6 ms·

The reason why microturbines are not taking off is, as you mentioned, low efficiency. "Not fantastic" is a bit of an understatement. Especially if you want the

by mppm 2y ago

The reason why microturbines are not taking off is, as you mentioned, low efficiency. "Not fantastic" is a bit of an understatement. Especially if you want the turbine to be reasonably cheap (no superalloys, etc) and if it runs below maximum capacity, you'd probably be happy to get 15-20% out of it, not even half of what is achievable with ICEs of the same size. There are not many applications where power-to-weight-ratio is important enough to overcome that limitation.

ahartmetz 2y ago
I just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?
- kragen 2y ago
  Ethanol, canola oil, or baby oil might be reasonable things to carry with you if you want to lighten your backpack or just reduce your risk of blindness.
  - ahartmetz 2y ago
    Well, obviously you are not supposed to drink it! For reasons that I don't know, methanol is more commonly used as fuel than ethanol. A nice thing about methanol and ethanol is that they evaporate without a trace if there is a minor spill. That is not true for most any distilled petroleum product or any vegetable oils.
    - dredmorbius 2y ago
      Lighter weights of petroleum oils (from petrol through natural gas) are highly volatile and will typically evaporate with minimal (though probably nonzero) residue. That's what makes them attractive as fuels generally as they require little persuasion to vapourise. OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels). Vegetable oils are nonvolatile, but also generally nontoxic and hence mostly environmentally benign. (You can choke a river or foul ground-dwelling creatures given sufficient quantities, but a few 100 ml won't cause major problems.)
      - jabl 2y ago
        > OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels). Anti-knock capability of a fuel has very little to do with how "lightweight" they are. Methane, the lightest hydrocarbon and gaseous at any kind of condition you'll find in an engine, has an octane rating of 120. And diesel fuel, substantially heavier than gasoline, as a much lower octane rating than gasoline.
        dredmorbius 2y ago
        Huh, I'd not known that about diesel. What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1). Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW. Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand. I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
        jabl 2y ago
        > What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1). Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline. What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well. > This information I'm remembering from Yergin's The Prize, FWIW. A pretty good book, I hear. I should read it. > Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand. Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed. If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW. > I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be. I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.
    - kragen 2y ago
      If you spill it, you might inhale a bunch by accident. Yeah, soaking your sleeping bag with canola oil would be a pretty bad problem. But a methanol or ethanol spill can also do significant damage. Xylene or citrus terpenes might be nicer, even if the lethal dose is lower than for ethanol.

by mppm 2y ago

ahartmetz 2y ago

I just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?

kragen 2y ago
Ethanol, canola oil, or baby oil might be reasonable things to carry with you if you want to lighten your backpack or just reduce your risk of blindness.
- ahartmetz 2y ago
  Well, obviously you are not supposed to drink it! For reasons that I don't know, methanol is more commonly used as fuel than ethanol. A nice thing about methanol and ethanol is that they evaporate without a trace if there is a minor spill. That is not true for most any distilled petroleum product or any vegetable oils.
  - dredmorbius 2y ago
    Lighter weights of petroleum oils (from petrol through natural gas) are highly volatile and will typically evaporate with minimal (though probably nonzero) residue. That's what makes them attractive as fuels generally as they require little persuasion to vapourise. OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels). Vegetable oils are nonvolatile, but also generally nontoxic and hence mostly environmentally benign. (You can choke a river or foul ground-dwelling creatures given sufficient quantities, but a few 100 ml won't cause major problems.)
    - jabl 2y ago
      > OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels). Anti-knock capability of a fuel has very little to do with how "lightweight" they are. Methane, the lightest hydrocarbon and gaseous at any kind of condition you'll find in an engine, has an octane rating of 120. And diesel fuel, substantially heavier than gasoline, as a much lower octane rating than gasoline.
      - dredmorbius 2y ago
        Huh, I'd not known that about diesel. What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1). Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW. Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand. I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
        jabl 2y ago
        > What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1). Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline. What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well. > This information I'm remembering from Yergin's The Prize, FWIW. A pretty good book, I hear. I should read it. > Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand. Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed. If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW. > I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be. I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.
  - kragen 2y ago
    If you spill it, you might inhale a bunch by accident. Yeah, soaking your sleeping bag with canola oil would be a pretty bad problem. But a methanol or ethanol spill can also do significant damage. Xylene or citrus terpenes might be nicer, even if the lethal dose is lower than for ethanol.