Latest / Mechanical Engineering Made Simple / (#99) The Myth of the Perfect Burn

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(#99) The Myth of the Perfect Burn · Mechanical Engineering Made Simple

• 0:00
  Welcome back to mechanical engineering made simple.
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  It's great to be back. Today I want to start by asking
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  everyone listening to do something for me.
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  Picture the engine in your car, or maybe a massive gas turbine
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  on a jet wing. When we think about what's
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  happening inside, we usually have this very sanitized high
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  school chemistry version of it. The textbook burn as we like to.
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  Call. It's this neat little equation.
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  You have fuel, you add air, you get a spark and poof, you get
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  heat, carbon dioxide and water. It's, you know, linear clean.
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  And it implies that if you just get that mixture right, you get
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  100% energy release and totally harmless exhaust.
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  Which is a lovely fiction. It really is.
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  If combustion were that simple, my job and the jobs of thousands
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  of combustion engineers, well, we just wouldn't exist because
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  the. Reality is less of a polite
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  handshake and more of a a riot. A battlefield, That's the word
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  for it. If you can zoom into the
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  molecular level inside that cylinder, you're not seeing a
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  one way street where reactants just become products.
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  No, you're seeing a chaotic state of what we call dynamic
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  equilibrium. And that concept, dynamic
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  equilibrium, that's really the heart of this deep dive, isn't?
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  It is. We're going to walk through the
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  story of a combustion reaction to answer some of the most
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  popular questions we see online. Specifically, why don't engines
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  get as hot as the math says they should?
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  And the big one, Where do all these nasty pollutants actually
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  come from if we're burning so-called clean fuel?
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  The answers you're saying boil down to two invisible forces.
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  Two invisible forces fighting against perfect efficiency.
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  First, you've got dissociation, which acts like an energy thief.
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  OK, the thief. And then you've got kinetics,
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  which is basically a cosmic speed limit.
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  All right, let's. Unpack this starting with the
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  thief dissociation. To get this we have to look at
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  equilibrium. Most of us remember the standard
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  equation, right? Carbon monoxide, Co plus oxygen,
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  O2. Makes carbon dioxide, CO2
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  stable. Done.
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  But that's the cold view. It implies that once CO2 forms,
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  it just stays that way. But inside an engine, we're
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  talking about temperatures pushing, you know, 2500, maybe
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  3000 Kelvin. That's incredibly hot.
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  At those energy levels, the product molecules themselves are
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  vibrating so violently that they start to shatter.
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  What The exhaust is kind of unburning itself.
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  In a way, yeah, that's a good way to put it.
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  The reaction actually becomes reversible.
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  CO2 splits back into carbon monoxide and oxygen, and water
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  vapor rips apart into hydroxyl radicals and hydrogen.
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  That process is dissociation, and it's all dictated by the
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  absolute grandfather of thermodynamics, Gibbs energy.
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  Right Gibbs, denoted by GI. Remember, the definition
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  involves enthalpy minus the temperature times entropy.
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  That's the one. And the key thing to remember is
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  that the universe is fundamentally lazy.
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  Any chemical system will always, always try to move toward the
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  state with the lowest possible Gibbs energy.
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  I always picture this is a ball rolling around a landscape of
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  hills and valleys. That's a perfect analogy.
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  The gas mixture is the ball and it's just looking for the
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  deepest valley it can find. And the composition of the gas,
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  how much fuel versus exhaust is the balls position?
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  Exactly. And the depth of the valley is
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  the energy state. So the mixture keeps reacting
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  forward, then backward, shifting its composition until it finds
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  the absolute bottom of that valley where the is 0.
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  That's the point where DG the change in Gibbs energy is 0.
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  That's equilibrium. Correct, but here's the catch
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  that trips people up that landscape.
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  It isn't static. The shape of the whole valley
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  changes depending on pressure and temperature.
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  So when you crank up the temperature in an engine, you're
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  basically warping the landscape. You're completely reshaving it.
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  The bottom of the valley moves, so at room temperature the
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  bottom might be mostly CO2, but at 3000 Kelvin the bottom shifts
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  to a mix of CO2, Co and oxygen. And what's the actual force
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  pushing the ball around this landscape?
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  That's something called chemical potential.
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  You can think of it like chemical pressure.
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  Just like air flows from high to low pressure, chemical species
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  flow from high chemical potential to low chemical
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  potential. The reactants are pushing to
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  become products, but at these high temperatures, the.
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  Products are pushing back. Precisely, and equilibrium is
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  just the ceasefire line where the chemical potential of the
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  reactants equals the chemical potential of the products.
