Re: Breaking News: New VAG 504.00 & 507.00 standa
but still nobody has given me a good solid engineering reason why it is bad idea
Generally speaking, a 2-valve OHV diesel
could have some of the benefits you've outlined previously; however, what you are failing to see is that VW engineers have chosen
pumpe-duese injection technology. Its main benefit is a shortened injection interval and higher pressures over a distributer pumped or common-railed system, if I understand correctly. Since the distance from cam lobe to PD plunger is so short, there is very little mechanical lash. Moreover, diesel fuel is a COMPRESSIBLE fluid. That means that over a fuel tube length of more than a few inches, it will exhibit the hydraulic compression-wave effect (a compression wave travels up and down the length, and the mass flow of fuel is concordantly slowed because you have to wait for the compression wave to travel. Addtionally, the wave spreads out the duration of the injection window in a manner related by the Navier-Stokes equation, dependent on temperature, viscosity, fuel makeup and additives, shear-forces and other factors.) This is one of the inherent design limitation of the VE pumped design. It is why VW engineers chose to not simply increase the injection pressures of the VE pump. It would have had a longer injection duration, and this would have complicated timing and reduced parameter flexibility for meeting emisions and fuel economy goals!
Recall that the Tdi's are injected rather late (retarded timing) to limit NOx production. Simply advancing the timing into the optimal range will accomplish a slight increase in power and fuel economy (if power not used all the time) in the VE design--(while NOx
will go up.) However, there is a limit to this, as the hydraulic effect means that the injection duration will have a fixed width unless you upgrade nozzles. This is the primary reason why nozzle upgrades are the
preffered first upgrade at tdiclub!
The engineers are trying to balance NOx limiting strategies with power and efficiency. If they were to try to activate a PD nozzle by rocker, in turn activated by pushrod, in turn activated by cam lobe, there would be more mechanical lash
in addition to the hydraulic effect. The net result would be an injection interval spread out MORE (and more variable(!)) than the current PD design. It would probably be WORSE than the VE design. Purely speculating, (since I don't have a supercomputer to solve the Navier-Stokes for this), I'd guess that the arrival time distribution would be gaussian or skewed gaussian, since there are ~16 differnt molecules in
standard D2.
I am quite confident that if VW engineers were given
carte-blanche to design a clean-sheet Tdi motor, they would ABSOLUTELY retain an overhead cam. Engineers aren't boneheads, you know. They have reasons for what they do.
Your ideas about 2-valve OHV design would certainly be fine for actuating valves but at increased parts cost. There's a reason no one is doing OHC. The next wave may be peizo-electric actuated direct injectors. Common rail also has room to grow and develop. The engineers still have lots of play in required parameters. Pumpe-duese opened up a new window of parameter 'play space' for the engineers. That is why VE has been abandoned. They'd run out of parameter 'play space' with that design. No way to meet future NOx emmissions goals and hit the other goals concurrently.
That's my 2 Wheatbacks.
Alrighty, back to 504/507 topic: Can't wait for them to get this deployed in NA
It will simplify things nicely!
PS: I think you should read the FAQ if you haven't in a while. It lays out the theory pretty well.
Edit: added post-script. Caveat: I'm no engineer--I'm a chemist, but I appreciate the complexity of the fluid dynamics involved.