The car market has been dominated by petrol and diesel in the last decades. Given the need to decarbonate the transport sector, alternatives fuel have emerged to reduce the exhaust emissions from vehicles. It includes CNG, hybrids and electric vehicles. This paragraph gives an overview of the main vehicles' fuels and drivetrains.
Vehicle dynamic
Several parameters define the motion of a vehicle. The movement is related to fuel consumption. The energy provided is converted into mechanical energy through the engine and transmitted to the wheels. The overall drive system (i.e., both the driving force and braking force) is controlled by the pedals to ensure a certain speed, acceleration or deceleration. While driving, three main external forces are applied to a vehicle: air drag, rolling resistance and grading (Figure 1). Air drag and air rolling resistance work against the movement. Air drag represents the impact of air on the vehicle. This resistance increases with the vehicle speed. Rolling resistance is due to friction between the tyres and the road. The weight plays an important role in this force. Increasing the vehicle's weight increases the rolling resistance. The grading force is dependent on the road slope. Due to gravity, it resists motion during uphill, whereas it contributes to the propulsion while downhill driving. All these forces that resist the movement contribute to the reduction of the energy efficiency of the vehicle. More energy, and thus higher fuel consumption, is needed to overcome those resistances. Figure 1: Overview of the different forces acting on a vehicle moving forward Source: (Jimenez et al. 2018)
Notes:
Faero is the air drag force which is the impact of the air on the vehicle
Fhc is the component of the vehicle weight that acts along the road slope (grading force)
Frr is the air rolling resistance which is due to frictions between the tyres and the road
Ft is the drive force
Fi is the acceleration force
Key parameters of vehicles contribute to the forces mentioned:
The overall weight of the vehicle (the curb weight, passengers, luggage, etc.): the heavier, the more energy is needed to put the vehicle in motion.
The rolling resistance coefficient, Cr: depends on both tyres and road condition such as materials, structure, temperature or pressure and act on the rolling resistance force.
The aerodynamics coefficient, Cd: defined by the shape of the vehicle and contribute to the air drag forces.
The frontal vehicle area contributes to the air drag force. The bigger, the higher the force is.
The vehicle speed: increases the effect of air drag.
The first four parameters are vehicle specific. Besides the mentioned resistances forces, the overall energy efficiency of the vehicle also depends on the engine efficiency and the transmission design. Losses through frictions appear along the powertrains.
Reference:
Jiménez, D.; Hernández, S.; Fraile-Ardanuy, J.; Serrano, J.; Fernández, R.; Álvarez, F. (2018) Modelling the Effect of Driving Events on Electrical Vehicle Energy Consumption Using Inertial Sensors in Smartphones. Energieshttps://doi.org/10.3390/en11020412
Combustion engine
The classic combustion engine system works as follows: petrol and diesel engines are made up of cylinders, also known as combustion chambers. In it, air and fuel are brought together and compressed by a piston, after which the mix ignites and releases energy. This energy creates an up-and-down motion of the piston, which is converted via the crankshaft into a rotating movement. This mechanical energy is then transferred to the wheels of the car via the powertrain.
In the simplest configuration, each cylinder is equipped with two valves: the intake valve, where air enters the cylinder, and the exhaust valve, through which the exhaust gases leave. In addition, each cylinder also has an injector, which brings fuel into the cylinder. The cylinder capacity of an engine is the difference between the volume above the piston's head when it is in its lowest and the volume when it is in the highest position, multiplied by the number of cylinders.
The classic passenger cars' engines are called four-stroke engines. This means that the engine goes through 4 phases ("stroke") (see figure 2), divided over 2 revolutions. An engine does approximately (depending on the engine type and the load) between 1,500 to 5,500 revolutions per minute (rpm), meaning 25 to 90 per second.
Both petrol (also known as Otto engines) and diesel engines follow this cycle. The difference between the two is in the ignition. With a gasoline engine, the mixture is detonated via a spark from the spark plug at the right time. In the case of a diesel engine, the mix ignites under the influence of the high pressure applied to it which is called self-ignition. Both processes also differs on the fuel-air ratio mix. It explains why it is impossible to drive a petrol engine on diesel fuel and vice versa.
An engine can have any number of cylinders, which can be arranged in various configurations: in-line engines (cylinders aligned in a row next to each other), boxer engines (cylinders opposite each other), radial engines (the cylinders are in a star shape), V-engines (the cylinders are in a V-shape, put in pairs), etc. For vehicles, in-line engine were initially chosen because of the simple construction. As all kinds of peripherals claimed their place under the hood, the more compact V-motors have been adopted and "flat" boxer engines to a lesser extent. However, the vast majority of our vehicle fleet consists of in-line engines. The number of cylinders mainly depends on the required power; currently, 4 cylinders are still chosen for most engines.
