Fuel Cell Technologies in Passenger Cars
Fuel Cell Technology
The last decade is characterized by the rapid development of research activity in the field of fuel cells, thus, creating the conditions for their uptake in electric systems. The basic fuel cells’ operating principle has been known for 150 years. However, a variety of technical and financial problems had kept them on the margins of technological applications (Brandon & Hart, 1999).
The basic difference between a fuel cell and batteries is that while batteries store energy, fuel cells directly convert a fuel’s chemical energy into electricity by an electrochemical reaction (i.e. Without requiring combustion or movement of mechanical parts) (Brandon & Hart, 1999):
Fuel + O2 / Catalyst Electric Energy + Heat + H2O (1)
Fuel cells can be characterized as the centers of a system that uses mostly hydrogen as fuel. They are the ones which undertake the conversion of the fuel into useful electric energy. Fuel cells are externally powered, by a fuel, typically hydrogen gas, subsequently generating energy for the duration of the fuel supply, and thus, never requires electric recharging (Brandon & Hart, 1999).
Fuel cells comprise of two electrodes, an anode and a cathode. These are separated by a membrane, which has the role of the electrolyte. Between the membrane and the electrodes lays the catalyst layer. A fuel cell system, which includes a fuel reformer, can utilize hydrogen from any carbohydrate fuel, i.e. from natural gas to methanol or even gasoline. Since fuel cell systems rely on chemistry and not combustion, their emissions are lower than that of the “cleaner” fuel combustion process systems (Brandon & Hart, 1999).
Fuel cells in vehicles. Fuel cell electric vehicles, storage take place in the fuel tank, as in conventional vehicles. While hydrogen has a higher energy density ( 42kWh/ kg) it is so light, so that a liter pressurized at 35 Mpa weighs only 31 grams and generates 1,3kWh. By contrast, gasoline has a lower energy density ( 14kWh/ kg), but a liter is equivalent to 8,3kWh (Brandon & Hart, 1999).
Admittedly, fuel cells and electric motors are more efficient than internal combustion engines. However, to provide an electric vehicle with a performance range of 500 km 6 kg of hydrogen is required. Compressed at 35 Mpa hydrogen will occupy 200 liters, while with the addition of pipes, valves and regulation and compression systems, the required storage space will be doubled (Vielstich, Lamm, Gasteiger, & Yokokawa, 2003).
Companies with significant expertise in nickel metal hydride batteries have used their knowledge to develop metal alloys that can save 7% of their weight in hydrogen at low pressure of 200 kPa. With this technology, 6 kg of hydrogen in 120 liters can be stored, i.e. at about twice the size of the reservoir used in conventional medium size vehicles (Vielstich, Lamm, Gasteiger, & Yokokawa, 2003).
However, the storage burden remains a major problem, while the production of hydrogen is another issue to be resolved, as there are no corresponding infrastructure or immediate prospects of its establishment (Brandon & Hart, 1999).
An alternative to hydrogen storage is its constant supply via the production of more manageable materials. Several automakers have turned to creating compact chemical systems for the production of hydrogen from common fuels. This alternative eliminates the infrastructure problems. However, it also creates complexity issues and pollution risk problems similar to those of conventional vehicles. Finally, the safety of hydrogen compared to gasoline during the movement of vehicles is still an open question (Vielstich, Lamm, Gasteiger, & Yokokawa, 2003).
Types of fuel cell technology. Fuel cells are generally classified by their electrolyte (the membrane separating the two electrodes). The characteristics of the electrolyte determine the optimum operating temperature and the fuel to be used to generate electricity (Brandon & Hart, 1999).
All fuel cells include a barrier that selectively allows ions to pass, eventually forcing the remaining electrons to pass through an external circuit. This electron flow is the one that generates electricity (Brandon & Hart, 1999).
In Table 1 the principal types of fuel cells used in passenger vehicles are summarized (Goodstal, 2013; Achara, 2011; Dyer, Moseley, Ogumi, Rand, & Scrosati, 2009; Apanel & Johnson, 2004), while in Table 2 their primary passenger-type transport application is presented (Bertau, Asinger, Plass, Schmidt, & Wernicke, 2014; Achara, 2011; Dambra, 2011; Dyer, Moseley, Ogumi, Rand, & Scrosati, 2009; Apanel & Johnson, 2004; International Energy Agency, 2004); Spath & Dayton, 2003):
Different Types of Fuel Cell Technologies Used in Passenger Vehicles
Passenger-type Transport Application of Different Types of Fuel Cell Technologies
Motor Control Strategies
In fuel cell vehicle applications two motor control strategies are prevalent: a) scalar, and b) vector control, whose two main categories are direct torque control (DTC) and field oriented control (FOC) (US Fuel Cell Council, 2001).
