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Author Topic: Engine Theory Part 3 - Valve Timing  (Read 5476 times)

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Fuse 19

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Engine Theory Part 3 - Valve Timing
« on: 30 September 2008, 09:05:03 »

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!

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Fuse 19

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Re: Engine Theory Part 3 - Valve Timing
« Reply #1 on: 30 September 2008, 09:14:15 »

Q and A

1) Is the effect of opening of the exhaust valve before BDC called "scavenging"? Or is that where the the momentum of the fresh mixture from the inlet pushes out exhaust gas around TDC from valve overlap?

Answer:

Closer to the latter.

The opening of the exhaust valve early is because there is still some significant pressure in the cylinder yet, because the piston is close to BDC and now slowing down its not imparting any significant energy to the stroke (if any at all). So, you open the exhaust valve and the gases shoot out the exhaust before you have to use any piston energy to pump them out.

Scavenging actually occurs at TDC just before the exhaust valve shuts. What has happened is that the main exhaust pulse is now traveling down the exhaust and behind it there is negative pressure (yes, you get a vaccum created in an exhaust which is why manifold leaks can affect lambda readings!)....its this negative pressure that scavenges the last bits of exhaust gas out of the cylinder.

If we consider the 2.5 V6 which has:

Bore: 81.6mm
Stroke: 79.6mm

Add now we calculate the cc:

= Pi x (Bore Diameter/2)2 x Stroke x No of cylinders

= Pi x (8.16cm/2)2 x 7.96cm x 6

= 2497cc

Now, that is the swept volume of the piston, there is actualy about another 37cc of volume in the cylinder head (the squish area or combustion bowl).

By scavenging the exhaust gases we can actually get a small increase of cc by creating a small vaccum in the bore as the inlet opens and by ensuring that all the exhaust gses are pumped out, incluing the squish area which is NOT swept by the piston
« Last Edit: 30 September 2008, 09:15:54 by Mark »
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