10 ms·

Why would it be bad to put hot air into the engine? For blast furnaces you want to preheat the air going in so that it does not cool down the furnace. Is it ju

Why would it be bad to put hot air into the engine? For blast furnaces you want to preheat the air going in so that it does not cool down the furnace. Is it just about cooling the engine down, or is it something else?

curveship 12y ago
cool air = more dense = more oxygen to burn
matthavener 12y ago
Cold air is denser. Denser air contains more oxygen for combustion, and won't get as hot after compression. In 4-stroke engines, this means more power.
Gracana 12y ago
The engine and turbo compress and heat the air charge immensely. Others have mentioned air density, but while dense air is nice, the increased temperature is the important part when it comes to high performance turbo applications. Everyone knows that overheating an engine is bad, but what actually happens when you push things too far? If your combustion chamber grows too hot, you can suffer from pre-ignition, where your fuel-air mixture ignites sooner than it should. This results in wasted power and increased temps as the engine has to compress the now-expanding combustion charge. What often follows pre-ignition (provided nothing has melted or blown up yet) is detonation. In normal combustion, you have a flame front that expands smoothly from the ignition spark and causes a smooth increase in pressure. It occurs quickly, but it's not an explosion. Detonation is the opposite. Pockets of fuel-air mixture in any part of the combustion chamber ignite explosively, sending out sharp shockwaves that are too fast to be absorbed by the rotating mass of the engine. If the detonations are too powerful or too frequent, they can cause catastrophic damage. There's plenty more that can be explained on the subject and I'm sure somebody can find something to correct, but that's the gist of it. The engine is already hot enough, higher intake temps make things worse, go too far and you get abnormal combustion which results in spontaneous disassembly.
sbierwagen 12y ago
All three of the replies to your comment are true, but they don't cover everything. Car engines are heat engines, they extract mechanical work from differences in temperature. Heat engine efficiency is n = 1 - sqrt(Tcold/Thot) If Tcold, the cold end, isn't cold enough, then you lose efficiency, and thus horsepower. A badly overheating engine loses a lot of power. If you can cool the compressed air back down to ambient, then your engine produces more power.
- grogers 12y ago
  This seems orthogonal to me. Tcold for the efficiency is the exhaust temp, not the intake. For pure efficiency, it is actually beneficial to capture waste heat in the exhaust to heat the intake air. But it destroys your power density, for the reasons already mentioned (hot air has less mass at the same pressure). I believe some automotive engines actually do this under light load conditions, but you wouldn't see it in F1.

curveship 12y ago

cool air = more dense = more oxygen to burn

matthavener 12y ago

Cold air is denser. Denser air contains more oxygen for combustion, and won't get as hot after compression. In 4-stroke engines, this means more power.

Gracana 12y ago

The engine and turbo compress and heat the air charge immensely. Others have mentioned air density, but while dense air is nice, the increased temperature is the important part when it comes to high performance turbo applications. Everyone knows that overheating an engine is bad, but what actually happens when you push things too far? If your combustion chamber grows too hot, you can suffer from pre-ignition, where your fuel-air mixture ignites sooner than it should. This results in wasted power and increased temps as the engine has to compress the now-expanding combustion charge. What often follows pre-ignition (provided nothing has melted or blown up yet) is detonation. In normal combustion, you have a flame front that expands smoothly from the ignition spark and causes a smooth increase in pressure. It occurs quickly, but it's not an explosion. Detonation is the opposite. Pockets of fuel-air mixture in any part of the combustion chamber ignite explosively, sending out sharp shockwaves that are too fast to be absorbed by the rotating mass of the engine. If the detonations are too powerful or too frequent, they can cause catastrophic damage. There's plenty more that can be explained on the subject and I'm sure somebody can find something to correct, but that's the gist of it. The engine is already hot enough, higher intake temps make things worse, go too far and you get abnormal combustion which results in spontaneous disassembly.

sbierwagen 12y ago

All three of the replies to your comment are true, but they don't cover everything. Car engines are heat engines, they extract mechanical work from differences in temperature. Heat engine efficiency is n = 1 - sqrt(Tcold/Thot) If Tcold, the cold end, isn't cold enough, then you lose efficiency, and thus horsepower. A badly overheating engine loses a lot of power. If you can cool the compressed air back down to ambient, then your engine produces more power.

grogers 12y ago
This seems orthogonal to me. Tcold for the efficiency is the exhaust temp, not the intake. For pure efficiency, it is actually beneficial to capture waste heat in the exhaust to heat the intake air. But it destroys your power density, for the reasons already mentioned (hot air has less mass at the same pressure). I believe some automotive engines actually do this under light load conditions, but you wouldn't see it in F1.