Reheat / afterburners
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Join Date: Jan 2002
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Reheat / afterburners
A question that occurred to me the other day...
An afterburner, as I understand it, is a fairly low-tech device whereby raw fuel is injected into the hot exhaust gases of a jet engine. The raw fuel burns, creating greatly increased thrust, although at the expense of greatly increased fuel burn rate.
I would have assumed that, for any well-designed and correctly-tuned internal combustion engine, the exhaust gases shouldn't contain any oxygen - the oxygen should all have been used up by ensuring that the fuel/air mix was correct to start with. However, if that was the case, the afterburner fuel wouldn't have anything to react with.
What am I missing? Are the just engines leaned off when afterburner is selected, allowing some oxygen to make it through?
Cheers for any pointers...
An afterburner, as I understand it, is a fairly low-tech device whereby raw fuel is injected into the hot exhaust gases of a jet engine. The raw fuel burns, creating greatly increased thrust, although at the expense of greatly increased fuel burn rate.
I would have assumed that, for any well-designed and correctly-tuned internal combustion engine, the exhaust gases shouldn't contain any oxygen - the oxygen should all have been used up by ensuring that the fuel/air mix was correct to start with. However, if that was the case, the afterburner fuel wouldn't have anything to react with.
What am I missing? Are the just engines leaned off when afterburner is selected, allowing some oxygen to make it through?
Cheers for any pointers...
Join Date: Sep 2002
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Gas turbine engines don't come close to using all the oxygen in the airflow; if they did the turbine temperatures would be ENORMOUS - both due to the huge amount of combustion going on and due to the lack of (unconsumed) air to use as cooling flow.
Gizajob
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Tuning a combustion process so that there is excess air is normal - in large industrial boilers for instance, control of excess air is vital to the running of the boiler.
Car engines are no different. As mentioned before, stoichiometric (perfect fuel/air mix) mixtures burn at very high temps and the engine management system will only allow those kind of mixtures under light load - if the engine is under heavy load, detonation may occur.
Car engines are no different. As mentioned before, stoichiometric (perfect fuel/air mix) mixtures burn at very high temps and the engine management system will only allow those kind of mixtures under light load - if the engine is under heavy load, detonation may occur.
