One approach to lowering the CO2 emission from traffic is the hybridization of vehicles. A hybrid vehicle uses two or more distinct power sources, i.e. hybrid electric vehicles (HEVs) combine an internal combustion engine and one or more electric motors. Vehicles employed in urban areas like small passenger cars, local delivery trucks and city busses benefit from hybridization and show substantially lower CO2 emissions, ranging from 23 to 43% depending on the traffic dynamics. For passenger cars there are various levels of hybridization possible all giving rise to various amount of CO2 emission reductions at different costs. Small passenger cars benefit the most from strong downsizing in combination with micro hybridization. Cars running most of their kilometers on motorways do not benefit from hybridization mostly because on motorways vehicles drive at more or less constant speeds. Hybrid vehicles are still more expensive than traditional vehicles using an internal combustion engine. They have the advantage of higher fuel efficiency and reduced CO2 emissions without additional infrastructure requirements.
At present, there is no single good solution to the problem of lowering CO2 emission in the transport sector. Currently, there are two major technical pathways to GHG emission reductions. The first pathway involves the deployment of low carbon alternative fuels like biofuels, LPG, LNG and CNG. The second technical pathway involves the improvement of the energy efficiency of the vehicles through downsizing of the engine and various levels of hybridization and electrification. These two technical pathways are complementary.
The most energy efficient vehicle available today is the electric vehicle. However, commercialization of full electric vehicles is still hampered by high purchase prices, short driving ranges and long recharging times. These facts have led to the construction of hybrid vehicles. A hybrid car combines an internal combustion engine with technologies used in full electric vehicles.
Hybrids achieve improved efficiencies by employing several techniques. An important technique is regenerative breaking. During breaking the kinetic energy of the vehicle is normally dissipated as heat in the breaking discs. However, in hybrid vehicles, electro motors attached to the wheels serve as generators during breaking converting the kinetic energy into electricity. In full hybrid vehicles the electro motors also propel the vehicle at relatively low speeds. The advantage of electro motors over combustion engines at low speeds is that even at low revolutions they can exert the maximum torque (pulling force). In contrast combustion engines need to go to much higher revolutions to exert their maximum torque. This makes electro motors much more efficient at low speeds than combustion engines.
The Office of Energy Efficiency and Renewable Energy (EERE) of the United States Department of Energy offers an animated explanation of how HEVs work and why they are more fuel efficient here .
There are four different levels of hybridization available in vehicles (Larsen, 2004):
Micro hybrids do not use electric motors to propel the vehicle. The electric motors are only used as generators for regenerative breaking to charge the car battery. These vehicles have small generators connected to the internal combustion engine lowering the friction and hence lowering the fuel consumption. Additionally, micro hybrids employ the start-stop system which switches the engine off during idling. (RDW, 2010)
Mild hybrids have electric motors which are used to propel the vehicle. However, they cannot drive solely electrically. Mild hybrids also employ regenerative breaking and the start-stop system. (RDW, 2010)
Full hybrid cars are parallel hybrids which can be propelled fully electric at low speeds and use the internal combustion engine at higher speeds or when the electric energy stored in the car battery is low. RDW, 2010)
Series hybrid cars are full electric vehicles which use the internal combustion engine as a generator to produce electricity. The powertrain of a series hybrid is identical to a battery electric vehicle and a hydrogen fuel cell vehicle. The only difference between these three electric vehicles is the source of the electricity produced by the car. (RDW, 2010)
Hybrid electric vehicles are most feasible for use in urban traffic, where there is a frequent need for breaking. The effect of the regenerative breaking and the use of electro motors of a hybrid car on CO2 emissions is shown in figure 1. According to this graph, hybrid vehicles have substantial tailpipe CO2 emission reductions only at relatively low speeds. The graph assumes that at speeds below 50 km/h, the vehicle is operated in an urban area with the corresponding traffic dynamics. [media:image:1] The improved efficiency of hybrid vehicles makes hybridization especially worthwhile for urban passenger cars, small trucks for local delivery and city busses. Hybrid vehicles do not show a significant improvement in fuel consumption when driven on highways. Vehicles which are mainly used on highways are best equipped with downsized diesel engines to obtain minimal fuel consumption.
A large advantage of hybrid vehicles compared to other options for reducing GHG emissions in transport is the fact that no additional infrastructure investments are required
The hybrid vehicle has left the large scale demonstration phase and is now in the first stage of commercialization. A variety of different HEVs are commercially available today.
