Diesel engines such as Volkswagen's are equipped with ceramic Diesel Particulate Filters) DPF that are basically honeycomb structures made of ceramic "foam." The little one on the Volkswagen costs $700 to replace. Soot accumulates and has to be burned off every few hundred miles. A tiny percentage of the soot isn't carbon and it eventually clogs the filter so it has to be replaced (typically at 120,000 to 150,000 miles. While near zero particulates aren't possible, the volkswagen diesel emits less of them than a Prius or Volt running on gasoline (the famous handkerchief test). But compensating for their emission is the fact that the Prius or Volt won't ever get stuck on the side of the road with a clogged DPF.
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Diesel engines such as Volkswagen's are equipped with ceramic Diesel Particulate Filters) DPF that are basically honeycomb structures made of ceramic "foam." The little one on the Volkswagen costs $700 to replace. Soot accumulates and has to be burned off every few hundred miles. A tiny percentage of the soot isn't carbon and it eventually clogs the filter so it has to be replaced (typically at 120,000 to 150,000 miles. While near zero particulates aren't possible, the volkswagen diesel emits less of them than a Prius or Volt running on gasoline (the famous handkerchief test). But compensating for their emission is the fact that the Prius or Volt won't ever get stuck on the side of the road with a clogged DPF.