ESE's works engine tuner

[bucketracer]

Trawling the net for opinions on spark duration.


http://spdispark.com/pages/spdi

Spark Duration

An inductive ignition system has a spark duration of appoximately 1 milisecond. A CDI system has a very short duration of around 50 micro seconds. In comparision, SPDi produces a spark that can be as long as you want. In theory the SPDi Spark could be continuous, but that probably wouldn't be a good idea. Generally our spark profiles produce a spark that is 1.5 to 2 ms in duration. Our testing has shown that the best results are achieved when the spark ends between 10 to 15 degrees past TDC.

[bucketracer]

http://plasmatronicsllc.com/spark.pdf



A bit about the shape of the electrodes and there effect on spark initiating a flame front.

[dinamik2t]

Aha! I 've read everything, thank you Bucketracer and Wob.
At first I was thinking everything in terms of power, as V*I -or I^2*R as Wob said-, and I was confused with the spark energy.
By the way, all those -ctance terms in electronics are so frustrating for foreigners

[Ocean1]

Our testing has shown that the best results are achieved when the spark ends between 10 to 15 degrees past TDC.

Buckets are limited to pump gas?

It matters. A top fuelie running 90% nitromethane 10% methanol will run over 60deg BTDC.

Change the fuel and the ground rules change dramatically.

[bucketracer]

all those -ctance terms in electronics are so frustrating for foreigners

Yes ... sorry about that. but I do think your doing very well with the English language.

Buckets are limited to pump gas?

No ... used to be but can now use Av gas.

A little water injection through the inlet system is on TeeZee's list of things to try. Mainly to cool the fuel mixture like methanol does.

[Frits Overmars]

A top fuelie running 90% nitromethane 10% methanol will run over 60deg BTDC. Change the fuel and the ground rules change dramatically.

I think the 60° bTDC has more to do with the ridiculous piston speeds and the even more ridiculous combustion chamber shapes found in those two-valve top fuelers.
It's more or less the same in Formula 1. There the piston speeds are a bit less ridiculous (25 m/s at 19,000 rpm with a 39,75 mm stroke) but the combustion chamber shape of an F-1 four-valver is not much better than that of a two-valve top-fueler because of the F-1's 98 mm bore and the much higher compression ratio of a petrol engine compared to a nitro-burner. An F-1 combustion chamber at TDC looks like someone dropped a pancake. About the only point in the F-1's favour is the central spark plug. Still they have to run over 50° ignition advance on fast-burning petrol.
Now let's return to two-strokes.

[Ocean1]

Now let's return to two-strokes.

Pistons can outrun flamefront on two strokes?

[Frits Overmars]

Pistons can outrun flamefront on two strokes?

That huge ignition advance may have led you to believe that the piston would otherwise run away from the flame. No, the flame will always catch up with the piston. But with high piston speeds, adversely shaped combustion volumes, insufficient squish, tumble or swirl, or less than optimal mixtures, combustion may proceed so slowly that the pressure rise comes too late for thermal efficiency. Roughly (very roughly) speaking, a quarter of the mixture should be burning by TDC.

[Ocean1]

No

O well, the message should be at least 10 characters long.
So: no, they can't.

Interesting to consider the implications a different fuel might entail for two strokes though.

So, lower rev's must make a two stroke less effective, but 7 or 8 times the fuel charge per intake stroke is a pretty good compensation...

Still, not allowed, so little point spending much time on it.

[Frits Overmars]

Lower revs make an engine less powerful, not less effective (the BMEP of a two-stroke Aprilia RSA125 is nearly 17; no naturally-aspirated petrol-burning four-stroke can match that, no matter how high they rev).

[husaberg]

Think of the two cdi units as battery's that supply "power" to the ignition coil.
Run two batteries in parrallel and the voltage stays the same - but potentially twice the current is available.
As the coil resistance ( plus inductive reactance) stays the same, this doubled current creates potentially more than twice the power in the coil.
as power = Watts = Current squared * R, the watts are more than doubled.
This translates differently in practice.
On a scope the initiall arc -over voltage and time is the same, but the actual burn time ( or the period where the stored energy in the capacitors bleeds down ) by the spark
ionising the molecules in the gap,increases around 3 fold.
The other method of increasing the spark energy is to increase the inductive reactance of the coil, this is why the RGV/Aprilia low primary resistance coils work well with a DC - CDI.
Even better is a huge Crane coil that is used in Nascar CDI systems.
A twin Ignitech driving one of these pulls 6A continuous and will destroy the ground electrode of a "normal" plug.

