Why phasing out diesel cars in Hong Kong is not the best route to cleaner air

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Heavy traffic on the way to the Cross-Harbour Tunnel in Wan Chai. Photo: Sam Tsang

Published: 5:52pm, 20 Oct 2018Updated: 5:51pm, 20 Oct 2018

In her 2018 policy address, Chief Executive Carrie Lam Cheng Yuet-ngor said the Environmental Protection Department would consult stakeholders and commission a study on the feasibility of halting all new registrations of diesel private cars. These vehicles make up roughly a fifth of the private car fleet. (“Do you have an old diesel vehicle? Hong Kong government may pay you to take it off the road”, October 10).

Hong Kong follows the European Union’s emissions standards grading scheme under which the cleanest vehicles are of Euro VI standards. A Hong Kong government source noted that Euro VI vehicles emit about 90 per cent less nitrogen dioxide than Euro IV vehicles and estimated that the government’s policy can offset 1,300 tonnes of nitrogen dioxide by 2024 by giving the owners of Euro IV vehicles incentives to upgrade to Euro VI ones by 2023.

Banning of vehicles that do not comply with certain standards has made news in recent months in Germany, with a court in Frankfurt ruling that the city must ban diesel cars that only meet Euro IV and older emission standards from February 2019.

A woman covers her nose in Causeway Bay. Euro VI vehicles emit about 90 per cent less nitrogen dioxide than Euro IV vehicles. Photo: Sam Tsang

Surprisingly few people realise that vehicles that run on diesel have engines where only enough fuel is injected to produce the power demanded, whereas the Otto cycle (petrol) engine always takes in a complete cylinder of fuel and air mixture capable of producing full power, even when idling, only restricting the production of power by throttling.

Red light on private cars needed in Hong Kong to curb roadside pollution

Thus the latter is always wasting fuel except when going full throttle, producing much more waste heat and carbon dioxide, the main gas responsible for global warming, than diesel engines. Additionally, because the Otto cycle engine operates at a much lower compression ratio than the diesel engine and produces much less torque, it consumes at least 30 per cent more fuel per distance traversed.