Deep Dive into Detroit DD13 Emissions System

Introduction

I wanted to take a moment to talk a little bit about the DD13 emissions system. Specifically, the Selective Catalyst Reduction (SCR) on Green House Gas (GHG)17 configurations. As a side project I was doing some datalogging using a CSS datalogger, and noticed a very interesting thing about how effective the SCR catalyst is and want to share those findings with you all. Like many people that will likely read this article I have always been told during training sessions that the air coming out of the vehicle is cleaner than what goes in on these new platforms. I can safely say that the data says otherwise. There are times during operation where this statement may be the case but there are likely more times when it is not.

The Data

As a short background the following data is not some detailed analysis of multiple vehicles for comparison. It is a very specific platform and vehicle during normal operation that most motorcoaches would undergo. I would consider doing a more detailed analysis with the proper funding, but I am limited in my current capabilities as a small business owner. With that out of the way let's look at what was observed. While looking back over the data I noticed that when the vehicle is idling (high idle or not) the amount of NOx emissions always climbs up and ends up being just as bad as what comes in. You can see this on the chart below. This chart is zoomed in on a specific time frame, so it is easy to see on a computer screen. The observation here is replicated over and over again in previous time frames and not an anomaly.

Engine RPM / Inlet and Outlet NOx

The engine RPM is charted in blue and used as an indicator that the vehicle was driving as the value is not constant. The inlet and outlet NOx are charted in green and yellow, and their scale is on the right side in Parts Per Million. It is very clear that the outlet NOx starts to climb only 10 minutes after the vehicles has been put into high idle. This increase is clearly associated with decreased exhaust system temperatures and shows the exact reason why idle laws are so important. Even the "clean" systems of today do not reduce the NOx emissions especially at low RPM and Low exhaust temperatures.

From my research on the topic, it is clear that the SCR catalysts is only effective when operating above temperatures of 227 Celsius. When putting the exhaust system temperature chart above the NOx chart you can see a clear line between the NOx rising and the temperature falling. Yellow line is SCR outlet temperature on the top graph, green line is SCR inlet temperature.

SCR NOx vs Exhaust Temperatures

There are a whole multitude of health effects and damage to the environment that come from NOx emissions, and the expensive exhausts systems that have been mandated are clearly only helping with a small portion of the problem. Pollution is and always was a larger problem in urban settings, and it appears that all of this new technology over the last few years as not made that much of a difference. Idle laws matter and besides saving you from expensive fines and excess fuel usage, they clearly have a long-term health benefit for the communities you operate in. I think one of the biggest reasons for excessive idling in the bus world is to keep the temperature down inside the vehicle, and that points out how important it is to maintain a properly functioning HVAC system. A system that works well will cool quickly or warm up quickly, if it takes a long time to get to temperature then the operator will compensate by idling more.

Summary

One of the other interesting things that have come out of this logging is, unless you really know the system, it is hard to properly troubleshoot. I am sure operators have been in the following situation before - "I have been using this bus for two weeks around town and now it went out for a long trip and suddenly has issues with the emissions system." There is a clear answer here; the system does not really work all the time so it is unable to warn you in advance that something might be wrong. If you plan to change the type of operation for a vehicle from urban to rural it is likely a good idea to perform and SCR efficiency test to confirm that everything is working well.

Telematics are a great place to start watching for excessive idling. If you need more specific details, then datalogging is a good choice. For more information and or develop a better understanding of your specific equipment, reach out.