Effect of EGR Flow on Cylinder Temperature and Pressure

Good post, legend on the map is not clear. It’s just “20%” so it doesn’t indicate flow or restriction.

 

Interesting. Some questions:
is this from a simulation you built?
what is the temperature of (piston?)? 900K prior to ignition seems pretty hot. I'd expect the gas to be below 100C, right? Aluminum melts around 900K, so any problems there?
Pressure prior to ignition = 80 bar. Am I reading that right? How can it be, with a compression ratio around 13 (guess)?

 

Thank you for posting that; the PriusChat discussions on that topic have long suffered from an absence of quantitative and sourced information.

So now we're part way to a better place: we have two pictures of quantitative information, but still nothing sourced. Where are the pictures from? If they are your own work, it would be good to know the materials and methods you used. If from another source, it would be helpful (and good form) to credit the source.

That's the kind of conclusory statement that at least the graphs could help us begin to evaluate, but there's sure a lot left to know. The left graph certainly shows about a 15% increase {100 to 115 bar) in peak pressure, over about 10 of the 720 degrees of the cycle. Likewise, the right one shows about a 10% effect on peak temperature, again over about a ten rotational-degree span. As for effects on the engine, some might be more sensitive to peak temperature and others simply to the average temperature over the cycle, which is both lower, and affected to a smaller extent. I'm sure the effects of variation in that range have been studied and that there is plenty to be found in a library. This is a start, but just enough of a start to show how important the engineering questions "how much?" and "compared to what?" always are.

? That's really cold.

 

LOL yeah, I meant 100C. I'll fix that. Thanks for the catch.
But even with the fix from K to C, the actual numbers there seem out of bounds, as I asked about. I mean, the engine would seem be melted, pre-ignition pressures close to peak pressures, ... Some questions about the numbers, as I asked above, still remain to me.

 

I've unhooked my EGR valve after the last episode of clogged EGR I just didn't want to risk blowing the head gasket with misfires. It seems the gen 3 is 1 misfire away from a blown head gasket. I haven't noticed any different mpg or driveability issues. Car has 296k miles on it 2010. I'm sure the emmissions are a touch higher but overall environment pollution is lower because I'm not junking a Prius or engine. Just some thoughts and a 50k mile no egr evaluation

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An interesting strategy, considering that unhooking the EGR valve is the same as having completely clogged EGR (with one difference, the car can tell the valve's unhooked, and goes to fail-safe extra-retarded ignition timing).

 

I'd be correct to assume 20% EGR is fully functional, unclogged? As it rolled out of the factory?

Thanks a lot for this, and I'd second: can you share details on how the graphs were produced, are they your experiements?

 

The ECM chooses how much EGR to administer from moment to moment, determined by driving conditions, from none, to probably around 20% in the specific conditions where it would call for that much.

Which does lead to another question about the graphic. It isn't clear what the engine operating conditions, such as load and RPM, were while gathering those different EGR% traces. In normal operation, different EGR% would be observed under different operating conditions.

If the graphic is from a source, I would expect a lot of the obvious questions would be answered there in the text surrounding the graphic.

 

It appears to be Figure 17 in A. Nemati, R. Barzegar, A. Khalil, and H. Khatamnezhad, “Decreasing the emissions of a partially premixed gasoline fueled compression ignition engine by means of injection characteristics and EGR,” Thermal Science 15(4) (2011): 939–952. doi:10.2298/tsci110227099n. That paper reports on work done using a computational fluid dynamics simulation of a Caterpillar 3401 gasoline engine.

 

That would shed some light on some of the questions about numbers, since that (according to the paper) is a heavy-duty gasoline engine throbbing away at 1300 rpm with a 16.1:1 compression ratio and bore ✕ stroke of 137.2 ✕ 165.1 mm (you could nearly shove a volleyball in there).

I'd encourage reading the paper because it's interesting and it isn't paywalled, so it's a good glimpse of the kind of detailed knowledge that is out there in libraries.

Their purpose with the section investigating EGR was to choose the ratio that would give them the best reduction of NOx emissions without sacrificing too much in specific fuel consumption. They evaluated 0% to 20% and concluded 15% was their sweet spot. Whether any of "those temperatures and pressures reduce the lifetime of the engine" wasn't an object of the study or a consideration in the results. Presumably if their analysis of the NOx vs. efficiency sweet spot had come up with 5% EGR rather than 15%, they would go with that and still be comfortably within the mechanical capabilities of the engine.

Though there would be some limits to applying a paper about some volleyball-pistoned Caterpillar engine to the behavior of a little Prius 2ZR, the basic ideas are there. The EGR is there for its effect on reducing combustion temperatures over a range that matters to NOx emissions. We knew that, and had discussed it repeatedly, before. We have pictures now, but they are pictures of stuff we already knew. What we didn't have was references on how that range of temperature or pressure variation compares to the mechanical limits of the engine, and that's where we are still.

 

The report is talking about a diesel engine (compression ignition) that is running on a gasoline mixture to explore reducing emissions. They are playing around with injection timing and altering EGR to do this.

Given the vastly different pressures and temperatures involved during combustion relative to the low powered spark ignition engine, and how it operates normally in the Prius, I'm not sure how much of comparison can really be drawn to what is happening in our cars.

 

Not sure that's entirely true since car runs great and mpg is same or better.

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But whether it's true or not depends only on whether it's true or not, and not on your theorizing about MPG effects.

And it can be determined from what's published about the operation of the valve in the Toyota manuals.

 

Maybe somebody should block the egr flow with a metal shim and still allow the egr valve to operate. It might be an interesting long term experiment especially when done in conjunction with a head gasket job. Ideally an early gen3 with original rings. Tim?

 

It would be interesting to do that just to see how long it takes for P0401 to show up. (Considering how finicky the driving pattern is that you have to go through to clear a P0401 once it shows up, maybe it might take a while to be detected in the first place, until you happen to drive a pattern that is like that. That would be consistent with a lot of people's reports that they found significant clogging with never a P0401 reported.)

In any event, once the computer calls out a P0401, it will retard the ignition timing from that point on to reduce risk of detonation, same as it does when the valve is electrically unplugged. That much is clearly stated in the manual.

 

My first egr clean at 200k miles never threw a code. It was so blocked we bought a new cooler. Eventually cleared the old one using a wire spun by a drill and plenty of solvent over a period of days.

 

Is that Repair Manual or New Features Manual? I've yet to find the latter, would like to.