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#326 2023-10-10 06:21:41

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 16,473

Re: Internal combustion engines for Mars

For kbd512 re #325

SearchTerm:CO O2 engine on Mars compared to electric battery for all terrain vehicle

This post introduces (or re-introduces) the idea of a small turbine engine as an alternative to piston engines which were in mind when this topic started.

The post includes specifics of chemistry, molecular values, mass values and relative cost/complexity/maintainability

***
For kbd512 re further development of the vehicle design with respect to energy investment...

Your post #325 envisions liquid CO and liquid O2 for the vehicle.

It takes energy to compress and cool gas until it is liquefied.

Is there any way to add the cost of the energy needed to liquefy the CO and O2 to the specifications for the CO vehicle vs a battery powered one?

The battery powered vehicle needs the energy to refill the batteries, plus a bit more due to whatever losses there may be.

Let's assume both designs draw power from solar arrays of the same size.  The performance difference will show up as time needed to refuel.

For the CO vehicle, the energy investment would include:

1) Capture, filter and compress CO2 from the atmosphere
2) Separate CO from O2 and save the output as gas
3) Liquefy both gases

Given your post figure of 8 hours for operation of the two vehicles, I'm wondering if it is possible to show three values that might be of interest to designers:

1) Energy to load up the battery pack
2) Energy to load up a CO/O2 vehicle designed to use compressed gas instead of liquefied gas
3) Energy to liquefy the gases in #2 - This energy would be added to #2 to give total for the liquefied version

I don't know the answer to how this comparison would turn out, but it sure would be interesting!

The trade off for the vehicle is tank size ... the tank size for the liquid gases would be smaller, and the tanks don't need to be as strong.

The tanks for the compressed gas version would be larger and stronger.  I'm thinking of the tanks for compressed air vehicles that a member showed us recently. The post included an image of a tank wrapping machine showing a wide ribbon of plastic material being wrapped by a wrapping machine to achieve an impressive capability with low weight.

(th)

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#327 2023-10-10 13:19:24

kbd512
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Registered: 2015-01-02
Posts: 7,270

Re: Internal combustion engines for Mars

Topsoe eCOs CO2 to CO Electro-Catalytic Converter

6-8kWh/Nm^3 of CO, so 1.229kg of CO.

1. That means 1,728kWh for manufacturing 354kg of CO and 202.2kg of O2 per day.  Direct liquefaction of CO and O2 will be somewhere around 1.5kWh to 1.7kWh/kg with a 8% efficient cryocooler (source is Linde- people who mass manufacture liquid cryogens here on Earth).
2. 601.8kWh daily power use for the CO liquefaction.
3. 343.74kWh daily power use for the O2 liquefaction.
4. 2,673.54kWh or 2.673.54MWh of total daily power draw for the CO and O2.
5. I don't know how much power is required to obtain liquid CO2 feedstock, because if it's done at night then it requires very little power.
6. 596.56kWh total daily power consumption for the battery, or about 4.48X better.

There's a problem with this simpleton calculation, though.  The battery cannot recharge from a solar panel at night, unless a second full-sized battery is also present.  There's another problem related to battery longevity.  If you discharge 100% of the battery capacity from Lithium-ion, cycle life is about 1,000 to 2,000 cycles, so 2,000 driving days tops, so 5.5 years before replacement.  In reality, it's 11 years before replacement because this silliness doesn't work without 2 complete 596.56kWh batteries, one on the charger, the other in the vehicle.

Assume you get an average of 238.175W/m^2 of solar panel surface area (35% efficient, twice as far from the Sun as Earth), and 1,429Wh/m^2/day.  This is realistically achievable under optimal conditions near the equator.

sCO2:
2,673,540Wh / 1,429Wh = 1,871m^2 of solar panel area
(3,742kg at 2kg/m^2 using Triple-Junction monocrystallane wafers on a honeycomb CFRP backer board, which I proposed before, to try to get the mass of Louis' all-solar setup down to something manageable, and then it still couldn't compete with nuclear)

Li-ion:
596,560Wh / 1,429Wh = 418m^2 of solar panel area (836kg)

5,965.6kg for pair of 596.56kWh batteries + 836kg of PV = 6,801.6kg for Li-ion and PV
350kg for sCO2 turbine setup in vehicle + 3,742kg of PV = 4,092kg for sCO2 and PV

I can afford to add extra cryogen tanks, weight for the electrolyzer equipment, insulation, power cables, etc.  Regardless, I'm coming in under the mass and volume of an equivalent battery powered system.

