The Great Generational Divide
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Will somebody please tell me what LSPI is abbreviated for ?
Originally Posted By: bbhero
I believe PeterPolyol knows a whole lot more about this than just about 99% of people on the board here.
Agreed and if he were making it all up, then he's better than 99% of people on the planet at bullshitting! Thanks for the post PeterPolyol.
I believe PeterPolyol knows a whole lot more about this than just about 99% of people on the board here.
Agreed and if he were making it all up, then he's better than 99% of people on the planet at bullshitting! Thanks for the post PeterPolyol.
I think I will avoid buying a DI Turbo vehicle as long as I can. Technology has created a whole new set of problems to worry about. I appreciate the technology, or the lack there-of, in my '64 GTO.
Originally Posted By: Merkava_4
Will somebody please tell me what LSPI is abbreviated for ?
Low speed pre-ignition.
Will somebody please tell me what LSPI is abbreviated for ?
Low speed pre-ignition.
Originally Posted By: Ducked
Originally Posted By: KL31
I also wonder as Bigj16 has, if Low Speed Pre Ignition results due to so much fuel in the oil. Makes sense to me, flash point reduced and fuel present increases chance of pre ignition.
Could be. OTOH, fuel enrichment is used as an antidote to "traditional" detonation, and it appears that at least some LSPI conditions involve detonation.
The oil is, however, relatively low octane. I wonder if fuel thinning of the oil (along with high boost pressures) is allowing more oil to enter the combustion chamber, where it promotes detonation.
Perhaps some experimental support for both those guesses.
http://papers.sae.org/2014-01-1224
The Effect of Oil Intrusion on Super Knock in Gasoline Engine
"Most of oils have been proved to have higher cetane number than n-heptane dose, indicating that the intruded oil is very liable to auto-ignition in a boosted engine."
"The slightly diluted oil (75% oil + 25% gasoline, for example) causes the heaviest knock."
It is experimental support though, i.e. they injected oil and got pre-ignition and knock. Doesn't mean its significant in real life. Abstract doesn't give any clues as to quantities.
Originally Posted By: KL31
I also wonder as Bigj16 has, if Low Speed Pre Ignition results due to so much fuel in the oil. Makes sense to me, flash point reduced and fuel present increases chance of pre ignition.
Could be. OTOH, fuel enrichment is used as an antidote to "traditional" detonation, and it appears that at least some LSPI conditions involve detonation.
The oil is, however, relatively low octane. I wonder if fuel thinning of the oil (along with high boost pressures) is allowing more oil to enter the combustion chamber, where it promotes detonation.
Perhaps some experimental support for both those guesses.
http://papers.sae.org/2014-01-1224
The Effect of Oil Intrusion on Super Knock in Gasoline Engine
"Most of oils have been proved to have higher cetane number than n-heptane dose, indicating that the intruded oil is very liable to auto-ignition in a boosted engine."
"The slightly diluted oil (75% oil + 25% gasoline, for example) causes the heaviest knock."
It is experimental support though, i.e. they injected oil and got pre-ignition and knock. Doesn't mean its significant in real life. Abstract doesn't give any clues as to quantities.
Originally Posted By: Ducked
Originally Posted By: Ducked
Originally Posted By: KL31
I also wonder as Bigj16 has, if Low Speed Pre Ignition results due to so much fuel in the oil. Makes sense to me, flash point reduced and fuel present increases chance of pre ignition.
Could be. OTOH, fuel enrichment is used as an antidote to "traditional" detonation, and it appears that at least some LSPI conditions involve detonation.
The oil is, however, relatively low octane. I wonder if fuel thinning of the oil (along with high boost pressures) is allowing more oil to enter the combustion chamber, where it promotes detonation.
Perhaps some experimental support for both those guesses.
http://papers.sae.org/2014-01-1224
The Effect of Oil Intrusion on Super Knock in Gasoline Engine
"Most of oils have been proved to have higher cetane number than n-heptane dose, indicating that the intruded oil is very liable to auto-ignition in a boosted engine."
