Two important methods of reducing greenhouse gas emissions in motor vehicles are that of mass reduction through materials substitution and the use of hybrid electric drive. The issue is that the use of hybrid electric drives increases the mass of the vehicle, contradicting the first efficiency goal. Both of these methods of increasing the efficiency of the vehicles in use increase the vehicles emissions in production; and so a trade off between the benefits of low production emissions and low usage emissions must be found to minimise the vehicles environmental impact. Hybrid electric vehicles (HEVs) can be reduced in mass not only through materials substitution, but through substitution of higher energy density battery types.
Using fleet based analysis which accounts for temporal effects, life cycle assessments for multiple combinations of conventional drive and hybrid electric drive vehicles with various levels of light alloy substitutions for steel, and different battery types has been conducted. A final comparison of a conventional Otto cycle powered vehicle to a hybrid electric vehicle is then done, using the most viable results from the light alloy substitution and battery type analyses.
The results for the light alloy substitution into conventional vehicles indicate that cast aluminium is the most viable method of mass reduction; and is particularly beneficial when using recycled aluminium in the product. Casting is consistently the most viable metal form to substitute, with panelling second, and beam structures being the least viable to substitute for a light alloy. Electrolytic magnesium is also viable in cast form, however is less so than aluminium, making aluminium a superior option. The results for light alloy substitution into HEVs is similar to that for conventional vehicles, however is scaled with longer crossover times; reflecting the lower savings in HEVs due to mass reduction. Titanium alloys are never determined to be an environmentally sound option.
Battery type selection compares the three most promising battery types; semi-sealed lead acid, nickel metal hydride (NiMH), and lithium ion cobalt (Li-ion). The results indicate that NiMH starts to save emissions compared to if the same HEV had lead acid batteries, after a period of 15 years. Li-ion batteries will always create more emissions than either lead acid or NiMH.