Laminar flow and ICE engines...

[maxSaleen]

I think you guys will find this interesting.

I'm having a discussion with my professor (Benjamin Fries, professor of fluid dyanamics at UCF) about the new direct injection fuel technologies that bosch offers for IC engines. I mention that they claim one of the benefits is better fuel atomization, he laughs and says that inside the combustion chamber of an engine, atomization is a pointless goal.

I ask why....

He goes on to explain that as compression increases, laminar flow increases at an exponential rate, meaning that the air and the fuel seperate.

Our conversation was ended abrubtly at this point as I had to make another class...

Does this sound correct to any of you guys? If I'm interpreting his statement correctly, does this mean that fuel tends to cling to/seek the walls of the combustion chamber?

If so...

Wouldn't it be a good idea to groove the walls of the combustion chamber to increase its surface area and decrease the laminar flow boundary between the air and the fuel? Kinda like what this guy did:

http://www.popsci.com/popsci/futurec...cbccdrcrd.html


I'm curious to see what you guys think...

[bobo5195]

I disagree, kinda. And I’m fairly sure based on your description that he is not too deep into engines. ICE engines are not lamina flow they are the most turbulent places you will find! Reynolds numbers over 50k, stupid amounts of flow variation. The flow in an engine differs immensely between stokes, it is completely different because it is so turbulent. The turbulent kinetic engine in an ICE engine is about 9MW per kg. It is converting turbulence to heat at the rate of 9MJ per kg of air per second! About 50 celicus of engine heat is direct losses to turbulence.

Fuel does stick on engine walls but only if you doing something wrong. You can still make a GDI engine that works very well even if this happens because the walls of the engine are very hot so it will vaporise due to that. The only thing is that time spans in an engine are short so it may not completely or whatever (must of nodded of for a second in lectures). My prof (Alex Taylor, fluid dynamics of ice engines at Imperial College London https://www3.imperial.ac.uk/portal/p...ema=PORTALLIVE
not a ditch digger - civil enger like fries :P ) did some work on Honda (they pay him a lot of money to do stuff like this) showing that one of their GDI engines did this despite that fact that it work quite well. He teaches a course on ICE engine design at our uni. A prof whos left (he earned a mint) called a prof Gosman developed the first program for working this out (and kinda half invented CFD in the process). It is now a CFD program called STAR CD your prof should of heard of that.

Air and fuel are separate in a GDI engine because the engineers want it that way. Modern GDI engines run exceptionally lean (say 40:1+) they even pump some exhaust gases back in to the engine to reduce the amount of oxygen. They do this because the charge is stratified. There are regions of stoichmetric (14:1 air to fuel) and above in the engine. By using ultra lean and stratified charge you can get very good NOx levels as the mean cylinder temperature is low and good efficiency and performance. Think their getting 30% more overall than a normal engine after using GDI. Something like 20% more bhp for a similar engine. In other words worth doing. There are some problems though which is why they don’t put GDI engines in mass production. Basically theres puddles of fuel that burn to produce very fine soot that you can’t filter. This soot is fairly inert which is good except that it can be very inert and immovable in your lungs! Which is not a good place for it to be.

The problem is getting these stratified areas in something as turbulent as an ICE engine takes a lot of effort think teams of Fluid mech professors and 10million+ budget and maybe a few supercomputers. Your goal is small pockets of fuel turbulence spreads out and to make sure the flow happens to move these exactly by the spark plug at the right time.

In short fuel is atomised and they atomise it exactly and that is why GDI engines work if they didn’t atomise it right then it would not right. Mitsubishi has an engine running at 55:1 afr. You cannot light 50:1 afr! It will not happen, it impossible if it is perfectly mixed so the engine has probably some areas of 12:1 and but mostly 100:1 say.

[maxSaleen]

I think that Fries (how'd you know he was a civil engineer? He works for/owns a civil engineering firm, CPH) was basically saying that most people/engineers don't fully appreciate the implications of pressure/heat.

He said something to the effect that if you could stop an IC engine right before ignition, and look inside the cylindar, you would see that the fuel was clinging to the cylindar wall, almost entirely seperated from the air. Apparently, as pressure increases, you loose the ability to dissolve one substance in another (read: fuel in air). This happens at an exponential rate (what Fries said...).

[bobo5195]

That is what Prof taylor does he has glass engines and uses lasers to calculate the flow. it is well cool and costs a lot of money to do. He fully understands the implications of pressure and heat.

Looked at his web page (he isnt even a prof is he is just a Mr).

Anyway it doesnt work as he says. The fuel is a fine mist and stuff but to call it seperated is a bit odd. I'll have to look at some combustion notes but it doesn't really work like that. Besides the flow in an engine is so turbulent that there is no way there could be large particules.

[bobo5195]

had a little look through my notes,

average level of fuel atomisation in GDI engine is 20-50 microns.
It is a design LAW that those droplets should not hit the wall. V bad if it does.
And charge that hits wall will evapourate as the wall is at 500c. Evapouration means that you may have some fuel in exhaust due to low AFR in air.