Cooled EGR vs Water Injection vs Super Lean

[Isaac Zackary]

So I'm increasing my CR from 7.5:1 to 9.5:1 in my 72 VW Bug. I'm going to try to use cooled EGR to control detonation for now. I'm making the valve so that it operates with the throttle so that it opens as much as the throttle valve does only it's about 10% the size of the throttle valve. I'm pulling it off further down the exhaust so that there's less pressure in an attempt to get around 10% to 15% EGR even at full throttle. I also have a fully adjustable digital CB Magnaspark distributor.

I was also contemplating water injection. But it seems WI usually decreases fuel economy. I suspect that it has to be finely metered on a gasoline engine in order to increase fuel economy.

One problem with both EGR and WI is that they can reduce fuel combustion increasing CO and HC emissions while decreasing fuel economy. One thought that I had was that if I used enough, then I could run lean, even at WOT. But that would likely cause even less fuel to actually burn, perhaps even causing misfires.

But then it ocured to me. EGR and water can be used because they are inert. An enriched AFR can also do the same thing since fuel without oxygen is inert and so boils off and lowers combustion temps. So why can't a very lean AFR be used. The excess air without fuel would be inert just like EGR only that a very lean AFR would provide more than enough oxygen for complete combustion. I've read where after a certain point (about a 16:1 AFR) combustion temps and NOx production are reduced dramatically. Since a gasoline engine can run on as low as an 18:1 AFR why not use that to reduce detonation? Of course that might not be enough, so it might have to be used along with water injection at WOT.

Any comments will be appreciated.

[Isaac Zackary]

This is what I'm talking about.

Could lean buning at WOT be possible with either the addition of cooled EGR or water injection?

Any thoughts would be appriciated, including the criticizm of those wondering why I'd even think of taking an air-cooled engine and increasing it's CR to way too high and then leaning it out to way too lean.

[Daox]

You can try it, but I don't think you'll have that great of success. If your engine can only run 18:1 in lean burn, you're not even going to get to 18:1 if you start injecting other stuff. The reason that the engine starts to miss at 18:1 is the charge is diluted so much that it simply doesn't propagate the flame front smoothly and evenly. There simply isn't enough fuel to support smooth even combustion. If you start adding water or EGR to the mix, its going to be even further diluted and/or quenched.

There are ways around this. Honda did pretty much all of these with their lean burn engines. Designing the head and combustion chamber for lean burn is one. This means lots of swirl in the cylinder and a higher compression ratio.

Water/EGR will definitely help if, for example, you're running 16:1 and hitting high EGTs. At this point adding water or EGR will help because you've still got good flame propagation.

[Isaac Zackary]

Originally Posted by Daox

You can try it, but I don't think you'll have that great of success. If your engine can only run 18:1 in lean burn, you're not even going to get to 18:1 if you start injecting other stuff. The reason that the engine starts to miss at 18:1 is the charge is diluted so much that it simply doesn't propagate the flame front smoothly and evenly. There simply isn't enough fuel to support smooth even combustion. If you start adding water or EGR to the mix, its going to be even further diluted and/or quenched.

There are ways around this. Honda did pretty much all of these with their lean burn engines. Designing the head and combustion chamber for lean burn is one. This means lots of swirl in the cylinder and a higher compression ratio.

Water/EGR will definitely help if, for example, you're running 16:1 and hitting high EGTs. At this point adding water or EGR will help because you've still got good flame propagation.

Thanks! That makes plenty of sense. So basically if an engine has a lean limit of 18:1 then that's an air+EGR+water to fuel ratio. In other words an 18:1 AFR plus 15% EGR is actually a 20.7:1 of everything to fuel ratio, which is beyond the limit. Interesting. Whereas a 16:1 AFR plus 15% EGR is 18.4:1.

Well, I'm going from a 7.5:1 CR to a 9.5:1 CR. That should help (or hidder). Also I'm shooting for a tight quench. I can put dual spark plugs, that is, two in each cylinder. Would that help? How do I get more swirl?

[Daox]

Dual spark plugs will help.

As for increasing swirl, I'm not sure there is too much you can do. I assume your engine only has 2 valves per cylinder. I believe what Honda did on their lean burn engines was close one intake valve before the other, thus making the intake charge swirl a bit more in the cylinder. Really, all you really want is an evenly mixed charge in the combustion chamber. You don't want any areas where the fuel is not dense enough to burn properly.

[Isaac Zackary]

Well then I better get things ready to tap those spark plug holes from the bottoms of the heads. In that case I'll be using two Ford TFI moduals to drive two Toyota 1.5 ohm coils. I've heard that that particular combination really works well with lean AFRs on a single spark plug. I'll need to figure out how to adapt a Nissan dual spark distributor cap and rotor to my distributor.

I'm sure this whole thing is going to really throw the ignition timing for a loop. Good thing I got a fully adjustable distributor.

I'm really working to get a well mixed charge with a carburetor. I supose that if push comes to shove I'll have to change to FI.

[Isaac Zackary]

Oh and yes. It only has one intake and one exhaust valve per cylinder. I suppose the only thing I could do would be to add aluminum around the valves in order to shroud them. That's a bit more than I'd like to do right now.

[oil pan 4]

Around 16:1 burns valves.

[Isaac Zackary]

Isn't stoichiometric the hottest?

[Daox]

Nope. NOx increases as combustion temperature increases.