"The slightly diluted oil (75% oil + 25% gasoline, for example) causes the heaviest knock."
It is experimental support though, i.e. they injected oil and got pre-ignition and knock. Doesn't mean its significant in real life. Abstract doesn't give any clues as to quantities.
Most of the studies I have see point toward a oil/gasoline mixture as the fuel. The process is a little more cloudy.
Your post makes sense.
Originally Posted By: Ducked
Originally Posted By: KL31
I also wonder as Bigj16 has, if Low Speed Pre Ignition results due to so much fuel in the oil. Makes sense to me, flash point reduced and fuel present increases chance of pre ignition.
Could be. OTOH, fuel enrichment is used as an antidote to "traditional" detonation, and it appears that at least some LSPI conditions involve detonation.
The oil is, however, relatively low octane. I wonder if fuel thinning of the oil (along with high boost pressures) is allowing more oil to enter the combustion chamber, where it promotes detonation.
Perhaps some experimental support for both those guesses.
http://papers.sae.org/2014-01-1224
The Effect of Oil Intrusion on Super Knock in Gasoline Engine
"Most of oils have been proved to have higher cetane number than n-heptane dose, indicating that the intruded oil is very liable to auto-ignition in a boosted engine."
"The slightly diluted oil (75% oil + 25% gasoline, for example) causes the heaviest knock."
It is experimental support though, i.e. they injected oil and got pre-ignition and knock. Doesn't mean its significant in real life. Abstract doesn't give any clues as to quantities.
Most of the studies I have see point toward a oil/gasoline mixture as the fuel. The process is a little more cloudy.
Your post makes sense.
Originally Posted By: Ducked
Originally Posted By: PeterPolyol
Gasoline in autoignition combusts at a much quicker rate than diesel, so when it detonates at a low RPM (regardless of the reason) it'll do very bad things, always.
I dunno enough about this to say you're wrong, but I find the "always" in that statement quite surprising.
Actually, come to think on't, I find the rest of it surprising too.
I'm not sure of the significance of the "much quicker rate" in "Gasoline in autoignition combusts at a much quicker rate than diesel" but diesel clearly has a much lower octane rating than petrol, so its much more likely to autoignite.
I THINK autoignite in this context is pretty much a synonym for detonation, but the terms get used so inconsistently that I can't be sure.
So you SEEM to be saying that petrol detonates more violently than diesel would, in the same engine, even though petrol has a higher octane rating.
That's surprising, if its true.
Originally Posted By: PeterPolyol
Gasoline in autoignition combusts at a much quicker rate than diesel, so when it detonates at a low RPM (regardless of the reason) it'll do very bad things, always.
I dunno enough about this to say you're wrong, but I find the "always" in that statement quite surprising.
Actually, come to think on't, I find the rest of it surprising too.
I'm not sure of the significance of the "much quicker rate" in "Gasoline in autoignition combusts at a much quicker rate than diesel" but diesel clearly has a much lower octane rating than petrol, so its much more likely to autoignite.
I THINK autoignite in this context is pretty much a synonym for detonation, but the terms get used so inconsistently that I can't be sure.
So you SEEM to be saying that petrol detonates more violently than diesel would, in the same engine, even though petrol has a higher octane rating.
That's surprising, if its true.
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Detonation is not a controlled flame front. It is explosive combustion. Gasoline is known to do this - go bang all at once - under certain conditions ...
Diesel ignites and burns through the injection cycle. It can not explode because it never has a fuel/air mixture in a cloud filling the whole cylinder ready to go anytime conditions permit ... Of course it has a cloud, but it's smaller, and fed by the injection event.
Diesel engines do not have an air throttle. They are full air flow all the time. The only limit on combustion power is the amount of fuel fed in. Small fuel, idle - max power, max fuel. The whole engine is controlled by the injection event.
Not sure direct injection gas is the same. It still relies on a spark, so that means there is injection and air mixing before the spark event ... That is a situation ready for detonation ...
