Greater fuel milage possible? - PeachParts Mercedes-Benz Forum

barry123400 · #1 04-09-2009, 01:17 AM

I have owned quite a few 1.6 litre volkswagon diesels. I hope anyone else that has owned one will speak up.

Usually on the larger Canadian gallon the normal milage was about 50 mpg. On the rare really hot days the milage would increase to about 70 Mpg.

This happened on my longer trips on really hot days. It did occur to me the probable cause was the much warmer air being ingested somehow was probably increasing the combustion efficiency.

At the time I could not think of any way to make this a somewhat more constant situation. Other than by stealing heat from the exhaust system. Of course the amount available from a diesel exhaust system is not very hot to start with. Plus what is there could be robbed away or depleted very fast. The engine does intake a large volume of air after all.

So the thought just dropped into limbo until I was starting to think about any possible ways to improve the 616 and 617 engines fuel milage the other day.

Could not a constant volume of pretty warm air be removed from the radiator? If an electric fan was placed ahead blowing air through the rad? Then a simple ductwork feeding into the air intake tube.?

This from a small collector placed directly in contact with the rear face of the raditor. Area of the collector could probably be say 4X6 inches and should not impair the normal cooling. Might even with a little effort incorporate it in the upper corner still using the mechanical fan. Although the electric fan and thermostat control might keep an average higher radiator temperature.

Since constant feed air over ninety degrees seemed to make so much difference on the 1.6 litre volkswagon engines. I wondered if the mercedes indirect injection engines might respond in a simular fashion?

To get disscussion going it would be nice if anyone owning or had owned an earlier volkswagon diesel had noticed this effect and disgarded it or paid little attention. Or has anyone ever noticed their mercedes diesel engine got higher fuel milage on a really hot day?

I am hoping someone out there has observed this with their mercedes diesels as an effect as well. If this turns out to be the case a pretty warm constant feed air from the radiator might be worth looking into in my opinion. We might even ram charge a little depending on the area of the duct intake.

The naturally aspired mercedes diesel gets less milage than the turbo. Might it just indicate just further combustion ineficiency compared to the turbo?

The above premise of course is based on the only signifigant differance that effected the fuel milage on a hot day was the higher air temperature. The lesser density of the air I would have thought to be negative.

tbomachines · #2 04-09-2009, 01:21 AM

My mother owns a 2000 TDI (the 1.9), we consistently get between 50-55 mpg, but in the winter it drops to about 45....not much, but I've noticed it happens. I always attributed it to the grumpiness that often accompanies the cold

compress ignite · #3 04-09-2009, 01:55 AM

tbomachines,

When they blend winter fuel to prevent fuel "Gelling" it reduces the available
stored energy in the fuel. So the milage suffers.

barry123400,

An interesting Observation/Theory. Unfortunately,I cannot supply verification.
The VW Golf 1.6 D I used to own was never subjected to milage computation.
(The biggest problem was remembering to EVER refuel.
It seemed like it ran for MONTHS on a single tank.)

Are you suggesting the Turboed MB Diesels enjoy better MPG due to the
Heated Intake Air created by the Compression of the Turbocharger?

Everybody,it seems, is fixated on "Cooling" the compressed Intake Air
to "Get More Air Molecules" into the Cylinder on Intake.
(Or is it to reduce internal engine Temperatures?)

Which if we think about it is strange,for Diesels run with Too Much Air
in their Air/Fuel Ratio almost all the time.(It's the nature of the beast)

Number_Cruncher · #4 04-09-2009, 04:40 AM

>>t did occur to me the probable cause was the much warmer air being ingested somehow was probably increasing the combustion efficiency.

A while ago, shortly after I joined this forum, I said something similar on a cool intake thread, and was shouted down.

However, this idea was used by Jaguar in their intake design for the V12 XJS. The idea being that at lower speeds, an intake behind the radiator would pick up the warm air, and there would be an efficiency benefit, but, at higher speeds, the air would be cooler, denser, and more useful for high power. I'll see if I can dig out some more info on the Jaguar idea.

The link about Jag intake systems:

http://www.jagweb.com/aj6eng/technics.php

#5 04-09-2009, 05:33 AM

Quote:

Originally Posted by Number_Cruncher

However, this idea was used by Jaguar in their intake design for the V12 XJS. The idea being that at lower speeds, an intake behind the radiator would pick up the warm air, and there would be an efficiency benefit, but, at higher speeds, the air would be cooler, denser, and more useful for high power. I'll see if I can dig out some more info on the Jaguar idea.

What works for a g@sser does not for a diesel. Thinner air on them means the computer can reduce the fuel injected. They also have the benefit of having to use more throttle with hot air which reduces pumping losses across the throttle plate.

Diesels don't care about air temperature, density or volume except at maximum power output.

t walgamuth · #6 04-09-2009, 06:25 AM

I think that warmer air is easier to push the car through being lighter....thus giving a benefit in mileage.

Just running in a dense pack of cars also seems to help mileage....the air is pushed along by the group of cars and makes less resistance.

Number_Cruncher · #7 04-09-2009, 03:05 PM

>>Diesels don't care about air temperature, density or volume except at maximum power output.

I don't think that's quite the complete picture. IC engines, whether diesel or petrol rely on changing the thermodynamic properties of the air they injest, so, there cannot be complete independence of the initial conditions of the air.

If you injest warm air, the conditions in the cylinder just as the diesel begins to be injected must be different from the case when the air injested is cold.

Although I agree that the pumping losses of a diesel are much lower than of a petrol (particularly at part load), they aren't zero, and reducing them will give rise to an efficiency gain, hence an economy gain.

pawoSD · #8 04-09-2009, 03:40 PM

It is possible to build a 70mpg car on the cheap, TopGear was able to:

http://www.topgear.com/us/features/more/we-build-a-70-mpg-car/

Check it out...

barry123400 · #9 04-09-2009, 05:34 PM

Correction for the american gallon. An older 1.6 volkswagon diesel should average about 42 miles per gallon normally on the highway. With a very hot day that could rise to very high 50s or low 60s per american gallon.

The reason I suspected there might be a crossover possibility was the turbos give better power as well as fuel milage. Both in the volkswagon as well as the mercedes.

I also believe the transmission and rear end ratios where the same in the volkswagon up until 1991. Not so with the non turbo mercedes compared to the turbo version. So it was not a gearing effect that punished the milage in the normal non turbo volkswagons.

Hopefully again another volkswagon owner may post that he observed this as well. At the current time I have not owned one for quite awhile now.

Just in case it comes up. The actual block operating temperature is no higher on a really hot day then an average day to my knowledge. The volkswagon cooling system is about the best I have ever owned.

You could probably take that car and run it in a serious desert enviroment with no issues cooling wise. The electric fan never seems to come on unless you are at idle for a long time. For practical purposes one might get by without a radiator fan most of the time.

The radiator was very large for the displacement of the engine. Also the heater design was superb. It delivered very hot air quite quickly. Put many gas cars to shame in that area. Some things were very well engineered on those cars.

Quite a few of us that have owned many examples think volkswagon has slowly gone downhill over the years in many respects.

rs899 · #10 04-10-2009, 06:50 AM

Barry-

I have been running my VW 1.6D pickup commuting for about 15 years now, and I really can't find very much variation in winter vs. summer MPG. I don't think I see more than about 2 MPG, but here in Florida daytime "winter" and summer only vary by about 25 degrees. I agree with most of your comments about the VW- they do have a robust cooling system (when maintained) and an efficient heater. And rebuilding the front suspension is a snap compared to a Mercedes.

Rick

cell · #11 04-10-2009, 09:43 AM

also keep in mind that air resistance goes down as the air heats up, so it takes less energy to blast down the highway at 3pm than it does at 3am.

also, your engine may have been running slightly hotter. there are SAE papers which show a linear (but small) increase in efficiency as engine operating temperature goes up.

this makes sense if you consider the rule about heat transfer -- that the rate of heat transfer between two bodies is related to their difference in temperature. that is, when touching something really hot to something really cold, they will exchange heat energy at a much higher rate than two bodies which only differ by 1 degree.

relative to the temperature of combustion, the water jacket in your engine is cold, so heat is constantly bleeding through the cylinder walls into the water jacket. that's heat which could have gone towards pushing down the piston, so any heat which leaks through the cylinder walls is lost efficiency.

thus, if your water jacket goes up in temperature, but the heat of combustion stays the same, then you will have a slightly slower rate of heat transfer through the cylinder walls, and slightly higher efficiency.

