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It's interesting how at the high end the piston engine increasingly looks like a jet turbine. In the 919 we essentially have more than half the energy created b

It's interesting how at the high end the piston engine increasingly looks like a jet turbine. In the 919 we essentially have more than half the energy created by a rotating set of turbos, capturing a part of the huge energies usually expelled down the exhaust, in way reminiscent of the rear (hot) turbines in a jet (in this case the "compression stage" is provided by the piston engine). The big advantage, of course, shared with wankel engines, is no waste of energy accelerating and decelerating pistons hundreds of times per second, and lower frictional losses too. Porsche captures this fact by making a small V4, not some big v6 or V8. In essence, we're getting closer to pure turbine style for extracting the available energy in the fuel, because the electric part of the engine creates a way of "buffering" the energy and releasing it on demand, which of course is the major problem with jet turbines (constant output difficult to modulate). Take it to its logical conclusion and, if turbines could be made small enough, we might no longer have a physical link between the fuel-burning part of the engine and the road. Not a ICE engineer myself so happy to be corrected but this seems to be where fuel-burning vehicle engine technology is headed (I note the proliferation of turbos in road cars).

sokoloff 11y ago
Jet engines don't dominate airline usage because of good fuel consumption (their BSFC [specific fuel consumption, or "how much fuel is needed to create a set amount of power?] is not competitive with piston engines). Jet engines dominate because of high power to mass, highly reliable power production, and a good match between power available and power required in different phases of flight. Non-flat-rated jet turbines lose power with altitude and temperature right off the deck. Some are flat-rated (higher thermodynamic power capability than physical power capability), but even those lose power at some altitude. This is a good match for high power required at takeoff and less power required at cruise altitudes.
- vegabook 11y ago
  why do most power stations use turbines and not pistons? There is no need for power to mass ratios there?
  - sokoloff 11y ago
    That's a great question. I do not know for certain, but I suspect the answer lies in reliability and much longer intervals between servicing and overhauls. Piston aircraft engines have a recommended time between overhaul of 1200-2000 hours (with outliers above and below). Jet aircraft engines have a recommended time between overhaul of several times that, with required inspections typically at slightly longer intervals than piston engine overhauls. (In a fixed install turbine plant, you might be able to skip the inspections as well, and just use trend monitoring data to determine when to take the turbine out of service for maintenance/overhaul. In an aircraft carrying people, the risk profile is a little different.)
  - msandford 11y ago
    There's a difference between turbine and jet engine. Natural gas peaker plants are very expensive to operate due to fuel cost. Most big power plants do use turbines to spin the generators but the turbine is fed steam, not hot combustion gas as in a jet engine.
  - Gravityloss 11y ago
    It depends, there are also many power plants that use diesel engines. Knowing about ship propulsion, compared to turbines, diesels have higher efficiency and longer times between overhaul. Turbines are lighter. Both mean that if you use turbines, you pay more for fuel and your ship must be docked more often which are a big nono in commercial shipping. Generally only the military uses turbines in ships. Sometimes they are installed in for example cruise ships if they need a lot of power and there's no space, / there are balance issues with diesels. Probably on the land side if you only run it rarely, or your natural gas is extremely cheap, then a less efficient turbine is fine. Generally power plant frame turbines are more old fashioned and less efficient than aircraft turbines.
rebootthesystem 11y ago
Interesting comment but I am having trouble wrapping my head around the idea. A turbocharger doesn't create energy. It's function is to increase the volumetric efficiency of the IC engine. How? By allowing more fuel to be pumped into the same size engine. It really is that simple. Ideal combustion of gasoline occurs at the stoichiometric ratio, which is 14.7 to 1 (roughly, lots of variables). Let's call it 15 to 1. That means you need 15 grams of air [1] for every gram of gasoline you pump into the engine. How do you generate more power? You burn more fuel per unit time. How do you burn more fuel per unit time? You build a bigger engine or you figure out how to pump more fuel into the same engine. What is the limiting factor in being able to pump more fuel into the same engine? You can't get enough air into a naturally aspirated engine in order to maintain the roughly 15 to 1 ratio. If you can't get more air into the engine you can't pump more fuel into it. Period. How do we get more air into the engine? You build a compressor. The compressor can be powered directly from the engine's drive shaft, in which case it is called a "supercharger", or it can be powered by exhaust gasses, which is known as a "turbocharger". A turbocharger doesn't turn a piston engine into a turbine. It's function is to forcefully shove more air into the combustion chamber in order to allow a proportionally greater amount of fuel to be burned per unit time, thereby producing more power per engine unit volume, often termed "volumetric efficiency". A greater volumetric efficiency does NOT mean an engine is more efficient. This is often a mistake made by consumers when buying cars. A turbocharger allows you to make more power with the same engine. It does not make it more efficient other than recovering some of the energy lost through exhaust. Having owned several turbocharged and twin turbocharged fire-breathing monsters over the years I can tell you fuel efficiency was never better than non-turbo cars and, in my case at least, never a criteria for buying them. The latter said with a big fucking grin. And I have the speeding ticket collection to prove it. officer: Do you know how fast you were going? I could not catch up to you until I turned on the lights and you slowed down. driver (not to be identified): Not sure. I stopped looking at the speedometer at 120. officer: If you want to get out of this ticket you have to let me take this thing for a spin. driver (now absolutely perplexed): Here are the keys, sir That was the longest uncomfortable 45 minute wait standing by a CHP patrol car in the middle of the California desert. The grin in the officer's face told the driver (not to be identified) all would be well. Warning issued. Set cruise control to 100 for the rest of the desert crossing. [1] Actually, I think it's oxygen, not air. I don't remember, it's been decades since I studied this. I'll just call it "air" as a generalized yet possibly inaccurate term.
msandford 11y ago
You actually don't waste any energy accelerating pistons as the system is balanced. If it wasn't you shake the engine apart due to huge amounts of vibration. The crankshaft has counter weights so that it is balanced and the individual pistons all balance each other out. It's like a see-saw, yes the mass isn't evenly distributed but it is balanced. And it takes only a small amount of force to make it oscillate relative to the force required to support it.
- vegabook 11y ago
  I stand corrected, and I think I've figured it out. Even without burning fuel, in a frictionless system, the decelerating piston would be putting its energy back into the crankshaft? So it draws energy then puts it back every 180 degrees? Takes energy on the acceleration, puts it back on the deceleration? I suppose the friction is the main loss source then?
  - aunty_helen 11y ago
    Conversion of linear motion to rotational motion is the loss you're looking for. To your point about effectively gas turbine/electric drive trains from your OP, the problem is efficency in burning the fuel. This comes from the lack of achievable compression ratio in a turbine system. 6:1 is good for a jet, 13:1 is good for a N/A piston engine (not sure what race engines run exactly). Higher compression ratio = better fuel efficency.

