Re: Flow vs Pressure
I personally like the PWM set-up.
Initially it was a Aquamist 2c. Since the race pump dropped in flow rate, I have replaced it with a 160PSI 2.xl/min pump of eBay for €26!. The harness works for the most part except that the pump flow indicator LED is now driven by the 12V line after the pump's micro switch.
The pump has a integrated pressure switch. It stops at about 10bar. Once pressure drops below 8 bar, the pump automatically activates itself. The switch closing fires also the LED. This is my flow indicator. As in the 2c system, I maintained the 30cc accumulator. This dampens the pump switching cycles.
So overall it behalfs like a fuel injection system with pump based pressure control. A bit more crude, but hey.
With this system you end up with a line that is permanently pressurized with your mix.
Next you need to control flow. For this purpose Aquamists sells very suitable valves. I have the older HSV (high speed valve). Now they sell a version called FAV (fast acting valve).
The valve is controlled by my ECU. I use a boost control map with PWM output for this purpose. Most ECUs feature auxiliary PWM maps. The boost control map in open loop mode allows to set a PWM percentage based on throttle position vs. rpm.
This allows me to deterministically set my injection onset and flow rates over the rpm range. The beauty is the proportionality of PWM. If I set it to 70%, I get about 70% of the max. flow rate, if I set it to 30%, I get about 30% flow. It makes the injection a very reproducible and mappable process. I do not need any Hobbs switches to control the system. I still have one to automatically switch to a non WMI map once rail pressure drops below like 5 bar.
Cableing is simple. 12V and ground to the loom, LED and tank level lines to the front., 1 wire from the ECU to control the HSV and 1 line for the map switch as a fail safety.
There is no separate controller and its associated wiring and adjustment knobs. It is all ECU controlled.
Under PWM control, you get spray even at low flow rates as during each opening of the HSV, a full pressure wave passes through the line to the jets.
I can test the pump by pressuring the throttle with the engine stopped.
Over the last 25% of the throttle the valve opens from 0 to 100%.
It works beautifully and you get spray over a big part of the control range.
Pump speed based systems only increase flow by about 40% for a doubling of the pressure.
This is probably why those 250PSI pumps are so popular. You need the dynamic range on the top to get a sufficent controllable flow range as at low pressures, spray is poor.
If you change jet size, all the scaling does not work anymore as pressure vs. flow is now changed. Exception are internal bypass pumps with a constant pressure.
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