Advertise on Bikeforums.net



User Tag List

Results 1 to 25 of 25
  1. #1
    la vache fantôme phantomcow2's Avatar
    Join Date
    Aug 2004
    Location
    NH
    Posts
    6,266
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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?
    C://dos
    C://dos.run
    run.dos.run

  2. #2
    You Know!? For Kids! jsharr's Avatar
    Join Date
    Apr 2005
    Location
    Just NW of Richardson Bike Mart
    My Bikes
    '05 Trek 1200 / '90 Trek 8000 / '? Falcon Europa
    Posts
    6,082
    Mentioned
    11 Post(s)
    Tagged
    3 Thread(s)
    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.
    Are you a registered member? Why not? Click here to register. It's free and only takes 27 seconds! Help out the forums, abide by our community guidelines.
    Quote Originally Posted by colorider View Post
    Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.

  3. #3
    steel lover
    Join Date
    Apr 2007
    Location
    Houston
    My Bikes
    Bianchi Alloro, Miyata 710, Fuji Espree Fixie convert
    Posts
    1,317
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  4. #4
    Hey guyz? Guyz? Wait up!! Siu Blue Wind's Avatar
    Join Date
    Apr 2005
    Posts
    10,473
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Hmm how about intake? How is cold air able to give more power?
    Quote Originally Posted by Buddha
    We are what we think. All that we are arises with our thoughts. With our thoughts, we make the world.
    Quote Originally Posted by making View Post
    Please dont outsmart the censor. That is a very expensive censor and every time one of you guys outsmart it it makes someone at the home office feel bad. We dont wanna do that. So dont cleverly disguise bad words.

  5. #5
    And he's single... jmarkley710's Avatar
    Join Date
    Dec 2006
    Location
    Tampa, FL
    My Bikes
    2006 Motobecane Mirage
    Posts
    291
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Cold air holds more oxygen molecules. So a CAI works by feeding cold air into the engine for the combustion process.
    Live in the Tampa Bay Area?
    Want to meet some people, join group rides, find local resources for cyclists?
    Check out Tampa Bay Cycling Group @
    http://sports.groups.yahoo.com/group/tampabaycycling/

  6. #6
    Pokes On Spokes JPradun's Avatar
    Join Date
    Jun 2005
    Location
    Madison, WI
    My Bikes
    Pedal Force ZX3
    Posts
    825
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.
    The quest for success: Collegiate Nationals:
    http://iliveonnitro.blogspot.com
    (Updated daily 3/26-4/4-: North Carolina training camp)

  7. #7
    Why not? EthanYQX's Avatar
    Join Date
    May 2007
    Location
    Canada
    My Bikes
    Giant STP 2
    Posts
    845
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Siu Blue Wind View Post
    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.
    "It is not the critic who counts."

  8. #8
    Senior Member mezza's Avatar
    Join Date
    Aug 2006
    Location
    On the intramaweb thing.
    My Bikes
    Steel geared. Steel Fix.
    Posts
    1,016
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by chevy42083 View Post
    there's more oxygen in the air when it is cooler.
    Then how come I ride faster when its warmer??

    Ooops, thread hijack!!
    Do you own or run a small/medium business in Australia?
    Grow your business GUARANTEED* www.jumpglobal.com.au

  9. #9
    Pretty Hate Machine Weeks's Avatar
    Join Date
    Aug 2006
    Location
    Down in it
    My Bikes
    Bianchi Brava
    Posts
    435
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by mezza View Post
    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.

  10. #10
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
    Join Date
    Dec 2002
    Posts
    9,163
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by mezza View Post
    Then how come I ride faster when its warmer??

    Ooops, thread hijack!!
    You wheel bearing grease isn't as thick when its warm

  11. #11
    You Know!? For Kids! jsharr's Avatar
    Join Date
    Apr 2005
    Location
    Just NW of Richardson Bike Mart
    My Bikes
    '05 Trek 1200 / '90 Trek 8000 / '? Falcon Europa
    Posts
    6,082
    Mentioned
    11 Post(s)
    Tagged
    3 Thread(s)
    Quote Originally Posted by Weeks View Post
    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?
    Are you a registered member? Why not? Click here to register. It's free and only takes 27 seconds! Help out the forums, abide by our community guidelines.
    Quote Originally Posted by colorider View Post
    Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.

