This image, referenced in this question, shows in green the pressure value in a jet engine:

enter image description here

The combustion section between compressor and turbine is where fuel is injected, mixed with air, and burnt. Why is there a pressure drop while combustion occurs and temperature suddenly increases by 1,500°C (to be compared with pressure increase after ignition in a reciprocating engine).


The air moves¹ in direction of decreasing pressure except where forced by the compressor. So the pressure must be decreasing to maintain desired flow. If the pressure was increasing, the flow would stop and reverse and the engine would stop operating.

In fact, that's exactly what happens if fuel is added too fast: the energy quickly raises, but the still slow-spinning turbine provides too much resistance, so pressure will increase above what the compressor can provide and the compressor will stall, the engine will emit loud bang and some flames from both ends and likely flame out as it runs out of oxygen for a moment.

As already explained, in the normal flow the released energy increases the velocity of the flow instead of pressure.

¹ More precisely accelerates. If the pressure was constant, it would keep moving. But it must not raise. It decreases slightly due to friction.


The question you linked to contains the answer to your question:

the small pressure drop in the combustor is caused by friction

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    $\begingroup$ @mins: You have a restricted volume in a piston engine, while the volume inside the combustor is open at the rear end. Granted, there are some turbine blades in the way, but in general jet engine design is about restricting flow more at the front and less at the rear, so the air knows which direction to flow. $\endgroup$ – Peter Kämpf Nov 20 '16 at 16:13

In a reciprocating engine, the gases are trapped until the exhaust valve opens, unlike a jet engine where it's an open exhaust. In a cylinder the gases are not accelerated. So, v rise in a jet, p rise in an engine cylinder. Temperature rise in both.


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