Pondering about flathead potentials

[Nikolas Ojala]

This is my interpretation about how a plasma plug works.

plasma_plug.png

• V1 The voltage supply of the ignition system, typically battery.
• R1 Ballast resistance, limiting the current in the L1 primary winding.
• break This device may be mechanical contact, but nowadays electronic semiconductor device.
• C1 This capacitor saves the break device, but in electronic ignition systems there are different circuits.
• L1 The coil stores sufficient spark energy. Once the break device breaks the primary circuit, the coil sends the stored energy to the spark plug.
• R2 Parasitic resistance of the cable. In a good cable this is nearly zero, but there are no zero resistance cables.
• L2 Parasitic inductance of the cable. The inductance is very small.
• R3 In a plasma plug this resistance is optimized. It is small but also it is bigger than parasitic R2 in a decent cable. The idea is that R3 would dominate, not R2.
• C2 In a ordinary spark plug this is nearly zero, or insigificant anyway. In a plasma plug this has been optimized so that C2 will store majority of spark energy.
• R4 This parasitic resistance is extremely small, intentionally.
• L3 This parasitic inductance is also very small.
• gap The spark gap is nonlinear circuit element. It is like a politician who says first "No never no no I said NO" but under sufficiently high stress suddenly "YES I said yes all the time." So the resistance of the gap changes first practically infinite, then sudddenly very low.

The idea of a plasma plug is to release spark energy in few nanoseconds, not milliseconds. This way the spark is very bright and emits ultraviolet light that ionises gas in the combustion chamber. Ideally it would ignite the whole fuel charge quickly, pretty much resembling the combustion in a HCCI engine, although the ignition is very different.

[pdq67]

Is it possible to direct inject gasoline with CR's in the diesel engine range and still hold detonation at bay??

The plasma ignition stuff got me to thinking is all and this would pertain to flatheads!

Is it possible to control the combustion of gasoline in such a way that it would push like black powder vs blow up like dynamite and detonate thus cracking pistons and rings and such..

And I really like the idea of the instantaneous combustion everywhere in the huge flathead combustion chamber using new ignition and HCCI technologies.

pdq67

[Circlotron]

The idea of a plasma plug is to release spark energy in few nanoseconds, not milliseconds. This way the spark is very bright and emits ultraviolet light that ionises gas in the combustion chamber.

Also propagates a shockwave a short distance ahead of the mixture about to be burnt.
I read those plasma ignitions make a bit of a difference in non-turbulent mixtures like you would find in a laboratory combustion test "bomb". Turbulent mixtures like on a real running engine, not as much of a difference. Would be interesting to try on a flathead though, for sure.

[Nikolas Ojala]

Is it possible to direct inject gasoline with CR's in the diesel engine range and still hold detonation at bay??

That sounds like partially premixed combustion, so I would say yes it is possible, although I have serious doubts about that possibility in a flathead.

Is it possible to control the combustion of gasoline in such a way that it would push like black powder vs blow up like dynamite and detonate thus cracking pistons and rings and such..

Before a detonation may happen in the combustion chamber the flame propagates slower than the pressure wave, and at some point in the combustion chamber where the flame has not reached yet, a Chapman–Jouguet condition occurs and detonation happens.

Now, just imagine that the ignition propagates faster than the speed of sound. That situation is fundamentally different than the condition that leads to detonation.

And I really like the idea of the instantaneous combustion everywhere in the huge flathead combustion chamber using new ignition and HCCI technologies.

Yes. Although it does not seem likely to make HCCI ever work in a flathead, there is still the possibility to improve spark ignition, either with a plasma plug, laser ignition or something else.

[Nikolas Ojala]

The idea of a plasma plug is to release spark energy in few nanoseconds, not milliseconds. This way the spark is very bright and emits ultraviolet light that ionises gas in the combustion chamber.

Also propagates a shockwave a short distance ahead of the mixture about to be burnt.

Light propagates at the speed of light. A shockwave propagates at the speed of sound.

I see an effective spark as a potential method to prevent detonation, although there is also a danger: Suppose that someone designed a bad engine that was prone to detonation. Then the bad design was saved by a superior ignition device. The engine runs fine as long as the ignition device works just fine.

[pdq67]

The idea of a plasma plug is to release spark energy in few nanoseconds, not milliseconds. This way the spark is very bright and emits ultraviolet light that ionises gas in the combustion chamber.

