Food for thought version 2

I would like to reinitiate this topic but hopefully maintain the thread, without unrelated comment.

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Subject: Food for thought ! Yesterday at 6:21 pm

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To any or all.
If ignition spark duration was in the order of 1milli second, then, at 12,500 rpm on a 54 bore the
whole combustion process would have been completed in less than that time frame !
This assumes a good quality fuel and a favourable, stoichiometric air/fuel ratio . I worked this out
for the RS Honda , so a lower reving Bantam , and particularly a 175 with it`s bigger bore , would
have a different time frame . Makes you think though , and does illustrate the critical necessity for purity of
trapped charge , high turbulence and spark intensity within a compact combustion chamber .

Hope this one runs a bit, we might have another "Heavy Crankshaft" if we are lucky! It could raise a
lot of peripheral, but related , issues as well !

Regards Trevor


john bass



Number of posts: 561
Age: 46
Localisation: Bensberg, Germany
Registration date: 2006-12-06

Subject: Good Quality fuel....??? Yesterday at 8:19 pm

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Comes back to that -- I think, Trevor -- every time: a fuel that has the properties of high anti-knock, low peak combustion pressure and long heat-release-rate period AFTER TDC***....

And 175 -- bigger bore with longer flame path? --- uh oh! I think I am out of my depth...?

Hum hum hum...

***{[(tetraethyl lead -- illegal...)]}

Whoops! afterburn -- once the fire starts is the spark duration important...?

Gentlemen,

A few starter questions;

In terms of total available combustion time, are you measuiring from spark initiation to exhaust port opening ?

How important do you feel combustion chamber shape is to the flame propogation process ?

Are offset and twin plug head designs of any real value.

James

Shape of the combustion chamber, amount of piston crown (`Bump´) clearance to inside head plus the amount of squish &swirl -- along with compression-ratio all contribiute to complete -- or incomplete -- combustion.

Swirl is a measure of how the fuel-air mix is moving into the flame-path either before the mixture gets to the combustion chamber and swirls during burning
or only as `Swirl in Chamber´ where the mixture is burning mostly in the combustion chamber -- all down to "tuning" of course...?
Squish is the final squashing of the mixture from above the piston to when the mixture is finally squeezed into the combustion chamber. The good tuner tries to get this happening before the fire gets really going because that becomes an increasing pressure above the piston BEFORE it gets to TDC -- hence reducing power output.
What the good tuner tries to do is get these aspects working in harmony so that combustion takes place mostly in the combustion chamber and then the gas expansion is happening after top dead centre....

Answering James, I think the twin plug -- offset plug -- works well with intentionally induced high swirl otherwise central plug seems the optimum.

I am of the opinion that the spark duration for most Cdi ignitions is in the region of 50 to 80 Micro seconds. The rise of the voltage (for a race ignition) is some in the order of 50 Mega Joules of instant energy, I believe the one I use is 52 Mjoules, so quite some spark!
Once the spark has fired the combustion process then I'm uncertain that any further sparks will actually help.
I tried a multi spark system on my bike some time ago, and just thought it didn't actually do anything, however I didn't try this on a dyno so the jury is definately out at the moment!
The speed of combustion certainly has an effect on the power output but can be offset by other factors, for instance Avgas is slow burn, so the timing for best power is well advanced (around 3 degrees) compared to pump petrol but some of the fuel does not burn and is evaporated instead, so giving a cooling effect. The slightly lower power is offset by higher compressions, and the timing, and the edvantage is a cooler running (and more reliable) motor.
Cheers,
Alan

Many years ago I was compelled to study a Rolls Royce tank engine which had dual magnetoes supplying two plug per cylinder, one plug giving a big fat spark and the other a series of trailing sparks... seemed a good idea at the time...

Much later (about 35 yrs or so...) when deep into development of diesel engines we switched the notion around and had a `pilot´ injection of a small amount of fuel that got the fire going before the Squish and TDC (good, because not too large a combustion pressure at the start) -- then the major period of injection followed --- during which was constant high injection pressure with high air swirl in bowl*** combustion... It wasn´t easy to achieve with old fashioned mechanical injection systems but now with computerised control it is happening.

