Quote:
Originally Posted by swineone
So I went out for another drive today, paying attention to ignition advance -- PID 0x0E (hex)/14 (dec).
So indeed, at lower boost, ignition advance is higher (more degrees before TDC), while at higher boost, ignition advance is lower (fewer degrees before TDC). Either way, while pulsing, it's always positive (before TDC). I assume none of this is news.
So you claim ignition is retarded (pushed closer to TDC) to avoid knock, and that makes sense: if you ignite too early, there is the risk of the flame front meeting the piston head while it's still compressing rather than expanding.
What I'm trying to understand is how that "dumps part of the useful energy that could have been extracted from combustion, out the exhaust". I imagine early ignition, when the flame front meets a compressing piston, besides being destructive (through knocking), would also waste useful energy by pushing the piston head in the opposite direction of its movement, therefore doing negative work. But if the flame front meets the piston head when it's already expanding, then isn't that doing positive work as expected? Where exactly is useful energy being dumped out the exhaust?
Sorry if it's a stupid question, but I really want to understand this. It would help me understand why my car could have a BSFC sweet spot at part load, and why it would be better to pulse at lower boost pressures rather than higher ones.
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Great observations!
You're right, it isn't news that ignition timing is more retarded (or less advanced) at higher boost. Just looking at timing numbers though, it's nearly impossible to glean much.
You're right about igniting things too early as having a negative impact on economy. There's a concept, called "MBT" (mean best torque), which refers to the perfect ignition timing at a given load and RPM, to get the most power out of combustion. Ignite the charge any earlier, and you get more negative work, with combustion pushing on the rising piston. Ignite it much later, and the flame is chasing the piston down the cylinder, never actually getting to push on it. It's a continuum, of course, but the idea is that there's a "perfect" time to ignite the charge to get the most power out of combustion.
It sounds like you already know a fair bit of this, so forgive me if I write a lot that isn't news to you, but the reason you can't glean much from just looking at timing numbers is that MBT is different for literally every point on the map, and for every engine. Further, combustion isn't instantaneous - it's a flame front that spreads from the point of ignition, and takes time to complete.
As more fuel burns, the heat (and therefore pressure) in the cylinder rises. If that pressure gets too high before combustion is complete, the remaining unburned fuel can auto-ignite, either in points or all at once. When the two flame fronts crash together, this is audible as a "knock" or "ping", and the resuoting shockwave can damage the engine. It also reduces fuel economy (and power) so even if you build an engine strong enough to handle it, you still wouldn't want it (with some notable exceptions).
Keeping cylinder pressure from getting too high is managed in several ways. Higher "octane" fuel is more resistant to detonation, but typically has some drawbacks (refining, cost, energy density), and what kind of fuel one can get varies from place to place. Others ways include: Reducing intake air temperature, dumping extra fuel to cool the chamber, intercoolers. Some combustion chamber designs are simply better at preventing detonation ("hemi" designs are among the worst, pentroof among the best). Lowering boost (dynamic compression) lowers peak pressure. Reducing static compression. Also, starting combustion later (retarding ignition timing) moves the point at which combustion competes to a later time, when the piston crown is farther down the cylinder, meaning the instantaneous chamber volume is larger when the maximum heat and energy has been released. Unfortunately, retarding timing to prevent knock may also start ignition too late to "capture" as much combustion energy as possible (past MBT), resulting in more combustion energy passing into the exhaust as heat, rather than doing useful work.
Some other interesting bits:
When cylinder pressure is higher (more air and fuel, or boost), the combustion event happens more quickly. One can think of it as the fuel and air molecules as being closer together, so it spreads between them faster and more easily. Likewise with a higher static compression ratio, the flame front literally has a smaller area to fill before complete combustion. Because of this, less ignition advance is needed for MBT (you can start it later because it finishes earlier), and a smaller portion of combustion happens while the piston is still rising, meaning there's less negative work that happens.
This is one of the reasons higher loads are generally more efficient. In most engines, and especially at low load, combustion needs to be started long before TDC. I believe at certain points in my engine's map, I have ignition starting as far as 48 degrees before the piston reaches the top of its compression stroke. At higher loads, it's more like 12-25° depending on RPM (flame speed is mostly constant, so if the piston is moving faster, you have to start combustion sooner).
Different fuels also burn at different rates. Alcohols generally burn faster than gasoline, so they need less ignition advance even with all else being equal.
There are also a few tricks engineers have found with engine design to help with this. One example is an offset crankshaft, where TDC doesn't occur at 0° crank rotation. While it results in an unbalanced engine, it helps sidewall loading on the combustion stroke, but more importantly, it offsets ignition timing so that much less negative work happens. Downsides include a lot more vibration and harshness, and the need for much lower redlines. Possibly engines built this way are also less able to take high boost, though I'm not certain.
There are probably some things I'm forgetting, and some things I haven't explained well, so I might post a bit more later if I think of anything.