Hybrid-Electric Vehicles

Hannu Jääskeläinen

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Abstract: In a hybrid powertrain, at least two means can provide power for the vehicle, with internal combustion engine/electric motor hybrids being the most common configuration. Compared to a vehicle with only an internal combustion engine, life cycle GHG reductions up to about 1/3 are possible with hybrid vehicles, depending on the duty cycle. Hybrid drivetrains can provide a number of functions that can be used to reduce fuel consumption and/or improve vehicle performance, such as regenerative braking, internal combustion engine–off operation, and electrically continuously variable transmission (eCVT) capability.


In a hybrid powertrain, at least two means exist to provide power for the vehicle. While internal combustion engine/electric motor hybrids are the most common, other possibilities include internal combustion engine/hydraulic, internal combustion engine/pneumatic, internal combustion engine/flywheel and fuel cell/battery electric. Most hybrid drivetrains also contain a means for energy storage (battery, capacitor, hydraulic reservoir, flywheel, etc.) but this is not essential.

Compared to a vehicle with only an internal combustion engine, life cycle GHG reductions of about 1/3 are possible with hybrid vehicles when sufficient urban driving is encountered [4445]. Pure-electric and plug-in hybrids that have a sufficiently large battery to allow some electric-only operation can provide additional GHG reductions, but the benefit is strongly dependent on additional parameters such as climate, the carbon intensity of the grid and the GHGs associated with battery production. Further discussion can be found elsewhere.

Hybrid vehicles make up a considerable portion of light-duty vehicle and urban bus sales. In 2020 in Europe, non-plug-in hybrid vehicles (HEV) represented 12% of new light-duty registrations, and plug-in hybrid vehicles (PHEV) 5%. This increased to 20% and 9% respectively in 2021 [5422][5423]. In the US, sales of both HEVs and PHEVs have also increased but make up a considerably smaller portion of light-duty vehicle sales than in Europe, Figure 1. In the US, there has been a considerable shift in vehicles being hybridized from cars to cross-over vehicles. In 2021, crossover vehicles constituted over 50% of HEV and PHEV sales while the proportion of HEV and PHEV sales made up by cars has decreased from ~100% in 2014 to about 25% in 2021 [5424][5425].

[SVG image]
Figure 1. US quarterly light-duty vehicle sales by powertrain, 2014-2021

(Source: US EIA)

According to American Public Transportation Association data, from about 2004 to 2014, sales of hybrid transit buses in the US increased annually until they reached about 18%, Figure 2. Between 2014 and 2021, sales have remained at about 18% of total bus sales between 71,000 to 72,000/y [5426].

While battery electric buses receive much attention, they are included in the “other” category of Figure 2 which also includes hydrogen and LPG buses. In 2020, the “other” category accounted for 1.4% of total APTA bus sales [5426].

[SVG image]
Figure 2. Proportion of US transit bus sales according to powertrain/fuel type

Electric, hydrogen and LPG buses included in “other” category

Hybrid drivetrains are also an attractive option for medium and heavy-duty vehicles and some types of nonroad equipment. They are also being adopted in marine applications where they are best suited for vessels with large variations in power demand, coastal trades and operations within emission control areas. The combination of diesel and electric drives can allow a number of benefits including: engine operation at its most efficient point regardless of power requirement or load, engine-off operation (required in some ports for specific maneuvers) and enabling the use of smaller engines to save fuel (a smaller diesel engine can be used in applications such as ferries where full power may only be required for relatively short periods of time and where shore power can be frequently accessed to charge batteries). The technology was also becoming applicable for some deep-sea shipping segments such as crane operations.