Scalar control (V/f (Volts per Hertz) control) strategies are widely applied in the industry since they are relatively simple to apply. Scalar control is based on an equivalent motor model applied in the permanent operation mode, which uses as control variables the magnitude and frequency of the supply voltage. However, the flux and torque depend both on the voltage and supply frequency. This is a major disadvantage, because due to the coupling, the control of one of the amplitudes affects the other, resulting in slow system response (International Energy Agency, 2004).
The problem of scalar control is faced by another method, more prevalent in recent years, i.e. vector control. This strategy removes the coupling between flux and torque. With vector control the stator current component (isd) corresponds to the exciting current of the deck engine, and thus, controls the magnetic flux irrespective of the torque. Also, the component isq, which corresponds to the rotor current, controls the torque. This strategy results in drive systems with an improved dynamic behavior, rapid transitions between operating points and an increased yield compared to scalar control (International Energy Agency, 2004). Additionally, the evolution of microcontrollers’ and digital signal processing systems’ technology has solved the problem of high computational power that vector control requires (because vector control is a relatively complex computational process). Consequently, the vector control strategy tends to prevail in the industry and is widely found in electrification systems (US Fuel Cell Council, 2001).
Comparison of CNG and LPG Fuels in Passenger’s Cars with respect to Fuel Characteristics, Conversion Technologies and Engine Control Technologies
CNG and LPG Fuel Characteristics
The term liquefied petroleum gas or LPG refers to all liquefied gases derived from petroleum. As a general term, it describes gaseous fuels, which consist mainly of hydrocarbons of 3 or 4 carbon atoms (C3 and C4), i.e. propane, propylene, normal butane, isobutane, isobutylene, butylene and ethane. 65% of global production of LPG is produced during the oil extraction process, while the remaining 35% is produced during petroleum fractional distillation and natural gas’s processing (US Environmental Protection Agency, 2014; Kojima & Lovei, 2001).
LPG used in passenger cars is in the form of a propane-butane mixture (20% propane and 80% butane). Propane is a hydrocarbon with 3 carbon atoms (C3H8), while butane a hydrocarbon with 4 carbon atoms (C4H10). LPG should not be confused with CNG, which is mainly methane, or with liquefied natural gas (LNG). LPG is colorless, odorless (its characteristic odor is added before consumption, so as for its presence in means of smell to be sensed in case of a leak), and 1.6 to 2.1 times heavier than air. The latter is the reason why that in case of a leak, LPG does not diffuse into the atmosphere and concentrates low to the ground, thus, being very dangerous for provoking an explosion (Demirbas, 2002).
LPG has a high octane number (105) and does not require the addition of lead or other additives. LPG’s ignition temperature in the air is 480 °C. Finally, LPG is stored and transported in liquid form under pressure via suitable steel tanks and bottles (LPG’s gaseous storage requires 250 times bigger spaces in comparison to its liquid storage) (Kojima & Lovei, 2001).
On the other hand, natural gas is a gaseous hydrocarbon mixture, which is mainly extracted from geological deposits, known as gas fields. Due to its properties, it is considered an ecological fuel. The main component of natural gas is methane (CH4: 70-90% by volume), which, however, coexists with significant quantities of ethane (C2H6: 5-15% by volume), propane and butane (C3H8 and C4H10: <5% by volume), as well as smaller quantities of carbon dioxide (CO2), nitrogen (N2), hydrogen (H2), helium (He), and hydrogen sulfide (H2S). Before using natural gas as fuel, it must undergo extensive processing to remove almost all materials other than methane (Demirbas, 2002).
As LPG, CNG is also colorless, odorless (its characteristic odor is added before consumption, so as for its presence in means of smell to be sensed in case of a leak), but is 0.6 times lighter than air. That is the reason why that in case of a leak CNG, in contrary to LPG, will diffuse into the atmosphere, thus, not being as dangerous for provoking an explosion (Gruden, 2003).
CNG’s ignition temperature in the air is 650 °C, i.e. higher than for the respective of LPG. However, CNG requires less energy for its ignition than LPG. Additionally, CNG has a higher octane number (120-130) than LPG. Finally, CNG is generally transported by pipelines or LNG carriers, while in some countries it is also carried by trucks (US Environmental Protection Agency, 2014).
