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Chat Area => General Discussion Area => Topic started by: Marks DTM Calib on 23 September 2008, 13:24:37

Title: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 23 September 2008, 13:24:37
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.

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!

Title: Re: Engine Theory 3 - Valve timing
Post by: cem_devecioglu on 23 September 2008, 13:29:29
Been waiting for that thread :y
Title: Re: Engine Theory 3 - Valve timing
Post by: cem_devecioglu on 23 September 2008, 13:44:31
So for a race engine the inlet and exhaust valves must overlap for high end power( which explains their bad idle) .But how they solve the problem of low power for low rpms if turbo is not used..

My initial estimation is variable valve timing.. :)
Title: Re: Engine Theory 3 - Valve timing
Post by: Kevin Wood on 23 September 2008, 14:17:22
Quote
So for a race engine the inlet and exhaust valves must overlap for high end power( which explains their bad idle) .But how they solve the problem of low power for low rpms if turbo is not used..

My initial estimation is variable valve timing.. :)

There's one easy answer. Don't use low RPM! It's not a problem then.  ;D

Race cars generally have a much closer ratio gearbox so you can keep the engine RPM high where you have the power.

Variable valve timing is handy where you want a wider power band because you can have a taste of both worlds.

Kevin
Title: Re: Engine Theory 3 - Valve timing
Post by: cem_devecioglu on 23 September 2008, 14:28:27
Quote
Quote
So for a race engine the inlet and exhaust valves must overlap for high end power( which explains their bad idle) .But how they solve the problem of low power for low rpms if turbo is not used..

My initial estimation is variable valve timing.. :)

There's one easy answer. Don't use low RPM! It's not a problem then.  ;D

Race cars generally have a much closer ratio gearbox so you can keep the engine RPM high where you have the power.

Variable valve timing is handy where you want a wider power band because you can have a taste of both worlds.

Kevin

Not for skinnies like me ;D
Title: Re: Engine Theory 3 - Valve timing
Post by: Andy B on 23 September 2008, 14:39:29
Well written Mark.


PS Whilst we have Center Parks and Builders? Center we spell it centre here in Blighty!  ;D  ;D  :y  :y  :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 23 September 2008, 18:08:06
Quote
Well written Mark.


PS Whilst we have Center Parks and Builders? Center we spell it centre here in Blighty!  ;D  ;D  :y  :y  :y


As an engineer......spelling is a secondary issue and of no major relevance to me :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Bandit127 on 23 September 2008, 19:10:28
Thanks for a great article Mark.

2 things come out of it for me...

A statement
That the gasses are moving so fast, their momentum is significant in valve timing.  :o

A question
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?

Thanks again.
Title: Re: Engine Theory 3 - Valve timing
Post by: Andy B on 23 September 2008, 20:52:31
Quote
....
As an engineer...... .....

You have letters after your name?  8-)  8-)  :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 23 September 2008, 22:02:19
Quote
Thanks for a great article Mark.

2 things come out of it for me...

A statement
That the gasses are moving so fast, their momentum is significant in valve timing.  :o

A question
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?

Thanks again.

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 significnat 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....
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 23 September 2008, 22:04:53
Quote
Quote
....
As an engineer...... .....

You have letters after your name?  8-)  8-)  :y

Of course, but i rarely use them!
Title: Re: Engine Theory 3 - Valve timing
Post by: Bandit127 on 23 September 2008, 22:32:53
Quote
Quote
Quote
....
As an engineer...... .....

You have letters after your name?  8-)  8-)  :y

Of course, but i rarely use them!
Engineers that do use them are usually sales engineers people.

The exception is Civil Engineers which pretty much have to be chartered to be able to practice. They usually use theirs.. CEng.

Where I am in manufacturing, CEng adds nothing except oldfashionedness. Which is a shame really as it is quite tough to get unless you have a Masters and a significantly responsible job to go with it.
Title: Re: Engine Theory 3 - Valve timing
Post by: Andy B on 23 September 2008, 23:22:56
Quote
.....
Of course, but i rarely use them!

 :y  :y  :y  :y  :y  ;)
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 08:11:01
Quote
Quote
Quote
Quote
....
As an engineer...... .....

You have letters after your name?  8-)  8-)  :y

Of course, but i rarely use them!
Engineers that do use them are usually sales engineers people.

The exception is Civil Engineers which pretty much have to be chartered to be able to practice. They usually use theirs.. CEng.

Where I am in manufacturing, CEng adds nothing except oldfashionedness. Which is a shame really as it is quite tough to get unless you have a Masters and a significantly responsible job to go with it.

I achieved CEng status many years ago but, I am not willing to keep paying the IEE a huge fee for nothing!
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 08:22:18
Just to add to the scavenging question

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  :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 08:58:19
The next article will be about about cam duration, quite a short easy one to once you can understand the above.  :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 20:38:05
Do we have any requests for a topic anybody would wish covering?
Title: Re: Engine Theory 3 - Valve timing
Post by: Entwood on 24 September 2008, 20:42:06
If you have nowt else to do ..... ( and the stuff so far is brilliant ..  :)  )

I've never understood the theory behind the multi-ram system .. ie . how simply changing the length of the air-intake changes the torque curve .....

