It looks like Singapore was right when it resisted the diesel trend which started enveloping Europe some 30 years ago.

It was only after much lobbying from German automotive groups such as Bosch, Daimler and Volkswagen that the Republic relented and started to ease up on its hard stance against diesel cars in recent years.

Before 2006, a special tax was levied on all diesel cars which was equivalent to six times their annual road tax. A 1.5-litre diesel would have attracted an annual levy of $4,774 back then.

Today, the same car, if it meets the Euro 5 emission standard, would attract an annual tax of only $1,282.

A recategorisation of the COE system in 2014, which put a 130bhp cap on Category A certificates, and a 2015 revision to the Carbon Emissions-based Vehicle Scheme (CEVS) made diesel cars even more attractive.

This is because diesel engines produce less CO2 and less horsepower (but higher torque) than their petrol equivalents.

But now, the government is reviewing the CEVS (as reported by The Straits Times three weeks ago). Besides CO2, other pollutants such as nitrogen oxide, hydrocarbons, carbon monoxide and particulate matter are likely to be measured too.

In the wake of the Volkswagen emission cheating scandal in late-2015, several other countries have also started re-examining their stand on diesel.

But even before this, studies have shown that cases of respiratory problems such as childhood asthma had been creeping up in some places, including Germany, where the share of diesel cars is as high as 60 per cent.

(The share is forecast to plunge to below 40 per cent this year in the wake of the VW scandal.)

Cities such as Paris, Mexico City, Madrid and Athens have also vowed to ban diesel vehicles altogether from 2025.

At a feedback session on Wednesday evening, Minister for the Environment and Water Resources Masagos Zulkifli hinted strongly at regulatory changes that would discourage diesel vehicles.

"It turns out that diesel, although it produces less carbon dioxide than usual cars, is not as clean as was earlier touted," he told about 40 participants from industry, non-governmental organisations and academia.

This means diesel cars will most likely be relegated to the back seat once again.

Are manufacturers that declare over-optimistic emission figures to blame?

Partly. The auto industry was, in fact, acting on a push by several governments to cut down on CO2, a major greenhouse gas that contributes to global warming.

In Singapore, diesel cars account for a small percentage of noxious fumes associated with diesel engines. Their numbers are relatively low, and so is the mileage they clock.

The main culprits are commercial vehicles, especially heavy trucks and buses. Taxis are next.

So, whatever regulatory changes are made must be relevant to these vehicles, too.

The roll-out of Euro 6 standard for diesel vehicles next January is not enough.

Singapore must consider re-introducing the diesel duty at the pumps.

Today, diesel vehicle buyers pay a fixed sum. Hence, there is little incentive for users to pay heed to fuel consumption - especially since diesel is significantly cheaper than petrol.

Re-introducing duty at the pumps will trigger a behavioural change and lead to less fuel burnt, and less pollutants emitted.

There would also be incentive for the authorities to clamp down on "white pumps".

Today, these pumps are widespread, dispensing fuel of questionable quality.

So, even if Singapore implements stricter vehicular standards, they are ineffective if end-users are not conscious about the cost of the fuel and if they continue to fill up with cheap alternatives that foul up their engines and the air.

Lastly, regulatory changes must retain the carbon element because CO2 emission is still a big concern. To that end, rethinking how electric car taxes should be calculated will go a long way.