The Mysteries of Combustion
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- mikenixon
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Re: The Mysteries of Combustion
Personally, I think how multi spark came to be and the problem it solved is fascinating. But it only has relevance in the context of CDI.
Let's see if I can say this better. Multi spark is an advantage only when it is a part of CDI, and only in a street context. This is because it fixes a weakness in CDI, not because multi spark is an advantage all by itself. If someone promotes a non-CDI as being a better performer because it is multi spark, I have to say that is untrue. As Eric pointed out, the most you could expect from a longer duration spark apart from the system also being CDI is a touch better throttle response and acceleration from a stop. But that is pretty theoretical and I have hot heard of any maker claiming that.
Let's see if I can say this better. Multi spark is an advantage only when it is a part of CDI, and only in a street context. This is because it fixes a weakness in CDI, not because multi spark is an advantage all by itself. If someone promotes a non-CDI as being a better performer because it is multi spark, I have to say that is untrue. As Eric pointed out, the most you could expect from a longer duration spark apart from the system also being CDI is a touch better throttle response and acceleration from a stop. But that is pretty theoretical and I have hot heard of any maker claiming that.
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- mikenixon
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Re: The Mysteries of Combustion
Yes, I agree. Outside of CDI, multi spark and/or longer duration (essentially the same thing) hold no promise of better performance. See my comment elsewhere.Whiskerfish wrote:If the spark is sufficient enough to initiate then after that the strength and duration and additional sparks should make no difference right?
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- mikenixon
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Re: The Mysteries of Combustion
Eric, not sure about that but remember the 60/40 rule. Inadequate spark needs more fuel to compensate, and lean fuel needs better ignition to compensate. They work together. So, a street spec engine, whether emissions era or not, has a far leaner mixture than does an offroad machine. Thus its ignition has more demand on it. A lean mixture does not burn as smoothly. A better ignition largely makes up for that. The problem with CDI in its beginning was it had a spark duration 1/3 as long as the other ignitions. This made it not work well with street bike carburetion, though it worked fine on snowmobiles of the time whose mixtures were plenty rich. The same situation held true for ATVs not long ago, before they went to fuel injection. Many had AC-CDI. Worked great because these are offroad vehicles not carbureted street-lean. But when they got fuel injection all of a sudden they were subject to the same mixture restrictions as street bikes, so the CDI had to go and was replaced with the same transistorized Kettering that all of Kawasaki's street bikes use.ericheath wrote:I have read that after the initial flame front starts and expands outwards, a vacuum is left and some unburned fuel fills that vacuum similar to a mushroom cloud. The additional spark(s) help burn that.
Remember, a diesel compresses air, not fuel. And not all that highly, either. A few spark ignition race machines are approaching diesel levels of compression.This article showed I was in the “detonation is the same as preignition” camp. Now I have to rethink what happens in a diesel.
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Re: The Mysteries of Combustion
Love your articles, Mike, but I always hear the sound of a jet roaring over my head.
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Re: The Mysteries of Combustion
Thank you Mike for the time and effort you take to pass on your wealth of knowledge to us mere mortals.
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Re: The Mysteries of Combustion
another interesting attempt to improve ignition was Honda's CVCC engine, which had a relatively rick mixture where the spark was and a relatively lean mixture in the main chamber.... the '77 civic I had generated lots of power and good gas mileage as well
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Re: The Mysteries of Combustion
Compound Vortex Controlled Combustion, if I remember correctly. Yes, innovative.rcmatt007 wrote:another interesting attempt to improve ignition was Honda's CVCC engine, which had a relatively rick mixture where the spark was and a relatively lean mixture in the main chamber.... the '77 civic I had generated lots of power and good gas mileage as well
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- mikenixon
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Re: The Mysteries of Combustion
An example of a dual plug engine, a modified Harley Evo. Built this in 1995. Owned by Cruiseamerica president Bob Smalley. Started out as a 1987 Harley-Davidson FXRS (Evo). Has the best cylinder heads available at that time, specially cast STD (brand) heads. The valves are repositioned and angles changed to 30o. Dual plugs (see the plug wires going between the pushrods?), bathtub shaped chambers (a design made famous by Jerry Branch). Ported by Carl Monroe of Carl's Speed Shop. I received mentoring on the build by Ron Dickey, owner of Axtell's. Has a 0.620" cam, 600 in-lb valve springs, Axtell cylinders, chrome moly pushrods, a Bandit clutch, a Tech starter, and a Japanese-made RevTech carburetor I mated to an S&S air cleaner as a result of several dyno hours configuring the intake. The only factory H-D parts were the crankcases and the lifters, and the lifters were replaced after the dyno runs. Over $30,000 went into this engine. The 13:1 CR bike cleared 125 rear wheel horsepower on almost its first dyno session and placed 3rd in its class at Daytona's 1995 Harley shootout. The bike later made 195 hp on the bottle (nitrous). This entirely civil, electric-starting bike was a joy to ride. After building it, I rode it for several days to shake it down, and during that time all I could think of was a piped V-Max with 1" handlebars. It had that kind of feel and acceleration.