Figure 2: The four phases of the internal combustion engine
The diesel engine performed better in terms of thermal efficiency, which is defined by the energy from the fuel's combustion that is available to move the car from the engine. While petrol engines can reach up to 38 % of efficiency, diesel engines can reach up to 42%. The goal from car manufacturer is to reach 50% for both engines. The overall powertrain efficiency, which includes the engine efficiency and losses on the transmission system (i.e., frictions), lies in the 13-20% range. (Hooftman, 2018)
Reference:
Hooftman, N. (2018)., The road towards a zero-carbon transportation system by 2050 - A comprehensive study for Belgium in a European context.
Alternative fuels
The automotive industry and research are transitioning towards alternative fuels and drive systems to make transport as clean as possible. The main developments in alternative fuels and drivetrain concepts for cars are depicted on this page.
Gaseous fuels
Engines with gaseous fuels hardly differ from classic fuel engines. Adjustments are mainly situated in the fuel tank, supply and injection. After all, only gas under high pressure (usually about 200 to 250 bar) needs to be stored and moved. There are several gaseous fuels:
CNG (Compressed Natural Gas) as fuel combines low CO2 emissions compared to petrol car (about 15-18% less on Well-to-Wheel approach (Hooftman, 2018)) with very low emissions of PM, CO and NOx. In addition, the methane stored as CNG can also be obtained from the fermentation of organic material, such as food crops or waste. With the latter, additional CO2 emission reductions could be achieved. One must keep an eye on the amount of unburned fuel in the exhaust gases since natural gas (methane) also contributes to the greenhouse effect. In addition to good combustion of the fuel, good functioning of the catalyst is necessary to remove unburned residues, which can offer a solution for this. Finally, the energy efficiency of CNG passenger cars is similar to petrol or diesel cars. However, the driving range is about 50% lower.
Hydrogen (H2) is often mentioned for fuel cell technology (see below). Nevertheless, using hydrogen as a gaseous fuel in a combustion engine is just as possible. In theory, the emissions consist only of water and small amounts of unburned hydrogen that are entirely harmless. The way hydrogen is produced (e.g., via electrolysis of water) will determine the environmental friendliness of this fuel, especially in terms of CO2 emissions.
LPG (Liquified Petroleum Gas) is produced from petroleum and consists of a mix of propane and butane. It is refuelled as liquid fuel under pressure, but under normal atmospheric pressure, the fuel is gaseous. In principle, any classic petrol car can be converted to an LPG vehicle, although specific engine characteristics (e.g., direct petrol injection) may complicate the process. The litre consumption of a car on LPG is 20 to 25% higher (depending on the mixing ratio propane/butane) than of the same car on gasoline. However, because the combustion of a litre of LPG releases 30% less CO2 emissions than a litre of petrol, the average CO2 emissions is reduced by approx. 15%. In addition, there is a slight reduction in NOx emissions compared to a petrol engine. Frequently heard objections to LPG are the limited tank capacity (autonomy) and the parking ban in some underground garages.
Biofuels
Biofuels and flex-fuel vehicles. When fuel is produced from organic material, one speaks of a biofuel. Biofuels are often produced from plant material. Some of the best-known biofuels are bioethanol, biodiesel and pure vegetable oil (PPO). Chemically, there is little difference between fossil fuels such as petrol and diesel and biofuels. Biofuels also consist mainly of carbon and hydrogen.
Nevertheless, biofuels emit less CO2 than fossil fuels. The reason is that the CO2 emitted while burning is equivalent to the CO2 amount absorbed during the plants' grow. However, in practice, CO2 will still be emitted in the transport and production of the fuel and will be indirectly induced by land-use change due to agricultural activities. Therefore, better CO2 reductions could be achieved by using waste products. Such biofuel are referred to as "second generation".
Due to their similar structure, bioethanol and biodiesel can be mixed with conventional fuels. No engine modification is necessary as long as this is done in small quantities (5% biodiesel in diesel, 10% bioethanol in gasoline). These mixes can be used in all vehicles that are currently driving around. Higher concentrations (e.g., E85: 85% ethanol and 15% petrol) require adjustments in the fuel supply and engine control and may only be used in cars that have been adapted for this purpose.
A special category of vehicles, the so-called flex-fuel vehicles, can run on any possible mixture of classic and biofuel. A sensor measures biofuel concentration, and the engine operation is adjusted. When biofuel filling stations are still difficult to find during the transitional period, these vehicles offer a user-friendly solution.
