[Diy_efi] Diy_efi Digest, Vol 1, Issue 524

[Marcell Gal]

Hi Mike,

Let's imagine a scenario:
We're at  maxTPS, driver wants max crank-Power possible.
 (not saying WOT since at drive-by-wire it is confusing  )
We're around max boost.
We want to save the turbo from overspinning.

There are 2 ways (currently discussed) for controlling boost:
- preTurbo wastegate (binary on/off or multistep or variable inlet)
- postCompressor throttle plate

If you use the wastegate you can maintain high crank-Power 
(especially if you have separate exhaust pipe for the wastegate).
If you use the throttle plate, you can only do this with
decrease in the crank-Power at the same time, which is unwanted.
(our second goal, keeping the Turbo RPM under control will 
result in a decreased P).

Why? Let's imagine 2 different sized turbos:
- a slow turbo => huge lag, unpleasant to drive
- a fast turbo => turbo _KEEPS_ACCELERATING_ at the given engine 
operating point * You can only keep it from overspinning if you
decrease engine-power: skip spark/injection or decrease
MAP _BELOW_THE_LEVEL_WHICH_YOU_WOULD_LIKE_FOR_MAX_POWER
=> you do not get maximum crank-power.

That's why some kind of wastegate or variable exhaust/turbo geometry
 is your friend. 
At least that's how I understand, it might not be absolutely
correct. Do you have some computational model for an engine to play with?
In opensource Java-sourcecode, eg.?

	Marcell

*  a very basic model:
for any crank-Power operating point (which relates to the energy of exhausts) 
there is an ** turboRPM the turbo-speed is converging to (not exactly in a
logarithmic fashion, but kindof). For a fast-spinup turbo at maxP this will
 be above safe turboRPMs, your turbo would explode. 

** Of course this turboRPM also depends on other factors (pressure ratios, T),
but the above would give a good starting point to start thinking about it


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