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  OK, that's the theory, but let's apply the So what factor?
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  If I'm designing a Formula One engine, why do I care if 5 or
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  10% of my molecules decide to break up?
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  You care because it steals your power.
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  This is why we call it the energy thief.
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  Breaking those chemical bonds, turning CO2 back into Co and
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  oxygen is endothermic. It absorbs heat.
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  It absorbs huge amounts of it. It sucks thermal energy right
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  out of the explosion. So let's say you do a quick
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  calculation on a napkin. Assuming perfect combustion,
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  your calculator says, congratulations, you're going to
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  hit 2950 Kelvin. Which means a massive pressure
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  spike, which means lots of torque on the crankshaft.
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  Right, but then you run the actual engine and you only
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  measure 2800 Kelvin. You've lost one 150° off the
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  top. And that energy didn't just leak
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  out through the cylinder walls. No, it was consumed internally
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  by the molecules tearing themselves apart, and that's a
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  direct loss in peak pressure, which is a direct loss in work
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  output. That's a huge efficiency hit,
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  and I see in the notes there's a metric for this alpha, the
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  degree of dissociation. Yeah, alpha is just a fraction
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  of the original molecule that's broken down.
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  If alpha is 0, you've got a stable molecule.
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  If alpha is 1, it's completely disintegrated.
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  And engineers have some levers to pull here, right?
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  We're not completely helpless. We have two main livers,
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  pressure and temperature, and they work against each other.
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  OK, walk us through the pressure side first.
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  So think about what happens when things dissociate.
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  Let's say 2CO2 molecules breakdown.
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  They turn into two Co molecules plus one O 2 molecule.
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  So you started with two particles and you end up with
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  three. Exactly.
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  Dissociation increases the total number of moles in the gas.
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  It wants to take up more space. O applying Alicia Teles
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  principle, if you squeeze the system, increase the pressure,
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  it's going to fight back. It will try to relieve that
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  stress by shifting toward the side with fewer molecules, which
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  is the non dissociated side. So a high compression ratio in
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  an engine actually helps suppress dissociation.
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  It does high pressure kind of keeps the family together, but
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  temperature does the complete opposite.
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  It adds violent kinetic energy and encourages the breakup.
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  That is such a cruel trade off, isn't it?
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  The Carnot cycle tells us we want high temperatures for the
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  best theoretical efficiency, but those same high temps trigger
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  the dissociation that steals that very efficiency.
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  Welcome to engineering. It's never just one variable,
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  and there's a neat little nuance here in the data Water is
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  tougher than carbon dioxide. Meaning H2O doesn't break down
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  as easily. Right, in your typical
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  hydrocarbon combustion, the water molecules dissociate a lot
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  less than the CO2 molecules at the same temperature.
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  The water is just more stable. OK, so we can't just guess at
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  this. If I'm building a simulation, I
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  need a number. How do we calculate where this
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  balance point actually is? For that we use the equilibrium
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  constant, which we call K sub P It's a value that tells us the
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  exact ratio of products to reactants at a specific
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  temperature based on their partial pressures.
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  And the sources mentioned the Van Hoff equation here.
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  The Van Hoff equation is just a beautiful way to show how that
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  case of P constant changes with temperature.
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  It linearizes the problem. If you plot the natural log of
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  case of P against one over temperature, you get a straight
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  line. That sounds great for a chart,
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  but looking at the actual math, this seems really hard to
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  calculate by hand. You're ending up with cubic
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  equations and systems of non linear equations.
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  It's a total nightmare to do by hand.
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  It looks like the method is basically guess and check.
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  That's essentially what it is. It's an iterative loop.
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  You see, you can't solve for the final temperature directly
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  because the final composition depends on the temperature.
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  But the temperature depends on the heat release, which depends
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  on the composition. It's a circular reference.
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  Exactly. So you have to guess a final
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  temperature. Then you calculate K sub P for
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  that temperature. Find the composition.
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  Then you check the energy balance.
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  Did the heat released match your guess?
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  And it probably didn't. Almost never in the first try.
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  So you adjust your guess and you do the whole loop again and
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  again until it converges. I'm very glad we have software
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  like Matlab or Python to handle that iterative part now.
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  We are too, but you have to understand the logics to set it
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  up. But here's the thing.
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  Even with a perfect computer model, equilibrium has one fatal
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  flaw, which is it assumes you have infinite time.
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  It assumes the molecules can just hang out, bump into each
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  other, and comfortably settle into that lowest energy state.
