How do you eco-drive a diesel ?

[tasdrouille]

Thanks connClark! I will indeed get a copy of this book. The sfc figures in the pic look like they're at full load though.

Ollie, I'd like to add that when I blocked the cross airflow to the IC, the IAT did not change by a significant amount. I could see a 10 degrees difference after a ~30 seconds hill climb at full boost, but nothing in regular city or highway driving.

[Duffman]

Originally Posted by Big Dave

I put a free-flowing exhaust (you could drop a Cokr can through it) and a free-flowing warm air intake and it didn't help a bit.

One thing you can do. Put a pyrometer on it and drive to keep EGT under 600 degrees F.

From what I have seen from your other posts, IMO you have moved your cruising RPM so low as to negate any benefits.

[instarx]

Originally Posted by Duffman

I think more so than in a gas engine there is an optimum intake air temperature in a diesel. Cold air is more dense and therefore has more O2 which is definately good for FE but a diesel also requires heat to run so a charge that is to cold may not run well either.

Diesel's do not need heat to run - they need compression. The compression provides the heat. In every circumstance in diesel engines, the colder the intake air the better the efficiency.

Well, re-considering - if it is very cold the diesel needs some additional heat from the glow plugs for a few seconds, but that's a special case. It never needs warm air.

[instarx]

Originally Posted by tasdrouille

The intercooler definitely help pack more air in, but I don't see how it improves fuel economy by itself. In fact it's a significant restriction in the intake. In my TDI there's a 3 psi pressure drop from the IC. Could you please provide a reference?

Ahhh, but that pressure drop you are measuring across the intercooler isn't from a restriction in the IC, it results from the cooling of the air and it is desirable. Pressure drops caused by flow restrictions are bad, but pressure drops caused by cooling are good.

When the very hot compressed air from the turbo is cooled by the IC its pressure (and volume) drops proportionally and so it contains more air molecules per volume than did the hot air (Boyle's Law). This allows even more O2 to be stuffed into your cylinders than if you didn't have the IC.

All things being equal, the IC allows you to use less boost to get the same number of air molecules into the cylinders - and that improves fuel economy (or power, whichever way you want to look at it).

[ConnClark]

Originally Posted by instarx

Ahhh, but that pressure drop you are measuring across the intercooler isn't from a restriction in the IC, it results from the cooling of the air and it is desirable. Pressure drops caused by flow restrictions are bad, but pressure drops caused by cooling are good.

When the very hot compressed air from the turbo is cooled by the IC its pressure (and volume) drops proportionally and so it contains more air molecules per volume than did the hot air (Boyle's Law). This allows even more O2 to be stuffed into your cylinders than if you didn't have the IC.

All things being equal, the IC allows you to use less boost to get the same number of air molecules into the cylinders - and that improves fuel economy (or power, whichever way you want to look at it).

Actually it isn't from cooling. You can have hot air on one side of a room and cold air on the other side and they will be at the same pressure. If it indeed there is a 3 psi drop across the intercooler he should really consider looking at an after market intercooler to reduce this loss as it a serious loss in useful energy. I also don't know where he measured the the pressure drop from. If it was right before and right after the intercooler then the drop is all too the intercooler. If it was between the outlet of the turbo and the manifold the plumbing route could be a big contributing factor. Yet another factor is the velocity of the air at the two points of measurement as this will have different static and dynamic pressure if they are different.

[instarx]

Originally Posted by ConnClark

Actually it isn't from cooling. You can have hot air on one side of a room and cold air on the other side and they will be at the same pressure. If it indeed there is a 3 psi drop across the intercooler he should really consider looking at an after market intercooler to reduce this loss as it a serious loss in useful energy. I also don't know where he measured the the pressure drop from. If it was right before and right after the intercooler then the drop is all too the intercooler. If it was between the outlet of the turbo and the manifold the plumbing route could be a big contributing factor. Yet another factor is the velocity of the air at the two points of measurement as this will have different static and dynamic pressure if they are different.

Sorry ConnClark, but you are mistaken. In a closed system as we are discussing, temperature does effect pressure, and it is easily measured because it is constant. A room is not a closed system so it is harder to measure, but in fact if you suddenly cooled one end of a hot room there would be airflow to the cold end because of the pressure differential. True, the pressure would equalize as the air mixed, but so would the temperature. If you took 10 pressure readings in the room you are in right now they would all be slightly different - and all directly related to the slightly different temperatures at each sample point.

As for magnitude, it seems reasonable to me. Three psi is only 1/10 of an atmosphere, and reducing the temperature of air from 250F to 150F can easily induce a change of 0.10 bar (particularly when it is pressurized to begin with). Trust me on this - I spent a lifetime correcting air sample measurements to STP (standard temperature and pressure) for human exposure monitoring studies. Note that I made my biggest assumption in this paragraph - that the air was cooled about 100F by the IC. If this is way off let me know and I will recalculate the pressure drop that would be expected across the IC.

