Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site cepu.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!ittvax!dcdwest!sdcsvax!sdcrdcf!trwrb!trwrba!cepu!scw From: scw@cepu.UUCP (Stephen C. Woods) Newsgroups: net.physics Subject: Re: Visible automobile exhaust on a cold day Message-ID: <421@cepu.UUCP> Date: Fri, 4-Jan-85 16:33:57 EST Article-I.D.: cepu.421 Posted: Fri Jan 4 16:33:57 1985 Date-Received: Mon, 7-Jan-85 02:43:23 EST References: <174@dmcnh.UUCP> Reply-To: scw@cepu.UUCP (Stephen C. Woods) Distribution: net Organization: VA Wadsworth Med. Center; LA CA Lines: 25 Summary: In article <174@dmcnh.UUCP> gts@dmcnh.UUCP writes: >Can anyone out there tell me why a car has visible exhaust while warming >up on a cold day; then as the car warms up, it becomes invisible. It is >possible to tell how long a car has been driving by noticing how much of >its exhaust is visible. Why is this? The 2 major components of automobile exaust are CO2 and H2O. The H2O is in the form of steam (vapor), this will condense into visible liquid when the air temp is low enough (in fact it will freeze into ice before it disipates if the tempature is low enough (~-30F)). The formation of visable condensation requires that vapor be in a high enough concentration to form droplets while condensing. The reason that the visible condensation vanishes as the engine/exaust system gets hot is that the concentration of vapor gets too low before the tempature drops enough to condense it. As an interesting side light, burning 1 Gal of gasoline (Petrol for our English friends) produces about 1.5 Gal of water (I misremember the exact amount but I know that it was more than the volume of fuel). Airships (rigid ones, that is Zepllens or Dirigables) condensed some of the water out of their exaust gas to replace the weight of the burned fuel. -- Stephen C. Woods (VA Wadsworth Med Ctr./UCLA Dept. of Neurology) uucp: { {ihnp4, uiucdcs}!bradley, hao, trwrb}!cepu!scw ARPA: cepu!scw@ucla-cs location: N 34 3' 9.1" W 118 27' 4.3"