Diesel ignites and burns through the injection cycle. It can not explode because it never has a fuel/air mixture in a cloud filling the whole cylinder ready to go anytime conditions permit ... Of course it has a cloud, but it's smaller, and fed by the injection event.
Diesel engines do not have an air throttle. They are full air flow all the time. The only limit on combustion power is the amount of fuel fed in. Small fuel, idle - max power, max fuel. The whole engine is controlled by the injection event.
Not sure direct injection gas is the same. It still relies on a spark, so that means there is injection and air mixing before the spark event ... That is a situation ready for detonation ...
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Originally Posted By: bigj_16
Low speed pre-ignition.
Thank You !
Low speed pre-ignition.
Thank You !
Pre-ignition and detonation are different phenomena, both involving abnormal combustion. Pre-ignition is combustion that occurs PRIOR to the spark plug firing, and detonation is abnormal combustion that occurs AFTER the spark plug fires and normal combustion has already commenced.
I see and hear the two terms used interchangeably frequently, and it's important to note that the causes and results of each phenomena are different.
I see and hear the two terms used interchangeably frequently, and it's important to note that the causes and results of each phenomena are different.
ZDDP is a quencher of LSPI and it's effects are exponential - so a little extra goes a long way in reducing LSPI, while Calcium detergent reduction is only linear in it's effects. (See the link Virtus_Probi gave on the first page).
Typical A3/B4 Euro synthetic oils like M1 or Edge 0W40 have ~ 1000 ppm zinc, and they are not exhaust cat killers, as MB, Audi and BMW have been running (and burning) these oils for years. They are thin 40 grades and a bit of fuel dilution will make them a 30 grade, so a bit of a viscosity buffer built in. Anything MB 229.5 rated has a low volatility of below 10%, plus B4 is a modern diesel rating - so soot handling is part of the oil design.
For cars with anti-knock sensors, it seems to be that fuel dilution happens more with low octane fuel when the ignition advance is reduced and maybe the fueling runs a touch richer. The problems appears less (from reports here) with higher octane fuel. Less gas in the oil sump means less gas/oil distillation effects and less oil going back into your fuel system.
Also the big thing about LSPI is that it happens a low engine speeds (low RPM). These auto boxes that race to the top gear and hold it there as long as possible are not doing anybody any favours with respect to LSPI. My auto has a "sport" mode which holds the gears for longer going up and drops the gears sooner going down. Much like I would do, if driving a manual transmission car.
If I had a TDI engine then I would run a high ZDDP A3/B4 Euro engine oil, run quality high octane fuel and keep the revs up when driving. Just like I have always done for all of my performance vehicles.
I think the big problem is when you get a TDI engine and put it in an economy car where people run economy fuel and fuel economy focused oil with a auto gearbox tuned for fuel economy.
Do high performance Euro sports cars suffer LSPI issues ? Probably, but the chances are you will never see it, as there is a different mindset to owning and driving these. We grabbed the engine design, but not the rest that goes with the engine.
Typical A3/B4 Euro synthetic oils like M1 or Edge 0W40 have ~ 1000 ppm zinc, and they are not exhaust cat killers, as MB, Audi and BMW have been running (and burning) these oils for years. They are thin 40 grades and a bit of fuel dilution will make them a 30 grade, so a bit of a viscosity buffer built in. Anything MB 229.5 rated has a low volatility of below 10%, plus B4 is a modern diesel rating - so soot handling is part of the oil design.
For cars with anti-knock sensors, it seems to be that fuel dilution happens more with low octane fuel when the ignition advance is reduced and maybe the fueling runs a touch richer. The problems appears less (from reports here) with higher octane fuel. Less gas in the oil sump means less gas/oil distillation effects and less oil going back into your fuel system.
Also the big thing about LSPI is that it happens a low engine speeds (low RPM). These auto boxes that race to the top gear and hold it there as long as possible are not doing anybody any favours with respect to LSPI. My auto has a "sport" mode which holds the gears for longer going up and drops the gears sooner going down. Much like I would do, if driving a manual transmission car.
