Engine technology thread.

[Dimman]

Originally Posted by Snaps

^ I would love to, but I don't have the knowledge...At most mine is basic:

Lift: How far the cam lobe moves the valve - higher lift = more area around the valve for the air/fuel mixture to flow = more power in most cases. Too much lift can turn a non-interference engine into an interference engine, which is generally bad.

Duration: How long the valve is held open (measured in degrees), larger duration = longer time for air/fuel mixture to fully fill the combustion chamber. In reality, there is an optimum duration for each engine dependant on how much the head flows, the displacement of the engine, etc. If the piston begins to travel toward TDC and there is less pressure in the cylinder (compared to the pressure in the intake ports), the valve should stay open until the point where there is no difference in pressure. However, this is limited by the movement of the piston -If the piston gets to TDC (combustion stroke) then the intake valve needs to be closed before the spark plug fires.

Overlap: The amount of time the intake and exhaust valves are open at the same moment, don't know too much about the benefits of increasing/decreasing this, so I'll leave that to someone else

Intake velocity: A higher Intake velocity means more air flow into the cylinder when the intake valve is open, this obviously means more fuel can be used, and more torque can be achieved. A high intake velocity at low rpm is achieved by using smaller intake ports, this means the air travels faster into the cylinder compared to an engine with larger intake ports. However, by having smaller intake ports, you 'choke' the engine at higher rpm where the engine needs to flow much more air. Small intake ports are more generally used in Family sedans, V8's, etc. where the aim is to achieve a higher torque for the type of car (i.e. family sedan is likely to be driven around town a lot, it is more efficient to have more torque in the lower rpm ranges).

Larger intake ports cause a slower Intake velocity at lower rpm, which causes less torque, but these engines usually create more torque in higher rpm's (as this is when the Intake velocity is high, and the engine is not being choked by small intake ports), because of the higher rpm, the engine creates more power with the available torque (Power = [Torque x RPM]/5252, increasing rpm while maintaining torque will result in a higher power). Engines with larger ports are generally used in racing, where the rpm's are always kept high, and the larger ports create the same torque at those higher rpm's.

Excellent, Snaps. I will expand on some of the whys and hows of these tomorrow.

[Snaps]

Awesome, would love to hear it... Being an Engineering student I'm always looking to expand my knowledge

[bofa]

Originally Posted by serialk11r

Hey guys forgive me if this is a stupid question, but how much restriction does the air filter create? I've never examined one, but seems to me like it would cause a lot of restriction. I was thinking, wait I've never seen people try to use multiple filters or a ginormous one to reduce restriction. Engines suck quite a bit of air through those so I imagine it could be significant?

Stock intakes are generally a bit restrictive. Each engine has a sweet spot, but once you pass it you get into diminishing returns and there's no more to be gained. Not all oem intakes are bad though. The dual intakes on the 350Z/370Z are hard to improve upon. I think we are seeing manufacturers get better at optimizing the "cheap fixes" like intakes in order to improve overall performance without much investment. I'll be interested to see what this car comes with.

Edit... doh it logged me out so I replied to an old post... lol

[Dimman]

Originally Posted by Snaps

Awesome, would love to hear it... Being an Engineering student I'm always looking to expand my knowledge

So I lied. Not going to have time to get in depth today when strippers are waiting.

But since you're an engineering student, I'll give a few ideas for you to look into.

1.) Airflow:Valve lift. Given that a valve lifted over a certain limit (.25D or something, I can't remember the exact ratio at the moment) will no longer be a restriction to airflow, why do some performance cams exceed that limit?

2.) Valve angle, overlap and interference. This is something to do with the 45 degrees valve angle of the Toyota 'G' heads. This isn't great. Can you think of why?

3.) In both the hi-po multi-valve motorcycle AND 2 valve domestic performance headwork, there is now a trend to sacrifice some airflow in exchange for more velocity in extreme performance heads. Why?

If I'm not too hung-over tomorrow, I'll expand.

[serialk11r]

Point 1 is just false, there is always restriction, depending on design whoever is making these high performance cams thinks they can flow even better. That's all I can see at the moment, there may be another reason.

[Snaps]

1) ^ I agree, I don't think there is ever NO restriction on air passing around the valve, BUT there is a point where the available flow around the valve is more than the available flow through the intake/exhaust port... At which point I would guess there is almost no benefit to going with more lift, and there may even be a detriment - as it means the air would slow down as it reaches the valve...