Last edited by mikenixon on Tue Jun 26, 2018 10:50 pm, edited 1 time in total.
Mike Nixon
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- mikenixon
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Re: The Mysteries of Combustion
Here is an in-progress image. The bike has a Dills electric golf cart battery. Had to section the oil tank to make it fit. The engine is stroked, requiring the front engine mount to be modified. While I built this bike for Sturgis, my wife brought me dinner most nights at the shop. I don't work like that any more. Life's too short.
Note the H-U-G-E pistons. STD 30o heads don't fit on stock cylinders. I don't recall the displacement.
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Re: The Mysteries of Combustion
Another project. That's a MKII Kawasaki KZ1000 engine. I built the big bore engine and wired the MSD, nitrous and rest of the electrical.
Mike Nixon
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Re: The Mysteries of Combustion
Another Harley I built. This one owned by a very successful contractor in town. Engine is relatively near stock, with just bathtub-reworked stock heads over 3.517" pistons, plus a mild Andrews EV79 cam, and of course, N2O. Note the nitrous bottle. Engine cases are polished. Handlebars illegal.
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Re: The Mysteries of Combustion
Mike, that was very interesting. Way over my head, but I did learn quite a bit. Since you were expounding about the mysteries of spark ignition combustion, I wonder if you could follow up with a treatise on the mysteries of compression ignition (diesels) combustion and also discuss multi pulse injection on them as well. Honor us with your wisdom.
Thanks much.
Thanks much.
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Re: The Mysteries of Combustion
77Gowing -- I beg ignorance. That is not an area I know a lot about, diesels and their unique form of fuel injection. I have serviced Kawasaki's diesel Mules, but that's about it.
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Re: The Mysteries of Combustion
I am not an expert in engines, but I did have to take chemistry before med school and a lot of this is basic chemistry. We all know that when you compress a gas (remember, gasoline vaporizes into a gas) the energy of the compression heats the gas (run your 5hp compressor while blasting the tank gets VERY warm). The other part of compression ignition is that as you force the molecules closer and closer and add heat they eventually start the chemical reaction that we call ignition or burning. There is a point you do not need a spark to have that happen.
An interesting factor in compression. If you simply take "air" and compress it and cool the tank, eventually the different components start to liquefy (each at a different amount of compression). This is how we end up with liquefied oxygen, or CO2 etc. As these gases "boil off" into vapor they take heat. For example every notice all the ice on the big O2 tanks outside of a hospital.
Hydrocarbons do not take nearly as much compression to liquefy, so butane (c-c-c-c) can be in a relatively thin walled container, while ethane with two carbons with many bonds needs a relatively strong container.
An interesting factor in compression. If you simply take "air" and compress it and cool the tank, eventually the different components start to liquefy (each at a different amount of compression). This is how we end up with liquefied oxygen, or CO2 etc. As these gases "boil off" into vapor they take heat. For example every notice all the ice on the big O2 tanks outside of a hospital.
Hydrocarbons do not take nearly as much compression to liquefy, so butane (c-c-c-c) can be in a relatively thin walled container, while ethane with two carbons with many bonds needs a relatively strong container.
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