I have always wondered about those lovely looking Coil packs from the Gsxr Srad and others.

I see they do have low primary resistance but are they up to it?
I realise that bigger is often better when it comes to coil performance as has been alluded to before.

from srad600 manual
primary 0.07-0.11 ohm
secondary 4.5-6.9 KOhm
peak over 100

coils/caps weigh in at 3.5 ounces.
?????????????????????\

I seem to remember quite a few cars having them also Falcons etc

[Frits Overmars]

I have always wondered about those lovely looking Coil packs from the Gsxr Srad and others. I seem to remember quite a few cars having them

I seem to remember my car (Suzuki Baleno 1.8) has them too. In fact I remember damn well how much they cost; I had to replace three of them because they died through overheating in those deep pits between the camshafts.
Once they have been too hot you can still run the engine under light load. But when you give it full throttle the spark has to overcome an increased resistance which somehow results in very late ignition. The engine then reacts by backfiring the airbox (filled with LPG) to smittereens.

[wobbly]

In the testing I did the twin CDI into the Crane had 40Kv arc-over voltage that dropped to 15Kv and discharged for 180Ms,this was over 3* the discharge period of a single cdi driving
a stock RGV coil.
The stock setup pulled 0.6A at 10,000, the twin setup driving the Crane pulled 10* that.
I dont think that this mega system for a single would be any advantage under normal conditions, but for the VERY rich combustion process used in a racing 2T on Methanol it certainly gets that
non optimal mixture fired up.
The twin fire idea does work well on a Honda RS125, using the stock coil,but I havnt tested a RGV coil back to back yet.

Re ignition advance and burn duration - the theoretical consideration or advantages or not of this stuff doesnt matter a shit.
It all comes down to at what point you fire the spark, and thus how much of the combustion energy ( a fixed amount ) is used to
create heat that increases the pressure on the piston, or, if a larger amount of this heat is discharged into the pipe.
The former scenario gives peak pressure close to TDC, the later has the peak well past TDC but dumps alot of heat into the exhaust, increasing the wave speed and thus overev power.

As a matter of interest EngMod throws up combustion burn periods varying from 30 to 90* of crank rotation.

Re spacing apart the twin firing points - i asked Ignitech about this, and technically it can be achieved by the newer processor to initiate a gap of say 100MSec between the firing points.
But they suggested that using the two capacitors and a coil with lots of Inductive Reactance would achieve the same effect.
They were right, as using these two elements of the system both increases the burn duration in the gap.
I had to use expensive spiral wound Magnecor wire to kill the RF of the plug lead, as the system killed the 5K resistor in the NGK racing caps ( over $100 each ) instantly, so I think I will use this
setup from now on, and use normal caps on the plug for all racing ignitions I build..

[dinamik2t]

I used to have some KV85 Magnecor wire on a coil once.. A little bit fat to screw on a 7mm-wire-hole coil.



Wob, what instruments do you use to measure the arc voltage and duration??
About the resistor, the problem lies to the cap only? NGK racing plugs, having 5kΩ along with a 0Ω cap, would be safe?
And finally, the cap resistor is removable?

[husaberg]

While we are talking plug caps i would not recommend anyone use these style NGK plug caps
They have a wire clip to attach to the plug and they always seem (For me anyway)to wear a notch in the plug resulting in a loose plug misfire.

I seem to remember my car (Suzuki Baleno 1.8) has them too. In fact I remember damn well how much they cost; I had to replace three of them because they died through overheating in those deep pits between the camshafts.
Once they have been too hot you can still run the engine under light load. But when you give it full throttle the spark has to overcome an increased resistance which somehow results in very late ignition. The engine then reacts by backfiring the airbox (filled with LPG) to smittereens.

Suzuki i would have thought this would have been more in keeping. Or a Saab two stroke?

I thought for a minute he was letting the misses drive then i realised a lot of the Continental types still drive on the wrong side of the road
The plug coils are quite cheap on EGay.

So has anyone tried one?