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Take a planet with an atmosphere similar to Earth's but, say, 9-10x as dense. What is this going to mean for the performance and design of the heat engines that are so familiar to us? Higher oxygen partial pressure... easier to burn? But higher air density means more heat conducted away... easier engine cooling, yes, but does it also hurt their performance? An internal combustion engine, instead of having something like 10:1 compression ratio, will only have about 2:1 (edit; assuming the engine is constructed to the same strength). Does this actually make a big difference, or does only the absolute difference in pressure matter? Likewise for a steam engine. Or (because I'm thinking particularly about aircraft here) a jet engine.

Tristan Klassen
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3 Answers3

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The effects won't be as dramatic as you might think

For piston engines increasing atmospheric pressure will cause a similar situation to turbo charging. You will create 10 times the power per density, compression ratio will remain the same. This means to get the same power your engines will have 1/10th the displacement, but that does not mean they will actually be smaller. Since your engines will be operating at higher pressures, they will be creating more mechanical stress on the engine block, which will need to be reinforced. Contrary to what others have said, you will not have to deal with increased fuel consumption. Fuel consumption per unit power will be the same since your displacement will be smaller. Only when you try to keep 1 atm engine displacements will you see a jump in fuel consumption (but power will rise equally). Of course trying to move quickly through such an atmosphere would require more power due to drag, but stationary engines will not have this concern. Similarly, back-pressure caused directly from the atmosphere will not be a problem, the concern here will be that drag through the exhaust pipe may build up easier, so you will want freer flowing exhausts.

Moving towards aircraft and jet engines in particular get a little more interesting. Let us start with how the atmosphere works. The higher you go up, the thinner the air. That atmosphere above the point where it reaches 1 atm pressure will look identical to our atmosphere. Planes will simply fly higher when speed or long distances are required. Most planes that reach high speeds at altitude can not do the same at sea level. Mach 2.5 fighter jets are often limited to Mach 1.2 at sea level. If you do want to fly fast at low altitude, besides the rules for piston engines, you will likely see changes to nozzle design. If your exhaust is supersonic, then the higher the pressure the smaller the nozzle. Aircraft that operate at many altitudes will need nozzles that adjust in size dramatically.

XRF
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    "Since your engines will be operating at higher pressures, they will be creating more mechanical stress on the engine block, which will need to be reinforced." Right. Can you make a practical IC engine to operate at ~100bar? What if you can't? "That atmosphere above the point where it reaches 1 atm pressure will look identical to our atmosphere." I recognize that, so I'm concerned with flight in the lower atmosphere.... "Planes will simply fly higher when speed or long distances are required." because every high-flying plane will have to get up to that altitude in the first place. – Tristan Klassen May 12 '19 at 19:41
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    @TristanKlassen, most current engines are made from gray iron because it's cheap and easy to work. It's also a weak, brittle material. Replace it with a suitable steel and you can easily get a ten-fold increase in strength. – Mark May 12 '19 at 21:40
  • @TristanKlassen You can find people running boost in that region with a quick google search. Their vehicles are not reliable, but they are also making a lot of heat along side the boost. On the other hand, fuel injectors can be running 300-400 atmospheres of pressure. Since your cylinder can be smaller, the total forces won't actually be 10x greater, and as Mark stated, you can use better materials for extra strength if need be. On the topic of aircraft, it is as I stated, to transit the lower atmosphere efficiently variable geometry nozzles would be needed. Airlines might even use them. – XRF May 12 '19 at 22:42
  • "You can find people running boost in that region with a quick google search." Wouldn't think of looking when I don't necessarily know what to search for, given that I don't drive and don't understand what people are saying when they talk about engines. – Tristan Klassen May 12 '19 at 22:48
  • @TristanKlassen The word boost means pressure above atmospheric added to the engine by some sort of a compressor in a forced induction engine (turbocharged or supercharged). It is typically measured in bar (almost identical to atm). 10 atm is the same pressure as 9 bar boost. Most people keep the amount of boost below 2 bar, but there are always exceptions. – XRF May 12 '19 at 23:13
  • I know that now... but that still doesn't give me the full picture. I'm trying to figure how performance figures for vehicles on such a world will compare to those on Earth at the same tech level, though I don't know what leads to the performance figures I know on Earth. – Tristan Klassen May 12 '19 at 23:20
  • @TristanKlassen Engine performance is governed by a combination of losses (friction, drag, etc.), combustion efficiency, and the metrics of the thermodynamic cycle being followed. Spark ignition piston engines (gas powered engines) use the Otto cycle, diesel engines use the Diesel cycle, jet engines use the Brayton cycle. In theory piston engine power is proportional to the rate of rotation, but because losses add up a high speeds, peak power is reached at a finite rpm. – XRF May 12 '19 at 23:44
  • Higher pressure in the heat engine is a non-issue: the atmosphere presses on the outer cylinder wall and provides exactly what's needed to counter the additional pressure from air intake (slightly more because air intake still needs to work against friction). Since you can work with much smaller displacement, you can use smaller cylinders and this means you can make the walls thinner without losing strength (volume vs. surface works in your favor here). – toolforger May 13 '19 at 05:42
  • @toolforger, no, it doesn't work that way. Here on Earth, a 10x compression ratio means you've got 10 bar of pressure inside the cylinder pressing against 1 bar of ambient pressure, for 9 bar of unbalanced pressure. In the proposed atmosphere, you've got 100 bar of pressure inside the cylinder pressing against 10 bar of ambient pressure, for 90 bar of unbalanced pressure. – Mark May 13 '19 at 06:47
  • @Mark You get exactly the pressure imbalance that you design the combustion chamber's geometry for :-) – toolforger May 13 '19 at 22:51
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Higher oxygen partial pressure... easier to burn?

