Ionisation of engine fuel/air mix

[snailboy]

@matt

As I understand it, this is because the lean combustion alters the heat transfer properties - rich combustion leaves an insulating unburnt gas layer at the cylinder walls. (I’d be interested if someone could clarify this!)

From what I’ve read, it’s because nitrogen is burning into nitrous oxides. Nitrogen burns at a much higher temperature than gasoline.

[Una_Persson]

@Fridgemagnet

One day I’ll tell the tale of how I ‘invented’* the capacitive ignition lead to boost spark energy, only to realise it was total bollocks.

*I’m not the first person to have come up with this idea, and there are even brands on the market you can buy. They are bollocks also. In light of this, I run some very excellent Magnecor leads, which are effective and respected.

Heh. I came up with an idea of using a variable capacitor, mounted on the dash, to allow a person at steady engine speed to super-tune-in their timing and try to super-optimize their engine economy. That didn’t work either, although I swear the concept was sound…of course, you had to keep turning the dial even if it did work, or have it cut out if the engine speed delta was greater than +/- 10%…

[Una_Persson]

@matt

Running an engine lean does improve efficiency slightly. It costs some power, and causes the engine to run hot despite the combustion itself being cooler! As I understand it, this is because the lean combustion alters the heat transfer properties - rich combustion leaves an insulating unburnt gas layer at the cylinder walls. (I’d be interested if someone could clarify this!) It may be possible to design an engine to run lean, but at the moment there’s no good reason why you should.

The first thing I’d ask is what you mean by the combustion being cooler - in everything from IC engines to 1000MW power plants, running lean generally increases the combustion flame temperature (and increases the NOx production; it’s not driven the other direction by NOx creating the temperature, see activation energy equations and curves to see why).

The second thing I’d mention is that there are engines which are designed to run lean, sometimes very much so, provided that you don’t need the most power and you can handle the NOx production.

[matt]

@Una Persson

The first thing I’d ask is what you mean by the combustion being cooler - in everything from IC engines to 1000MW power plants, running lean generally increases the combustion flame temperature (and increases the NOx production; it’s not driven the other direction by NOx creating the temperature, see activation energy equations and curves to see why).

I’ll happily take your word for it, Una, but I don’t understand how that works. I would have thought stochiometric burning would give the maximum flame temperature - complete reaction with minimum diluents. Burn lean and you have unconsumed oxygen and additional air inerts in the flame. Why doesn’t that make it cooler?

Burn rich and you have unconsumed fuel in the flame, so I guess I can see why burning a tad lean might actually be hotter than burning a tad rich, depending on heat capacities, but other than that I must be missing something here!

[Una_Persson]

@matt

I’ll happily take your word for it, Una, but I don’t understand how that works. I would have thought stochiometric burning would give the maximum flame temperature - complete reaction with minimum diluents. Burn lean and you have unconsumed oxygen and additional air inerts in the flame. Why doesn’t that make it cooler?

Burn rich and you have unconsumed fuel in the flame, so I guess I can see why burning a tad lean might actually be hotter than burning a tad rich, depending on heat capacities, but other than that I must be missing something here!

I think the difference here is I’m thinking reality, where due to poor mixing, radicals, and creation of non-ideal products and such you always need excess air levels to get the complete combustion in the first place, and you may be thinking of on-paper, where stoichiometric burning is best. I need to look in my books for the actual “best” excess air level for a gasoline IC engine, but for things such as coal it’s about 120-130% stoichiometric, and for oil about 105-110%, and natural gas from 102-106%. There’s also a side issue of having enough of a mean partial pressure of oxygen in the real combustion process througout the combustion cycle…but I really don’t want to go down that path.

I think we might just be on the different levels of actual combustion versus theoretical.

[Fridgemagnet]

You know, I’ve never really thought about the mechanism behind combustion temperatures with respect to fuel/air ratio. Some thought provoking insights going on here…

Back in the days when you could alter the carb mix by a tweak of a single jet, there was a rule of thumb that it’s safer to burn a little too rich than a little too lean. Too rich and your ports may get a little sooty, but too lean and there’s a very real danger of burning out the valve seats.

I’ve had occasions when I’ve temporarily run an engine very lean*, and it really kills the power, puts big lumps in the torque vs. rpm curve, and is very prone to stalling at idle speeds.

• To frig a pass on an old car in the annual emissions test. I used to do this by removing the air filter, adding an overdose of octane-booster to the fuel, and tweaking the carb to run lean. It would just about run on air, with just a sniff of fuel vapour. But I don’t recommend it.

[am77494]

Another child left behind :smack: