Sunday, April 21, 2013
On Apr 20, 2013, at 5:56 PM, Baldur Gislason <firstname.lastname@example.org> wrote:
On the latest post about listening for deto, I figured I'd chime in.
Saturday, April 20, 2013
Our copper tube detonation sensor—which functions much like a stethoscope—has been the subject of much discussion with DTR members who operate their own engine and track dynos. This will provide clarity and understanding about how we use it. Sean Ray installed the copper tube here several years ago. He’s a calibration engineer at Delphi in Rochester, NY, and learned of this from Korean Hyundi engineers who came to work with him on one of their engines. They didn’t have total faith in their electronic knock sensors, and they brought a roll of ¼” copper tubing so they could listen to the engine. After Sean heard the audible clicking sound of the Hyundi engine detonating, he knew that was what we needed at DynoTech Research. Today, every million dollar dyno cel at Delphi is fitted with a $25 roll of copper tubing with a megaphone-like funnel soldered to the end in the control room for amplification.
The copper tube must be solidly attached to the top end of the engine to transfer the sound of deto. There is a ¼” electrical lug soldered onto the end for bolting to the engines. On sled engines the head is good, and on engines with thermostat housing, the 6 mm bolts attaching bolts there are ideal. Arctic Cat twins have no thermostat housings on the heads, but have a larger metric tapped hole near the center of the head, said to be used at the factory for a lifting lug. For that, I’ve made a steel adaptor that’s shown attached to the copper lug. For Cats we can also hear deto clearly if we bolt the ¼” copper lug to any one of the 6mm exhaust valve bolts, too. The steel adaptor also can be used on head bolts/ studs if length is adequate. That works well on SkiDoo Etec engines. Sleds with individual heads and more than two cylinders should have the lug attached to the coolant outlet manifold on the center cylinder. The sound of deto carries well through the coolant manifold, from one cylinder to another.
The copper tube is routed into the control room through the wall separating it from the engine dyno room. Sean uses a 2’ long piece of 1” ID clear plastic hose to keep the copper from resting against the drywall (and possibly dampening out some of the clicks). And of course care must be exercised when routing the copper tube through the engine compartment to keep the copper from rubbing against anything.
Four-stroke engines’ detonation causes loud audible clicks in the control room when the lug or adaptor is bolted to any exhaust manifold stud.
The following photo shows how Sean took the foam insulation out of the left side of a set of cheap Harbor Freight ear protectors. Then he drilled and tapped the plastic to 1/8th NPT and inserted a ¼” hose barb. Then Tygon fuel line connects the copper tube hanging in the control room and does a nice job transferring clicks to our left ear.
The headphones are only necessary on loud race engines. The control room is pretty well insulated so on most trail engines the headphones are unnecessary. On quiet engines, detonation clicks are very audible coming off the outside surface of the copper tube itself, and can be heard by everyone in the control room.
OBSERVATIONS AFTER SEVERAL YEARS OF LISTENING TO DETONATION:
This setup has saved many hundreds of pistons by allowing us to abort a test before damage occurs from detonation. Stock hilldraggers must run pump gas, and tuning for max HP with pump gas with lean A/F (max power is usually 13/1 but sometimes an engine will detonate with mixtures even richer than that.) and lots of timing can take engines into knock before HP peaks.
What’s deto sound like? It’s very much like the sound of a high voltage spark you get if you pull a plug wire off of a running engine and hold it close to the plug—sort of a loud “snap” that is unmistakable. When we monitor engine knock with Arctic Cat or Polaris EFI software if we hear two loud clicks during a dyno test, both will usually pull a couple of degrees of timing and the dyno test is usually aborted since we know HP will be down.
On sleds with no knock protection a click or two at torque peak is acceptable since the deto will usually subside at high RPM. From experience on two-strokes two or three random midrange clicks may be OK but 8 clicks is not—you probably won’t hear the ninth click because the engine will be “tightening up” by then! And it's a combination of rapid clicks over an extended period of time that wrecks parts. The highest BMEP mod two-stroke engines are usually octane-limited. Any engine that is going to make close to 200 psi BMEP usually must have 116 motor octane high RVP fuel to do that. Our ears tell us that!
Four-strokes are more forgiving than two-strokes when it comes to tolerating detonation. But it’s still wise to abort any test where clicks are heard and reduce timing, or add fuel to cool combustion chambers. Big loads of fogger-nozzle N2O often detonate severely for a fraction of a second, when activated, because with most systems the N2O gets into the engine a fraction of a second before the fuel does. Once the proper amount of fuel joins the N2O in the intake, the deto subsides! It sounds awful, and probably doesn't help longevity of parts, but we have learned to accept that. We don't hear that with Boondocker EFI N2O systems, because with those the fuel gets there immediately.
FOOLPROOF TESTING? Not really. We still benefit from watching the real time graph on the dyno while testing. When tuning for max HP, it’s wise to abort a test that is lower in midrange HP than the prior test—even with no knock. We still can seize engines from too-lean mixtures caused by the wrong jet selection (or partially plugged main jet) or EFI tune, and too-often by stale, low RVP race gas that wont vaporize adequately in the combustion chambers. Severely lean engines might not detonate. And my own theory is that I don’t think we can hear preignition which can be more harmful that deto, with some hot spot like an overheated plug ground strap igniting the mixture WAY too early. Prignition is often preceeded by detonation which scours away the cooling boundary layer of air insulating parts from 5000 degree F combustion gases. Once a hot spot occurs from, say, six clicks of deto, then quiet preignition can begin and really make a mess of things. But something like stale race gas can create extremely lean mixtures in the combustion chambers even when our meters are showing safe numbers. If to much of the stale gas doesn’t vaporize and burn until it gets out into the exhaust, then what measures as 12/1 A/F ratio might net out to be 15/1 in the combustion chamber—within the “explosive limit” (no misfire) of 10/1-17/1 but lean enough to lose power and perhaps stick a piston without detonating.
So it pays to watch the numbers and the HP curve during testing. But if we had this great listening device 26 years ago, perhaps 1000 pistons could have been saved!
How about field testing with a short copper tube bolted to the engine? I don’t know why it wouldn’t work, but I don’t know anyone who has tried it. My neighbors JD Powersports amplify the sound of deto while creating Cat turbo tunes on their SuperFlow dyno by using electronic “Chassis Ears” that clips onto their copper tube then sends a signal to a set of Bose earphones. Maybe some sort of BlueTooth deal could be used with Chassis Ears inside a helmet. Or, maybe a custom fitted ear protector insert (like those used by pro racers) could be fitted with a ¼” hose barb. Then the flexible Tygon hose could be used to allow listening while riding.
If someone comes up with a functioning copper tube deal for field testing, please send a picture that I can post here!