LPG & CNG
On a cycle representing congested urban traffic, both LPG and CNG outperform petrol powered vehicles on emissions of carbon monoxide (CO). Indeed, emissions of CO from CNG powered vehicles are of the same order as those emitted by diesel vehicles.
However, emissions of total hydrocarbons (THC) from CNG vehicles are relatively high because of methane, the major component of natural gas. Although methane is a small contributor to the formation of low level ozone it is a major factor in global warming. Emissions of NOx and particulates from both LPG and CNG powered vehicles are significantly lower than those from diesel vehicles.
Moreover, emissions of NOx from CNG vehicles are half those from equivalent petrol engined vehicles. A recent study using a small delivery van fitted with a three way catalyst and capable of switching between CNG and petrol, showed that on a modified EU emission test cycle, emissions of CO, non-methane hydrocarbons (NMHC) and NOx were 76%, 88% and 83% respectively lower with CNG than with petrol. Using data from other studies CNG also compares favourably with emissions from equivalent sized diesel-engined vehicles.
City Diesel
City diesel is a petroleum based lower emission diesel developed in Sweden but now available in many European Countries including the UK. Exhaust emissions from vehicles fuelled with city diesel compare favourably with exhaust emissions from equivalent vehicles fuelled with conventional diesel.
Particulates Reduced By 34-84%
The main benefit of city diesel is that its combustion reduces particulate emissions by 34 - 84% depending on engine type, duty cycle, test basis and type of particulate measured. An additional benefit of city diesel is that it is a low sulphur fuel, which is necessary for the optimum running of oxidation catalytic converters.
The main reason drivers refuse to run full time on City Diesel, is the extra cost per litter. London's Mayor should be looking at either a subsidy or zero rating this product, in much the same way as the London Bus companies are currently receiving such benefits.
On 12th of September, Mayor of London Boris Johnson again bragged in the Assembly that he has cleaned up emissions by introducing a taxi age limit. Both DEFRA and Kings Collage have produced concrete evidence that the new taxis are actually causing more pollution than the older taxis. There has been no reduction in pollution.
A complete and utter lie by Mayor Johnson
See 40 minutes in to hear Boris Johnson lying about the taxi age limit
http://www.london.gov.uk/mayor-assembly/london-assembly/webcasts