88 replies to this topic

[Ray Bell]

I've been musing over this for a long time...

Lots of pistons have flycut valve reliefs in them. It seems to me that these must have an affect on combustion in some way... whether it be that they might retain some burned gases, or that they interrupt the swirl that might be designed in or whatever.

Any ideas? Any knowledge of testing etc?

[malbear]

Originally posted by Ray Bell
I've been musing over this for a long time...

Lots of pistons have flycut valve reliefs in them. It seems to me that these must have an affect on combustion in some way... whether it be that they might retain some burned gases, or that they interrupt the swirl that might be designed in or whatever.

Any ideas? Any knowledge of testing etc?

In F1 engines the flycuts are the combustion chamber . They have to be to attain a high compression ratio with such a large bore short stroke engine. Combustion efficiency could not be good.
Has that set the cat amongst the pigeons?

[Greg Locock]

Mal, why don't you introduce yourself?

[Ray Bell]

I thought he did years ago?

But I also think that was a different username... certainly it was before 2002.

[malbear]

Greg Locock,
Just to clear it up Malbeare and malbear are one and the same. I had a bit of trouble trying to find my password so I re registered,
http://www.jack-brabham-engines.com
http://www.sixstroke.com/pageone.htm

[Ray Bell]

And so this page introduces the Mal Beare of our earlier days...

With quotes from my cousin, I see.

[Bill Sherwood]

HI Mal.
I'm at home at the moment, I should pop around for a visit.
I also need to get a block cryo-treated and you're the man to talk to.;)

[Ray Bell]

And now... is there any discussion on the affect of flycuts on combustion?

[knighty]

given the choice of either fly cuts or a large intruder, flycuts seem to be the best compromise, this is why the latest f1 engines have such a narrow included valve angles, over the years it has changed from about 35degrees for a Cosworth DFV.....to about 18 degrees for the latest F1 kit.......hence at 18 degrees, the flycuts are each only about 9 degrees each - quite piffling really........in my experience, big intruders really kill power, you need the flycuts in order to get the required squish and compression, without a decent compression you are going nowhere........I hope thats of use to you ;-)

[malbear]

knighty,
here is a quote from an article I found. I think that the decreasing valve angle has contributed to good gas flow and this is where the power is coming from not necessarily the combustion chamber shape.

"The Testastretta engine fitted to the Ducati 998R 2002 version, the bore is 104 mm.
Unfortunately, such a large bore currently causes combustion problems with dramatically decreased efficiency.
This stems fundamentally from the need to augment the injection advance and from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter. The "shape factor" is a critical synthetic value to check a combustion chamber's good operation, and a good indicator of its compactness and "thermal efficiency".
It should be borne in mind that aspirated racing engines require rather extreme valve lift and overlap angles, therefore, cavities are made in the piston crowns to prevent contact with the half-open valves. The combustion chamber is therefore practically contained in the piston cavities, such cavities becoming bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high specific power engines."

[Ray Bell]

Yeah, that's the way I was thinking...

They are nothing more than inconvenient necessities. I guess the guy who works out a way to lessen their impact on combustion will make the next big advance?

[desmo]

It appears to me that much of the piston crown area between the valve reliefs has become a squish area in F1 engines and that each cylinder now essentially has two combustion chambers, one for each intake/exhaust valve pair with some limited interaction between the intake pairs but very little on the exhaust side. The flame fronts travel a similar distance from the central sparkplug location compared to a "conventional" pentroof 4-valve design, but the actual combustion area geometries are somewhat closer to optimal as a result.

[shaun979]

What is a big intruder?

[malbear]

shaun979,
Basically an intruder is any lump on the piston that is above the head line.
Bring back the Herron head all is forgiven. No valve angle and no protrusions.

[Ray Bell]

Originally posted by malbear
.....Bring back the Heron head all is forgiven. No valve angle and no protrusions.

You've been talking to Phil Irving again, haven't you?

[shaun979]

Originally posted by malbear
shaun979,
Basically an intruder is any lump on the piston that is above the head line.
Bring back the Herron head all is forgiven. No valve angle and no protrusions.

Ah ok. The thing I don't understand is why knighty speaks of flycuts and large chamber intrustions (protrustions) as if they are two approaches to a problem. Aren't they totally different? A flycut is an intrusion into a piston, whereas a a chamber intrustion is a protrusion from the piston.

given the choice of either fly cuts or a large intruder, flycuts seem to be the best compromise

Thanks for any clarification.