sokoloff 11y ago

Jet engines don't dominate airline usage because of good fuel consumption (their BSFC [specific fuel consumption, or "how much fuel is needed to create a set amount of power?] is not competitive with piston engines). Jet engines dominate because of high power to mass, highly reliable power production, and a good match between power available and power required in different phases of flight. Non-flat-rated jet turbines lose power with altitude and temperature right off the deck. Some are flat-rated (higher thermodynamic power capability than physical power capability), but even those lose power at some altitude. This is a good match for high power required at takeoff and less power required at cruise altitudes.

vegabook 11y ago
why do most power stations use turbines and not pistons? There is no need for power to mass ratios there?
- sokoloff 11y ago
  That's a great question. I do not know for certain, but I suspect the answer lies in reliability and much longer intervals between servicing and overhauls. Piston aircraft engines have a recommended time between overhaul of 1200-2000 hours (with outliers above and below). Jet aircraft engines have a recommended time between overhaul of several times that, with required inspections typically at slightly longer intervals than piston engine overhauls. (In a fixed install turbine plant, you might be able to skip the inspections as well, and just use trend monitoring data to determine when to take the turbine out of service for maintenance/overhaul. In an aircraft carrying people, the risk profile is a little different.)
- msandford 11y ago
  There's a difference between turbine and jet engine. Natural gas peaker plants are very expensive to operate due to fuel cost. Most big power plants do use turbines to spin the generators but the turbine is fed steam, not hot combustion gas as in a jet engine.
- Gravityloss 11y ago
  It depends, there are also many power plants that use diesel engines. Knowing about ship propulsion, compared to turbines, diesels have higher efficiency and longer times between overhaul. Turbines are lighter. Both mean that if you use turbines, you pay more for fuel and your ship must be docked more often which are a big nono in commercial shipping. Generally only the military uses turbines in ships. Sometimes they are installed in for example cruise ships if they need a lot of power and there's no space, / there are balance issues with diesels. Probably on the land side if you only run it rarely, or your natural gas is extremely cheap, then a less efficient turbine is fine. Generally power plant frame turbines are more old fashioned and less efficient than aircraft turbines.

