technique

Perhaps all plug-in hybrids open each cap to add energy in a different way. It has a button near the fuel tank neck, the main task of which is to start venting the excess pressure.

For some cars, the fuel tank cover opens from inside the car, somewhere from the driver’s seat, while for others, it opens from the outside using a finger recess or by pushing on the cover itself. However, plug-in hybrids, with absolute exceptions, have the same opening method – while the cover of the charging socket opens by pressing, the cover of the fuel tank neck has a button in the interior.

One can also notice this apparent illogicality in extremely expensive plug-in hybrids, such as the BMW M760e, where both covers could be expected to be fully motorized. There is a specific reason for the fact that the fuel tank cap on plug-in hybrids is unlocked from the interior – the tank needs to be vented.

The car companies that I approached with a question about the reason for this method of opening all more or less agreed in their answer – unlike cars with conventional or even full-hybrid drives, the fuel tank is pressurized. It is the excess pressure of gasoline vapors that must be released before the car allows the opening of the tank neck and subsequent refueling.

The vapor pressure in the tank rises simply because gasoline is volatile. This means that it evaporates on its own and basically, regardless of the temperature or its amount in the tank, the vapors will gradually create roughly the same pressure. According to Dalibor Krejčík from Peugeot, it is not big, it is at most tenths of a bar.

The excess pressure is released through an activated carbon filter, just like in gasoline-only cars. However, the activated carbon fills up at a certain moment and would no longer fulfill its function. This is not a problem with conventional cars, because coal is “cleaned” by venting vapors into the engine, where they are burned together with gasoline, Krejčík continues.

However, plug-in hybrids are expected to run on electricity only for several days, so if the tank were vented continuously, the coal would become saturated and cease to perform its function. This can be solved either by enlarging the filter and adding more activated carbon, or by making a pressure tank with a thicker wall. And most of the automakers opted for the second option.

In practice, it looks like for the driver, when he stops at a gas station, he presses a button in the interior, and before he gets out and goes around the car to the tank neck, it is vented and he can insert the gun. Of course, if any pressure needs to be vented; if you run on gas for a long time before refueling, releasing the excess pressure may not be necessary. The pressure may drop as gasoline is drawn from the tank.

“With plug-in hybrids, the fuel tank is not continuously vented to the atmosphere through an activated carbon filter. It is hermetically sealed by an electromagnetic valve. Therefore, it is reinforced and works with vacuum/overpressure in the range of -100 to +300 mbar. A tank pressure sensor is added to the pump module.

When the opening button is pressed, the closing solenoid valve opens and connects the tank with the filter and activated carbon to equalize the pressure in the tank with the atmospheric pressure. Only then will the fuel tank opening occur. A petrol car ‘cleans’ the activated carbon filter continuously by drawing ‘fake’ air into the intake and through the activated carbon filter. However, a PHEV can only run purely electrically, so the activated carbon filter would not be cleaned. That is why the tank is hermetically sealed,” says Sylva Marounková, representative of the Fiat and Jeep car companies on the Czech market.

Hyundai representative David Pavlíček agrees: “This is to prevent the release of poisonous hydrocarbons from the tank of a stationary gasoline vehicle. Vehicles have long been equipped with carbon filters that absorb vaporized gases, which are then used to operate the vehicle. However, the limits with the EURO 6d-temp emission standard have recently been tightened, when it is no longer allowed to release two grams of gases into the air once every 24 hours, but now every 48 hours.”

Michaela Sklenářová from Škodovka adds one more reason why the pressure must be released from the tank before the throttle can be opened: “If the vehicle does not start the internal combustion engine, the EVAP system is not activated and therefore the tank is not relieved of the excess pressure of gasoline vapors that gradually accumulate. Therefore, the opening is electrically controlled to release the pressure from the tank and prevent the user from being splashed with fuel; as a result of the pressure, a small amount of fuel could spray out of the tank neck.

So the reason is clear and purely technical. Of course – why isn’t the cover of the charging socket also unlocked from the interior of the car with a button? Here we will rather guess, but it seems quite clear – a mechanical button under the cover is cheaper and simpler – and therefore more reliable, even in case of frost – than any electronic system.

2024-01-18 17:55:00
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