Flat-tops or `D`s

Flat-tops/chamber geometry

A subject that, thus far, has not had much of an airing on the Forum.

One thing that has been firmly established though, is that a flat-top piston running with a conventional domed chamber will give inferior results over a domed piston. The plug needs to fire combustion gas at the point of maximum turbulence for maximum flame speed and a rapid and complete burn. Flat-tops with a parallel squish band will not do this. The squished end gases will exit horizontally, a long way from the plug electrodes and max turbulence. The only practical method to get the plug down into the more optimal position is with a full toroid profile to the chamber geometry.

These cad pictures give an idea of what a full toroid looks like. The upper image of the double picture shows the Aprilia chamber as raced, but is more bath-tub than toroid. The lower shows a flat-top toroid chamber using the same volume and comp ratio, the plug is in the correct place but surface area is greater. Always compromises to be made. The lowest of the three images is a water cooled cylinder head, the toroid is easier to appraise in an `actual` context.

On a top flight engine, the squish velocity needs to be up around 30-35mtrs/sec, with a 50% band area, running a clearance of .65mm, closer if you can avoid contact! Piston to plug centre electrode dimension needs to be down to around 6mm! Don`t use anything less than a NGK 7376 10 grade fine electrode race plug, I use a 10.5 and the plug doubles up as an additional heat exchanger. Race engines need race quality plugs, no debate! Assuming around 60* of crank travel from initiation of combustion to the effective termination of burn, we have 300* of travel remaining to shed some heat from the head. Very necessary on any race engine and much more so on an air-cooled with no directional control over cooling air flow. In fact I would recommend a 10 grade minimum for any Bantam race engine, excess heat can be an unpredictable killer and a 10 will cope much better in conducting excess heat away to fins or water.

It has been suggested that the protrusion into the toroidal chamber can leave un-burned residuals at the roof of the blend radius, particularly when the piston is descending and offering an easier path to the expanding flame front. I have no way of confirming this but it`s not difficult to imagining that happening. Combustion is an incredibly difficult subject to analyse, particularly so within the dynamics of a high revving engine where the event is over in what seems a nano-second.

Flat-tops also raise the issue of transfer flow attachment to the piston crown around bdc that cools the piston and assists coherent directional control of the in-coming transfer streams to form the rear, upward column of fresh scavenge charge. A dome provides for this, a flat-top, by and large, won`t. Any thing that keeps a delicate Bantam engine on the right side of self-destruction is to be welcomed.

It has to be said though, there are a lot of combinations of crown and chamber shapes that will work together, but the question you must ask is, would something else be more efficient and make more power? Flat- top pistons only give the best output when in conjunction with a toroid chamber. So is this combination better than any other pairing, that question could be up for debate?

Trevor

Could this be the stimulus for the profile?rofile?

That's not a 'D' - more like a 'C'.

You may be right john, but I am having more trouble with the stimulus than the lettering!

Trevor

Just to keep as wide a spectrum of inclusion as possible this a toroid with a dome!

In our Montreal diesel engine we had the "Re-entrant Toroidal" combustion bowl in the piston which looked like that picture of a woman who was ALL TEETH, you showed and a lot of fuss was made over the VIB -- velocity in RET bowl -- er -- eh? -- now what was it I was saying....?

... something to do with Tractive Resistance or resistance-and-friction -- something like that.

I keep getting messages of people I have known, dying -- two from my old army school just last week -- and I say to myself, if I pop-off sudden like, what can I claim to have done that was worthwhile in my life? -- and having forgotten everything there only be Bantam Racing which I have never been able to forget ---

--- particularly the characters....

Cheers! (Even forgotten my own name!!!....)

John. if you were in the armed services , then i think thats job done mate


so, squish bands.. . 90% of detonation caused by these.. got to be another way to produce turbulance to help with conbustion... although in a conversation with a bantam racer he commented that a particular engine had detoed quite badly yet showed no signs of this when looking at the piston. Detonation or piston clipping head ? suppose either would leave its mark... did the old style ,half tennis ball shaped chamberd head, suffer from this problem?

