Talk:Specific fuel consumption (thrust)

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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,

,

but the pressure ratio is defined as the pressure increase

.

Using the combined gas law we get:

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)

Contents 1 SFC table 2 Conversions table/explanation needed 3 Comparing jets and pistons 4 Article needs to be split

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)