TE 250 (X Lite ) Question

Am currently rebuilding a TE 250 for a friend of mine. It's a fuel injected model and when I was setting the timing something occured to me.

How does the Mikuni ECU know when the piston is at TDC on the compression stroke versus when it is at TDC on the exhaust stroke ?

I know these engines are wasted spark so it sparks on the exhaust stroke too, but how does it know when to inject the fuel ? There are no cam sensors on this bike so how does it always inject fuel during the intake stroke rather than during the power stroke ?

(A carburettor is old school and simple and only delivers fuel when air is drawn through it (i.e. during the intake stroke).)

Anyone got any ideas ??

The flywheel has "metal knobs" on it, There is one missing. The magentic pickup sends the spark and injects the gas based on that.

Thanks for your reply.
Yes I know the flywheel has knobs on it with one missing, but the thing is that this 'missing' spot can pass the pickup coil at both TDC compression stroke and TDC exhaust stroke. Does it use manifold pressure sensor to ascertain when there is vacuum in the intake to determine the intake stroke versus the power stroke ?

Thanks again

hey r_little would you or anyone else happen to know where i could get a kick start for my husky tc 250 2004...i need the whole assembly.Cheers

No, the bike sparks every rotation. It's call a "wasted spark" system.

First of all, let me state categorically: I do not know. So the rest is pure speculation; if anyone knows for a fact please educate me (us).

ferguscawley, since we all understand the wasted spark system (although I don't know of any modern engine using it... lawnmowers maybe), the question remains for the fuel injection: how & when. There is no cam sensor.

I would GUESS that the ECU can tell the difference between the spark on compression and the spark on exhaust. It may take as little as 4 strokes to determine this (1 cycle, 2 crank revolutions, 1 cam revolution.... whatever you wanna call it). Hell, the difference might be big enough that during the first firing the ECU would know what stroke it's on.

After that- everything would be golden. Hell, you wouldn't even do the wasted spark thing anymore (maybe even reverse polarity on the plug for a few microseconds to redeposit electrode metal, during ignition). I am assuming the fuel spray happens a millisecond or two before ignition depending on engine speed, temperature, and air density.

Computers are fast, and can do lotsa stuff in microseconds (millionths)... millisecond events are spent mostly waiting- doing nothing.

Again: this is all speculation on my part.

Let me add a bit: When I say the ECU can tell the difference between a spark igniting on compression as to a spark jumping on the exhaust stroke, I mean the ionization time, current growth and drop-off and other factors that differentiate between the 2 different sparks. In essence the spark plug is a sensor for the ECU.

Once you've established the compression stroke, there's no need to fire on the exhaust stroke.

Maybe.

[sheesh- I can't believe I hafta go through the approval thang again.... I hate not being able to edit my posts]

Thanks for the replies guys

Would be easy enough to find out I guess.

Use an oscilloscope or even grab the spark pug lead when the motor goes to TDC on exhaust stroke and see if you get shocked.

Any volunteers?

That's interesting. Thanks for that.

Fergus

Thanks Jon- so you're saying the injector is spraying FOUR times per one full engine cycle (4strokes, 2 revolutions) on a single cylinder engine? hell, if that's true, you could have a base clock independent the actual engine speed (sorta a PWM type thing; and only widths varying depending on the rpm et al). But it seems like your mixture will vary within a cycle, depending on the instantaneous air velocity. But OTOH maybe that wouldn't matter in the real world. Hmmm... I'll hafta think about this a bit. And perhaps become edumajacated about efi finally.

BTW, betcha you know this: a carb is not drawing fuel constantly; there are tiny pulsations that coincide with the intake stroke/valve train closures . In fact, on the INTAKE side of the carb, there is a "cloud" (standoff from resonance) of fuel caused by these pulsations at certain rpm's. But I understand your model with regards to the TB.

ferguscawley- if this is indeed how the fi works, think of it as being able to spray constantly while the engine is running, with micro-adjustments of fuel volume in purt'near real-time. Doesn't need engine timing info: the ECU just richens it up when the air velocity is high (low pressure caused by intake stroke) and leans 'er out when the air velocity is low (also adjusted using info from other sensors of course: tps, air temp, cyl temp, rpm). Of course, IRL there are discrete sprays but, still, it's a neat model.