gts@dmcnh.UUCP (12/28/84)
[posted to net.cooks.line-eaters] 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? Thanks in advance. +-------------------------------------+ +------------------------------------+ | USENET: decvax!ittvax!sii!dmcnh!gts | | DISCLAIMER: The content of this | | USMail: 14F Hampshire Dr. | | message is the sole responsibility | | Nashua, NH 03063 | | dmcnh!gts and does not necessarily | | NEBell: (603) 880-2069 | | reflect the policies of Datamedia. | +-------------------------------------+ +------------------------------------+
scw@cepu.UUCP (Stephen C. Woods) (01/05/85)
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"
marcus@pyuxt.UUCP (M. G. Hand) (01/09/85)
In Article-I.D.: cepu.421 scw@cepu.UUCP (Stephen C. Woods) writes: > The 2 major components of automobile exaust are CO2 and H2O.... > 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. Thats about the right order of magnitude, but of course the precise figure depends on the fuel being burned, and the extent of combustion: Lets assume we're burning pure iso-octane (with an octane rating, therefore of 100). The combustion reaction is: C8H18 + 12.5O2 ---> 8CO2 + 9H2O which we all know never quite gets achieved (hence that noxious CO floating around). So 1 mole of octane produces 9 moles of water. The molecular weights of these are 8*12 + 18*1 = 114, and 2*1 + 16 = 18, and lets further assume that the densities of the two liquids are water - 1000 kg m-3 octane - 980 kg m-3 (actually this is a wild guess but as everyone knows, gas floats on water, so in the absence of a Handbook of Physical Constants...) Therefore, 114/980 m-3 octane generates 18*9/1000 m-3 water or 1 volume of octane will produce 18*9*98/11400 = 1.39 volumes of water "m-3" means "per cubic metre", ie m raised to the power of -3. If, as is likely the density of octane were a little higher more water would be produced. The presence of longer chain homologues would tend to decrease the amount produced, as would cyclic and more unsaturated molecules. A-level chemistry i should think!!! -- Marcus Hand {ihnp4!}pyuxt!marcus