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Old 06-14-2008, 09:32 PM   #1
fleabag
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Default Why do car engines run hot?

What I'm really asking is, why has it been determined by engine manufacturers that their engines should operate from 180F to 250F? Do the lower power "economy" engines operate at the lower end of the spectrum while the "performance" engines run at the higher end? Is the reason for running hot but not 500F hot is because it's a compromise between performance and not having such things as seals, lubricants, bearings and whatnot failing prematurely with their current design, warranting higher quality/more expensive materials?

Wouldn't it be more efficient to run an engine (I'm talking about otto cycle here) at 100F-150F? Sure you wouldn't get the same kind of performance per se, but it should be consuming far less energy.

Why doesn't engine coolant boil at temperatures such as 250F? Or are there such great variances in what can be considered the engine "temperature" that the coolant may only be 180F, the exhaust manifold is 1200F, the cylinder heads are 800F and the block is 250F?

I'm confused about a lot of things and I've been wondering what is considered the "engine" temperature and why certain things are over 6 times hotter than the very area in which all this heat is being generated in.

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Old 06-14-2008, 10:20 PM   #2
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Default Why do car engines run hot?

One reason you want the engine oil hot is to "burn off" water that accumulates in the oil so parts don't rust. I believe water also contributes to sludge buildup.

Coolant doesn't boil at 250 degrees because it's under pressure... that's why you never want to remove the radiator cap from a hot engine... it'll release the pressure and instantly boil causing sort of a geyser.

And yes, different parts of the engine operate at different temperatures, especially when you compare the exhaust manifold to the intake manifold.

In general, when referring to the engine temperature, they're looking at the temperature of the coolant as it exits the engine and goes into the radiator.
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Old 06-14-2008, 10:46 PM   #3
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Default Why do car engines run hot?

Not sure if this applies to full size engines in our cars but in small 2 stroke engines, there is always that sweet spot as far as air/fuel mixture.

Running too rich means you will be bogged down and run cool, too lean and you'll overheat.

I'm sure it's not so simple in car engines as we have ECU's to constantly adjust air/fuel mixture.
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Old 06-14-2008, 11:07 PM   #4
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Default Why do car engines run hot?

Quote:
Originally posted by: BouZouki
Not sure if this applies to full size engines in our cars but in small 2 stroke engines, there is always that sweet spot as far as air/fuel mixture.

Running too rich means you will be bogged down and run cool, too lean and you'll overheat.

I'm sure it's not so simple in car engines as we have ECU's to constantly adjust air/fuel mixture.
14.7:1 is what is generally considered the optimal air:fuel ratio but it varies. If you're looking for power, it's probably more like 12:1. If you're looking for economy, it wouldn't be uncommon to see 16:1 at cruising speeds without much load on the engine. That doesn't have a whole lot to do with the temperature of the engine as a whole though.
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Old 06-14-2008, 11:45 PM   #5
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Default Why do car engines run hot?

Car engines put out more power if you can get the compressed air/gas mix to get the largest change in temperature possible. They'd love to push up the temperature a bit, but they can't. Like you said, there is limits to the temperatures that bearings, seals, and other components will take. Also, the engine oil has a limit. Even the high end oils used in air craft turbines max out at 400 degrees F. It won't burn at that temperature, it will start leaving deposits and gum everything up.

Coolants don't boil at 212 degrees F for two reasons. First, antifreeze actually raises the boiling point. Second, 212 is only the boiling point at normal atmospheric pressure. Cars have a pressure cap that raises the pressure, raising the boiling point.
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Old 06-15-2008, 07:46 AM   #6
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Default Why do car engines run hot?

You get more hp from a car engine, the cooler it can be run. That is one reason on
turbos, they run the intake air thru an intercooler and why most hot rodders will run
a Lower T-Stat (about 10 degs cooler) than the stock value. The main reason for
engine temps in the 190-200 deg range are for emissions (easier to clean up emissions
at those temps) and also, if the Engineers want that block temp, then the Cooling System, Radiator
Fan & Water Pump can be smaller, lighter and Cheaper.
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Old 06-15-2008, 08:40 AM   #7
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Default Why do car engines run hot?

higher temps would increase thermal efficiency, but decrease material strength, among other things, especially when you start looking at long-term operation.
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Old 06-15-2008, 08:41 AM   #8
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Default Why do car engines run hot?

higher temperatures also result in much higher NOx emissions.

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Old 06-15-2008, 11:49 AM   #9
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Default Why do car engines run hot?