CNG and LPG Conversion Technologies
CNG passenger cars are based on the same principles used in petrol passenger cars. In particular, similar to petrol, CNG is mixed with air in the cylinder included in the four-stroke engine. It is subsequently ignited via a spark plug so as for the up and down movement of the piston to be enabled (it has to be stressed that the ignition temperature of CNG is different than that of petrol) (Harris, 2016).
Nonetheless, apart from the above similarities, there are some conversions that have to take place so as for a CNG passenger car to work effectively. In particular, the conversions refer to a) the fuel storage, b) the engine, c) the chassis, and d) refueling (Harris, 2016).
The first conversion, associated with the fuel storage, refers to the need of the pumping of the compressed natural gas into high-pressure cylinders of a tube shape, which most commonly attach to the top, rear or undercarriage of the passenger vehicle (it has to be noted that natural gas is prior to pumping compressed in the fueling station to about 207-248 bars). The newest technology of cylinders is light-weight cylinders, known as Integrated Storage Systems (ISSs). These cylinders are all-composite, which are put inside a fiberglass shell and impact-absorbing foam in order to be protected from damage in case of an accident. At the same time, they are constructed with a small diameter in order for three of them to be able to be placed together in a way that resembles a common petrol tank (Harris, 2016).
As soon as the CNG engine is started, natural gas is transferred from the storage of the cylinders, and then to the regulator, which is situated near the engine, so as for its pressure to be reduced. Here is when the second conversion, associated with the engine takes place. In particular, this conversion refers to the installation of a) a multipoint gaseous fuel injection system, which is responsible for the introduction of the fuel into the cylinders, b) sensors and computers responsible for the adjusting of the fuel-air mixture in order for the gas to burn effectively after ignition, c) forged aluminum high-compression pistons, d) hardened nickel-tungsten exhaust valve seats, and e) a catalytic converter explicitly for methane (Harris, 2016).
Another potential conversion refers to the chassis and the taking away of the spare tire and jack (so as for a flat floor plan to become feasible) and the introduction of run flat tires, whose intention is to compensate for the lack of the spare tire and jack. Concurrently, there may be need of changing the lateral-link suspension of the patrol vehicle with a semi-trailing arm suspension in order to provide additional space for the fuel storage containers in the rear undercarriage (this latter conversion takes place without, however, jeopardizing the comfortable trip of the passengers) (Harris, 2016).
Finally, the refueling position of a patrol car may have to be changed so as to be positioned at the front of the passenger car. Additionally, the refueling time may be the same as petrol cars, i.e. Via the fast-fill pump method, or may last from 5 to 8 hours when the slow-fill pump method is applied (Harris, 2016).
On the other hand, the conversion of a petrol car into an LPG one is a more complex procedure, thus, requiring skilled and knowledgeable mechanics. It has to be noted that conversion usually takes place in the form of a dual-fuel conversion, which basically includes the introduction of a) the proper tank, b) a fill point, c) a solenoid valve, d) a regulator (also called vaporizer), e) a mixer, f) automatic switching between gas and petrol, and g) an emulator (Grabianowski, 2016).
More specifically, the first conversion refers to the adding of LPG (propane) tank, which, since the old fuel (petrol) tank is not removed, increases the total space occupied for fuel storage. This tank, which is usually positioned in the trunk, is either a) of a “torpedo” form, which due to its higher capacity occupies more space, or b) of a “donut” form, which is generally a smaller tank that usually occupies only the space used in conventional petrol cars for the storing of the spare tire (Grabianowski, 2016).
The insertion of the extra tank has to be followed by the drilling of a full point to the passenger car’s body, which has to be connected to the tank. Subsequently, a solenoid valve has to be introduced on the fuel line between the engine and the tank so as for the flow of LPG to be stopped in the case when the car is powered through petrol or when the engine is shut down (Grabianowski, 2016).
The next conversion refers to the regulator, whose role is to utilize the heat generated from the passenger car’s cooling fluids in order to transform propane to its gas form. The regulator is also employed to stop LPG’s flow, something achieved via the electronic circuit incorporated to the regulator. It has to be noted that the regulator’s size is typically smaller than that of the conventional carburetor, and thus, space inside the engine compartment can be easily found (Grabianowski, 2016).
The above system has to be accompanied by the appropriate switching system, which by being connected to the passenger car’s electrical system will enable for the switching between the two fuels (Grabianowski, 2016).
Finally, in case the passenger car uses an electronic injection system, an electronic emulator will most probably required so as for the proper signals informing on the dual-fuel passenger car’s appropriate operation to be applied (Grabianowski, 2016).
