ngk replacement sparkplug

[Mike C]

How do you know what direction the compressed gas  mixture is coming from with the piston squish present near TDC?

[Bfg]

 

On 8/20/2019 at 8:04 PM, DaveN said:

I also see that some folk recommend 'indexing' the plug so that the electrode gap is facing the incoming fuel/air mixture.

I reckon someone's been pulling your dangly    .. not us though .. eh ? 

..as spark plugs do not only arc across their end tip..  If that were the case then the central electrode would only erode on that one side.

 

Aside from which, as all schoolboys used to know - the four stroke cycle is in effect :   suck, squeeze, pop, and fart.

So even if it were practical to 'index' the electrode's gap - it might only make a slightest difference *  if induction was immediately followed by the big Pop.  But as there's a full crank cycle inbetween the fuel-air mix being drawn in & downwards by the lower pressure behind a descending piston, and then being compressed (9 : 1 ? ) into a tiny space all around the spark plug ..before the ignition is fired - the spark plug simply needs to face downwards and be angled towards the centre of the combustion chamber to be indexed.                                   * and that difference would be the time it takes for the ignited air-fuel mix / flame path to travel just the diameter of the centre electrode.

Of course, as Mike says..  some engines utilise squish effect (this is used on the post-war 500cc twin cylinder Sunbeam motorcycles I'm rather fond of).  But even then, there's a broad wave-front of air-fuel mix (..the full width / diameter of the cylinder) to ignite ..so again the spark plug just needs to be nearest the middle of its combustion chamber..  

Three or Four valves per cylinder facilitates putting the spark-plug in the centre of the combustion chamber, so the flame front has shortest distance to travel / combust all the fuel mix in the shortest possible time (..important for very high revving engines).  And twin spark plugs were a popular tuning option offered on motorcycles at one time (..I gather very few customers though). 

 

[DaveN]

 .........(this is used on the post-war 500cc twin cylinder Sunbeam motorcycles I'm rather fond of). 

Rebuilt one o’ them with my father many moons ago....HBO 590. I see it was last mot’d in 2012 so it is still around. It was a complete basket case when we collected it. Twas an S8 and never been used for hauling a chair. 

As for indexing plugs... have a google some odd people reckon it’s worth it! Me.. I just gap ‘em and wind them in...any old plug will do providing it starts and goes flat out...(well not really, when I got my car it has some Champion thingys that I left for a couple of years then swapped them out for ngk’s of some description. Are they better? Can’t say I’ve noticed any difference but hey ho!

[Peter Cobbold]

Ah, turbulence during combustion. A huge topic that I attempted to summarsie in an IWE talk

https://supertrarged.wordpress.com/2016/07/21/iwe-2016-technical-seminar/

Note use of engine with square cylinder !

Peter

[Bfg]

 

1 hour ago, Peter Cobbold said:

Ah, turbulence during combustion. A huge topic that I attempted to summarsie in an IWE talk

from your link to a Power-Point-Presentation (which I cannot read) I see this was in 2016  ..which accounts why I didn't know why you were giving this talk at this year's IWE !

..shame as I would very much liked to have attend.  But at the same time glad you didn't give it this year and I didn't know about it or otherwise foolishly missed it ! 

 

But for those wishing to read more here's a link to Peter's most illuminating  < Combustion Basics > page.

Thanks Peter.

[Peter Cobbold]

2 hours ago, Bfg said:

 

from your link to a Power-Point-Presentation (which I cannot read) I see this was in 2016  ..which accounts why I didn't know why you were giving this talk at this year's IWE !

..shame as I would very much liked to have attend.  But at the same time glad you didn't give it this year and I didn't know about it or otherwise foolishly missed it ! 

 

But for those wishing to read more here's a link to Peter's most illuminating  < Combustion Basics > page.

Thanks Peter.

Thanks Bfg !!  The 2016 talk was  barely audible over the kids disco. In the 2017 talk on 'How an SU works' I lost a battle with a tannoy outside. So its good when a forumite voices appreciation of what at the time I regarded as disasters. Thank you for reading!

Peter