It means higher fuel consumption and higher produced power per cycle

But higher air density means more heat conducted away... easier engine cooling, yes, but does it also hurt their performance?

An ICE is based on the expansion of the gas. That the gas is heated is just a nuisance, since one has to cool the combustion chamber. Indeed there have been design were water is injected into the chamber to absorb the heat and increase expansion thanks to the formation of vapor. So additional cooling is not an issue.

An internal combustion engine, instead of having something like 10:1 compression ratio, will only have about 2:1.

Wrong. The compression ratio of an ICE doesn't depend on the intake pressure, but on the volume ratio between the upper and lower point of the cycle. Those do not change with air pressure.

Same general reasoning holds for steam engine and jet engine.

Incidentally, increasing the intake pressure is the exact reason for applying a turbo to ICE.

L.Dutch
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    "Wrong. The compression ratio of an ICE doesn't depend on the intake pressure, but on the volume ratio between the upper and lower point of the cycle. Those do not change with air pressure." But, assuming material strength is the limit, the same engine would burst if you tried to compress a 10bar atmosphere to 100bar. – Tristan Klassen May 12 '19 at 18:11
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    My point is, they're not (I think) going to be able to build ~10:1 compression ratio engines with our technology level. – Tristan Klassen May 12 '19 at 18:14
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    @TristanKlassen, speaking as someone who's been involved (peripherally) in engine design, they're not going to have any trouble building 10:1 compression-ratio engines with our technology. Engine manufacturers have been shaving cylinder walls as thin as possible to make engines cheaper, lighter, and more efficient. If you need an engine that can stand 10 MPa at the end of the compression stroke rather than 1 MPa, you just add more metal. (You could also use stronger alloys than the junk iron commonly used, but that would raise the manufacturing cost.) – Mark May 12 '19 at 21:30
  • But the sort of things I want to know are... Assume you build a reinforced engine to operate in this atmosphere. It'll be larger and heavier relative to its displacement. It'll also generate more power relative to its displacement. (XRF notes both of these things.) How will the power/weight of the engine compare to an engine designed for Earth? – Tristan Klassen May 12 '19 at 22:54
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Have you ever done something particularly foolish as a child?

I have. Oh, yes... I've done some whoppers. And one of them was pounding a potato into a tail pipe. As you can imagine, the engine failed to start.

1. Increased Fuel Consumption

Engine efficiency drops like a rock. That means small engines we love today (like my weed trimmer) might not even start, because the back-pressure on the exhaust is too high. The solution? A bigger bang. That means I'm increasing fuel consumption to overcome the increased back pressure.

2. Increased Cooling Required

I believe (I could be wrong) that higher air pressure does not improve cooling. In fact, it makes heat dissipation worse. If increasing surrounding density helped with cooling, computers would have a rock sitting on top of them rather than a fan. Heat propagates fairly slowly through a static atmosphere, which acts (like pretty much all things) as a thermal insulator. Increasing its density is like adding more insulation to the walls of your house. In fact, air-gap insulation is very effective. It's why people staple plastic over their windows during the winter. That gap of static air is an insulator. Therefore, you have a bigger problem with heat removal, which means a bigger radiator.

3. Decreased need for air flow

You are correct that greater air density means more oxygen per cubic centimeter. Therefore, the air draw requirements would not be as large.

4. Increased air filtering

But, as a nasty byproduct, you need a larger air filter because the higher density of air will push crap through an air filter more readily than here on good ol' Earth. You need a way to disperse that pressure, too. That means a bigger filter (more surface area) with greater density filtration. So, in the long run, #3 and #4 are a wash.

5. Increased Fuel Consumption — Again

Finally, you're pushing whatever the engine is moving through thicker air. This might not have a massive impact, but the reality is that the higher the pressure (air density), the harder it is to push an object through it. That means you need an even bigger bang than #1 requires to overcome the additional air resistance.

