BMEP and gearing for economy

MrDave

Veteran Member
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Sep 5, 2002
Location
1300 km NW of nowhere
Came across the BMEP chart from a different post,
and I've been thinking.

Looking at the BMEP with HP values:


If I'm currently at 2750 rpm at ~100 kph on the highway
(I have a totally different tranny mounted compared to
the regular TDI), so let's assume it's taking 50hp to drive
the car that speed.

If I were to install different gearing that would take me
down to 2250 rpm for the same speed, assuming the same 50hp
to drive the car along, my BMEP values change from 225 to
205, following the 50hp line. That's a difference of
~9.75%. So my fuel economy should increase ~9.75%?

Following further, dropping the rpm to 1750 would give me a
bmep of 197, for a difference of 13.6%, so my economy should
increase 13.6%.

But looking at the BMEP curve, is engine is right at
it's power limits there? Looking at where the bmep limit
for that rpm, there's not a whole lot of HP left for passing
or dealing with a headwind.

Am I reading this right?

This is, of course, assuming that it takes 50HP to move the
car along at that speed and disregarding the differing driveline losses due to different engine speeds.

If considering driveline/engine losses, assuming a constant 'x'hp loss per revolution, then 2750 rpm is 2750x, and 1750 rpm is 1750x,for a difference of ~36%, meaning
that the engine power requirements may drop an assumed
10hp, so the car would require 40hp to drive it, leaving a
bmep of 199, or a difference of 11.5% in the bmep, so 11.5%
increase in economy, and another 10+ hp left for passing?

Am I on the ball here, or out to lunch?


-Dave
 

GoFaster

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Location
Brampton, Ontario, Canada
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2006 Jetta TDI
At 100 km/h it probably takes more like 20 or 25 hp to move the vehicle; could be considerably more or less depending on wind direction and slope!

Otherwise your thinking is in the right direction. 25 hp at 2750 rpm is around 270 g/kWh; 25 hp at 2250 rpm is around 240 or 245 g/kWh. It's tempting to take it to extremes and ask for 25 hp at 1400 rpm and get below 210 g/kWh, but rest assured that gearing this tall wouldn't be very pleasant ... Fuel consumption cannot be the only consideration when selecting gear ratios, as you've noted ...

Realistically, you probably want to be able to climb a hill of several percent grade in top gear. At 100 km/h with a vehicle weighing 1500 kg loaded, it takes 33 horsepower to go up a 6% grade and that doesn't account for losses in the driveline. Add that to the 25 hp for friction and aerodynamics, and add 15% for transmission losses and you need about 67 hp to do that. That's available around 2250 rpm, and that's very close to the gearing and available performance from a stock A4-chassis 90hp TDI.
 

watercop

Veteran Member
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Aug 13, 2002
Location
Clay County, FL
I boosted my gearing and gained mpg along the lines of what is described above.

As a practical matter I have yet to encounter a hill I couldn't "hold", that is, maintain speed, as long as I'm at or above 1800 rpm.

I believe the interstate system grades are limited for the most part, to 6%. I have run the Blue Ridge Parkway and used 4th gear quite a bit, climbing at 40-50 mph with no problem. I really avoid applying a lot of "gas" pedal below 1600 rpm.
 

MrDave

Veteran Member
Joined
Sep 5, 2002
Location
1300 km NW of nowhere
Watercop:
3.16 final and 0.72 gearset from the 02a transmission, right?

I'm in the midst of transmission ponderings.
I'm currently running a 3.89 final with a .71 5th.
Combined with the 195-50-15 tires, makes for high revs
on the highway, and I feel my fuel economy is suffering
as well.

I'm waiting for a 3.42 final with a .71 5th which will
be fantastic.

Oh yeah, A4 TDI in an A1 Rabbit, mated to an 020 tranny.

My next step after the 3.42 will be an 02A with the
3.16, and I'm wondering if I should throw a 0.72 gearset
in as well (as vs it's current 0.76).

I'm wondering if the .72 will be worth the added expense,
and what the drivability will be like.

Quick table:
transmission
code final 5th rpm @100kph
020 3.89 .71 2550
020 3.67 .71 2400
020 3.42 .71 2250
02A 3.16 .76 2200
02A 3.16 .72 2100
02J 3.39 .76 2375
02M 2.61 .91 1950

This is all for 205/50/15 rubber.

-Dave
 

gdr703

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Jun 13, 2002
Location
Vancouver, Canada
TDI
Golf 2 door 2002 Indigo
Yes I believe your reasoning is right.
Looking at the model that has been hacked together to consider rolling resistance and aerodynamics, it indicates a HP requirement of 15.1 HP to maintain a steady 100km/hr on a good dry level road surface etc etc etc, and assuming fuel consumption is as that chart indicates etc. etc.

