Pre-detonation ????  
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Pre-detonation ???? - 6/18/2008 1:57:21 AM   
dakamat


 

Posts: 141
Joined: 3/23/2004
From: Dolbeau, QC, CANADA
Status: offline 		Is somebody could tell me how i can tell that an engine make pre-detonation??
Tonight i've start the break-in on my new RB TM928. I've make an idle first tank, keeping the temps at 220. In the RB owner manual, they claim that this engine is shim for 25% nitro, and if we go up to 30%, we must put an aditional 0.01" head shim.
I've this shim in my tool box, but i don't know how to tell if my engine need it.

Help please.

Thank you very much

David

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RE: Pre-detonation ???? - 6/18/2008 1:04:51 PM   
Argess



Posts: 410
Joined: 1/12/2006
From: Pleasantville, NS, CANADA
Status: offline 		If you can't get a good tune without the temperature running away, you may need to add the shim.

All engines run at diferent temps, so it's a hard call by using a certain temp as a guide.

Here's how I see it:

After a long run, you bring the vehicle back in and listen. If the idle sounds funny......sort of sizzling, or it has been described as a frying egg sound, then you may be having pre-ignition (or detonation....never was sure of the difference).

I found on one of my engines that always used to overheat, even though it wasn't running lean, that pre-ignition started around 325F, but of course that's different for different engines.

(in reply to dakamat)
       Post #: 2

RE: Pre-detonation ???? - 6/18/2008 2:56:37 PM   
ugly duck


 

Posts: 357
Joined: 3/28/2008
From: Calgary, AB, CANADA
Status: offline 		If you're really interested:

Detonation is from spontaneous combustion of the mixture in the cylinder. The burn typically is started at a controlled point and at a controlled time, and is typically started while the piston is still moving up (very slightly - it's darn near the top of it's motion). It takes a bit of time for the flame to really get movin', which is why it's started BTDC. The flame kernel starts out small and burns it's way outward, like a ripple on a pond. As the mixture is consumed and the flame front spreads, pressure and heat are generated. If the pressure and heat become great enough, pockets of air/fuel might start to burn on their own, without actually being consumed by the existing flame front. This causes the pressure and temperature to climb even faster, which can start more pockets to spontaneously light up. Because the piston is sitting near TDC at the time, there's nowhere for the pressure to go except to bend the rod or hammer out the bearings. Detonation happens post-ignition.

Pre-Ignition is when neither pressure nor heat exist in excessive quantities, but there is a hot spot that causes the mixture to burn at much lower temperatures. The mix will start to burn WELL before TDC, and it is SILENT. The mixture is burned and all the hot gas that normally drives the piston down the bore is generated long before the piston reaches TDC, and the piston is trying to compress all that gas. Double plus ungood. You cannot hear pre-ignition, you only feel it as a power loss and you see it in a melted piston. Pre-ignition happens before normal ignition, hence the name.

Pre-detonation is... normal combustion?

Glow plug nitro engines exist only because of pre-ignition and detonation. Pressure, heat, and the hot spot (glow plug) are all necessary in order to light off the mixture, but it's done in such a way that the combustion is predictable and in a controlled manner. If the glow plug is too hot, you'll ignitie the mixture too soon and you can cause excessively high pressures causing UNWANTED detonation. Also if the fuel is too volatile (mixture) or not resistant to detonation (high nitro or oil content), you can also get unwanted detonation. Detonation can be so violent that it will actually blow pieces of piston off the crown and stick them to things like the glow plug. Pitting of the piston crown can be an indication of detonation, as can a bent connecting rod or worn out rod/pin bushings.

Anyway, if the manufacturer suggests that you need a certain thickness of head shim to run a certain fuel, I'd use it. Especially if you already have it.

< Message edited by ugly duck -- 6/18/2008 2:58:07 PM >

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(in reply to Argess)
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RE: Pre-detonation ???? - 6/18/2008 6:03:47 PM   
Argess



Posts: 410
Joined: 1/12/2006
From: Pleasantville, NS, CANADA
Status: offline 		Thanks for the explanation. They do seem rather similiar. Detonation almost sounds like normal diesal engines and pre-ignition sounds almost like normal glow-plug operation, but I guess they both happen when you don't want it to, vs. normal operation.

(in reply to ugly duck)
       Post #: 4

RE: Pre-detonation ???? - 6/18/2008 8:30:40 PM   
ugly duck


 

Posts: 357
Joined: 3/28/2008
From: Calgary, AB, CANADA
Status: offline 		It's a common misconception that diesels run on detonation: Detonation happens in a gasoline engine because there's unburned air/fuel mixture in remote locations of the combustion chamber as the mixture is being ignited. In a diesel, fuel is being injected directly into the cylinder just before TDC and burning as it's being injected, so there's no possibility for uncontrolled combustion. The heat from compression and the vapourization of the injector allows the fuel to burn without aid, though the glow plug on a diesel is there, mainly for cold starts.

Glow engines do run based on preignition, but the preignition happens just about exactly when you want it to, so no damage is done. The damage from classic preignition comes from the piston trying to compress a fully or partially burned mixture, while with a glow engine it "preignites" just before TDC, right when you would want a spark plug to light the mixture.

< Message edited by ugly duck -- 6/18/2008 8:32:50 PM >

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(in reply to Argess)
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RE: Pre-detonation ???? - 6/19/2008 3:23:44 AM   
dakamat


 

Posts: 141
Joined: 3/23/2004
From: Dolbeau, QC, CANADA
Status: offline 		Thank you very much ugly duck, i'll not take a chance and use a 0.01" shim as the manual say

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Ofna Hyper 7 pro, Hyper .21, 8 ports, Thunder Tiger Ducati nitro bike, Team Iwata R/C body painting

(in reply to ugly duck)
       Post #: 6

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