PRV V6 combustion burn characteristic

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M Faulks
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PRV V6 combustion burn characteristic

Post by M Faulks »

Guy,

I would appreciate your advice on the burn characteristic from the photos below.

Background:
This engine is the 2.5ltr turbo V6 PRV block utilised in a Renault Alpine 310 rally car - in this case it has been setup as twin turbo, and up to 1 bar over-pressure. The engine needs rebuilding, purchased from ex-racer and driven for around 200 road miles of varying road conditions before pulling down. The engine had a fresh build before, but stored for many years and various problems were encountered in the short shake down tests. The carbon signature is from these few road miles, and does give a reasonably good indication of what is going on I believe. The fuel mixture has not been calibrated from the settings it was received and this will need sorting as separate issue. The engine is a long rod (>2:1 ratio), short stroke 63mm, 91mm bore, and late ‘60s / 70s design.

The head and pistons are standard components, there has been some porting performed but I don't think significantly affecting anything related to the burn efficiency / tumble / swirl characteristic. The piston has a small raised crown, but doesn't encroach into the chamber significantly; the piston is below deck height at TDC.

Brief:
From my understanding when I pulled the head I was surprised to see what I understood to be quenched combustion at the extremity of the chamber behind the plug. Again from my understanding of what I was looking at, likely little combustion turbulence given the very localised burn carbon signature. I believe the combustion kernel growth is towards the hot exhaust valve given the higher activation energy of the entrainment, and the opposite case on the inlet side where it looks almost quenched. I assumed the over-rich mixture would have slowed combustion events, but should I expect the combustion characteristic and extent to be significantly different if this were closer to ideal?

Is my understanding correct, or am I misreading the information? Indeed, can I really read from this reliably, although all the cylinders display similar trait? The only difference I noticed is that in the hotter cylinders the carbon trace did extend further out. I do appreciate the idle running could significantly modify what can be deduced, as a more scientific full power run and engine cut wasn’t performed.

Am I correct in understanding there is little in the way of combustion turbulence here, and that with more rapid and complete combustion (and hopefully) the yield of power could be improved?

Given the intake runner and short inlet port design in the head, I believe there is little to be gained here, so is there anything else I could potentially try?

I note the Peugeot 205 GTI 1600 has a similar plug boss arrangement, and had thought of extended nose plugs as a possible option, but likely only small improvement?

Any advice, thoughts or insights would be much appreciated.

Thanks
Martin
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Guy Croft
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

Well considered post, Martin..!

My first thoughts are that the absence of any squish might be the cause of the concentration of the burn in the middle of the chamber, however it equally as well could be that the carbon deposition is nothing to do with flamefront development (which may actually be quite good) and is in fact the result of reverse flow of ex gas due to high back-pressure coupled with high lift at tdc on the overlap event. I say this because that (black) deposition is rather well confined to the two valve regions...

A picture of the piston might be helpful.

GC

edited 1235hrs 01 Oct due to typo (bold)
M Faulks
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

Guy,

Thanks, this seems more likely. I will measure the cam lift curves, overlap / lift and report back. Actually I would like to discuss this engine further, as you know I have spent sometime on these. In the later 3lr n/a versions the combustion chamber is very similar in shape and form, however, the plug boss is projected than recessed, and hence, the plug electrode is much further into the centre of the chamber.

The inlet port is the same long, tall drop SSR as the Peugeot 106 you are already familiar. 30 deg seat, with a full radius insert. I would like to discuss the head porting and manifold arrangements, I have moulds of the ports to aid in discussion. I know this is slightly off-topic, so if you prefer I could start a separate thread in the road / competition page or where ever appropriate to present the information I have gathered thus far?

Thanks,
Martin
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Guy Croft
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

Keep it here Martin, it's an interesting one.

Thanks for the piston photo, just as I thought, not much squish there betw head and crown.

30 deg valve angles are useless here, they need to go for a start.

Please note my typo above.