If I had a TDI engine then I would run a high ZDDP A3/B4 Euro engine oil, run quality high octane fuel and keep the revs up when driving. Just like I have always done for all of my performance vehicles.
I think the big problem is when you get a TDI engine and put it in an economy car where people run economy fuel and fuel economy focused oil with a auto gearbox tuned for fuel economy.
Do high performance Euro sports cars suffer LSPI issues ? Probably, but the chances are you will never see it, as there is a different mindset to owning and driving these. We grabbed the engine design, but not the rest that goes with the engine.
Originally Posted By: LEONAR
Pre-ignition and detonation are different phenomena, both involving abnormal combustion. Pre-ignition is combustion that occurs PRIOR to the spark plug firing, and detonation is abnormal combustion that occurs AFTER the spark plug fires and normal combustion has already commenced.
I see and hear the two terms used interchangeably frequently, and it's important to note that the causes and results of each phenomena are different.
This is partly true, and describes the "classic" situation, but logically the two are not mutually exclusive, since one definition is purely a matter of timing.
I think in LSPI "practice", at least sometimes, detonation can be initiated by pre-ignition instead of the timed spark event, and therefore can and does happen before it.
Detonation is normally restricted to close to TDC because it requires faily high compression, but the danger zone is likely to be widened by turbo or supercharging boost. Pre-ignition can happen anytime after fuelling which is partly why its more dangerous.
Pre-ignition and detonation are different phenomena, both involving abnormal combustion. Pre-ignition is combustion that occurs PRIOR to the spark plug firing, and detonation is abnormal combustion that occurs AFTER the spark plug fires and normal combustion has already commenced.
I see and hear the two terms used interchangeably frequently, and it's important to note that the causes and results of each phenomena are different.
This is partly true, and describes the "classic" situation, but logically the two are not mutually exclusive, since one definition is purely a matter of timing.
I think in LSPI "practice", at least sometimes, detonation can be initiated by pre-ignition instead of the timed spark event, and therefore can and does happen before it.
Detonation is normally restricted to close to TDC because it requires faily high compression, but the danger zone is likely to be widened by turbo or supercharging boost. Pre-ignition can happen anytime after fuelling which is partly why its more dangerous.
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Hey guys. I'll try to spare you all the yapping and word soup so here goes
Originally Posted By: Ducked
The general drift of apparently informed comment seems to be that engines will often be quite tolerant of detonation, (as opposed to pre-ignition).
Pre-ignition refers to timing only. Normally, combustion begins at the firing of the spark plug.
Detonation OTOH refers to a combustion mode, and is the other of two main gasoline combustion modes (the first one being 'normal combustion'). Detonation can occur before or after spark timing.
Normally, we need the charge to burn in a progressive manner, starting with the flame kernel around the plug and expanding outward to the edges. I'll call this "flame-front" combustion and it is a separate kind of combustion than detonation. Conventional gasoline engines are meant to operate on flame-front combustion only.
Detonation decribes a condition where the active charge simultaneously experiences auto-ignition. That means the entire mixture detonates all at once, not progressively. So instead of a hot-spot triggering normal flame-front combustion prematurely as in Pre-ignition, the heat and pressure of compression "detonates" the entire thing all at once like dynamite, like an IDI diesel engine. As I mentioned eariler, gasoline in detonation "pops" much quicker and brutally than diesel in detonation. An old school HCCI IDI diesel engine can hold up to burning (detonating) diesel, but if you ran it on gasoline it would disintegrate. This is noteworthy when discussing the practical challenges of SCCI. Burning gasoline like diesel in ANY engine is no walk in the park.
Originally Posted By: Ducked
The general drift of apparently informed comment seems to be that engines will often be quite tolerant of detonation, (as opposed to pre-ignition).
Pre-ignition refers to timing only. Normally, combustion begins at the firing of the spark plug.
Detonation OTOH refers to a combustion mode, and is the other of two main gasoline combustion modes (the first one being 'normal combustion'). Detonation can occur before or after spark timing.
