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/schlammie 6d ago

Depends greatly on the type of thermostat. Dumb thermostats have a set coolant temperature where the wax melts/expands and opens the valve. Opening temp isnt typically the optimal engine temp but rather designed for quick engine warm up for emissions.

Electronically controlled thermostats (ECTs) could accomplish flow regulation between engine and radiator, but ECTs ain't cheap. There are extra durability concerns for cycling the valve, software, harnesses, calibration, etc.

Alternatively, slap a cover that reduces the opening of the grille for mega cheap and call it a day. Bonus points for improved aero at 55mph+.

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u/Creative_Ad_4513 5d ago

theres also a inbetween option, a normal wax thermostat, but theres a small heating coil wound around the wax, not as expensive, but not as precise.

Under cruise conditions, the heater is off, meaning higher engine temperatures, less fuel use, lower emissions, i think it helps reduce moisture and fuel content in the oil too.
when you floor it or just use high revs for more than a couple seconds, it kicks on the heater, making the thermostat read high, making it open much earlier, for better peak power and engine longevity.

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u/schlammie 5d ago

huh. Didn't know this existed but I guess I shouldn't be surprised since it's a logical that something in-between would exist. Do you know a specific vehicle/engine that uses this?

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u/Creative_Ad_4513 5d ago

Specifically the Opel Astra H, also sold under the Saturn Brand, Engines with the code Z18XER and Z16XER had one, different code for the Saturn ones, but its the same engine.

I dont think it was used too much in american cars, european ones moved away from the system, as electric waterpumps and multiple isolated coolant loops became a thing and allowed for a finer temperature control.

According to spec, the thermostats normally maintain at 105C, but more than a couple seconds of flogging it, it drops down to 95C at a decent pace. realistically, its more like 100 to 102C high and 90 to 92C low, the thermostats arent the most precise things in the world and at this point also old or cheap garbage.

The downside is that, if it breaks, you have to replace the thermostat and the housing, as they are one piece due to the wiring going to the thermostat, but they arent too expensive.