[knighty]

Hmmmmm……..decreasing the valve angle makes the gas flow better........ I’m not saying the guys wrong, but the more upright the valves are, the more bent the ports need to be, for under bonnet packaging reasons........ which normally restrict flow, but it could easily be argued that this bend in the ports could help the tumble effect of the incoming charge, more tumble is a good thing......obviously F1 type installations dont have the bend packaging problem though!

In my mutterings about the compromise between intruders and fly-cuts………. try to imagine a wide valve layout, say 50 degrees included angle, the space created in the combustion chamber would be massive, in order to get a sensible compression with sensible valve openings, you would need a big intruder, with quite deep fly cuts, don’t forget the deeper the fly-cuts, the less the compression – not a good situation to be in!.........now consider an 18 degree included valve angle, the combustion chamber shape will be a lot shallower, requiring a much shallower intruder and fly-cuts to get the same compression as a 50 degree head…….

my honest thinking about big intruders is they act like a knife to the combustion gas force, and direct the main force away from the centre of the piston and create a sort of splitting swirt effect, but I’m only guessing on that one, I haven’t seen any data to support it, other than the fact that big intruder engines never make the power that a small intruder engine makes, I have seen this happen of BTCC, WRC and S1600 engines on several occasions…….am I making sense now?

[malbear]

The may fireball design is an inversion of the Herron design , still has verticle valves but the exhaust valve is short with the combustion chamber underneath. the only flycut on the piston is for the intake valve, and the piston has no protrusions as it is virtually a flat top. A good compact combustion chamber burns with good thermal efficiency as the surface to volume ratio is low. The engine just dosent breath as well as a 4 valve head.

[McGuire]

I suppose everyone is familiar with the double-flycut pistons mass-produced for the OEMs etc (two pairs of flycuts so they can be installed in the cylinder either way). IMO they may cost you a few hp but I can't prove it.

Here is a design by Jim McFarland for an outift called High Tech Pistons. He says the dimples induce turbulence. Does it work? I don't know.

[Ray Bell]

Originally posted by McGuire
I suppose everyone is familiar with the double-flycut pistons mass-produced for the OEMs etc (two pairs of flycuts so they can be installed in the cylinder either way). IMO they may cost you a few hp but I can't prove it.....

It was actually this issue that brought it to a head for me, McGuire...

The Poly has a flycut each side of the piston, but it's only there for the inlet valve. The exhaust valve doesn't come in conflict with the piston, normally, so doesn't need one... apart from the fact that it's at a different angle.

So when Ben orders his pistons, he'll be getting them without flycuts... he'll then flycut half of them for one bank and the other half for the other bank. Just one flycut in each and they'll therefore be handed for each bank.

That's an interesting proposition you've put up there too... wonder how it all works?

[McGuire]

Originally posted by Ray Bell
That's an interesting proposition you've put up there too... wonder how it all works?

The idea is the air tumbles as it runs over/into the holes I guess. Whether the specific tumble is effective or even desirable, I don't know. You will note the piston has a particular dimple pattern ( I have seen several). How the individual pattern is established I don't know either. Seems sort of hocus-pocus to me, but I can't knock it as I haven't touched it.

[McGuire]

Originally posted by desmo
It appears to me that much of the piston crown area between the valve reliefs has become a squish area in F1 engines

That is a great photo you posted there Desmo. The carbon buildup is a virtual map of the combustion pattern.

[shaun979]

Thanks for the explanation knighty

Originally posted by knighty
the more upright the valves are, the more bent the ports need to be, for under bonnet packaging reasons........ which normally restrict flow

On the other hand, if the valve angle is high, then the port angle is far off from the bore axis and airflow will be compromised.

more tumble is a good thing

I think that for a given engine and a given operating condition, there is a point of optimal mixture motion, whether swirl, tumble, or a mix of both - above or below which power drops off. It is sad that, generally, mixture motion varies inversely with airflow, so I don't think we can say more tumble or more swirl is always a good thing.

[shaun979]

Originally posted by McGuire


That is a great photo you posted there Desmo. The carbon buildup is a virtual map of the combustion pattern.

Dear McGuire, could you spare a couple minutes to share what you see in the carbon patterns please? I read something about reading it a while ago, but have since forgotten. I wish I had remembered.

TIA

[Ray Bell]

Second the motion...

Errr... regarding port angle... there's a fair bit of freedom on that score, I'd say. We are talking about racing engines, they have seperate inlets, I'm sure they can work that part out. Look at the 640 Repcos and the late BRM V8s, for instance.

[shaun979]

Originally posted by Ray Bell
Errr... regarding port angle... there's a fair bit of freedom on that score, I'd say. We are talking about racing engines, they have seperate inlets, I'm sure they can work that part out. Look at the 640 Repcos and the late BRM V8s, for instance.