Normally, we need the charge to burn in a progressive manner, starting with the flame kernel around the plug and expanding outward to the edges. I'll call this "flame-front" combustion and it is a separate kind of combustion than detonation. Conventional gasoline engines are meant to operate on flame-front combustion only.
Detonation decribes a condition where the active charge simultaneously experiences auto-ignition. That means the entire mixture detonates all at once, not progressively. So instead of a hot-spot triggering normal flame-front combustion prematurely as in Pre-ignition, the heat and pressure of compression "detonates" the entire thing all at once like dynamite, like an IDI diesel engine. As I mentioned eariler, gasoline in detonation "pops" much quicker and brutally than diesel in detonation. An old school HCCI IDI diesel engine can hold up to burning (detonating) diesel, but if you ran it on gasoline it would disintegrate. This is noteworthy when discussing the practical challenges of SCCI. Burning gasoline like diesel in ANY engine is no walk in the park.
Originally Posted By: Ducked
So you SEEM to be saying that petrol detonates more violently than diesel would, in the same engine, even though petrol has a higher octane rating.
That's surprising, if its true.
It's counter-intuitive at first glace, yes but shouldn't be surprising. Octane/AKI and Cetane ratings only describe the auto-ignition temperature of the fuel, not other properties like volatility and reactivity. Until SCCI hits the market, gasoline engines should never experience autoignition of the gasoline, but since gasoline is blended to an AKI spec, there are limits on the resistance to autoignition and those limits are sometimes crossed in the real world.
The auto-ignition aspect of LSPI seems to be implicating high-cetane/low-octane species mixing with the fuel charge, like engine oil.
So you SEEM to be saying that petrol detonates more violently than diesel would, in the same engine, even though petrol has a higher octane rating.
That's surprising, if its true.
It's counter-intuitive at first glace, yes but shouldn't be surprising. Octane/AKI and Cetane ratings only describe the auto-ignition temperature of the fuel, not other properties like volatility and reactivity. Until SCCI hits the market, gasoline engines should never experience autoignition of the gasoline, but since gasoline is blended to an AKI spec, there are limits on the resistance to autoignition and those limits are sometimes crossed in the real world.
The auto-ignition aspect of LSPI seems to be implicating high-cetane/low-octane species mixing with the fuel charge, like engine oil.
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Originally Posted By: BrocLuno
Detonation is not a controlled flame front. It is explosive combustion. Gasoline is known to do this - go bang all at once - under certain conditions ...
Diesel ignites and burns through the injection cycle. It can not explode because it never has a fuel/air mixture in a cloud filling the whole cylinder ready to go anytime conditions permit ... Of course it has a cloud, but it's smaller, and fed by the injection event.
Diesel engines do not have an air throttle. They are full air flow all the time. The only limit on combustion power is the amount of fuel fed in. Small fuel, idle - max power, max fuel. The whole engine is controlled by the injection event.
Not sure direct injection gas is the same. It still relies on a spark, so that means there is injection and air mixing before the spark event ... That is a situation ready for detonation ...
Sorry broc I think I repeated what you correctly said. You're right.
LEONAR - Right on
Ducked, your last post pretty much covers it.
Exactly right in that pre-igntion is only a matter of timing.
MCompact - Nice one
SR5 - Great info. I'm exicted about the new oil formulas
Detonation is not a controlled flame front. It is explosive combustion. Gasoline is known to do this - go bang all at once - under certain conditions ...
Diesel ignites and burns through the injection cycle. It can not explode because it never has a fuel/air mixture in a cloud filling the whole cylinder ready to go anytime conditions permit ... Of course it has a cloud, but it's smaller, and fed by the injection event.
Diesel engines do not have an air throttle. They are full air flow all the time. The only limit on combustion power is the amount of fuel fed in. Small fuel, idle - max power, max fuel. The whole engine is controlled by the injection event.
Not sure direct injection gas is the same. It still relies on a spark, so that means there is injection and air mixing before the spark event ... That is a situation ready for detonation ...
Sorry broc I think I repeated what you correctly said. You're right.
LEONAR - Right on
Ducked, your last post pretty much covers it.
