modifying the diesel for more economy

[Vekke]

You should also try out 5w-30 oils if you have a VAG TDI engine and trans oils change wont hurt either. I could see 1-2 MPG gain after I changed engine and transmission oils.

[roflwaffle]

Originally Posted by diesel_john

roflwaffle, correct me if i'm wrong, you obviously know a lot more about pumps. how much timing advance is too much? when EGT's drop? On the mechanical system should i raise the internal bypass pressure?

Well, I've read a lot about pumps at least.

You can kinda sorta set it via pump pressure and flow rate, but it isn't super accurate. The main idea though is that wear and thinner fuels almost certainly retard dynamic timing, so increasing internal pump pressure is better than nothing. On a slightly related note, this is also why people with VE pumps on biodiesel may see better or the same mileage even though biodiesel has less energy per gallon. With a more viscous fuel, the advance mechanism works closer to stock spec. Ideally if you can check timing you can compare a change in that to mileage, but even w/o checking it you can still make small changes and see if anything improves, using the normal timing to fine tune.

I don't know if anyone has measured changes in IP pressure to EGTs, but it should help, along with excessive clattering, in determining if you have too much advance.

[Deezler]

Originally Posted by diesel_john

I am surprised that the sweet spot is located at so little advance for 1500 RPM.

So we can advance the timing because the EGR slows the combustion rate.

I was a little surprised also, but if you think about it, it kind of makes sense. There is an inherent ignition delay. While the start of ignition is dependent on the pressure in the cylinder to heat the air and fire it off, its also a function of time for air/fuel to mix and get burning.

At low engine speeds, there is more time per degree of crank angle revolution. Thus in the same amount of crank rotation, more time has passed for the combustion to begin. At higher speeds, the time is much shorter, so you must inject earlier to ensure combustion has begun at the proper crank angle. Any cylinder pressure rise before top dead center is only working against the piston anyway, and lowering the mean effective pressure (usable power).

We also know EGR increases ignition delay. This would allow you to run more injection advance, but how can we be sure that we are getting down to the target O2 concentrations on the figures provided? The air we are pulling into the engine starts at around 21% O2. Assuming that re-circulated exhaust is now nearly 0% O2, and that the TDI engine can re-direct up to, say, 30% EGR rate, you would get down to ~15% O2 conc. My numbers could be way off though.

Ernie, I don't think you can draw any conclusive evidence of either ignition delay or rate of burn directly from these graphs since they only show the end result of total combustion efficiency. They probably correlate though.

Those graphs are great, btw, and thank you for posting them again Tas. But I find a couple points troubling:

1) If you notice on figure 28 "work conversion efficiency", there is a dark red area up in the >20% O2 concentration, slanting back from 5 deg SOI towards even earlier injection. On the first plot, figure 27, "combustion efficiency", this is also the highest % area. Why then, on the final "fuel conversion efficiency" plot does this obviously overlapping area not produce good results? This means that the fuel conversion efficiency is not so simple as overlap between the first two figures. I'll have to read the whole paper and learn up.

2) The experimental engine used is quite similar to a modern diesel, EXCEPT:
- It only has 800 bar of injection pressure. Modern common rail engines run up to 3 times that amount of pressure, greatly affecting fuel atomization and injection control.
- It was running 1.2 bar of intake pressure (is this absolute or ABOVE ambient?)
- The intake temperature is 90 degrees Celsius! That is very, very hot. My TDI typically runs more like 30-50 C. That is going to have a huge effect upon the combustion process.
- Is also has a swirl ratio of 3.77. I'm not too sure, but I think this is a lot.

Anyway, this is a terrific discussion, lets keep it going.

Good call on the fluids Vekke, I have some new synth trans fluid waiting to go in, and definitely plan on using 5w30 oil during the winter from now on!

[Deezler]

Whoa, they allowed fueling rate to vary by up to 10%. That kind of throws a wrench in the whole thing. I'll keep reading...

[diesel_john]

Originally Posted by roflwaffle

Well, I've read a lot about pumps at least.

