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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old March 25th, 2008, 17:54   #16
tnp
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This is really cool stuff, guys. Thanks a million.

Anyone know how to tell what BMEP is at any point in time? I agree that one isn't going to be at full load all the time. I'd love to figure out if I can manage to keep it in the sweet spot.

At worst, maybe it is something one could calculate from a few of the variables.

I have VAG-COM, but this variable/figure doesn't appear to be available. I'm assuming there is some fancy formula out there that I could have Excel calculate after the data is logged, if it isn't readily available.
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Old March 25th, 2008, 19:21   #17
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Please don't forget vehicle dynamics when thinking about BSFC. The best part throttle BSFC is not likely to be adjacent to the vehicle drag curve.

There is also another practical factor. The cost per MPH ratio. In other words, if slowing from 70 to 50 nets 8MPG more but completing a 500 mile weekly distance takes 2.8 additional hours. How much are those nearly 3 hours worth to you? At what point do you decide the extra speed is worth it?

45MPG at 70MPH, 500 miles = 11.1 gal/7hr 8min
53MPG at 50MPH, 500 miles = 9.4 gal/10 hours

In my mind, those 3 hours are worth well more than the 7 additional dollars it cost to go fast.

I brought this up because the somewhat varying BSFC numbers have little bearing on overall cost per mile.
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Old March 26th, 2008, 02:16   #18
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I have something...

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Old March 26th, 2008, 02:19   #19
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Quote:
Originally Posted by cujet
45MPG at 70MPH, 500 miles = 11.1 gal/7hr 8min
53MPG at 50MPH, 500 miles = 9.4 gal/10 hours

In my mind, those 3 hours are worth well more than the 7 additional dollars it cost to go fast.

I brought this up because the somewhat varying BSFC numbers have little bearing on overall cost per mile.
In the same way it has been shown time and time again that slowing down in traffic has been shown NOT to increase commute times on the average in direct proportion to the degree of slowing down. SLowing down might bring with it other benefits, too...
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Old March 26th, 2008, 06:50   #20
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Quite a spread sheet you have there TDI! And, I agree, on short trips, slower speed does not affect the ETA by much.

However, if your average speed changes considerably, your weekly time savings can be significant.

The BSFC of a diesel engine is generally better than an gas engine. So, across the operating range, the fuel consumed ends up being much less. As much as 30% in some cases. Remember that the BSFC (overall) difference between comparable diesel and gas engines is not on the order of 50% as so many claim here. While my 06 ranges between 35 and 42MPG and a gas variant 50% less, they are not comparable. The gas car has WAY more power and much more internal friction due to displacement.

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Old March 26th, 2008, 07:58   #21
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Quote:
Originally Posted by tnp
Anyone know how to tell what BMEP is at any point in time?
I have VAG-COM, but this variable/figure doesn't appear to be available. I'm assuming there is some fancy formula out there that I could have Excel calculate after the data is logged, if it isn't readily available.

BSFC= consumption / power


power = (engine speed * torque) / 9549.27


consumption (g/h)
engine speed (rpm)
torque (Nm)


All these three are available in the measuring blocks

Also

Torque (lb ft) = (BMEP (psi) x displacement (cu in))/150.8

or

BMEP = 4 * 3.141632 * torque / engine displacement

displacement (cm3)
MEP (pascals)
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Old March 26th, 2008, 09:18   #22
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Quote:
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LOL! A lot of trucks run the same road and you should see what's left of those deer when they collide. Meat explosions!
I used to be a "bobby big-rigger" and when I'd see deer I would hold my course and turn the wind shield wipers in case of blood splatter..
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Old March 26th, 2008, 10:51   #23
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ok so i do alot of highway driving. all thid data is making my brain hurt. How fast should I go or what RPM's should I maintain to get the best gas mileage.
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Old March 26th, 2008, 11:46   #24
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Quote:
Originally Posted by samuelc
ok so i do alot of highway driving. all thid data is making my brain hurt. How fast should I go or what RPM's should I maintain to get the best gas mileage.
40 mph in your highest gear will get you within spitting distance of best possible steady state MPG in most cars.

One guy on the Corvette Forum ran some careful tests on a 400 hp C6 Corvette on dead flat roads and hit best MPG at about 40 mph which was 900 rpm in 6th, so it even holds for some modern high performance cars.
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Old March 26th, 2008, 18:13   #25
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Thanks, TDIMeister. That's a great spreadsheet you have.
Thanks, No One of Consequence. I can now calculate the load with my VAG-COM data.

Should be fun to crunch some numbers and see where it goes! :-)

I'm hoping I can get another 5-10 MPG by just understanding this better.
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Old March 28th, 2008, 20:17   #26
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Anyone know how much the plot (like what TDIMeister posted in post #14) changes with a performance tune?

My assumption is that variables like injection timing have a significant effect on the shape and values of the curves whereas things like nozzles don't have much effect at all.

