Lean is less dangerous than rich?

[raz]

I've always heard that too lean is more dangerous than too rich. Now I happened to find this interesting article linked from Innovate Motorsports' forum. It says that's nonsense.

 

Very short summary:

• Max EGT (exhaust gas temp) is at stoichometric
  
• Max CHT (cylinder head temp) is slightly richer than stoich (as is best power)
  
• You really want to avoid high CHT, not EGT, to avoid damage
  
• Tuning the mixture to, in this example, 50°F EGT leaner than stoich will result in 25°F lower CHT than tuning equally richer than stoich
  
• OK, so you lost power. But now you compensate the power loss by using more throttle (this is for cruising of course, not WOT). After doing that you end up with same power, much less fuel consumption and much lower CHT than using the "best power" way of tuning.
  

In the airplane example, fuel consumption was lowered with 10% and CHT lowered with 35°F with equal power output, compared to when mixture was tuned for max power.

 

The problem is that running this lean won't really turn out smoothly on our twins unless they are extremely well synced. We would get rough running that is "often and wrongly characterized as lean misfire". But it is not misfire and it won't burn valves or make little holes in our pistons.

 

This was new to me but I'm sure it's not to all of you. Any objections? Is there any way this does not apply to our engines? Otherwise, if nothing else it tells me I shouldn't fear experimenting with really lean cruising. Worst case is rough running, not burnt valves.

[emry]

Most automakers now employ lean burn fuel curves for steady state crusing and idling. The curves then switch to a richer setting as required by calculated load. Advances in combustion chamber design and catalytic converters has keep the otherwise poor emissions in check. Honda has been doing heavy development of extreme lean burn technology for use in their hybrid drivetrains and other economy cars. Ratios of around 22:1 are fairly common now with as high as 50:1 being used for future direct injection engines.

[NPS]

Raz

That's very interesting and useful to know. I think much depends on combustion chamber design as emry says.

 

I've had alot of experiance playing around with crusing mixture setting on my Sporti with Cliff's MY16M. I found that on this engine anything leaner than about 14:1 caused a distinct roughness with no apparent fuel saving.

[GuzziMoto]

One difference is air craft engines run steady state, no accellerating and decellerating all the time like a motorcycle. But I'm sure there is still some that applies. As far as lean burn tech, atleast some of it uses stratified charge technology. Honda is big on that.

[Tom M]

I seized a few 2 strokes due to too lean or partially clogged main jets. I never heard of one seizing from being too rich

[Garsdad]

My Subaru is an "Ultra-low emission vehicle" and it shuts off the fuel injectors anytime the car is coasting in gear, and runs super-lean whenever it can. It stumbles on-and-off throttle, like when I am cruising down my street at barely above idle, and it keeps shutting off the injectors, then turning them on, then off again... you can watch the fuel mileage display- every time they are off, it flashes "99.9mpg". Makes the car lurch around like I don't know how to drive a stick. Really fun when you lift off the throttle and apply a little brake to slow, then a second later the power is cut and you suddenly lurch to a near-stop.

 

OK, it may cut emissions a little, but I can't see much of a difference in fuel mileage between mine and previous generations that other family members own that don't do this. It is a pain in the ass. If my bike ran this poorly and was this abrupt in part-throttle transitions I would just hammer the throttle all the time; kind of the same way to defeat the 1-4 shift in GM 6-speeds. As it is, if you see an 07 or newer Subaru lurching around like the driver can't drive a stick, cut them some slack- it really is the car!

[GuzziMoto]

That may be because in a two stroke lean or rich doesn't just affect the cht, it also affects whether the engine has enough oil to lubricate it. But while I heard about the lean/rich temp thing before, I always heard that the temp curve on the rich side was much steaper then on the lean side(ie, a little rich was cooler then a little lean). But I'm no expert.

[raz]

Thank you all.

 

I wonder, just as food for thought, if Cliff's dual closed-loop setup could result in a good enough engine sync to run 15,5:1 in selected map areas without much roughness.