Quote:
Originally posted by: bruceb
You get more hp from a car engine, the cooler it can be run. That is one reason on
turbos, they run the intake air thru an intercooler and why most hot rodders will run
a Lower T-Stat (about 10 degs cooler) than the stock value. The main reason for
engine temps in the 190-200 deg range are for emissions (easier to clean up emissions
at those temps) and also, if the Engineers want that block temp, then the Cooling System, Radiator
Fan & Water Pump can be smaller, lighter and Cheaper.
That has nothing to do with the temperature of the engine, they are only cooling the air. Cooler air is more dense, meaning more oxygen for the same volume of air. More oxygen lets you put in more fuel, getting more power.

It is not necessarily more efficient from a fuel standpoint. This is because for that extra hp you are burning extra gas (assuming you have a fuel injected system, carbs will dump the same amount in with hot or cold air).

They have intercoolers on turbocharged engines because you are running the intake air through a compressor which is powered by hot exhaust. The compression and being close to the exhaust cause the intake air to heat up. The intercooler is just to try and get the air back down to normal intake air temps.



Fenix is right, higher temps make thermal efficiency go up. It's all about the otto cycle. And as canadageek mentioned, high temperatures makes NOx go up but modern catalytic converters can help a great deal with that.
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Old 06-16-2008, 12:42 PM   #10
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Default Why do car engines run hot?

No car should be running 250 degree coolant temps. That's into "very bad" territory. The upper red line on most coolant gauges is between 230 and 240 degrees.

Most factory thermostats are set at ~190 degrees. As others have said, they run at that temperature because it's a favorable tradeoff between longevity of the engine, power production, and emissions.

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Old 06-16-2008, 02:16 PM   #11
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Default Why do car engines run hot?

Quote:
Originally posted by: Jeff7181
Coolant doesn't boil at 250 degrees because it's under pressure... that's why you never want to remove the radiator cap from a hot engine... it'll release the pressure and instantly boil causing sort of a geyser.
And it's quite a sight to see, too, it takes longer for that geyser to dry up than you'd think. A friend of mine pulled his cap at a gas station once when he was having overheating problems. IIRC, the geyser went a good 15 feet up in the air.
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Old 06-16-2008, 03:19 PM   #12
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Default Why do car engines run hot?

Higher change in temps also require looser tolerences which also inherit less effiecientcy.
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Old 06-16-2008, 08:31 PM   #13
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Default Why do car engines run hot?

Quote:
Originally posted by: nakedfrog
Quote:
Originally posted by: Jeff7181
Coolant doesn't boil at 250 degrees because it's under pressure... that's why you never want to remove the radiator cap from a hot engine... it'll release the pressure and instantly boil causing sort of a geyser.
And it's quite a sight to see, too, it takes longer for that geyser to dry up than you'd think. A friend of mine pulled his cap at a gas station once when he was having overheating problems. IIRC, the geyser went a good 15 feet up in the air.
If he got away without massive scalding he's very lucky. You shouldn't touch a hot pressurized coolant system, having your skin burned off by superheated steam and coolant would be uncomfortable to say the least.
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Old 06-16-2008, 08:37 PM   #14
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Default Why do car engines run hot?

I will be careful about de-capping my radiator when hot, yikes!
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Old 06-17-2008, 01:38 AM   #15
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Default Why do car engines run hot?

Quote:
Originally posted by: Skoorb
I will be careful about de-capping my radiator when hot, yikes!
Don't be careful about it, simply don't do it.
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Old 06-17-2008, 08:08 AM   #16
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Default Why do car engines run hot?

They burn gas.
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Old 06-17-2008, 12:28 PM   #17
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Default Why do car engines run hot?

Quote:
Originally posted by: BouZouki
Running too rich means you will be bogged down and run cool, too lean and you'll overheat.
I used run into this on my dirt bike too. Text
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Old 06-19-2008, 01:08 PM   #18
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Default Why do car engines run hot?

Quote:
Originally posted by: mooseracing
Higher change in temps also require looser tolerences which also inherit less effiecientcy.
So wouldn't that mean that if you could set your engine running temperature to be 140F-170F, wouldn't that be more efficient? That way the engine wouldn't have to warm up quite as much.
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Old 06-19-2008, 01:09 PM   #19
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Default Why do car engines run hot?

Quote:
Originally posted by: potato28
They burn gas.
Yeah that whole combustion thing.. its usually hot
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Old 06-19-2008, 08:35 PM   #20
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Default Why do car engines run hot?

Quote:
Originally posted by: fleabag
Quote:
Originally posted by: mooseracing
Higher change in temps also require looser tolerences which also inherit less effiecientcy.
So wouldn't that mean that if you could set your engine running temperature to be 140F-170F, wouldn't that be more efficient? That way the engine wouldn't have to warm up quite as much.
The thermodynamic cycle wouldn't work for crap. It's a balance between tolerances and the efficiency of the thermodynamic cycle.

Turbines run at ridiculous temperatures with tight clearances, so it can be done. However, that turbine also can cost $20k+ for a smallish one. You do the math.
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