Assume the weight of the power control electronics for sCO2 PV is 4.5X heavier than Li-ion (can't easily estimate weight for this- AC vs DC, voltage and amperage, etc, all greatly affect the numbers for both solutions).  No weight is added to the battery vehicle for power control electronics, or electric motors, or radiators, nor the structural support hardware for a much heavier battery- even on Mars this still matters.  That is how low the energy density of Lithium-ion batteries is, when compared to a system that must produce oxidizer and fuel from scratch.  A vacuum-rated battery pack is nowhere near 200Wh/kg at the pack level, either, more like 100Wh/kg.

We'll use every trick in the book to reduce the weight of the battery vehicle and increase the power efficiency of the sCO2 vehicle, but at the end of the day energy density differences between a fuel 4X worse than Methane vs a Li-ion battery show how far behind batteries are compared to any type of combustion engine.

We're talking about liquid cryogen vs batteries, not compressed gases, but it wouldn't matter if we were using compressed gas, and I've shown this before with Meth-Ox.  You cannot overcome a 5X to 10X energy density deficit of CO / O2 vs Li-ion.  It's not possible.  I know our battery and solar panel advocates want it to be possible, but simple math shows that it's not.  Batteries are best reserved for laptops, cell phones, flashlights, radios, light duty indoor-use power tools, and starting real engines.  They are not acceptable substitutes for combustion engines, even when you synthesize the fuel from scratch, even when you have to store the fuel and oxidizer, even when the fuel and oxidizer are not even 1/4 as energy dense as Methane.

If weight matters, and on Mars it matters quite a lot, then you get drastically more "bang for your buck" using combustion engines.  That's why Mars Direct featured a CH4 / O2 combustion engine rover.  Dr Zubrin is an aerospace engineer who is unencumbered by ideology.  He does things that make sense from a energy / mass / volume / cost perspective.

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#328 2023-10-10 14:41:58

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 3,285

Re: Internal combustion engines for Mars

Kbd512, a good analysis.  I have often wondered if it would make sense landing fuel factories on Mars, along the routes that a rover would take to reach scientifically interesting sites.  These would be small units, probably no more than a few hundred kg each.  We could ship a thousand of them to low Mars orbit in a single starship payload and then land them where we expect to use them.  They would each be equipped with solar panels or a kilopower unit and would convert Martian CO2 into cryogenic CO and O2.  The rover would drive between fuel stations, which could be spaced a couple of hundred miles apart.

Regarding the solar electric solution for vehicles.  How would the analysis stack up for a direct solar electric powered vehicle, without batteries?  This would involve directly powering a DC motor using solar panel.  Speed would be a function of sunlight intensity, so there would be no driving at night and probably no driving outside of an 8 hour window every 24 hours.  The power available to the vehicle would be limited by the surface area of panels that could be mounted on the vehicle.  So I would imagine this would be slow, probably no more than human walking speed.  A bit like being stuck in first gear.  But a vehicle like this would not need to carry propellant.  Dust storms could be the thing that screws it up.


