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Old 09-03-07, 05:57 PM   #1
phantomcow2
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Why is a hot engine more efficient?

Okay here is my understanding about efficiencies in an internal combustion engine...
You've got stored energy in the large molecules of petrol. This gets burned in hopes of extracting kinetic energy, but a lot of it gets converted into thermal energy (waste).
So why is that a hot engine will be more efficient, especially if that heat is there in the first place because of waste? Is it because the heat allows the fuel to vaporize more completely?
Also, if one was to make cylinders, pistons, and other engine components out of some material with a higher thermal resistance than is currently in place, would engine efficiency increase?
Am I way off base here?
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Old 09-03-07, 06:37 PM   #2
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seems to me, that it takes energy to heat up the engine. The engine is sacrificing horsepower to generate this heat. Once the engine is at operating temperature, more of the fuel can be converted to horsepower.
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Old 09-03-07, 07:05 PM   #3
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For one... yes...Fuel atomizes better when it is hotter.
There are a TON of other factors though. For example... there's more oxygen in the air when it is cooler. Or I guess, more air per square unit when it is cooler.
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Old 09-03-07, 07:07 PM   #4
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Hmm how about intake? How is cold air able to give more power?
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Old 09-03-07, 07:10 PM   #5
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Cold air holds more oxygen molecules. So a CAI works by feeding cold air into the engine for the combustion process.
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Old 09-03-07, 07:11 PM   #6
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Cold air is more dense.

There are a lot of factors, another being that a warm engine has looser clearances (more slack between piston/sleeves, and in bearings, etc) and, thus, less friction.
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Old 09-03-07, 08:23 PM   #7
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Hmm how about intake? How is cold air able to give more power?
Cold air is denser, so more oxygen to mix with more fuel to make more power. Same way a turbo, blower or nitrous makes power, basically. By adding more oxygen particles.
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Old 09-03-07, 11:59 PM   #8
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there's more oxygen in the air when it is cooler.
Then how come I ride faster when its warmer??

Ooops, thread hijack!!
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Old 09-04-07, 04:34 AM   #9
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Then how come I ride faster when its warmer??

Ooops, thread hijack!!
your brain plays mind games with you dude. i'm not like a..brain..doctor or anything though.
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Old 09-04-07, 04:37 AM   #10
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Then how come I ride faster when its warmer??

Ooops, thread hijack!!
You wheel bearing grease isn't as thick when its warm
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Old 09-04-07, 08:06 AM   #11
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your brain plays mind games with you dude. i'm not like a..brain..doctor or anything though.
So...... you just stayed in a Holiday Inn Express?
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Old 09-04-07, 08:23 AM   #12
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As far as I can see, the question hasn't yet been answered. I doubt it has anything to do with power used to heat up the engine, since thermal efficiency is higher for a hotter engine even when temperature has reached a steady state. Same thing with friction and other such things.

A jet engine is also more efficient when its operating temperature is higher (AFAIK). This is why they now run modern jet engines at temperatures (in some sections) higher than the melting point of the metal in the blades and vanes. Cool air is blown inside the blades and vanes to keep the metal cool enough, while still allowing the temperature surrounding them to be higher.

There must be some more fundamental reason for why higher operating temperature generally means higher thermal efficiency. However, I'm no engineer, so I can't tell you what it is...
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Old 09-04-07, 08:46 AM   #13
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A more simple reason could simply be due to tighter tolerances in a warmed engine as the various metals expand. The space between the piston rings and the cylinder wall will be tighter when they are at operating temperature. Less blowby, etc.

I know this is ONE factor. I'm sure there are more.

