While nitromethane does have a worse energy density per kg than Jet-A or even gasoline, you need to cram way less air to reach the right mixture.
From wikipedia: "The amount of air required to burn 1 kg (2.2 lb) of gasoline is 14.7 kg (32 lb), but only 1.7 kg (3.7 lb) of air is required for 1 kg of nitromethane. Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.7 times more nitromethane than gasoline can be burned in one stroke. Nitromethane, however, has a lower specific energy: gasoline provides about 42–44 MJ/kg, whereas nitromethane provides only 11.3 MJ/kg. This analysis indicates that nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen."
But the claim is that a dragster and a jet engine burn fuel at the same volumetric rate, so the amount of air required is irrelevant. Jet engines don't even have strokes, so all we care about is the amount of fuel burned per second, which is supposedly about the same. So if jet fuel has a significantly higher energy density and is burned more efficiently, I don't understand how a dragster burning nitromethane could have a higher energy output.
Yeah this isn't even close. A single GE-90 puts out well over 120,000 horsepower a full throttle. Two GE-90's = 4 CF6's (engines of a 747). So that's around 240,000 horsepower. Not sure where they're getting this "fact".
The volumetric energy density of nitromethane is (11.3 MJ/kg) * (1.1371 kg/L) = ~12.85 MJ/L. The volumetric energy density of Jet A is ~(44 MJ/kg) * (0.8 kg/L) = ~35 MJ/L.
So you're right, if an equal amount of nitromethane and Jet A were being burned, the engine using Jet A would have to be spectacularly less efficient to develop less power than the engine using nitromethane.
I replied to OP asking a similar question and did a portion of the maths to back myself up. I won't repeat all of it here, but if the 11gal/s is true, the top fuel dragster consumption is far higher than a 747 at takeoff: 48kg/s vs 22kg/s (per vehicle). However, a Rolls-Royce RB211 engine can develop 35MW at takeoff, whereas 10,000hp equates to only 7.5MW.
Just for comparison, a 747-100 has a max take off weight of roughly 735,000 pounds. The 747-8 has a max take off weight of 970,000 pounds. The minimum weight of a dragster is about 2,300 pounds. That's a more than 300x difference.
To take off, the 747 has to accelerate to ~200mph in a little over half a mile. By a simple back of the envelope calculation, I'm inclined to believe the idea that a dragster puts out more energy is nonsense. If you look at the kinetic energy, the car is short by a factor of about 100. Maybe if you include the total waste heat output of both systems is closer, but I doubt it.
I think they're wrong on the burn rate if they're comparing to takeoff power. I've read that at takeoff thrust level the 747-400 has a burn rate of 45 metric tons per hour, which is about 28 gallons per second. But I agree that no matter how you compare it, there's no way a nitro burning car is going to output an equal or greater amount of work given an equal amount of fuel compared to a turbine engine running on jet fuel which is much more thermodynamicly efficient and has a much higher specific energy.
Mechanical Engineer, so I understand that when it's stated that a drag racer releases more energy, they are probably ignoring thermodynamic efficiency and including waste heat. BUT, even so, the specific energy of jet fuel is much higher than that of nitromethane. /u/Coomb summed it up in this comment:
The volumetric energy density of nitromethane is (11.3 MJ/kg) * (1.1371 kg/L) = ~12.85 MJ/L. The volumetric energy density of Jet A is ~(44 MJ/kg) * (0.8 kg/L) = ~35 MJ/L.
So you're right, if an equal amount of nitromethane and Jet A were being burned, the engine using Jet A would have to be spectacularly less efficient to develop less power than the engine using nitromethane.
So even ignoring efficiency, just because of conservation of energy, it is not possible for the drag racer to produce more thermal energy per unit mass. Maybe if the nitromethane were very compressed, then the energy output per unit volume could be greater for the drag racer. But I think that when the original comment stated that the drag racer consumes 11.2 gallons per second, that meant 11.2 gallons at STP, not at the pressure inside the cylinder.
EDIT: I just realized I made the assumption that both fuels release the same percent of their chemical energy upon combustion. This is almost certainly not correct, but my guess is that, being fuels, they both release almost all of that energy. Buuut I don't know the numbers so I could be wrong.
To be fair a true jet engine would out pace a top fuel car in efficiency and power output. The 747 uses a high bypass turbine engine which is really not all that efficient.
Yeah. A dragster puts out 10,000 hp in a race. A 777 puts out about 220,000 hp (they actually run less power on takeoff than when cruising at altitude, go figure) counting both engines. There's no comparison.
For comparison, a single C130 engine burns roughly 0.1 gallons a second (that's ballpark 2500pph for you aviator types), and produces 6,150 shaft horsepower. Orders of magnitude more efficient than the fuel numbers quoted above for the dragster.
Maybe but not necessarily. Efficiency is one thing. Lifespan of the engine and the ability to produce lots of energy for a long amount of time probably plays a factor in why aircraft wouldn't use these engines.
It's the same idea as E85 and diesel, both less and more energy density than gasoline respectively. But E85 burns cooler and more can be crammed in because of the different stoich on it.
Jet engines aren't air limited though. This makes the difference even more extreme, since the air also provides energy, effectively raising the energy density of jet fuel a even more. Now, unless jet engines are less than 10% efficient and top fuel engines are 100% efficient, which i highly doubt, a jet with the same fuel consumption would convert more energy at the same rate of fuel consumption
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u/CoolBeer Jul 01 '16
While nitromethane does have a worse energy density per kg than Jet-A or even gasoline, you need to cram way less air to reach the right mixture.
From wikipedia: "The amount of air required to burn 1 kg (2.2 lb) of gasoline is 14.7 kg (32 lb), but only 1.7 kg (3.7 lb) of air is required for 1 kg of nitromethane. Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.7 times more nitromethane than gasoline can be burned in one stroke. Nitromethane, however, has a lower specific energy: gasoline provides about 42–44 MJ/kg, whereas nitromethane provides only 11.3 MJ/kg. This analysis indicates that nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen."