Electric Buses and the Mysterious Tailpipe

One of the beautiful new electric busses!

The New Kids on the Block

This past Earth Day, the City of Ithaca (where I currently live) unveiled 7 new all-electric buses and incorporated them into their bus fleet. The plan is to have the entire 55 bus fleet all-electric by 2035 and this was a monumental step in that direction. As a frequent bus-rider and infrastructure nerd, I was thrilled to see this upgrade; electric buses are far quieter than their diesel counterparts, have more rear visibility (they have rear windows since there is no engine to block the view), can act as an emergency power source, diminish congestion, offer an option for those who can’t drive, and, of course, result in zero tailpipe emissions including particulate matter, improving city air quality and reducing climate impact.

Short Tailpipe

The other day, I was gazing lovingly at one of the new electric buses as it went along its route when I noticed something interesting; the bus had a tiny little tailpipe. This is bizarre because, as a zero emissions vehicle, there should be nothing for the bus to exhaust. I was curious enough to look up what this tailpipe does and I’ll save you the time: each of these electric buses has a diesel heater on board to heat the bus during the coldest days of winter.

The small tailpipe for the diesel heater on board

Most buses run on diesel, and these engines generate massive amounts of heat because they run on miniature explosions for propulsion. Heating one of these buses is merely a matter of diverting some of that heat into the cabin of the bus. Electric buses, by contrast, don’t generate nearly as much heat, and so heat must be purposefully generated. Originally, Proterra heated their buses with the batteries, but in very cold climates (such as Ithaca) this was draining the battery too fast. To heat one of these modern buses, it would be take about 30-40% of the battery, making this unfeasible if you actually want to use the bus to move people around. To make a bus that is usable in winter, Proterra equips buses in its “Cold Weather Package” with a small diesel heater to warm up the cabin on the coldest days that exhausts through that tiny tailpipe pictured to the left. In case you were wondering, cooling the cabin is easier, requiring only about 5% of the total onboard energy, so no auxiliary power source is needed for this.

While some authorities claim that these buses locally emit zero greenhouse gasses, I contend that isn’t true, based on what those words mean. Regardless, though, utilizing a small diesel generator on some cold days is far better for the planet than gallons of diesel being guzzled by most buses.

This was the first electric bus I ever rode! I had a giddy conversation with the bus driver about the experience.

Long Tailpipe

But are electric buses better for the planet than diesel busses? Obviously, for those that live in cities, the noise reduction and air quality improvements are nice, but the electricity that powers those vehicles has to come from somewhere and, in some places, that somewhere is coal plants. If electric buses draw enough energy on a coal-powered grid, they could actually be accelerating climate change rather than working to mitigate it. This is actually a common argument against electric vehicles (EVs); that if they are plugged into coal-powered grids, they’re ultimately worse for the planet (silly environmentalists!). This argument is called the “Long Tailpipe,” and is genuine conundrum for many. While an affirmative answer to this question wouldn’t negate all the benefits of electric buses, the answer should be important to environmentalists like me who like to have informed opinions.

First, we should consider what is worse for the planet, a traditional diesel bus chugging along its route or an electric bus on that same route charged via coal-derived power. Luckily, some folks have already crunched the numbers for me: the average city diesel bus gets 6.1 miles per gallon while, by contrast, the Proterra ZX5 buses driving around Ithaca can get up to 23.1 miles per gallon equivalent (or mpge, a cross-fuel measure of efficiency) with the average probably somewhere between 17-23 mpge (low end based on this 10 year old data which, of course, doesn’t factor in the technological improvements Proterra has made in the past decade, and high end based on Proterra’s own estimates, which I assume are generous). While impossible to get a perfect apples-to-apples comparison, these estimates use the average energy makeup in the United States, so if you’re in the United States, electric buses are far more efficient per unit of energy than diesel busses. That agrees with what the Union of Concerned Scientists found as well! So that settles it then, if you live in the US, you can rest assured that your quiet, public-transit commute is better for your lungs and for the planet.

USA! USA! USA!

But wait a second, what if you don’t live in the United States? Or what if you live in an area only utilizing coal energy, not the average energy makeup of the United States?! Good question! While it used to be true that an EV drawing energy from coal generation was emitting more emissions than an internal combustion car compared over the same distance, that has stopped being true due to advances in both energy generation and EV manufacturing. Further, the United States is far from the best nation in the world when it comes to having a renewable grid. If your country is completely reliant on fossil fuels in a place like South Africa where about 92% of all energy is non-renewable, the total reduction in emissions mostly tracks with the efficiency of the vehicle, but in a place like Paraguay, where about 99% of electricity generation is hydropower, this emissions reduction is substantial. If I can leave you with anything let it be this: talk to your local leaders about upgrading the city bus fleet and advocate to your national leaders to improve your nation’s grid to one that is powered on renewable energy. One you will feel day-to-day, the other will change all of our futures.

A look backward at a look forward.

The Eternal Tailpipe

So electric busses are great, but what about those little diesel heaters? Will that be a perennial issue, an eternal remnant of the days of yore? In 2099, will a single pumpjack bob in the boondocks of Texas to ensure that bus riders stay warm in the north? No! Just like how using an air source heat pump to heat your home can be better for the planet, your health, and your wallet, such systems have been developed for busses! These systems use far less energy per unit of heat generated and have already been installed in some buses in Europe, meaning that someday soon we may see a truly zero-emission bus, even in the coldest parts of the world.