Exactly right in that pre-igntion is only a matter of timing.
MCompact - Nice one
SR5 - Great info. I'm exicted about the new oil formulas
Originally Posted By: PimTac
Thanks for the informative post. Your reference to SCCI sparked my curiosity. I have read that Mazda’s new SkyActiv X engine which utilizes SCCI will have up to 18:1 compression. They are also going with a supercharger instead of a turbocharger. Are you saying that LSPI will not be a issue with this engine?
I'd say LSPI should not be an issue. Considering the engine will be switching between conventional flame-front ignition and SCCI means that during SCCI, detonation of a very lean mixture will actually be intended! The ECU that controls it will be so fast, that it can make several changes if it had to during the course of a fraction of a stroke.
The SCCI part works by staging the injectors very precisely. The fuel needed for the one stroke will be divided up into several injection events, sort of like how modern CR diesels do but for different purposes.
CR Diesels inject, let's say two pilot injections, and main injection and post injections to produce a desired 'mean combustion pressure'. With CRDI diesels, the high cetane fuel begins to burn the moment it emerges from the injector, thus the use of staged injections to achieve a smoother "pressure curve".
With SCCI, the focus of mutiple staged injections is solely on placing the fuel exactly where it needs to be. So there are some similarities to PCCI as well with an initial (lean) injection during the intake stroke, this allows time for air fuel mixture to occur. There are other injections too, but I can't remember the purpose off the top right now, but as I recall some are to drive sprayed fuel into a tumble flow or something.
Anyway, the magical final injection and 'spark-controlled' detonation of the lean mixture occurs very tightly around TDC and timing this right so that the resultant combustion pressure peak happens at the right time is the real magic.
In SCCI mode, right around TDC, the direct injector shoots a tiny little bit of fuel around the spark plug to make a small volume in the immediate area around the electrode a stoich, reliably ignited mixture, surrounded by a very lean 'cusion' mixture which has been brought (by the ECU via boost pressure, valve timing) right to the threshold of auto-ignition. So once that small stoich area aroun the plug lights, the total cylinder pressure is bumped up just over the auto-ignition point of the lean mix and boom goes the dynamite. So as you can see, they've turned the horrible, nightmarish detonation into an actual normal operating mode! The amount of reactive fuel charge is carefully measured to produce the desired torque, and so there really can't be LSPI and subsequent destructive detonation because the fuel is so lean.
The only way LSPI could occur (and this is a stretch) maybe an excess of oil or high cetane species alters the auto-ignition temp of the lean AF mixture- which I'd imagine has to be pretty significant. And then again, an appreciable amount of gasoline would not be present to make it a problem, so I still don't think it will happen but it's possible I suppose.
The ECUs on these things are supposed to use ion sensing extensively to monitor combustion quality/speed and where the peak cylinder pressure occurs, because the computer MUST determine the rate of burn of the current fuel mix.
Someone may be asking "why bother with the convolusions and complication? why don't they just lean burn and be done with it?"
Well that's because of NOx emissions and ignition reliability. Lean mixtures don't ignite very easily or reliably, thus the employment of a high static compression ratio which can be called upon when required. Also, a true HCCI gasoline engine using heat and pressure as the sole ignition source is extremely hard to control detonation timing on. The CRDI diesel mode (using injection as timing - fuel burning the instant it's sprayed) would bring the diesel engine problems with it; not allow combustion temps to be controlled, not allow the AF mixture to, well, mix properly resulting in soot and partial combustion, of yeah and would require an engine to be built heavier than a diesel! So that's not happening.
SCCI allows the lean AF mixture to be brought reasonable close to autoignition compression temp/pressure with a tiny flame kernel to push it over the top, and a very small amount of gasoline to be used to achieve good, mean combustion pressures
If anyone is as excited as I am you can follow the SCCI technical progress here
http://stks.freshpatents.com/Mazda-Motor-Corporation-nm1.php
*I have not reviewed any of the materials today so there may be inaccuracy
Thanks for the informative post. Your reference to SCCI sparked my curiosity. I have read that Mazda’s new SkyActiv X engine which utilizes SCCI will have up to 18:1 compression. They are also going with a supercharger instead of a turbocharger. Are you saying that LSPI will not be a issue with this engine?