JBH
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    "Heat propagates fairly slowly through a static atmosphere, which acts (like pretty much all things) as a thermal insulator." But a non-confined atmosphere can convect. ""I believe (I could be wrong) that higher air pressure does not improve cooling. In fact, it makes heat dissipation worse. If increasing surrounding density helped with cooling, computers would have a rock sitting on top of them rather than a fan." Actually, solids are used for cooling -- look up 'heat pipe'. And remember how some old supercomputers were flooded with non-conductive fluorine-based liquid coolants. – Tristan Klassen May 12 '19 at 18:21
  • @TristanKlassen how does any of that relate to the thicker atmosphere? Almost everything can convect. Increase density and convection efficiency drops. How does that help the OP? And go read what a heat pipe is. Liquid vaporizes (aka, "cools" sweat does this) and resolidifies at the other end (it isn't just a solid). A heat pipe is little more than an air conditioner without the compressor. Once again, what does that have to do with the increased atmospheric pressure of the OP's question? I'll stand by my answer. – JBH May 12 '19 at 20:46
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    Higher pressure improves cooling in any realistic scenario, by increasing the mass of cool air moving past the hot object. Nobody in their right mind tries to cool an object in a stationary atmosphere because forced-air cooling and even unassisted convection are so much more effective. – Mark May 12 '19 at 21:33
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    @Mark higher air flow improves cooling. Higher pressure does not. I'm open to correction, but we might need to find some math to prove it. Also, of course nobody does it. Forced air flow is always used. but it was the simplest way of explaining the difference between atmospheric densities. – JBH May 12 '19 at 21:45
  • @TristanKlassen Do not worry about the potato effect: The potato works by not budging, so you have a pressure buildup on the exhaust side until the exhaust has the same pressure as the combustion chamber, preventing fresh air from entering. A normal Otto has 8-18 bar, plus 10 bar from the air intake, so it will push the exhaust at 18-28 bar. – toolforger May 13 '19 at 05:54
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    @JBH higher density means higher air flow - not in terms of volume, but in terms of heat capacity. At a given airspeed anyway - if you have forced cooling with a fan, the denser air requires more work to accelerate the air mass, I'd expect the effects to cancel pretty evenly. -- Do you have a reference that explains how denser air makes cooling harder? This would help making the answers better. – toolforger May 13 '19 at 05:58
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    Notice that we try to use vacuum for best heat insulation, e.g. in vacuum flasks (Thermos). Compressed gas (always?) transfers heat better than uncompressed. – JimmyB May 13 '19 at 10:11
  • "because the back-pressure on the exhaust is too high" - That doesn't make any sense. Notice that the very same atmospheric pressure that causes part of the back-pressure is present all around the engine, the exhaust and, most importantly, inside the engine, i.e. below the pistons, so that part of the back-pressure just cancels out. – JimmyB May 13 '19 at 10:13
  • @JimmyB that's not how it works. A controlled explosion happens in the combustion chamber. That controlled explosion is principally pushing against the cam shaft - but it's also pushing against the exhaust manifold with valves controlling the flow of new air in and old air out. But since those valves aren't open for both paths simultaneously, that explosion must push against the higher air pressure in the exhaust path. That's why covering the exhaust stops the engine. The explosion doesn't have enough force to clear the exhaust, leaving CO in the combustion chamber, choking the next explosion. – JBH May 13 '19 at 14:02
  • @JimmyB, a picture is worth 1,000 words (and I had to be concise to the point that I might be imprecise with a couple of points). Check out this animated graphic of how an engine works and notice the stage where the exhaust is being pushed out. That stage is pushing against the higher air pressure. It doesn't "cancel out." – JBH May 13 '19 at 14:05
  • @JBH Yes, it does cancel out, because there's also 10 atm pushing the piston from below. The potatoe stalls/breaks the engine because it only increases the back-pressure in the exhaust without the countering force from the crankcase which on earth remains at 1 atm. – JimmyB May 13 '19 at 14:58
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    @JBH Following your logic, at 10 atm the valves would have to hold back those 10 atm pressure from entering the combustion chamber, but because there's 10 atm on both sides of each valve, the net force (pressure differential) is 0. Equivalently, it would be harder to crank an engine at higher ambient pressure and easier at lower pressures. This is not the case, except for marginally higer losses due to more mass being moved. – JimmyB May 13 '19 at 15:00
  • @JBH Imagine a simple water pump which can produce e.g. 1 Bar of pressure. Will it stall if operated 100m (~10 Bar) under water? No, because the water enters the pump at 10 Bar and is output to an environment that's also at 10 Bar. – JimmyB May 13 '19 at 15:06
  • In total, points #1, #2 and #4 from your answer are incorrect. – JimmyB May 13 '19 at 15:19
  • @JimmyB, well... you can always post your own answer. Neither the community nor the OP will mind. – JBH May 14 '19 at 00:30