Purpose of High rpm Low Torque Engines

Discussion in 'Gas Engines' started by baboonslayer, Dec 21, 2010.

  1. Yellowjacket
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    Yellowjacket Senior Member

    Turbines aren't generally as efficient as diesels. A typical high speed diesel has a specific fuel consumption (SFC) of .36-.38 lb/hp-hr (which equates to a thermal efficiency of just under 40%). That is, for each .38 pounds of fuel you burn you get one horspepower hour of work out of it. Large gas turbines can get close to this SFC, but only at high power. At part power you will burn more (on the order of .48 lb/hp-hr), and small turbines (under 500 hp) burn, at best, .52 lb/hp-hr at full power and .70 at half power. Older turbines, like the Lycoming enigne that is popular with the go fast boat crowd are pretty thirsty too, with full power SFC of about .56 lb/hp-hr. Multiply the SFC by the horsepower and that is your fuel burn in pounds per hour. So a 1300 hp Lyc with an SFC of .56 is burning just over 700 pounds, or about 100 gallons per hour. You have to need the low weight of the turbine to want to burn that much fuel. There are people who will, but then there are people who light their cigars with $100 bills too. Just not very many of them.

    While there are some low speed diesels that claim 50% thermal efficiency, that is only obtained with a very low speed machine, and the power to weight ratio of those engines is horrible. Think about a boat with most of its payload being a huge lump of an engine. Such an engine only makes sense in sationary units, or low speed displacement hulls were a few tons of extra engine weight doesn't make a difference. Also, since you are essentially buying a huge expensive engine and not turning it very fast, you are paying a premium up front to save some fuel later. Not many people are willing to do to that, so the market for such engines is also relatively small.
     
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  2. Joakim
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    Joakim Senior Member

    LHV is used in (almost) all thermal system efficiency calculation, since the water vapor in the flue gas can not be used in typical cases. If you would like to use it, you would need to condense all the water vapor from the flue gas to water liquid. Just dropping the temperature below boiling point is not enough. It will only condense part of the water vapor. In order to condense all of it, you need to drop the temperature to about freezing point. What would you do with heat at such a low temperature? Melt ice?

    Part of that energy is used at some power plants to preheat air etc. But then you have a problem with wet flue gases and you need to have a glass fiber stack to avoid corrosion.
     
  3. RonL
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    RonL Junior Member

    I would move it through a volume of liquid propane, the gas pressure produced and controlled at volume and pressure from 100 PSI or more, can drive a sliding vane type motor(s), after work performed and expansion of the propane gas it can move through an exchanger system that uses the temperature of the sea water and a slight pressure to return to liquid.
     
  4. rxcomposite
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    rxcomposite Senior Member

    It is typically called a "marine gear" because it has only two ratios. Forward and reverse. Gonzo just shortened it to gear. Some marine gears are available with 2 speed forward. Not all marine gears have 1:1 ratio for reverse.

    "gearbox" is commonly used in the automotive world. It just get carried over to the marine world.
     
    Last edited: Dec 27, 2010
  5. rxcomposite
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    rxcomposite Senior Member

    I checked my source, Jane's High Speed Marine Craft, 27th edition and entered the Sfc of various turbines. to compare, I used a generic diesel with (as per marine engineering book, 1972) has a standard Sfc of 0.5 lb/hp/hr (I used 0.494 figure to be exact).

    This is what I am getting;

    Engine type lb/hp/hr gr/kWh
    Generic diesel consumption 0.494 301
    Taurus Marine (Max rpm) 0.433 263 13,000 rpm
    Taurus Marine (continuous duty) 0.436 265 11,000 rpm
    Taurus Marine (lowest rpm) 0.822 500 6,000 rpm
    LM 2500 0.372 226
    LM 600 0.444 270
    LM 1600 0.372 226
    Textron TF 40 0.372 226

    Interesting to note that the Taurus Marine turbine has the low Sfc in higher range but shoots up more than 0.8 lb/hp/hr at low rpm. The efficient band however is very narrow.

    If the modern turbo diesel now is more efficient at 0.36 to 0.38 lb/hp/hr as mentioned by Yellowjacket then the turbines are no longer competitive.
     
  6. Joakim
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    Joakim Senior Member

    1972 is quite outdated for fuel efficiency of any engine or power plant. As well as using non-SI units...

    Small marine (10-50 hp) diesels seem to have specific consumption of about 250-280 g/kWh (0.40-0.45 lb/hph). I don't think you can buy a marine diesel as poor as 0.5 lb/hph (310 g/kWh) anymore. A bit bigger ones (100-500 hp), which typically are turbocharged are around 190-230 g/kWh (0.31-0.37). All of the most modern model are below 200 g/kWh (0.32).

    Then the real big ones used for ships and power plants are 160-190 g/kWh (0.26-0.31).

    The best turbine I found was 46% efficient at full power (GE LMS100, 100 MW, 140 000 hp). It is 10% more efficient than any other GE turbine. This is equivalent to 180 g/kWh (0.29) for LMS100 and 200 g/kWh (0.32) for the next best model.