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#329 2023-10-10 17:32:50

kbd512
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Posts: 7,270

Re: Internal combustion engines for Mars

M1068 Standard Integrated Command Post System Carrier (M113 APC Variant)
Length: 4.85m
Width: 2.7m
Height: 2.7m
Ground Clearance 435mm
Combat Weight: 12,172kg (Earth); 4,625kg (Mars)
Horsepower Required: 275hp (Earth); 104.5hp (Mars- this is where the 100hp / 74.57kW figure comes from)
Ground Pressure: 8.68psi (Earth); 3.2984psi (Mars- it "floats" across sand that a human foot would sink into)
Engine:
Detroit Diesel 6V53T
Dry Weight - 769kg
Displacement - 5.2L
Power - 212hp gross at 2,800rpm
Torque - 492ft-lb at 1,300rpm
Engine Dimensions - 991mmL x 940mmW x 1041mmH
Max Level Road Speed: 40mph (Earth); 25mph (Mars; 100% off-road)

m1068front.jpg

m1068rear.jpg

A fold-out 400m^2 solar array on its roof, using a tilt / swivel truss structure, could nominally provide 95,270Wh per hour, during a 6 hour timeframe, when the sun shines the brightest (10AM or 11AM to 4PM or 5PM).  The rest of the power is for life support, keeping batteries and tracks warm, sensors, radios, airlock, etc.

What effect would 800kg at least 5m above the ground, not including wiring and support structure have on this vehicle's stability?

I dunno, but you never want to climb or descend a hill at a perpendicular angle.  I can tell you that much.  If thin film with a scratch-resistant "Gorilla Glass" covering was much lighter than 2kg/m^2, then perhaps that would reduce stability concerns.

For sCO2 power, a solar thermal reflector can supply 612.45Wt/m^2 (because we're not attempting a 35% efficient conversion).  Let's say we get 306.225W of thermal power (50% efficiency).  That reduces solar array size to 243.5m^2 (15.6m x 15.6m), which is somewhat less unwieldy, but then the material has to keep the semi-flexible solar concentrator focused on the hot spot while the vehicle is bouncing around.

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#330 2023-10-12 01:15:47

kbd512
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Posts: 7,270

Re: Internal combustion engines for Mars

A Strontium-90 power source providing direct heat to a sCO2 gas turbine, would probably weigh about half as much as the battery.  It would require a larger radiator than the CO / O2 option, but it's still less of a hassle than CO / O2 synthesis or recharging batteries with photovoltaics, and it's made from a nuclear waste product.

Strontium-90 Titanate (SrTiO3)
Melting Point: 2,080C
Density: 5.11g/cm^3
Heat Production: 450W/kg
Surface Temperature: 700C to 800C

149,140W / 450W/kg = 331.42kg (at 50% efficiency, this is how much SrTiO3 you need)
331,422.22g / 5.11g/cm^3 = 64,858cm^3 / 64.858L (40.2cm by 40.2cm by 40.2cm)

At 715C, the prototype 2-stage 250kW sCO2 gas turbines achieved 50% thermal-to-mechanical efficiency.  A 250kW sCO2 gas turbine wheel is about the same diameter as the US Quarter Dollar coin.  If we want minimum weight, minimum hassle, and guaranteed thermal performance, then this is the way to do it.  We could have 4 RTGs for redundancy, with each one providing 1/4 of the total thermal power input, 4 sCO2 turbines for redundancy in case one fails, each capable of full output, with associated valves and piping to cut out a failed turbine or coolant loop, plus smaller fully redundant gas turbines to supply electrical power to the life support equipment.

This won't be a solution for all types of vehicles, but for long-range surface exploration / transport vehicle can use Strontium to avoid the need to lug a bunch of extra equipment with the vehicle.  Short range ATVs for quick transits between pressurized base structures can use compressed CO2 and Sr90 heat sources.  Base construction equipment can use direct solar with power cables and batteries, with backup sCO2 gas turbine generators or fuel cells.  I think these solutions use the best mix of technology capabilities where they're most appropriate.

The whole air transport thing that GW alluded to elsewhere?  Well...  I don't think we're going to do much flying.  As GW pointed out, it's very dangerous to fly so fast so close to the surface.  If a reliable nuclear decay heat powered land transport is available, then it obviates the need for high-speed air transport between colonies.  If ace pilots want to try their luck providing air transport of medical supplies or transfer of injured personnel back to the main base, then we can test the practicality of rocket-powered hoppers or propeller-driven aerodynamic lift / rocket VTOL aircraft.  I don't think helicopters are practical at the scale required.