I'd be curious to try using something like Evan's NPG+ coolant and run the engine at higher temperatures (say 300F) and see if the difference is measurable.

argus445 · #12 04-10-2009, 09:56 AM

Quote:

Originally Posted by barry123400

Correction for the american gallon. An older 1.6 volkswagon diesel should average about 42 miles per gallon normally on the highway. With a very hot day that could rise to very high 50s or low 60s per american gallon.
.

Your saying on a hot day when air density falls u get about 25-35% fuel economy increase???

I've never seen much difference except in wintertime when the btu's in fuel drops about 4-5mpg other wise i get 38mpg period in the 1.6 vw's n/a and 1.9 tdi both.

blacknight · #13 04-10-2009, 10:00 AM

No one has mentioned thermal expansion of fuel. If you fill up on a cool Canadian morning then drive during the hot day your mileage will go up substantially.

Bajaman · #14 04-10-2009, 10:37 AM

The idea that warm air will give better efficiency flies in the face of common well established theories of thermodynamics. Carnot efficiency is based after all on the temperature difference between the source and sink. Warm air will decrease this difference thus decreasing efficiency. This is for the heat engine itself. Other factors such as wind resistance and throttling behavior may however have a greater effect. The engineers put a cold air intake on the vehicle for a reason.

barry123400 · #15 04-10-2009, 05:34 PM

Quote:

Originally Posted by Bajaman

The idea that warm air will give better efficiency flies in the face of common well established theories of thermodynamics. Carnot efficiency is based after all on the temperature difference between the source and sink. Warm air will decrease this difference thus decreasing efficiency. This is for the heat engine itself. Other factors such as wind resistance and throttling behavior may however have a greater effect. The engineers put a cold air intake on the vehicle for a reason.

I could not agree more that it seems unreasonable. That is why I posted this old observation of mine. Needless to say I posted the extreme observation. It occured on a long trip to a distant salvage yard on a really hot day. Of course on that occassion the vendors pumps might have been defective is possible as well.

It really got my attention anyways. Living in Florida might not give enough of a tempreature swing though to show much effect.

Another thing I noticed was a lot of americans seemed to claim better milage than was typical here with these as well over time. Where the claims flawed or a result of operating in a hotter enviroment? Many claimed to aproach or duplicate the same fuel economy as here on a gallon that was about 8/10s of a litre smaller.

The 2000 jetta tdi for example never varied much from new till the time my wife totalled it. I really do not know what caused this effect. I was just aware if I took the 1.6 diesels on a long trip in really hot weather the fuel milage was better. Even took it for granted. Never the tdi or if so too small to get your attention.

I did not check it every time unfortunatlly just felt it when I refuelled. Fuel expansion was not an issue in my mind.

Milage does decline on these in really cool weather as well. Not only winter fuel I think as there is a greater decline percentage spread than other diesels. A lot of 1.6 owners have noticed that.

I guess the only way is for me to incorporate a hotter air intake system and log milage with no bias. See what results. Hopefully I will have a little more spare time to do this.

Milage being worse I am even prepared for. I still want to attempt it. The problem will come if it is greater as it does fly in the face of knowledge. Less dense hotter air is counterproductive to good milage. Decreases performance.

This is what we understood with gas engines anyways. Less oxygen ingested and they burn all of it that is available.

This leads us back to the non turbo and turbo 617s. I think there is no doubt by most members the non turbo suffers quite a miles per gallon defeciency in comparison to the turbo engine with more power. I can understand the more power with the turbo easily enough but the efficiency is somehow improved for the milage component as well. It should be less.

The 1.6 volkswagon diesel has the same issue just not as pronounced with their turbo and non turbo set up.