Electric and Hybrid vehicles
Electric vehicles are no longer powered by a classic combustion engine but by an electric motor. An electric motor converts electrical energy into mechanical energy (i.e., rotational movement), which is transmitted to the wheels. This engine has some advantages over the classic combustion engine. Firstly, energy is only used when the motor needs to deliver power. When stationary, an electric motor does not consume electricity. Secondly, the motor can also be used as a generator. For example, during braking, part of the kinetic energy can be converted back into electricity (this is called braking energy recuperation or 'regenerative braking').
It also has a better overall powertrain efficiency (about 60-80% compared to 13-20%) than an ICE vehicle. The limitation of this technology lies in the storage of electricity. Currently, this is done in batteries . It has two main disadvantages that hinder the EV adoption from drivers: limited capacity (and therefore the limited driving range of the vehicle) and long recharging times (of the order of 4 to 8 hours). In the future, these issues will be addressed by new developments in battery technology and electricity storage, such as better energy density or fast and ultra-fast charging will offer a solution. Currently, EV driving range lies between 250 to 400km, which is still way less than ICE cars. The electric motor does not produce emissions at the exhaust, but one must take into account the emissions during electricity production.
Hybrid vehicles are vehicles with two different energy conversion systems. Today, this term is mainly used for a vehicle with both a classic combustion engine and an electric motor. In this way, the disadvantages in terms of the autonomy of the electric motor are compensated by the combustion engine, while the advantages of the electric motor are exploited. There are two main configurations of hybrid engines:
Series configuration (Figure 3): only the electric motor drives the wheels. The combustion engine is only used to produce electricity for the electric motor. This means that the combustion engine must only run when the battery drops below a certain level.
Figure 3: Series hybrid drivetrains Source: Hooftman (2018)
Parallel configuration (Figure 4): both motors drive the wheels. When the combustion engine does not have to provide motion to the wheels, it can be connected to the electric motor to produce electricity. Figure 4: Parallel hybrid drivetrains Source: Hooftman (2018)
In addition to these configurations, one also speaks of microhybrides. These have a classic combustion engine to drive the wheels, but they have a small electric motor that only serves to recover braking energy. The stored electricity is then used to drive on-board equipment. These cars are also equipped with a start-stop system. The engine is automatically shut down at a halt, and then started again with the help of the electricity from that electric motor.
Hybrid technology reduces vehicle consumption, especially in start-stop traffic. In addition, many hybrids can drive purely electric for a short time, which reduces the vehicle's emissions to zero. A good example of this is the 'plug-in hybrids' or PHEVs, hybrid cars whose batteries can also be charged via an ordinary plug. These cars can drive fully electric over shorter distances (about 50 km), such as daily commuting. However, due to the presence of a combustion engine, this configuration allows traveling longer distances without recharging the battery. Therefore, the petrol or diesel consumption of PHEVs depends very much on the willingness and possibilities to recharge the batteries.
Fuel cell Electric vehicle
Fuel cell vehicles are powered by an electric motor. The electricity for this engine is produced in the fuel cell. The fuel cell consists of two electrodes and a membrane. Hydrogen is passed along one electrode, oxygen along with the other. Hydrogen atoms can only pass through the membrane after releasing an electron to the electrode (anode). When the hydrogen atoms have passed through the membrane, they combine with oxygen by absorbing an electron from the other electrode (cathode) and forming water. This creates a flow of electrons (electricity) between the two electrodes, which can be used to drive the electric motor or stored in a battery.
The advantage of the fuel cell is that it converts the energy from the fuel into electricity instead of heat. As a result, less energy is lost, so more energy is extracted from the fuel than the classic combustion engine. The efficiency of the drivetrains is higher than for ICE cars and ranges between 22-48% (Hooftman, 2018). In theory, fuel cell vehicles themselves only produce water as an emission, although the emissions during hydrogen production must be taken into account.
Overview of the different vehicle's drivetrain and performances
The table summarised some key parameters from the main passenger cars' powertrains.
Table 1: Overview of the main fuel and vehicle powertrains characteristics. *HEV: Toyota Corolla 2020, FCEV: Toyota Mirai 2019, Others: VW Gold 2020 Sources : Hooftman (2018), IEA (2020), Toyota Europe Newsroom (2021), CTCN (2016)
Figure 5: Powertrain efficiency of hybrid vehicles as a function of the hybridation degree of the car Source: Hooftman (2018)
Hooftman, N. (2018)., The road towards a zero-carbon transportation system by 2050 - A comprehensive study for Belgium in a European context.
IEA. (2020, May). Average price and driving range of BEVs, 2010-2019.
Toyota Europe Newsroom. (2021). Toyota Mirai breaks the world record for distance driven with one fill of hydrogen.
UN Climate Technology Centre and Network. (2016). Compressed Natural Gas (CNG) as fuel.
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