Hypothetical musings......

[RNDDUDE]

First off let me say that I am a big proponent of clean air and low-emission vehicles. This is NOT an attempt to circumvent any emissions equipment.This is more about trying to mine the forum expertice on the effects of emission control strategies and their impact on fuel mileage.

So I was wondering...if you were to take a CR engined car, and could remove all post-combustion after-treatments...EGR, DPF, cats, resonators, mufflers, etc....and could tune the ECU to do no regens and be tuned for economy, what sort of gains theoretically could be realized? Or would a PD engine offer better potential? I am thinking that other than the DPF and it's required fuel-rich regens, there probably aren't a lot of appreciable gains to be had. Your thoughts are encouraged......

 

[raitchison]

Does the DPF even need fuel rich regen cycles if you regularly drive a decent amount of distance on the highway?

I know in my case I drive 25 miles each way to work and while my afternoon commute is usually in traffic in the morning I'm generally 70+ the whole way.

 

[VeeDubTDI]

Golfs and Jettas are seeing +10 MPG.

Nobody has done a Passat that we know of, but the improvement would be less because it is more efficient to begin with.

 

[RNDDUDE]

raitchison...I do believe it always does require a late cycle fuel injection to temporarily bring the EGT up enough to make the DPF move from trapping soot to burning it into ash. I doubt that it would survive a constant temp high enough to burn soot continiously.

 

[NickBeek]

And there are regens that occur on a timed basis whether the soot level warrants a regen or not... That is the one that I don't get at all. Why regen if the sensors are not telling the ecm that it is needed.

EGR also takes a toll on F/E in my opinion. If that could be removed or the amount of EGR cut back economy will rise.

 

[RNDDUDE]

^ I don't believe there is a sensor that would relay that info. I believe the sensor that is there only will indicate when the ash level is at a level that would indicate "full of ash, can't accept any more, get new DPF" but i could be wrong. I need to check my self-study guide for more info....

 

[TDI4evah]

NickBeek, why do you think EGR will hurt mileage? I agree it's a PITA with the intake manifold plugging, but EGR helps warm the engine up more quickly in two ways. One is the EGR<->water heat exchanger, designed to cool the exhaust, but the heat from the exhaust goes into the water. That helps warm the water up more quickly.

The other way it helps is by raising combustion chamber temperatures when the engine is cold. Higher temps mean faster and more complete combustion.

Once the engine is up to temperature I don't see it making much difference either way. But I'm all ears if you have a good reason for believing otherwise.

 

[josh8loop]

Your favorite tuner also most likely has a tune that includes "Dynamic EGR" or whatever moniker they give theirs. The Dynamic EGR allows normal EGR flow for fast warm ups etc, until the coolant temperature reaches a certain point where the EGR flow is cut off. Very nice strategy IMHO.

 

[VWBeamer]

It's not that the recirculated gases hurt MPGs, it's the stratagey that VW uses to get the motor to draw in the EGR.

Most TDi have a Anti shutter valve before the egr port. VW uses the ASV as a throttle to reduce fresh air coming in, so that the motor will draw the reciculated exhaust gas.

The ASV acts as a throttle plate and introduces a pumping loss, much like the throttle plate on a gas engine.

NickBeek, why do you think EGR will hurt
mileage? I agree it's a PITA with the intake manifold plugging, but EGR helps warm the engine up more quickly in two ways. One is the EGR<->water heat exchanger, designed to cool the exhaust, but the heat from the exhaust goes into the water. That helps warm the water up more quickly.

The other way it helps is by raising combustion chamber temperatures when the engine is cold. Higher temps mean faster and more complete combustion.

Once the engine is up to temperature I don't see it making much difference either way. But I'm all ears if you have a good reason for believing otherwise.

 

[NickBeek]

I am basing my statement from experiences people I know have had when building performance diesel engines. They invariably get better mileage after deleting EGR. The other thing that makes me believe this, is the newer generation of diesel pickups all incorporate DEF which allows reduced EGR duty cycle. Fuel economy is rising as a result. I realized none of this is scientific, but it makes good sense to me.