G
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

Thanks Guy, apologies for this one being out in the cold for so long. Well this project has been taking some time, and I started another PRV engine build in between this one as a test mule effectively. Hence, I have been collecting more evidence, before commencing a maximum effort engine build, and I have been finding consistent results across the engine series. There are 3ltr and 2.5ltr head castings in the 2V form, and I have used both to gather similarities and any differences that would give me clues. Anecdotally I have noticed that on setting ignition timing for both, the 3ltr version required a lot more ignition timing to make torque, albeit it has the shorter connecting rod and hence ratio. As a side note, Alejandro in his excellent post on the Renault Fuego, here: viewtopic.php?f=4&t=2135 may have a similar issue, but likely less pronounced. The chambers are of the same basic form, however the chamber is obviously enlarged for the 3ltr with the 93mm bore opposed the 91mm of the 2.5, and further, the valve sizes are larger. The valve inclination is the same in both heads.

The 3ltr does have the advantage that the valve seat angle is 45 deg, reference earlier comments Guy.

In forced induction trim, and OE valve timing the events have very little overlap, inlet and exhaust 0.18mm at TDC - forced induction profile. I don’t see many signs of reversion in the inlet port, so believe that this is limited. I do see however, the fuel boil present from the simple batch fire of the injectors, and I believe that poor fuel atomisation is another issue. Looking at the photo below, I think the distinct clean triangle shape on the chamber is fuel wash, would this seem reasonable?

Image
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This is a standard 2.5ltr head (standard fuelling and normal road use, just an engine I have parted), and with the OE 30 deg seat angle. I would suspect 45 deg inlet seats would help keep the fuel in suspension in the air stream (wet flow), but this is my summation than anything I have gleaned any evidence.

I have another series of photos from engines I have again split, this time 3ltr. The following series is taken from the heads that were quickly checked on your bench. Looking at the burn characteristic I think there is much for me to learn here, and a few mistakes. I wish to point out that the fuelling was not optimised and gasket issues caused oil and water leaks. This is evident; the cause was over ambitious chamber modifications on my part, and using two different manufacturers gaskets for the ones use to open the chamber, and then those used finally to make the build. So some lessons there that I have taken on board, however, I believe there is still valuable information to see regardless of the impact from this.

Image
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I think that there is likely a similar chamber wash / wet flow issue with the 3ltr head. Increased fuel flow for increased engine performance appears to exasperate this (the injectors are very close to the back of the inlet valve, and single high flow fuel injectors are probably not the best configuration). I had considered going to a 52 or a 55 deg inlet valve seat to control the fuel separation from the flow, but without wet flow tests this would be difficult to assess. Further, I suspect I would need increased lift to regain flow. Guy, any suggestions you can make here, ideas or directions, especially on the fuel burn?

Image
Image

There is an interesting characteristic swirl pattern around the bottom of the inlet valve close to the blind side of the plug. I believe the lack of carbon is simply the lack of burnt fuel, or lack of available atomised fuel to burn in this region (or possibly quenched). In some ways suggests that airflow in this head is less critical than achieving and keeping an atomised and homogeneous fuel distribution to achieve better performance. As you mentioned before Guy, there is a lack of squish, and likely very hard to achieve as well. I had been thinking on the lines of actually keeping the piston crown well away from the chamber contours to promote better flame travel, and accept there would no squish present, but giving a faster less impinged flame front instead. The piston crown photo is taken from another engine of the same series, and this exhibits the same characteristic on the inlet side, lower part of the picture.

Image

I have spent a number of hours tuning these units now, and detonation is easily achieved. Looking at the information from the pictures, and given the chamber and piston design where small crevice volumes are created, I believe its likely free radicals are probably poorly scavenged, and thus leaving residuals each cycle. These then form centres in the end gasses to initiate detonation. The following piston crown and side photo show the inlet side where the piston ring lands have collapsed squeezing out the oil scrapper through detonation. These photos were taken from a failed engine from another club member, and used with permission.

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I’m in the process of building another engine for a track car I intend to compete, and the heads are prepared, but looking at this again, I’m a little reluctant to move to assemble if I need to make changes.

