Turbo or N/A for Max Fuel Economy?

[vwmike]

I think this arguement could be summed up best like this:

- All else remaining equal, a turbo will not increase fuel economy in and of itself. But, in the the instance that the unit of work is beyond the normal capabilities and puts such strain on the engine to acheive that goal that it is pushed beyond it's area of peak volumetric efficiency, a turbo may increase mileage as it allows the engine to operate further within it's range of efficiency.

So what does that mean?

- If you live in an area that is relatively flat, and/or the vehicle is largely operated at low speed then mileage would be better without the turbo.

- If you live in an area with a lot of hills which frequently require more engine RPM's than can be efficiently attained (ie - Downshifting) then you are probably better off with the turbo.

[Master ACiD]

i can agree with that.

[VWRacer]

You're right Master ACiD, there is no getting something for nothing. However, the case of turbocharging a diesel doesn't fall into that category. But rather than argue about it I decided to do some additional research to find out if I was on the right track, and so dug out my copy of Sir Harry Ricardo's The High-Speed Internal Combustion Engine, to see what he has to say about diesels. It turns out that he devotes 14 pages to just boosted diesels. The pages in question are filled with graphs and summaries of his research, but in the end he boils it down to 8 main points:

1. Detonation and pre-ignition are not present, permitting higher compression ratios than with spark-ignition engines. (We already knew that, of course.)

2. A greater charge density results in a shorter ignition delay and more complete combustion.

3. Cooling the intake charge yields greater charge density, permitting greater power up to the mechanical limits of the engine, but at the cost of greater ignition delay as compared to a hot intake charge.

4. The higher the boost pressure the less sentitive the engine to cetane number.

5. Intercooling a C.I. engine results in a relatively greater gain in thermal efficiency than it does for a S.I. engine.

6. C.I. engines increase their oxygen utilization rate up to 5% (from 82% to 86%) with added boost (comparing atmo to 3 bar of boost).

7. Adding boost alone results in a increase in brake effective mean pressure (BMEP). Just 10 lbs of boost (0.67 bar) resulted in an increase in BMEP of more than 19%.

8. Turbocharging will, at almost all load ranges, result in greater gains in mechanical and thermodynamic efficiency than are absorbed by the exhaust energy used to drive the turbo. [Emphasis added.]

Ricardo's final point can be considered as primarily the result of points 6 and 7. Namely, that combustion is more efficient in boosted diesel engines, and that increasing the mass of the air taken into the engine directly results in an increase in BMEP (what I wrote above as "the turbo increases the mass of the working fluid" resulting in "a greater proportion of the energy released during combustion in the form of torque." I just didn't use technical language.)

[fspGTD]

Those are some really interesting bits of info, Stan!

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.

That is true, but for every upwards stroke there is a downwards stroke.  While the extra cylinder pressure and pre-turbine backpressure fight the piston as it moves up during the compression and exhaust strokes, (like you said - sapping power and torque from the crankshaft,) the extra cylinder pressure and intake boost pressure help the piston on its way down during intake and power strokes.  So it seems then that that the net effect on the crankshaft through all four strokes would be zero, provided that the turbo could put out as much boost pressure as there is pre-turbine backpressure.

Which EPA diesel fuel economy ratings you are referring to?

[Master ACiD]

both of these subjects on compression ratios and intercoolers are not involved in the original topic, and the spesfic diesels we are talking about dont even come with intercoolers. heck their intakes are about 3 inches from the turbo!

now back to my question, why is the epa's established fuel economy ratings rate vw turbo diesels 5mpg lower than the same car and engine without a turbo?

check it out for youreself, at fueleconomy.gov. they list alot of cars including turbo and non turbo vw's, and the turbo engines are listed with lower mpg. the 85 jetta is the one i am spesfically talking about.

[vwmike]

now back to my question, why is the epa's established fuel economy ratings rate vw turbo diesels 5mpg lower than the same car and engine without a turbo?

I'd guess that could be because they rated those cars when they were new and the speed limit was 55mph. An N/A diesel can do 55, no problem. But 65+ might be an issue going up hills and such which makes you downshift and then your economy goes out the window.