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  And inside an engine cylinder, Comfortable is the last word I'd
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  use, right? Let's think about the clock.
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  If your engine is running at 6000 RPM, that's 100 revolutions
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  a second. The entire 4 stroke cycle
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  happens in 10 milliseconds. And the actual combustion event
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  maybe? Two to three milliseconds.
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  That is just insanely fast. It's faster than the chemistry
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  can keep up with sometimes. And that brings us to our second
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  invisible force, chemical kinetics.
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  The speed limit. Exactly.
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  Equilibrium tells you what would happen if you waited forever.
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  Kinetics tells you what actually happens in the three
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  milliseconds you've got. So thermodynamics is the
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  destination, but kinetics is the rush hour traffic.
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  I like that. That's good.
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  Reactions are a contact sport. They only happen when molecules
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  physically collide. So first you need a crowded
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  room. We call that molar
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  concentration. The higher the density, the more
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  collisions. But just bumping into each other
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  isn't enough, is it? No.
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  If they just gently bump, they bounce off.
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  Nothing happens. They need to smash into each
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  other with enough force to actually shatter their existing
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  bonds so new ones can form. And that minimum energy
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  threshold is the activation energy E.
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  Right, and this is all governed by the Arrhenius equation.
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  It is the famous rate constant K = a times E to the power of
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  negative E over RT. It looks scary, but you're
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  right, it tells a simple story. The A part is just how often
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  they collide. And the exponential term is the
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  gatekeeper. It tells you what fraction of
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  those collisions actually have enough energy to get over that
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  activation hurdle. And if the temperature T drops,
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  that exponential term crashes towards 0 incredibly fast.
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  And this the speed limit. This is where the story gets
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  darker, isn't it? Because this lack of time is the
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  origin story for most of our pollutants.
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  It is. Let's talk about thermal and
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  onitric oxide. OK, so in the air all around us,
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  nitrogen and oxygen are like strangers on a subway.
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  They're in the same space, but they totally ignore each other.
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  At room temperature, yes, the activation entity to break that
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  triple bonded nitrogen molecule is just massive.
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  But inside the engine at those peak flame temperatures, 3000
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  Kelvin. We have enough energy.
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  The Arrhenius gatekeeper opens the door.
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  Right, the Zeldovich mechanism kicks in.
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  It's a chain reaction where free nitrogen atoms start attacking
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  oxygen molecules and suddenly you're making NO.
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  But wait. Following our equilibrium logic,
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  as the piston moves down, the gas expands and cools.
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  The energy valley should shift back.
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  The NO should want to break up and go back to being harmless N2
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  and O2. It wants to.
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  Thermodynamics says it absolutely should, but kinetics
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  says sorry, you're too slow. The expansion happens too
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  quickly. Way too quickly as the
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  temperature plummets during that expansion stroke.
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  Look at the Arrhenius equation again.
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  The rate for the reverse reaction breaking down NO, just
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  hits a wall. The rate goes to effectively 0.
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  So the NO gets trapped. It's frozen in the mixture.
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  We call it frozen equilibrium. The chemical composition is
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  literally frozen in its high temperature state even though
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  the gas itself is cooled down. The NO is a fossil of the
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  combustion peak. And it gets pumped out the
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  tailpipe before it ever has time to decompose.
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  That's a really powerful image. The pollution is a snapshot of
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  the chaos that didn't have time to clean itself up.
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  Exactly. And it's not just no think about
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  soot or particulates. This happens in rich zones of
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  the chamber. Rich meaning too much fuel, not
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  enough air, right? Like in a diesel engine near the
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  surface of an injected fuel droplet, it's super rich,
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  there's not enough oxygen to go around, so the carbon atoms,
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  they just start bonding with each other.
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  Forming chains and then sheets and clumps, creating those
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  little sip particles. And once those solid particles
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  form, burning them back up takes even more time and heat.
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  If the cycle ends before that can happen, you get black smoke.
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  And sulfur dioxide SO2. That was a simpler villain.
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  It's just an impurity in the fuel.
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  If there's sulfur in your diesel or gasoline, it burns and
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  becomes SO2, which leads to acid rain.
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  The only fix there is to clean the fuel at the refinery.
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  And then there's the big one, carbon dioxide.
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  The one that's unavoidable. That one is the fundamental
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  product. It's not a mistake of kinetics.
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  If you burn a hydrocarbon you are going to get CO2.
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  The only way to stop making it is to burn less fuel or burn a
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  fuel with no carbon, like hydrogen.
