May 2012

Tuesday, May 29, 2012

DynoTech : Kevin Cameron-the history of combustion chamber design, and applying that info to help engine performance

From Jim to Kevin Cameron:


If it's not too late, could "goin with the flow" [the name of the next tech article KC is writing for us explaining how fuel vaporization happens] also address sleds with EFI with injectors fitted to the throttle bodies, and maybe compare how fuel is vaporized coming out of an injector into the airstream compared to carbs like your original goin with the flow addressed? I think that perhaps half of the hotrod sleds we tune here are EFI and we need to understand the difference, if any. I cringe when guys come here to tune sleds with bored out EFI throttle bodies. We can spend hours power commander tuning part throttle fuel because the big throttle bodies add lots of air at 1/8th to 1/2 throttle compared to stock size, then often add zero air and zero HP at 100% throttle (maybe even less?). And probably those big throttle bodies with lower velocity at the same CFM do a worse job of vaporization. So we waste many hours of machining/ dyno time for zero power increase which is no fun, but helps pay the mortgage.


I can't recall if I told you about AMA #2 Kenny Coolbeth coming here from CT to have a local engine guy Ron Jewell (RLJ Race engines) modify his Kaw 450 flattrack bikes (Kenny had seen some good running RLJ bikes). His previous dyno guy was making him "happy" dyno sheets but Kenny was trying to race with unhappy engines. Kenny came here with I think a 57 DJHP dyno sheet from the engine guy from NH, and the 49ish DJHP we made here with that same bike matched the way the bike was going down the straightaways. Ron Jewell has modified Kenny's new bikes, making 57ish DJHP here, but with similar porting/ cam timing/ compression ratio can't quite match the power Ron gets from the Hondas and Yamahas (low-mid 60's), even though his flowbench tells him the Kaw CFM is nearly identical to his mod yamahas and Hondas. The Hondas and Yamahas have flat HP curves from 9000-11000 and the Kaw peaks at 9000ish then drops significantly as revs climb. Ron says the Kaws have higher piston domes than the others, w/ similar comp ratio. Early on with Kenny's 450 Ron dropped from 13.8/1 to 13.2/1 with less dome intrusion into the combustion chambers and picked up 4hp so he plans to go even lower. It reminds me of the time I bored my stock GS1100 out to 1150 with 11/1 domed pistons instead of flat 10/1 pistons and slowed the bike down. Bigger bore and domes pistons = less HP! I'll let you know Ron Jewell’s KX450 project works out. Regards, Jim C


From Kevin to Jim:


