Would intercooler improve mileage on a 99 GM turbo diesel? 2

[mrwizz]

I've done considerable tinkering with a 99 GM 6.5 turbo diesel automatic suburban trying to improve mileage. The starting mileage was 18.5 to 19 mpg at 70 miles per hour with the air cond. on. I keep the tires at 55 lbs. The best I can get with some changes is 22.5 mpg. Would an intercooler be worth the trouble? The intake manifold runs about the same temp. as the thermostat - 195 F. The boost at 70 mph is about 3.5 lbs. and the engine is turning 1900 rpms. Has someone tried this?

I installed a rig so that I could eliminate ideas faster by having an accurate gauge of intantanous fuel mileage. I installed a digital volt meter across one section of the accelerator pedal resistance pot (drive by wire) that feeds the engine management processor. The higher the number (0 to 5 volts), the less fuel is being used. Small changes like drafting another vehicle or turning off the air gives a noticable change. Prior to the "rig", I had been led to several wrong conclusions of a theory under test because of measurement errors caused by fuel density changes.

 

[MikeHalloran]

[ My '02 Explorer has a fuel mileage estimator that delivers an instantaneous value just after you hit 'reset'. My current record is 62 mpg. Going downhill, of course. Cruise in Southern states is ~17..19 mpg on gasoline. It can also display an 'estimated miles to E', which clearly does not respond to the reset button, and seems to be pretty accurate so far. ]

Oh. Back to your question. If I understand this stuff correctly, and I'm usually a little off the mark, you can get more torque with intercooling, which would allow you to run a taller axle, which would allow you to run the engine at a lower rpm at the same vehicle speed, which would give you better mileage. .... and would make it more difficult to tow and accelerate, so you really need to add a gear to the transmission, or a selectable overdrive. Which gets expensive pretty fast.

I suppose you could evaluate if a taller gear would help by borrowing a set of larger tires. I don't honestly know if the intercooler would help all by itself. Fogging some water into the air filter might tell you that.



Mike Halloran
Pembroke Pines, FL, USA

 

[SMOKEY44211]

There is something to be gained via the intercooler by virtue of lower boost pressure for the same amount of mass air flow. I don't think the small amount of fuel savings would offset the cost of the intercooler and associated plumbing to make it cost effective.--------Phil

 

[TheBlacksmith]

The intercooler also will increase the resistance to flow in the intake system, so while clearly you can get more horsepower with the intercooler and cooler, denser air at WOT, at cruise speeds it's probably a wash.

 

[Fabrico]

The need for an intercooler is linked to the need for a turbo. If the turbo is working hard the intercooler will be getting a lot done too. If you have the vehicle stricly for cruising and mileage experiments, they may both be a wash.

 

[mrwizz]

Interesting idea Mike to borrow some big wheels for a cheap gear change test. Thanks

Fabrico: I have another GM diesel with nearly the same gearing and weight but without turbo. The non turbo one clearly runs out of air (black smoke) during most speed changes. The mileage on the non turbo isn't much less but it can't get out of it's own way in many situations where a little more power is needed. So, the turbo without an intercooler was a definite improvement. Thanks

 

[Majik]

Since I'm in the Midsize truck diesel engine business we have customers call all the time about intercoolers on those trucks. Most only consider the intercooler once they're pushing the stock turbo to 10-12lbs. Never once have I ever had a customer report a mileage gain with an intercooler. That's the stats on the street I guess.

 

[mrwizz]

Thanks Majik, that's the experienced input I was looking for. I had guessed that there would be no gain because I don't make any black smoke. No smoke means plenty of air to fully burn the fuel. More air would not change mileage because there is nothing left to burn with it.

Thanks everyone for your input.

 

[globi5]

Most time is usually spent at cruising, when there's little or no boost. If there's no boost there's nothing to cool either and in this case an intercooler is just some extra weight taken for a stroll.

However, with an intercooler one could also consider to increase compression ratio (less boost but same mass flow). With an increased compression ratio the engine would be more efficient at cruising and therefore increase mileage. (Of course the mileage gain would never be worth the extra costs.)

 

[rmw]

I suspect that if Intercoolers didn't produce better fuel milage most 18 wheelers wouldn't be dragging their extra weight around.

In theory, you would get better fuel mileage. Practically speaking, would you ever recover your investment....well that's another calculation.

