5 ms·

>more fuel means less compression is tolerated before knocking Generally, no. Knocking happens from pre-detonation, that's usually caused by heat from compres

>more fuel means less compression is tolerated before knocking Generally, no. Knocking happens from pre-detonation, that's usually caused by heat from compression causing the fuel/air mix to ignite before it's triggered by spark. To avoid this engines will run a fuel/air mix that is not stoicheometrically ideal, to make the mixture less likely to ignite early. It is safer to run a fuel rich mix than to run a fuel lean mix as it keeps combustion chamber pressures low (unburned fuel takes heat out of the exhaust). It is more economical and more ecologically friendly to run fuel lean since you paid for that unburned fuel and it's kinda gross. In general, more fuel than ideal means more resistance to knock. But these things are complex. EDIT: Knocking happens from pre-detonation. Knocking can also happen from predestination, like in the case of turbocharged Subarus.

to11mtm 2y ago
> EDIT: Knocking happens from pre-detonation. Knocking can also happen from predestination, like in the case of turbocharged Subarus. Edit of the week IMO.
MrDunham 2y ago
Friendly correction for others because your auto correct failed you... "predestination", pretty sure parent meant "pre-detonation" A.k.a. Autoignition aka "it goes boom before you planned on it" Only adding this as it's a pretty crucial word for understanding the comment.
- rpmisms 2y ago
  That fuel is totally depraved, and has no hope of salvation.
  - cout 2y ago
    This is why TULIP oil makes a great fuel additive.
    - rpmisms 2y ago
      I totally, unconditionally, agree. My own limits do not account for the irresistibility of this combination. I hope you persevere with this message.
- i_am_jl 2y ago
  Ah you're right, thank you!
magicalhippo 2y ago
Ah yes, read that engine knock article the wrong way around, guess my mind was drawing on intuition from things like gun powder where more stuff crammed more tightly together is worse. So then, if they're doing a non-ideal initial burn it would have to be a lean lower-compression burn, followed up by a higher-compression secondary burn?
- i_am_jl 2y ago
  I am sure that I don't understand the specifics of what Porsche is doing, but the article says the cycle is intake-compression-power-compression-power-exhaust. For that to be the case they have to be burning the same fuel and air mixture twice, and I don't understand the chemistry there enough to even speculate how it works. I think it's safe to assume that the second stroke is burning incomplete combustion products left over from the first stroke. I think that the second compression stroke would have to be higher compression than the first in order to get more complete combustion of what's left behind.
  - Ccecil 2y ago
    That actually makes a lot of sense merging with some comments from above. Second stroke goes lower and there are some ports to add air which are not accessed during the initial compression stroke...so the stroke is longer (higher compression) and more air is added to help with the reburn. Kinda sounds like combining the idea of the Miller cycle with a variable compression/stroke setup (see Nissan). There are a lot of ideas out there that create gains individually...glad to see them being combined more and more in modern engines. (ex. VRIS, VVT, DFI,) I personally think there is still another few decades of playing around with ICE to be done...not sure it will be viable for the market...but the research will lead to a lot more interesting engineering.
    - lloeki 2y ago
      > Miller cycle Heh all along I'm wondering, what kind of thermodynamic cycle is it? By six strokes, does it mean there are actually distinct new phases to the PV graphs compared to Atkinson/Carnot/Miller? Or is it just masquerading a well-known cycle underneath six strokes, only some parts are being optimised?
hatsunearu 2y ago
>Knocking happens from pre-detonation, that's usually caused by heat from compression causing the fuel/air mix to ignite before it's triggered by spark. No, it usually happens because the normal flamefront from the spark causes a rise in pressure that triggers compression-ignition in other parts of the cylinder. It's not solely from the compression, usually. That scenario is rare primarily because as you reduce the knock margin, you'd hit knock from what I said before you get to the state where it's so bad it ignites from compression alone. https://www.researchgate.net/figure/In-cylinder-pressure-trace-for-regular-and-knocking-combustion-from-Heywood-1988_fig7_282849935 https://www.researchgate.net/figure/In-cylinder-pressure-tra... Look at this picture; this is a typical waveform of cylinder pressure vs. crank angle. The spark happens 28 degrees before TDC, so basically the left edge of each of the graphs. As the flamefront consumes the air-fuel mixture inside the cylinder after the spark, the cylinder pressure gradually rises. During knock events, the cylinder pressure as risen by the normal combustion process gets to a point where it starts igniting the fuel elsewhere in the cylinder, away from the gradually expanding flamefront. This causes rapid combustion which causes the pressure to rise suddenly, which causes damage to the engine (if severe enough)
cout 2y ago
This edit is brilliant!
mattpallissard 2y ago
> caused by heat from compression causing the fuel/air mix to ignite before it's triggered by spark. I would think the usual cause is the timing was off and the spark plug fires before the cylinder reaches TDC.

to11mtm 2y ago

> EDIT: Knocking happens from pre-detonation. Knocking can also happen from predestination, like in the case of turbocharged Subarus. Edit of the week IMO.

