OK, can anyone out there explain the physics of detonation to me? This has no bearing on my Z and is purely out of interest to satisfy my own curiosity.

 

Why does detonation occur because of lean conditions and not rich conditions. In my ignorance I would have thought putting extra fuel into a hot place would be more likely to cause premature combustion than putting less fuel in. This is apparently not the case but I would be interested to know why this is.

 

Also, I think I'm correct in saying that advancing the ignition too much causes detonation. If detonation is the fuel igniting at the wrong point, it would occur to me that igniting it sooner is going to be beneficial, igniting it before it self ignites at the wrong point. I know this is wrong but I would love an expert's explanation as to why.

 

Sorry for being far too inquisitive and not just accepting the facts, shutting up and getting on with it!

 

Mike

Detonation can occur for several reasons. Here are two.

 

If the ignition timing is advanced too far the the fuel is ignited and burns before the the pistion has reached tdc, causing it to try to force the piston back the way it came. Ideally the fuel should ignite just before tdc and as it burns push the piston downward (forward) on the completion of the stroke.

 

Secondly, if the engine is running really lean the spark plug gets too hot, and begins to act like a glowpug in a diesel engine, igniting the fuel before the spark and cuasing a similar problem as before. You often notice this with old cars when they 'run on' after the ignition is turned off.

Mikey...go here...

 

So what is Detonation?

 

Well that answered that for me. So am I correct in thinking that the point of advancing ignition is so that the fuel can start to burn before TDC but the "shock wave" of burning fuel/air only meets the piston at TDC or just after? If this is the case, how does higher octane fuel alleviate this? Does the air/fuel mixture somehow burn slower, thus reaching the piston later?

I think that higher octane petrol has a higher burning temperature, so the heat alone inside the cylinder is not enough to ignite the fuel

Mike

As I understand it, and could be wrong, is that the timing needs to be advanced for increasing rpms cause the time for the explosion to reach effective motive pressure on the piston is roughly constant but the piston is moving faster and faster so you start the bang earlier and earlier but never too early. However the effective cylinder pressure related to how much fuel and air is in there also affects the explosion wave front speed. So light load needs more advance time than heavy load. a correct fuel air mix at high pressure as at WOT is more easy to ignite than weak fuel air and pressure rises rapidly towards tdc. So glowing carbon deposits or shit on spark plugs or low octane fuel can also early ignite giving detonation. Pre-ignition with lots of fuel air means max energy before tdc that will burn thru a piston crown in a flash. You wont hear the pinking. Also runing weak mixture has the effect of increasing combustion chamber temp again leading to glowing carbon deposits in a dirty engine or hot spark plug tip hence weak mix pre-ignition. Cool plugs can help but only if you run the engine hot enough to burn off normal deposits on the plug during pottering around. Its a compromise for road use. Max efficiency is usually as near to the pre-ignite point as possible. I think TT Zs flood in rich fuel mix at high loads and rpm. Extra fuel helps cool the charge too but is wastefull. Twin spark plug engines can increase the efficiency of planning the bang for max effect like on Alfas. Octane rating in fuel only rates how resistant it is to auto ignition due to heat and pressure. I still dont believe theres any more energy in your gallon of high octane. In fact I think its less cause theres more additives and octane enhancers therefore slightly less hydrocarbon in your gallon.

Mike - you are absolutely bang on with your last statement - the calorific value (energy) of higher octane fuels is actually less than, say, normal unleaded. Its the fact that the engine can be pushed further to get more charge into the combustion chamber before detonatin occurs, that high octane fuel gives you more power - not the fuel enery itself

 

CheerZ,

 

Andy

Maybe some use?

http://300zx.co.uk/200sx/ubb/Forum2/HTML/000810.html

Maybe some use?

http://300zx.co.uk/200sx/ubb/Forum2/HTML/000810.html

To get a mental image of the difference between low and high octane fuels, imagine pouring lighter fluid onto your hand and then sticking a flame to it. It would ignite for a micro-second but not actually burn you. This is an extreme version of low octane fuel.

 

If you did the same with diesel, the flame would not actually ignite it. If however, you somehow COULD get it to light then it would burn far more intensely and would hurt quite a lot !

 

The diesel is more like a high octane fuel in that it is harder to ignite but has far more energy produced when it does.

 

High octane fuel will not ignite as quickly, hence it reduces detonation.

 

Two things to note before the literalists and lawyers amongst you respond :

 

1) The difference between 95 & 97/98 octane petrol is tiny compared to the difference between lighter fluid & diesel - I'm just using it for illustration purposes !

 

2) For the lawyers : setting fire to fuels in your hand is not big and it's not clever. I do not endorse it and in fact positively discourage it. And given that it's Friday night and I can guess what state many of you will be in when you read this, ALCOHOL DOES NOT MAKE YOU FIREPROOF !

 

Dave