Can you explain further please? I don't quite understand. Fair bit of freedom to do what with the port angle? No matter what is done to the inlets or port, the air is still going to have to make a turn toward bore axis in the final stages of travel and that's where the loss lies no?

I just took Karl Ludvigsen's book off the shelf to look at the BRM V8s and am not sure what you mean.

Thanks Ray

[Ray Bell]

What I am saying is that there is the freedom to have the inlets come out of the head anywhere...

Well, you wouldn't want them disturbing the airstream, but they don't have to turn sharply if you don't want them to. The BRM had them sticking straight up, right, between the cams. You might not do that any more, but you might have them come out equally close to the other side of the cam.

[shaun979]

Thanks, I understand that you can design to have the inlet anywhere. My point is that at high valve angles, the air will have to make the turn towards bore axis no matter the shape or trajectory of the port, and no matter where the inlet is placed. There is no escaping this which is why racing valve angles have been getting lower and lower over time.
Best regards

[Ray Bell]

Sorry... I'd be misreading your question... of course, you have a point there.

[McGuire]

Originally posted by shaun979


Dear McGuire, could you spare a couple minutes to share what you see in the carbon patterns please? I read something about reading it a while ago, but have since forgotten. I wish I had remembered.

TIA

it's pretty simple, the carbon buildup pattern indicates the predominant areas of combustion. For example, Desmo naturally intuited (quite correctly IMO) that the bare sections on the piston were strong squish areas. Detonation and fuel washdown can also influence the carbon pattern but I don't see that here.

[cosworth bdg]

Piston flycuts have a large influence on burn-rate of the incoming charge, just look at the v10 of current world champions RENAULT . In 2005 ,this v10 GP engine ran flat top pistons, yet still maintained power along with fuel mileage....

[shaun979]

Originally posted by McGuire


it's pretty simple, the carbon buildup pattern indicates the predominant areas of combustion. For example, Desmo naturally intuited (quite correctly IMO) that the bare sections on the piston were strong squish areas. Detonation and fuel washdown can also influence the carbon pattern but I don't see that here.

Ah thanks.. I finally recalled where I had read about carbon reading and it was off Reher Morrison's site in an article by Darin Morgan. Excerpt below...

The fluorescent dye allowed us to see the behavior of the air/fuel mixture in fine detail. We could watch as a vortex would form, grow, and move around the chamber like a miniature tornado as we adjusted the valve lift. We could spot areas where the vortices joined to form a cyclone of fuel and air. We could see where flow was turbulent, and where it was stagnant. We felt like blind men who had been given the gift of sight.

The first thing that came to light was the myth of fuel wash. Like most racers, I believed that clean areas on the chamber walls and piston dome indicated where fuel had fallen out of suspension and cleaned off the carbon. Wet flow testing revealed the truth: The shiny areas are where there is the least amount of fuel. This lean mixture burns quickly and completely. In fact, the most fuel fallout occurs where there is a dark, sooty burn pattern on the chamber and piston. That is where the fuel falls out of suspension, creating a vortex. In this condition, the fuel burns, but it burns too slowly and too late in the cycle to create usable cylinder pressure.

Usually the burn pattern that we see on the chambers and pistons mimics what we see on the wet flow bench at the upper ranges of valve lift. For example, our Pro Stock engines use a cam with 1-inch valve lift, and the burn signature in the chamber is almost identical to what we observe on the wet flow bench at .800-inch valve lift.

[shaun979]

Originally posted by cosworth bdg
Piston flycuts have a large influence on burn-rate of the incoming charge, just look at the v10 of current world champions RENAULT . In 2005 ,this v10 GP engine ran flat top pistons, yet still maintained power along with fuel mileage....

I am not so sure we can infer that much from Renault's strong performance since it is a combination of many other factors too.

[cosworth bdg]

Originally posted by shaun979


I am not so sure we can infer that much from Renault's strong performance since it is a combination of many other factors too.

Iwhole heartely agree with your statement .

[desmo]

Originally posted by cosworth bdg
Piston flycuts have a large influence on burn-rate of the incoming charge, just look at the v10 of current world champions RENAULT . In 2005 ,this v10 GP engine ran flat top pistons, yet still maintained power along with fuel mileage....

May I ask on what, if anything, you base your assertion that Renault ran flat top pistons? I'm skeptical to say the least.

[McGuire]

Originally posted by shaun979


Ah thanks.. I finally recalled where I had read about carbon reading and it was off Reher Morrison's site in an article by Darin Morgan. Excerpt below...

