V12 Diesel

Discussion in 'Volkswagen' started by gofastclint, Jan 2, 2009.

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  1. Mar 19, 2015 #41

    B100

    B100

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    Hi Ross thanks for you reply, it seems we are communicating better which always good for all [​IMG]

    "We agree that the BSFC for 4 stroke diesels is better than most SI engines- no argument. Here pump gas is around .83c/ L and diesel is 1.05. This means the diesel is not going to save any money over a comparable SI powered car."

    Lol...Luky you! here in Portugal and in most other European countries its the other way round, diesel is cheaper than gasoline! here is a link that should make you guys across the pond realise how good you have it[​IMG]

    https://www.energy.eu/fuelprices/ (these are reference prices, they can be a little cheaper or expensive)

    Here you only have unleaded 95 @ 1,48 euro/L and 98 @ about 10 cents more! Diesel costs 1,25euro/L

    Just for the record my home made Biodiesel has a final production cost between 30 and 40 cents/ L ! so you can understand why I bother to make it and dream of a flying machine powered by it !

    Depending on where you are in Europe diesel powers between 50 and 65 % of all traffic on the road even in countries where diesel is more expensive! Just stating a fact that explains why the best diesel engines are developed here!

    "At the low specific output levels you are talking here, SI engines easily last a very long time also. Our shop turbo 240SX has 330,000 km on it now after 17 years of being turbocharged, beat on at the racetrack etc. Compression is still excellent, oil consumption 0.5L in 10,000 km. I just change oil in it and drive it. I think you will find that engine life in the aero application (continuous high power) is reduced considerably compared to automotive, especially when the hp is increased over stock levels. My point was simply that diesels can't match the same power to weight ratios as SI engines and weight is important in aircraft. Any turbo engine can have better power to weight ratios simply by increasing manifold pressure."

    Well Ross there are good and bad engines out there, both SI and diesel , but the point is the engine examples I mention have decent power to weight ratios that are stock and can maintain high power output all day without undue stress! So you can increase power levels safely between 20 to 25% and some makers actually offer that as an option! But you dont need to , because the power is there already ! While you are right that Si engines can be boosted to produce more power then diesels that comes at greater cost in terms of fuel cosumption and engine life ! and lets not forget that torque plays a part as well ! (no wish to go into the torque vs hp debate) but in aircraft applications it seems to be a bonus in certain conditions .

    So correct me if I´m wrong take off on 80 to 100% power for 2 minutes, then back of to 75 to 80 % on climb , and cruise on 60 to 65 % power !
    In a current diesel engine that translates into 3600 to 4000 rpm then back of to 3000 to 3200 rpm and cruise at 2400 to 2600 rpm !
    For a diesel engine that is easy, in fact they thrive on stable power settings , when I drive across Europe my diesel loves it , I set cruise control for 2400 rpm (good for 120kph). In Germany I push it a bit 2800 rpm and keep there for hours! no problem and this stability also delivers great fuel economy!
    My point is a diesel is built precisely for long periods of high work loads sustained by their natural low end torque that is constant from as low as 1800rpm , and max out at around 2800 rpm, which is apparently the sweet spot for best prop efficiency.
     
  2. Mar 19, 2015 #42

    rv6ejguy

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    There are many stock, naturally aspirated SI engines today which exceed 90hp/L specific output- about the same as the best turbocharged stock diesels. They are also loafing along on the highway, even at 150km/h and they easily go 300- 400,000 km. Add turbos to any of these and you can have staggering power as we see in cars like the Nissan GTR (158hp/L stock, 526hp/L modified).

    Cruising on the highway at 120 kph demands only about 10% power on an average small car with a 200hp engine so your cruise conditions in the aircraft are 5 to 6 times higher than on the road, this means at similar rpms, manifold pressure must be 5-6 times higher- more stress for sure.

    To be clear yet again, diesels do not produce more torque than the same sized SI engine at the same manifold pressure- complete fallacy. Find me one current production engine that does. Apples and oranges to compare turbo and non-turbo engines in this regard. Area under the hp curve determines the work any engine can perform. Diesels do not do well in this comparison. Variable geometry turbos are responsible for the low torque peak rpm on many modern engines (1300 rpm for the BMW 135i), has nothing to do with being a diesel or not. The new M3 puts out 425hp and 406 lb./ft. of torque at about HALF the boost pressure of the 535D turbo diesel (255hp and 413 lb. ft.)

    Finally, we see today that modern SI powered cars like the Prius 4 get better highway fuel economy than a similar diesel like the VW Passat TDI SE. Highway cruising keeps the electric motors out of the comparison. With direct injection and compression ratios around 14 to 1 plus other technologies like BMW's Valvetronic, modern SI engines are also very impressive for fuel economy.
     
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  3. Mar 19, 2015 #43

    BJC

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    One of the challenges in comparing alternative engines to proven purpose-designed airplane engines is making a realistic comparison. To be realistic, the entire propulsion system needs to be considered for multiple parameters, including weight, complexity / reliability, impact on drag, volume required, fuel systm requirments, etc. Although there are successful alternative engines flying, it is very hard to beat the overall value of a proven purpose-designed airplane engine. But hope springs eternal.


    BJC
     
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  4. Mar 19, 2015 #44

    BJC

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    I consistently got 55 to 56 MPG in my 1983 VW diesel Rabbitt, and it had room for 5 full sized adults. The only problems were slow acceleration and inability to drive with an airspeed of more than about 75 MPH.

    Oh, and the emissions seemed to bother the US EPA.


    BJC
     
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  5. Mar 19, 2015 #45

    Himat

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    I do not quite agree on that statement. In Europe the small van is a popular car, and they have for they size decent load carrying capability and often quite small engines. One example, here in pick up truck format, the Fiat Doblo:
    doblo5-stor.jpg

    The specifications varies, bit the smallest engine is a 1,3l turbo diesel rated at 66kW (90hp).
    Top speed of the car is 158km/h or close to 100miles a hour.
    Now, said car with the smallest engine is certified to carry a one ton load and pull a one ton trailer.
    With 130km/h and 70 miles a hour speed limits common on European roads, this engine will see something different than easy cruising at 10% of rated power. (Ok, there is often a lower speed limit if towing a trailer, but that one is not always adhered to.)
    I'll expect quite a bit of pedal to the metal driving.
     
  6. Mar 19, 2015 #46

    rv6ejguy

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    My post said car, not van and 200hp not 90hp. Obviously the smaller the engine and the higher the drag of the vehicle, the higher percentage of full power it takes to go X speed. Same goes for higher weight dragging up a mountain. Just comparing an average 200 hp/ 3000lb. car load cycle at 120 kph on a level road and this situation is nowhere near what a cruising aircraft would require for hp.
     
  7. Mar 19, 2015 #47

    Himat

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    Yes, I did read your example, my point was more to provide another example that show that how a car engine is used can not be generalized, not from either example. The engine in question is used in a lot of different small to medium size cars, but is probably worked hardest in the small vans. I probably could have found an example with a similar car with a small spark ignition petrol engine.

    As is the cost and availability of different fuels. Around here diesel fuel is cheapest, even more so if it is not for a road vehicle use as there is less tax on diesel fuel not intended for use in road vehicles. And while talking taxes, in Europe many countries taxes cars by the engine size, both capacity and power output. Small cheap cars then tend to have small engines. To keep up with the traffic the engines then get worked hard.
     
    Last edited: Mar 19, 2015
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  8. Mar 20, 2015 #48

    B100

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    Really BJC ! are you serious ? are you trying to compare a 1983 era diesel engine with current engines? I don't want to be offensive but that is ridiculous! Please, be serious do your homework, and get your facts straight!
    What you call a Rabbit on that side of the pond is what is known world wide as a VW Golf ! so while back in 83 that was true, today your "rabbit" lol....would be more like a hare....! And updated specs of current diesel VW rabbits/Golf would be closer to this :

    Volkswagen Golf Golf 2.0 TDI 150 GT 5d full specification - BBC Top Gear or the latest 120 cu in engine putting out a miserable 181 hp :roll:

    But please, dont take my word for it, check it out yourself :

    Volkswagen Golf Mk7 - Wikipedia, the free encyclopedia

    Lol...I guess with these bunnies you would have no problem with " slow acceleration and inability to drive with an airspeed of more than about 75 MPH." Would you?

    Now I am aware that you dont get the best engine options on that side of the pond but that doesn't mean they dont exist!
     
  9. Mar 20, 2015 #49

    B100

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    Grrrrrrrr........just finished a long detailed reply......and lost it when trying to post it :mad2: So here the short version!

    "Cruising on the highway at 120 kph demands only about 10% power on an average small car with a 200hp engine so your cruise conditions in the aircraft are 5 to 6 times higher than on the road, this means at similar rpms, manifold pressure must be 5-6 times higher- more stress for sure."

