Pop quiz: Why do boosted engines need a more powerful spark?

[James Snover]

I was going to post the pic, but I know what she looks like now.
Age has not been kind to her.

There is work being done, but it is by the other cylinders. Add in the lack of pressure due to the exhaust valve(s) opening.

The piston traveling on the up stroke does not have power compared to the other pistons rotating the crank.

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But again, WHY is it not making power at that point? What fact of life, the universe, and everything, makes it _physically impossible_ to produce power even though it is at it's highest temperatures and pressure?

 

[Coiled03]

Correct ... but _why_ is no work being done at this point? What ... principle ... prevents it? Prohibits it, in fact?

As I explained, at TDC when there's instantaneously no motion, there's no distance being traveled, thus no work being done. End of story.

If you're looking for the principle that's related to that, it's conservation of energy, aka the first law of thermodynamics, which states energy can neither be created, nor destroyed, it can only change states. All the kinetic energy of of the piston has been converted to heat energy, and potential energy.

 

[PhoenixM3]

I blame the democrats

I was gonna say collusion.....

 

[James Snover]

As I explained, at TDC when there's instantaneously no motion, there's no distance being traveled, thus no work being done. End of story.

If you're looking for the principle that's related to that, it's conservation of energy, aka the first law of thermodynamics, which states energy can neither be created, nor destroyed, it can only change states. All the kinetic energy of of the piston has been converted to heat energy, and potential energy.

Cloooose. Think mechanically.

 

[derklug]

You are pushing straight down on the crank at TDC, once the crank rolls over TDC you get angular momentum started. The most torque should be at 90 past TDC

 

[pdm]

The rod axis is at near zero distance from crank centerline.

 

[pdm]

Beat me to it. I got distracted

 

[gimmie11s]

Trunk space. Definitely lack of trunk space. That’s why.


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[08mojo]

Correct ... but _why_ is no work being done at this point? What ... principle ... prevents it? Prohibits it, in fact?

Others beat me to it...

W=f*d, in this case d=0 therefore W=0

Angular momentum, stored in the flywheel, keeps it going.

 

[Coiled03]

Cloooose. Think mechanically.

Uhh, no. I'm right.

EDIT: You asked for the principle that prevents it from doing work. That's what I explained. If you want to talk about the mechanics or kinematics of the system, that's a different question.

 

[earico]

Not just close, you're dead-on right! I'd expected this to take a lot longer, it usually does when I try it at parties.

Air is a great insulator. The more of it there is, the more difficult it is to form a spark. In WWII, they quickly found that as airplanes flew higher and higher, they had all kinds of arcing problems in the ignition systems. Below 5,000 feet, the engine ran like a bat out of hell; above 25,000 and the engine ran like a heart-patient on one lung. The answer was ... pressurized ignition harnesses! Taking a feed right off the supercharger, increasing the pressure surrounding the wires fixed the arcing.

Add in that gasoline (and alcohol) are very forgiving fuels. It takes a spark to make them burn, in most ordinary cases. No spark, no burn. (Unless it's the Chrysler dieseling gas-burning four bangers from the '80's, but that's a different story about poor combustion chamber engineering and ineffective cooling).

So, in a boosted engine, when the air-fuel charge is being compressed and then ignited, the pressure in the cylinder is now many times atmospheric pressure, and if you don't have a powerful enough ignition ... the spark is not able to even form. It doesn't get "blown out." It never even forms!

Another observation: it is even more difficult to make a spark in a fluid. There is a kind of medical machine called a lithotripter. The old ones used banks of gigantic, water-cooled capacitors as big an average man's chest (I kid you not), to make a spark form in water, framed against a specially-shaped parabloic dish, which converted that energy into a focused blast of a particular frequency that _just happened_ to be the resonant frequency of ... kidney stones! And also by luck, that particular burst of energy was largely (but not entirely!) unreactive to soft tissue of the kidneys. So they could break up kidney stones without surgery and let the body remove them on its own. Like xrays, it did do some harm to surrounding tissue, but nothing on the level of surgery! Modern lithotripters use staggering high voltage and high frequencies enabled by modern electronics to do the same thing. And I'm glad of it. Those huge old caps scared me. There were ways to handle them safely. But one mistake, one moment's inattention, and you'd be dead before you even felt any pain.

ANYHOW: That's why boosted engines need a more powerful spark, despite the fact the air-fuel charge in the combustion chamber wants to burn a hundred times more than when you're stuck in stop n' go traffic, idling. Without a spark, it just can't. And without a big shot of power, it can't make a spark.

Dad was a Hawk missile tech in the Army in Korea. When he got out he became an electrician and electronics geek. So growing up he taught me things and told me about how air gets ionized. Fascinating stuff. Love this kind of stuff, James. Thanks for elaborating!

 

[pdm]

And that’s why you can read the spark like with a scope and tell where the spark is occurring (inside or outside the combustion chamber) among other things

 

[CV355]

1) Air = Resistance. Denser Air = More Resistance
2) Sin(0) = 0 ; Sin(90) = 1 ; Zero vector of force to translate to torque at TDC

I always laughed at the "air blows out the spark under boost" myth.

 

[Tifosi2003GT]

Not reading this whole thread.. So the candle doesn't burn out?

 

[4.698gt]

So my related question to this whole matter is this the same principle that applies to coolant in the pressurized cooling system with the boiling point increasing as pressure increases?

 

[James Snover]

Uhh, no. I'm right.

EDIT: You asked for the principle that prevents it from doing work. That's what I explained. If you want to talk about the mechanics or kinematics of the system, that's a different question.

There is a term I’m looking for. Three of you (including you) have come so close to it, explained it correctly, even. But not named it.


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[Revvv]

There is a term I’m looking for. Three of you (including you) have come so close to it, explained it correctly, even. But not named it.


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My brain is tired. My list of physics terminology is shorter than usual today.

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[CV355]

There is a term I’m looking for. Three of you (including you) have come so close to it, explained it correctly, even. But not named it.

Because 100% adiabatic efficiency is impossible.

 

[08mojo]

There is a term I’m looking for. Three of you (including you) have come so close to it, explained it correctly, even. But not named it.


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Moment? As in the length of the moment arm, which is 0 and TDC.

 

[James Snover]

Moment? As in the length of the moment arm, which is 0 and TDC.

SO CLOSE!

I'm going to be tied up this evening, so, the answer is:

Mechanical advantage. Leverage would also have been acceptable.

Several of you described it perfectly correctly, and for all the right reasons.

Anyone want another one? OK. And it is related to the preceding question:

It is well known that in most modern reciprocating engines, the exhaust valve is almost always opened well before the piston hits BDC. If there is still pressure in the cylinder (and there is!), WHY do they open the valve and let the pressure escape before the piston has reached the end of its travel to BDC?