A follow on from that was the Texaco System of spark plug with fuel-injector in the bowl together -- with the bowl in the piston, of course!

OK! so I hear yapping that its not ICengine & not Bantam.

No!

But I am thinking that the idea of a fuel injector, as Direct Injection, together with spark plug in the Bantam cylinder head would really be a step forward.
Costly though. Need an injection pump and an oil-pump and lube-system but short-stroke, reed valve and FAIRINGS have been allowed -- SO WHY NOT!!!!???


*** bowl in the piston crown

Uh oh!!!

You answered your own question John!
"Costly though..." Says it all!

The Bantam Formula is what it is....
Cheers,
Alan

Right Alan! -- but worth a try just for the heck of it -- could actually have a low pressure injector in the top end of a transfer port and have computer controlled
fuel injection ...

Why dunn I just shut up!!!

52 mJ is a lot of Joules Alan which backs my point about importamce of strength of the spark.

I just mugged-it-up in my Bosch bible:-

"Only 0.2mJ spark energy is required when the air-fuel mixture is stoichiometric (chemicall correct for complete combustion) yet OVER 3mJ are necessary for a rich or weak mixture... So you have a safety factor of >70%.

Point of interest is how is such a strong spark achieved?

Bosch do some very good racing spark plugs -- unfortunately I cannot remember
the code....

The 52 mJ is stated and tested for the system I use, and I have the graph that proves the system is the same from virtually zero revs to 20K plus rpm.
If you watch my starting technique in a morning before practice you will see that I do not need much assistance! I flick the back wheel to get the engine up and running, no starter rollers required here! also when the engine is warm, a single turn of the crank and provided there is no engine trauma the engine starts....
It is dangerous to play with the crank rotor/sensor system whilst the ignition is turned on, mainly because the engine could easily start with a flick of the rotor and take your finger off!! For safety I always unfasten the battery as well.
Thats what I call ignition!
The ignition is a Cdi type but not sure how the voltages are acheived, you could research this from Google.
Cheers,
Alan

What a difference? what advance in technology ?-- Icarus-1´s ignition `high´ voltage spark at 6 to 8000volts and Alan´s at 40,000 ...!!

Wow!

My Dad used to shut off his single-cylinder engine -- in a show-off manner -- by shorting out the spark plug with his hand. Good job the Cdi system hadn´t been invented then or my dear old Dad would have committed suicide by the way he fooled with motorbike sparks. He nearly did though. That was by jokingly giving Mum a shock with his magneto fixed in the vice and She obediently holding the brass ferrule of the HT cable. He spun the sprocket, She flung her arms out like Al Jolsen singing `Mammy´, her left arm caught him across the face, Dad´s specs were knocked off his nose to fall on the vice and star-crack the lens on one side!

Alan! all I could find on how 40,000 volts is achieved over say the best system of the early 80´s, 15,000volts by transistor era -- was an "Amplifier" in the system which amplifies by a factor 100:1.
I just wonder how such is achieved -- except perhaps by GUESSING that `electrical resonance´ is invloved, which is using the discharging of one
capacitor to excite another capacitor -- the first exciting the second into resonance which then discharges ...????

Perhaps Trevor can explain....

Hi James,

Sorry , a little late , but i`ve had a busy time here .
Clever change of location, a nice ploy , i never would have thought of that , worked so far though !

Total combustion time for the average race engine is in the order of 40* of crank rotation , this will vary with crank/rod geometry, bore size,
fuel spec and even engine to engine. One millisecond in our example engine represents about 70* degrees of crank rotatation, therefore
effective combustion is over well inside the time frame. From this it`s easy to see that much of the spark duration is wasted, but it`s
reasuring to know that modern igns are well up to the task expected of them , unlike the battery/ coil and contact breakers of the sixties .