    No ! dont agree! my TDI has 115 hp , set @ 2400rpm = 120 kph that is for a loaded weight of around 2100kg so with your numbers.....no way !
    Average car engine in Europe is around 100 hp or less ! We cant afford the big inefficient gas engines you have over there ! Oh! and there is a reason that 60% of all cars on the road are diesel!
    Himat:
    "As is the cost and availability of different fuels. Around here diesel fuel is cheapest, even more so if it is not for a road vehicle use as there is less tax on diesel fuel not intended for use in road vehicles. And while talking taxes, in Europe many countries taxes cars by the engine size, both capacity and power output. Small cheap cars then tend to have small engines. To keep up with the traffic the engines then get worked hard."

    Amen to that and Himat forgot another big component too! emissions taxes ! CO2! so....diesels burn less fossil fuel = less CO2 taxes (some countries have different standards for both fuels)

    Quick example: 1,6L engines 100hp , one gas , one diesel! Diesel emits less CO2 pays less tax!
    Boost them as you say; Diesel easy 140 hp = same or lower fuel burn! Gas up to 180 hp = more fuel= more emissions= more taxes !

    As for engine work load , well consider my real world personal example! on the road my engine is propelling 2,1 tonnes @ 120km on 2400rpm and has to go uphill , cope with the road surface/friction, and air resistance/drag and weather which the plane can fly around/over (adverse weather) . The plane ,(certainly lighter than the car) simply has to deal with drag, with the benefit of altitude and thinner air.
    So I ask, in these conditions which engine is working harder, considering the constant load both are subjected to maintain the RPM set ?

    My lost post had a lot about you BMW comparison , but short of it is that you guys in Canada and the US get very limited engine options on the European cars sold there, this mostly because of your lower fuel standards in gas and diesel ! Our current M3 3,0L diesel comes stock with 313hp !

    "Finally, we see today that modern SI powered cars like the Prius 4 get better highway fuel economy than a similar diesel like the VW Passat TDI SE. Highway cruising keeps the electric motors out of the comparison. With direct injection and compression ratios around 14 to 1 plus other technologies like BMW's Valvetronic, modern SI engines are also very impressive for fuel economy. "


    Again , you are right but it only applies to Canada and US because of the mentioned limited engines available! In Europe quite a few diesels beat the Prius in terms of fuel economy . I like you am also amazed at the latest development in fuel injection tech, but Ross you know that dew to limitations involving fuel to air and compression ratios SI engines can never reach the efficiency of a diesel, and while 14 to one is impressive, its about the upper limit for a gas engine and low by diesel standards. And since you insist on comparing apples & apples , a Prius is smaller and lighter than a Passat sedan or wagon ( I actually own one Passat)

    Hope this helps to put the discussion into context![​IMG]
     
  10. Mar 20, 2015 #50

    Kingfisher

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    Last edited: Mar 20, 2015
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  11. Mar 20, 2015 #51

    cheapracer

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    Only the context you seek but many of us try to stick to real world context.

    I actually spend a lot of time and money researching these options and the math for a diesel simply isn't there - and that's why there is next to no diesel options being offered.

    As this forum is almost specific to 2 seat light aircraft of around 300 to 400kgs, there just isn't a realistic place for an engine that weighs an extra 80 to 100 kgs, or in other words, the weight of one entire passenger, besides the re-engineering of the aircraft. Real world math.

    That's not what he said, try to keep up.
     
    Last edited: Mar 20, 2015
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  12. Mar 20, 2015 #52

    BJC

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    What are you flying "on that side of the pond" B100?


    BJC
     
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  13. Mar 20, 2015 #53

    rv6ejguy

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    In Europe where diesel is cheaper than gasoline, diesel makes a lot more sense than it does over on this side of the pond, hence you have a much higher percentage of diesel cars there. Same goes for diesel aircraft where 100LL is scarce or expensive or both and diesel or Jet fuel is available and cheaper. No argument there. Over here, economically they make less sense especially in aircraft where the engines are double to triple the cost of similar hp SI engines.
     