Mixture in the end zone of the squish band burns later in the combustion cycle. Deto occurs after cumulative heat/pressure cycles, chemical changes have matured to form free radicals and then remaining mixture is pre-heated in advance of the true flame front and spontaneously detonates at sonic speed. All gas events that take place in an engine do so at the local speed of sound, as temps rise so do their relative speeds. It is this effect that scours the piston crown edges of flecks of aluminium and gives the tell-tale physical signs and that are audible enough for a det-counter to pickup. So if combustion can be made fast enough you can out-run this chemically induced and complex process, paradoxically squish speeds combustion!
The only safe squish clearance then has to be Zero, no gas...no deto!

Because the true hemispherical chamber provides for such pedestrian combustion burn and flame speed, the piston has left the scene before deto can set in. The old F1 engines revving to 20,000rpm were much the same, despite the colossal temperatures involved deto was never a problem, as soon as it started the piston had gone, temps dropped and deto stopped. Deto is time related and at 20,000 revs, there is precious little of it.

There are other issues that can bring on deto and all are directly thermal ones, but none are squish related.
Pushing the boundaries of maximum engine power can often leave it on the verge of detonation, doesn`t take much to go over the edge.

Trevor

A couple of seasons back Steve`s RS was `hired as a back up` to a team competing at Donnington in a BSB meeting.
During the Friday practice the team bike rider reported that the det-counter was lighting up like a Christmas tree. The top was taken off the engine but no evidence of deto could be seen, on piston or head, so suspecting the counter itself a replacement was fitted, but the results were the same in the next practice. Lots of head scratch ensued. Six laps into the final session of the day, a gearbox bearing failed!

The det counter was working fine, but the message it was sending was interpreted in the wrong way!

Steve`s RS went out to practice the next day, but the det-counter barely showed any lights?

The team bike was repaired, and raced in the main event, the det-counter behaved itself, but the rider didn`t, he crashed half way into the race!

Such are the vicissitudes of bike racing!

Trevor

Hi Trevor!

Refering to your penultimate post re Det´annat´, the adage was, "She was going so well, I´d say at her BEST, just before she seized up..."

Uh oh! I´ve forgot. What were we talking about ....?

Cheers!

The 125 Aprilia GP engine was set up with .7mm static squish clearance, during racing the dynamic clearance reduced to almost nothing. Revving to 14,500 would make the piston just clip the head, revs were therefore limited to 14,250; perfect! This is perhaps why Jan Theil always maintained that the engine could run all day at max revs in complete safety, free from thermal problems. Mind you, pumping 60 ltrs/min of coolant through must have helped just a bit, but
closing the throttle and easing the load on the engine at those revs brought instant destruction.
Perhaps best then not to have too much slack movement in the bottom end?

1LTR/MIN PER HP is the pumping regime to aim for to keep things cool!



Trevor

A 250hp  Volvo diesel had its Bump-Clearnce [piston crown to head clearance (or squish because the bowl was in the piston)] at 0.65mm plus-or-minus a gnat´s winkle and mechanics and others used to marvel at the lack of carbon on the piston crown ... fact is  that in some cases the piston  crowns were as if clean aluminium  and this was not detrimental to reliabilty ....

That Volvo was one of the "cleanest" diesels in the late 70´s to early 80´s and it was to wonder why other diesel engine companies stuck to 1mm  with + or -  0.2mm limits when the difference between 1mm and 0,7mm bump clearance  had clearly shown a detriment in exhaust pollutants.  

