Drivers in the U.S. have longer commutes than those in Europe and Asia, on average.

In the U.K., short commutes in vehicles with combustion engines—whether gasoline or diesel—pose particularly tough emission problems.

That's because the exhaust aftertreatment systems have to warm up fully before they cut emissions to the levels required under EU regulations.

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The problem has been highlighted by the British consultancy Emission Analytics, which routinely analyzes the data from its various EQUA Index sectors to highlight different emission statistics and challenges.

The company's trenchant analyses of real-world emission and fuel-consumption data has gotten considerable attention, and it has now expanded into the U.S. as well.

This week, it noted that according to British government data, the average distance of a car journey in inner London was just 1.5 miles.

Portable Emissions Measurement Systems (PEMS) on a Peugeot 308

Portable Emissions Measurement Systems (PEMS) on a Peugeot 308

The implications of that figure, as well as the fact that more than half of all vehicle trips in the U.K. are under 5 miles, are dire for limiting emissions of nitrogen oxides (NOx). As the company wrote:

The average daily distance driven in [U.K.] passenger cars is not sufficient for a vehicle’s pollution control system to warm up and become fully functional.

The resultant high levels of cold start NOx emissions, from both gasoline and diesel engines, could provide an additional challenge for urban air quality initiatives such as the proposed Clean Air Zones in the UK.

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Unfortunately, the majority of vehicles Emissions Analytics has tested take more than 5 minutes to warm up their emission-control systems fully.

It's the same reason that in hybrids and plug-in hybrids, when the engine kicks on for the first time after a period of electric-only operation, it usually stays on for 2 to 5 minutes before switching the car back to the default mode of all-electric power.

While those engines are specially fitted and tuned to warm up exceptionally quickly and remain low-emission in frequent on/off cycles, they still must heat their catalytic converters to temperatures of several hundred degrees F.

Portable Emissions Measurement System (PEMS) (Photo by Millbrook Proving Ground)

Portable Emissions Measurement System (PEMS) (Photo by Millbrook Proving Ground)

Across all the cars Emissions Analytics has tested, it calculated that even after 5 minutes, the average NOx emitted by diesel engines of all ages is 7 times the legal limit for new cars today(0.08 g/km) when fully warmed up and about 30 percent more than that when cold.

Gasoline engines suffer from much higher NOx emissions when cold versus when they're warm.

According to the company's calculations, gasoline engines are 4 times over that limit when cold, but emit less than half the maximum permissible NOx once fully warmed up.

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As more cars are fitted with batteries that permit travel of 12 to 50 miles solely on electricity, this problem is largely dealt with for short-distance urban journeys.

But as the study indicates, vehicles without the ability to move at least 10 miles in urban settings solely under electric power will continue to emit vastly higher quantities of NOx on the short journeys that are half of all usage in the U.K.

That data clearly has some policy implications that would seem to favor plug-in electric cars.


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