However, the one overriding requirement for fast , complete combustion is turbulence, create all you can !
If combustion was entirely laminar the engine would struggle to get over 1,000rpm , the burn rate would be just too slow !
The principle contributors to tuburbulence are, the scavenge process, piston motion, the squish process and the returning gas , propelled by
the exhaust plugging pulse .
It is also worth pointing out that quoted c/r values take no account of the amount of gas actually in the combustion chamber at trapping .
A 12-1 ratio with 5cc trapped is a totaly different proposition to 12-1 with 10cc trapped, but the geometric ratio remains the same .

Chamber profile must influence burn rates to some extent , but the two stroke world has settled on the hemisphere and squish band
for the good reason that it works very well .It`s efficient, has a favourable volume to surface area ratio , and from our point of view
is easy to calculate a different value , and consequently , manufacture on available machinery.
There is a new kid on the block in the shape of a torriodal profile , but i have yet to see any published evidence that it out performs
the hemisphere . Could it be your area of expertise John ?

I have never tried a twin plug arrangement on a conventional two stroke bike engine, but i do have a Merco 61 model aircraft engine
with a twin plug head . Running on one glow plug is fine , running on two the whole thing is so much sweeter that it seems like a
different engine, but bear in mind though, this is run on Methanol and not petrol , with a totally different burn rate. Occationally i treat
it to a mix with 10% nitromethane content , and that , is a whole new ball game !

Keep compressing that gas , regards Trevor

Come to think of it I seem to be going thro´ my 3rd childhood -- missus says 5th... and from what with your playing with model aeroplane engines -- Trevor...???
-- you are just a big Kid at heart!!

I will be castigated (ouch! painful, but I don´t care!) for this which follows `cos its 4-stroke diesel ... but Trevor did ask -- and what more encouragement do I need...?

Combustion chamber shapes were my playthings 26-35 years back. In Montreal our new diesel engine finally*** sported the, "Re-Entrant Toroidal" chamber; bowl in flat-top piston; extremely high swirl -- vertical-oval, spiralling inlet port with raised `blister´ close to inlet valve; piston-bump clearance at 0.85 -1.0mm !

I cannot send pictures so I shall have to describe the re-entrant toroidal shape in words -- as best I can. Perkins had a flat bottom toroidal with sloping inward sides and earlier Ford diesel had the toroidal with flat bottom and vertical sides -- ALL IN THE PISTON CROWN.

Imagination again: Cut the piston vertically in half and the cross-section looks like a pair of circular balls in a `sack´ with a pip poking upward in the middle between them. Above the balls the `sack´ has sloping sides coming inwards to a `throat´-- a parallel hole -- with its diameter about 75% of the maximum dia of the two balls with the injector poking down into this so that its four spray holes spray roughly into the middle of the toroidal path.
So what you have there during the period of injection and heat release is like a pre-combustion chamber inside the top of the piston where the squish followed by the
high swirl has a fiendish furnace like activity rushing around the toroid -- INSIDE
the piston crown ....

We lost the HUMMVEE contract to GM´s Hummer and the above design with its
complementary fuel-injection system was sold to Ford...

The above configuration is absolutely no use whatever to Bantam engine developers because the bowl in piston cannot work with the LOOP SCAVENGE effect...

But this precombustion chamber idea appeals to me -- along with direct fuel injection and spark plug directly in the combustion chamber á la the Texico system which gave the best overall performance of seven different combustion systems.
This chamber could be in the cylinder head ....

Hummm!?
It doesen´t look possible, at the moment, in a Bantam engine arrangement
but I cannot stop thinking about it....

***intake port was rectangular to start with in Cologne! --

Dead right there John !
When i was a child i spake as a child , thought like a child and reasoned as a child . When i became a man
i carried on my childish ways .
Indeed, my wife continually admonishes me with an exasperated enquirey as to when i might "grow up ".
I mean , damn it , thats for old folk ! !

Now where did i put my copy of the-- Ladybird book of the two stroke engine ?