Raymer Manned Mars Plane

Vstall: 115kts
Vcruise: 150kts

Max Gross: 6,000lbs
Wing Area: 2,080ft^2
Wing Loading: 2.885lbs/ft^2

Dr Raymer is postulating 500Wh/kg batteries by 2030, which don't presently exist, so I'm substituting H2 fuel cells.  ZeroAvia reports that their test stacks achieve 2.5kW/kg in their 20kW test stacks.  Hydrogen provides 33.33kW/kg, we're going to assume 65% efficency, so 21.67kWh/kg of H2.  His notional 834lb 500Wh/kg (417kWh) battery is being replaced by 19.25kg of LH2 and 154kg of LH2, or 382lbs of reactants for the fuel cell.  If we include the weight of the fuel cell, we're at 470lbs.  Batteries need to achieve 1,000kWh/kg before we have lighter batteries.  If the cryogen tanks are 5% the cryogen mass, then we're at 489lbs total.  He's showing cruise power of only 31.2hp / 23,266W.  I'm not sure if that's correct or not, because it seems quite low, but I'll run with his numbers since I'm not a PhD aerospace engineer.

Scratch that!  We already have single-use Aluminum-air batteries that achieve 1,200Wh/kg.  It then takes 12kWh to 15kWh of electricity to convert the Alumina, back into Aluminum, but Aluminum-air batteries already exist and have existed at that power density level for years now.  They're not practical for Earth-bound cars, but a Mars airplane can justify the 15X energy input premium.  Jackpot.  Batteries are finally helping us out by doing a better job than competing alternatives.  Go wild, battery advocates.  We finally found an application where batteries are superior to competing alternatives.  We do need to carry a more modest supply of O2 with us, because oxidation of the Aluminum makes the electricity.

I don't like the crazy amount of wingtip flex in his 345ft wingspan aircraft, so I'm specifying a box wing / biplane configuration.  As Dr Raymer indicated, it will have a significantly greater amount of drag, but a box wing is not as unwieldy, fragile, and heavy.  The 2,000km of range is great, but flying such a small aircraft for 6 hours is impractical.  4 hours of flying is more practical.  Even if the box wing reduces the range to 1,000km, a more practical aircraft is still worth the tradeoff.  It's still a 141ft wingspan, which is a little nuts for a 2-seater.  A 345ft wingspan is grossly impractical, though.  A C-130's wingspan is only 132ft.

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#331 2023-10-12 16:01:13

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 3,285

Re: Internal combustion engines for Mars

The problem with Sr-90 as an RTG heat source is bremsstrahlung emissions.  The strontium nucleus has a high charge density, and any beta electrons approaching it are deflected at wide angles, causing them to emit x-rays.  This is why beta shielding is usually light elements like carbon and hydrogen.  In terms of abundance, Sr-90 would be a far more practical RTG isotope than 238Pu.  But those pesky x-rays make it difficult to handle. Pure Sr-90 has power density of 0.95W/g.  So a tonne of Sr-90 will give you 1MWth.  Strontium titanate achieves about half of that.  One option would be Sr-Li liquid alloy.
https://link.springer.com/article/10.1007/BF02877514

This has the added advantage of being liquid, allowing convective heat transfer.  Sr-90 is some 15x more massive than Li-6, so even a high molar proportion of lithium will only weakly dilute the mass power density.  A high molar concentration of Li ensures that most of the beta deceleration is accomplished by a light element, reducing bremsstrahlung emissions.  This allows the GT to avoud the need for shielding.  A gas turbine engine on Mars could consist of Sr-Li within steel or nickel tubes, which would be in a tapered chamber directly behind the compressor.  The heated CO2 would then pass through the turbine, a portion of whose work would drive the compressor through an axial shaft.

Because we are relying upon heat transfer from solid tubes into a gas stream, a high compression ratio is highly desirable, as heat transfer is proportional to gas density.  The low temperature of the Martian atmosphere, which is far beneath critical point of CO2, makes this achieable without the compressor eating up all of the turbine work.  For this reason, a nuclear or RTG gas turbine may work much better on Mars than on Earth.  Although the Martian atmosphere is thinner, the cold CO2 does not behave like an ideal gas.  Compared to air, far less work will be required to achieve a volume change.

