Why can't you replace EGR?

Why can't you replace the EGR system with something else?
It seems like the new 2010 engines with EGR/DOC/DPF/SCR we should be able to replace the EGR system with something else.
Two ideas I have are an additional SCR system, to nab that final 15% of NOx that apperently one SCR system can't grab.
The other idea, is water-injection, either a basic methanol-30% water-70% setup to reduce peak cylinder combustion temperature, or something more advanced, like water+hydrogen injection.

The new trucks only use EGR at a 15% rate. It seems like the new trucks would be a lot more reliable if we could ditch that part of the system.

 

Well, on looking at a pure SCR solution for NOX control, Cummin's in Australia had a SCR only solution for EU stage 4.
Iveco, a European class 8 truck manufacturer, has their trucks only use SCR, and promote it as a strong selling point.
So the tech is out there, but it seems the manufacturers in general are more concerned with DEF fluid usage than fuel economy or engine reliability and durability.

The reason I was suggesting hydrogen wasn't because I think it's an answer as an alternative fuel or as part of a duel fuel system.

With water/methanol injectiuon, water does two things, it creates better combustion, and lower peak cylinder combustion temperature, however too much water can cause "queuching" which feels like engine knock but is really poor fuel combustion. Methanol helps the diesel fuel combust more completely, because of it high burn rate.
Ideally, you wouldn't even need to modify the EGR system, mechanically or electronically. The EGR system should detect the lower combustion temperature, and use significantly less or no EGR, the same way it self moderates based on the quality of the air (low vs high altitude) and quality of the fuel (summer fuel in Texas vs Winter fuel in Quebec).
The problem with water/methanol is three things: increased cost, maintenance issues, and availability. Cost is obvious, either bought online premixed or just using blue winter windshield was fluid, it costs more than straight potable water. Maintence wise, menthol alcohol causes problems with corrosion. That issue presumably gets worse if you use windshield washer fluid, which is the most available, which includes 1% detergent.
With hydrogen injection alongside water injection you wouldn't have those problems, however systems that produce hydrogen through electrolysis are typically pretty expensive.
A water-hydrogen injection system would be a fairly convoluted setup. It requires a watertank with water heater, an onboard household kitchen sink-type water distillation system, a water injection system, a hydrogen injection system (including electrolysis system), calibrating the system to boost-pressure/EGT-guage, and regularily getting potable water from a RV center or truck repair shop sliping some junoir apprentice a $20 under the counter.
That's expensive, convoluted, with multiple systems that could fail. It doesn't eliminate EGR entirely, just reduces peak cylinder combustion temperature with more complete combustion, to dramatically reduce the amount of soot getting recirculated back into the engine, keeping the DPF clean and preventing EGR cooler failure because if EGT's are reduced the system should be using less (or no) EGR, and the exhaust gas that is recirculated through the EGR cooler will be of a significantly decreased volume and tempuature, so the EGR cooler will be less likely to crack and leak. Granted, you could just use hydrogen injection by itself, from a hydrogen tank, but all that would do is reduce the amout of Soot(PM) being recirculated into the engine. It would decrease issues with incomplete fuel combustion resulting in less PM congesting DPF and reducing cylinder life, but like hell would I want to be in a crash with a truck that has a hydrogen tank a DPF. That would be a #### bomb driving down the highway.

So you see how much more simply a duel SCR system would be? Using Detroit Diesel as an example, they have a "One Box" and "Two Box" exhaust system. The One Box includes a DPF, DOC, and SCR system all in one. The Two Box has one one system comprising of the DPF and DOC, with a seperate system for the SCR. Seems like it would be simple to buy the second half of the two box system, the SCR system, pipe it in, and delete the EGR, and there you go. Better fuel economy, no EGR issues, so no DPF issues caused by high rates of PM formation from EGR, much longer engine life, much more reliable engine, ability to isolate between engine problems and emission problems, and no further maitence other than filling the DEF tank. I don't see why this isn't a no-brainer solution.

 

Bump

 

So why couldn't you use a SCR retrofit system on a truck that already has EGR-DPF-DOC-SCR. There are retrofit SCR systems out there, Cummin's has one, Detroit has a two box emission system with the second box being the SCR system. You'd think a second pass through a SCR system would get that last bit of NOx.

I think this would be worth looking into. The same way that electronic piracy has faded behind legal streaming services, because those services are low cost, legal, and high quality picture/sound, I think the emission deletes will fade off when there is a legal alternative system, even if it's aftermarket.
People are either risking fines for deletion, or out of fear of fines, spending commonly tens of thousands of dollars on EGR afflicted engines.

I've looked as extensively as I can, and on the aftermarket, products are either for pre-emission engines or these crazy complicated duel-fuel or electric hybrid system. No one, has focused on the EGR. No one, has said EGR is an abomination that reduces fuel economy, reliability, and durability, and we are going to focus on an addon that can replace that environmental function of the engine and legally eliminate the EGR.

And it's going to have to come from the aftermarket, because the OEMs focus on reducing production cost for volume sales as the most important aspect of their design has most of the OEMs engineers stuck with using EGR because the accountants tell them to.

 

" Looking back now (in theory) you could get rid of the egr and use a scr to control nox output but why would you? The scr system would cost a lot more to maintain and the fact that you would have to use more def to control the output of the nox through the scr"

Because the EGR is causes extreme wear, reduced fuel efficiently and is prone to catastrophic failures to the engine and DPF.
Pre SCR engines used something like 40-50% EGR, SCR engines use 15%. I don't think that eliminating the EGR would require substantially more DEF, but even if it did, the benefits should be worth it I'd think. SCR would have to be maintained, but I rather pay to do that, than pay for regular expensive engine maintence because of EGR. So I simply disagree that it would cost more to maitain a SCR system than the combined cost of EGR components and engine components that fail because of EGR. Even if the SCR system just failed, you've pretty much isolated the problem. A mechanics time can easily cost you more than the parts, if you're getting 5 MPG because of an EGR issue, good luck finding a solution. Most shops will just throw parts at it. You can spend mountains of money and not solve the problem, just temporarily patch symptoms.
If the SCR fails, take it, fix it, done. That's the advantage.
SCR trucks are solid compared to EGR trucks, but not enough, it just makes sense to push that concept one step further.