Of course change your gearing and you'll run out of flexibility to accelerate, up hill, in the rain, against the wind, with fat sticky tackies, in fifth, but then all you have to do is change down a gear. and in the meantime you've won some mpg's
cheers
hth
 

watercop

Veteran Member
Joined
Aug 13, 2002
Location
Clay County, FL
Mr Dave,

I would dearly love to sell you my 3.38 gearset - only about 6 months / 19,000 easy highway miles on it...

I think my trans is an 02J, and I think gearing parts from an 02a and an 02j are cross compatible
 

TDIMeister

Phd of TDIClub Enthusiast, Moderator at Large
Joined
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Location
Canada
TDI
TDI
The 02A transmission installed on A3 Jetta and B4 Passat TDIs (trans code CTN) actually had an 02J differential with 3.16 R&P ratio.
 

GoFaster

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Location
Brampton, Ontario, Canada
TDI
2006 Jetta TDI
For the end user, the horsepower versus BSFC comparison is the only one that's at all useful. For a given horsepower requirement, follow the curve until you find the lowest BSFC, and according to this, that's the rpm you should be operating the engine to get best fuel consumption. All it's going to tell you is that if you require less than about 20 horsepower, use the highest possible gear. If you need between 20 and 50 horses, the rpm at which BSFC is lowest rises with power demand to around 2000 rpm. Beyond that, you're limited by the torque curve ... pedal to the metal in the highest gear that meets the horsepower requirement is the most economical choice.

Remember, running an engine at more than light load and close to idle speed is murder on clutches, engine mounts, driveline, crank and rod bearings, and in this case, potentially the turbocharger.
 

gdr703

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Location
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TDI
Golf 2 door 2002 Indigo
quote
more than light load and close to idle speed
endquote

Increase the cetane of the fuel decreases the stress on the engine, by reducing ignition delay, makes for a smoother engine operation.

not sure about you "is murder to" comment. Torque is torque, right?
 

RogueTDI

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Location
San Diego
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1998 Jetta TDI Black
I have to agree 100% with GoFaster's last quote here. And it is a great thing. Just imagine your typical driving patterns, then use the chart to estimate the best operating regime for a given power requirement.

Generally, power requirement just changes with speed on flat roads, and also if you are driving on a grade.

What a great find. I will be making good use of this chart.
 

GoFaster

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Location
Brampton, Ontario, Canada
TDI
2006 Jetta TDI
quote
more than light load and close to idle speed
endquote

Increase the cetane of the fuel decreases the stress on the engine, by reducing ignition delay, makes for a smoother engine operation.

not sure about you "is murder to" comment. Torque is torque, right?
This is not correct. We are dealing with a 4 cylinder engine with very high compression. You can do anything you want with the cetane rating, and it won't change the fact that at 1200 rpm you are getting power delivery at 40 hammer-blows per second, all transmitted one hammer-blow at a time through the clutch and driveline and engine mounts. Increase revs to 2400 rpm but with lighter load, and you are getting much smaller hammer-blows at a faster rate. It's easier for everything in the driveline to deal with that situation.

At 2400 rpm compared to 1200 rpm, if cylinder pressure is the same (remember - diesel is unthrottled!!) it takes the same amount of energy to do a compression stroke. But the kinetic energy of the crankshaft and flywheel is 4 times greater (goes up with square of speed). So the amount of speed that the crankshaft fluctuates as a result of that compression stroke is (roughly) a quarter as much, and the amount that the engine shakes back and forth in its mountings is (roughly) a quarter as much. Same goes for the vibration damper inside the clutch, it's much more capable of dealing with a fast rate of little pulses than a slow rate of mighty hammerblows.

As for main bearings and rod bearings ... at higher revs the oil pump is pumping more, and for a given load and geometry situation the hydrodynamic lubrication in the main bearings and rod bearings has a mininum operating speed (which depends on the oil viscosity and the clearances and a ton of other factors).

It's easier on an engine and driveline to spin away with light load and higher revs than at heavy load and low revs, as long as the speed is within what's permissible for the engine. I doubt if the dynamic forces associated with the inertia of the piston reach the same magnitude as the combustion forces until close to redline ...

As noted previously, thermal efficiency isn't the only consideration required ...
 

RogueTDI

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1998 Jetta TDI Black
I was just looking at this map and wondering...


Shouldnt the "isoload" lines (lines of constant load/power) be more or less straight? I mean, if you increase rpm and maintain constant power, shouldnt torque, or BMEP, go down linearly?


