In my meeting today with the MBTA about the air quality inside T buses, I suggested that the fact that we sometimes see exhaust smoke coming from underneath the buses, despite the fact that the buses’ exhaust pipes are on top of the buses, might indicate a broken exhaust system which might explain how exhaust fumes are getting into bus passenger compartments. As evidence of this, I pointed to an article by Doug Tillberg on TransitBoston.com.
In response, the T employees at the meeting explained to me that it’s actually normal for exhaust smoke to come from underneath the buses, because there are actually two engines in the back of the bus that burn diesel fuel, one of which exhausts out the pipe at the top of the bus, and the other out from underneath.
This is because T buses have auxiliary power units (APUs) that run on diesel fuel. I was told at today’s meeting that the a bus’s APU is used to keep the antifreeze at the appropriate temperature so that it keeps circulating properly. I suspect the APU is also used to warm up the engine block and fuel system so that the bus can be started more easily in cold weather; the alternative would be to plug the buses in overnight during the winter, like my parents used to do with our car during the many cold nights in Minnesota, where I grew up.
There are two important facts to know about the exhaust from bus APUs:
Fact #1: Although the exhaust from the main diesel engine is scrubbed by a particulate filter, the APU exhaust is not. In other words, the APU spews completely unfiltered diesel exhaust into the atmosphere on an ongoing basis. I raised this issue in today’s meeting after learning of the existence of the APUs, and I was told that the amount of pollution from the APUs is inconsequential because they are much smaller than the main engine and run only when needed, not constantly.
However, the T doesn’t actually know how much pollution is caused by APU exhaust, because…
Fact #2: The T’s emissions monitoring and control program only tests emissions from the exhaust pipe. Doug Tillberg wrote about this as well, pointing out that testing emissions from the exhaust pipe doesn’t do any good if the exhaust system is broken and leaking exhaust from somewhere else. Doug is right — a leaky exhaust system will actually score better on its emissions test than an intact one — but Doug didn’t know when he wrote his article that every bus, not just the buses with broken exhaust systems, produces unfiltered exhaust (from its APU) by design.
The T folks with whom I met today said that the APU allows the main engine to run more efficiently and use less fuel, such that even without a filter for the APU exhaust, there is a net drop in emissions as a result of having an APU. That’s certainly plausible, but it begs the question: why the heck doesn’t the design of these buses include a filter for the APU exhaust or route the APU exhaust through the main filter? I honestly don’t know what to make of this.