Last edited by Calliban (2023-10-12 16:12:12)


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#332 2023-10-12 18:48:24

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Internal combustion engines for Mars

If we are going to use heat energy from a burn chamberwhen using turbines coupled with electric generation from the motion that is created does yield a bit more than the typical 25% but we can also do more with the chambers heat by wrapping it with vapor coils to obsorb the heat to make more energy and then the exhauist heat can also be captured for the same purpose. That said we can alter the current KRUSTY design to perform this function of power from heat.

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#333 2023-10-12 23:32:12

kbd512
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Posts: 7,270

Re: Internal combustion engines for Mars

Calliban,

I never thought about the possibility of using a light molten alloy, but better power density and reduced core volume and weight is very attractive.  I still don't think you can get away with no X-ray shielding.  Maybe it can be reduced by quite a bit, though.  The Sr90 is still going to interact with the walls of the storage container, producing X-rays.  My initial thought process was that the exterior would be properly shielded and we'd have SrTiO3 plates with holes for the CO2 to flow through, much like NERVA's core.  Regardless, the volume claim inside the vehicle is much lower compared to competing solutions.

What are your thoughts on using Indium-Tin-Bismuth for thermal power transfer?

The vehicle could have a tank of molten metal surrounding the RTG, to draw excess power from, in case the vehicle needs a burst of energy for climbing hills, operating hydraulic dozer blades or grading blades or power shovel attachments, providing thermal power transfer to the base's backup thermal power storage when not being driven, a heat source for compressed CO2 power tools, etc?

We're losing thermal power whenever the vehicle is not being driven, so we may as well have a method to transfer surplus power for other applications.

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#334 2023-11-15 18:20:38

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Internal combustion engines for Mars

GW Johnson wrote:

What I found was poor Isp performance with LOX-LCO,  due to low c*.  But if it is readily available,  you may not care about that.  It won't get you into orbit or halfway around the planet as a rocket propellant,  but it might serve for short-range "local" stuff. 

I suspect that in some sort of internal combustion engine,  it would perform about like methanol.  Just remember,  that on Mars,  you have to carry the oxygen (which outweighs the fuel),  and a diluent gas (which outweighs the oxygen),  to operate internal combustion devices as we know them here. 

If you are willing to undertake the extensive redesign to handle 500-1000 C higher flame temperatures,  you can delete the diluent gas. All fuels burn with air at about the same temperatures at the stoichiometric mixtures,  whether low or high energy.

GW

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#335 2023-11-16 00:30:10

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 3,285

Re: Internal combustion engines for Mars

Some gas turbines have combustion chamber temperatures exceeding 1500°C.  The blades are single nickel alloy crystals with cooling tubes actually cast into them.  Very impressive.  Unfortunately, adiabatic flame temperature of most fuels approach 3000°C in pure O2.  There aren't many metals that can withstand those temperatures for very long without severe erosion.
https://www.thoughtco.com/flame-tempera … ble-607307

Maybe instead of using CO2 or N2 as diluent gas, we could use water?  If water can be recondensed from the exhaust, we can reuse it and reduce the mass that must be carried.  The problem here is that a large radiator is needed to cool the exhaust gases to the condensation point of water.  Carbon dioxide is a more obvious choice for diluent gas on Mars, because the vehicle can refill an LCO2 tank from the air when it is parked.  The waste CO2 can be vented back into the air without cooling, carrying waste heat with it.

Last edited by Calliban (2023-11-16 00:38:54)


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#336 2023-11-17 11:19:21

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,385
Website

Re: Internal combustion engines for Mars

The gas temperature in a turbine's combustor cans is not uniform.  There is a hot spot in the core where the mixture is just right,  that can approach 4000 F,  but things are cooler all around it.  Right at the forward dome,  it is too rich to ignite,  being nearly all fuel.  Air is added from the sides as you proceed down the can,  and amounts to temperature-lowering dilution air in the downstream portion of the can.  Mixed gas temperature must be down to the allowable turbine inlet temperature range by the time it leaves the can,  which is under 2200 F,  and under 2000 F in older designs.