In the days of carbs the idea was as short and unobstructed intake as possible to get as much air in as you could !!!

 :-/ :-/ :-/ :-/
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 20:53:49
Quote
If you have nowt else to do ..... ( and the stuff so far is brilliant ..  :)  )

I've never understood the theory behind the multi-ram system .. ie . how simply changing the length of the air-intake changes the torque curve .....

In the days of carbs the idea was as short and unobstructed intake as possible to get as much air in as you could !!!

 :-/ :-/ :-/ :-/

The multiram is a spin off from the above.

We mention in the above how the mass and momentum of the inlet gases help to fill the cylinders.

So, if we have a short large inlet (i.e. a motor bike) we have slow moving air at low rpm and fast moving air at high rpm.....this results in low torque at low rpm and lots at the top revs.

If we now make the inlet long and smaller (i.e. the Rover V8) we have fast moving air at low rpm giving lots of torque and at high rpm the air flow is restricted.

If we then look at the multiram and plenum setup we have a few possabilities

1) Both Valves shut

We have each cylinder feeding off half a plenum and a single long medium size inlet tube. The result is lots of torque at low revs

2) Both valves open

We have each cylinder feeding off a full plenum and the inlet tube is now the two in parralel. The is result torque at the top end.

Plus two other which are switched in at around 3000-4000 rpm to smooth the transition (so its not sudden like on the old straight six boat anchor from the Senator)

So, we are doing little more than varying the size and length of the inlet to keep the air velocity up throughout the rev range.....and the reasons why we do this are as per the valve timing above :y
Title: Re: Engine Theory 3 - Valve timing
Post by: Entwood on 24 September 2008, 21:03:44
Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...
Title: Re: Engine Theory 3 - Valve timing
Post by: Marks DTM Calib on 24 September 2008, 21:09:14
Quote

Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...

Pretty much, if you read about the valve timing above, we want to ensure we get as full a charge in the cylinders as possible with max atomisation of the fuel........and that does not work at all well with slow moving air!
Title: Re: Engine Theory 3 - Valve timing
Post by: Bandit127 on 24 September 2008, 21:22:58
How about inlet pressure?

This could be used to discuss (K&N type) filters, port polishing and turbos?

Does the 'resonance' of the air box fit in here? It is certainly important fo highly tuned motorbike engines.

Title: Re: Engine Theory 3 - Valve timing
Post by: Mr Skrunts on 26 September 2008, 20:53:41
Quote
Do we have any requests for a topic anybody would wish covering?

Just thinking out loud to a degree,  but at some stage I would be interested in your thoughts on Blue printing, Balancing and Gas flowing.

The reason I ask. in the days of reading the car mags like CCC there were plenty of articles on the like of the Blydenstein engines and the differences of the Satge 1.2. and 3 cams, big valve heads etc.

These words dont appear very often these days, is it that todays motors are more efficiently produced and this work is not required, or is it something that motorsprt just take for granted.

Would any of the above be beneficial whilst work was being done on a Miggy.

Title: Re: Engine Theory 3 - Valve timing
Post by: Kevin Wood on 26 September 2008, 21:35:00
Quote
Quote

Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...

Pretty much, if you read about the valve timing above, we want to ensure we get as full a charge in the cylinders as possible with max atomisation of the fuel........and that does not work at all well with slow moving air!

There's also a resonance effect with intake tract length to each cylinder (which the multirams vary by isolating the 2 banks at differing lengths). In addition to the pressure pulse that Mark mentioned with regard to the exhaust system, the same thing happens in reverse in the intake. As a cylinder takes a "gulp" of air a pulse travels back down the intake away from the valve, is reflected from the entrance to the tract, and then travels back up it again. If its arrival at the intake valve coincides with the intake valve opening again you have a convenient high pressure pulse behind the valve which has a supercharging effect on the engine.

This only works in a couple of very narrow RPM bands, of course, but if you can tune it to happen where the engine has a "flat spot" for other reasons, such as cam characteristics or poor exhaust scavenging you can make a more tractable engine with a flatter torque curve. Alternatively, for a highly tuned engine, you can arrange it to happen at the top end of the engine's characteristics for more power.

Ideal intake length depends on RPM (the frequency of the pulses), so you have F1 engines revving to 18K RPM with little stubby trumpets over each valve (high resonant frequency), and intake manifolds with great big long ducts on lazier engines (low resonant frequency).

Kevin
Title: Re: Engine Theory 3 - Valve timing
Post by: Bandit127 on 26 September 2008, 21:51:15
What's going on with the EXUP valves fitted to Yamaha bikes then? Honda adopted these for the 'Blades too. It's a butterfly valve fitted in the exhaust that is virtually closed at low revs and open at high revs.

I understand that they increase low rev torque and work on resonance but don't know how.