Why mpg testing is difficult and variable...

[ZugyNA]

There are many variations on the approaches to engine and fuel management design by engineers from many different cultures...the only common ground in the US being the fuel used and the EPA emissions regs.

Also each vehicle might have a different total mileage and state of tune....and possibly other modifications from stock.

So a modification that might increase mpg on one vehicle...won't necessarily have the same effect on another.

So this might be why using an A-B-A testing method with statistical analysis might be a kind of overkill. Though with a good mpg readout method and a good test route...that might not be so relative to one particular car.

Main point is: even if you do careful testing of a mod on one car and find it doesn't increase mpg...that doesn't mean this mod might not work on another car.

1) you might not have set it up right...the devil is in the details?

2) this kind of mod just might not work on this particular car?

So...to try and extrapolate from testing an mpg mod on one car to all cars...is NOT statistically acceptable?

[Mighty Mira]

Originally Posted by ZugyNA

So...to try and extrapolate from testing an mpg mod on one car to all cars...is NOT statistically acceptable?

Yes, it is more complex than simply saying x mod increases mpg by y percentage. It's because fuel economy is not a simple function.

Fuel is wasted in several different areas:

Load at highway speed:
1) In heating up the air through turbulence (high CdA)
2) In heating up the tyres and the road (high Crr tyres)

Engine/transmission efficiency:
3) In an engine that is in too high an rpm range for the load
4) In an engine that is too large for the load.
5) In too many, inefficient parasitic loads on the engine (alternator, pumps, etc)

In braking:
6) Through an overly heavy car
7) Because we heat up brake pads/discs/drums instead of returning kinetic energy to a battery.

2, 6 and 7 mainly apply in city driving. The rest apply to highway driving.

FE in a car can be visualized as being constrained by three bottlenecks. Attack one bottleneck, and eventually you approach diminishing returns. For example, start off with a car built like a brick and geared for city driving. Do some aero mods, and the highway performance will improve. Do more mods and it may not improve the FE much, because the gearing will be much too high for the steady state load.

Note that in each of those different areas, it is possible to measure the variables and so compute a function that will give a good approximation as to expected FE. However, it's more complex than saying "Do this and get X increase".

Now, since most driving is done in the city, it makes sense for car companies to attack the braking issue, and the only way to do that is with electric technology - you can't turn kinetic energy back into gasoline again. Hence the hybrid car - one with both a gas and electric motor.

However, if you can solve the other problems (i.e. decreasing the load on the engine), you can do away with the gasoline engine entirely and go solely to electrical, as range is a major factor in EVs not catching on.

I would say that as gs.org gets more of an understanding, people will learn how to measure and understand the effect of the different variables in their car, in the same way as people currently understand the interplay between different variables in producing a fast quarter mile time etc.

[cheapybob]

Originally Posted by Mighty Mira

Yes, it is more complex than simply saying x mod increases mpg by y percentage. It's because fuel economy is not a simple function.

Fuel is wasted in several different areas:

Load at highway speed:
1) In heating up the air through turbulence (high CdA)
2) In heating up the tyres and the road (high Crr tyres)

Engine/transmission efficiency:
3) In an engine that is in too high an rpm range for the load
4) In an engine that is too large for the load.
5) In too many, inefficient parasitic loads on the engine (alternator, pumps, etc)

In braking:
6) Through an overly heavy car
7) Because we heat up brake pads/discs/drums instead of returning kinetic energy to a battery.

2, 6 and 7 mainly apply in city driving. The rest apply to highway driving.

FE in a car can be visualized as being constrained by three bottlenecks. Attack one bottleneck, and eventually you approach diminishing returns. For example, start off with a car built like a brick and geared for city driving. Do some aero mods, and the highway performance will improve. Do more mods and it may not improve the FE much, because the gearing will be much too high for the steady state load.

Note that in each of those different areas, it is possible to measure the variables and so compute a function that will give a good approximation as to expected FE. However, it's more complex than saying "Do this and get X increase".

Now, since most driving is done in the city, it makes sense for car companies to attack the braking issue, and the only way to do that is with electric technology - you can't turn kinetic energy back into gasoline again. Hence the hybrid car - one with both a gas and electric motor.

However, if you can solve the other problems (i.e. decreasing the load on the engine), you can do away with the gasoline engine entirely and go solely to electrical, as range is a major factor in EVs not catching on.

I would say that as gs.org gets more of an understanding, people will learn how to measure and understand the effect of the different variables in their car, in the same way as people currently understand the interplay between different variables in producing a fast quarter mile time etc.

All those things are variables, and the more variables you have the less likely that consistant results will result. Skip the attempt at "city" testing, IMO. Its hopeless to get non-variable conditions. Even on a highway test its difficult.