The combustor can is a double-wall device:  a pressure vessel containing air from the compressor,  with a perforated liner.  The fuel is sprayed inside the perforated liner,  at its forward end,  and (usually) combustion within that perforated liner is spark ignited just downstream from there,  where the mixture is just rich of stoichiometric.  The perforated liner is bathed in compressed air on its outer surface,  and by combustion gases on its interior surface.  That air can only be moderately hot from compression (both inlet ram and compressor stages) before it is effectively no longer the "cooling air" that allows the perforated liner to survive.

That over-temperature risk in the combustor cans,  the risk to the turbine blades,  the risks to the compressor blades,  and the risks to afterburner and nozzle components are why gas turbine engines for flight to Mach 3 are so rare and expensive.  There have been only 3 so far that qualify as production engines:  in the XB-70 (Mach 3.0),  the SR-71 (Mach 3.2),  and the Mig-25 (Mach 3.5).  The Mig-25 engine had only a 500-hour service life,  at the end of which you did not overhaul it,  you replaced it with a new one.

GW

Last edited by GW Johnson (2023-11-17 11:22:17)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#337 2024-01-23 09:42:10

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Internal combustion engines for Mars

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#338 2024-01-23 10:53:39

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,385
Website

Re: Internal combustion engines for Mars

That's the first time I've ever seen the 6-stroke cycle for an internal combustion engine.  The water gets vaporized by the residual exhaust gases after the exhaust valve closes,  and the resulting steam can't get out,  so it pushes down on the piston for the second power stroke. 

The second exhaust stroke has to clear out the steam so that it does not quench the fuel-air mixture from igniting in the intake and compression strokes.  That's a big risk to successful running,  by the way. 

I don't know that I agree about the need for distilled water,  but it will at least have to be potable water.  Which is going to get scarcer as the population gets larger and the climate keeps warming.  Just something else to worry about. 

The water is a second,  smaller flow of liquid into the engine,  and it has to come from a second tank.  This thing still burns a fossil fuel.  That is a technology at least conceptually posing the risk of putting the wrong liquids in the tanks. 

With the steam pulses out the exhaust valve,  I rather doubt that our existing emission control technologies will work right.   The steam will be a much higher percentage of the exhaust stream on average,  and at discrete points in time,  it will dominate.  Our catalytic converters may not work right.

Although,  the cat converter should stay cleaner of life-shortening soot,  due to the cleaning action of the steam.  If it doesn't work right,  then NOx is a very potent source of greenhouse warming,  in addition to being a source of health effects and acid rain. Something else to worry about.

I'm no expert,  and I could be wrong about these things.  But they need to be worried about,  and that's what the "engineering development" of this thing would be all about.  That's expensive,  time-consuming work.  It cannot be both fast and accurate.

BTW,  this is not the "water engine" that some crack it up to be.  There are other things also claimed to be "water engines".  But there is no such thing,  that would violate the 2nd Law of Thermodynamics! 

Why do I say that?  It costs more to split the water into hydrogen and oxygen than you can get back from re-burning them,  because NOTHING is 100% efficient.

There are some things that come close to high efficiency,  but NONE of them have ever been heat engines of any kind.  The upper bound on heat engine efficiency is the Carnot efficiency.  It rarely even approaches 30%.  Properly evaluated,  the high temperature in the Carnot efficiency is your combustion temperature,  and the low temperature in it is the exhaust gas temperature,  NOT the ambient atmospheric temperature! 

Why,  when so many use the atmosphere temperature?  Because what goes on downstream of the exhaust valve is UTTERLY IRRELEVANT to whatever happens inside the cylinder,  as long as the pressure is near atmospheric.  And what goes on inside the cylinder is what thermodynamic modeling is all about. 

