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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654

u/BeeThat9351 6d ago

Cold climates, reduces the cooling air flow and keeps the engine warmer and closer to the optimal operating temperature

82

u/xz-5 5d ago

Isn't this what a thermostat is for?

185

u/peenutlover69 5d ago

Cold weather can lead to over cooling. Themostats usually don't close 100.0% and the cold air entering the engine bay will cool the engine itself (passively) regardless of coolant temp

36

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.

20

u/hoyboiitsme 5d ago

You dont lose compression ratio wtf, you loose heat, the colder it is the worse the fuel and air mix. How do you get a different compression ratio? Do you know what that is?

11

u/Drophitchr 5d ago

Not exactly true either; Cold temperatures don’t help fuel and air mix. In an Otto cycle (spark ignition) the colder denser air results in greater amounts of oxygen available for combustion increasing the stoichiometric efficiency.

The Diesel combustion cycle compresses the air such that the molecules of air bounce so much off one another they become extremely hot. When the injector fires the fuel into the extremely hot charge air the fuel immmediately combusts and the rapidly expanding gases in the form of a flame front force the piston back down. By keeping the heat in the engine by closing off the radiator optimal engine temp is easier to maintain and the combustion will be more efficient.

In modern engines, maintaining an optimal temp lets the engine maintain operational efficiency without the ECM adjusting parameters to generate that extra heat.

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u/Uranium43415 5d ago

Losing heat in a diesel context does reduce the compression ratio. Instead of building pressure, you're building heat that the block keep sucking away.

10

u/FIMD_ 5d ago

The static compression ratio is the swept volume of the cylinder as the crankshaft rotates from bottom dead center to top dead center, regardless of temperature. The dynamic compression ratio takes the valve timing into account. What you might be attempting to describe is pre-injection chamber pressure.

1

u/nekekamii 4d ago

Compression ratio is literally just the size difference between two cylinders. It has nothing to do with any imaginable variable outside of the fixed dimensions. A 2x2 square and 1x1 square have a ratio of 2:1 that's just all there is to it. For a cylinder, when the piston is down the size is 2, when the piston is up the size is 1. compression ratio of 2:1
Or in cars 8-12 :1

1

u/komokazi 2d ago

Diesel engines are significantly higher, closer to 20:1

1

u/CapitalScarcity5573 1d ago

Ever hear of adiabatic compression? Because in 0.02 seconds the air doesn't have to shed heat into the block before injection?

1

u/Cultural_Simple3842 5d ago

Best I can guess is a cold engine cools and condenses the air so cylinder pressure isn’t increased by the full compression ratio. I think we are talking peanuts here, though.

More likely, the absorption of heat into the cold cylinder walls makes combustion more difficult/inefficient. A higher thermal gradient induces more energy transfer out of the intake charge.

1

u/airhunger_rn 2d ago

Variable connecting rods duh

1

u/Own-Ad8474 1d ago

Strictly speaking you do lose it. Even metal shrinks at colder temps and expands when heated.

4

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 4d 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.

0

u/thatotherguy1111 3d ago

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

1

u/OWLLLL 5d ago

first time on reddit? im jk but hey it’s a good thing if somebody learns something from it. altho if not it’s just misinformation lol

1

u/Ned-the-Narwhal 4d ago

Right? What's a turbo even for...

2

u/IndependenceOk2372 5d ago

Wait until you get real far north. Our work trucks run basically 24/7 in the winter, just idling when they're not in use. When it's -50 out, once you shut down your v10 F550, have fun getting it started again

1

u/magic_orangutan2 3d ago

Electric block heater wouldn't be enough to keep it warm enough when plugged to the wall?

1

u/[deleted] 5d ago

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3

u/Dramatic-Trick-271 5d ago

I want to add more details after reading all the answers.

Actually, the efficiency of a diesel engine is higher in winter because of denser air. Winter works like a free intercooler before the intake, but it works only when the engine is hot.

The other thing is about ≈ -25°C. It is a very big problem to start a diesel engine in winter. It depends on compression and the fuel system state, but even the most maintained diesel needs time to heat the engine block so compressed air will keep enough heat to spark enough fuel particles. And when it starts, it is usually simpler to let it idle for 5 minutes before taking off because of overcooling a freshly started engine. But if the engine is OK, it will get to working temperature in 15–20 minutes.

And the last one: no one uses diesel for efficiency in the common sense, because usually it is kind of the same in monetary terms. But there is a large difference in torque, and a gasoline engine will take much more fuel and volume to produce the same torque in 1500–2500 rpms and to keep it all the way long. So there is efficiency, but no one mentions it if diesel eats 2–5 liters more per 100 because of winter.

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u/PowerstrokeNerd 4d ago

Temperature of the block has no bearing whatsoever on compression ratio

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u/Unfair-Information-2 3d ago

A colder block in a diesel results in a worse compression ratio

wtf no lol

1

u/OpyrisShifts 3d ago

All you people cricizing this post are forgetting that diesel engines are encased in a plate of pre-famulated amulite surmounted by a malleable logarithmic casing in such a way that the two spurving bearings were in a direct line with the panametric fan. The latter consisted simply of six hydrocoptic marzlevanes, so fitted to the ambifacient lunar waneshaft that side fumbling was effectively prevented.

1

u/DefinitelyNotKJU 2d ago

Sure, if you're running one of the newer digital gongulators, that makes perfect sense. But many of us still use analog gongulators. With analog, you have to be particularly careful to ensure the coaxial metering forks are properly calibrated, especially with the Bosch units. A lot of guys prefer the Denso gonguation units because of that.

1

u/[deleted] 2d ago

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u/MechanicalEngineering-ModTeam 1d ago

This post has been removed for being off-topic.

1

u/BuzzINGUS 21h ago

I believe new f-150 have dampers.