[Matt Timion]

Originally Posted by cheapybob

All those things are variables, and the more variables you have the less likely that consistant results will result. Skip the attempt at "city" testing, IMO. Its hopeless to get non-variable conditions. Even on a highway test its difficult.

Agreed... besides, when you only test highway you get better bragging rights :P

Seriously though, an extra stop in the city could cause you your new MPG record. When on the freeway/interstate you can easily use the same route, same lane, etc. The variables you'll have (temperature, weather, etc.) can be quantified and can be taken out of the equation.

[cheapybob]

Originally Posted by Matt Timion

Agreed... besides, when you only test highway you get better bragging rights :P

Seriously though, an extra stop in the city could cause you your new MPG record. When on the freeway/interstate you can easily use the same route, same lane, etc. The variables you'll have (temperature, weather, etc.) can be quantified and can be taken out of the equation.

If you really were willing to put in the effort, you could come up with a test route to run in an empty parking lot with timed idling stops and consistent throttle amount and shift/braking points. You'd also need an accurate scangage or equiv for this to work.

And its true that highway is more fun because the numbers are higher.

[krousdb]

Originally Posted by cheapybob

And its true that highway is more fun because the numbers are higher.

City numbers are better for me....

[Mighty Mira]

Originally Posted by cheapybob

All those things are variables, and the more variables you have the less likely that consistant results will result.

While that is true, it is possible to isolate those variables. If you know the Cr of the tyres, it is possible on a calm day to do a high speed coast down test and accurately assess the change in Cd. This will be very accurate if you have an electronic device to assist your testing.

You should be able to isolate the Cr in much the same way, with a coast down test at low speed.

Again, with the appropriate electonic apparatus, it should be well possible to measure the efficiency of the engine at different rpm and throttle points, and an intelligent decision can be made on how to gear it.

All this requires knowing exactly how your intented modification will improve fuel economy, and directly measuring it, because it IS possible.

Which is why I'm inherently skeptical of the various engine additives. If it was cost effective, oil companies would have already done it by now. But the reason those things sell is because they are cheap to produce and the profit margins on those things are high. Americans in general want a "quick fix". And that's why you will never see a map of engine efficiency vs rpm and throttle position included with any of the fuel economy nostrums marketed to the gullible - they can't produce one. And so they are careful to make their claims ambiguous enough and offer a guarantee if it doesn't work, so that they can't be sued for false advertising.

Originally Posted by cheapybob

Skip the attempt at "city" testing, IMO. Its hopeless to get non-variable conditions. Even on a highway test its difficult.

Agreed.

[ZugyNA]

Originally Posted by Mighty Mira

If it was cost effective, oil companies would have already done it by now.

What you need to know?

The EPA is controlled by the White House and the Legislature. Both mostly represent big oil, car makers, and other business interests. People get elected by using $$ from these interests.

So the refiners still refine poorer quality fuels...automakers are not made to improve mileage...the whole corn into ethanol thing is a subsidy for the farmers and Cargill?

Also...think about the taxes gasoline/diesel bring in. If they seriously do something like passing stricter CARB standards...there goes the tax base?

They wanted to pass a tax on hybrids cause they wouldn't be paying their fair share for road repairs....seeing the light?

[diamondlarry]

Originally Posted by Mighty Mira
If it was cost effective, oil companies would have already done it by now.

Originally Posted by ZugyNA

What you need to know?

The EPA is controlled by the White House and the Legislature. Both mostly represent big oil, car makers, and other business interests. People get elected by using $$ from these interests.

So the refiners still refine poorer quality fuels...automakers are not made to improve mileage...the whole corn into ethanol thing is a subsidy for the farmers and Cargill?

Also...think about the taxes gasoline/diesel bring in. If they seriously do something like passing stricter CARB standards...there goes the tax base?

They wanted to pass a tax on hybrids cause they wouldn't be paying their fair share for road repairs....seeing the light?

It isn't cost effective for them to do it. They will never do those things because it would benfit/be cost effective for us not them.

[95metro]

Originally Posted by ZugyNA

So a modification that might increase mpg on one vehicle...won't necessarily have the same effect on another.

So this might be why using an A-B-A testing method with statistical analysis might be a kind of overkill.

I agree that FE results from modifications will vary from vehicle to vehicle. Some vehicles may see a lot, some just a little.

But all results need to be repeatable and that's where A-B-A testing comes in. To even say a device works on one specific vehicle needs to be able to be proven over and over - at any time on any day in any season.

With something as highly variable as FE I don't find statistical analysis to be overkill at all. It gives you the ability to make educated guesstimates much closer to the truth than evidence with unestablished baselines will.

Since FE is already variable and testing adds more variables you have to have accurate data to build on to ascertain baselines, margins of error, and to be able to separate real results from flukes and assumptions.