Dieselgate was trying to avoid using DEF claiming you could get compliant NOx output without it [1]. You can if you reduce power and efficiency during the test

Dieselgate was trying to avoid using DEF claiming you could get compliant NOx output without it [1]. You can if you reduce power and efficiency during the test cycle, if you want good power and efficiency while having compliant NOx output you need DEF on a diesel engine and it works very well but does increase complexity and cost. https://en.wikipedia.org/wiki/Volkswagen_emissions_scandal https://en.wikipedia.org/wiki/Volkswagen_emissions_scandal