Better economy in 4th than 5th?

Hi,

This seems strange to me but (using the computer readout) at about 60mph I get better fuel consumption in 4th than 5th gear.

The car is re-mapped.

Any ideas?

Robert
 

MikeMarsUK

A2OC Donor
Hi,

This seems strange to me but (using the computer readout) at about 60mph I get better fuel consumption in 4th than 5th gear.

The car is re-mapped.

Any ideas?

Robert
The instant mpg display isn't perfect, it tries to guess fuel usage based on various parameters (it isn't told the actual amount of fuel used). This is doubly the case if you have been remapped because some of the parameters which the map adjusts were used to calculate the fuel consumption. Similarly, if the mass-air-flow sensor is reading high or low it will also affect the calculation.

If you google for a 'brake specific fuel consumption' / AKA BSFC chart, this shows the fuel consumption at various RPMs and engine loads.

http://ecomodder.com/wiki/index.php...aps#Volkswagen_Jetta_TDI_1.9L_ALH_1999.5-2003

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

It's not very easy to read I'm afraid. You have to look at the contour lines at the RPM (x-axis) and HP (blue lines) to find the amount of fuel being burned to generate a certain horsepower. But at reasonable RPMs and engine-loads 5th-gear is almost always more efficient than 4th.
 
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J

jp.oog

Guest
It is a mine field for sure. The 4th 5th thing is odd. Sometimes I'll drop to 4th at 50mph to prevent the engine labouring. A labouring engine can cause economy issues
 

spike

Well-Known Member
Traditionally the sweet spot for fuel economy on most engines is around peak torque rpm.
As MM indicated, the fuel consumption curves are the bible but as a rough guide, for the same road speed,higher rpm increase internal engine friction and parasitic losses of engine driven accessories, which in turn increases fuel consumption. It is however possible that dropping a gear allows the engine to run in that sweet spot and still outweigh the negatives of the higher rpm.

One critical thing in doing your 'gear' experiments is maintaining exactly the same road speed in each gear. BHP is proportional to the square of wind resistance and the cube of speed, so dropping an odd mph while doing the comparison can affect the results by quite a margin.
For example dropping from 50 to 48 mph uses 12% less power

Cheers Spike
 

Birchall

Dick Chown Award 2016
I am not so sure that there is anything odd about it at all.

If you put the car in fifth gear before you have attained the right speed, the engine will be labouring slightly and not running at its prime.

If you ever look at power bands from dyno runs etc. you will see that the BHP and Torque have a curve and before the optimum rev band the power is lower than while on the optimum, Similarly when you go over the optimum range the power drops off again.

The MPG follows that band closely too, because the power is coming from the fuel being fully burnt, so if you change gear too early you have dropped away from the optimum efficiency and so it is not strange at all that 4th gear can be more economical than 5th is certain rev ranges and conditions.

I hope you can see the logic behind this, because I can see why it is easy to assume that the higher the gear the more MPG you get. When that is generally true but not in all conditions and rev ranges.

To use an extreme example, try taking off in fifth and see how many MPG you get!

Sorry, just got in and rushed this reply, I see that Spike has already said this but in a far more simple and readable way!!!

Steve B
 
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tagscuderia

Member
Whilst most of the theory is easy to comprehend, make it forced induction and it's a whole new ball game! A turbocharged small displacement diesel is somewhat schizophrenic for me. *In theory, for maximum efficiency you don't want to be running into the turbo so with your re-mapped thus higher-output engine, it's perhaps not surprising that at "only" 60mph 4th is >= 5th.
A diesel's strength is at low RPM and whilst forced induction is all the rage now, that's only to allow and offset the use of smaller engines with lower displacement.
BUT! I'm not an engine designer and there are so many thousands of variables that affect engine efficiency.
 

MikeMarsUK

A2OC Donor
... *In theory, for maximum efficiency you don't want to be running into the turbo so with your re-mapped thus higher-output engine, it's perhaps not surprising that at "only" 60mph 4th is >= 5th. ...
Just theoretically - Isn't that the other way around, you might be using the turbo at higher rpm (4th), and potentially not using it at the lower rpm (5th).

But in practice, at 60mph I'm pretty sure you're using the turbo regardless of whether you are in 4th or 5th or even a super-long 6th. I think it starts to spool much earlier, at 1500-ish, with a peak around 2200ish (please correct if you have an accurate number).
 

