I have read about dished pistons being ideal for the flame when gas/air is burned. What about the other strokes of the piston and there effects? When the air is pulled in, and then compressed, what are effects? Are aerodynamics considered when pistons are designed? I am thinking mostly about non-symmetrical pistons like a domed one with valve reliefs.

 

I was under the impression that current thinking is for flat tops and small combustion chambers as most conducive to efficient combustion (good flame travel). I'd guess a dish increases the "combustion space" and therefore increaes the time required for complete combustion. But if anyone knows to the contrary, I'd be happy to learn from them. The problem with using flat tops is that most heads have too big a chamber to allow the desired CR with a large cam and domes are used (race engines primarily). Street motors using pump fuel and relaitvel small camscan usually use flat tops and still get the desired CR.

Now, on a blower car, increased combustion space has a "hidden benefit". This is one reason blower cars get away with such low static CR. David Vizard describes this advantage in terms of VE. Anyone want so show off and figure out how increasing the size of the combustion space could increase "apparent" VE under boost? It's an instructive exercise towards understanding what the heck is going on.

Thoughts in this vein also lead to thinking about ignition advance. This one is easier, but anyone care to expound on how the size of the combustion space and the efficiency of flame travel effect optimum ignition advance for maximum horsepower? If the speed of flame travel was infinitely high (ie. instantaneous) what would be optimum ignition advance? And why.

For whatever reason, stuff like this interests me.

Rich

 

...Not to think too deeply on the subject...but wouldnt you have to study the power curve for the flame itself? And then the power curve of the flame acting on a single piston? So that for a specific air/fuel amount (amount not, ratio), starting at a certain piston position, when ignited, the piston travel xx inches (the maximum effective range of the flame, for the given conditions). Then you can find the acceration of the piston. Then maybe the power being produced after each millimeter the piston moves.
Then you can go back and redo the experiemnt so that you start with the piston in a different position? And use that to determine the best time for the spark?

I may be wrong about the dished piston thing, I read it somewhere, and it most likely was not a credible source (the internet).

 

Just the thought of studying a flame front hurts my head. I can think of about 10 variables for that theoretical equation off the top of my head and I'm probably not even close. Mixture motion, swirl, tumble, octane, compression, spark duration, air/fuel mixture...........

Yeah, let's do that. You let me know when you have it all figured out 77.

-Mindgame

 

I'll take a guess at the question about blowers and dished pistons.....

There's more cylinder volume TOTAL with a dish. More room near TDC when the intake opens, allowing the boost to start filling the cylinder when the intake opens and more room near BDC to keep that cylinder filling going under boost just before the intake valve closes. More volume = more room to keep filling up with boost = more power.

Am I anywhere near close on that?

 

Yes. If you make the area above the piston crown greater on an NA motor, the mass of fuel and air entering on each intake stroke will not change much. And the lower CR will decrease combustion efficiency. If the manifold is under positive pressure the whole time the intake valve is open, there will be a larger mass of fuel and air in the cylinder at IVC. So, the VE will actually increase. Sounds paradoxical, and it is indeed counterintuitive when the fram of reference is NA. Being stuck in the NA mindset is why some people are so reluctant to drop the CR of a boosted motor.

Rich

 

Rich, is it JUST about volume, or does shape enter into it?

In other words, if you keep roughly the same volume but change the shape in a way that gives the intake valve "more room" will that still have the same effect on VE?

Also, beyond VE; does the type of fuel matter? In other words; are the chamber volume needs of an alcohol motor different from a gas motor? (Both under pressure.)

 

Well, combustion chamber designs that unshroud the valves clearly work better. The piston crown is fairly far away from the intake valve on the intake stroke though, so I doubt it has much effect. Maybe a HUGE dome would make a difference, but otherwise I'd guesstimate "no".

As far as fuel, when you are talking alcohol blower cars, you are talking race cars. In that case, there are a large number of differences in what you are trying to achieve when compared to a street car running pump fuel. Look at a blown alcohol Hemi. Needless to say, these things make good hp. The coumbustion chamber shape sucks. How do they do it? High compression, huge amounts of boost, and great flow through huge heads. Different ballgame altogether. Could they make more power with a different paradigm? Maybe. But guys who run these things are pretty conservative and most tend to stay with what works. At that level, getting the power to the ground is usually the issue anyway, not necessarily making more hp.

Rich

 

Well I'm personally going to try to change a paradigm. Alcohol has been a race fuel, not a street fuel; because it is frankly a PITA. You know what I'm talking about.

I believe that E85 changes that paradigm and I plan to put my money where my mouth is. I'm actually in the parts accumulation process of that now.

So when I'm talking about alcohol blower cars now, I really am looking at the street and at relatively low HP. (~750)

Flex fuel vehicles in the showroom are not optimized for either gasoline or alcohol, which is part of the reason they have suck lousy efficiency. Or at least, that's what I believe to be the case.

That's essentially where my question comes from.
It's reasonable to expect that E85 will behave differently than gasoline. That doesn't mean it will, but I think I'd rather ask the questions BEFORE I spend thousands of dollars only to find out that I have made design decisions that are sub-optimal.

After all, I'm going out on a limb here anyway, even more than Big Show is, because at least gasoline burining, street-going, 4-valve engines are faily well understood.