  14. Mar 20, 2015 #54

    rv6ejguy

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    Of course there are smaller and lighter diesel cars than the Prius which give better fuel economy. Just making a comparison of roughly equal sized cars SI and diesel. The interior volume of the Passat and Prius are almost identical, the Passat does weigh about 400 more lbs. The Prius got better mileage than the Passat and probably would even with 400 more lbs. on board. You say inefficient but today SI engines with modern tech have narrowed the gap considerably with diesels. Back in 2007ish, the SAE World Congress recognized in a statement that new SI technologies were rapidly bringing SI BSFC numbers close to diesel engines. Actually, if you compare the best, latest, SI passenger car engines to diesels on a BTU level in the fuel, the SI engines match or exceed the diesel in this regard. Diesel fuel has about 13% more energy content per unit volume. The energy content per unit weight is very similar between the two fuels. There is a rapidly diminishing return in thermal efficiency gain with compression ratio. Modern diesels have actually seen a reduction in CRs whereas modern SI engines have seen an increase- which has narrowed the efficiency gap lately along with other new technologies.

    Let's take my 2002 BMW 330ci with 3L SI engine. At 120 kph on a level road, it's using 6L/100km. Converting this to hp and using .45 BSFC, that's 21hp used to overcome rolling and air resistance. A Lycoming O-360 develops 135hp at 75% power cruise. That's 6.4 times the hp of the BMW at 120 kph so you see, the load cycle is much higher in aviation use. Most aircraft generally never cruise at power settings below 50%- usually 65-75%. The BMW example- mid sized car, 3500 lbs is using 9.3% power to cruise at 120 kph. I would expect the BMW to last 400,000-500,000 km cruising under these conditions. Let's say 3000-4000 hours conservatively. I'm not sure it would last that long in my RV6 pulling 60% power (135hp).

    We see the Thielert and Austro 2L aero diesels based off a Mercedes design rated at 155 and 170hp respectively- they have not pushed the ratings up with higher boost pressures since they know longevity is likely to suffer. Initially, the Thielert 1.7 had many piston/ ring issues and many engines were being pulled even prior to 1000 hours because of excessive oil consumption. They eventually solved the problems on these and the 2L is now working very well according to most reports. Generally on auto conversions in aircraft, we do not uprate hp over the stock levels since we know this is just going to reduce life and possibly reliability. The Austro and Thielert engineers are no dummies and understand this. Derating engines is a time proven way to extend life. Few knowledgeable engine people would agree that uprating an engine (diesel or SI) would have no impact on longevity at high, continuous output levels like we see in aviation.

    The M3 SI turbo still has 110 more hp than the M3 diesel and uses less than half the boost pressure to do it.

    The diesel has better performance at low loads than SI engines (outside of developments like BMW's Valvetronic). This is a fair advantage in light load auto applications. The BSFC gaps closes at or near WOT like we use in aircraft. The BSFC of the Thielert is .36, only marginally better than a Conti 550 running LOP at about .375.

    As I've said on other forums regarding engine types, fly whatever engine you like and what turns your crank so to speak. I just find the diesel worship guys often seem to think that EVERYONE should be flying a diesel. I just point out that a diesel is not better than an SI engine is all regards for aircraft in all areas of the world and for all missions. They have their place as do SI engines.
     
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  15. Mar 20, 2015 #55

    rv6ejguy

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  16. Mar 20, 2015 #56

    cheapracer

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    Ross, I compliment you on your patience, sadly it seems I've run out of it in recent times.
     
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  17. Mar 20, 2015 #57

    B100

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    LOl..... my sofa! but I have to admit that your reply is a funny way of manning up to the fact that you Rabbit example was just ridiculous! :gig:

    Dixit!
     
  18. Mar 20, 2015 #58

    B100

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    Yes ! Indeed Cheapracer, You probably know all, and best, and everyone knows that progress comes from closed minds and not pushing boundaries and new possibilities ! Fortunately those pesky bicycle builders known as the Wright brothers didn't think like you !
     
  19. Mar 20, 2015 #59

    B100

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  20. Mar 20, 2015 #60

    cheapracer

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    Ironically it was very fortunate that others did not think like the Wrights, and that the Wrights blocked others from following their methodology turned out to be a good thing forcing individuals to discover far superior solutions. Don't try using history as a weapon unless it's properly loaded.

    I have "pushed the boundaries" many a time and my aviation based patents/applications are all about "new possibilities". I spend time and money researching stuff like the possibilities of diesels and in fact, a product of mine being currently developed is advantageous in it's market place through it being diesel powered (modern 2.0 turbo common rail), so overtones of me having anti-diesel bias or ignorance in anyway is nonsense, but it's just not a fit for HBA typical light aircraft. It always comes back to the math and 2+2 needs to equal 4.


    You have an unfortunate short history here of not liking other people actually having experience in matters overriding your veritable delusions, but anyway, no problem as I don't actually answer for you, there's many others reading here as well and I'm sure they enjoying reading and learning, especially from Ross's posts which are book worthy.
     
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