Euro 6 trucks far greener on NOx than diesel cars

By Categories: NewsPublished On: Tuesday 31 January 2017

The organisation that revealed the Volkswagen ‘emissionsgate’ scandal has given the Euro 6 heavy-duty diesel engines fitted to trucks and buses a clean bill of health as far as nitrogen oxides (NOx) emissions are concerned – confirming that their exhausts output considerably less NOx than those of diesel-engined passenger cars for each kilometre travelled.

Tests carried out by the International Council on Clean Transportation in the USA revealed that ‘real-life’ emissions of NOx from Volkswagen cars exceeded limits set by the US Environmental Protection Agency by factors of up to 35.

Subsequent investigations revealed software on the offending cars had been programmed to ‘game’ the tests by producing low NOx emissions only when the engine was following the mandated test cycle.

Consequent ‘real-life’ tests carried out by researchers revealed that, on average, Euro 6 diesel cars emit six to seven times the mandated NOx limit of 80mg/km. Various environmental groups have responded by calling for blanket bans on ‘dirty diesels’ across large cities.

The ICCT has subsequently acquired data from ‘real-life’ tests on 24 heavy-duty Euro 6 vehicles, including buses, tractor units and rigid trucks. Data came from chassis-dyno tests conducted in Finland, and German type approval in-service conformity testing.

While the makes and models of heavy-duty vehicles tested in Finland are not revealed, they consisted of four tractor units, one rigid truck and six buses, enough given the relatively small number of major engine manufacturers active in the market for a representative sample across the sector.

“On average, NOx emissions from diesel cars are more than double those of diesel trucks on a per-kilometre basis, even though CO2 emissions—which are proportional to fuel consumption—for heavy-duty vehicles are five times those of cars,” the ICCT report, titled NOx emissions from heavy-duty and light-duty diesel vehicles in the EU: Comparison of real-world performance and current type-approval requirements, said.

“On an engine load basis, heavy-duty diesel vehicles are roughly ten times better than light-duty diesels at reducing NOx emissions. This indicates that tailpipe NOx emissions have been essentially decoupled from CO2 emissions, so that increases in CO2 emissions—that is, in fuel consumption—do not necessarily produce an increase in NOx emissions,” it continued.

The report highlights the effectiveness of the selective catalytic reduction (SCR) technology, now universally employed in truck and bus engines, in reducing real-world tailpipe NOx emissions, and contrasts this with the real-life impracticalities of the lean NOx trap, which uses a platinum catalyst for the short-term storage of NOx, fitted to many passenger cars.

Lean NOx traps require frequent regeneration, in intervals as short as a few seconds apart, which is activated by injecting extra fuel. This causes the NOx to react with surplus hydrocarbon molecules to convert into nitrogen and water.

An engineer wishing to ‘game’ the light-duty emissions test could confine regeneration fuel injection events to conditions found within the test cycle, giving the car better driveability and reduced fuel consumption when operating out of test conditions, at the expense of virtually uncontrolled NOx emissions.

The report suggests that the bulk and cost of SCR systems has deterred car manufacturers from specifying them for engines of less than 2.0 litre displacement.

Truck and bus emissions have benefited from the tightening of heavy-duty diesel testing. ICCT acknowledges that Euro 4 and 5 tests for heavy-duty vehicles did not include significant amounts of cold-temperature or light-load running, whereas Euro 6 included a cold start, lower engine loads and significant idling time.

Consequently, Euro 4 and 5 heavy-duty engines have been found to exceed NOx limits in real-life urban use, whereas Euro 6 heavy-duty diesels generally do not.

Another important change with the adoption of Euro 6 for heavy-duty diesels was the addition of a so-called ‘off-cycle’ test to prevent ‘gaming’.

Additional random engine-speed versus torque and load combinations falling outside the test cycle, but within a specified range, are tested to ensure that the emissions control system is not narrowly calibrated to meet emissions limits while being operated on the test cycle but then exceed the limits (to benefit fuel consumption or torque output) as soon as the engine is being operated off the type-approval test cycle.

Since the manufacturer cannot know ahead of time the random off-cycle test points, the engine emissions control system must be designed to function over a large part of the duty cycle.

Bus and truck operators and their respective trade groups are likely to welcome the ICCT findings, which may prove useful in challenging future attempts to restrict heavy-duty Euro 6 diesel vehicle operation in cities on the grounds of excessive NOx emissions.

The Road Haulage Association (RHA) said media reports on the findings: “support a key policy message from the RHA, which we have been making to central and local government in the air quality debate.”

The full report can be read here.