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Life-Cycle Emissions Analysis of Fuels for Light Vehicles

Tom Beer1,2, Tim Grant3, Harry Watson4 and Doina Olaru1, May 2004

1 CSIRO Atmospheric Research, Aspendale, Vic.
2 CSIRO Environmental Risk Network, Aspendale, Vic.
3 RMIT Centre for Design, Melbourne, Vic.
4 University of Melbourne, Department of Mechanical and Manufacturing Engineering, Parkville, Vic.

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Executive Summary

This study extends the work done for the Australian Greenhouse Office (AGO) on the life-cycle assessment of emissions from heavy vehicles. It does so by using the same methodology and applying it to light vehicles. Petrol, diesel, LPG (dual-fuel) and CNG fuels and a number of vehicle technologies were examined.

On a full-fuel cycle basis, when vehicles are normalised to remove mass differences, the lowest greenhouse gas (GHG) emissions are from hybrid electric vehicles. Diesel vehicles emit less exbodied GHG (exbodied emissions are the sum of the pre-combustion emissions and the tailpipe emissions) than petrol, LPG or CNG vehicles, which also means that a diesel-hybrid would have lower exbodied GHG emissions than a petrolhybrid. Diesel vehicles also have lower exbodied emissions of carbon monoxide and nonmethanic volatile organic compounds (NMVOC) than petrol, LPG, and CNG. However, diesel vehicles emit more particulate matter than any other fuel class.

Exbodied LPG emissions are below those of the equivalent class of petrol vehicle for all types of fuels (propane and autogas) and for all emissions except for carbon monoxide. The equivalent class of petrol vehicle means that second generation LPG vehicles1 are compared with ULP vehicles, whereas third generation LPG vehicles are compared with PULP vehicles. These findings refer to dual-fuel LPG vehicles manufactured on the production line or post-equipped under the control of the car manufacturer. We expect after-market conversions to LPG to perform more poorly, but would also expect dedicated single-fuel LPG to perform better.

CNG vehicles have lower GHG emissions than petrol and second generation LPG vehicles, but higher emissions than diesel and third generation LPG vehicles. Third generation LPG vehicles have the lowest NMVOC, NOx and PM emissions. CNG emissions of NOx and PM are comparable with third generation LPG whereas CNG emissions of NMVOC are slightly higher.

However, these results depend on the drive cycle used to examine the emissions. The above conclusions are based on the European Drive Cycle (EDC) that is required under ADR 79. Under the Artemis Drive Cycle recently introduced as a test drive cycle in Europe, the GHG tailpipe emissions of CNG are less than those of diesel vehicles, whereas the reverse is the case under the EDC and the Australian Urban Drive Cycle (AUDC). This indicates that vehicle technology and catalytic converter technology need to be very tightly designed for optimum performance and minimum emissions. It is for this reason that we expect that dedicated LPG vehicles should be able to be more tightly designed and thus have lower emissions than dual-fuel vehicles.

Present day health concerns associated with motor vehicle emissions are predominantly focussed on particulate matter (PM10, PM2.5, PM1). LPG (third generation) vehicles have the lowest tailpipe emissions of PM10, but on a life-cycle basis the PM10 emissions from LPG and CNG are comparable, and are less than those from diesel, petrol or even hybrid vehicles.

We examined the effect of vehicle mass by examining the exbodied emissions to be expected from a compact-sized vehicle of approximately 1000 kg - compared with the reference family-sized vehicle of 1,700 kg mass. The same relativities hold in both cases, but the absolute values of the emissions are much lower in the case of smaller cars. Thus the reference ULP (less than 150 ppm sulfur) vehicle emits 349 g CO2-e per km on a fullfuel cycle basis; the equivalent Euro 4 PULP vehicle (with less than 50 ppm sulfur) emits 289 g CO2-e per km, whereas a petrol hybrid of the same mass emits 200 g CO2-e per km. However, a compact Euro 4 PULP vehicle of 1130 kg emits 191 g CO2-e per km, a petrol hybrid such as the 2003 Prius emits 128 g CO2-e per km, whereas the Honda insight (950 kg) emits only 101 g CO2-e per km.

A comparison of exbodied emissions is provided in charts ES.1-ES.5, and is also presented in Section 7.2 of the report. The bar charts show the emissions per km for family vehicles (1,700 kg).

The effects of vehicle mass are most marked in the case of fuel consumption and GHG emissions. Emissions of the criteria pollutants are more dependent on vehicle technologies and emission control systems.

A summary of the results of the analysis of emissions per km is presented in Table ES.1.

Table ES.1 - Relative performance of fuels analysed in the study (ULP Euro 3 as baseline)
PULP Euro 4 - - - - -
ULS PULP Euro 4 - - - - -
LS Diesel Euro 4 - - - - ++
ULS Diesel Euro 4 - - - - ++
LPG Autogas 2nd gen. - - + - -
LPG Propane 2nd gen. - - - - -
LPG Autogas 3rd gen. - - - - -
LPG Propane 3rd gen. - - - - -
CNG - - - - -
Hybrid PULP - - - - -
Hybrid Diesel - - - - -

Legend: - significantly lower2 (than the reference fuel); - lower; = much the same; + higher; ++ significantly higher.

Numerous data gaps were revealed during this study. There were:

These findings suggest that further investigation is necessary. The statistical inference is confined to very few data under Australian conditions. This is an area of future research and we recommend testing the requisite vehicles on the same drive cycle, with the four fuels examined in the present study.

Family-sized Australian car

Figure ES.1 - Exbodied greenhouse emissions from family-sized vehicles European Drive Cycle

Figure ES.1 - Exbodied greenhouse emissions from family-sized vehicles (European Drive Cycle)

Figure ES.2 - Exbodied carbon monoxide emissions from family-sized vehicles (European Drive Cycle)

Figure ES.2 - Exbodied carbon monoxide emissions from family-sized vehicles (European Drive Cycle)

Figure ES.3 - Exbodied NOx emissions from family-sized vehicles (European Drive Cycle)

Figure ES.3 - Exbodied NOx emissions from family-sized vehicles (European Drive Cycle)

Figure ES.4 - Exbodied hydrocarbon (NMVOC) emissions from family-sized vehicles (European Drive Cycle)

Figure ES.4 - Exbodied hydrocarbon (NMVOC) emissions from family-sized vehicles (European Drive Cycle)

Figure ES.5 - Particle (PM10) emissions from family-sized vehicles (European Drive Cycle)

Figure ES.5 - Particle (PM10) emissions from family-sized vehicles (European Drive Cycle)

1 Second generation LPG vehicles have electronic control, and the third generation LPG combine advanced fuel injection technology with advanced electronic management features. For more details, the reader can consult Anyon (2002).
2 Significantly lower is < 0.5*base value; lower is below 0.95 of the base value; about the same is within 0.95 and 1.05 of the base value; higher means >1.5*base value; significantly higher means > 2*base value.
3 Dual-fuel vehicles (in Australian terminology) are known as bi-fuel vehicles in the UK.

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