Friday 2010-12-10 Dev. video update and Discussion

[T}{OR]

Originally Posted by W32Blaster

well not as simple as that.

The ideal mixture is about 14:1, also called lambda=1.

Almost correct. The ideal stoichiometric mixture ratio is 14,7:1 => or 15:1. When we're talking in mass, not volume.

There are two different lambdas in petrol (Otto) engines: lambda(air) & lambda(charge)*.

*not sure if this is the correct technical term in English.

In a petrol engine lambda 1 should always be constant (=1), whereas lambda(charge) can go up to 7 (with supercharged and turbocharged engines), otherwise it also equals 1.

Lambda of 1.0 is at stoichiometry, rich mixtures are less than 1.0, and lean mixtures are greater than 1.0.


Like Azimech, I am also curious of what carburetors are you talking about. The advantages of direct fuel injection over carburetors are numerous. If not, car engines would be using them even today.

Originally Posted by Sternjaeger

erm... no, in a nutshell: mixture has different ratios, which need to be varied according to your altitude (leaner/richer mixture), besides a 50/50 mixture ratio would probably send your valves into orbit (Disclaimer: just wanted to make a joke here, please don't take it literally and start posting copy/paste engineering blabber..)
What Richie said just doesn't make sense: engines won't produce different flames if they're direct injected or carburator injected. Or maybe I got it wrong in the first place.

Fair enough. I forgot to write one thing - at ground level. Or even better: at 20° C & 1,013 bar (Standard atmospheric pressure: the conditions at which every engine is constructed and tested).

As for the rest (at least to my understanding, I am more into car engines), there is no such thing as 50/50 mixture. Maybe that is a phrase used by pilots and maintenance crews - they usually don't follow correct technical terms anyway. What I am trying to say (and to comment on your reply), there is an ideal ratio needed for the fuel inside the cylinder to burn most efficiently / completely (as previously mentioned). As you go higher air density is decreasing. Thus (in non supercharged engines) you need to lower the amount of fuel inside the cylinder to achieve the desired ratio or else you risk engine damage and various other things I do not want to get into now. The added downside of that is that engine power drops as well. To compensate for that - superchargers were invented. They compress air so that you can have the very same atmospheric pressure inside your cylinder as if you were flying on sea level. They can also be used to increase engine power (this is the easiest way to do it) - compress the air enough and more fuel can be put in the cylinder.

I could go on, posting diagrams, formulas etc. - but even 100 posts would be enough...

Anyway, what Richie meat was something else. With carburetors you can't get this ideal stoichiometric mixture ratio of 15:1 throughout whole RPM range because most carburetors are tuned for a certain RPM. This results in imperfect chemical reaction when fuel is burning inside the cylinder - hence probably more variable flames coming out of the exhaust, especially when changing RPM.

Originally Posted by Spinnetti

ideal mixture is still basically the same at any altitude. The difference is that the air density decreases as you go up, requiring mixture adjustments to maintain the same air/fuel ratio. Some planes had auto compensation, but most did not, so mechanical tune and operator capability also come into play here.

Absolutely true. What I posted above. ^^

Originally Posted by Letum

I have a sneaky suspicion that Thor might know what he is talking about when it comes to engines...

I sure hope so...