Talk:Thrust-specific fuel consumption

compression ratio and pressure ratio

This page incorrectly confuses compression ratio (used for positive displacement engines) and pressure ratio, used for turbines. See the table below for conversion. From this it is easy to see that a diesel engine operating at a CR of 25:1 has a much higher pressure ratio than an advanced aircraft turbine engine.

CR	1:1	3:1	5:1	10:1	15:1	20:1	25:1	35:1
PR	1:1	2:1	10:1	22:1	40:1	56:1	75:1	110:1

I partly agree with Mr. Anonymous here. The compression ratio is defined via the volume reduction,

$CR={\frac {V_{1}}{V_{2}}}$,

but the pressure ratio is defined as the pressure increase

$PR={\frac {P_{2}}{P_{1}}}$.

Using the combined gas law we get:

${\frac {P_{1}V_{1}}{T_{1}}}={\frac {P_{2}V_{2}}{T_{2}}}\Rightarrow {\frac {V_{1}}{V_{2}}}={\frac {T_{1}}{T_{2}}}{\frac {P_{2}}{P_{1}}}\Leftrightarrow CR={\frac {T_{1}}{T_{2}}}PR$

Since T₂ is much higher than T₁ (compressing gases puts work into them, i.e. heats them up), CR is much lower than PR. - Alureiter 17:01, 22 October 2005 (UTC)

Not so obvious to me. The idea behind both diesel and turbine engines is that you compress fuel and air, ignite the mixture (increasing the temperature) and then let it expand. In addition, the number of molecules often changes during combustion such that PV=NkT isn't valid. Gah4 (talk) 23:00, 3 February 2021 (UTC)

SFC table

I expanded the table. I think it's easier to reference existing examples than blurry values, so I use it. The table is nearly fully referenced by wikipedia articles. For the MAN thermal efficiency system, I asked an employee there to confirm, but it's not as good as it could be. It is an interesting value, though: the maximum efficiency of a reciprocating engine as of today.--Marc Lacoste 23:34, 25 October 2006 (UTC)

it's a pretty good table, but I think the entry in service needs to be a single value and then if you want to change the end service year as a separate column, that would be cool but the data doesn't match the values well... I also think the trent entry needs to be broken up by model, the Trent 300 is pretty different from the Trent XWB.

Maybe putting in 3? trent entries to bracket the 80's, 90's and 21st century might be more informative. --Patbahn (talk) 20:57, 27 April 2020 (UTC)

Wow that's a long discussion! The different Trent models are shown in Rolls-Royce Trent#Specifications including TSFC, from the same source (except later T1000/7000 and TXWB). The range is a good way to show the bracket, the table here should not detail every variant if it is detailed in the linked article, and should not offer more space to one or the other model (eg: if the Trent variants are shown, the CF6 or PW4000 variants should be also). Columns are in the source for WP:Verifiability, adding other columns would have to be referenced.--Marc Lacoste (talk) 05:29, 28 April 2020 (UTC)

there is an error in the RB211 tsfc data it is near equal for ground and cruise. where the thermodynics and comparable engines would demand a significant difference. cruise tsfc is invariably higher.ZwergAlw (talk) 17:57, 26 February 2021 (UTC) imu a typo. .5... values should be .3... values.

Conversions table/explanation needed

It seems to me we need to be able to explain how to convert between lb/h/lbf and all the other metrics, including describing how to get to specific impulse.WolfKeeper 16:20, 4 September 2007 (UTC)

The conversion table also needs a little note or something explaining how lb/hr/lbf is often simplified to 1/hr in practice. SkycraftAero (talk) 15:14, 21 August 2013 (UTC)

inclusion of "/{g/kNs =s/m}" directly after "g/kNs" is visually confusing, especially when not also included in the imperial row — Preceding unsigned comment added by 104.156.75.106 (talk) 18:12, 11 February 2020 (UTC)

Comparing jets and pistons

Modern jet engines have truly fantastic overall compression ratios reaching into the 30's, yet they have SFC's that are still lower than gasoline-powered piston engines. The explanation above explains this, but not in a form that is suitable for inclusion in the article body (IMHO, it's a little technical). I think an explanation needs to be part of this article, does anyone have suggestions for the wording? Maury (talk) 16:42, 1 March 2008 (UTC)

It's the other way around, piston engines aren't as efficient as jet engines per se, but piston engines usually have a more efficient drive train, but if you attach them to a propeller, then they're not as efficient as a good turboprop.16:48, 1 March 2008 (UTC) —Preceding unsigned comment added by Wolfkeeper (talk • contribs)

That's not true. Turbines use a constant-pressure combustion systems that is simply not as efficient as the constant-volume system in a piston engine. That's why you have things like the Orenda OE600 replacing things like the Pratt & Whitney Canada PT6, because these days fuel costs are becoming more and more important, important enough to override the reliability and maintenance costs. There have been a few attempts at contant-volume jet engines, the Heinkel HeS 40 for instance, but none of these have reached production. Maury (talk) 18:15, 1 March 2008 (UTC)

The latest jet airliners get the equivalent of 90 mpg. I admit it's a lot to do with them simply getting to the destination a lot more quickly I suppose, but jet engines sit in the middle of a system, and it's the system efficiency you really care about. And I completely don't understand your point about constant pressure at all, both jet engines and piston engines are analysed using PV diagrams; if you follow the air through the cycle of a jet engine then it's certainly not constant pressure.- (User) WolfKeeper (Talk) 20:01, 1 March 2008 (UTC)

Ok, I admit that for lower speed, lower altitude work, a piston engine can compete or even beat a jet engine, but high speed there's no contest, piston engines don't seem to do high speed at all well.- (User) WolfKeeper (Talk) 20:01, 1 March 2008 (UTC)

That's because of the propeller, not the engine. Propellers are designed to work subsonically, and as the aircraft as a whole starts to approach the speed of sound the speed of the propeller, which is aircraft speed + engine rotational speed starts hitting this limit fairly early. Up to about 400 mph you can get over 80% of the engine power into the prop, but faster than that and it starts dropping off very rapidly. It's possible to make a propeller that works supersonically, see here, but this dramatically lowers slower-speed performance. The only example of such a design to enter production is the Soviet Bear bomber, other designs like the XF-88 and NASA's UHB went nowhere. See propfan.

As to the PV diagrams, you're absolutely right, pistons and turbines are both examined using one -- and if you do so you will notice that turbines have less area inside the curve, see [1] and [2]. It's that vertical bar on the left that's missing on the Brayton cycle that makes a piston engine better than a jet. It's fundamental to the physics, turbines simply cannot match pistons. There are ways you can reduce the difference, using recombiners and preheaters, but they greatly increase the complexity of the engine, which is the whole reason you use a jet in the first place. And it's not "constant pressure", it's "constant pressure combustion".

Your theory that constant pressure combustion is inherently less efficient than intermittent is self-evidently wrong. Rockets use continuous combustion and achieve engine efficiencies of up to 65% or even more. Ultimate efficiency is to do with combustion temperatures rather than pressures and combustion modes; as any student of Carnot should know very well indeed.- (User) WolfKeeper (Talk) 17:10, 3 March 2008 (UTC)

Large jets have good fuel economy because they are large. Don't forget that if you put four people in your car with likely gets more than 80 mpg, and that's the way they quote the jets. If your car was scaled up to seat 500 people it would be even more efficient. We have such vehicles, they are called trains, and you can see here that they are far more efficient than any aircraft. Maury (talk) 13:16, 3 March 2008 (UTC)

That was recently studied, and even they, they're only more efficient if run at unrealistically high capacity, which by and large they aren't- (User) WolfKeeper (Talk) 17:10, 3 March 2008 (UTC)

(undent)WK, did you read any one of the articles I linked to? I don't see much reason carrying on this thread otherwise. Maury (talk) 19:27, 3 March 2008 (UTC)

On the contrary, I suggest you read this and then ask yourself why you are comparing a diesel engine (which so far as I know isn't usually for aeroengines) with a jet engine which are typically optimised for low weight.- (User) WolfKeeper (Talk) 19:40, 3 March 2008 (UTC)

If you can show me where engine weight figures into the SFC formula, then you'll have a point. But it doesn't, so you don't. I won't presume that you are conceding the actual point about cycle efficiency, but I do presume that you are not going to address it, so I'll end end my participation in the thread here. Maury (talk) 17:51, 6 March 2008 (UTC)

It comes in because the wing has a L/D ratio, and which gives the ratio of weight to thrust. The amount of fuel needed to carry it comes from the SFC. So a heavier engine needs more fuel.- (User) WolfKeeper (Talk) 05:26, 18 May 2008 (UTC)

SFC in isolation is not a particularly useful number.- (User) WolfKeeper (Talk) 05:26, 18 May 2008 (UTC)

Article needs to be split

There's two numbers here, SFC of thrust engines and SFC of shaft engines. They're not the same.