Also propagates a shockwave a short distance ahead of the mixture about to be burnt.

Light propagates at the speed of light. A shockwave propagates at the speed of sound.

I see an effective spark as a potential method to prevent detonation, although there is also a danger: Suppose that someone designed a bad engine that was prone to detonation. Then the bad design was saved by a superior ignition device. The engine runs fine as long as the ignition device works just fine.

But isn't this the way everything is supposed to work.

A dizzy breaks down and the engine quits running anyway, doesn't it? Fuel filter gets clogged up, etc., etc...

pdq67

[Nikolas Ojala]

I see an effective spark as a potential method to prevent detonation, although there is also a danger: Suppose that someone designed a bad engine that was prone to detonation. Then the bad design was saved by a superior ignition device. The engine runs fine as long as the ignition device works just fine.

But isn't this the way everything is supposed to work.

Yes, indeed. Just think about the HCCI I already hinted. A pure HCCI engine runs fine if the electronic control unit calculates everything right. If it doesn't the engine self-destructs. (Not quite like space ship Nostromo in the Alien movie, but anyway.)

[peejay]

I see an effective spark as a potential method to prevent detonation, although there is also a danger: Suppose that someone designed a bad engine that was prone to detonation. Then the bad design was saved by a superior ignition device. The engine runs fine as long as the ignition device works just fine.

But isn't this the way everything is supposed to work.

Yes, indeed. Just think about the HCCI I already hinted. A pure HCCI engine runs fine if the electronic control unit calculates everything right. If it doesn't the engine self-destructs. (Not quite like space ship Nostromo in the Alien movie, but anyway.)

Well, we'll see... Mazda is shipping them starting next year.

They still have spark plugs but they are mainly there for high load and as "backup" in case the compression ignition misfires.

There is a lot of emergent tech in these little babies! If I'm reading the press releases right, they are monitoring chamber pressure in real time to watch combustion speed/stability and making changes in real time as necessary. Think of the computing power required! And mass produced cylinder pressure sensors, the aftermarket will have a lot of fun with that!

[140Air]

The idea of a plasma plug is to release spark energy in few nanoseconds, not milliseconds. This way the spark is very bright and emits ultraviolet light that ionises gas in the combustion chamber.

Also propagates a shockwave a short distance ahead of the mixture about to be burnt.

Light propagates at the speed of light. A shockwave propagates at the speed of sound.

Sound travels at the speed of sound. A shock wave is not a sound wave.

[MadBill]

...and by definition, it travels faster than the S.O.S.

[Nikolas Ojala]

Anyway, if the ignition propagates faster than a shock wave could, that will spoil the otherwise possible detonation.

[140Air]

Anyway, if the ignition propagates faster than a shock wave could, that will spoil the otherwise possible detonation.

Hmm, what we want is a burn propagation that is slower than a shock wave! If ignition is caused by intense(!) UV traveling at the speed of light through the chamber we will get virtually an instantaneous reaction of the whole mixture. This will have an effect greater than setting off the gasoline/air mixture energy equivalent of a quantity of the highest explosive known.

[gruntguru]

HCCI is one example of combustion propagation faster than sound. The problems caused by detonation arise from the fact that the flame front is travelling AT the speed of sound. This creates a pressure front/shock wave that grows in slope and magnitude as it travels across the chamber. When it reaches the edge of the chamber it is violent enough to do mechanical damage and equally important - it breaks through the stagnant gas boundary layer and exposes metal to the burning gas.

So flame speed faster than Mach 1 is not as bat as a flame travelling at Mach 1.

[Nikolas Ojala]

I ran into a situation where there was a need for a super compact engine.
Out of curiosity I modeled up a flat head with various valve combinations.
I hoped it might do better but this is what I found:

With 1 intake and 1 exhaust at 10:1 the flow is down about 35% compared to a simple OHV chamber.
With 2 intake and 2 exhaust at 10:1 the flow is down about 35%
With 3 intake and 3 exhaust at 10:1 the flow is down about 45%
With 4 intake and 3 exhaust at 10:1 the flow is down about 50% Even with near 150% curtain area; throat area was small.

I appreciate your research effort and conclusion represented in clear numbers.

[vannik]

HCCI is one example of combustion propagation faster than sound.

Where did you get this info? My understanding is that at best HCCI ignites in more than one place simultaneously but at no time does the flame front exceed the local SoS.