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Turbo or N/A for Max Fuel Economy?
by
Northboundtrain
on 25 Aug, 2005 08:14
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The next project I have in mind is building a 1.6L engine for the very best fuel economy I can possibly get. I'm wondering if it's worth the effort to find a turbo block (very hard to come by around here), or, for my purposes, whether an n/a set up would do nearly just as well (I already have a block).
I clearly see how a turbo allows more fuel to be burned for more power, but I've never quite understood whether a turbo makes the engine run more efficiently under a given load.
Any insights greatly appreciated.
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#1
by
vwmike
on 25 Aug, 2005 10:37
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Well, I can tell you my experiences -
I had an 81 Rabbit pickup with an NA 1.6 diesel. The fuel screw was untampered with, and I got about 38 mpg. The compression of the engine was mostly in spec, but not the best.
My 88 Jetta TD gets 40 mpg and is by no means fast but it is MUCH faster than the truck was. I can go up grades on the freeway at or above the speed limit which the truck had a hard time doing. Compression is good on the jetta at 475 across. The only things I have done so far are maintenance and a set of GTD nozzles with a bit higher pop pressure.
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#2
by
BlackTieTD
on 25 Aug, 2005 10:38
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in theory a turbo should get better economy.
in practice that rarely happens, in my experience. i guess it depends on your driving habits.
if you're going for max fuel economy, then i imagine the overall budget of the project is also a concern (its money out of your pocket one way or another) so i would stick with the NA diesel. cheaper to build, less parts to break. will be slow, but you want economy so you won't be mashing the pedal anyway. my 2 cents.
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#3
by
DieselsRcool
on 25 Aug, 2005 12:03
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For best MPG lust use your na engine and pump. Add a turbo and don't turn up the fuel. Add a larger exhaust and use TD injectors. Install a cool air intake if you don't already have one. be sure your rings and valves are in great shape. Set the timing to turbo specs. Use stock size 13" tires.
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#4
by
Northboundtrain
on 25 Aug, 2005 20:19
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DRcool,
Since this will be a rebuild, what kind of pistons should I use? Turbo or N/A?
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#5
by
houseofdiesel
on 26 Aug, 2005 07:11
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N/A by far-gets way better economy if your careful. I was getting 1400kms per 50L (850 miles on 13 gallons) reliably and consistant on my 87 Jetta keeping it under 65 mph daily commute. Totally stock engine but modified exhaust (cherry bomb muffler-free flowing).
The turbo engine excells when you need the power-if your not under boost the fuel economy suffers cause the intake/exhaust are alot more restricted. Of course you do have the extra power when needed...
If your living in the mountains go with a turbo you need it and will get better economy when climbing.
I have owned close to 20 diesel VW's, the n/a diesels always far outshined the turbos when it came to fuel economy even on the same commute/speed. I live in a pretty flat area though, if your in the mountains I think a turbo would be needed or atleast the close ratio trans.
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#6
by
Master ACiD
on 26 Aug, 2005 09:36
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reading through corky bells "maximum boost" book (amazing book btw, you should all get a copy) , you cant get somthing for nothing, no such thing as free energy and the turbo will decrease mpg under all conditions., even idling at a stop light.
normally it is not very much though, if you have a properly sized (large) turbo which has minimal backpressure off boost and reduced backpressure on boost you wont take a large mpg hit. if you run a dinky little k26 you would see alot of backpressure which is not good for efficency.
the only time you may see a mpg gain is under extreme load, such as hill climbing or pulling a huge load, because a turbo engine can have the power to get the job done and get out of that situation quicker than a n/a engine, which will be bogged down and under load for a greater length of time thus having a lower mpg over an extended legth of time.
imagine a vw pickup loaded up with stuff in the bed, going up a mountain side at 65mph. say it peaks the mountain in 15 minutes then we would have 15 minutes of boost action. imagine now that same truck with a n/a engine, running in a lower gear at 25 or 30mph with the petal floored, it now takes 30-40 minutes to go over the same distance.
30-40 minutes of full n/a throttle would probably cost more fuel than 15 minutes of boosted action. the boosted engine uses more fuel, but it gets the job done substancially quicker.
of course, with the hundreds of variables in engine design, hill grades etc, one cannot possibly give a definitive answer without knowing all the facts.
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#7
by
fspGTD
on 26 Aug, 2005 09:53
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It actually is possible to get "something for nothing" with a turbocharger, because a large part of the energy it uses otherwise literally goes out the tailpipe as waste heat. True, a turbo can cause backpressure, which the crankshaft must work against. But when the turbo is working right, it will put a substantial amount of this energy back into the crank via boost pressure on the intake stroke. In some cases, a well-matched turbocharger will even add more pressure to the intake stroke than it adds restriction on the exhaust stroke.
The main practical reason a turbodiesel will get better fuel economy than a naturally aspirated is that the engine can be loaded more and run at a lower RPM, where the engine will operate at an improved efficiency (lower brake specific fuel consumption.)
PS - this has been covered before:
http://www.vwdiesel.net/phpBB/viewtopic.php?t=649
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#8
by
vwmike
on 26 Aug, 2005 11:18
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When I put in the GTD nozzles and messed with the LDA pin the car smoked a lot so I turned down the fuel screw a tiny bit as a quick fix. It made the car substantially slower and actually hurt my mileage. I had to rev the engine higher to make the same amount of power which increased the fuel consumption. IMO, an NA diesel would be fine if most of the time you drive in the city, but I wouldn't want to deal with the lack of power on the freeway again. If you want to go 60 mph on the freeway here you're going to get killed and I had to drive the truck with my foot to the floor on the freeway. It would slow down to 60 or below whenever I hit any sort of real grade. I just found it incredibly frustrating and I don't think I'd want to wish that on anyone.