The problem is there's a whole bunch of links...- (User) WolfKeeper (Talk) 05:26, 18 May 2008 (UTC)

I've done it.- (User) WolfKeeper (Talk) 06:33, 18 May 2008 (UTC)

Reminds me of being in a flight museum and noticing the description of a jet engine, with thrust in pounds and kilograms. I asked about this, and they noted that the official Smithsonian guidelines say pounds and newtons (as I expected). But then he mentioned it was the other way for piston engines. It seems that piston aircraft engines are measured in horsepower and kilowatts. Since thrust changes with pitch, especially for a variable pitch propeller, using thrust doesn't work so well with piston engines. So, yes, it isn't easy to compare them. Gah4 (talk) 05:34, 20 June 2023 (UTC)

One question

Why is specific fuel consumption double at altitude, when compared to take/off? I suppose it is because of pressure ratio? Could we build a more efficient engine, that would be used only for optimal cruise, that would have SFC at altitude equal to T/O?--Pawlin (talk) 15:23, 21 April 2014 (UTC)

When travelling at cruise altitude and velocity the jet engines must slow the air that goes through the core in order to compress it prior to mixing with fuel and burning. Much fuel is required to maintain velocity.

The engines at the start of the runway add velocity to static air, so greater thrust, acceleration and efficiency.

Ramjets work without compressors but are optimal from Mach 3 to Mach 6. At lower speeds you need to ensure that the jetstream goes out the back of the jet and not out the direction from where the air came in. 120.21.212.41 (talk) 04:24, 17 February 2023 (UTC)

Note near the top: The fuel consumption per mile or per kilometre is a more appropriate comparison for aircraft that travel at very different speeds. Consider a jet at the beginning of the runway, not (yet) moving. Fuel consumption per kilometer is infinite. And very high for taxiing around the airport, even if the TSFC is low. Engines are optimal at cruise altitude, with the appropriate measure. Gah4 (talk) 18:20, 17 February 2023 (UTC)

why is the table edited out? GGeorgee (talk) 10:41, 19 June 2023 (UTC)

Concorde example

The example confuses lbf (pounds force) with lb*f (pound feet) for the units of SFC. As this is the Thrust Specific FC article, it is awkward to introduce a section on Power SFC.

Furthermore, discussion about comparisons using velocity are covered more succintly and higher up in the article. — Preceding unsigned comment added by 104.156.75.106 (talk) 18:08, 11 February 2020 (UTC)

Rethinking this position, then entire section on Significance of SFC just skips the significance of SFC. It starts off by saying there are mitigating factors without actually addressing why one might use it and then outlying how that is too simplified a view. The Concorde example just exacerbates the lack of cohesion by introducing units without showing the math. — Preceding unsigned comment added by 104.156.75.106 (talk) 18:30, 11 February 2020 (UTC)

I think we ought to thrash imperial units and just use SI as a single table, with an entierly separate section for imperial if it must be used. The SI system should not be stained by imperialists measuring force in pounds. — Preceding unsigned comment added by 85.224.164.101 (talk) 11:52, 25 May 2020 (UTC)

I was sometime ago trying to figure out why some values had so many digits. I had traced it through Europe to FAA documents, the latter in pounds. That is US pounds, which may or may not be an imperial unit. I suspect much goes through US agencies, or companies, and so is in pounds. (Not that we can't convert them.) Gah4 (talk) 00:04, 20 June 2023 (UTC)

The Imperial pound is used in two ways - the pound force (lbf) and the pound mass (lbm). There is no US pound that might be different from the Imperial pound. You are thinking of fluid measure - the US gallon and the Imperial gallon. (The latter is 20% larger than the US gallon.) Dolphin (t) 00:42, 20 June 2023 (UTC)

OK not different, but not the same name. From Pound_(mass) the avoirdupois pound is used in the British imperial and US customary systems. So it is not, for example, the US imperial pound. But yes, I do remember when Canada sold gasoline in imperial gallons, though before I was old enough to drive. Gah4 (talk) 05:14, 20 June 2023 (UTC)

SFC/speed

There is a section explaining the need to compare SFC/speed, which isn't convincing to me. Slower craft obviously need lower thrust. Well, I think we need the actual scaling law for turbine engines. How do thrust and fuel consumption scale with size? Gah4 (talk) 23:09, 3 February 2021 (UTC)

Thrust is a force, not a power.--Marc Lacoste (talk) 07:37, 4 February 2021 (UTC)

I don't see where I said otherwise. OK, say you take an existing jet engine design and scale it down in size by a linear factor of two. All dimensions are half. How does the thrust scale? How does the SFC scale? And how does the power (which isn't thrust or force) scale? Gah4 (talk) 09:30, 4 February 2021 (UTC)

Turbofans have a pretty linear thrust to weight ratio, around 5-6, no gain with size here. But blade Engineering tolerances are constant, so efficiency is better with a larger engine for similar eras (see table).--Marc Lacoste (talk) 11:59, 4 February 2021 (UTC)