A wide variety of powertrains are available in the market today, serving different purposes and customers’ needs. The selection criteria of the most suitable vehicle vary and usually include economic, environmental, and societal factors, as well as personal needs and preferences. Detailed technical knowledge is lacking from most of the consumers / potential vehicle buyers; hence, a simplified overview of the different available powertrains and their main characteristics with regard to air pollutant emissions is provided below.

The chain of components involved in the process of getting the vehicle to move is as follows:

The first step in the powertrain cycle is to switch the engine on in order to convert the stored fuel (energy) of the vehicle into motion (and losses). This energy is then transferred to the wheels, which is where the transmission (consisting of the gearbox and clutch), flywheel and driveshaft come in. Referred to collectively as the ‘drivetrain’, these are the parts that convert the stored fuel (energy) of the vehicle and transmit it (using gear ratios) as torque – or rotational force – to the wheels, which are set in motion.

A powertrain’s exact design depends on a number of factors. However, the factor which is more relevant to emissions and environmental performance of the vehicle, is the type of the engine and how the power is provided to the wheels. Specifically, the following three main categories can be identified:

• Internal combustion engine vehicles (ICEV): power to the wheels is only provided directly by an internal combustion engine (for example, a petrol or diesel engine).
• Hybrid electric vehicles (HEV): power to the wheels is provided by an internal combustion engine and one or more electrical motor(s) in different configurations.
• Electric vehicles (EV): power to the wheels is provided by one or more electrical motor(s).

Vehicles using fossil fuels – no electric powertrain

Petrol vehicles

Petrol vehicles use spark ignition (SI) engines, designed to run on petrol, which is mixed with air and the combustible mixture is ignited by a spark plug to produce power. Continuous developments keep these engines a better choice for small and medium sized vehicles due to their lower torque demand.

In terms of fuel efficiency, all but the most advanced SI engines cannot achieve maximum efficiencies above 30-35% in steady-state operation. On average, the energy conversion efficiency of SI engines is in the order of 18-20% (Ntziachristos and Dilara, 2012). The rest of the energy is lost in engine and drivetrain inefficiencies, while a small proportion is also used to power various vehicle accessories, e.g. radio and air conditioning (EEA, 2016).

In terms of pollutants, petrol vehicles contribute by more than 70% to total CO and VOC emissions in the EU. The following picture shows that petrol cars are by far the largest contributor, being responsible for more than half of total emissions.

_{Contribution of the different vehicle and fuel types to the total CO (left) and VOC (right) emissions from road transport. Source: Own calculations with the COPERT model}

Diesel vehicles

Diesel vehicles use compression ignition (CI) engines, designed to run on diesel. In these engines, the fuel is self-ignited after pressure and temperature inside the combustion chamber exceed a certain limit. Diesel engines require higher pressures to operate and combustion is slower than spark ignition ones. Hence, they are better suited to larger applications where inertial forces of large engine components demand lower speed operation.

In terms of fuel economy, compared to a spark ignition engine, compression ignition engines offer higher efficiency.  The energy conversion efficiency of CI engines is typically in the order of 25-30% for diesel cars (Ntziachristos and Dilara, 2012).

It is known that diesel vehicles are responsible for the majority of NOx emissions from road transport. In the EU, this share exceeds 95%, as depicted in this picture. Diesel passenger cars are responsible for about half of the total NOx and PM emissions.

_{Contribution of the different vehicle and fuel types to the total NOx (left) and PM (right) emissions from road transport. Source: Own calculations with the COPERT model}

Liquefied Petroleum Gas (LPG) vehicles

Liquefied petroleum gas is a mixture of propane and butane and a range of other hydrocarbon traces. When it is stored, it is a colourless liquid which, during the combustion process, evaporates into a gas. LPG can fully replace petrol in existing SI engines, with minimum conversion effort. LPG retrofits are probably the most frequent kind of retrofit today in Europe, as owners try to benefit from lower prices per unit of energy.

Natural Gas (NG) vehicles

Natural gas is mainly methane and can be easily combusted using typical spark ignition engines. Natural gas may be stored on board the vehicle either in liquid (LNG) or in compressed (CNG) form. This depends on the specific application and does not lead to any fundamental differences in usability and vehicle performance. In general, LNG is typically used in heavy-duty vehicles that travel long distances and CNG is more commonly used in passenger cars. The fuel efficiency of natural gas is comparable to petrol.

Natural gas vehicles are usually found in the market as bi-fuel ones, since they have the option of running on two fuels, usually petrol and CNG (in light-duty vehicles), but configurations with diesel can also be found (in heavyduty vehicles). The two fuels are stored in separate tanks and the engine runs on one fuel at a time, or both simultaneously. Bi-fuel vehicles have the capability to switch back and forth from one fuel to the other, manually or automatically.

Vehicles using biofuels – no electric powertrain

Biofuels are important for reducing the carbon intensity of transport fuels. The carbon intensity of most biofuels is significantly lower compared to the fossil fuels they substitute and depends mainly on the production pathway and feedstock used for their production. Biofuels can have some air pollutant benefits too, but these are rather small compared to the reduced carbon emissions.

Ethanol – Flexi-fuel vehicles (FFV)

Ethanol (EtOH), or more exactly bioethanol, is produced by the fermentation of vegetable sugars and can be used as a replacement of petrol in spark ignition engines. Blends of up to 85% bioethanol in normal petrol (E85) are already in use for some years now in flexi-fuel vehicles (FFV). Higher ratios are not used because the lower vapour pressure of ethanol compared to petrol makes it hard to start the engine when ambient tempera­ture drops.

