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Ignition Curves
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Posted by: Swiss---------------------
Hey Guys,
Hard to get information sometimes but I figure that this is one of the places to ask. Seems to be a lot of attention is being given lately to programmable ignition systems that can use different curves to enhance different parts of the powerband. I was asking Paul Olmstead of Powroll questions about this back in the mid '70s and he DID say that optimizing the ignition at different rpms could enhance power, but at that time any application was mostly limited by mechanical technology. What I am asking those in the KNOW is what are the primary differences in the programming that are responsible for the changes between a strong mid-range power ignition and a top-end system etc..?? Since much of this is engine specific, I am not asking for the numbers for say a YZ426F (don't own one anyway). Just the theories behind the changes. This handlebar switch technology seems really neat, but you have to know something about what you are doing to make it work. And I am SURE that some know a lot more than others!
Thanks,
Swiss
Posted by: Rich Rohrich---------------------
The basic idea behind ignition advance on any conventional piston engine is to try and time the peak cylinder pressure to occur so as to apply the pressure on the descending piston in the most mechanically efficient way. This tends to be in the range of 15-20 degrees ATDC. As rpm increases and
less time is available for charge burning to take place the engine will need to fire earlier (more advance) to account for the diminished time. The overall idea is to use the minimum timing necessary to obtain the best possible torque. This is sometimes referred to as MBT (Minimum for Best Torque).
Higher engines speeds reduce the overall burn time by virtue of additional turbulence in the combustion chamber, less time available to lose heat to the surrounding chamber surfaces, higher flame speeds, and increased mixture density. As a general rule, the first half of combustion 0-50% burned, flame speeds rise in direct proportion to rpm, while the 50-100% burned time flame speeds rise exponentially with rpm. As an example Honda has shown
in some of their research a flame speed of 45 m/sec at 8000 rpm, with an increase to nearly 200m/sec at 16,000 rpm on a small bore test engine. In engines with narrow valve included angles, and efficient combustion chambers these factors can offset the reduced time available for combustion to take place, and retarding the timing will actually produce more torque. We are starting to see newer four-strokes take advantage of this fact and using more advanced digital ignitions that have the ability to retard/advance the timing in a non-linear fashion. By monitoring RPM and throttle opening (which influences Manifold Absolute Pressure or MAP) it's possible to have a smarter advance curve that more closely follows the actual requirements of the engine, rather than just advancing with RPM. Full engine management systems monitor additional parameters to take this control to a higher level still. This is a VERY general explanation of the four-stroke side of things.
Two-strokes tend to have fairly specific reasons to retard the timing at high speeds. Including the above points, we can add, more efficient scavenging at high speeds which improves mixture density and speeds combustion, plus the mixture density as a whole is much more dense than on a four-stroke because of crankcase compression. Dense mixtures burn considerably faster, so less timing is needed. Once the engine is operating in the range of the pipe tuning, the volumetric efficiency will be at it's highest which will obviously be the point where maximum cylinder filling occurs. Again this will improve the mixture density and speed things up, so less timing is needed. In a two-stroke race engine this tends occur at high rpm Two-stroke exhaust valve equipped engines will have higher dynamic compression ratios at high rpm which will also help reduce the burn time. But probably the most important reason for retarding the high speed timing on a two-stroke is to use some of the available combustion heat to manipulate the pipe wave timing and improve the scavenging/ram tuning. Less efficient combustion tends to raise exhaust temperatures, so they are effictively trading off some thermal/combustion efficiency to improve the scavenging and increase the delivery ratio of fresh charge. Like most things in engine design, it's a tradeoff based primarliy on the intended use of the engine. More radical designs like the RS roadrace have fairly narrow power bands, so they use these effects to maximum advantage. Add to all this the basic simplicity of a two-stroke combustion chamber, which by it's very nature
will provide a fairly fast burn rate and it's pretty apparent that there are probably more similarities than differences between the two engine types.
I hope this helps some.
Posted by: Swiss---------------------
Thanks Rick,
It is a good start. I have known most of that for years. What I am really looking for is info on how, say the dual switchable systems work and what compromises they generally use in a Stage1-Torque/Stage2-rpm.max power situation. A LONG time ago (years) I would have said to dyno test the engine, and then custom curve the ignition to match the BEST power at ALL rpms. But I understand now that there CAN be times when you would want more grunt at the bottom and then less high rpm "hit" for traction control etc.. We used to depend on the throttle for this, but with the engine power steadily climbing each year, having some "controller" handle a bit of it seems worthwhile. Open terrain, max power. Rocks and sand, boost the low and mid-range etc.. Question is WHERE do you go when you "de-tune" say the upper rpms while boosting the bottom and mid? With a points only system or even a crank trigger and std. linear advance, you CAN custom tune the curve a bit, but it is NOT going to vary for rpm or throttle opening like the newer systems. I know that a lot of these programmable systems are hitting the market now, and it would be great to have some real knowledge out here about what you can do and what effects it will most likely produce.
Swiss
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