Valve timing plays an important role in any engine and is quite relevant to the V6 s it can be adjusted (to an extent) thanks to the idler set used on the V6
The cam has a simple function in life and that is to control the opening and closing of the valves.
So, lets consider the ottoman engine again.
- Piston is at TDC (Top dead center) and as it moves down, the inlet valve opens as part of the inlet stroke (Suck)
- Piston reaches BDC (Bottom dead center) and the inlet valve closes.
- Compression stroke - Both valves closed (Squeeze)
- Power stroke - both vales closed, the piston moves down (Bang),
- At BDC the exhaust valve opens, the piston moves up (Blow)
- At TDC the exhaust valve shuts again
So, thats the theory and, as you might expect, its nothing like the real world. Why you may ask, the reason is that the various items involved in the process (fuel, air, exhaust gases etc) have mass and don't respond to instantly to the piston movement!
OK, lets start with a simple rule and very simple reasons why this is a rule
The timing of the inlet valve is more important than the timing of the exhaust valve.This is because on most engines the inlet stroke relies on creating a vacuum to draw the mix into the chamber where as the exhaust gases are pushed out by the piston!
So, lets consider the inlet to start
[size=14]Inlet valve opening[/size]This is actually less important than the closing but, lets try to picture whats happening!
If we open the valve before TDC the result will be some valve overlap because the exhaust valve will still be open also. Now what you have to consider is that the exhaust gasses are flying out the exhaust and creating a vacuum behind them as they go. Plus we also have a mass of moving air in the inlet manifold forcing it self into the chamber, this will draw some fuel air mix into the cylinder even though the piston is still traveling up-wards!.
Whats more, the faster the greater the mass and speed in the inlet, the earlier we can open the inlet valve and this is where variable valve timing can really play a part!
Inlet valve opening summary
Opening the inlet valve |
Earlier | Later |
More power at higher rpm | Less power |
More overlap with exhaust valve | More torque at lower rpm |
Lower flexibility | Better emissions! |
Poorer response at low rpm | |
[size=14]Inlet valve closing[/size]Again, because the incoming mixture has momentum and mass, we can actually close the inlet valve after BDC and still get mixture in as the piston start to rise!
And its this that has the biggest impact on power!
Inlet valve closing summary
Closing the inlet valve |
Earlier | Later |
Less power | More power at higher rpm |
Higher compression ratio at low rpm | Compression ratio improves with higher rpm |
Better torque | max. torque in higher rpm band |
More flexible engine | |
[size=14]Exhaust valve opening[/size]As stated in the rule above, the exhaust valve timing is less critical than the inlet but, you can still get some gains by playing with it!
So, in the case of the outlet, its the piston that pushes the exhaust gases out and this requires energy (pumping losses).
So, if we consider what is happening at the bottom of the power stroke (Bang), as the piston approaches BDC, there is very little power to be gained (most has already been taken and the piston is now slowing down) so we can open the exhaust valve a little earlier which will quickly reduce the cylinder pressure and create a fast pulse of exhaust gas through the exhaust. This then reduces the amount of pressure required to pump the gases out as a fair chunk of them already gone!
The down side is that the gases are hotter and the exhaust valve will run hotter as a result (note though that on a multi-valve engine we already have a benefit here - read the previous related thread!)
Opening the exhaust valve |
Earlier | Later |
Less pumping losses, bit more power | More pumping losses,less power |
Hotter outlet valve | More complete combustion, less emissions |
More chance of pre-ignition | Lower exhaust temperature! |
[size=14]Exhaust Valve Closing.[/size]OK, if we leave the exhaust valve open slightly beyond TDC we can now get those inlet gases to push the last of the exhaust gases out ensuring we get even more fuel air mix on the inlet stroke (Suck).
What we don't want to do is leave it open to long or some of the inlet gases will pass through to the exhaust. This results in popping and banging at low revs and lumpy idle (and I am sure we have seen this on race engines many times!)
Closing the exhaust valve |
Earlier | Later |
Less overlap with inlet valve | Part of intake mixture goes straight into exhaust at lower rpm (engine is "Off cam") |
More flexible engine at low rpm | More high end power |
Less power at higher rpm | Less flexible engine at low rpm |
More torque at low rpm | Low torque at lower rpm |
Improved emissions | |
So, to summarise, valve timing is very complex and all about trade offs!