Biodiesel and other diesel substitutes

Biodiesel refers to the blend of fatty acid methyl esters (FAME) used today in blending with fossil diesel. Biodiesel may be combusted up to a certain proportion, limited to 7% by volume (noted as B7) under the current Fuel Quality Directive, in a diesel engine, without any mechanical conversion or other adaptations. However, due to the complexity of diesel combustion and diesel fuel system, neat biodiesel (i.e. B100) is not used and is not expected to be used as a diesel replacement (Ntziachristos and Dilara, 2012). Biodiesel is considered as a renewable fuel and it is usually produced by new and used vegetable oils, animal fats and, sometimes, recycled restaurant grease.

Other advanced biofuels used (mixed) with fossil diesel are hydrotreated vegetable oil (HVO), gas to liquid (GtL) and biomass to liquid (BtL). HVO is a renewable diesel fuel made by a refinery-based process converting vegetable oils to paraffins. Animal fats are also suitable for feedstocks. Because they are hydrocarbons, they largely meet conventional diesel fuel requirements. GtL fuels are produced from natural gas using the Fischer-Tropsch process in sites near gas reserves around the world. BtL uses biomass such as woodchips as feedstock to gasification and the Fischer-Tropsch process.

Other liquid fuels substitutes

In addition to fuels of fossil or biogenic origin, liquid fuels can be also produced synthetically, with the help of renewable energy. The Power to Liquid (PtL) concept is based on the conversion of renewable energy to liquid fuels. In addition to having a higher energy density than traditional fossil and biofuels, PtL can offer carbon neutral fuels for the transport sector.

Hybrid electric vehicles

Petrol-hybrid and diesel-hybrid vehicles

The fundamental difference between a hybrid vehicle and a vehicle powered only by an internal combustion engine is that the former combines both an ICE and an electrical motor to power the wheels. In a strong (full) hybrid vehicle, the electric motor and an internal combustion engine are connected in parallel (most common configuration) or in series (less common) and can both deliver power to the wheels. In a mild hybrid vehicle, there is no pure electric mode available for the driver to select (as in the case of a full hybrid one).

For the charging of the batteries of these vehicles, there is no external charging system; this is done through the internal combustion engine and regenerative braking. Hybrid vehicles can deliver certain environmental benefits compared to conventional ones, since they usually reduce fuel consumption and pollutant emissions. The level of emission reduction varies, depending on the exact configuration of the hybrid system.

Plug-in Hybrid Electric Vehicle (PHEV)

In a Plug-in Hybrid Electric Vehicle (PHEV) the primary energy sources are both liquid fuel and electricity from the national grid stored in the vehicle’s battery. The battery is charged by an adapter that connects the vehicle to the mains. Power to the wheels is provided both by the internal combustion engine and one or more electrical motors. The environmental impact of PHEV depends heavily on their operation mode. On one hand, the all-electric mode results in effectively zero tailpipe emissions; on the other hand, relying on the internal combustion engine can lead to emission levels comparable or even higher than those of conventional vehicles (as a result of increased weight due to the battery and the rest of the electric powertrain components). As the all-electric range of most current PHEVs is limited to an average of around 50 km, these vehicles are best suited for short everyday trips such as for commuting, where they can be zero-emission vehicles. The emissions advantage is substantially reduced, or even completely disappear, when PHEVs are used mostly on longer trips, beyond their all-electric range.

Pure electric vehicles

Battery Electric Vehicle (BEV)

A battery electric vehicle (BEV) involves only an electrical powertrain and no internal combustion engine. Energy is stored in batteries in the form of chemical energy which produces electricity and, upon demand, this energy is delivered to the electrical motor which powers the wheels. Electrical motors have a real-world efficiency that may exceed 80% (EEA, 2016) and produce zero tailpipe emissions. However, abrasion of car tyres and brakes still create pollution (as it does in other vehicles). Heavy technology investments and continuous improvements in recent years have resulted in the introduction of a large range of electric vehicles in the market by all vehicle manufacturers. Financial and other incentives are contributing to a quick uptake of these vehicles in the European market.

Fuel cell electric vehicles (FCEV)

Fuel cell electric vehicles usually use hydrogen (H₂), which can be used as a fuel in fuel cells. Hydrogen has received large interest recently as a fuel that could fully replace carbon-based fuels. However, the cost-efficiency of this pathway is at present lower compared to battery electric vehicles. In addition, limitations in the supporting infrastructure needed for producing and distributing hydrogen, together with the cost of fuel cell production, coupled with a limited availability of models, result in very low sales of FCEV passenger cars at European level. In any case, hydrogen fuelled vehicles remain as a technical possibility, and could be a mid-term decarbonisation solution for vehicles that are seen as difficult to electrify, such as long-haul trucks.

Summarising the main types of electrified powertrains, the following figure shows the energy pathway for the various vehicle types, i.e. how the fuel/energy is delivered to the vehicle, where it is stored, and how the energy is transferred to the wheels. Four vehicle types are shown in this figure, namely:

• A BEV and a FCEV, which are pure electric vehicles, i.e. there is no internal combustion engine.
• A PHEV and a HEV, which are hybrid electric vehicles.

In addition, BEV and PHEV are also labelled as plug-in electric vehicles (PEV), since in both vehicle types the battery is charged by an adapter that connects the vehicle to the mains. On the other hand, this is not the case for HEV and FCEV.

_{Types of electrified powertrains. Source: (Gaton, 2018)}