[fspGTD]

I wouldn't make too much out of what seems like one data point (1985 VW Jetta).  The test results of these kinds of general fuel economy tests simply vary too much (potentially for a number of reasons) to take them literally.  There is also other fuel economy data that contradicts the 1985 naturally aspirated 5mpg highway fuel economy lead.

If you go to the source of the data, the EPA, and take a look around there, at some other data points:
http://www.epa.gov/otaq/fereport.htm
...for example check out the 1983 model year data, comparing the VW diesels.  You'll find the turbo-diesel is sometimes equal, sometimes a little better, sometimes a little worse than the naturally aspirated diesel, and the numbers vary much more depending on transmission equipment than presence of turbo:

estimated miles per gallon for 1983 models years
1983 Jetta Diesel, 5-speed manual: 42 mpg
1983 Jetta Turbo-Diesel, 5-speed manual: 43 mpg
1983 Jetta Diesel, 3-speed auto: 37 mpg
1983 Jetta Turbo-Diesel, 3-speed auto: 36 mpg
1983 Rabbit Diesel, 4-speed manual: 50 mpg
1983 Rabbit Diesel, 4-speed manual: 48 mpg
1983 Rabbit Diesel, 5-speed manual: 43 mpg
1983 Rabbit Turbo-Diesel, 5-speed manual: 43 mpg
1983 Rabbit Diesel, 3-speed auto: 35 mpg
1983 Rabbit Turbo-Diesel, 3-speed auto: 36 mpg

miles per gallon for 1984 model years (est, cmb, hwy)
-49 state-
1984 Jetta Diesel, 5-speed manual: 43/47/53
1984 Jetta Turbo-Diesel, 5-speed manual: 41/46/54
1984 Rabbit Diesel, 3-speed auto: 35/39/44
1984 Rabbit Diesel, 4-speed manual: 31/36/45
1984 Rabbit Diesel, 5-speed manual: 44/49/57

-california only-
1984 Jetta Diesel, 4-speed manual: 46/51/58
1984 Jetta Turbo-Diesel, 5-speed manual: 41/46/54
1984 Jetta Turbo-Diesel, 5-speed manual: 40/46/54
1984 Rabbit Diesel, 4-speed manual: 47/52/61

IMO... EPA says that they only test 10% of the actual figures... I'll bet they just rolled the 1984 Jetta data into 1985.  You can see that the 1984 Jetta test is comparing a 4-speed manual naturally aspirated diesel to a 5-speed turbo-diesel.  The 4-speed actually gives better numbers on EPA's test.  One possible explanation.  Another possible expanation, is that tests simple have too great of a margin of error to be accurate enough to tell which engine is more efficiency.  They seem vary when we wouldn't expect them to, so either the tests have a wide margin for error, and/or the production tolerances vary from one car to the next enough that it causes these variations.

I believe that a more accurate test of engine efficiency would be the brake specific fuel consumption plots published by VW powertrain engineers in SAE papers (see other thread for this.)  These tests rule out transmission and other equipment differences including the body style, etc, and focus simply on answering the question: how efficient is just the motor itself in producing power at various engine RPMs (plotted on the x axis) and loads (plotted on the y-axis)?

[VWRacer]

Jake, what is your Dad's VW diesel pickup's fuel economy post-turbo compared to the same engine pre-turbo?

[fspGTD]

Dad's 1.6lVNTD (Rabbit sedan, by the way) is now getting a touch over 50 miles per gallon.  That is an average reading spanning several tanks by the way, so it should be a quite accurate.  Unfortuntely, we don't know what it got before the turbo was added, because the odometer had a broken tooth, and so it skipped.  But we are pretty confident that it didn't get anywhere near that mileage before the turbo was added.  It was smoking quite a bit then, and now it is tuned to be just on the verge of smoke during all out wide open thottle.  The 50 miles per gallon is also abnormally high for what is expected of a naturally aspirated diesel Rabbit, which I'd say is generally low 40's for the way it is used.

