Random thought on mileage

Theory question here:

If your 2003 Jetta TDI Wagon loses MPG with a rack and bikes, what will happen with the same set-up on a 2003 Jetta GASSER Wagon? Will it drop the same MPG, will it drop the same percentage, or will it drop differently?

Thoughts, theories, thesises?

I'm betting same percentage.

I'm guessing the wagon would have slightly increased MPG vs. the sedan.

Wagon vs. Wagon I would think is negligible.

I could be wrong.

velocity, negligible how? By percentage, or would it drop say 3 MPG for both? Or maybe it would drop by a percentage?

Maybe I should have stated it as positive proof that it's one way or another. That way someone would come in with their own proof that I'm wrong. At least that would be actaul discussion.

I would say the JSW would take more of a hit on mpg.Because it has rounded edges so more will air hit the bikes and rack.

MonsterTDI09 said:

I would say the JSW would take more of a hit on mpg.Because it has rounded edges so more will air hit the bikes and rack.

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I'm thinking of exact model cars with different engines.

My suggestion is to do a real life test. There are too many variables to get a good answer any other way.

In real life, I doubt if the difference is worth the worry.

It was just a thought to try to get folks to discuss something.

Oh well.

Unsubscribed.

I agree with Joe.

The only way that there'd be a real difference is if one vehicle started with much better aerodynamics than the other. Both vehicle's aerodynamics would be ruined, hence the vehicle which originally had the better aerodynamics would end up with a higher % change. However actual fuel consumption from both would be similar.

But as Joe says, there are far too many variables to make this a realistic comparison. If you want to carry bikes around, drive slowly as you are able, and that'll reduce the impact as much as possible.

The JSW of that vintage, interestingly enough, got 1 mpg more on each EPA cycle than the sedan.

My guess is that the reduction in fuel mileage would follow the same percentage, and I really would not expect it to be noticeably different when comparing like models.

whitedog said:

Theory question here:

If your 2003 Jetta TDI Wagon loses MPG with a rack and bikes, what will happen with the same set-up on a 2003 Jetta GASSER Wagon? Will it drop the same MPG, will it drop the same percentage, or will it drop differently?

Thoughts, theories, thesises?

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Aerodynamics, wind resistance would be the same- as such a like amount of energy would be required to overcome regardless if gas or diesel. So the difference would solely be how much fuel each engine require to overcome the resistance. I am sure it could be calculated if one had wind tunnel test results and accurate rates for power to wheel ratios fuel consumption for each engine. Diesel have much greater "pulling power" for unit of fuel consumed, my guess would be slightly greater impacts the gas engine.

Question: With the same increase in load, which will have a greater hit on FE and why?

Answer: One would need the bsfc maps for both engines but I would put a wager on the tdi taking the smaller hit in FE.

whitedog said:

Theory question here:

If your 2003 Jetta TDI Wagon loses MPG with a rack and bikes, what will happen with the same set-up on a 2003 Jetta GASSER Wagon? Will it drop the same MPG, will it drop the same percentage, or will it drop differently?

Thoughts, theories, thesises?

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Seems some of those responding got Wagon mixed up with Sedan!

Re-read above. Whitedog didn't ask anything about a Sedan. He is asking of a comparison between to very very similar vehicles (Jetta Station Wagons).........one being a TDI Diesel and the Other being a GASSER!

My guess is that the percentage loss would be very close to the same!

AndyBees said:

Seems some of those responding got Wagon mixed up with Sedan!
...

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Yeah, I didn't even notice the word 'gasser' (although obviously it's a fairly meaningless word over here, where 'gas' means lpg (liquid-petroleum-gas), and petrol means petroleum! Why the us uses 'gas' to refer to petrol I really don't understand ).

But still, aerodynamics would still be ruined for both vehicles, and both would have a dramatically increased aero drag as a result. So it becomes the question "Which has the higher change in fuel consumption, diesel going from 15hp to 50hp, or petrol going from 15hp to 50hp?"

... to which I reply "I haven't the foggiest, I haven't owned a petrol car in more than a decade!"

Gas is short gasoline and LPG we call it propane.

greengeeker said:

Question: With the same increase in load, which will have a greater hit on FE and why?

Answer: One would need the bsfc maps for both engines but I would put a wager on the tdi taking the smaller hit in FE.

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Excellent point regarding (BFSC).

Bill

I think it's an interesting question. There is a good chance that the "gasser" might take a smaller FE % hit than the diesel. Is the gasser a 1.8T or a 2.8 24V?

manual_tranny said:

I think it's an interesting question. There is a good chance that the "gasser" might take a smaller FE % hit than the diesel. Is the gasser a 1.8T or a 2.8 24V?

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I don't think the gasser would win out.Because the gasser would have use a higher rpm to do same work as diesel.The diesel rpm would change very little if at all.


What the deal with your picture?Looks like you took a killer hit of somthing?

MonsterTDI09 said:

I don't think the gasser would win out.Because the gasser would have use a higher rpm to do same work as diesel.The diesel rpm would change very little if at all.

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RPM is not the only factor involved with fuel efficiency.

Lets give this some thought. My ALH can return 50-60mpg at stock height with no accessories installed. If I lift the vehicle 2" and install my roof-rack, I'm down to 30-40mpg with the same speeds. That's a FE hit of up to 50%.

Lets say I get into my VR6, which averages 23-27mpg. In order for it to take as big of a % FE hit as my TDI, it would have to get as low as 13.8-18mpg when lifted with a roof rack on. I have a very hard time believing that I would get as low as 13.8mpg with my VR6.

MonsterTDI09 said:

What the deal with your picture?

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Never heard of Salvadore Dali?

MonsterTDI09 said:

Looks like you took a killer hit of somthing?

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No, it doesn't.

I'm going to agree with Monster, although it's purely speculation on my part.

The diesel putting out more torque at the same rpm would more easily overcome the higher aero load. The gas engine would have to run at higher RPM's to generate the same torque. Therefore it should burn more fuel.

Ok rebuttal time....

NickBeek said:

I'm going to agree with Monster, although it's purely speculation on my part.

The diesel putting out more torque at the same rpm would more easily overcome the higher aero load. The gas engine would have to run at higher RPM's to generate the same torque. Therefore it should burn more fuel.

Ok rebuttal time....

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Well, before rebutting, I will point out that I think the most important variable is WHICH gas engine are we talking about?

That will determine what sort torques the engine makes at various rpm, and what sort of FE the engine gets normally.

In the case of the VR6, it's got 6 cylinders, so it's already wasting some fuel in normal driving that might become useful at a higher load.

Also, I am playing devil's advocate for the sake of discussion.

Agreed, not enough info to be definitive.

My guess is that if you are talking about the turbo gasser, it would be a similar percentage. I am betting that the non-turbo would be a greater percentage. I am basing that on the increase in torque output from the turbo gasser. I think the non-turbo would take more go pedal to keep speed as comparing either to the TDI.

It will drop differently. I think.

IMHO, greengeeker was onto it. If you are increasing load on an engine, where all else is the same vehicle-wise, you are putting the engine into a (slightly) different operating regime. So, one question is, where does that fall on the BSFC chart?

Generally, increasing the load on the TDI puts it into a more efficient operating part of the BFSC curve. I don't know what happens on the curve for a gasoline engine, but I would guess it also gets more efficient, but not by as much, and so then the TDI would suffer a slightly smaller hit (percentage-wise) than the gasser.

Drag from a roof rack and bikes is not going to be that big, unless you are going highway speeds. It would be equivalent to towing, or driving with underinflated tires...anything that required the engine to supply more work.