United Nations Environment Programme

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Advanced vehicles available today

Comparison of clean diesel vehicles, CNG vehicles, and HEVs

There are three major aspects to be considered when choosing an alternative vehicle; emissions, energy, and economics

All currently available alternative vehicles have fewer emissions compared with standard vehicles, and can also meet future stringent emissions reduction criteria (such as EURO V, which comes into effect in 2008, and American Tier II standards, which will come into effect in 2009). However, owing to more stable engine operation conditions, HEVs have an advantage over conventional engine design in terms of emissions. CNG vehicles are preferable to diesel vehicles due to the use of cleaner burning fuel. The size of this difference depends on the exact nature of the fuel and utilization of emissions control technologies.


The major difference between the alternative vehicles is the reduction potential for fuel consumption and thus CO2 emissions. In Figure 2 below, the fuel reduction potential of HEVs, CNG and clean diesel vehicle technologies is given.

Figure 9: A comparison of the fuel use reductions among the different alternatives. Source: UNEP

For HEV passenger cars, the potential reduction in energy use is substantial, ranging from 30-40% reduction when compared with petrol vehicles. For HEV city buses and delivery trucks – which have a lot of stop-and-go movement – the fuel reduction is around 25-30% for state-of-the art hybrid buses/delivery trucks when compared with standard diesel buses.

The benefits of HEVs depend on the circumstances under which they are driven (i.e. the drive cycle). The more stop-and-go traffic, the greater the fuel reductions. Using HEV technology on a highway truck provides minimal benefits.

Replacing petrol vehicles with CNG passenger cars can reduce energy use by approximately 10% in terms of the petrol equivalent. Replacing diesel vehicles with CNG buses and delivery trucks uses more energy, because diesel buses are highly efficient. A purpose-built CNG bus that replaces a diesel bus is estimated to use 10-15% more energy, whereas a retrofitted CNG bus is estimated to use about 10-40% more energy.

Clean diesel vehicles. Diesel engines are inherently more efficient than conventional petrol engines; for passenger cars, benefits are estimated to be an approximate 20% reduction in energy use. Heavy-duty vehicles already use diesel technology, so the benefits with “clean diesels” arise through a sharp reduction of regulated pollutants.

The economics/life-cycle cost

Buying a hybrid vehicle or a CNG vehicle is more costly, but given the lower fuel consumption, the total life-cycle cost can be equal or lower than that of a conventional car. The life-cycle cost does not only include the cost of buying the vehicle, but is comprised of the total costs accrued during the vehicle lifetime, including costs such as fuelling and maintenance.

The costs of buying and fuelling HEVs. Currently, a HEV passenger car costs approximately $3000 -$6000 more than a conventional model. This estimated cost does not include government rebates and incentives offered in some countries, which can be anywhere from $300 to $2500. The first HEV buses on the market in year 2000 were up to 30% more expensive than diesel buses. However, the costs of HEV buses and trucks vary greatly among manufacturers, and as the market matures, the price is decreasing.
Fuel costs are usually assumed to be between 30-40% of the total operating costs of a truck (including the cost of the driver), so buying a HEV truck with 25-35% less fuel consumption can be profitable. The “pay-back time” for the extra purchase price of a HEV can be calculated based on mileage, fuel prices, and actual fuel savings.

The costs of buying and fuelling CNG vehicles.Compared with standard vehicles, the extra cost for CNG passenger cars (usually sold as bi-fuel vehicles) are similar to that for HEVs. CNG trucks and buses are estimated to cost 10-20% more than comparable diesel trucks/buses, but in this case the prices are also very uncertain. CNG vehicles consume up to 10% more fuel compared to standard vehicles, but CNG fuel is generally less expensive. However, the infrastructure needed to fill the trucks/cars is costly. The profitability of this option depends heavily on price of CNG fuel and on the cost for establishing a network of filling stations.

The costs of buying and fuelling a clean diesel vehicle. Currently, the extra cost of buying a passenger diesel car with a particulate filter is around $300-$500. At the moment, the extra cost of buying an Euro V truck is not known, and will depend on factors such as how many are ordered, etc.. As soon as the Euro V standard is mandatory for vehicles, the price difference will disappear.

Maintenance costs for HEVs and CNG vehicles: While the maintenance costs of CNG and HEV passenger cars are comparable or even lower than their diesel and petrol counterparts, the maintenance costs of CNG and HEV diesel buses and trucks have initially been reported as being higher. These higher costs are attributed mainly to the fact that larger HEV and CNG vehicles are newer and much fewer in numbers.

  • New York City Transit reported maintenance costs for the HEV buses were almost twice as high as with ordinary diesel buses. However, these buses were prototypes and it is expected that maintenance costs will drop significantly with experience and development.
  • The delivery company UPS reported that their CNG delivery trucks had also higher maintenance costs compared to diesel delivery trucks, but most of this had to do with the introduction of a new vehicle technology, see UPS example.
  • Eletra in Brazil is, however, projecting that maintenance costs will be 32% lower for their hybrid bus, compared with a similar diesel bus. Furthermore, the engine system is expected to have a lifetime of 15 years, as compared to seven years for a standard diesel bus engine (before overhaul).

In principle, CNG and HEV diesel buses should not have higher maintenance costs than standard diesel buses. With time, these costs will decrease as development continues. This has been the case with maintenance costs for HEV and CNG passenger cars, which have been around longer.