In my just-prior post, I was about a decade out-of-date in my understanding of the gas vapor recovery systems installed on U.S. gas pumps. I’m going to correct that here.
- Gasoline vapors are a major contributor to photochemical smog, and, in particular, to the creation of ground-level ozone.
- Once upon a time, the U.S. EPA required that gas pumps in some urban areas have vapor-recovery nozzles. These were designed to collect the gasoline vapors that would otherwise just pour out of your car’s gas tank as you refilled it.
- By and large, these were only mandated in areas that could not meet federal air pollution standards for ground-level ozone.
- But by 2006, virtually all new cars and trucks were equipped with on-board vapor recovery systems. They collected their own gasoline vapors during refueling, stored them, and burned them
- In 2012, the EPA dropped its requirement for vapor-recovery nozzles (reference). At that point, so many cars had the new on-board systems that the vapor-recovery nozzles no longer offered sufficient benefit to justify their cost.
- Whether or not gas stations were required to keep up those vapor recovery systems was left up to the states. For example, Virginia chose to decommission all those vapor recovery systems in 2017 (reference).
The upshot is that gas pumps in my area haven’t had those vapor-recovery nozzles for more than half a decade. They still have some sort of rubber cup on the fuel nozzle, but I guess that’s just to prevent splashback or possibly to aid the car’s own on-board recovery system.
Some implications
With this, a lot of things now click into place.
Vehicle fuel tank filler necks now have an elastic seal in them. I’m sure that older cars did not have those. That seal is required on a modern car because the on-board vapor recovery system needs a tight seal against the gas nozzle. That’s the only way to make sure that the gas vapors in the tank end up in the on-board charcoal canister.
The standard advice of “Don’t top off your tank” now has a new rationale. In the ancient past, that was the advice because gas expands as it warms, and if you topped off your tank in summer, you’d end up spilling gas out the fuel filler as you drove down the road. Now, that advice is there to protect your on-board vapor recovery system. If you top off you tank, you can end up shoving liquid gasoline into your vapor recovery system, something it was not designed for.
My old two-gallon gas can produced four gallons of gasoline vapors before I even considered spills, venting, and the gasoline-permeability of the plastic. Every time I filled that at the gas pump (since 2017), that displaced the two gallons of vapors, in the gas can, into the atmosphere. And then, as I repeatedly filled the tank on the mower, that sums up to another two gallons of gasoline vapor displaced into the atmosphere.
How does that compare to gasoline vapor emissions from cars?
The EPA estimates that these on-board vapor recovery systems capture about 98% of gasoline vapors, at least according to this presentation. The same source shows that the EPA estimates an average of 0.32 grams of gasoline spilled per gallon dispensed at a typical gas station.
The U.S. averages about 650 gallons of gasoline consumed per licensed driver. Based on that, a year’s worth of fill-ups, for the typical licensed driver in the U.S., would generate:
- 13 gallons of gas vapor spilled directly into the atmosphere (2% of that 650 gallons).
- Another 12 gallons of gas vapor due to the average 208 grams of fuel spilled (0.32 g/ gallon).
In other words, the average driver with properly-functioning vapor recovery equipment and average diligence about spilling gasoline will generate about 25 gallons of gasoline vapors annually.
In that context, the 4+ gallons of gas vapor directly emitted by my old gas can seems quite material. Particularly because my wife and I now exclusively drive her Prius Prime. We seem to use on-order-of 40 gallons of gas a year, with the rest of our travel being electric. From that 40 gallons, based on those EPA averages, we’d only be emitting about 1.5 gallons of gasoline vapor per year. So that, in our household, the lawn mower and old gas can were responsible for far more gas vapor emissions than our car was.
That said, it’s worth noting that the lawn mower — even with the old gas can — is nowhere as bad as the average American passenger vehicle, in terms of venting gas vapor to the atmosphere as a result of refueling. That’s not because the lawn equipment is clean — it’s not. That because the average driver uses such a vast quantity of gasoline. Even those small fractional losses during refueling add up to far more gasoline vapor than the lawn mower / old gas can emit in a season.
But that’s only for refueling-related gasoline vapor losses. That does not include any gasoline vapor losses by the mower during operation. For example, losses through the charcoal-filled gas cap, losses from the vented carb bowl after engine shutoff, and so on. I still need to track those down.
The new gas can ought to eliminate half of those “displacement” gas vapor emissions. The new can vents through the end of the pouring spout, so it’s “inhaling” the gas fumes out of the tank as it puts new gas into the tank.
The only gas vapor that will be directly emitted as a result of displacing vapor during refueling will be from refilling the gas can, at the local gas pump. That, because our gas pumps no longer have vapor-recovery nozzles. And apparently haven’t had them since 2017.