GW

Last edited by GW Johnson (2024-01-23 10:57:57)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#339 2024-01-23 11:54:55

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 3,285

Re: Internal combustion engines for Mars

One wonders why the designers would opt for such a complicated cycle?  Why not pass the exhaust gases through a heat exchanger and keep the steam and exhaust gases seperate?  For that matter, the primary stage could be a small GT with better P/W than a piston engine.  GT powered cars are nothing new.  Power can be spectacular, but fuel economy has always been on the low side of mediocre.  A two-stage engine combining a GT with steam cycle, with a heat exchanger between the two, would probably beat a piston engine on both P/W and fuel economy.  I can't imagine that this idea hasn't occured to the people that design new cars.  I would imagine development costs have prevented this innovation.


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#340 2024-01-24 13:49:38

kbd512
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Registered: 2015-01-02
Posts: 7,270

Re: Internal combustion engines for Mars

Is anyone else here familiar with turbo-compounding?

The big radial engines of WWII fame used it to good effect after the war, before jet engines took over airliner service.  The Wright R-3350 was a pretty famous example of turbo-compounding, as-used in the Douglas piston engine airliners, as well as the Boeing B-29.  As far as engine efficiency is concerned, recovering the energy in the waste heat of the exhaust gas is a great place to start.  Ye olde radials connected an output shaft from the turbo to the crankshaft, but that's not the only way to do it.  The turbo can spin an electric generator and it can spin-up a flywheel as well, for faster acceleration using a less powerful engine that runs at full rated output at all times for sake of thermodynamic efficiency.

The Pratt & Whitney PT-6A turboprop is an example where the exhaust gas spins a free turbine not connected to a turbine wheel aft of the hot section, so that it doesn't need to "spin" at the exact same speed as the primary shaft that the compressor section is connected to.  The Garrett TPE-331 (or whatever they call it now) is an example of a turboprop engine where the compressor and hot section are all connected to the same shaft.  General Electric also makes very large geared turbofan engines that prevent the tips of the fan blades (which provides most of the thrust, but is much larger in diameter than the compressor section fan blades, and would thus ordinarily be moving at much higher velocity if everything was directly connected on one shaft), from going supersonic.

Anyway, turbos and gearing allows different parts of an engine to spin at very different speeds so the component in question can perform more optimally, increasing overall combustion efficiency by facilitating airflow through that explosively-driven air pump.

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#341 2024-01-25 12:55:13

GW Johnson
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From: McGregor, Texas USA
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Posts: 5,385
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Re: Internal combustion engines for Mars

2+ decades ago when I worked at Baylor U in the Aviation Sciences department,  the department had a test truck with a PT-6 and a "club" prop mounted on it,  to use as a test facility to evaluate alternate turbine fuels.  I designed the steel mount for the engine,  and the steel outrigger stabilizers that took the loads into the pavement.  I also set the mounting angle such that the occupants in the truck bed half-box that was the test cabin,  would survive either an engine fire or an uncontained engine core failure. 

I used that facility with permission while teaching an aircraft systems course.  I brought the students out and had them operate the PT-6 and prop pitch as part of teaching how gas turbine engines worked.  The idea was to make the dry classroom content "real" to students training to be future airline pilots,  who would otherwise get most of their training flying piston aircraft.  My students all told me (and others),  that that class was the very best one they had,  at Baylor. 

Experimentally,  I tested biodiesel blends with Jet-A in that PT-6.  I would diagnose combustion problems by stepping into the blast and sniffing the odors.  I could tell the source of the biodiesel:  converted grease from the fast food places smelled like French fries cooking,  and converted tallow from the slaughterhouse smelled like barbecue cooking. Any time the burner cans were not fully efficient,  you could smell kerosene in the stream.

It turns out that the biodiesel in the blend actually rejuvenates old fuel bladders that are getting stiff.  That was a pleasant surprise to me and the FAA.  That result was a part of how I was able to put a Beech King Air into "experimental" for flight tests of the experimental fuels,  and still bring it back to "standard",  all in 6 months.  The other part was exposing only one engine,  and doing a hot section overhaul on that exposed engine before going back to "standard". 

This was all 1998-vintage stuff,  long before the current "sustainable" fuels. 

GW

Last edited by GW Johnson (2024-01-25 12:58:38)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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