# Maximum amount of energy from a gallon of gasoline **URL:** \ **Category:** Factual Questions\ **Created:** [June 14, 2002, 12:16am UTC](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953 "2002-06-14T00:16:06Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![City-lee](https://avatars.discourse-cdn.com/v4/letter/c/9de053/32.png) [@City-lee](https://boards.straightdope.com/u/City-lee)\ **Post date:** [June 14, 2002, 10:39pm UTC](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953/21 "2002-06-14T22:39:08Z")
I believe both civilian and military piston engine heavy aircraft used water injection for max takeoff power, usually mixed with isopropyl alcohol to prevent freezing. There used to be an injection kit available for automotive use that used an electric fuel pump to squirt a very fine stream of water into the carb barrel(s). A vacuum sensor regulated the amount, none at high vacuum(idle condition) and max at low vacuum(wide open throttle). I have used it on a 1973 3/4 ton Dodge pickup. In operation the water injection allows a much further advanced timing than normal specs because it effectively increases the apparent octane of the gasoline thus preventing preignition (knocking) The engine runs 30 to 50 degrees cooler and the horse power increases enough to run in a higher gear pulling a very heavy boat and camper up very steep hills. I also get a noticable an increase in gas milage. ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [June 14, 2002, 11:06pm UTC](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953/22 "2002-06-14T23:06:48Z")
Today I read an article about people who try to the most mpg out of their cars. A man who owns a Honda Insight reportedly got 97mpg on a recent trip! ---
**Author:** ![Boo\_Boo\_Foo](https://avatars.discourse-cdn.com/v4/letter/b/e274bd/32.png) [@Boo\_Boo\_Foo](https://boards.straightdope.com/u/Boo_Boo_Foo)\ **Post date:** [June 14, 2002, 11:21pm UTC](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953/23 "2002-06-14T23:21:05Z")
City-lee? Out of interest, how often do you need to top up your water supply for your inlet manifold injector? Also, what happens to your engine if the water supply runs out? And Popup? Thank you for your kind compliment earlier. I too have learnt things here as well, which I just love. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [June 15, 2002, 2:36am UTC](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953/24 "2002-06-15T02:36:47Z")
> [@](#): > > \*Originally posted by andy\_fl \* > \*\ ***The real chemistry of Combustion:** > > CxHy + zO2 + 3.7N2—\> aC02 + bCO + cC + dCnHm + yH2O + eNOx + HEAT\*\* Wimp. My combustion calculations for coal consider the following: aC + bH[sub]2[/sub] + cS + dN[sub]2(fuel)[/sub] + eH[sub]2[/sub]O + fAsh + gO[sub]2(fuel)[/sub] + hCl + iO[sub]2(air)[/sub] + jN[sub]2(airl)[/sub] + kH[sub]2[/sub]O[sub]air[/sub] + lAr —\> aaC[sub]unburned combustible, as defined in ASME PTC 4.1[/sub] + abCO + acCO[sub]2[/sub] + baH[sub]2[/sub] + bbH + bcH[sub]2[/sub]O + bdH[sub]2[/sub]O[sub]2[/sub] + caS + cbSO + ccSO[sub]2[/sub] + cdSO[sub]3[/sub] + sulfur capture in Ash + Ash + daCl + dbHCl + eaN + ebN[sub]2[/sub] + ecNO + edNO[sub]2[/sub] + eeN[sub]2[/sub]O + faO[sub]2[/sub] + fbO[sub]3[/sub] + gaAr…geeze, have I got it all? > [@](#): > > **To get the highest temperature, ideally there should be exactly the amount of Oxygen (air) required to convert the CxHy to CO2 and H2O (this is called the stoichiometric amount). However, this never happens in real life because this gives rise to unburnt carbon (soot particles etc. ) SO some excess air is always use (Usually 10% ). Now you cannot use more excess air because after combustion if more free oxygen is available , you’ll have more NOx ( not desirable). Hence, the amount of excess air is an optimization, (less is bad and more is bad too)** FYI, real-world natural gas combustion runs at about 2-5% excess air, oil at 3-8% excess air, and coal at 9% (extreme case) to 40% (extreme case), with a more typical range of 15-25%. While more excess air normally leads to more NOx, note that this is not always the case - combustion tuning can do very interesting things in practice. But still, what you say is a good rule of thumb. > [@](#): > > **2\> Use a gas turbine and a high temperature. Yes gas turbines can withstand high temperatures but the problem is that at higher temperatures NOx is formed more and more. So that limits the temperature you can go upto unless you use pure oxygen.** However, if your fuel has fuel-bound nitrogen, using pure oxygen will still result in a substantial amount of N[sub]2[/sub] —\> NO conversion. > [@](#): > > **4\> Recycle the exhaust gases to preheat the air / fuel coming into the engine. This is done in big turbines and furnaces. But heating air is not too economic in small engines.** And there’s that nasty problem of detonation, which is tied quite a bit to the inlet air temperature. Una [Previous page](https://boards.straightdope.com/t/maximum-amount-of-energy-from-a-gallon-of-gasoline/113953.md?page=1)