A little while ago I blogged about the widespread practice by automakers in the EU of reducing nitrogen oxide (NOx) emission controls at ambient temperatures below 10 °C–17 °C and after a hot restart. As I pointed out there, shutting down the emission controls on the pretext that the ambient temperature was too low, or when the vehicle was restarted hot, would be illegal in the United States. That it may not be in Europe—at least, it’s not an open-and-shut question—underscores a crucial ambiguity in the EU regulation that should be clarified, as we’ve noted repeatedly (most thoroughly here).
Now Opel has admitted that it is reducing the emission controls based on three different conditions other than ambient temperature or restart. Opel says that its “engines are in compliance with the legal requirements” in Europe. But, once again, they would not be legal in the US, and once again the difference is a powerful argument for clarifying how the ban on vehicle emission test defeat devices is administered in the EU regulation.
Let’s take a look at Opel’s justifications for what it's doing, at least as far as we can tell from their statement, and discuss how those would be regarded under the U.S. regulation.
Shutting off emission control at high road speeds
Opel states: “Ammonia, created by the injected AdBlue additive, can no longer be stored in the catalytic converter at temperatures above 400 °C, this can lead to ammonia slip from the exhaust pipe.” So, in the Opel engines, “AdBlue dosage is reduced at temperatures above 400 °C as the SCR catalytic converter loses its storage capability at high temperatures. This is also done to avoid ammonia slip and releasing it into the environment.”
It is true that release of poisonous ammonia gases must be prevented, something that has to be factored into the design and operation of the selective catalytic reduction (SCR) system. But there are well-known, widely available technical fixes for this.
Commercially available zeolite catalysts can work at temperatures well above 450 °C. Thus, if Opel needs to shut off urea (AdBlue is a brand name for urea) above 400 °C, they must be using an inferior catalyst. In the U.S., this would not be allowed. As the Environmental Protection Agency's guidance states (Advisory Circular 24-3): “If an AECD [auxiliary emissions control device—EPA’s term for what the EU regulation defines simply as a "defeat device”] is expected to cause an excessive increase in any regulated pollutant, EPA will consider whether design alternatives are available which would make the engine/emission control system less susceptible to the need for an AECD that increases emissions to the extent of the proposed AECD.”
Ammonia slip can also be controlled by installing an ammonia-slip catalyst after the SCR catalyst. In fact, this is common practice in the U.S. Shutting off urea-injection because of a failure to install an ammonia-slip catalyst would also fail EPA’s design requirement and be classified as an illegal defeat device.
But that isn't the only problem here. Opel is reducing the AdBlue dosage because of catalyst temperature, which apparently it is neither measuring nor estimating very rigorously: “The speed values [referring to 145km/h in the question] were chosen as representative for a critical combination of exhaust gas mass flow rate and exhaust gas temperature of around 350°C at 140km/h and 400°C at 145km/h.”
Even if it were the case that at high speeds the catalyst temperatures are too high to work, under the U.S. regulation Opel would have to directly measure catalyst temperature or estimate catalyst temperature based upon all relevant factors, not simply reduce urea injection at speeds above 145km/h. Ambient temperature, road grade, and the amount of time at a given vehicle speed also affect the catalyst temperature. Catalyst temperature does not instantly rise to 400°C the second the vehicle gets to 145 km/h, but gradually follows changes in engine load. In the U.S. EPA will grant an exemption to protect the engine “only to the extent such strategies are necessary … [and] represent the minimum [employment of the defeat device] necessary.”
Reducing emission control at high altitude
Opel claims: “Internal combustion requires an adequate air/fuel balance to ensure stable operation at altitude and to avoid excessive sooting which is regulated in addition to NOx. Even at higher altitudes, the engine needs to be supplied with sufficient oxygen to prevent extensive sooting. This effort to protect the engine renders it an appropriate mechanism within the system.”
All vehicles have air flow sensors, as this is a critical input to many engine controls. Opel appears to be saying that they ignore their own measurement of air (and oxygen) flow and, instead, reduce EGR based on an arbitrary altitude. In the U.S., EPA would require Opel to define the minimum necessary oxygen flow, not just use barometric pressure as a surrogate. Also, as with ammonia slip, there is an easy technological fix: increase the turbocharger boost a bit to provide additional oxygen. What Opel is describing was a problem decades ago when diesels did not have turbochargers. Even if Opel hasn’t updated its high altitude calibrations in 20 years, others have, and under the U.S. regulation Opel would be required to deploy such an available fix instead of changing the emission controls. In fact, all diesels sold in the U.S. pass the same emission standards during testing in the Rocky Mountains, at 2000m elevation.
Reducing exhaust gas recirculation at engine speeds above 2,400 rpm
Opel states: “The functionality of the exhaust gas recirculation physically depends on the operating states of the EGR system components, the turbo charger with variable turbine geometry, the temperature in the inlet manifold and the heat dissipation of the vehicle cooling system. Furthermore, the use of EGR under certain circumstances in the combustion chamber can lead to unwanted emissions effects or an instability of the combustion.”
This is an accurate and insightful statement. But Opel didn’t revise the EGR calibration based upon a careful assessment of these interacting factors. They just changed EGR calibration above 2,400 rpm. This would not be allowed in the U.S., because the U.S. regulation requires that the alternative emission control device—defeat device, in the parlance of the EU regulation—remain in use for only the minimum amount of time necessary to manage whatever operating condition required it to be deployed in the first place. And, in fact, the FTP-75 test used in the U.S. has higher speeds and loads than the NEDC test used in Europe, requiring higher engine rpm to drive the test cycle, and manufacturers have no trouble meeting more stringent emission standards.
Properly calibrating vehicles is difficult and time consuming
In their statement, Opel emphasized that there are 17,000 adjustable parameters, functional parameters, and values that modern engine control systems have to monitor and manage. Yes, properly calibrating engine and emission controls is difficult and time consuming. In fact, I ended my previous blog with a similar discussion. But using shortcuts to make calibration easier is not allowed in the U.S., where many manufacturers (including Opel's parent company, General Motors) have proven that they can maintain low emissions and excellent durability at far higher catalyst temperatures, altitudes, and engine speeds than those that Opel's statement cites as justification for reducing the emission controls on its engines in operation.