eletric blow off

[AbatelliCristian]

some cars have electric blow off valve.
There are 2 map. Before and after the butterfly.
Is possible implement a strategy?
if pressure1 + "x"> pressure2 open blowoff

[hardline]

custom analog sensor input, then a generic IO.

[AbatelliCristian]

custom analog sensor input, then a generic IO.

It 's impossible. I need to compare and map1 map2, whereas a maximum value of difference

[elaw]

I can think of a way to do it that would not be quite perfect but would probably work...

Get a low-range pressure switch (say one that closes above 3 PSI) and plumb it to the turbo outlet or somewhere else before the throttle plate. Then configure the MS with a generic output that turns the output on when MAP < 100. Wire the generic output, in series with the pressure switch, to the blowoff/diverter valve. The result would be that the valve is energized whenever MAP is below atmospheric (assuming you're driving near sea level where atmospheric pressure is ~100 kPa) and there's boost at the turbocharger outlet.

Possibly the worst part of the above scheme is it doesn't deal with changing altitude well, as the manifold-pressure condition is based on absolute manifold pressure and not gauge pressure. You could get around that by hooking up a gauge-pressure sensor to the manifold and set it up in MS as a generic sensor, then use that to drive the generic output.

[AbatelliCristian]

The simply way is open bolws off when map < 90kpa... but for perfect working and max spool "I think " I need to compare map1-map2. In this way we can get to the surge limit during partial TPS.
If you can put this strategy is also a good upgrade for MS.

[masterx81]

Exists in the market differential pressure sensors, you can use one of them, input in the ms, and open the valve when you prefer via a generic output...

[dontz125]

Exists in the market differential pressure sensors, you can use one of them, input in the ms, and open the valve when you prefer via a generic output...

dP sensors tend to be finicky creatures. The ones I've seen on Mouser and Digikey tend to have very low tolerance for reverse pressure; things are fine so long as P1 < P2, but if P2 < P1 by even a little, you can ruin the sensor. There are dP switches and sensors out there that can take significant reversing loads, but these tend to be industrial designs that 1) use AC current, and 2) tend to be big and bulky.

You're probably better off using two MAP sensors and a comparator; since both sensors are always under 'positive' pressure, there's no chance of reverse pressure damaging one or the other.

[masterx81]

I think that the pressure before the butterfly will be always major or equal than the pressure after...
Else 2 normal absolute pressure and a comparator will be for sure a valid alternative.

[AbatelliCristian]

but is complex insert the strategy? 2 standard map and no external circuit. And there's the possibility of calibration the difference pressure.

[dontz125]

Here's a circuit diagram simulating 2 MPXH6250A MAP sensors; the dark blue trace (V2) is before the butterflies, while the green trace (V3) is after the butterflies. The red trace is the difference in voltage, while the light blue trace is the output of the comparator. U1 and U2 are acting as buffers, and help to keep the signal from sagging as it goes through the R1-R4 resistor network.

Both sensors start at about 0.5v, representing ~ 40 kPa at idle. As the boost increases, the after-throttle trace drops off at 2.5 seconds, simulating the driver closing the throttle. The voltage difference climbs, until at about 0.45v / 30kPa difference, the comparator activates its output.

The comparator reference value is set by the voltage divider R5 and R6, and would likely be replaced by a pot in real life. The 3 op amps are rail-to-rail designs; the first hour or so I was working at this, I was baffled by the huge offset I was getting. Switching to a rail-to-rail device (capable of true zero output) solved that.

dP Blow off.JPG

[jsmcortina]

Subtracting one sensor from another is easy to do in software. Handling this in a generalised manner needs more thought though.

James

[AbatelliCristian]

in stock engine with small-medium turbocharger, blow off is important for surge limits.
example:
rpm 5500, tps 20-30% gear 5, map 70-80kpa. if you measured pressure before the throttle, is possible 180kpa... this is the big problem because the engine sucks low lb/h, but the turbocharger is in pressure and work in "surge" area.
I think is important the difference map1 and map2. If we can decide "what's max difference", we can have the max pressure (and low spool during acceleration)at the surge limits....

this is my experience..
many years ago I have exploded the turbocharger without blow-off,in 5 gear, high rpm, low tps.... during a long way.

I hope my english

I hope my English is comprehensible

[dontz125]

Sustained high-rev / low-throttle operation is bad for the engine anyway. Many high-revving engines depend on compression forces to balance the mass of the piston trying to fly off the end of the rod.

[AbatelliCristian]

Sustained high-rev / low-throttle operation is bad for the engine anyway. Many high-revving engines depend on compression forces to balance the mass of the piston trying to fly off the end of the rod.

this is other question... is mechanical problem. And I've never seen a broken engine for low tps and high rpm.... stock engine no work in this condition?
I have see broken engine for too high rev limits, but not for low tps...

[masterx81]

And using a low tension spring on a normal blow off, an electric valve on the vacuum line to the bov, knowing the rpm of the turbo and the pressure we can regulate the bov so that vent only the air necessary to exclude the surge.
The jbperf iox if i'm not wrong can log an high speed channel, but how measure the compressor speed?
Reflective paint on a blade?

[jsmcortina]

Some turbos come with a speed sensor facility. Probably using a VR sensor. See also the shaft speed system in MS3.

James

[masterx81]

I've seen a borg warner kit and a garrett kit. The garrett costs much more...
Both read the speed from the aluminium blades (maybe measures eddy current magnetic field?). I think that they can be installed virtually on every compressor cover.
Would be nice to use the compressor speed for command the blow off (with a small rpm/pressure vs %bov opening) so that the surge can be avoided with precision. Maybe can be used also a strategy to avoid turbo overspeed or include the compressor speed with the control of the wastegate...