Fuel system vacuum pump, what does it do?
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Depressurizing the gas tank keeps your gas fresh longer. Some people advocate using a gas additive product like Stabil to add another level of confidence in the freshness of the gas that you haul around month-after-month while driving on EV power.
If you're one of those people who have added gas to your Clarity (or someone who just tries all the buttons), you'll have noticed the delay in opening the gas-filler door. I don't know why the Clarity can't simply open the door and release the vacuum without the delay. Would a sudden release in the vacuum cause gas to spill out if you opened the gas-filler door when the tank is already filled (or over-filled)?
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ClarityDoc
Active Member
A vacuum cannot be released, since it's an absence. Relieving the vacuum, i.e. sucking air into the tank, might be something they engineered - and the fill port might be the wettest (i.e. worst) path for that. Not sure but I hope they might have a dry, perhaps filtered, path for air to enter when repressurizing (on opening).
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neal adkins
Active Member
My camry hybrid did had the same delay. I wonder if this coild be a safety precaution to avoid the possibility of a spark from static electricity or because of the batteries being close to fuel tank.
In an ICE car, warm days cause the gasoline vapor pressure to increase. To prevent escaping vapor pollution, the gas tank is attached to a carbon granule canister. The carbon absorbs the gasoline vapors. When the engine is run, the intake manifold develops a vacuum. It is attached to the canister, and it sucks the vapors our of the carbon and they get burned by combustion. Since the engine runs frequently, it keeps the carbon from getting saturated.
A PHEV may go long times without the ICE turning on to empty the canister of absorbed gasoline vapors. I'm wondering if the vacuum pump is for emptying (or filling) the canister. But if the vapor pressure builds up over time, and the vacuum pump empties it, where does the pump exhaust go? You can't release it to the air, you can't burn it in the engine if the engine doesn't run. So, what is is connected to (input and output) and when does it run? And why?
A PHEV may go long times without the ICE turning on to empty the canister of absorbed gasoline vapors. I'm wondering if the vacuum pump is for emptying (or filling) the canister. But if the vapor pressure builds up over time, and the vacuum pump empties it, where does the pump exhaust go? You can't release it to the air, you can't burn it in the engine if the engine doesn't run. So, what is is connected to (input and output) and when does it run? And why?
Are we sure it is under partial vacuum? I got the impression it was retaining pressure. This makes sense to me, as when the vapors are created with temperature rise, rather than venting them or chemically absorbing them, they can just be retained at higher vapor pressure, and will re-condense into the gasoline if the temperature falls. If gasoline is burned then the free space in the gas tank will increase, reducing the vapor pressure without actually losing any vapor.
Keeping the vapor to re-condense also makes sense since it would keep the gasoline mixture from changing - more volatile compounds escape to vapor more readily, and this is more of an issue if more time elapses before adding fresh gasoline. Under this theory vapor would end up being vented when depressurizing to refuel, but not between refuelings, and the tank has the most free vapor space if it is refueled near empty.
This could be all wrong - I'm not a chemist, but I am a physicist.
Keeping the vapor to re-condense also makes sense since it would keep the gasoline mixture from changing - more volatile compounds escape to vapor more readily, and this is more of an issue if more time elapses before adding fresh gasoline. Under this theory vapor would end up being vented when depressurizing to refuel, but not between refuelings, and the tank has the most free vapor space if it is refueled near empty.
This could be all wrong - I'm not a chemist, but I am a physicist.
Groves Cooke
Active Member
"I got the impression it was retaining pressure."
You are correct, there is no vacuum; the fuel system is pressurized. Here is a quote frpm the owners manual:
"This vehicle has a pressurized fuel system.
When the fuel vapor pressure inside the fuel tank is
high, it takes about 10 seconds for the vehicle to vent
the fuel tank. Once the pressure is vented, the driver
information interface changes from Please Wait to
Ready.
If the filler nozzle keeps turning off when the tank is
not full, there may be a problem with the pump's fuel
vapor recovery system. Try filling at another pump. If
this does not fix the problem, consult an authorized
Honda Clarity Plug-In Hybrid dealer.
Do not continue to add fuel after the filler nozzle has
automatically stopped. Additional fuel can exceed
the full tank capacity.
The filler nozzle automatically stops to leave space in
the fuel tank so that fuel does not overflow as a result
of changes in air temperature."
You are correct, there is no vacuum; the fuel system is pressurized. Here is a quote frpm the owners manual:
"This vehicle has a pressurized fuel system.
When the fuel vapor pressure inside the fuel tank is
high, it takes about 10 seconds for the vehicle to vent
the fuel tank. Once the pressure is vented, the driver
information interface changes from Please Wait to
Ready.
If the filler nozzle keeps turning off when the tank is
not full, there may be a problem with the pump's fuel
vapor recovery system. Try filling at another pump. If
this does not fix the problem, consult an authorized
Honda Clarity Plug-In Hybrid dealer.
Do not continue to add fuel after the filler nozzle has
automatically stopped. Additional fuel can exceed
the full tank capacity.
The filler nozzle automatically stops to leave space in
the fuel tank so that fuel does not overflow as a result
of changes in air temperature."
Hello fellow physicist. Gasoline vapors in the tank will be under a relative vacuum or pressure depending on ambient temperature. In this weather, my plastic gas cans crumple inward. In the summer, if left in sunlight, they bulge outward. So, it depends.
From the owners manual, describing sounds unique to the Clarity: "Noise from Under Vehicle: This noise is caused by the vacuum pump inside the fuel evaporation leakage check module (ELCM). Depending on conditions, the pump will come on for about 15 minutes about 5 to 10 hours after engine shutoff. This noise is just normal vehicle operation and don’t indicate a vehicle problem."
I don't know of other cars using a vacuum pump as part of the fuel system. My original question stands: What does this pump do? What is it connected to on the input? What is it connected to on the output?
neal adkins
Active Member
Wouldn't it make sense to suck it from the canister and put the vapors back into the fuel tank? This would also repressurize the fuel tank after the the fuel condenses after cooling down at night. Im definately not an expert though.
SOLVED, nothing unique or special. Full explanation of similar system here https://www.motor.com/wp-content/uploads/2016/02/6_EvapSystem_0216.pdf
TL;DNR: When I read that the vacuum noise was "unique to the Clarity", I assumed this fuel system was unique, perhaps because the gasoline might stay unused for so long in the Clarity. Nope. This is just a pretty standard way of checking for leaks in a car's fuel vapor system as required by current emission regulations. What a disappointment. OTOH, I learned how fuel vapor leak detection worked, so that's fun.
TL;DNR: When I read that the vacuum noise was "unique to the Clarity", I assumed this fuel system was unique, perhaps because the gasoline might stay unused for so long in the Clarity. Nope. This is just a pretty standard way of checking for leaks in a car's fuel vapor system as required by current emission regulations. What a disappointment. OTOH, I learned how fuel vapor leak detection worked, so that's fun.