Thoughts?
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Old March 29th, 2008, 01:48   #27
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Most chip tuning does not touch the part load operating range. They only increase fuelling values, boost and timing at points near- and beyond the nominal stock full-load curve. Where this is the case, the curves of constant BSFC will remain exactly where the stock curves are up to the stock full-load line, except the new full-load line will be shifted up. For a small increment above stock, it is possible to extrapolate the BSFC chart, but it's impossible to do it accurately by this method because only the tuner knows what tables he has changed, and what effect these changes have on BSFC.

What is clear is that, as you increase fuelling by lengthening injection duration, you reduce the indicated efficiency, because combustion duration is getting longer and deviating from a constant-volume process that gives the best thermal efficiency at a given compression ratio. As you increase the relative AFR lambda, the expansion specific heat ratio goes down, also reducing efficiency (increasing BSFC). As you request a stock turbocharger to produce greater boost and furthermore operate at higher outputs at high RPMs, the operating point shifts to a point of diminishing total turbocharger efficiency, and choking may also begin to occur. Exhaust back pressures therefore increase in relation to boost pressure, hampering gas exchange and increasing pumping losses, and therefore further reducing BSFC. It is for these and other reasons that the point of minimum BSFC rarely coincides exactly with the maximum load line as engineers would intuitively expect (friction, pumping losses are at a minimum in relation to the brake mean effective pressure -- IMEP = BMEP + FMEP + PMEP), but rather shy of the max. load line, and then the curves "loop around" to increasing BSFC at a given RPM and further increasing load.


Advancing the injection timing will tend to reduce BSFCs to a point, at the expense of increased peak cylinder pressure, NOx emissions and Diesel clatter (cause by increased ignition delay).

I said earlier that chip tuners normally do not touch the part load maps. Where they do, they usually advance the injection timing maps, so if one were to draw new BSFC graphs, for most given operating points in part load, one would expect slightly lower BSFC values. It is for this reason that many people who drive chipped cars report better fuel economy when driven normally. It may be a real effect of the tuning, not be a placebo.
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Old March 30th, 2008, 14:19   #28
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Quote:
Originally Posted by xs650
40 mph in your highest gear will get you within spitting distance of best possible steady state MPG in most cars.
And accelerating at full load in the vicinity of 1500 RPM will get you close to an even better dynamic MPG. Hence, the power of pulse and glide. Alternating between full throttle and no throttle keeps the engine operating at either its best possible efficiency, or its lowest possible consumption.

Which leaves me wondering, is this just the case for diesel? We are warned against "jack rabbit starts", but the TDI performs most efficiently under load, so a quick start (minus burning rubber) is probably the best bet.
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Old April 2nd, 2008, 21:23   #29
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Keep in mind folks, fuel economy is NOT THE ONLY CONSIDERATION when selecting gear ratios.

If you select a top gear tall enough to get close to the best BSFC point in a normal highway driving speed range, the acceleration and response characteristics WILL be unacceptable to the average driver. (No top-gear hill-climbing ability or available acceleration without a downshift.) The feeling of lugging (torque pulsations) will be quite apparent, and unpleasant to most drivers.

Past experience with the VNT15 turbochargers in particular has found that requesting much engine load below approx 1700 rpm puts the turbo compressor uncomfortably close to the surge regime, which is bad for turbo longevity, particularly if this is done at high altitude. Heavy load at low revs also increases stresses on engine mounts, gearboxes, clutches, etc., because the internal vibration damper in the clutch is not really capable of accepting the inherent load pulsations of a 4 cylinder high compression engine at low revs. When revs drop too low, the power delivery through the driveline stops resembling a smooth flow and starts resembling a series of hammer-blows.

High torque at low revs is not good for parts that are hydrodynamically lubricated, either ... like your crank and rod bearings. These engines don't have a reputation for blowing up bottom ends, but everything has its limits.

It's better to perhaps spend a wee bit more on fuel, but get acceptable drivability and vibration properties, and not smash up turbochargers, clutches, and engine mounts.
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Old April 3rd, 2008, 11:33   #30
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Hi,

Just looking at the graph with the blue load lines on the top, and comparing to my normal driving habits and situations:

* 30mph in 5th gear, 1000rpm (steady city traffic)

Ideal at 10hp load, but if I understand the graph I'd be better in 1250rpm (4th?) if the load is 20hp, or 1500rpm if the load is 30hp. What HP load would my Passat be under if the 30mph is in steady traffic on the level?

* 50mph in 5th gear, 1650 rpm (motorway)

Looks about perfect

* Climbing a steep hill (varying between 1:10 to 1:20) at 40mph

... what gear / rpm would be most efficient here? I can't tell the engine load! The engine can cope with 5th gear on the slope, but is smoother in 4th.

* Pulse and glide

I experimented with this (glide in neutral with engine on) but without much success, most likely due to bad technique (such as over-pulsing it and needing to brake, interferring with other traffic, using inappropriate gears, etc).

Obviously the Passat's 115hp PD engine will have a different BSFC map to the Beatle's, but hopefully it is similar enough overall.
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