Over here in the US Diesels are required to have a very low NOx emission level, which is why many of them have an urea injection system installed into the exhaust system.
It's a bit pricey, and it's only really practical on larger cars (pick-ups and small SUVs) because of the additional plumbing involved as well as the need for the reservoir for the urea fluid. This is part of what led to the huge kerfuffle with Volkswagen when it came out their TDi's were a sham.
It's a bit pricey, and it's only really practical on larger cars (pick-ups and small SUVs) because of the additional plumbing involved as well as the need for the reservoir for the urea fluid. This is part of what led to the huge kerfuffle with Volkswagen when it came out their TDi's were a sham.