Gas Ported Pistons

[Brian Coat]

HardingFV32, now that I actually looked at a few pics of NASCAR pistons, I saw no vertical ports. So your research may well be right. EDIT: I have seen lateral gas ports on pictures of NASCAR pistons though.

Also, maybe I'm making my point wrongly to say NHRA PS is like a spec motor. Its a very stable rule book and pretty restrictive, so everyone has picked all the low hanging fruit and is looking for tiny gains. So they try stuff like this.

Rather than wondering why F1 people don't use vertical gas ports I'm wondering why they'd need to or want to?

[olefud]

HardingFV32, now that I actually looked at a few pics of NASCAR pistons, I saw no vertical ports. So your research may well be right.

Also, maybe I'm making my point wrongly to say NHRA PS is like a spec motor. Its a very stable rule book and pretty restrictive, so everyone has picked all the low hanging fruit and is looking for tiny gains. So they try stuff like this.

Rather than wondering why F1 people don't use vertical gas ports I'm wondering why they'd need to or want to?

Clearance at the top of the piston causes dead (no burn) spaces and compromised stabilization of the ring. Gas ports cause less of the former and don’t affect ring stabilization, cooling and heat transfer.

[hardingfv32]

wondering why they'd need to or want to?

This is would require a well design test of a gas port system which I have never seen published. While drag racing engine builders are very capable, I do not think any of them carry out research at the level required to valid a gas port system.

I suspect that most engine manufactures have developed data indicating the correct amount of cylinder sealing vs ring drag. That the current ring and piston designs allow them to meet these requirements.

Brian

[riff_raff]

Go back and take a look at the first photo in the OP. If you look closely you'll note there is a narrow groove cut into the land face between the first and second compression ring grooves. This is a "buffer groove", and its purpose is to provide added volume behind the top compression ring so that the combustion gas leaking past the ring end gap does not result in enough of a pressure build-up between the first and second rings to cause the top ring to lose contact with its lower groove face (ie. "flutter").

[Brian Coat]

This is an interesting overall review on the subject of racing piston rings, with comments relevant to F1.

http://www.ret-monitor.com/articles/wp- ... /rings.pdf

[hardingfv32]

I have also seen research indicating the groove between the first and second compression ring grooves is for oil accumulation.

I have never seen actual tests verifying the effectiveness of this combustion gas accumulation zone. I have always been suspect of the theory.

Brian

[Brian Coat]

I have also seen research indicating the groove between the first and second compression ring grooves is for oil accumulation.

I have never seen actual tests verifying the effectiveness of this combustion gas accumulation zone. I have always been suspect of the theory.

Brian

Hopefully this will be of interest.

MIT have measured it working dynamically on an engine with highly instrumented pistons.

The RET (Race Engine Technology) article I posted, includes discussion of 2nd land grooves and how they work.

Mahle offers the following in a book on pistons and testing which they edited/co-published with MTZ (Motoren Technische Zeitschrift). "Brief periods of excessively high pressure between the first and second rings can cause the first piston ring to be forced away from the lower flank of the groove and thereby increase the lube oil consumption due to “reverse blowby” A change in volume between these two rings in the form of grooves chamfers or recesses can lead to faster and more immediate pressure equalization in the intermediate ring space"

Having seen various brands of piston-design snake-oil, I'm quite sceptical too but if RET, Mahle and MTZ say something is real and MIT have measured it and then I start to get convinced.

[riff_raff]

I have also seen research indicating the groove between the first and second compression ring grooves is for oil accumulation....I have never seen actual tests verifying the effectiveness of this combustion gas accumulation zone. I have always been suspect of the theory.

The principle of adding buffer (or accumulator) volume between the 1st and 2nd compression rings to minimize flutter is valid, and it has been verified by both analysis and testing. But as to how much benefit it provides depends on the specific operating conditions and design of a particular engine. Engines with high cycle pressures (turbocharged, supercharged) and/or high piston acceleration rates (long stroke, high rpm) would obviously benefit more. But it can be difficult to provide additional buffer/accumulator volume between the 1st and 2nd rings on race pistons with minimal deck height.



Compare this to the spacing between the 1st and 2nd ring grooves on a modern high-performance diesel engine piston that has more vertical space available:



If you want to understand how a buffer/accumulator volume is effective at minimizing ring flutter, first plot the combustion gas flow back and forth across the orifice area bounded by the top ring end gap in the tangential direction and the clearance between the lower groove land and cylinder wall in the radial direction, between TDC and BDC of the combustion stroke. What you'll find is that due to the much higher delta-P across the ring gap orifice just after TDC vs nearer BDC, the peak gas flow rate is much greater in the initial direction than in the return direction. Then plot the net axial gas pressure force acting on the ring against the axial inertia force and the radial friction force on the ring over a cycle to see if/when the bottom ring face loses contact with the mating groove land surface.

[hardingfv32]

What would you say the white dots are in the accumulation groove in the above illustration? It would seem to me that the best way to relieve pressure in this zone would be ports through the piston into the crankcase zone. Would not access to good crankcase vacuum be a benefit to the accumulation groove? I have never seen this port design before on any piston.

Brian

[riff_raff]

Don't know what those white dots represent. It would not be a good idea to have vent holes in that location since it would allow lube oil to migrate around the oil control rings on the intake down stroke and would result in excessive oil consumption.

Here's an image of the accumulator groove on a typical race piston:

[gruntguru]

Don't know what those white dots represent. It would not be a good idea to have vent holes in that location since it would allow lube oil to migrate around the oil control rings on the intake down stroke and would result in excessive oil consumption.

Not to mention rendering the second ring useless as a compression seal.

[autogyro]

Don't know what those white dots represent. It would not be a good idea to have vent holes in that location since it would allow lube oil to migrate around the oil control rings on the intake down stroke and would result in excessive oil consumption.

Not to mention rendering the second ring useless as a compression seal.

Perhaps they are blind and to increase the groove volume?

[safeaschuck]

I believe vertical gas ports foul up and become blocked too soon to make them useful for much other than drag pistons.

[riff_raff]

Take a look at this 4-stroke, 4V race piston. Note the radial grooves around the top land between the top ring groove and the piston deck. This arrangement of grooves/ridges is designed to reduce the gas flow thru the gap between the top land and cylinder wall. Of course this is only helpful if the dynamic forces acting on the top ring do not produce flutter given the reduced gas pressure.

[Brian Coat]

The clogging is a concern but probably not as great as we think - e.g. midget racers use them and only swap pistons after 100+ (?) miles.

I'm more inclined to think they are not used because they are not beneficial in most applications, including F1.