I recall when son Slick was spannering for Austrian,  `Kamakazi Gustav´ (Reiner) GP privateer  he (Slick) complained of spending  several night  hours trying to achieve 0.5mm bump clearance and even when they´d got to 0.55mm  Gustav would insist on trying  again and they´d come up with 0.6 -or even 0,65mm .... I forget which make of 500 twin it was...  They used both plastacine and soft solder as the measuring medium  and Slick could not get through to Gustav that they should pack it in at 05.oo am because they had only an hour before they´d be driving to the GP race venue. Gustav was a tough nut who seemed to need less 2 hours sleep a night ... He mades 12th place   in practice for the senior GP  which was good for a privateer but he discovered that 11th place was held by another private entry so he went out in last session of practice intent on  doing  better. He really piled it up... The bike was a wreck. Slick thought he´d  be driving alone back to Stuttgart but a battered and bruised Gustav was delivered by the ambulance people back to his caravan. The man was in no state to even stand up and Slick expected him to say drive me home but  Gustav instructed Slick to get cracking and sort out the bike whilst Gustav nursed his headache and bruises. There was much to do and Slick had another sleepless night getting the bike sorted. That GP (I forget which one -- but it was in Europe...) was MCN famous for Gustav baulking the leaders when they were trying to lap him...  I think it was Wayne Rainey & his adversary-of-the-day (name also forgotten). Gustav was disqualified with a fine.... I think nowadays he would not have been allowed to race-- much less discharge himself from the medical post.

Cheers!
   .

John,
Old Gustav sounds rather like Steve with the RS, he obsessively swaps base gaskets until the squish clearance is .65mm, almost ignoring the other variables that could be more significant!

Despite it`s importance has anyone ever analysed poor performance and irrefutably attributed it to the squish being a thou out, I doubt it, trouble is spanner men can never win ?

Cheers, Trevor

Right Trevor!
We established that anything  below 0.7mm Bump-Clearance (Squish) on diesels with bowl in piston and toroidal renetrant shape improved exhaust emissions. At around 1.0mm the performace and sfc were the same ...  It was at 1.2mm bump-clearance that one could state there was a deprimental difference in all three -- sfc, power and emissions. What made the diesel unpopular was the particulates in the exhaust and that problem has been revived ...

Slick was still in his teens and idolised Gustav to start with but   this being so finicky -- (as you say, there were probably more important aspects ignored because of it) in  trying for the impossible (and measuring with both plasticine and soft solder) that Slick was happy to get away and work for Jimmy Wells who told me -- over the phone -- that he was knocking my son into shape and had nicknamed him "Slick" -- as in an Ocean Oil-Spill -- because no matter where he was working, or  what on,  the boy  was always standing in a pool of oil...

Later when Slick was interviewed as Foggy´s mechanic he told the interviewer he was called Slick because he was extra-quick, reliable  & secure... There was an instance of  Slick saving an engine when an Italian mechanic "changed the engine oil" between practice sessions and had only  DRAINED the engine oil. Foggy was actaully on his way to the Pits gate when  Slick stopped him and put things right. I remenber John Koshinsky (spelling?) was also a bit unkind about Italian mechanics in MCN....  

Cheers!

John ....

In the early TZ 250 head I saw the `squish band` was non existent. Well, about an eighth of an inch. There were, of course, attempts to fill and reprofile them employing a conventional squish. The result was severe and very destructive detonation. It made me wonder about the effectiveness of squish at all. I thought that, providing the gas was compressed within the limits of the design overall; ignition timing, r.p.m. gas used, water or air cooled and considering the dangers of overheating, all should be OK. The design still raised enough questions that I never answered until recently.(The majority of my work was on production and racing machinery of all kinds. I never saw a head that was unusual, just some of the volumes) My grandsons are scooter and DT enthusiasts and I get to see their standard and race kitted motors. The DT head answered some of the questions It looked conventional except for the squish band which had been scooped out as though to fit a 1/4/" sealing ring. I wasn`t allowed to measure it but it appeared to answer those basic questions. If it is useful for racing I wouldn`t know. The local Yamaha agent may be kind enough to let you have a look at one. All the best. KISS

Welcome to the Forum KISS,
First post at the age of 85, many congratulations, I hope this is the first of many contributions, you must have many an interesting tale to tell.

Fortunately technology has moved on rapidly from the early Yamaha days. Today, using the high combustion turbulence that an active squish action can provide, power levels have risen sharply, with the measurable squish velocity making a vital contribution. It is now considered an absolute essential, with the norm being a 50% band.

Regards, Trevor