EDIT: Whoops. Wasnt thinking. Actually, it should not be linear. That is the classic plot of an inverse relationship. The curves are fine.
 

gdr703

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Location
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TDI
Golf 2 door 2002 Indigo
GoFaster said
the fact that at 1200 rpm you are getting power delivery at 40 hammer-blows per second, all transmitted one hammer-blow at a time through the clutch and driveline and engine mounts. Increase revs to 2400 rpm

Good point. At the same load, but at twice the rpm then the individual piston strokes need be half as powerful. Makes for less stress per stroke. Thanks for pointing that out. That's true of the piston, crank, big ends, etc. the gearbox sees the same torque.

My point about the Cetane is that increased Cetane Number fuel reduces the ignition delay. There is less fuel when combustion begins, this encourages a more even burn, it reduces the pressure peak in the cylinder, thus reducing the momentary highest pressure - stress that the piston, crank, etc sees. makes for a smoother engine operation, and reduced stress per stroke as there is a more even presuure on the piston and less of a "hammer blow"
hth
 

RogueTDI

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First, I would love to know the exact source for this chart. It plainly appears to be for our engines, but it would be nice to be certain.


Now, some interesting conclusions, based on this chart, that contradict some assumptions I have heard, and believed.

The main assumption underlies the claim that driving full throttle is bad for fuel economy. The data in this chart plainly disagree with that. The assumption is that once the engine reaches some maximum level of BMEP (throttle, or load) presumably defined by some smoke limit, pushing the throttle further just dumps in excess fuel that only increases the fuel/air ratio, and thus smoke and fuel consumption. While a typical older turbodiesel might have had such a map, our engines appear not to have this characteristic, at virtually any rpm. In fact, it would appear that for the most part, full throttle at almost any rpm gives the optimum fuel consumption, particularly around 2000 rpm.

What do you think? This contradicts what I have heard, but maybe what we've been hearing is based on experience with older engines.
 

gdr703

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Yes, the chart clearly states its best for economy if you need to drive up an 8% hill, then do it in 5th with a heavy right foot at a relaxed rpm, rather than in 4th (same speed, at higher rpm) with a lighter right foot.

But then again, apparently we'll be stressing the engine more, (can the engine take it?) and potentially making more soot??

hey ho.
 

MrDave

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Location
1300 km NW of nowhere
Personally, I don't fully grasp the chart. Maybe it's just the figuring of it all, but if I'm driving along using 40 hp, I'm using less fuel than than if I were driving along using 50 hp to travel the same speed. However, the engine may not be running at its maximum efficiency, ie: not getting the maximum use of the energy available.

I can see the engine being more efficient under load, as in: less parasitic engine losses, but it should use more fuel under load.

If you're rolling along using no fuel, you have infinite fuel mileage. If you're sitting idling, you have zero mileage. The less fuel injected for a given rpm, the higher the mileage.

It's all very interesting. I'm hoping more people can stick their head in on this one, maybe we can all gain the proper understanding.

btw, I had a 1.6IDI diesel, and it returned 42 mpg if it was driven like a grandpa or if it was driven hard.

-Dave
 

gdr703

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Perhaps it would help that at a steady speed, good road, etc, 40mph = 5hp, 50mph = 10hp, and at 60 mph = 15hp, and at 70mph = 24hp. (or thereabouts)
You only need 40hp at about 85mph steady speed, or at other speeds when accelerating to overtake, or driving up hill, for example.
So look lower down the chart for "normal" operating environments.
 

John Kuhn

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Rubicon, WI
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'14 red JSW TDI base model
The key to understanding the chart is to recognize that for any given condition, a certain amount of power is required, and that there are an infinite number of combinations of rpm and torque (BMEP) to achieve that power. The colored curved lines represent power. So just say it takes 20 hp to go 60 mph. You can get 20 hp at 4000 rpm or 20 hp at 2000 rpm, by varying the load (BMEP,torque). At 4000 rpm, the black squiggly lines show a relative fuel consumption of 400 g/kw-hr. If you follow the horsepower line over to the left and see where it crosses the 2000 rpm line, and look to see what the fuel consumption is at that point, it would be between 230 and 250 g/kw-hr. That is a lot less fuel used at 2000 rpm. The graph shows that the lowest fuel consumption for 20 hp would occur at about 1300 rpm, if you could gear it like that. But as somebody suggested, such gearing would leave no reserve for hills, headwinds, etc. The graph does suggest that in almost all cases, the lowest fuel consumption for a given power is at the lowest rpm. Hope that helps.
 

