Lean Burn

[HUMINA]

Looking for a little help in understanding something about running an engine lean. My understanding is that running an engine a little lean (maybe around 15 or 16:1 ratio) will result in a hot running engine.

How lean can an engine be and still run even if it isn't running well? Basically what is the air to fuel ratio limit where combustion won't happen. At some point it seems that running lean would result in an engine running cooler because there is excess air.

If heat (and less power) is the concern with running lean, could an engine be set up to run extremely lean in combination with water injection to help keep temperatures down? Then, if the engine and water injection are being controlled, at WOT the engine could go back rich for power.

I am sure there is a large hole in the idea ... just having trouble finding it at the moment.

 

[rbohm]

8) a lean mixture doesnt always mean overheating, but it can be a cause. as for how lean a mixture can be and have an engine still run, that depends on how efficient the engine is overall.

 

[XPC66]

Yeah the conjugate heat transfer can have an inverted bathtub curve, but is generally lower with lean mixture (same as it is with a richer than stoich mix, but for different reasons). This is because the flame front is slower, higher temperature while burning, the peak pressure lower and later. With gas recirc your flame front can be down to 0.25m/s.

The CHT drops because the overall liberated heat is less, but the exhaust valves suffer because during blowdown the temperature of the exiting gas is higher than design.

Don't confuse CHT with EGT.

 

[HUMINA]

Don't confuse CHT with EGT.

Alright ... EGT is Exhaust Gas Temperature. If my memory serves me right, CHT is Cylinder Head Temperature. If I understood you, a lean mixture would leave me with a lower CHT because less fuel is being burned, therefore less heat is being put into the head. However, the gas that does burn is burning hotter so the exhaust temperature can be higher.

Did some digging and my Internal Combustion Engine textbook states that in a homogenous mixture gas will only combust upto a 19:1 air to fuel ratio. To have a lean burn engine at higher AF ratios, you have to have some sort of stratified charge in the cylinder.

Back to my original thought, it seems to me that water injection would have to keep the EGT down to do any good in the case of lean burn. What I have read with testing of Water Injection, it keeps CHT down but does not have a big impact on EGT.

 

[Pinhead]

Water injection will further slow the burn. While water injection may save your valves, power will greatly suffer. If you have control over your mixture, inject hydrogen/hydroxy/browns gas, etc into the incoming air stream. This will speed the flame front back up, add H O radicals and greatly increase the engine's efficiency. Injecting hydrogen/oxygen into a homogenized mixture of gasoline results in a twofold increase in flame front speed.

 

[XPC66]

CHT = conjugate heat transfer, which in effect is manifested in increased jacket temperature. You wouldn't necessarily see and change if the radiator and fan are sized adequately.

Direct injection is the go if you want to reduce fueling without the EGT problem.

 

[jamyers]

In the "For What It's Worth" department:

* Ford sixes have apparently about the coldest combustion chambers ever (they *need* the hottest plugs made).

* My stock 200 runs very well with idle and cruise A/F ratios around 15.5 or 16. It likes to idle a bit richer - like 14.5 or so, but I can't tell any difference on the two temp gauges no matter what the AFRs are (maybe when summer kicks in full swing).

 

[HUMINA]

I guess my thought through all of this was can highway fuel economy be improved by setting up a car to run very lean while cruising. If using a fuel injection set up like MegaSquirt the same car should be able to run lean while cruising then running rich under WOT.

Don't know that the potential increase would be worth the trouble. If stoichiometric is 14.7 and gasoline will only ignite up to 19 ... a potential 30% reduction in fuel is possible. The again, running lean will reduce power possibly requiring a greater throttle opening. Don't know that in reality much of an improvement would really result.

 

[rbohm]

8) dont equate a lean mixture with improvement in fuel economy. you saw one issue in that a lean mixture can mean less power produced by each cylinder, and thus requiring more throttle pressure to maintain the same rpm, resulting in a loss of fuel economy. another thing that happens is that you can get a lean miss, and that also reduces fuel economy.

 

[Lazy JW]

Chrysler threw a whole bunch of money at their "Lean Burn" program back in the 70's. Those engines worked kinda-sorta ok when they were new but were VERY difficult to maintain "down the road".

One good thing to come out of it was vastly improved ignitions.
Joe

 

[Pinhead]

8) dont equate a lean mixture with improvement in fuel economy. you saw one issue in that a lean mixture can mean less power produced by each cylinder, and thus requiring more throttle pressure to maintain the same rpm, resulting in a loss of fuel economy. another thing that happens is that you can get a lean miss, and that also reduces fuel economy.

Lean mixtures burn more slowly and are more prone to misfire. Once you get to the point of losing power, your engine is also losing efficiency. Add something to speed the burn back up and you can run much leaner and the combustion efficiency will be maintained for longer. Hence H-O injection...

 

[TeddyXY71]

I vaguely remember from something I saw sometime ago that Toyota (or was it Mitsubishy?) had looked into lean burn motors where they were geting down to the 20:1 levels. However, to get any level of good combustion you needed a very homogeneous fuel air charge entering the chambers. This required some rather good engineering in the head/valve/piston shape department. A little beyond the typical level of technology available in our older motors.

Teddy

 

[HUMINA]

From what I have read, the latest lean burn engine technology focuses on a very stratified charge. Engineers are trying to create a more typical AF ratio around the spark plug with only air surrounding it.

As I think was said before, sounds like a great application of direct injection.

 

[rbohm]

8) the thing about direct injection is that you are able to get extremely tight control over you air/fuel mixture ratio for two reasons;

1: since you are injecting fuel only, and directly into the chamber, you are not wasting fuel by bouncing it off the backside of the intake valve.

2: since you are running air only on the intake side, you can tune the intake runners for maximum air velocity in all rpm ranges.