MrDunham 2y ago

Friendly correction for others because your auto correct failed you... "predestination", pretty sure parent meant "pre-detonation" A.k.a. Autoignition aka "it goes boom before you planned on it" Only adding this as it's a pretty crucial word for understanding the comment.

rpmisms 2y ago
That fuel is totally depraved, and has no hope of salvation.
- cout 2y ago
  This is why TULIP oil makes a great fuel additive.
  - rpmisms 2y ago
    I totally, unconditionally, agree. My own limits do not account for the irresistibility of this combination. I hope you persevere with this message.
i_am_jl 2y ago
Ah you're right, thank you!

magicalhippo 2y ago

Ah yes, read that engine knock article the wrong way around, guess my mind was drawing on intuition from things like gun powder where more stuff crammed more tightly together is worse. So then, if they're doing a non-ideal initial burn it would have to be a lean lower-compression burn, followed up by a higher-compression secondary burn?

i_am_jl 2y ago
I am sure that I don't understand the specifics of what Porsche is doing, but the article says the cycle is intake-compression-power-compression-power-exhaust. For that to be the case they have to be burning the same fuel and air mixture twice, and I don't understand the chemistry there enough to even speculate how it works. I think it's safe to assume that the second stroke is burning incomplete combustion products left over from the first stroke. I think that the second compression stroke would have to be higher compression than the first in order to get more complete combustion of what's left behind.
- Ccecil 2y ago
  That actually makes a lot of sense merging with some comments from above. Second stroke goes lower and there are some ports to add air which are not accessed during the initial compression stroke...so the stroke is longer (higher compression) and more air is added to help with the reburn. Kinda sounds like combining the idea of the Miller cycle with a variable compression/stroke setup (see Nissan). There are a lot of ideas out there that create gains individually...glad to see them being combined more and more in modern engines. (ex. VRIS, VVT, DFI,) I personally think there is still another few decades of playing around with ICE to be done...not sure it will be viable for the market...but the research will lead to a lot more interesting engineering.
  - lloeki 2y ago
    > Miller cycle Heh all along I'm wondering, what kind of thermodynamic cycle is it? By six strokes, does it mean there are actually distinct new phases to the PV graphs compared to Atkinson/Carnot/Miller? Or is it just masquerading a well-known cycle underneath six strokes, only some parts are being optimised?

hatsunearu 2y ago

>Knocking happens from pre-detonation, that's usually caused by heat from compression causing the fuel/air mix to ignite before it's triggered by spark. No, it usually happens because the normal flamefront from the spark causes a rise in pressure that triggers compression-ignition in other parts of the cylinder. It's not solely from the compression, usually. That scenario is rare primarily because as you reduce the knock margin, you'd hit knock from what I said before you get to the state where it's so bad it ignites from compression alone. https://www.researchgate.net/figure/In-cylinder-pressure-trace-for-regular-and-knocking-combustion-from-Heywood-1988_fig7_282849935 https://www.researchgate.net/figure/In-cylinder-pressure-tra... Look at this picture; this is a typical waveform of cylinder pressure vs. crank angle. The spark happens 28 degrees before TDC, so basically the left edge of each of the graphs. As the flamefront consumes the air-fuel mixture inside the cylinder after the spark, the cylinder pressure gradually rises. During knock events, the cylinder pressure as risen by the normal combustion process gets to a point where it starts igniting the fuel elsewhere in the cylinder, away from the gradually expanding flamefront. This causes rapid combustion which causes the pressure to rise suddenly, which causes damage to the engine (if severe enough)

cout 2y ago

This edit is brilliant!

mattpallissard 2y ago

> caused by heat from compression causing the fuel/air mix to ignite before it's triggered by spark. I would think the usual cause is the timing was off and the spark plug fires before the cylinder reaches TDC.