  12. #12
    Videre non videri
    Join Date
    Sep 2004
    Location
    Gothenburg, Sweden
    My Bikes
    1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike
    Posts
    3,202
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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...

  13. #13
    Cactuar! sneefy's Avatar
    Join Date
    Dec 2006
    Location
    Wisconsin
    My Bikes
    '04 Iron Horse Hollowpoint, '01 Giant Boulder with all non-original parts, '99 Trek 7500 (frame), Mid '90's? Wheeler MTB frame (anybody want it?), '96 Specialized Crossroads, '86 Schwinn Predator Qualifier, 1976 Viscount Aerospace Pro (no death fork)
    Posts
    42
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  14. #14
    Señor Member USAZorro's Avatar
    Join Date
    Oct 2004
    Location
    Carlisle, PA
    My Bikes
    1954 Hetchins M.O., 1959 Viking Severn Valley, 1970 Raleigh Pro, 1972 Fuji "The Finest", 1974 Raleigh Superbe&Comp, 1976 Raleigh Team Pro, 1994 Trek 830 MTB, 2000 Bob Jackson Arrowhead, Unicycle
    Posts
    13,034
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.
    The search for inner peace continues...

  15. #15
    Bike ≠ Car ≠ Ped. BarracksSi's Avatar
    Join Date
    Jul 2007
    Location
    Washington, DC
    My Bikes
    Some bikes. Hell, they're all the same, ain't they?
    Posts
    13,857
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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).

  16. #16
    Pretty Hate Machine Weeks's Avatar
    Join Date
    Aug 2006
    Location
    Down in it
    My Bikes
    Bianchi Brava
    Posts
    435
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by jsharr View Post
    So...... you just stayed in a Holiday Inn Express?
    Hahah you got me

  17. #17
    The Site Administrator: Currently at home recovering from a couple of strokes,please contact my assistnt admins for forum issues Tom Stormcrowe's Avatar
    Join Date
    Mar 2006
    Location
    South Florida
    My Bikes
    Techna Wheelchair and a Sun EZ 3 Recumbent Trike
    Posts
    16,012
    Mentioned
    10 Post(s)
    Tagged
    4 Thread(s)
    Quote Originally Posted by mezza View Post
    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.
    on light duty due to illness; please contact my assistants for forum issues. They are Siu Blue Wind, or CbadRider or the other 3 star folk. I am currently at home recovering from a couple of strokes. I am making good progress, happily.


    . “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche

    "We can judge the heart of a man by his treatment of animals." - Immanuel Kant

  18. #18
    almost kosher
    Join Date
    Aug 2005
    Posts
    398
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  19. #19
    That darn Yankee TexasGuy's Avatar
    Join Date
    Jun 2005
    Location
    West West Fort Worth
    My Bikes
    Mongoose XR-100, Eros Bianchi
    Posts
    4,272
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Speaking of which. I'm probably due for an oil change.
    Life is about hanging onto what you think is important and finding out what really is important.
    "Stop Ruining my joke!", "No, a joke implies humor attached at no additional cost"
    So many sayings, so little sig space.

  20. #20
    Videre non videri
    Join Date
    Sep 2004
    Location
    Gothenburg, Sweden
    My Bikes
    1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike
    Posts
    3,202
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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...

  21. #21
    No Rocket Surgeon eubi's Avatar
    Join Date
    Jan 2005
    Location
    Corona and S. El Monte, CA
    My Bikes
    Cannondale D600, Dahon Speed T7
    Posts
    1,648
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.
    Fewer Cars, more handlebars!

  22. #22
    Junior Member
    Join Date
    May 2006
    Posts
    5
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  23. #23
    hello roadfix's Avatar
    Join Date
    May 2003
    Location
    Los Angeles
    My Bikes
    thank you for asking
    Posts
    18,502
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    less friction of all internal parts...

  24. #24
    Senior Member
    Join Date
    Aug 2006
    Posts
    998
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by eubi View Post
    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.

  25. #25
    Trans-Urban Velocommando ax0n's Avatar
    Join Date
    Nov 2006
    Location
    Lenexa, KS
    My Bikes
    06 Trek 1200 - 98 DB Outlook - 99 DB Sorrento
    Posts
    2,400
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.
    ax0n: Geeky and bikey
    My latest tip: Carrying your laptop
    My latest geeky project: Ethernet-testing cuff links

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •