* Typical battery size across models currently on the market. The CO2 reduction from BEVs is based on switching from an average internal combustion engine emissions to a zero-emissions BEV.
The table above shows that mild hybrids are clearly the most efficient method of CO2 reduction, followed by full hybrids, given scarce battery production capacity. Plug-in hybrids are the next most effective after that, but only if they are operated entirely on battery, which is hard to enforce in practice. BEVs have the lowest efficiency, primarily due to requiring disproportionately large batteries to accommodate relatively infrequent, extreme usage cases where the driver will otherwise suffer range anxiety.
This analysis ignores the upstream CO2 in fuel extraction, refining and transportation, as well as in the production and distribution of electricity. Some studies suggest the upstream CO2 of the electricity is greater than for gasoline, but the relative efficiency calculations here implicitly assume they are equal.
Showing the distribution of performance by individual model, the chart below relates the efficiency of CO2 reduction by battery size. Mild and full hybrids are the most efficient on average, but there is significant variation within the class – demonstrating that individual vehicle selection remains at least as important as generic powertrain. The same chart also demonstrates the advantage in CO2 reduction that the US currently holds.
In terms of the trajectory to total CO2 reduction, a transition from gasoline internal combustion engine to full gasoline hybrid can reduce emissions by 34%. As it will take time to increase the supply of full hybrids, there are two routes to short-term CO2 reduction that are viable more quickly. First, a switch from gasoline to diesel internal combustion engine in practice reduces CO2 emissions by 11% at the tailpipe. A second step then to a diesel mild hybrid delivers a further 6% reduction. The final swap to full hybrid delivers another 16%, making 34% in total. Alternatively, a direct switch from gasoline to gasoline mild hybrid can deliver 11%, followed by a further 23% in moving to full hybrid. Therefore, there are immediate-term options for significant CO2 reduction, involving both gasoline and diesel powertrains – the former more suitable in the European market, the latter in the US due to the current mix of fuel utilisation.
It is at any regulatory stages beyond the 37.5% fleet reduction that fuller electrification would be required, as there are limits to the total CO2 reduction that hybrids can deliver. However, by 2030 the EU and US would have had more time to develop expanded, cleaner electricity generation capacity, enhanced distribution grid, and addressed the supply chain issues around the scarce materials in batteries. Not neglecting also that consumer education and acceptance are required to remove barriers to adoption. An alternative scenario by 2030 is that the availability and price of renewable electricity may have fallen to a level at which hydrogen fuel cell vehicles become economic viable, which avoid some of the environmental and geopolitical issues created by largescale battery production.
In summary, this data strongly suggests that policy unilaterally favouring one technology solution may be deeply inefficient and perhaps even the wrong eventual solution. A better approach would be to use real-world data to allow competing technologies to flourish as they can evidence genuine CO2 reductions, delivered as soon as possible.