MikeMarsUK

A2OC Donor
... The MPG follows that band closely too, because the power is coming from the fuel being fully burnt, so if you change gear too early you have dropped away from the optimum efficiency and so it is not strange at all that 4th gear can be more economical than 5th is certain rev ranges and conditions. ...
Yes, I've tried to illustrate this point in the graph here in red (apologies for the wobbly line):

View attachment 19639 (edited)

This graph needs a lot! of explanation because it's trying to display a 3-dimensional data set.

The X axis is RPM, the Y axis is torque (sort-of). The engine load is the proportion between the torque line and the X-axis (i.e., 100% engine load = max torque for a given rpm, 50% engine load is half-way down).

The consumption figures are found on the ISO contour lines, varying here between 197 and 500. You have to imagine that the fuel consumption is on the Z axis poking out of the paper. Higher number = higher fuel consumption per horsepower, so lower is better.

197 is the most efficient point anywhere on the map (1.75k rpm at about 95% engine-load), this is the torque peak that Spike mentioned for this particular engine (the map is for the classic 1.9TDI with a variable vane turbo, not remapped. A simpler turbo would have a torque peak at a higher rpm, say 2200 typically).
The least efficient points are the highest numbers, for example 500 at 4.5k with low load (= revving the engine to the red line in neutral).

Say we are driving steadily at 60mph on the flat, using a steady 30hp to do so. So we follow the blue 30hp line to find the efficiency at particular RPMs.

4k = 320, 3k = around 270, 2.5k = around 245, 2k 220, 1.5k about 204, 1.25k around 208, and the engine can't supply 30hp below that rpm.

The wobbly red line illustrates Steve's point - engine efficiency gets worse as rpm drops beyond the line. So at 30hp, you are worse off at 1250rpm than you were at 1500 rpm. But you are better off at 1500 rpm than 2000 rpm. If you are going up a hill / into a headwind / etc you will need higher hp and therefore you'd be looking at a different blue line.

If you are on the wrong side of the red line this is often called 'lugging' and the engine struggles.

This only takes into account the engine efficiency. Engine load at very low rpm is not particulary good for the gearbox / flywheel even if the engine itself is happy.


Sorry this graph is so tricky, pretty sure that my explanation will just cause confusion lol.
 
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tagscuderia

Member
Just theoretically - Isn't that the other way around, you might be using the turbo at higher rpm (4th), and potentially not using it at the lower rpm (5th).
Note to self, don't post when tired, such gibberish! I did say that turbos made it a minefield :rolleyes:.
Really can't think of a reason why 4th would be >= 5th... Hopefully your graph will straighten things out for my noggin!
Given that a CVT benefits MPG, it does back up everything that you and Steve have said thus far.
 
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MikeMarsUK

A2OC Donor
... Hopefully your graph will straighten things out for my noggin!
Ha ha not sure about that, it's really a very complicated diagram ... Hopefully now showing in the post above.


Given that a CVT benefits MPG, it does back up everything that you and Steve have said thus far.
If you look at the eco modder link, one of the many diagrams shows a Prius map with the rpms that the cvt uses. Basically really low rpm and then going across the map at about 90% engine load.
 

tagscuderia

Member
Ha ha not sure about that, it's really a very complicated diagram ... Hopefully now showing in the post above.
I don't know, take a bit of time and it's perfectly legible. It did take me about 5 minutes to work out what all the lines were and what they showed but actually, it's one of the simpler BSFC maps; getting headaches looking at some of them!
Remembered my train of thought regards 4th vs 5th:
the engine is now producing extra power and torque, keeping the stock gearing presumably leads to road speed being attained at lower revs thus lesser turbo pressure. Mapping will have altered where the engine is producing power efficiently which at that particular speed may close the gap and/or introduce a crossover.
Not saying that that explains it but at nearing midnight, that was making a lot of sense!
As per getting into the efficiency of the turbo at different RPMs, I don't think that I've got it in me...
 
Hi,

This seems strange to me but (using the computer readout) at about 60mph I get better fuel consumption in 4th than 5th gear.

The car is re-mapped.

Any ideas?

Robert
Just to clarify. The tests have been conducted over a number of times. On a flat piece of motorway the speed has been set using the cruise control and the fuel consumption has been measured using the instant and average mpg computer readings. The fuel consumption is invariably lower by a significant amount using 4th gear.

This is not what I would have expected.

The engine was re-mapped by AMD not long after new.

Regards

Robert
 

MikeMarsUK

A2OC Donor
Probably either an artifact of the way it's estimating fuel usage, or most likely a side-effect of the remap on the way that the fuel usage is calculated (rather than the actual fuel usage).


-- Edit:
I'd forgotten the remap
 
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