- Micro hybridization is often used for relatively small cars in combination with strongly downsized internal combustion engines. Examples are the VW Polo Bluemotion and the VW Up. The VW Bluemotion motion has a type approval (the process which ensures that vehicles meet applicable safety and environmental standards) value for the tail pipe CO2 emission of 87 grams/km (RDW, 2010).
- Mild hybridization is mainly used for midsized vehicles. Examples are the Honda Insight and the Honda Civic. The Honda Insight has a type approval value for the tailpipe CO2 emission of about 105 grams/km (RDW, 2010)
- Full hybridization is mainly used for bigger cars. Examples are the Toyota Prius and variousmodels by BMW and Mercedes. The Toyota Prius has tailpipe CO2 emission of about 92 grams/km
- Series hybrids are not available yet but will become so in the near future. Examples are the Opel Flextreme, Opel Ampera and the GM volt.
Hybrid vehicles only have substantially lower CO2 emissions when they are operated in urban areas (see figure 1). This fact makes hybridization an especially suitable tool for lowering CO2 and NOx emission from city buses and local delivery trucks. In a 2006 study by NREL the performance of hybrid diesel buses has been evaluated (Chandler, 2006). The fuel consumption of thirty new diesel articulated buses was compared with the fuel consumption of ten hybrid diesel buses over a period of 12 months. Depending on the traffic dynamics, the hybrid buses have 23 – 43% lower CO2 emission and 18-39% lower NOx emissions compared to similar new non hybrid diesel articulated buses.
A second group of vehicles which can substantially benefit from hybridization are the trucks used for local delivery in urban areas. In a 2009 study by NREL six hybrid delivery trucks were compared to six standard diesel delivery trucks over a period of 12 months (Lammert, 2009). The fuel economy of the hybrid trucks was about 29% better than the fuel economy of the standard diesel delivery trucks. However, a 29% increase in NOx emission was found.
It is difficult to assign a CO2 emission reduction to (full) hybrid passenger vehicles. A full hybrid vehicle driven most of its kilometers in urban areas can have CO2 emission reductions up to 25% (Passier et al, 2007). However, a passenger cars driving most of its kilometers on motorways will at most have very little CO2 emission reduction. The higher weight of the vehicle, because of the additional electro motors and car batteries, may even lead to higher CO2 emissions than comparable non hybrid vehicles. Therefore, for example city taxis are a good niche market for (full) hybrid vehicles.
Micro hybridization is the cheapest solution for passenger cars to benefit from fuel saving technologies like the start-stop system and regenerative breaking. In contrast, full hybridization of vehicles is still a relative expensive technique. Additional costs for the full hybridization of a light passenger car range from US$3,000 to US$6,000 (UNEP, 2009). The costs of maintenance of full hybrid cars are expected to be equal to non-hybrid vehicles. It is expected however, that the overall costs over the lifetime of a vehicle are lower for a hybrid vehicle due to better fuel efficiency.
The purchase costs of a hybrid bus can be 30% higher than a comparable non hybrid bus (Chandler and Walkowics, 2006). But the total operating costs of a hybrid bus are approximately 15% lower than for a non-hybrid bus, which implies that the initial 30% higher purchase costs can be earned back by lower operating costs over the lifetime of the bus.
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group .]
Project developers of projects deploying hybrid electric vehicles can use the following CDM methodology: AMS-III.C.: Emission reductions by electric and hybrid vehicles .
Chandler, K. and Walkowics, K. (2006). King County Metro Transit Hybrid Articulated Buses: Final Evaluation Results. NREL/TP-540-40585, available at http://www.nrel.gov/vehiclesandfuels/fleettest/pdfs/40585.pdf 
Lammert, K. (2009): Twelve-Month Evaluation of PS Diesel Hybrid Electric Delivery Vans. NREL/TP-540-44134, available at http://www.nrel.gov/docs/fy10osti/44134.pdf 
Larsen, R.P. (2004). An overview of Hybrid Vehicle Technologies. Argonne National Laboratory, Center for Transportation Research, 2004.
Passier, G., F.V. Conte, S. Smets, F. Badin, A. Brouwer, M. Alaküla, D. Santini (2007). Status Overview of Hybrid and Electrical Vehicle Technology 2007; Final report of Phase III, Annex VII, IEA. TNO, Delft, The Netherlands
RDW (2010). Brandstofverbruiksboekje 2010. available at http://www.rdw.nl/nl/voertuigeigenaar/auto/kopen_en_verkopen/milieu_en_verbruik/ 
UNEP, 2009. Hybrid electric vehicles - An overview of current technology and its application in developing and transitional countries. United Nations Environment Progamme, Nairobi, Kenya.