DC-2/DC-3 Engine exhaust question

An interesting chart from a postwar SAE Journal showing that the largest single use of the potential energy in fuel in a Wright engine is exhaust heat. This was in connection with utilising exhaust driven turbo chargers, but explains why things glow hot ! What is also amazing is the work done to manage differential expansion of engine materials, so the engine would not seize or fall apart.

About your question about early jet fighters not glowing red….

The aft fuselages were lined with asbestos.
Also, the combustion section of the jet is several feet from the exhaust area…unlike radials and many turbo installations.

That’s a great explanation Tony. Thanks for that.
Further to your point, is listening to airliners landing.
When most utilize their thrust reversers, the only flow that gets deflected is the cold stream.
The now exposed hot stream, is the majority of the noise that you hear due to the shear you described.

Andy

Hi TonyT – PM sent

That certainly does help Tony – thank you
Keith

Even older engines tend to be a bypass engine, it is only the very early stuff that isn’t a bypass engine, which puts all the air through the combustion chamber a bypass however is double skinned, and a portion of the air from the compressor though still warm from compressing passes over the core engine with the combustion chamber inside it and and cools as it goes, it then mixes with the exhaust air.

Ok let me explain a large percentage of the noise you hear from a jet engine, it the crackle is caused by air shear, that is the hot exhaust flowing out at high speed and rubbing against the cold air surrounding it, by putting a warm layer of slower moving gas between them you can reduce that noise, hence why airliners now have high ratio bypass engines.

In these such as the RB211 the large fan which in effect is a ducted prop pushes cool air backwards and produces about 70% of the thrust of the engine, the remaining 30% produced goes through the core engine, combustion chamber and spins the turbines which absorb some of its output and the joins the airflow, the large mass of cool air moving rearwards forms a barrier against the hot gas moving at roughly the same speed and reduces the noise, which is one reason engines evolved that way, that plus efficiency, so you do not see much heat.. I say turbines because you have multiple stages, an ideal would be one turbine for each stage of compressor, because a compressor is more efficient at near supersonic tip speeds, but the weight etc would be ridiculous, so you have 2 or 3 of them running different compressor stages such as the fan, a low pressure compressor and a high pressure compressor.

Does that help?

Many thanks for the responses – I wondered why the metal around the exhaust of say early jet fighters didn’t turn red – is this because of different types of metal?

Yup, a friend flying a twin across the channel at night was perturbed to see the white glow of the turbos through the cowling.

Exhaust gases are hot. That is normal for any piston engine.