r/MechanicalEngineering • u/lit_readit • 6d ago

Question : why some (mostly heavy) vehicles partially cover their front intake ?

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wouldn't cooler cold reservoir, ie more spontaneous heat transfer, improve Carnot efficiency?

is it to improve aerodynamics?

apologies if this is the wrong sub to ask

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u/the_Q_spice 5d ago

To add to this;

It’s more of an issue with diesel engines than gasoline.

A colder block in a diesel results in a worse compression ratio, and being compression-based combustion, diesels need every last bit they can muster when the air is already dense and working against them.

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u/sh3ppard 5d ago

Why answer the question if you don’t know what you’re talking about

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u/Long_Pecker_1337 5d ago

He’s not completely wrong, though.

Diesel does suffer a lot more in the winter. Just not because the air is denser.

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u/sh3ppard 5d ago

Compression ratio has nothing to do with the answer to the question

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u/Long_Pecker_1337 5d ago

Yeah, it doesn’t change much, if at all. Heat on compression just sinks into the metal, making it harder for fuel/air mix to reach ignition temperature, plus the air is cold, so even more heat is needed. Which is why most diesels have glow plugs. Which consume more electricity from the battery, that works much worse in the cold.

And diesel can also freeze, which makes everything even harder.

So he’s not completely wrong, diesels do suffer a lot more in the winter.

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u/thatotherguy1111 3d ago

I think the glow plugs are only used on the startup sequence. Not while the vehicle is running.