Talk:Brake specific fuel consumption

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I find having numbers smaller then 1 confusing if they aren't preceded by a zero. i.e. .0123 I'd prefer 0.0123 Any thoughts? --Stripy42 14:18, 17 April 2007 (UTC) Also what is 9549.27 about?

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" A reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running at its torque peak"

is demonstrably untrue, if you look at the bsfc map of a petrol engine. Also the article is self contradictory, the quoted best efficiency for a gasoline engine is refuted by the table in the article (and by the Prius' engine). Greg Locock (talk) 04:51, 20 May 2008 (UTC)

[edit] Jet engine figures for GE-CF6

I think I've got the calculation right. Basically power is energy per unit time, but energy is force times distance. Distance is speed per second. Hence if you multiply the N produced by the speed, you get power. Once you have power you divide by 3600,000 to get kWh. Stuff like that.- (User) WolfKeeper (Talk) 05:48, 20 May 2008 (UTC)

[edit] combined cycle efficiency

Sorry, this does not belong in this article. Brake power refers to shaft power. Combined cycle systems use the heat output as part of their claimed useful output. Compound cycles and such like are ok, as electric power is effectively shaft power. What do you think? Greg Locock (talk) 12:39, 20 May 2008 (UTC)

[edit] Posting of the Prius THS II BSFC figures

Greglocock posted the Prius THS II figures in the table on engine BSFC. Even though the figures are interesting, I thought the inclusion of the Toyota used engine being the 1NZ-FXE engine which has a BSFC of 245g/(kW-h) by itself, should also be posted. The table posted is of engines only and not of engines and electric motors combined (hybrids). All other engines in the table would show higher efficiency used with hybrid type setups. A reference to the BSFC figure of the Toyota engine is: http://www.electrifyingtimes.com/priustechspecs.html

The BSFC is for the engine alone. The reference for it is an SAE paper, for heaven's sake, not some blog. Greg Locock (talk) 07:11, 29 May 2008 (UTC)

What does anyone think on this matter? --Revetec (talk) 03:22, 29 May 2008 (UTC)

You reveal a disturbing lack of familiarity with the engineering literature. Greg Locock (talk) 07:11, 29 May 2008 (UTC)

[edit] Use of LHV to give energy efficiency

The gravimetric energy content of a fuel is the amount of heat energy released in combustion, per unit of mass. This is also referred to as the "higher heating value" or "HHV". "Lower Heating Value" or "LHV" is similar, except that the numerator is equal to the energy content minus the heat that the combustion products give up in cooling from an arbitrary temperature (which is agreed by convention) back to the original standard temperature.

LHV was developed for use in comparing the efficiencies of "non-condensing" equipment, hence the seemingly arbitrary decision to ignore a small but non-trivial portion of the actual energy in the fuel. (The temperature chosen is always ABOVE the condensation temperature of water, and much of the ignored energy is the latent heat of water.)

The efficiency of an engine is defined as the ratio of fuel energy consumed to used to energy produced. Thus, HHV must be used to give an accurate value, since it is the energy actually provided by combustion of the fuel, with none of it arbitrarily "forgotten". Because LHV was such a familiar term, one sometimes sees it used in giving the efficiency of engines, but it is technically an error.

I suggest that the article make reference to HHV. Mark.camp (talk) 02:08, 4 June 2008 (UTC)

I sort of agree, but logically the only valid rejection temperature to use would be absolute zero. Which would make the comparison practically useless. Since hardly anyone quotes LHV figures it will be harder to source refs. Greg Locock (talk) 02:20, 4 June 2008 (UTC)
The standard technique for measuring heats of reaction (including HHV) requires that the reactants start at standard temperature and pressure, are combusted in a bomb calorimeter (which ensures that no net work is done on or by the environment), and are returned to STP, which ensures that all of the energy released is measured, and that no energy added from the environment is measured.
So, with HHV, no arbitrary assumptions are needed concerning the rejection temperature. Does this address your concern that HHV may not measure the "true" energy of the combustion? Mark.camp (talk) 20:54, 4 June 2008 (UTC)


Actually I can't really see much of an advantage. The order would not change, the absolute numbers would, but you wouldn't be able to do comparisons with numbers from the literature, so people would pile in with 'better' efficiency figures that are measured on the usual basis. So overall, yes you are probably right, but changing it will create more problems than it solves. You could add another column to the table I suppose. Greg Locock (talk) 05:44, 4 June 2008 (UTC)
Thanks, Greg, I'll noodle on it.
Re comparisons between engines: You are right that the order would not change for two engines both using the same fuel. However, for shaft engines using other fuels such as hydrogen, the error introduced by using LHV is significantly greater than the error for gasoline. Fossil fuels create a lot of carbon dioxide, which remains a gas at STP, so LHV doesn't "forget" its "latent energy" or heat of condensation. Hydrogen burns to pure water, so 100% of the hydrogen energy that remains as combustion products' latent heat in the LHV methodology is arbitrarily considered to be non-existent--bad science. Actually, the fact that LHV introduces false comparative efficiency values for engines with different fuels is one of the arguments presented in the literature for eliminating LHV.
To your second point, I like the idea of giving both HHV and LHV. Then, comparisons with the widest variety of literature could be done. (I see a mixture of HHV and LLV used in the literature, and much of the literature gives both HHV and LHV.) Mark.camp (talk) 20:54, 4 June 2008 (UTC)
Thanks for the explanation. I entirely agree. Cheers Greg Locock (talk) 02:31, 5 June 2008 (UTC)