As for the backpressure issue - 80% of the energy which spools the turbo is actually heat energy. There is not sufficient pressure to really create boost without lots of fuel. After seeing how slow my car can be to spool, I don't think the stock turbo is really all that restrictive. If it was, you could see boost by revving the engine in neutral..
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#9
by
fspGTD
on 26 Aug, 2005 11:44
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I agree that a 1.6lTD turbo has a pretty free-flowing turbine section. (And that's part of the reason why the thing won't spool very quickly!) It turns out that the cross-sectional area of the most restrictive area of a 1.6lTD K24 turbine housing is about equivalent to a 1.75" diameter exhaust pipe. According to some measurements I took, there was a ring with a width of 8mm at a diameter of 59mm:
This gives 2.3 square inches.
Granted, the gasses are vortexing through that area not going straight through, so the effective area is probably significantly less. No doubt there is some pre-turbine backpressure happening when boost pressure is being developed, but the whole idea is that this energy is captured by the turbine wheel (and re-delivered to the engine through the compressor.)
----------------------
According to holset's web site, turbocompounding allows even greater engine efficiency to be achieved by a turbodiesel (46% instead of 42%). That is basically a turbine wheel in the exhaust but it is connected to the crankshaft mechanically by gears. So it basically captures exhaust gas energy and uses it to mechanically drive the crankshaft.
link:
http://www.holset.co.uk/files/2_5_1_3-turbocompound%20system.php
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#10
by
Master ACiD
on 26 Aug, 2005 12:36
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i dont think you are measuring the turbine section quite right. just the outside diameter of a pipe means nothing without taking into account what is inside it. in this case, a turbine wheel.
its that ratio of parts which combined with other readings can indicate how much packpressure you might see for a given engine.
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#11
by
Northboundtrain
on 28 Aug, 2005 07:25
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Thanks for all the info so far.
fspGTD: I read the other thread, thanks for the heads up.
So tell me if I'm on track. A turbo essentially makes a smaller engine larger and more powerful by increasing the volume of air that flows through the engine and increasing the effective compression ratio.
I know that given the same load or horsepower requirement, a smaller engine running closer to it's maximum torque will be more fuel efficient than a larger engine. But if the horsepower requirement increases, and the smaller engine has to increase its rpm beyond it's maximum torque curve in order to meet the hp demand, then it becomes less fuel efficient relative to the larger engine. So as the hp requirement increases, at some point the larger engine then becomes more fuel efficient than the smaller one.
Therefore, is it safe to say that the answer to my original question of whether turbo or n/a is more fuel efficient is: "depends on your load or horsepower requirement"? And if this is true, then suppose my horsepower requirement is maintaining 65 mph on a flat highway. Is this a big enough load to make the "bigger" turbo engine more efficient than the "smaller" n/a engine?
Again, thanks for the help
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#12
by
VWRacer
on 28 Aug, 2005 08:34
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It never got specifically addressed on the other thread, but I am fairly confident that the reason simply turbocharging a mechanically injected diesel engine can yield an increase in fuel economy is that the turbo increases the mass of the working fluid. As Jake notes, the turbo recovers some of the otherwise lost heat (energy) in the form of air pumped back into the cylinder. Even if combustion is no more efficient (though it probably is) the heat of combustion heats up a greater mass of air to push on the piston during the power stroke. This results in the recovery of a greater proportion of the energy released during combustion in the form of torque.
Master ACiD, Corky Bell was talking about gas engines when he made that remark. Specifically, on page 13 in response to the question
Will the turbocharger hurt my mileage?, he writes "
Yes. The turbo, when installed as an aftermarket item on a spark-ignition engine, is not an economizer and cannot be construed as such." [Emphasis added.] Bell carefully distinguishes between spark-ignition and compression-ignition engines, as well as between OEM and aftermarket installations. FWIW, as long as I stay off the boost in my OEM turbocharged Eagle Talon, it delivers the same fuel economy as the naturally aspirated version of the same car/engine combo. On-boost is another matter, of course...
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#13
by
Justin
on 28 Aug, 2005 14:13
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internal combustion engines are all based upon heat, the larger the difference in heat for intake and exhaust the more power that will be produced (thats why intercooling increases power, well that and a denser charche of air), so if you recapture heat, or the use of heat to pressureize the engine then the engine will be more efficient
its all about thermodynamics
later
Justin
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#14
by
Master ACiD
on 29 Aug, 2005 02:08
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you still cant get somthing for nothing though. more boost means more pressure that a piston must compress on the compression stroke. this fights torque. also backpressure fights torque. the key to making more torque instead of fighting it is burning more fuel, and to do that you need more air, where the turbo comes into play, and is why it can make more torque than it takes.
howcome the only people to have done real scientific testing on fuel economny, the epa, rated the fuel economy for a turbo vw lower, by as much as 5mpg in some cases, than a non turbo vw? shouldnt it be the other way around if what these people say is true about turbos making diesels more efficent?
im not really argueing that a turbo isnt a good thing, because i firmly believe a diesel vehicle without a turbo is almost a crime, but im just saying you cant get somthing for nothing. weither its gas or diesel or steam, no turbo is 100% efficent, not even close. thats kinda what i meant by refering to corkys book. yeah i know he only talks about spark engines, but some of the points, espically his writings on efficency are valid to any kind of engine.