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I had considered chamber welding, but think this may be a little hit or miss in having the desired effect ultimately; especially as I’m not sure what would be the best model to take as baseline design to copy. I’m open to advice, as I believe I’m reaching the limits of my own immediate technology, and possibly seeing the limits of the head and its configuration when originally designed?
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Guy Croft
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

I think you're placing too much emphasis on the injection per-se.

I am not sure one can enhance the inj phase by altering the valve angles but I can tell you that except at very low lifts (<4mm) 30 deg seats are hopeless for flow in case I have examined (Peugeot 8v, Volvo 8v Alfa 8v - see att). If the air gets in better (!) one can reasonably assume the gasoline will too. The scouring near the inlet valve is certainly indictative of fuel spray at that locality but why there is so much carbon deposition at all is another matter. Burn issue?

One thing that can certainly upset injection and the burn too is the in-cylinder state at inj and during firing/exhausting and it is possible that the header (ex manifold) setup is causing errant ex port/primary wave effects cylinder-to-cylinder, ie: one cylinder interferring with another. That can cause, for what it's worth, such random cycle-cycle firing that it's impossible to calibrate the engine with any degree of accuracy and only temp measurement in each primary pipe (during testing) can point you at that cause. High static ex back-pressure (never mind waves - which are sonic) will cause similar chaos. By random I mean the cylinder never fills the same twice, nothing like.

As for detonation it is 'effect' not 'cause' and the fundamental cause may be over-lean burn or engine overheating or even the very random effects mentioned above. Of course if the gasket cannot contain the normal cylinder peak pressure it will lift or blow and let water in but if det is present it will blow or a piston will get fractured or both. I see little point in altering the cc by welding (for more squish) to try and stop detonation - although I accept you think perhaps it may improve burn. But if the burn is being upset by the ex waves that won't help.

Hope this offers some clues in the first instance.
Attachments
old Alfa Spider 8V TC head on test &amp; dev at GCRE, see how the 45deg inlet seats massively outperform the std 30 deg at the more important (higher) lifts. This head also suffers from very poor ex flow in std trim (about 63% E/I). When you mod these heads you MUST start with the ex ports and base the inlet flow target on that to get E/I 72% or BETTER. Inevitably you find the inlets don't need significant alteration, except on the SSR. The OE guide loss is massive too.
old Alfa Spider 8V TC head on test & dev at GCRE, see how the 45deg inlet seats massively outperform the std 30 deg at the more important (higher) lifts. This head also suffers from very poor ex flow in std trim (about 63% E/I). When you mod these heads you MUST start with the ex ports and base the inlet flow target on that to get E/I 72% or BETTER. Inevitably you find the inlets don't need significant alteration, except on the SSR. The OE guide loss is massive too.
SS Alfa 8v TC flowtest.JPG (80.38 KiB) Viewed 16055 times
SS Alfa_ v low ex flow as expected.JPG
SS Alfa_ v low ex flow as expected.JPG (119.66 KiB) Viewed 16055 times
M Faulks
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

Thanks Guy, I really appreciate your input and insight, this has really triggered me into some further thinking. I think you are most probably right. The engine is out of the car at the moment, so I could pull the heads, and further weld in EGT bosses for all 6 headers. Essentially the manifold is a log with short connecting branches, and the effects you mention must play a major role.

Is the Volvo 8V similar in design to the PRV with respect to the chamber and valve arrangements? Any flow data that I could compare the previous numbers we took?

The OE inlet valve lift on the 30 deg seat is 8mm, and my current cam spec is lifting to 9.5mm. So the flow and lift ties in quite well with the convergence on your Alfa graph. I suspect the added reversion at low lift isn’t especially beneficial as the performance envelope is pushed either?
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

A log type exhaust manifold is a very cussid thing to work with. Such types behave as a damper so ruining any beneficial wave behaviour. Thus one has to be rather, er, 'circumspect' about ex valve event, or to put it another way - run very conservative ex overlap and cam timing.