I'd say LSPI should not be an issue. Considering the engine will be switching between conventional flame-front ignition and SCCI means that during SCCI, detonation of a very lean mixture will actually be intended! The ECU that controls it will be so fast, that it can make several changes if it had to during the course of a fraction of a stroke.
The SCCI part works by staging the injectors very precisely. The fuel needed for the one stroke will be divided up into several injection events, sort of like how modern CR diesels do but for different purposes.
CR Diesels inject, let's say two pilot injections, and main injection and post injections to produce a desired 'mean combustion pressure'. With CRDI diesels, the high cetane fuel begins to burn the moment it emerges from the injector, thus the use of staged injections to achieve a smoother "pressure curve".
With SCCI, the focus of mutiple staged injections is solely on placing the fuel exactly where it needs to be. So there are some similarities to PCCI as well with an initial (lean) injection during the intake stroke, this allows time for air fuel mixture to occur. There are other injections too, but I can't remember the purpose off the top right now, but as I recall some are to drive sprayed fuel into a tumble flow or something.
Anyway, the magical final injection and 'spark-controlled' detonation of the lean mixture occurs very tightly around TDC and timing this right so that the resultant combustion pressure peak happens at the right time is the real magic.
In SCCI mode, right around TDC, the direct injector shoots a tiny little bit of fuel around the spark plug to make a small volume in the immediate area around the electrode a stoich, reliably ignited mixture, surrounded by a very lean 'cusion' mixture which has been brought (by the ECU via boost pressure, valve timing) right to the threshold of auto-ignition. So once that small stoich area aroun the plug lights, the total cylinder pressure is bumped up just over the auto-ignition point of the lean mix and boom goes the dynamite. So as you can see, they've turned the horrible, nightmarish detonation into an actual normal operating mode! The amount of reactive fuel charge is carefully measured to produce the desired torque, and so there really can't be LSPI and subsequent destructive detonation because the fuel is so lean.
The only way LSPI could occur (and this is a stretch) maybe an excess of oil or high cetane species alters the auto-ignition temp of the lean AF mixture- which I'd imagine has to be pretty significant. And then again, an appreciable amount of gasoline would not be present to make it a problem, so I still don't think it will happen but it's possible I suppose.
The ECUs on these things are supposed to use ion sensing extensively to monitor combustion quality/speed and where the peak cylinder pressure occurs, because the computer MUST determine the rate of burn of the current fuel mix.
Someone may be asking "why bother with the convolusions and complication? why don't they just lean burn and be done with it?"
Well that's because of NOx emissions and ignition reliability. Lean mixtures don't ignite very easily or reliably, thus the employment of a high static compression ratio which can be called upon when required. Also, a true HCCI gasoline engine using heat and pressure as the sole ignition source is extremely hard to control detonation timing on. The CRDI diesel mode (using injection as timing - fuel burning the instant it's sprayed) would bring the diesel engine problems with it; not allow combustion temps to be controlled, not allow the AF mixture to, well, mix properly resulting in soot and partial combustion, of yeah and would require an engine to be built heavier than a diesel! So that's not happening.
SCCI allows the lean AF mixture to be brought reasonable close to autoignition compression temp/pressure with a tiny flame kernel to push it over the top, and a very small amount of gasoline to be used to achieve good, mean combustion pressures
If anyone is as excited as I am you can follow the SCCI technical progress here
http://stks.freshpatents.com/Mazda-Motor-Corporation-nm1.php
*I have not reviewed any of the materials today so there may be inaccuracy
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Thank you for that detailed and informative response. I do follow the Mazda process somewhat but I am far from an expert, just an interested consumer.
The computer that runs the awd system in my CX-5 supposedly makes 200 decisions a second. It’s all fascinating stuff.
The computer that runs the awd system in my CX-5 supposedly makes 200 decisions a second. It’s all fascinating stuff.
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