    So the very best huge turbines are almost as efficient as standard huge diesels.

    But at a bit smaller scale turbines are quite inefficient. E.g. GE LM2500+ Marine turbine (30 MW, 41 000 hp) has only 36% efficiency, thus equivalent to 230 g/kWh (0.38).

    So it seems your turbine values are still quite up to date (except for the huge ones), but diesel values are very far from that.
     
  7. CDK
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    CDK retired engineer

    I beg to differ Joakim.
    The question was about a TDI engine and its amazing performance at low rpm. The key is that it is a diesel, so no throttle and no narrow mixture boundaries.

    Gas engine employ turbo chargers to increase peak power and generally do nothing below 3000 rpm because controlling the mixture is much more complicated compared to a diesel. But I'm sure you know all about that.
     
  8. Joakim
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    Joakim Senior Member

    Not all gas turbo's are like that. Like I said mine has constant torque starting from about 2000 rpm. It has a small turbo, that doesn't need high rpm and at the same time is too small for high output. It is a 2.0 liter engine, which has only 150 hp despite the turbo. The idea has been to increase the low rpm output, not high rpm output. It feels like a 3 liter engine at low rpm, but lacks the power at high rpm. And it is better than most diesels at 60->120 km/h acceleration at the biggest gear (does it in 15 seconds).

    You can easily get 150 hp without a turbo from a 2.0 liter engine. And they did something like 500 hp from the same block with a very different turbo when it was used in the rally car's during 80's.

    And the nice performance of modern car diesels is all due to turbo. Non-charged car diesels are quite lazy at low rpm (well actually all rpm).

    Attached are the power and torque curves for this gas engine with and without a turbo. See the big difference at lower rpm.
     

    Attached Files:

  9. Joakim
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    Joakim Senior Member

    And here are some power and torque curves for diesels. The first two are based on the same block as the gas engine in the previous post. A 2.1 liter diesel with and without a turbo.

    The last one is a very modern 2.0 liter HDI engine, very similar to TDI. It would be quite identical in performance compared to my gas engine. The peak power is almost the same, but the diesel has much more torque. Due to different rpm range the gearing is very different, thus about the same acceleration using the biggest gear or going through the gears.

    Then a modern 2.0 liter gasoline engine. Notice how flat the torque curve is.

    The last one is a modern 1.6 liter gasoline engine with and without turbo charging. Note the very high torque already at 1400 rpm. Here are more details about this engine: http://www.psa-peugeot-citroen.com/document/presse_dossier/PK_PSA_BMW1103281940.pdf
     

    Attached Files:

  10. powerabout
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    powerabout Senior Member

    You cant find marine engineers with turbine experience is another reason ships have dropped them even LNG carriers are now built with diesels ( also because the price of gas has gone up)
    The gearbox cost alot also with the massive reduction and huge size.
    Guessing, but I assume they cannot meet emission regs either..anyone add to that?

    Turbines fit well when you want light weight and instant full power

    PS re piston side thrust......crosshead/trunk pistons in large 2 stokes have no side thrust they separated the rotation from the up and down
     
  11. FAST FRED
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    FAST FRED Senior Member

    "1900rpm i would be using about 54 hp, doing 58mph and getting about 58mpg (or about 1gph)"

    NO, at 58 mph you might have the ability to create 54 hp to get you up a hill , however at 1 gph you are probably creating 18-20hp at best.

    54 HP from a gallon of diesel is impossible today, perhaps in 2110.

    FF
     
  12. Joakim
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    Joakim Senior Member

    Not even then, since at 100% efficiency one gallon of diesel per hour is equivalent to only 51 HP.
     
  13. powerabout
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    powerabout Senior Member

    There will be better diesel by 2110
     
  14. rxcomposite
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    rxcomposite Senior Member

    Thanks Joakim,

    I will update my database and will include the LMS series you gave. I also found a reliable author who uses 0.357 lb/hp/hr for diesel. Seems reasonable enough for a baseline.

    I work in SI but the problem is most of my books came from the U.S. Also, the USAnians here wants it in English syste, so need to convert always.;)
     

  15. Steve W
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    Steve W Senior Member

    Ok Fred,i think got it, this stuff is great info for the mathematically challenged like myself. So,using 7.13lbs as the weight of a gal of diesel and using actual fuel consumption figures from a recent road trip from Minnesota to Virginia and back, on the way out with 2 people at a 70mph average we achieved 51mpg so 1.18gph,8.41lbs of fuel using about 23.4hp. Now on the return trip we were towing a 19ft beach cat, not real heavy,maybe 800 - 900lbs but significant windage. We achieved 30mpg through the mountains of west Virginia so 2gph, 14.26lbs/hr, using close to 40hp. interestingly we used slightly more fuel for the rest of the trip even though not many hills but a lot of cross and headwinds. Do these figures look about right?
    Steve.
     
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