British racing singles of the 1930s started life with flat-topped pistons and compression in the 4.5 - 6-to-one range. This allowed them to pretty much copy the hemi 2V chamber pioneered by Fiat in their 1922 GP car engine, setting the two valves at a 90-100 degree included angle. But fuel octane number rose through the 1930s as Britain legalized use of the violent poison/antiknock tetraethyl lead and combined best available leaded gasoline with 50% "benzole", which was a by-product of coke production. Benzole was a catch-as-catch-can mixture of benzene, toluene, and xylene - all highly anti-knock aromatic compounds. With all that new ON, compression ratios could go up. It was a lot cheaper to make a new piston than a new head, so up, up went piston domes. Combustion had never been all that fast in OHV engines - there was no way to have squish as in flatheads/sidevalves. Along came Harry Weslake with a little help from the tangential intake port, which converted high intake velocity into rotary swirl around the cylinder axis. THis "stored" intake energy for later use as turbulence to accelerate combustion. Now a little math - making a true hemi chamber exactly doubles the surface area of the combustion bowl, as compared with the area of a disc whose diameter is the cylinder bore. And adding the matching piston dome did something similar to piston crown area - increased it a lot. This put piston temperature up, discouraging people from increasing bores and decreasing strokes for a long time. In 1950, here came Pole Leo Kuzmicki, working for Norton. If you imagine the tall piston dome as being made of fudge, he essentially pushed it down, forcing it outward, closer to the head surface everywhere except where valve clearance was needed. In those regions he brought the piston as close to the head as mechanically possible, creating OHV squish for the first time. As he pushed the top of the piston dome down, he created room above the now-flat piston top in which intake motion could persist all the way to TDC without being damped out by friction between moving gas and close-by metal surfaces. He transformed the old, slow "half an orange peel" combustion chamber into a faster-burning, much more compact chamber that was basically just the valve cutouts plus spark-plug area. The greatly improved Norton would have defeated the new Gilera-4s in GP racing that year, but Dunlop tires came apart on a couple of fast tracks and prevented what would otherwise have been runaway wins. Norton came back in 1951 and did the job - beating a potentially much more powerful Gilera. People today, in the 4-valve era, forget this great lesson - that just cramming a bunch of mixture up into a tight, badly-shaped combustion chamber and setting it off does not equal power. Or, as the late Keith Duckworth put it, "People are mesmerized by airflow, never reflecting that they must burn all that air and fuel they are getting into their engines." When Duckworth applied the 4V version of Kuzmicki's concept, the result was a flat-topped piston, a narrow valve angle, and a strict separation between as-close-as-possible squish and the most open, roomy combustion space. When Duckworth applied this concept to his DFV V8 GP car engine of 1967, it was able to defeat higher-revving V-12s. In place of Weslake's tangential intake, he biased his intakes to produce downdraft so that air flowed from the intake valves, across to the far cylinder wall, then down to the pison, across its crown, and back up the near cylinder wall. This, which he called "barrel motion",, is now called "tumble". The problem today is that too few builders realize there must be room in the combustion chamber for the turbulence needed for fast combustion. They just add material to the piston wherever it is easiest until they get the 13.8-to-one or whatever ratio their buddies told them they had to have. The piston now comes so close to the head that there really is NO combustion space. Any tumble-generated turbulence is damped out as the piston rises close to TDC, so they are having to use very long ignition timings for best torque. To a certain extent, this compromise must be tolerated, but the Kuzmicki/Duckworth idea has to be kept in mind at all times; make room for combustion turbulence. In some cases, like the truly terrible 5V Yamahas, the compromise really bites, so you can have either acceleration (from high compression that kills flame speed on top, causing weak peak power) or top-end (by lowering the compression enough to get back some top-end flame speed), but not both. When I asked Claudio Domenicali at Ducati how they have been able to shorten stroke again and again and still have competitive engines, while both Suzuki and Kawasaki have made new, shorter-stroke models that were slower than previous longer-stroke versions, he replied, "I cannot speak for other manufacturers, but in our case, we use a device like a small anemometer, placed in the cylinder. Then we vary the intake downdraft angle and port sizes until we get the tumble motion that our experience shows to be necessary." Sure, nothin' to it! Anyway, that is the modern combustion chamber conundrum in a nutshell. It really hurts in F1, where bore/stroke is 2.5, and they end up with ignition timings up in the 60s. Another problem is a social one. Racers don't mind being considered "advanced", but no one like to be thought "retarded". But where combustion is concerned, the more ignition timing your engine needs, the worse its combustion is revealed to be. Some people just can't get past the old idea that needing a lot of ignition advance is good. The reverse is true. A classic example of a bad engine is the old Honda 450 twin of the 1960s. Its tall piston dome and 78-degree valve angle made it into a heat-gatherer, and air just hates to go into a burning hot cylinder. It is delightful to be rid of air cooling at last! I have to go on another trip weekend after this, but am resolved to write the vaporization article you have asked for thereafter. I've just finished writing a "50-engines book", so there is more time available for other things. KC




Update from Jim to Kevin


I forwarded your info to Ron Jewel who has accepted that lower compression might be better if it rids the combustion chamber of much of that annoying dome. As the following graph shows, very little low end torque as lost, but torque and power from HP peak to rev limit was greatly improved (that's where Kenny needs the extra power on mile flat tracks!). This is a great improvement, but the Honda and Yamaha 450s that RLJ mods (both with smaller combustion chambers and flatter pistons at 13.8/1) make about the same HP at 9000, but then continue to climb and flatten out with 63-65 HP right to the rev limiters. And Ron notes that his Superflow Flowbench indicates all three brand ports being nearly identical after porting is completed. So work remains to be done (perhaps in the combustion chamber to copy the shape and dimensions of the others) to make the Kawasakis match the performance of the Hondas and Yamahas. Then Sunday (yes I work on weekends/holidays--mortgages and taxes must be paid!) I had a fellow dyno tune (on my 902 shaft dyno of course) a Kawasaki KZ1000 turned into a 1750cc NHRA pro stock dragbike that seems to have a similar dilemma. The owner expected peak HP to occur at 13-14000 RPM, but after 10,500 RPM, HP disappointingly began to tail off--not unlike the drop we see in the KX450 with domed piston. This drag engine has a huge cast MTC head with hemi combustion chambers that are each surely chock full of dome at TDC to obtain the diesel-like compression ratio these guys run. Perhaps what we have learned from your history lesson, and from RLJ's and my own actual experience can help the ProStock bike owner find the 50 extra HP he needs to be competitive! Thanks much for the great help, as always. Regards, Jim C



KC reply:The drag race guys historically want a lot of compression to snatch the tire loose and get that 60-foot time.


This may be the place where a person has to decide whether a really big project is warranted- i.e., reducing the vlave included angle and moving the valves apart to create a flatter chamber with adequate valve area.


That graph is eloquent! It says what I have tried to explain to myslef and others in the past- that torque gains from compression at low revs can esily turn into slow combustion, heat loss, and fading torque at higher revs. I was at the World Superbike event in Utah at the weekend and hd the opportunity to talk to Herr Gobmeier, who directs the BMW 1000 Superbike program. For this year (when they are finally successful) they reduced intake port sizes to shift breathing down-scale to increase acceleration and result in a less harsh torque curve that tires can tolerate. They have also reshaped their combustion chamber while slightly reducing compression ratio.