As for the reason that performance cams exceed that limit, I'm not sure.. maybe more lift allows a slightly smoother airflow and therefore more mixture into the cylinder. The only other reason I can think of is that because the cam lobes must be smoothly curved to lift and lower the valve smoothly, the cams are 'overshot' to allow longer for the valve to be at the point where it is no longer a restriction. (i.e. the point where there is no restriction is not at the vertex of the lobe, but some points before and after that, and anything between those points will flow with no restriction).

2) I have very limited knowledge in this... I have thought about what valve angle would produce the smoothest flow entering/exiting the cylinder, but haven't come up with any definitive answer. I know there is an angle that is the most efficient for each specific cylinder, but if I was to try and apply that as a generalisation, I'm sure I would be wrong. I would love to hear more about it though!

3) Also guessing a lot here... Maybe the higher velocity causes a higher pressure in the intake port/runner when the intake valve closes, and this higher pressure area could either help to force air back into the cylinder when the valve opens again, or could help to slightly pressurise the plenum in order to force more air into other cylinder's when the valves for other cylinder's open? However, I'm not too sure about the travel of pressure in the plenum (speed of sound in air) compared with the speed of the valves opening and closing, and whether or not the valve for the cylinder would open before the pressure wave has a chance to travel back out into the plenum, down another runner, and into another intake port...

[Dimman]

Originally Posted by Snaps

1) ^ I agree, I don't think there is ever NO restriction on air passing around the valve, BUT there is a point where the available flow around the valve is more than the available flow through the intake/exhaust port... At which point I would guess there is almost no benefit to going with more lift, and there may even be a detriment - as it means the air would slow down as it reaches the valve...

As for the reason that performance cams exceed that limit, I'm not sure.. maybe more lift allows a slightly smoother airflow and therefore more mixture into the cylinder. The only other reason I can think of is that because the cam lobes must be smoothly curved to lift and lower the valve smoothly, the cams are 'overshot' to allow longer for the valve to be at the point where it is no longer a restriction. (i.e. the point where there is no restriction is not at the vertex of the lobe, but some points before and after that, and anything between those points will flow with no restriction).

First off there IS a point where lifting the valve higher off the seat offers no gain in flow. Not necessarily that the valve is not obstructing flow (stem is still in the way no matter what).

1) Bingo. Maximum valve lift (and any flow associated with it) is instantaneous, only at the apex. So extra lift beyond the flow limit, allows the valve to be at maximum flow for longer.

Originally Posted by Snaps

2) I have very limited knowledge in this... I have thought about what valve angle would produce the smoothest flow entering/exiting the cylinder, but haven't come up with any definitive answer. I know there is an angle that is the most efficient for each specific cylinder, but if I was to try and apply that as a generalisation, I'm sure I would be wrong. I would love to hear more about it though!

2) I've heard from a flow point of view the angle affects whether the combustion charge 'swirls' or 'tumbles' (and tumble is favoured by the pent-roof multivalve combustion chamber) but that isn't the critical issue. As the valve angle increases any lift of the valves is more towards each other (intake to exhaust). So a problem that bikes ran into because of their need for both high lift AND overlap is that the interference was the valves smacking into each other during the overlap period, and not into pistons. So to maximize the cam use, valve angles need to be narrower (sport bikes are around 20ish degrees).

Originally Posted by Snaps

3) Also guessing a lot here... Maybe the higher velocity causes a higher pressure in the intake port/runner when the intake valve closes, and this higher pressure area could either help to force air back into the cylinder when the valve opens again, or could help to slightly pressurise the plenum in order to force more air into other cylinder's when the valves for other cylinder's open? However, I'm not too sure about the travel of pressure in the plenum (speed of sound in air) compared with the speed of the valves opening and closing, and whether or not the valve for the cylinder would open before the pressure wave has a chance to travel back out into the plenum, down another runner, and into another intake port...

3) How higher velocities are used is that they can take advantage of even longer durations. (What is the force formula again? 1/2 mass x velocity^2?) What they are finding is that at high velocity the intake charge has enough inertia to continue stuffing itself into the chamber even as the piston begins compression (as long as the cams duration take advantage of this). The trade off (where the 'this is for power, but you lose torque' comes in) is a low intake velocity (lower rpm) is that the longer duration will push out the low energy, slow intake charge back into the port, losing power. This is where profile switching systems like VTEC allow for such aggressive high rpm profiles without losing too much on the bottom (VTEC kickin' in yo!).

