Street Smarts: EPA takes aim at diesel derates

Few diesel emissions regulations have been as instantly and universally unpopular as engine derates.

(Photo: Chad Elmore)

In 2010, selective catalytic reduction (SCR) systems were introduced as an emissions control solution for diesel engines. The system worked by spraying hot diesel exhaust gas with a urea-based solution called diesel exhaust fluid (DEF).

When DEF is sprayed into hot diesel exhaust, the water evaporates and the urea breaks down into ammonia and carbon dioxide. The ammonia then passes through the SCR catalyst, where it reacts with nitrogen oxides (NOx) in the exhaust. That chemical reaction converts the NOx into harmless nitrogen and water vapor.

SCR systems are standard equipment on diesel trucks worldwide today. But SCR was a highly controversial technology in 2010. Drivers hated the systems. Fleets hated them. Technicians hated them. And much of that hatred flowed from a single issue: diesel engine derates.

There was a real concern among OEMs and regulators that people would simply opt to not purchase and add DEF to an SCR system, negating its emission control capabilities. In order to “induce” operators to keep the DEF tank full, OEMs developed a fault-based system that would derate an engine if it was emitting excessive emissions into the air.

An empty DEF tank or even bad diesel exhaust fluid sensor, damaged wiring or another emissions system fault could quickly turn an otherwise serviceable truck into an expensive roadside ornament. Engine power disappeared and road speed was limited to a “limp mode” by the truck’s electronic control systems. Eventually, the truck often had to be towed to a dealer to diagnose a relatively minor problem.

Death of the Diesel Derate

Now the Trump administration wants derates gone.

In July, the U.S. Environmental Protection Agency (EPA) proposed eliminating DEF-related engine power reductions and vehicle speed restrictions on newly manufactured trucks. Instead of gradually reducing a vehicle to a crawl, the proposed system would rely on visual warnings and recurring audible alarms to tell drivers that the DEF tank is empty or that something is wrong with the SCR system.

(Photo: Chad Elmore)

If finalized, the change would apply to DEF shortages, poor-quality DEF, sensor failures and other SCR-related faults.

Importantly, EPA is not eliminating DEF. Nor is it eliminating SCR or backing away from the 2027 heavy-duty NOx standard of 0.035 grams per brake horsepower-hour. It is simply proposing to remove the regulatory hammer intended to make sure drivers keep DEF in the tank.

Allen Schaeffer, executive director of the Engine Technology Forum, was involved in industry discussions leading up to the introduction of SCR and DEF in North America in 2010. He recalls plenty of concern about how severe the derates would be and what might happen as emissions systems aged. But he notes there was no obvious alternative.

So, derates became the enforcement mechanism nobody liked – but everybody lived with.

And that raises an interesting question: Just how serious was the problem EPA is now trying to solve?

Heavy-duty fleets have certainly experienced DEF system failures. Some have paid towing bills, lost loads and watched productive trucks sit at dealerships because of malfunctioning sensors or other aftertreatment problems.

But derates never became the crippling, industry-wide operational crisis that their current political profile might suggest.

Schaeffer noted that after 15 years of on-highway SCR operation – and more than a decade in most off-road applications – the industry has not seen consistent, widespread demands from heavy-duty fleets to abolish derates.

“If derates were that big of a problem, we’d have all heard about it by now,” he told me. “And this just hasn’t been a big deal – at least for the heavy-duty diesel industry.”

Which makes me wonder if this policy push originated somewhere other than traditional heavy-duty trucking.

The private-owner pickup and medium-duty market seems a likely candidate. Emissions deletes and tampering have long been more culturally and politically visible in that world, where some owners regard federal control of their diesel engines as government intrusion.

Add the ongoing right-to-repair fight in agriculture and construction equipment, and DEF derates become tailor-made for populist politics: an unpopular government mandate that can disable an expensive piece of machinery because of a relatively inexpensive sensor.

But Schaeffer notes that doesn’t mean there are no legitimate safety concerns about engine derates. He says that motor coach operators made a particularly strong case during EPA hearings on diesel engine derates. A derated coach can strand 50 or 60 passengers hundreds of miles from home – a reality that raises very real safety concerns.

A Financial Incentive for DEF

But for large, professionally managed truck fleets, eliminating derates probably won’t change much.

Jack Roberts is a Tuscaloosa, Ala.-based independent journalist and licensed commercial driver with more than 20 years’ experience covering the North American and global trucking industries.

Those fleets have no compelling business reason to stop buying DEF, Schaeffer says. Running without it risks damaging expensive components, jeopardizing warranty coverage and turning on dashboard warnings that maintenance departments cannot responsibly ignore.

There is also a strong financial incentive to keep SCR systems functioning properly.

SCR allowed manufacturers to optimize diesel engines for fuel economy and performance while treating NOx downstream in the exhaust. Schaeffer pointed to research estimating that SCR-based emissions systems delivered approximately a 3% to 5% efficiency improvement compared with competing emissions strategies of the period.

DEF costs money, he admits. But it also helps diesel engines use significantly less fuel.

An emissions sensor failure should not reduce a perfectly serviceable truck to 5 mph. And it looks now like the simple fuel economy boost provided by DEF is more than enough incentive for heavy-duty fleets to keep SCR systems in good working order.

For now, the Trump EPA is rolling back the unpopular diesel derate regulatory hammer. But that only leads to harder questions that will have to be answered in the future: What policy will replace it? And will that replacement will be strong enough to keep everyone putting DEF into their tanks?