In terms of emissions efficiency, the cats have to be hot in order to do their job effectively. Cold catalytic converters do very little.
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Old 09-04-07, 08:50 AM   #14
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I can't say for sure, but it seems a given that an engine will heat up, and will be running within a range of "hot" temperatures. I'd presume that the engineers who design engines would take this into account and make the engines more efficient when they're in the temperature range that's inevitable.
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Old 09-04-07, 09:20 AM   #15
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When drag racing, a cold engine -- specifically, a cold intake manifold -- is better because it makes more power. Same reasons as posted above -- cold air being denser, etc. All else being equal (track temperature, driver skill, tire grip, etc), the colder engine will put down the quicker run. If you're not running nitrous or methanol injection, and even if you're using a cold-air intake, the easiest way to get cooler intake temps is to let the engine cool off. You could also put an ice bag on the manifold or attempt to cool the engine bay by turning on the radiator fans to circulate air.

Anywhere else, a warm engine is preferred. It gets better lubrication. Good oil flow also helps avoid oil starvation during hard cornering (a baffled oil pan helps, too) and maintains proper operation of variable valvetrain systems.

Basically... more power is made with cooler intake temperatures, more power is made through hotter & more complete combustion, and less power is lost when good lubrication is achieved (which is where the idea of a "warm engine is more efficient" happens).
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Old 09-04-07, 10:00 AM   #16
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So...... you just stayed in a Holiday Inn Express?
Hahah you got me
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Old 09-04-07, 10:51 AM   #17
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Then how come I ride faster when its warmer??

Ooops, thread hijack!!
Lower air density....it is a factor!

OK, PC2, yes there are alternative engine materials, like ceramic composite. There are experimental small displacement ceramic engines that are air cooled, and don't require lubrication. The ceramic block runs best when HOT! The material has such a low friction coefficient that it doesn't require oil at all other than the fuel in the cylinders.
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Old 09-04-07, 12:08 PM   #18
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http://www.centuryperformance.com/coolingsys.asp

start here fledgling gearheads. good stuff hidden within. check out the lubricants tech article too. you'll never ignore an oil change again.
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Old 09-04-07, 12:54 PM   #19
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Speaking of which. I'm probably due for an oil change.
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Old 09-04-07, 01:11 PM   #20
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Considering this place is usually crawling with engineers of all kinds, I'm surprised not one of them has popped in to provide us with an answer yet...
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Old 09-04-07, 02:59 PM   #21
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I believe I was the one that gave PC this idea...

I recall that one of the advantages of a ceramic engine is that:

1. It didn't need a cooling system

2. An IC engine runs more efficiently when hot.

That being said, I'm not sure how much more efficient or anything else. Combustion labs were performed, written up and quickly forgotten. Sorry PC.
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Old 09-04-07, 04:14 PM   #22
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the thermodynamic answer is efficiency is essentially mechanical energy output divided by the total energy available in the fuel burned. since a hot engine needs to be better insulated to maintain the higher temperature for a similar amount of fuel burned, theres less heat energy wasted and therefore more efficiency. the chemical answer is that for a chemical reaction to take place, a certain engergy threshold must be reached. a hotter engine has the fuel starting out closer to this threshold so less energy needs to be expended to make the gas ignite. what you wind up with is the gas needing less input energy and combusting more completely assuming you can get enough air mixed in with the fuel.
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Old 09-04-07, 04:20 PM   #23
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less friction of all internal parts...
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Old 09-04-07, 05:04 PM   #24
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I believe I was the one that gave PC this idea...

I recall that one of the advantages of a ceramic engine is that:

1. It didn't need a cooling system

2. An IC engine runs more efficiently when hot.

That being said, I'm not sure how much more efficient or anything else. Combustion labs were performed, written up and quickly forgotten. Sorry PC.
I wonder what happened to that technology. I remember a guy able to easily lift a ceramic V8, and how the engine was just quite happy at high temperatures compared to a normal metal one.
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Old 09-04-07, 05:42 PM   #25
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I think pc2 needs a physics and electronics class first, then auto shop, thermodynamics and/or thermal fluid sciences class. Then going to Skip Barber wouldn't hurt after he's been through all that.
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