The main idea though is that wear and thinner fuels almost certainly retard dynamic timing, so increasing internal pump pressure is better than nothing. On a slightly related note, this is also why people with VE pumps on biodiesel may see better or the same mileage even though biodiesel has less energy per gallon. With a more viscous fuel, the advance mechanism works closer to stock spec. Ideally if you can check timing you can compare a change in that to mileage, but even w/o checking it you can still make small changes and see if anything improves, using the normal timing to fine tune.

I don't know if anyone has measured changes in IP pressure to EGTs, but it should help, along with excessive clattering, in determining if you have too much advance.

I had not thought about fuel viscosity changing timing. That introduces another parameter, fuel temperature.

[roflwaffle]

That's why there's a return system as opposed to a dead head IIRC.

[tasdrouille]

Deezler,

point 1) I said "more or less" because you need to add an heat loss map that was not shown in the paper.

From the paper: "The estimated combustion efficiency and the work conversion efficiency are shown in Figs. 27 and 28, respectively. From Eq.(12), we can also reconstruct a map of the heat-loss efficiency required to produce the measured fuel conversion efficiencies."

There are more heat losses at low EGR levels and advanced timing because of higher peak flame temperatures (2700 K at 21% O2 vs ~2100 K at 14% O2). See fig 6.

Whoa, they allowed fueling rate to vary by up to 10%. That kind of throws a wrench in the whole thing. I'll keep reading...

How so? They controlled to keep IMEP at 3 bar. They just say that if if took more than a 10% adjustment in fueling rate to maintain their IMEP value the tests were stopped. You can chose to have IMEP vary at fixed fueling rate or have fueling vary at fixed IMEP as you change the EGR and SOI parameters, no?

Ernie,

"IGNITION DELAY
Ignition delay is influenced by both physical and chemical factors, and is thus affected by characteristics of the fuel injection equipment (injection pressure, nozzle hole diameter), injection timing (ambient temperature and density) and charge dilution (EGR rate) [22, 23]. Here, ignition delay is defined as the time from SOI to start-of- combustion (SOC), as determined by the crank angle at which 10% of the cumulative heat release occurred. This crank-angle correlates well with the beginning of rapid, high-temperature heat release. Figure 16 shows that, at the same injection timing, the ignition delay increases with decreasing O2 concentration."

[Deezler]

Yeah Tas, I probably should have read the paper in more detail before throwing down all that skepticism. After re-reading it, it's a pretty awesome study.

I can concur that the benefits of the moderate charge dilution (EGR) appear to be from lower gas temps = lower heat loss, and allowing for greater injection advance gives better work transfer from the cylinder pressure. The heat loss relation they mention in conjunction here is actually up in the middle of the paper.

I suppose I would have preferred if they kept fueling constant and allowed the IMEP to vary though. IMEP is dependent not just on peak cylinder pressures, or locations of peak pressure, but really the entire shape of the pressure curve through the entire compression and expansion stroke. Varying fueling is going to change the combustion signature, and even if total IMEP is matched, the shape of the curve might be different, thereby affecting heat loss and work conversion throughout the cycle. If you kept fueling constant, then you allow the cylinder pressure curve to vary as it naturally would from the different SOI and EGR parameters, and have a fairer look at effect upon fuel consumption, in my opinion. Yes, they are measuring instantaneous fuel consumption and including that into the fuel conversion efficiency calculations, but if the combustion had varied significantly to meet the same IMEP, than it may not have been optimal or fully representative of how the engine would run on its own. Tricky stuff, for sure. Probably trade-offs either way.

Earlier in the paper they show two figures, effect of charge dilution upon cylinder pressure with FIXED injection timing, and effect of injection timing on cyl Pressure with fixed dilution (10% O2).



IMEP goes up when theres more area under the curve after TDC. Because IMEP is proportional to brake power output (with constant friction and pumping losses), you can see from figure 4 that if one cannot adjust their injection timing, increasing EGR definitely will hurt economy. On the other hand, in the second plot with high EGR, if you can re-advance your timing, this effect is nullified. I think this sums up the whole debate on whether EGR helps economy or not: ONLY if you can advance your injection timing to take advantage of it.

[tasdrouille]

Great post Deezler, I agree completely.

[diesel_john]

I read about multi-pulse injection, is there gain in efficiency with this technology or is it aimed at emissions?