rebootthesystem 11y ago

Interesting comment but I am having trouble wrapping my head around the idea. A turbocharger doesn't create energy. It's function is to increase the volumetric efficiency of the IC engine. How? By allowing more fuel to be pumped into the same size engine. It really is that simple. Ideal combustion of gasoline occurs at the stoichiometric ratio, which is 14.7 to 1 (roughly, lots of variables). Let's call it 15 to 1. That means you need 15 grams of air [1] for every gram of gasoline you pump into the engine. How do you generate more power? You burn more fuel per unit time. How do you burn more fuel per unit time? You build a bigger engine or you figure out how to pump more fuel into the same engine. What is the limiting factor in being able to pump more fuel into the same engine? You can't get enough air into a naturally aspirated engine in order to maintain the roughly 15 to 1 ratio. If you can't get more air into the engine you can't pump more fuel into it. Period. How do we get more air into the engine? You build a compressor. The compressor can be powered directly from the engine's drive shaft, in which case it is called a "supercharger", or it can be powered by exhaust gasses, which is known as a "turbocharger". A turbocharger doesn't turn a piston engine into a turbine. It's function is to forcefully shove more air into the combustion chamber in order to allow a proportionally greater amount of fuel to be burned per unit time, thereby producing more power per engine unit volume, often termed "volumetric efficiency". A greater volumetric efficiency does NOT mean an engine is more efficient. This is often a mistake made by consumers when buying cars. A turbocharger allows you to make more power with the same engine. It does not make it more efficient other than recovering some of the energy lost through exhaust. Having owned several turbocharged and twin turbocharged fire-breathing monsters over the years I can tell you fuel efficiency was never better than non-turbo cars and, in my case at least, never a criteria for buying them. The latter said with a big fucking grin. And I have the speeding ticket collection to prove it. officer: Do you know how fast you were going? I could not catch up to you until I turned on the lights and you slowed down. driver (not to be identified): Not sure. I stopped looking at the speedometer at 120. officer: If you want to get out of this ticket you have to let me take this thing for a spin. driver (now absolutely perplexed): Here are the keys, sir That was the longest uncomfortable 45 minute wait standing by a CHP patrol car in the middle of the California desert. The grin in the officer's face told the driver (not to be identified) all would be well. Warning issued. Set cruise control to 100 for the rest of the desert crossing. [1] Actually, I think it's oxygen, not air. I don't remember, it's been decades since I studied this. I'll just call it "air" as a generalized yet possibly inaccurate term.

msandford 11y ago

You actually don't waste any energy accelerating pistons as the system is balanced. If it wasn't you shake the engine apart due to huge amounts of vibration. The crankshaft has counter weights so that it is balanced and the individual pistons all balance each other out. It's like a see-saw, yes the mass isn't evenly distributed but it is balanced. And it takes only a small amount of force to make it oscillate relative to the force required to support it.

vegabook 11y ago
I stand corrected, and I think I've figured it out. Even without burning fuel, in a frictionless system, the decelerating piston would be putting its energy back into the crankshaft? So it draws energy then puts it back every 180 degrees? Takes energy on the acceleration, puts it back on the deceleration? I suppose the friction is the main loss source then?
- aunty_helen 11y ago
  Conversion of linear motion to rotational motion is the loss you're looking for. To your point about effectively gas turbine/electric drive trains from your OP, the problem is efficency in burning the fuel. This comes from the lack of achievable compression ratio in a turbine system. 6:1 is good for a jet, 13:1 is good for a N/A piston engine (not sure what race engines run exactly). Higher compression ratio = better fuel efficency.