ASM Community

Hi everyone,

    I want to write a software on an ignition system for a 125cc four stroke engine.The purpose of my program is to produce a variable time delay for spark to occur in the ignition system. As the engine starts, pulses from the alternator assembly via the pulser coil(positioned at 36degree BTDC) will be collected. The time from the 1st and 2nd pulse will determined the speed of the engine. By using the speed of the engine, the program will call a suitable delay value for spark to occur. This means that spark will only be produce after the 2nd pulse and one spark is wasted.

    Its been 2 weeks since ive started the project, learning about the ignition system and how to program a PIC using c or assembly. Ive decided to program PIC16F84 with Wiz-C by using assembly language because it is quite easy to understand. So far i was able to do simple task such as flashing LED with a push button.


*** some back ground on the ignition system
Ideally a mixture of air and fuel needs to be burn exactly at the top dead centre(TDC) of the combustion chamber. In reality, a fixed quantity of air and fuel needs a particular time to combust. Although this can be solve by allowing the spark to occur before TDC say at 36 degrees before TDC, the engine however have a range of operating speed. Thus there needs to be a  variable delay system(depends on the speed of the engine) where combustion occur before TDC. 

*** Information about my program

Assume that the engine can run up to 8000rpm.
At engine speed of 1200 RPM;

1200 / 60 = 20 revs per second
1 / 20 = 50mS per rev
1 rev = 360 degrees

36 degrees at 1200 RPM = (36 / 360) X 50 = 5mS

So  at max rpm, delay = 0.
    at min rpm( 0-1200rpm), delay should be lower than 5mS say 4mS

The program have 8 set of delay values.
8000rpm= 0mS
7000rpm=0.5mS
6000rpm=1mS
5000rpm=0.5mS
4000rpm=2mS
3000rpm=2.5mS
2000rpm=3mS
1200rpm or lower =fixed at 4mS


      I came across a website that has an assembly code which is similar to the one that i want. I cant really follow the logic of the code and was wondering if someone can give me a simple example plus explanation on how to use timers to calculate rpm and use data tables to retrieve corresponding delay output using assembly language. I know this is too much but i really appreciate all the help that i can get.



www.transmic.net

The 16F84 is quite limited, so the author of that ignition system had to inline everything and he chose not use interrupts (because he'd have to do silly things to get a 16-bit timer out of the clobbered single 8-bit timer).

Could you explain why at 8000rpm the delay is 0ms? And why do you divide by 10 (I know, 36 degrees,.. but the timing...).

Actually, like in the image I've attached, could you graphically represent:
*assuming that when the rotor rotates, the angle increases, like in 0 degrees, 1 degrees, 2 degrees, 3 degrees.... 359 degrees, 0 degrees,..."
- when the rotor is at 0 degrees
- when the 36-degree (or is it -36 degrees? ) signal is pulsed.
- when we must spark.
- when we must stop the spark

I'm eager to help, it's just that I don't know the timing. (no time to research :P )
I have much more experience with the newer PICs, that cost just as much - yet have much more facilities (internal 16-bit timers, robust interrupts, 10x faster). But I think I can help with writing the code for that F84, or at least outlining what should be written.

Btw, using only 8 values of delay is quite limiting and probably will waste fuel, right? It's just as easy to implement ~200 values of delay, imho.

Do you think the following HLL outline will work? Especially about CurPhase, I don't see a way to determine which phase the engine is in.

// Assume 8MHz clock, so 2 million instructions per second


#define TICK_CYCLES 20 // how many cycles the "tick" loop takes, including the branching

/*	
	1 tick = 10us,  100 ticks=1ms
	1000 RPM = 60ms per revolution	= 6000 ticks
	2000 RPM = 30ms per revolution	= 3000 ticks
	4000 RPM = 15ms per revolution  = 1500 ticks
	8000 RPM = 7.5ms per revolution =  750 ticks
*/



unsigned short TimeBetweenCoilPulses = 0xFFFF;

unsigned short CurTickCount=0;
unsigned short PrevTickCount=0x8000;

unsigned short Time_TillSpark=0;
unsigned short Time_TillUnspark=0;


void ON_RB0_INTERRUPT(){	
	TimeBetweenCoilPulses = CurTickCount-PrevTickCount;
	PrevTickCount = CurTickCount;
	_outSparkPin = 0;
	
	static byte CurPhase=0; // how do we KNOW when we're being pulsed about which phase?! In 50% of the startups, we'd be out of phase!
	CurPhase++;
	if(!(CurPhase&1))return;
	
	
	
	if(TimeBetweenCoilPulses<=750){ // 8000+ RPM
		Time_TillSpark = 0;
		_outSparkPin = 1;
	}else if(TimeBetweenCoilPulses>6000){ // less than 1000 RPM
		Time_TillSpark = 525;
	}else{ // 1000-8000 RPM, TimeBetwCP is [750;6000]
		Time_TillSpark = TimeBetweenCoilPulses/10 - 75;
	}
	Time_TillUnspark = Time_TillSpark+100;
	
}



void main(){
	SetupInterrupt();
	_outSparkPin = 0;
	
	for(;;){ //--------[ this is the "tick" loop ! ]--------[
		DisableGlobalInterrupt();
		if(Time_TillSpark){
			Time_TillSpark--;
			if(Time_TillSpark==0) _outSparkPin = 1;
		}
		if(Time_TillUnspark){
			Time_TillUnspark--;
			if(Time_TillUnspark==0) _outSparkPin = 0;
		}
		EnableGlobalInterrupt();
		NOP; // filler-in. If an interrupt happened in the last ~20 cycles, we'll be interrupted now
		
		
		CurTickCount++;
	}
	//-------------------------------------------------------/
}

:shock:

Wouldn't you simply get pinking or detonation?

Maybe a two-stroke would be a safer bet....at least to start with.

What Happens When the Timing Is Advanced  :shock:

http://www.miata.net/garage/KnowYourCar/S10_Timing.html

Hi eek,

From what ive read (googled wasted spark), wasted spark wont lead to detonation and pinging since in exhaust stroke, air and fuel is not compressed but the extra spark lowers the lifespan of the spark plug(but i stand to be corrected).The engine that im working on uses CDI type of ignition, where in lower revs sparks occur 13 degree BTDC, 38 degree at Higher RPM. The main purpose of what im doing is to emulate this system by using PIC, so that if i want to use a fuel with higher octane, i can just pug in the timing curve (if it is provided).

Your wasted spark is fine.
I used to have a 2CV 652cc with electronic ignition, which was a flat twin and a sensor on the flywheel would set off the coil and it would ignite one on the power cycle while the opposite cylinder was on the exhaust stroke, which saved bothering with a distributer.

--------------------------------------
Visa Special - Club (two cylinder) 1978-1988

From the start in September 1978 until the end in 1988, the Visa was available with a two cylinder aircooled engine. This engine is similar to the engines which have been used on other two-cylinder Citroën models (like the 2CV) for many years. The principal differences are that its capacity has been increased to 652cc and that it is equipped with an electronic ignition system, controlled by a computer.
The Club is the more luxurious model of the two with some extras like adjustable seats, parcel shelf, side overriding strips, a grille with chrome accents and stainless steel wheel covers. The two cylinder models do not have a rear spoiler.
http://www.citroenvisa.net/visa3.htm
----------------------------------------------------

The computer was for timing advance and engine loading purposes.

What I find unusual about your wee 125 is it seems to NOT have any timing advance system as standard.

From these 125cc examples it looks like about 20 degrees BTDC is the norm.

http://www.jmp.fi/dmcs/English.htm