RogueTDI

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Location
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1998 Jetta TDI Black
Yes, the chart clearly states its best for economy if you need to drive up an 8% hill, then do it in 5th with a heavy right foot at a relaxed rpm, rather than in 4th (same speed, at higher rpm) with a lighter right foot.

But then again, apparently we'll be stressing the engine more, (can the engine take it?) and potentially making more soot??

hey ho.
Ah. But we are not really making more soot. Just expelling more for a shorter time, thus more noticable. But the net amount to achieve a given change in speed is the same, if not less, as the chart suggests. If we were making inherently more soot, the SFC would go down, but the map says otherwise.


About the fuel economy, I wonder how prevalent the notion is that heavier throttle on our engines will reduce fuel economy. If this is the common belief, we should draw people's attention to this "new" conclusion.
 

RogueTDI

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The key to understanding the chart is to recognize that for any given condition, a certain amount of power is required, and that there are an infinite number of combinations of rpm and torque (BMEP) to achieve that power. The colored curved lines represent power. So just say it takes 20 hp to go 60 mph. You can get 20 hp at 4000 rpm or 20 hp at 2000 rpm, by varying the load (BMEP,torque). At 4000 rpm, the black squiggly lines show a relative fuel consumption of 400 g/kw-hr. If you follow the horsepower line over to the left and see where it crosses the 2000 rpm line, and look to see what the fuel consumption is at that point, it would be between 230 and 250 g/kw-hr. That is a lot less fuel used at 2000 rpm. The graph shows that the lowest fuel consumption for 20 hp would occur at about 1300 rpm, if you could gear it like that. But as somebody suggested, such gearing would leave no reserve for hills, headwinds, etc. The graph does suggest that in almost all cases, the lowest fuel consumption for a given power is at the lowest rpm. Hope that helps.
Just a minor technical corrections/comment:

Load is not varied, rpm is - as you acknowledge, the power requirement is given, and it IS the load.


The map also suggests that full throttle is most efficient at almost any rpm, which I find to be an interesting, if not surprising, realization.
 

RogueTDI

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MrDave:

Personally, I don't fully grasp the chart. Maybe it's just the figuring of it all, but if I'm driving along using 40 hp, I'm using less fuel than than if I were driving along using 50 hp to travel the same speed.
Yes, of course, because you are making more power. However, driving at a constant speed for a given condition requires only one power requirement, i.e. 40hp, not two, so a 50HP output would cause acceleration.

However, the engine may not be running at its maximum efficiency, ie: not getting the maximum use of the energy available.

I can see the engine being more efficient under load, as in: less parasitic engine losses, but it should use more fuel under load.
Yes, but the specific fuel consumption goes down, in grams/kWh. That is what the chart tells us.

If you're rolling along using no fuel, you have infinite fuel mileage. If you're sitting idling, you have zero mileage. The less fuel injected for a given rpm, the higher the mileage.

It's all very interesting. I'm hoping more people can stick their head in on this one, maybe we can all gain the proper understanding.

btw, I had a 1.6IDI diesel, and it returned 42 mpg if it was driven like a grandpa or if it was driven hard.

-Dave

The explaination John Kuhn gives is very good. Follow that.

Essentially, there is nothing to figure out with this chart. You simply determine what your power requirement is, then locate its locus on the chart (the lines of constant power). Find a location on that line that intersects with the lowest fuel consumption value, and that coordinate tells you, and defines, the ideal rpm for optimal fuel consumption.

Since we have gearing limitations, you just have to select the gear that most closely gives the ideal rpm.
 

Boundless

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Jan 3, 2001
The chart appears to say the for a given power level, the lowest possible RPM to produce that power level will result in the highest fuel economy.

Please don't dwell on the rare cases where this deviates, but overall, lower RPM results in better fuel consumption.
 

RogueTDI

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The chart appears to say the for a given power level, the lowest possible RPM to produce that power level will result in the highest fuel economy.

Please don't dwell on the rare cases where this deviates, but overall, lower RPM results in better fuel consumption.
Yes, for the most part, this is true, and not surprising.

But why do you defend that particular point so specifically refering to the "rare" cases? It just so happens that the one case where it actually IS an issue is where so many people here have believed incorrectly. It seems those rare cases to which you refer occur just above idle, where a lot of people have long been driving at moderate speeds in high gear, thinking they were saving fuel.

While for the most part it is true that lower rpm is better, the low rpm end of the chart clearly suggests that driving around below about 1300rpm in 5th gear, depending on instantaneous power requirements, may not generally be the most economical practice. I know from my years at this forum that this probably defies a long lived consensus.
 
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