Yep, and here he is also talking about the new Mondello wet flow bench. I have been promised a look at the thing in operation and I am really looking forward to it.

[McGuire]

Originally posted by desmo


May I ask on what, if anything, you base your assertion that Renault ran flat top pistons? I'm skeptical to say the least.

I can't see how they could ever get enough compression.

[desmo]

Originally posted by McGuire


I can't see how they could ever get enough compression.

Exactly. It simply isn't an option. The question was rhetorical as I don't expect a reply, I'm reasonably sure the original assertion was based either on a misunderstanding or baseless.

[TestaRoasta]

He just wanted attention, lets give him a hand for playing

http://blog.gtroc.co...archives/cat57/

[NTSOS]

TestaRoasta

http://blog.gtroc.co...archives/cat57/

I clicked on the above URL and while interesting, some of the text is
replaced with square boxes............the photos are also great, but the
descriptions are again, mostly square boxes.

What am I doing wrong!

John

[desmo]

Originally posted by NTSOS
TestaRoasta

http://blog.gtroc.co...archives/cat57/

I clicked on the above URL and while interesting, some of the text is
replaced with square boxes............the photos are also great, but the
descriptions are again, mostly square boxes.

What am I doing wrong!

John

It's Japanese. If you've got Google Toolbar, just right click on the field of the page and select "Translate into English" Most of the F1 engine bits appear to be from the Ferrari F399. All the thumbnail image links appear to be dead however. There's something about halfway down the page relating to Renault, video I guess, that might mislead one into thinking the Renault uses flat top pistons. Maybe, it's another apparent dead link.

Here's a link to the translated page:

http://translate.goo...archives/cat57/

[NTSOS]

Ok....desmo, thanks so much!

I found this on one of his links!

What an interesting suspension!



You just gotta love the Carrera GT

John

[cosworth bdg]

Originally posted by desmo


It's Japanese. If you've got Google Toolbar, just right click on the field of the page and select "Translate into English" Most of the F1 engine bits appear to be from the Ferrari F399. All the thumbnail image links appear to be dead however. There's something about halfway down the page relating to Renault, video I guess, that might mislead one into thinking the Renault uses flat top pistons. Maybe, it's another apparent dead link.

Here's a link to the translated page:

http://translate.goo...archives/cat57/

Renault do use flat top pistons ,Regards BDG

[desmo]

Without a virtual reinvention of the 4 valve head it's imposssible in F1 given the need to keep a reasonable CR, the valves need to protrude well below the gasket plane near TDC at lifts and overlaps sufficient to flow at high rpm.

I'd welcome an explanation of why what I say isn't the case of course...

[Ray Bell]

They used curved valve stems?

[malbear]

http://img83.imagesh...43/5valv5qq.jpg






DESMO

"Without a virtual reinvention of the 4 valve head it's imposssible in F1 given the need to keep a reasonable CR, the valves need to protrude well below the gasket plane near TDC at lifts and overlaps sufficient to flow at high rpm.

The ideal combustion chamber shape is a sphere with an ignition source in the center. Since this is mechanicly impossible the next best thing would be a hemisphere with a spark plug in the center at the top, as in a twostroke head with an anular squish band and no protrusions or hot exhaust valves or flycuts.

[NTSOS]

[Halfwitt]

Originally posted by cosworth bdg
Renault do use flat top pistons ,Regards BDG

Maybe in a Laguna or a Clio. If they use flat-topped pistons in their F1 engine, I'll eat my hat, and yours for desert.

[desmo]

Originally posted by malbear

DESMO
The ideal combustion chamber shape is a sphere with an ignition source in the center. Since this is mechanicly impossible the next best thing would be a hemisphere with a spark plug in the center at the top, as in a twostroke head with an annular squish band and no protrusions or hot exhaust valves or flycuts.

Totally agree there. Hard to top a proper two stroke SI engine's combustion chamber's geometry. Four strokes necessarily get pretty compromised there when B/S gets too oversquare and valve lifts near or exceed half the IV diameter. The present designs, however imperfect theoretically, at least get the job done.

[desmo]

Originally posted by NTSOS

Here's a shot I took of the 1989 Ferrari F1 engine, you can see the three intakes through the (rather odd) throttle opening:

http://members.atlasf1.com/desmo/

I've got another pic I took of an experimental desmodromic 5 valve F1 head Ferrari did about somewhere...

From what little I've read the advantages of five valve heads are most readily seen at low valve lifts where the total intake valve circumfrences (greater for a five valve all else being more or less equal) are most telling on the discharge coefficients, and once the valves get much off their seats the four valve solution regains the edge.