Define Engine TIMING

[Zak33]

OK, we got air, we got fuel mixed in a perfect ratio.....we got an open valve....

Lets do 4 cylinders....and wording this is gonna be hard.

Lets stick with 2 valves per cylinder, cam belted, electronic ignition (ie brief description of timing and WHY its like it is....)

This is really hard to word without pictures, but hit us with your best shots

Damn fine work so far on the other threads

[Big RICHARD]

Timing allows us to prevent horrible torque forces from twisting camshafts and to provide smoother power output.

[Zak33]

No....I get where you're going, but I wanna know about valve and piston timing...

[Big RICHARD]

You want to know how to calculate firing order and valve timings?

[IainB]

Man, I REALLY wanna give this a go, but im at work so cant do much detail.....
Anyway - Zak you mentioned Inlet open, therefore we know the piston (only going to have time to describe one cylinder though!) is on the downward stroke to fill the cylinder (I would also assume that the exhaust would be closed/closing by now (closing creating a scavenge effect) ).
On the next rotation the fuel/air mixture would start being compressed, and then at a pre-determined point - most probably off a crank driven sensor (being electronic ignition) or even off an old distributor (if this is being used as a trigeer only - thinking MSD systems here!!), the HT coil would release its energy, creating a spark at the spark plug.
If this is correctly timed the flame front of the now ignited fuel/air mixture (remembering that this isnt an 'instant' burn) should coincide with the piston being at TDC - Top Dead Centre, then providing the force to push the piston back down the bore.
If this is incorrect - in as much as the spark arrives early, the fuel / air mixture would complete its burn cycle before TDC - therefore trying to send the piston back down the bore before TDC - not good for your engine!! or if after TDC, a serious lack of power (as the fuel/air would be ignited after compression)
Feel free to rip to pieces

[sdp]

Just to elaborate a little on what Eazieb said there is an 'overlap' after the exhaust stroke where the inlet valve opens before the exhaust valve is fully closed. This helps the induction stroke because of the vacumn in the exhaust adding to the suction and it means that all of the exhaust gasses exit the bore.

I think the camshaft rotates at half-engine speed, but anyway, it's what makes the valves open and close; eccentric lobes on the cam (assuming ohc) push down on bucket tappets which in turn push down on the valves to open them. The valves are sprung so that they return to the closed postion after the cams rotates past the opening point. Valve bounce can occur at high revs - the springs aren't strong enough to get the valve closed on time, and worst case secenario is valve meets piston crown.

[IainB]

Pants - thats what I get for replying quickly at work - only covered ignition timing at not physical engine timing
Me bad - sorry !!
Thanks sdp

[Zak33]

Not bad men...not bad AT ALL

Good work.....anyone wanna add to it?

Or question it, so we can better explain ?