If at all possible junk them and go to a pair of 3-1 tubular headers with min 27" pipe chord length. Even if it doesn't produce a perfect engine in the first instance you will have a core unit that is much more repsonsive to tuning of all types - hardware and calibraiton.

G
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

Thanks Guy. Yes agree, and the header length you mention is very similar to what I had in mind for a second build, a number of 24 - 26 inches came to mind from before. The aim is to get closer to a genuine 400hp forced induction motor in 2.9ltr form, with broad useable torque spread than chasing necessarily a high headline power figure and poor average torque. I had in mind stretching the stroke out to the later crank gaining an additional 10mm, giving 73mm. What header pipe diameters would you suggest as a starting point?

Should I keep this thread here or move it? I have been reviewing mid range turbos (350 – 450hp capable, various compressor and turbine combinations), and was going to do some back to back testing for transient performance. It would be interesting to gain experience from others as well. The rules for the particular class of racing limit the engine arrangement to that intend by the manufacturer, so although a natural change to twin on the V6 would be better for transient performance, this would not be allowed given the original arrangement was single. Current plan is to get the car race ready on the current hardware, and then build a second engine as the package needs to get more competitive.
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

Primaries - valve throat diameter (or port diameter if bigger) plus 5mm.

BTW - You will never be able to refine the log setup without advanced simulation and then all it will tell you is you are wasting your time!

G
M Faulks
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

Guy thanks for the previous recommendations.

I have been quietly working away on this one, and I did get the bi-turbo finished. So now back on trying to develop the unit performance in forced induction form, and specifically the heads and burn.

I did some quick ink blots, best way I could think of describing it, as the following pictures show, and I believe this confirms the burn patterns I posted previously. The “fuel” is thrown straight on making little attempt to turn, and indeed straight at the guide seemingly from where ever I introduced the ink mist at the port entry. I haven’t yet repeated the test with the original 30 deg valve and seat arrangement, probably pointless, but this is the pattern of a head modified to a 46mm inlet valve from the OE 44mm, and 45 deg seats as discussed. I now have a peak flow at full lift of 119 cfm taken at 10”, and I will graph in Excel and post the measurements I have taken for the different stages. However, much to catch up, but I will fill in the details slowly as it has taken me a while to get here. As an aside I did take some quick videos of flow flag activity in the port and have uploaded to YouTube that are not public listed, I could add the links if that would assist examination if appropriate? I have some flow questions as well that I feel are likely somewhat related, as I think to push on the performance of this unit I need to gain mixture motion balanced with flow potential.

The “fuel” pattern seems to proceeds straight for my test cylinder wall, and doesn’t make any turn. It’s not really surprising given the velocity over the SSR, as you know it is the same form as the other PSA / Douvrin engines of that generation and especially the Peugeot 8V. So any advice as how to improve this, or pointers to investigate would be much appreciated. I have D shaped the bottom of the inlet port to reduce the velocity over the turn, flatter wider section on the floor of the port. Picked up some useful higher lift flow, but didn’t have much if any impact with where my spray pattern ended up.
attempt to get a fuel flow pattern, and see if this corresponded with the chamber carbon and wash characteristics above
attempt to get a fuel flow pattern, and see if this corresponded with the chamber carbon and wash characteristics above
ink pattern.jpg (97.98 KiB) Viewed 15435 times
corresponding pattern on the back of the valve
corresponding pattern on the back of the valve
valve pattern1.jpg (57.94 KiB) Viewed 15435 times
same as above, some bias present
same as above, some bias present
valve pattern2.jpg (63.01 KiB) Viewed 15435 times
attempt to form a single trace line across the SSR.  I believe this likely corresponds well with the characteristic clean (fuel washed) area seen in the carbon on the chambers above
attempt to form a single trace line across the SSR. I believe this likely corresponds well with the characteristic clean (fuel washed) area seen in the carbon on the chambers above
trace line.jpg (86.17 KiB) Viewed 15435 times
I’ll come back to these points with more information; however, I have been looking at options to achieve a tighter combustion space with squish areas. It remains one area I would like to attempt to improve or at least trial to gain some information and learning with this unit whether good or bad. One option was welding the chamber as mentioned earlier, heat treat and then machine and match to an appropriate piston. Another that I recently rolled around my mind was to mill the chamber inside the fire ring diameter to give a flat land area to form two squish zones plug side and opposite the plug where the chamber is closest to the piston. I have a set of high CR unfinished flat top pistons intended for a Pinto application that are on an over-bore size, which matches to the PRV liners with a clean-up bore and hone. Matching these with suitable rods I could place the piston crown of this assembly 5mm above the deck (excluding the gasket). This is beyond what I have done in the past, where typically this has been in the order of 1mm. This would notionally allow me to pocket machine the heads to match the piston protrusion (and necessary stretch and rock clearance to suit), and thus gain squish area and a more compact combustion space located tight to the plug and valves. It does appear that there is sufficient wall thickness, but would need to check in detail. I would need to machine a slot across the crown to set CR and valve clearance. Areas of concern:

