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arghx7 · 02-08-2014, 04:51 PM

Some rules of thumb to understand about cam timing:

Overlap

1)When the overlap occurs before top dead center intake, with the intake valve opening early, air/mixture pushes back into the intake port. This is because the piston is still rising in the exhaust stroke when the intake valve opens. This will throw hot exhaust gases back into the intake port. It can reduce pumping losses, improve vaporization of fuel on the back of the valves (port injection), and could potentially increase the chances of forming carbon deposits.

2)When the overlap occurs after top dead center (intake valve opens after top dead center intake, exhaust valve is still open), the piston will tend to draw exhaust gases back into the cylinder. This is common to reduce NOx emissions and improve fuel economy.

3) If intake pressure exceeds instantaneous in-cylinder and exhaust pressures, scavenging will occur. Extra air will blow through the cylinder, in the process evacuating residual gases from the chamber. This will also increase the mass flow to a turbocharger wheel.

Intake Valve Timing

4) Geometrically speaking, max effective volume and compression ratio occur when the intake valve closes right around bottom dead center (give or take say 10 degrees).

5) Closing after bottom dead center reduces effective volume, effective compression ratio, and mixture temperature at TDC firing. This is because the piston has pushed air back out the intake valve. This will reduce the tendency to knock and in some instances could be called atkinson cycle or miller cycle.

6) Closing before bottom dead center has some similar effects as late closing, with lower effective compression, but mixture temperature can increase and so can knocking. Early intake valve closing is a form of dethrottling.

7) The air flowing into the cylinder has inertia. By closing the intake valve later at high speeds, more time is allowed to fill the cylinder up. Filling time vs effective compression ratio are the two major and in some ways competing aspects of intake closing timing.

8) Intake valve opening timing is a little less important for high speed power and cylinder filling. Opening later in the intake stroke, when the piston is at higher speeds, improves motion in the combustion chamber. However there can also be an increase in pumping work. Late intake valve opening is generally not good for power or torque, but it is good for in-cylinder motion and stable combustion.

Exhaust Valve Timing

9) The exhaust valve event is divided into two phases: the blowdown phase and the scavenge or evacuation phase--whatever you want to call the second one. Basically, at exhaust valve open there is a big high pressure pulse released, and that pressure pulse is bigger the earlier you open the exhaust valve.

10) Early blowdown can help relieve pressure in the cylinder, but depending on the exhaust manifold design blowdown can cause interference with other cylinders' exhaust pulses. It's one of the reasons why you have 4-2-1 headers and twin scroll turbos on I4 engines.

11) The earlier you open the exhaust valve, the more expansion work is wasted, but in many cases the less pumping work to move exhaust gases out. Late exhaust valve opening is better for increasing expansion work, at the expense of additional pumping work. It's one of the principles of variable exhaust lift systems.

11) Later exhaust valve closing timing can give more time to allow exhaust gases to escape, but too late could potentially draw gases back in (besides possible unwanted overlap).

Keep in mind this sort of "textbook" PV diagram:

Now when you're actually tuning the thing in the vehicle, you basically are doing what @mad_sb did in his very fine thread on cam phaser sweeps. With a turbo though it gets more complicated, because you want a lot of overlap down low to spool the turbo through scavenging. But keep those effects in mind when you are looking at valve timing diagrams and cam phasing maps.

02-08-2014, 04:51 PM	#8
arghx7 Senior Member Join Date: Nov 2011 Drives: car Location: cold Posts: 599 Thanks: 72 Thanked 610 Times in 185 Posts Mentioned: 33 Post(s) Tagged: 0 Thread(s)	Some rules of thumb to understand about cam timing: Overlap 1)When the overlap occurs before top dead center intake, with the intake valve opening early, air/mixture pushes back into the intake port. This is because the piston is still rising in the exhaust stroke when the intake valve opens. This will throw hot exhaust gases back into the intake port. It can reduce pumping losses, improve vaporization of fuel on the back of the valves (port injection), and could potentially increase the chances of forming carbon deposits. 2)When the overlap occurs after top dead center (intake valve opens after top dead center intake, exhaust valve is still open), the piston will tend to draw exhaust gases back into the cylinder. This is common to reduce NOx emissions and improve fuel economy. 3) If intake pressure exceeds instantaneous in-cylinder and exhaust pressures, scavenging will occur. Extra air will blow through the cylinder, in the process evacuating residual gases from the chamber. This will also increase the mass flow to a turbocharger wheel. Intake Valve Timing 4) Geometrically speaking, max effective volume and compression ratio occur when the intake valve closes right around bottom dead center (give or take say 10 degrees). 5) Closing after bottom dead center reduces effective volume, effective compression ratio, and mixture temperature at TDC firing. This is because the piston has pushed air back out the intake valve. This will reduce the tendency to knock and in some instances could be called atkinson cycle or miller cycle. 6) Closing before bottom dead center has some similar effects as late closing, with lower effective compression, but mixture temperature can increase and so can knocking. Early intake valve closing is a form of dethrottling. 7) The air flowing into the cylinder has inertia. By closing the intake valve later at high speeds, more time is allowed to fill the cylinder up. Filling time vs effective compression ratio are the two major and in some ways competing aspects of intake closing timing. 8) Intake valve opening timing is a little less important for high speed power and cylinder filling. Opening later in the intake stroke, when the piston is at higher speeds, improves motion in the combustion chamber. However there can also be an increase in pumping work. Late intake valve opening is generally not good for power or torque, but it is good for in-cylinder motion and stable combustion. Exhaust Valve Timing 9) The exhaust valve event is divided into two phases: the blowdown phase and the scavenge or evacuation phase--whatever you want to call the second one. Basically, at exhaust valve open there is a big high pressure pulse released, and that pressure pulse is bigger the earlier you open the exhaust valve. 10) Early blowdown can help relieve pressure in the cylinder, but depending on the exhaust manifold design blowdown can cause interference with other cylinders' exhaust pulses. It's one of the reasons why you have 4-2-1 headers and twin scroll turbos on I4 engines. 11) The earlier you open the exhaust valve, the more expansion work is wasted, but in many cases the less pumping work to move exhaust gases out. Late exhaust valve opening is better for increasing expansion work, at the expense of additional pumping work. It's one of the principles of variable exhaust lift systems. 11) Later exhaust valve closing timing can give more time to allow exhaust gases to escape, but too late could potentially draw gases back in (besides possible unwanted overlap). Keep in mind this sort of "textbook" PV diagram: Now when you're actually tuning the thing in the vehicle, you basically are doing what @mad_sb did in his very fine thread on cam phaser sweeps. With a turbo though it gets more complicated, because you want a lot of overlap down low to spool the turbo through scavenging. But keep those effects in mind when you are looking at valve timing diagrams and cam phasing maps. Attached Images Last edited by arghx7; 02-08-2014 at 05:02 PM.