They cool off the air charge. The turbo working at any boost level adds heat to the air that it pumps. A cooler air charge means more air gets injected into the cylinder. Since air is the working fluid in this thermodynamic cycle, the more that there is for the burning fuel to heat up, the more efficient your engine is. The more efficient your engine is, the less fuel it takes to produce the amount of work necessary to go the speed you want to go. End of story.

rmw

 

[kenvlach]

From thermodynamic considerations, it is wrong to simply equate mechanical efficiency with mpg. I suggest the comparison with 18-wheelers is inappropriate. While cruising, the Suburban's engine makes more power than it needs, way overpowered compared to an 18-wheeler. More air mass = more fuel burned. The GM 6.5 turbo diesel has an intake air temperature sensor, an intake air mass flow sensor, electronic control of each fuel injection, and an exhaust oxygen sensor (correct me if I'm wrong). Less air intake = less fuel burned. Fewer HP wasted through the radiator & tailpipe. Same effect as a taller gear or larger tires, so more mpg as long as the engine doesn't overheat or the tranny doesn't downshift. Anything wrong with this argument?

 

[Fabrico]

A turbo and intercooler are not neutral/advantageous, they are disadvantageous/neutral/advantageous. mrwizz' pickup is more in the lower range, an 18 wheeler more in the upper.

 

[rmw]

The surburban's engine makes more power than it needs?

More air mass=more fuel burned?

Less air intake=less fuel burned?

et al.

Where do I start?

rmw

 

[kenvlach]

For sure the engine computer will lean out the fuel-air mixture when less power is needed. But, what's the point of just forcing more intake air in & getting more back pressure out (except when more power is needed)?

 

[globi5]

rmv. The question was not whether an intercooler increases efficiency per se. If the entire system (including engine) is designed accordingly it will definitely improve efficiency. However, the question was whether effiency is improved in this particular case (without adapting the engine) and I doubt it will because as already mentioned boost at cruising is pretty low and therefore the intercooler is not very effective. In addition, a lower temperature in the intake would also lower temperature at compression and slow down burn rate and therefore possibly even reduce efficiency. Of course and as I already mentioned, if one would increase compression ratio at the same time, temperature at compression could be kept the same and efficiency could therefore be improved at cruising.

 

[NakedTestMonkey]

I would look to removing mass off of the vehicle and reducing drag if you are at highway speeds constantly (ie. driving slightly slower can net big gains in drag reduction). You can also develop a curved front air dam to help re-direct airflow away from high drag areas such as tires. I've seen CFD that has shown that even your Home Depot Racing (think garden trim) self-built air dams can produce small amounts of drag reduction in passenger cars. For a taller truck I'd imagine a larger dam would help a fair amount given the underbody drag.

Adding bigger wheels and tires will make it necessary to produce more torque per moment time in acceleration. You will have a reduction in city milage for a gain in highway milage. I personally don't feel this is a beneficial solution unless the vehicle is rarely used for city driving or towing.

 

[Majik]

while I think many are right in adding the intercoolers to semi's etc etc. Your comparing apples to oranges. Semi's run much lower static compression and a whole lot more boost. in the range of 17:1-18:1 @ 30+psi. The 6.5L Turbo diesel was designed to be a high compression low boost setup running 21.5:1 engine and a whooping 5-6lbs of boost (MAX) from the factory. Peak boost is really only seen between 2300 and 3300rpm. The fact is the air just isn't really that hot enough to intercool. Air intake temps are in the 150 or so degree range on a bad day. There's oh so little to be gained by removing a 9" straight pipe from the turbo to the intake and replacing it with about 8-10' of twisting pipes and couplers and all this other stuff throw in an intercooler and then route it back up. The only real mass produced intercooler for this vehicle mounts in replacement of the skid plate at more than a 45 degree angle. So it's at a crappy angle for efficency and the intercooler is marginally sized. Home made kits with a hacked up radiator core support and a FMIC off of a cummins or powerstroke seem to have better gains but require definite sawzall action to your truck. I like the analytical aproace to the problem but in reality it there may be a slight advantage but the customer most likely will not see a 1mpg gain. Once the boost on the stock turbo is turned up into the 15psi range then we will see gains in adding intercoolers not only in HP but in MPG. Usually this is outweighed by the customer getting a heavier right foot with more HP on tap.

 

[rmw]

Majik,

So, bottom line, your post above is that it will increase his fuel mileage, notwithstanding all the other factors to be considered, right?

rmw