1. top ring protection / endurance, as it would then be 3mm from the top of the deck where the ditch cut runs across for valve clearance (two sections of the periphery).
2. potential issue of the crevice volume now consisting of the top ring area, and that of the piston protrusion into the head from 2.5 to maximum 3.5mm depending balance between piston crown machining, and depth of pocket machined into the head to achieve squish land area. This would require test machining to see whether it was even worth doing I guess. I have spare heads I can trial.

I guess my question is does this even sound sensible? The crevice volume and potential increased detonation sensitivity concerns me, this may be unfounded but I have nothing to say whether it would or wouldn’t be in practice? I think I can deal with the top ring issue, or certainly accept that it would be a concept test for a dyno trial, and then get appropriately machined pistons for a second build. But open question, although possible, is it worth going that far and testing?
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M Faulks
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Re: PRV V6 combustion burn characteristic

Post by M Faulks »

I missed a couple of pictures that may add a little more information. The wet-out pattern in the carbon sits with quite good alignment with the convergence pattern below, accepting the limitations of my experiment and the steady nature of the flow in the inlet tract.
pattern on chamber after a few more trials, wet-out at the point the streams collide
pattern on chamber after a few more trials, wet-out at the point the streams collide
convergence pattern.jpg (71.32 KiB) Viewed 15408 times
with matching valve
with matching valve
convergence pattern matched with valve.jpg (66.89 KiB) Viewed 15408 times
Then a colouration pattern in an old head of the same type (unknown porting, but probably not too significant in this respect):
an old head with witness colouration after some running
an old head with witness colouration after some running
colouration of an old port.jpg (58.17 KiB) Viewed 15408 times
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Guy Croft
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

Post the links sure.

Nothwithstanding your reasoned post & methodology you are worrying unnecessarily about detonation. If you keep the charge temperature at 35degC max it won't happen with that chamber. I would certainly not try and induce more squish: the burn rate will be so much higher anyway with the pressure-charging. Fuel octane v boost is an issue of course. Achieving 35 deg will tax you plenty. If it tries to knock pile in the gasoline to cool it down and retard the igntion - fast. Measurement of individual pipe EGT will be essential.

As for the direction of flow - interesting - but there is nothing at all you can do about it. Efforts to divert the flow in a preferential way will only lead to a general loss of mass flowrate. At least you can wind up the pressure to compensate. This is common to every head I have worked on incl styles like yours.

My strong advice is bang it together and get it on a dyno.

G
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Re: PRV V6 combustion burn characteristic

Post by Brit01 »

If you keep the charge temperature at 35degC max it won't happen with that chamber.
Are you referring to the air intake temperature here Guy? This summer we had 55 degrees in the sun and near the surface of the road. 39 in the shade. Under those conditions I guess we just need to drive smoothly and softly without any excessive load.
My strong advice is bang it together and get it on a dyno.
'Bang it together'? Sir Guy I can't imagine in a million years you would ever bang an engine together. Lol.
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Re: PRV V6 combustion burn characteristic

Post by Guy Croft »

Haha Chris - join my Facebook and meet the real hacker behind the myth!

G
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