Thursday, November 30, 2006

AV - Ignition Timing

With regard to ignition...

Got a match? Gopher, kitchen, safety... any match will do. You’ll need a few of them for what follows.

Strike a match and measure how long it takes for the chemicals to burn off. Just hold it vertically and count-down starting from the pop of ignition until all of the chemicals are gone. You may chant ‘one-potato, two-potato...’ if you wish :-)

Do that several times and you will see that for same amount of chemical, it takes the same amount of time.

Now try to make it burn faster. Or slower. Blowing (gently) on the flame should give it more oxygen whilst holding it in the steam from a kettle should give it less but the odds are neither will effect the burn-time because the chemicals are a balanced mixture of fuel and oxidizer. That’s what’s referred to as a ‘stoichiometric’ mixture.

As with the match, the fuel-air mixture in a gasoline-fueled internal combustion engine does not explode, it merely burns. Or should :-) If it does explode (ie, detonatation) you’ve got a serious problem on your hands.

Although the match experiment isn’t very precise it offers a hint that combustion of a given quantity of mixture not only takes a certain amount of time, that amount of time is virtually fixed for a given quantity of material. If we set aside the temperature of combustion, which I am doing deliberately for the purpose of this explanation, the only way to alter the amount of time it takes to burn a given quantity of fuel is to alter the composition of the mixture. The key point here is that for a given engine and within the parameters already mentioned (ie, temperature and mixture ratio) combustion takes approximately the same amount of time regardless of engine rpm.

Now consider a spark-ignited Otto-cycle engine.

Even with a cylinder of large displacement, when the fuel-air mixture is compressed, combustion takes only a few thousandths of a second - - a brief flash is all you’ll see through the quartz head of a Test Engine. What happens during that brief flash is the heart & soul of understanding internal combustion engines..

During that brief flash all of the fuel combined with all of the oxygen to produce a given quanta of heat, raising the temperature of the residual gases in the combustion chamber, most of which are nitrogen, to several thousand degrees, at least momentarily and nearest the core. But that brief flash of heat also serves to raise the pressure inside the combustion chamber. Which is good. But only if the pressure rises in an orderly fashion - - and only if the peak pressure occurs after the piston has reached the Top Dead Center point of its up & down movement. If peak pressure occurs too early we might as well go home.

A little bit early isn’t too bad. It wastes power but the engine will still run. Here’s why: Each cylinder of an Otto-cycle engine has only one power pulse for every two revolutions of the crankshaft and that pulse lasts for less than ninety degrees of rotation. The energy needed to rotate the engine through the other 630 degrees has to come from other cylinders or some storage mechanism, such as a flywheel. Whatever method is used, it is sized for the slowest speed at which you want the engine to run, meaning there will always be some amount of excess energy at any higher speed. When peak pressure occurs a little bit early some of that stored energy will be used to get the piston past TDC. Under those conditions the engine’s efficiency is low and fuel consumption is high but the thing will still run.

But if the pressure peak occurs too early, there won’t be enough energy in the system to overcome the timing error and the thing will fail to run, often signaling it’s disgust with a back-fire.

By the same token, we don’t want the pressure peak to occur too late. If the pressure does not peak until the piston is already descending - - which it will do even without a power pulse, thanks to the momentum inherent in the Otto-cycle design - - much of the pressure we’ve worked so hard to produce will be dissipated without doing any useful work; the amount of torque available at the output will fall. When peak pressure occurs too late, the engine will still run but not very efficiently in the thermal sense, and its top speed will be limited, since any increase comes at the a further reduction in torque.

Notice here the distinction between initiation of ignition - - when the spark occurs - - and the moment of peak pressure. Although sequentially related these are two separate events, the interval between them determined by a number of factors such as the shape of the combustion chamber, the octane rating of the fuel, the point of ignition and so on. Most confusion associated with engine tuning stems from addressing only ignition timing and ignoring the timing of the resultant pressure curves.

It should be obvious that an efficient engine is more desirable than an inefficient engine. An efficient engine burns less fuel to produce the same power as an inefficient engine. Efficient engines also tend to last longer. What isn’t so obvious, especially with an antique design such as the air cooled Volkswagen, is that a remarkable improvement in thermal efficiency may be achieved by focusing the keenest attention to the myriad details which contribute to its inefficiency, such as the timing of the cam, valves and ignition, proper waste-heat management and so forth.

At the very least this message should have made two things immediately apparent: Ignition must occur at some time prior to the need for peak pressure, and the precise moment of ignition must vary according to the rpm of the engine.

Which is why I don’t use magnetos. Or any other ignition system having a fixed firing point.

Yeah, I know - - it flys jus’ fine. The question you gotta ask yourself is, how much better could it fly - - and how much fuel are you pissing away.


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