti knock. Higher octane is a slower more controlled burn. 86, 88, and 91 ignite quickly like a bomb, boom, and expells all of it's energy at the time of combustion. E85 ignites and the flame front not only pushs, but follows, and fills the entire combustion chamber which is what makes it was safer inmo, in other words leaded unleaded ignite quickly almost punching the piston, E85 burns so slow it literally pushs the piston down the entire way burning all if the charge which q16 doesn't due." Octane and flame speed are not really related. Octane is the resistance to exploding. Flame speed varies based on a lot of other factors than octane rating. While higher octane fuel has been shown to have a slightly slower flame speed than lower octane fuel, the difference is smalled compared to flame speed of gasoline vs ethonal vs diesel. One thing that affects flame speed the most is lamba, or basically your a/f ratio. All combustion processes should be a controlled burn and not an "explosion" in your cylinder. "I believe pre ignition and detonation both happen before the spark plug fires, and are essentially a result of poor poor tuning, uneducated mods and it an Improper functioning engine. " No - pre-ignition happens before the plug fires. Detonation happens after the plug fires. Details to follow. Pre ignition is simple. It's when the combustion of air and fuel is initiated by something before the plug fires -FTFY "Detonation is combustion caused by excessive cylinder pressure, this is happening constantly in a diesel truck. They don't have spark plugs so how do they ignite there af mixture. Cylinder pressure. There are a couple things that cause this to happen. One of the fearures of our Z's help prevent this is vtc adding more overlap. Overlap is when the intake and exhaust valve are open at the same time allowing the scavenge affect to clear the cylinder of the previously combusted gas's. This scavenging only happens at the top of the combustion chamber between the valves wear the spark plug is and should pull all if the previous charge out of the chamber. If all the gas's don't escape due to poor scavaging during overlap due to various reasons now there is an large area of the combustion chamber already occupied by a possibly ignitable charge, and we just force more combustible air eventually ignited before the spark plug due to excessive cylinder pressure, and we put more fuel ontop of that which means the spark plug can't efficiently burn everything, poor scavenging doesn't clear the chamber the excessive pressure causes gas's build up and pressure ignites part of the charge before the spark plug can. Then the plug fires igniting the second charge so it's two kabooms which is where the idea flame fronts crashing. So detonation is essentially two kabooms real quick. The first combustion untimed uncontrolled and ignited by pressure
when the piston is traveling upward causing the engine to want to stop in place, and this is one crush rod bearings and this is where the term two flame fronts crashing comes from. The second by the spark plug and this is usually what cracks the rings. So in a way detonation can be a type pre ignition, that would cause crazy cracking of parts forming edges, that would eventually get hot from a lean inefficient engine, start glowing orange like a glow plug and igniting the charge before the plug. " No. No for just about everything in those paragraphs.
You have pre-ignition pretty well defined. It mainly occurs when you get a hot spot in your cylinder. It could be the plug itself is staying too hot (hence colder heat range plugs), carbon build up holding in heat, or anything else staying hot enough to ignite the fuel charge BEFORE the plug fires. End effects are all the same. You end up with a really nice round hole in the dead center of your piston. Because the charge fires before its supposed to, its basically ramming your piston into a solid wall a couple thousand times a minute. Your engine will only last seconds to minutes with pre-ignition.
Diesels don't operate with detonation. Its a misconception a lot of people have. They operate under a princaple of pressure induced combustion, but that is not the same as detonation. Diesels have direction injection, something that only a few gas engines have incorporated yet. A diesel builds cylinder pressure and then injects fuel that instantly ignites. Because the injected fuel ignites immediately its almost impossible to have detonation in a deisel. Gas engines are different. The air and fuel are mixed in the intake runner. As soon as the intake valve opens, a combusable mixtures fills the entire cylinder. Your ideas on scavaging are close, but a little simple. Over lap effects cylinder swirl. Cylinder swirl determines how the cylinder fills up with new air/fuel mixture. As far as detonation goes though, you can assume the entire cylinder is filled at once. So how do we get detonation. Once the plug ignites the mixture it starts expanding from the plug outwards to the cylinder walls. There are two "fronts" that are expanding though. A flame front which travels at a relatively slow speed, and a pressure front, which travels much faster. Normaly, as the pressure front reaches the cylinder wall, it will induce combustion from too high a pressure. You now have two flame fronts travelling towards each other and when they hit, you get your "ping" that is a telltale sign of detonation. This is why diesels don't have "detonation". Because the fuel is ignited immediately, it is very very hard to have two seperate areas of the chamber ignite at the same time and interfere with each other. It is possible, but its much harder to do. Thats also why higher octane can prevent detonation. Because the higher the octane, the higher that pressure front has to be in order to ignite from compression. Retarding your timing allows the piston travel further downward on the power stroke and lower the overall pressure in the cylinder before that pressure front hits the wall. Either way will help get rid of detonation, but one doesn't lower your performance. Its also why an engine designed for 87 doesn't gain anything by running 93 octane, unless you advance your timing or do other mods. Most modern cars have knock sensors that will retard timing when knock is detectted. So while running 93 in a car built for 87 doesn't gain you anything, running 87 in a car built for 93 can cause performance losses. You'll get that performance back when you switch back to 93, which is why some people claim to gain horsepower with higher octane. One last note, poor scaveging will actually help reduce detonation. If you leave a bunch of already burnt gasses in your cylinder, its going to be harder for the pressure front to move quickly and efficiently across your cylinder. Think of it like running in air and then running in a pool. The more "stuff" in the cylinder that can't burn will hold back the pressure from getting through. You're going to make a ton less power though because for every particle of already burnt leftovers in your cylinder, there's less room for fresh air and gas. Thats why leaded fuel has higher octane than unleaded. The lead acts as a damper for the pressure wave, slowing it down and making it harder to ignite the fuel in far off reaches of the cylinder. It literally just gets in the way.
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12:54:00 03/17/14