I can also add experience with my Rabbit with swapping the engine from a naturally aspirated 1.6 liter to a turbo-diesel 1.6.  Back when it was naturally aspirated, it got on average low 40's miles per gallon (41-42 mpg.)  After I swapped in the 1.6lTD (stock), I remember the mileage jumped up a modest but noticeable amount, around 2-3 miles per gallon, ending somewhere around 43-44 miles per gallon.  (I also measured religiously at every tank and averaged across several sequential tanks, so was able to track any changes quite well.)  But after adding the turbo, it seems the old transmission wasn't using the torque to it's fullest most efficient operating point at freeway speeds.  This especially became apparent after opening up the exhaust, adding an intercooler, and increasing the boost pressure somewhat.  So I swapped in a 5-speed that maintained the stock .71 fifth gear, but a lower-ratio final drive (IIRC it was a 3.34 final drive replacing the 3.89 of the stock "FF" tranny), causing a drop in freeway cruising RPMs from 3000 RPM to about 2500, or about 15% lower.  The fuel economy shot up to 48 miles per gallon with the "longer legs" and the higher and more efficient torque area that was enabled by the turbo.  The RPMs at cruising speed on the freeway were now at a seemingly ideal 2500rpm instead of the stock transmission's 3000rpm.

It turned out that proper gearing was key to maximizing the potential efficiency offered by the turbo-diesel.  Looking at the brake specific fuel consumption plots, you can see how efficiency depends highly on load and RPM, and why reducing the RPMs and increasing the load is so helpful.

[Master ACiD]

i would love to look at those epa text files but i open them in note pad, its too difficult to sort through that stuff to bother. i cant seem to open a .rpt file.

i can tell you this, when my dad put a turbo on his rabbit truck, he straight up  lost 6mpg, which pretty much coincides with what epa says.

[jtanguay]

how does your dad drive now that he has a turbo?  did he tune the engine properly?

I bet your dad has poor mileage from trying to keep the car in boost while giving it more fuel to spool the turbo.  Get a nice tiny turbo like the VNT-15  

[Mark(The Miser)UK]

i would love to look at those epa text files but i open them in note pad, its too difficult to sort through that stuff to bother. i cant seem to open a .rpt file.

i can tell you this, when my dad put a turbo on his rabbit truck, he straight up  lost 6mpg, which pretty much coincides with what epa says.

Yes Jake how do you open a rpt file?The site offers pdf stuff only!

My Quantum hand book also gives improved mileage for TD over N/A. Best at about 56mph  10% or so.

[fspGTD]

Yes Jake how do you open a rpt file?The site offers pdf stuff only!

Mark - I just opened the .rpt file that is inside the .zip up in notepad like a .txt file.  Any text file reader should work.  Just make sure you open up the .zip file first and copy the file inside to your local machine.  You might find it helpful to rename it to add on a ".txt" to the end to get it to open properly.

MasterACID: Best I can figure, assuming you guys measured your fuel economy correctly and accurately, (and with no lead foot! :wink:), is that there is something wrong with the turbo equipment that you installed.  If it was all working properly, you should not have noticed a fuel economy decrease, if anything there should be a small to medium increase, like what I found when I did a 1.6lTD swap.  Hard to diagnose the problem without details about what was done during the turbo conversion though.

[Master ACiD]

what was done on the conversion, me dad replaced the original engine with a fully rebuilt turbo diesel. the turbo engine was bored 20 thou, gone over inside and out.  the head was totally rebuilt and surfaced with all new parts as needed. the injection pump, injectors and turbo sent to a bosch diesel specialist for inspection and repair. turns out everything was ok except he needed 3 injectors rebuilt.

i honestly dont think you can get somthing for nothing. if that was true then every diesel in the world would have a turbo. but theres plenty of diesel forklifts, road graders, generators, waterpumps etc that dont have turbos, even large equipment like my ford 4.4 liter i4 diesel power head has no turbo.

[vwmike]

i honestly dont think you can get somthing for nothing. if that was true then every diesel in the world would have a turbo. but theres plenty of diesel forklifts, road graders, generators, waterpumps etc that dont have turbos, even large equipment like my ford 4.4 liter i4 diesel power head has no turbo.

That's an entirely different subject. That is marketing and price point at work. Adding on the turbo increases the cost of production and to the consumer. If it isn't absolutely necessary to meet the demands then you'd be lucky to actually even see it as an option.