Hope that gives you some things to think about...

[Snaps]

Originally Posted by Dimman

First off there IS a point where lifting the valve higher off the seat offers no gain in flow. Not necessarily that the valve is not obstructing flow (stem is still in the way no matter what).

That's pretty much what I was saying, wasn't it lol? That there is no reason lifting the valve to flow any more than the ports can?

Originally Posted by Dimman

1) Bingo. Maximum valve lift (and any flow associated with it) is instantaneous, only at the apex. So extra lift beyond the flow limit, allows the valve to be at maximum flow for longer.

Woohoo!

Originally Posted by Dimman

2) I've heard from a flow point of view the angle affects whether the combustion charge 'swirls' or 'tumbles' (and tumble is favoured by the pent-roof multivalve combustion chamber) but that isn't the critical issue. As the valve angle increases any lift of the valves is more towards each other (intake to exhaust). So a problem that bikes ran into because of their need for both high lift AND overlap is that the interference was the valves smacking into each other during the overlap period, and not into pistons. So to maximize the cam use, valve angles need to be narrower (sport bikes are around 20ish degrees).

This was the other point I thought of, though for some reason forgot about when I got around to writing the post haha. Obviously different combustion chamber shapes prefer different angles for the valves to produce the best airflow for the cylinder, but is there any generalisation that can be applied to that? I would assume that having the valves at very small or very large angles to the perpendicular of the piston travel would be detrimental to performance, but to hear that sports bike's use ~20 degrees kinda throws that out a little...

Originally Posted by Dimman

3) How higher velocities are used is that they can take advantage of even longer durations. (What is the force formula again? 1/2 mass x velocity^2?) What they are finding is that at high velocity the intake charge has enough inertia to continue stuffing itself into the chamber even as the piston begins compression (as long as the cams duration take advantage of this). The trade off (where the 'this is for power, but you lose torque' comes in) is a low intake velocity (lower rpm) is that the longer duration will push out the low energy, slow intake charge back into the port, losing power. This is where profile switching systems like VTEC allow for such aggressive high rpm profiles without losing too much on the bottom (VTEC kickin' in yo!).

Hope that gives you some things to think about...

So really, to have a more perfect engine you would want a continously variable duration? I assume a similar effect could be achieved with the use of variable lift? I can't think of any, but I'm going to have a search on variable duration systems and see if anything turns up...

[Dimman]

Originally Posted by Snaps

So really, to have a more perfect engine you would want a continously variable duration? I assume a similar effect could be achieved with the use of variable lift? I can't think of any, but I'm going to have a search on variable duration systems and see if anything turns up...

I was looking at the Fiat MultiAir video and thinking about this. One of the things it showed was instantly dropping lift instead of following the cam's curve. The variable lift systems are giving me headaches, as I just started to get a handle on normal cam operations...

[Snaps]

The only variable lift system I know how it works is Toyota's valvematic... And that seems reasonably simple...

Having a look at Fiat's MultiAir now...it's pretty cool, I have wondered why manufacturers haven't used more electronically-controlled valve systems, but it seems like they're finally getting into it now... Still, that's a pretty complicated system, and it will also take me some time to fully get my head around it!

[Exage]

Originally Posted by Dimman

I was looking at the Fiat MultiAir video and thinking about this. One of the things it showed was instantly dropping lift instead of following the cam's curve. The variable lift systems are giving me headaches, as I just started to get a handle on normal cam operations...

What will really bake your noodle is that there are engines (albeit not in the automotive sector) that currently have infinitely variable timing adjustable on the fly by computer.

It's kind of like the Fiat MultiAir system except they don't feature a camshaft with profiles for valve actuation, instead the computer controls a valve for pressurized lubricating oil for valve actuation based on crank angle and other sensors with pneumatic valve return.

The sensors tell the computer the appropriate time for engine fuel injection and valve actuation. However you can play around with valve lift, duration, timing, as well as injector timing and pulse width, off of a base map for; fuel efficiency, power or tweaks for optimization and engine longevity.

[Snaps]

Wow... saves the need for having specific 'performance' or 'efficiency' cams and just gets rid of cams all together XD

[serialk11r]

Except that kind of system sounds like it consumes quite a bit of power compared to cams...

[Exage]

It does, but power and torque are not primary issues for this particular engine, emissions and maintenance are.

I don't think you'll would see this system adopted by the automotive industry. It's just an interesting tidbit I thought I'd share.