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  So this all comes together into what we call the engineer's
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  dilemma. It's a nasty optimization
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  problem. To get maximum power and
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  efficiency, thermodynamics demands high temperatures.
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  But high temperatures cause dissociation which steals your
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  energy, and high temperatures activate the Zelda Vich
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  mechanism which creates smog. You're fighting physics on 2
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  fronts. You push for efficiency, you get
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  pollution, you lower the temperature to cut pollution,
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  you lose efficiency, and you burn more fuel.
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  It's a vicious cycle. So what's the fix?
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  How have we been cheating the system?
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  For the last 40 years, the main answer has been to clean up the
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  mess afterwards. That's the catalytic converter.
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  The Magic Platinum box and the exhaust.
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  It's a little chemical factory. It takes the unburned fuel, the
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  Co and the NO, and it forces them to finish the reactions
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  that kinetics froze inside the engine.
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  But I've heard they're incredibly fussy.
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  They are. They need an almost perfectly
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  balanced or a stoichiometric mixture of air and fuel to work.
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  We call it Lambda equals 1. Why is that?
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  If you're too rich, there's no oxygen left to burn the leftover
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  fuel in the catalyst. If you're too lean, there's too
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  much oxygen and you can't chemically reduce the NO back to
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  nitrogen. Which explains why cars have
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  oxygen sensors and computers constantly adjusting the fuel
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  injection. They're trying to walk that
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  razor's edge. Thousands of times a second.
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  There are other strategies like lean burn where you run with
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  lots of excess air. Which lowers the peak
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  temperature. Right, it acts like a heat sink.
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  Lower temp means almost no NO formation, which is great and
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  it's sufficient, but your standard three-way catalyst
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  stops working in that oxygen rich exhaust.
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  So then you have to invent whole new complex catalyst systems.
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  It's a never ending game of whack a mole.
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  It feels like we're just bound by the limits of a heat engine.
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  We are. We're trapped by the second law
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  of thermodynamics. As long as we're making heat to
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  push a piston, we're stuck in this cycle of dissociation and
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  frozen kinetics. So is.
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  There a way to just flip the table to exit this game
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  completely? There is, and it brings us right
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  back to Gibbs Energy, the fuel cell.
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  Now a fuel cell isn't a heat engine at all.
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  Not at all. It's an electrochemical device
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  doesn't burn the fuel. It takes the chemical energy,
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  the Gibbs free energy, and converts it directly into
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  electricity. So it completely skips the fire
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  step. Which means it skips the Carnot
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  efficiency limit, and because it runs at much lower temperatures,
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  you don't have dissociation stealing energy, and you
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  certainly don't have the Zeldovich mechanism creating
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  thermal NO. So in theory, it solves both of
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  the invisible villain problems we started with.
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  In theory, yes. Now they have their own huge
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  engineering challenges, cost, durability, hydrogen storage.
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  But from a pure thermodynamic standpoint, they're a much more
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  elegant way to get the energy out of a molecule.
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  It really changes your perspective.
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  You think you're just pressing A pedal to go faster?
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  But what you're really doing is orchestrating a high speed
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  molecular war, trying to manage a chaotic explosion, freeze time
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  to capture the work, and then scrub the casualties out of the
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  exhaust. It gives you a lot of respect
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  for the engineers who make that happen seamlessly for hundreds
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  of thousands of miles. It's a miracle of control
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  engineering that it works at all.
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  So to recap, the perfect burn is a myth.
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  Real combustion is a balancing act between the energy stolen by
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  dissociation and the ticking clock of kinetics.
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  And while we've gotten very good at masking those imperfections,
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  the underlying physics hasn't changed.
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  Which leaves us with a final thought to Mull over.
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  We're in this massive push for 0 emission vehicles like EVs, but
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  we have to remember the whole system if the electricity for
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  that EV comes from a natural gas power plant.
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  You haven't eliminated the dissociation or the kinetics
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  problems, you've just moved them from your tailpipe to a
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  smokestack 100 miles away. The chemistry doesn't disappear,
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  it just relocates. The only way to truly beat the
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  limits of the heat engine is to stop using heat engines all
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  together. Whether that's solar wind or
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  electrochemical cells, yeah, you have to step out of the cylinder
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  entirely. That's the only way to win the
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  game. A heavy thought to end on if you
• 17:20
  haven't already, get over to our YouTube channel Mechanical
• 17:22
  Engineering Made Simple, where we post hands on how to videos,
• 17:25
  visual breakdowns, and practical engineering walkthroughs you can
• 17:28
  actually use. A family that solves together
• 17:31
  stays together.