Evaporative Emissions Purge Solenoid
The factory Rover evaporative emissions carbon canister purge solenoid was originally 'modulated' on and off when coolant temp is above 70c, engine is above 1500rpm and MAP below 30kpa.
- solenoid resistance ?
- 24.8ohm @ ~10c
- what happens if 12V is applied to the solenoid ?
- Valve is open
- why didn't they use a solenoid with lower current if 100% is never applied ?
- a series resistor can also be used to limit current
Documentation found on Rover MEMS control strategy
- Carbon filter solenoid valve (CFSV) A CFSV and activated carbon canister will also be employed to aid evaporative emission control (see illustration 14.7). The carbon canister stores fuel vapours until the CFSV is actuated by MEMS.
- the engine temperature is above 70°C
- AND engine RPM > 1500
- and the MAP<30 kPa.
When the CFSV is actuated by MEMS, the valve is modulated on and off
- what frequency ?
- anyone's guess without scoping..... Not going to happen, I no longer have the OEM ECU.
- Google search brings up Land Rover workshop manuals that state 10hz for purge valve frequency. The valve in this application may be very different?
- 25hz is minimum configurable in Secondary PWM
and fuel vapours are drawn into the inlet manifold to be burnt by the engine during normal combustion. So that engine performance will not be affected, the CFSV remains closed during cold engine operation and also during engine idle (makes sense indeed).
Using a diagnostic tool to access the Rover MEMS ECU live parameters, there is a page that shows 'Purge Duty' as 56%. This value never seemed to change in all the times I ever reviewed it. For lack of information on this function I will make the assumption that this is the duty cycle of the purge solenoid. Whether this value changes for different engine speeds is unknown at this time, so will simply assume a fixed value at this time.
Legacy
Previously, going back to firmware 1.0.73 I simply energised the solenoid on RPM and MAP. slightly later firmware releases up to 1.1.47 and beyond introduced the coolant temp into the Misc output config which helped. I've not had this car running since 2010 and a lot has changed in firmware since!!
Current
I intend to use secondary PWM in Absolute mode and configure 56% duty above 1500rpm, below 6000rpm and below 30kpa (~engine breaking). I had thought about using Misc2 output to energise a relay to cut the 12v feed to the solenoid based on coolant temp alone to emulate the original Rover strategy, but the extra hardware and wiring doesn't justify it in this application. It would be neat to add a function in the Firmware to allow Misc output to function through the secondary PWM table, but I'm happy enough the way it is at the moment.
- How do others implement evaporative hydrocarbon emissions systems on newer vehicles?
- RPM >1600 (anything messing with idle is undesired)
- CLT>65C (less important; kinda defeats one of the important usecase is to extract some fumes soon after startup on a summer beach)
- MAP < 50 kPa (cruising is usually 38..44 kPa) condition or engine breaking (<32 kPa) or similar
- many simply do not connect
- Connecting with a very tiny restriction would also work (the engine needs some air anyway, 10..13% of the idle charge coming from there should not cause any problem, if it disturbs idle, than RPM>1600 solenoid should handle it )
- Bernoulli airspeed for deltap=100kPa -35 kPa = 65000 Pa v= sqrt(65000/0.6) = 329 m/s
- because p*V = m/2 * v*v (m/V = 1.2kg / m3 for air)
- so 1.5 .. 1.7mm diameter drill or max 2,4 mm2 to get into the ballpark of 3 * 0.4 * 0.1 * 800/60/2 = 0.8 liter / sec
- wether mechanical or electric or electronic solution: only enable "EVAP" after tuning (and capturing logs) and see (idle) changes. Tuning normally and slightly richer (due to any reason) is usually less annoying for the driver than slightly leaner.
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