One could - from a narrow perspective[1] - argue that diesel engines on plant oil would be carbon-neutral but not that they are emission free. In fact: (older) diesel engines are being banned from cities (e.g. Antwerp, Belgium) not because of CO2-emissions but due to emissions of particulates (and NOx). CO2 is a global problem, but particulates and NOx are local problems (and hence more likely to trigger local political action).

[1] it is often argued that biomass/biofuel is not entirely carbon-neutral due to several reasons: the inherent lifecycle means some CO2 is in the atmosphere at any point; production is often based on CO2-emitting processes ... Secondly some other issues pop up as well: (indirect) land use change, competition with food ...