Static pressure and dynamic pressure (also known as velocity pressure, VP) are independent of each other. VP is a measurement used to measure the inertia of moving air (usualy in a duct), and has no purpose when determining the pressure drop across a restriction such as, say, a filter or IC. In this situation VP values do not complicate the situation because they are simply not applicable.

I read the post that he was getting a pressure drop measured across the IC, hence he blamed restrictive airflow in the IC. If the pressure measurements were taken other places it might change things, but those were not the parameters of the problem, and the pressure drop across the IC is almost surely due to the temperature drop.

[ConnClark]

Originally Posted by instarx

Sorry ConnClark, but you are mistaken. In a closed system as we are discussing,

But we are talking about an open loop system. An intercooler reduces the work required by the compressor to compress a given mass of air. It does not reduce the energy recovered by the turbine. This results in an increased mass of air compressed to balance the energy of what is recovered by the turbine. By definition this is an Open loop cycle.

temperature does effect pressure, and it is easily measured because it is constant. A room is not a closed system so it is harder to measure,

but in fact if you suddenly cooled one end of a hot room there would be airflow to the cold end because of the pressure differential. True, the pressure would equalize as the air mixed, but so would the temperature. If you took 10 pressure readings in the room you are in right now they would all be slightly different - and all directly related to the slightly different temperatures at each sample point.

However they would all have the same pressure. Pressure also equalizes far faster than temperature.

As for magnitude, it seems reasonable to me. Three psi is only 1/10 of an atmosphere,

Since mean pressure at sea level is 14.7, 3 psi is about 20.4% or about 1/5th of an atmosphere.

and reducing the temperature of air from 250F to 150F can easily induce a change of 0.10 bar (particularly when it is pressurized to begin with). Trust me on this - I spent a lifetime correcting air sample measurements to STP (standard temperature and pressure) for human exposure monitoring studies. Note that I made my biggest assumption in this paragraph - that the air was cooled about 100F by the IC. If this is way off let me know and I will recalculate the pressure drop that would be expected across the IC.

I will save you this trouble as it sounds like you already have a life time of work to recalculate since you assumed 1 atmosphere equaled 30 psi.

Static pressure and dynamic pressure (also known as velocity pressure, VP) are independent of each other. VP is a measurement used to measure the inertia of moving air (usualy in a duct), and has no purpose when determining the pressure drop across a restriction such as, say, a filter or IC. In this situation VP values do not complicate the situation because they are simply not applicable.

If this is so then please explain the attached picture.

I read the post that he was getting a pressure drop measured across the IC, hence he blamed restrictive airflow in the IC. If the pressure measurements were taken other places it might change things, but those were not the parameters of the problem, and the pressure drop across the IC is almost surely due to the temperature drop.

Once again I disagree. Any drop in pressure would be due to air flow restriction compounded by differences in velocity of the air flow at he points of measurements. Try playing around with a few simple computational fluid dynamics simulations that vary in temperature in a closed system. You will find that pressure is equal or does equalize very quickly. If you wish to invest the time, OpenFoam is a good package it just is very complex and has a steep learnung curve.

[tasdrouille]

You are right that plumbing probably played a role in the pressure drop I measured. I taped into existing lines at the turbo outlet going to the boost control solenoid and the post IC line going to the ECU.

[MazdaMatt]

physics aside... I also am looking for ways to more efficiently drive my big (huge) diesel truck. It is less aerodynamic than a brick and weighs between 5000 and 8000 pounds depending on the load. Constant throttle is no good because if i'm doing 80km/h and i reach a SLIGHT slope, i will lose 15km/h and drop down to a lower gear, then that throttle position can't maintain the high revs and i slow down even more. On a downhill, constant throttle may gain me 5km/h.

Should i be gradually accelerating towards a hill? should i be easing off the throttle during the downhills so i'm using less gas? If i drop throttle on the downhill it will not coast, it will slow down quickly and downshift.

It seems my THREE-SPEED tranny shift 95% based on speed and just barely based on load. ie, accelerating up to 65, i can drop throttle and it will upshift, or i could not drop throttle and it will shift at 70. If i DO drop throttle, by the time i'm in the new gear, i'm only doing 61-62 and any amount of throttle will cause it to downshift again... I kind of hate life when i'm driving this p.o.s.

I will not be investing in bolt-ons and upgrades. Yes, i WANT the turbo 5-speed manual, but i have the 7.3L N/A 3-speed auto... there is no changing that.

I keep my tires at sidewall max of 65psi and I drive 85-90 on the highway (100km/h) and I drive 70 in the 80km/h zones. My last trip was ~23L/100km (that's NOT mpg! my car gets about 6.8L/100km)

[TestDrive]

Is make, model year relevant?
You don't mention rpm - do you have a tach?