 

For a little while I tried cutting injection completely when coasting (zeroed out row 0 at and above 2300 rpm). There was some more or less cool decel popping but I had no issues with on-off-stumbling. I stopped that experiment only due to the massive engine brake that also was a result. It's violent enough as-is. Next project is to put row 0 (except for lowest rpm) on closed loop with a 15,5 target. Why? Er, because I can?

[Guest Nogbad]

Lean combustion takes longer, and the flame front is slower moving. The reason you can get the exhaust headers glowing and the valves burning if you are very lean is not the temperature, but the fact that combustion hasn't completed in the available time and the gases are still reacting as they are expelled. To run lean you need a good combustion chamber with a lot of turbulence to increase the reaction rate, and you need to establish a large flame front quickly when the plug fires. The Guzzi head just doesn't do this very well.

 

There IS such a thing as a lean misfire. A lean misfire occurs when there is insufficient concentration of mixture around the plug to get a properly established flame front, or when concentration gradients in the mixture cause partial extingiushment before combustion completes. Ultimately, if the mixture is very lean it won't ignite at all and will be expelled unburnt.

 

Stratified charge and direct injection gets around these problems because it allows reliable ignition in a localised area that provides a high energy richer flame intially to get the rest of the leaner surrounding mixture reacting.

[Guest Phil_P]

I've been toying with running an 'economy' map on my Power Commandered Sporti at part throttle cruise. The advantage of the PC setup is that you can use a toggle switch on the bars to flick from one to the other, and using the LCD screen you can select from as many maps as you can squeeze onto a memory card. Doing so in gloves on the move however, would be tricky.

[Dan M]

Just wanted to chime in here. Everything Nogbad said. Also, The mix burns completely at 14.7:1. Going lean increases HC emissions (unburned fuel) and going rich increases CO emissions. (partially burned fuel) either way, unless your combustion chamber is of good modern design (MG's is not), straying from Stoich will generally use more fuel for the given power output. Most often, lean misfires show up upon throttle changes with a pop indicating the fuel is being lit in the exhaust. Also, the popping people hear on deceleration is from fuel not burning in the combustion chamber and lighting off in the exhaust. Either way it's fuel wasted.

 

Edit: as GuzziMoto said, aircraft engines run is a very narrow rev range, with a rather constant throttle opening. No where near the changes in RPM & throttle opening as our engines. Part of the need for proper mixture besides economy & emissions is driveability. Go extra lean and you will learn the meaning of lean misfire.

[dlaing]

I've been toying with running an 'economy' map on my Power Commandered Sporti at part throttle cruise. The advantage of the PC setup is that you can use a toggle switch on the bars to flick from one to the other, and using the LCD screen you can select from as many maps as you can squeeze onto a memory card. Doing so in gloves on the move however, would be tricky.

Wow! I did not realize that there were so many optional gadgets for the PCIII.

Did you also get the Wideband Commander?

[dlaing]

Where do you suppose maximum CHT and EGT are on this chart?

My guess is maximum CHT is at about 15:1 and maximum EGT is at about 16:1.

 

Some things to keep in mind are that critical temperature can be considered more related to load than mixture. Also timing has an effect too.

I'd like to see more data!!!

[Guest Phil_P]

Wow! I did not realize that there were so many optional gadgets for the PCIII.

Did you also get the Wideband Commander?

 

Not got the wideband Commander, just the Standard PC III USB from Todd Eagan. I picked up the LCD display on Fleabay from the US too. Us saps in the UK usually pay the same for stuff like that in UK Pounds as you pay in $! Even with import duty and shipping it's usually cheaper to buy from over the water.

[luhbo]

... Next project is to put row 0 (except for lowest rpm) on closed loop with a 15,5 target. Why? Er, because I can? ....

 

That makes no sense! At closed throttle most of the air/oxygen in the exhaust is coming from the back end, furthermore you don't really have a working combustion in this situation. If you now try to achieve a whatever AFR you'll need LOTs of fuel. BTW, advancing the spark to 50° helps to prevent poping with throttle closed.

 

@ Dave e.a.: running an engine leaner or richer always needs a different spark advance. As a matter of fact the PChwatever is not able to alter the spark advance - so I'd ignore this thing in this thread. Thanks.

 

Hubert