CNG and LPG Engine Control Technologies
The engine control technology applied in dual-fuel passenger cars are the same, whether talking about an LPG-petrol or a CNG-petrol passenger car. In particular, in all contemporary dual-fuel engines and in order for optimum conditions in either of the two fuels to be met (i.e. For an operation near a stoichiometric air to fuel ratio), utilize an oxygen sensor in the exhaust gas, which provides the control signal for a “closed-loop” or the feedback control scheme. In spite of the fuel’s composition, this control system, which is based on the checking of the exhaust oxygen content, has to be able to retain the engine’s air to fuel ratio at stoichiometries (IEA ETSAP, 2010; J.E. Sinor Consultants, Inc., 1994).
On the other hand, the engine control technology used for solely CNG or LPG passenger cars is a more complicated electronic control unit, which comprises of a) a water temperature sensor signal, b) an intake manifold pressure sensor, c) an intake air temperature sensor, d) a gas temperature sensor signal, e) a gas pressure sensor signal, f) a sensor crank shaft position sensor signal ("CNG/LPG Engine Control Systems", 2016; IEA ETSAP, 2010).
Diesel Passenger Vehicles and their Pollutants
Diesel Passenger Vehicle Pollutants
The composition of the pollutant emissions of diesel passenger vehicles varies and depends on several factors, such as the fuel quality, the engine type, the driving behavior, whether or not anti-pollution devices have been installed and so on. In any case, however, the exhaust gases of diesel vehicles contain particles and a mixture of diverse compounds, which are in a gaseous phase (Faiz, Weaver, & Walsh, 1997).
In respect to the gaseous pollutants, except carbon dioxide (CO2), carbon monoxide (CO), nitrogen (NOx) and sulfur (SOx) compounds and hydrocarbons (HC) of a low molecular weight are included. From a toxicological point of view, of particular interest are the emitted aldehydes (as, for example, formaldehyde, acetaldehyde, and acrolein), benzene, 1, 3-butadiene, polycyclic aromatic hydrocarbons (PAHs) and Nitro-polycyclic aromatic hydrocarbons (Faiz, Weaver, & Walsh, 1997).
The particulate phase of emissions includes elemental carbon, particulate matter (PM), adsorbed organic compounds, small amounts of sulfates and nitrates and other minerals, and trace elements. From a toxicological point of view, of particular interest are PAHs, the Nitro-polycyclic aromatic hydrocarbons and the oxidized derivatives of PAHs. Usually, PAHs and their derivatives constitute less than 1% of the total mass of diesel-derived particles. However, their importance is crucial, as many of these compounds are carcinogenic and mutagenic (Faiz, Weaver, & Walsh, 1997).
Diesel Passenger Vehicle Pollutants’ Impacts
Impacts on health. The usual daily inhalation of diesel vehicle exhaust can worsen health. The effects can be acute or long-term, while among these effects ranks lung cancer (Krzyzanowski, Kuna-Dibbert, & Schneider, 2005).
Even the short-lived exposure to oil-derived exhausts can cause effects of a respiratory, neurological and immunological nature. Even exposure of one hour can cause irritation to the eyes, throat and bronchi. Symptoms also include headaches and nausea, cough, phlegm release and increased response to known allergens. Epidemiological studies of workers in diesel-bus parking places, as well as the Quarrymen exposed to diesel exhausts have shown decreased lung function, increased coughing, wheezing, and chest tightness. On the other hand, long-term exposure to diesel exhausts is associated with increased incidence of bronchitis, cough, phlegm release and reduced lung function (Krzyzanowski, Kuna-Dibbert, & Schneider, 2005).
Finally, numerous studies have demonstrated the carcinogenic effects of diesel exhausts. This effect has been recognized by several official bodies (National Institute for Occupational Safety and Health (NIOSH), International Agency for Research on Cancer (IARC), World Health Organization (WHO), and US Environmental Protection Agency (EPA) among others) since 1988. The carcinogenic effects of diesel exhausts are mainly manifested as lung cancer, which may occur in pollutant levels consistent with those found in urban centers (Krzyzanowski, Kuna-Dibbert, & Schneider, 2005).
Impact on climate change. Nowadays, the prevailing opinion presents diesel engines as a “weapon” to combat the greenhouse effect and to avoid dangerous climate change. This is because diesel vehicles emit proportionately less carbon dioxide (CO2) from the corresponding petrol cars. In particular, a diesel passenger car emits on average 30-35% less CO2 than an equivalent petrol passenger vehicle. Unfortunately, climate change is a result not only of CO2, but also the product and other greenhouse gases, many of which as explained above are emitted by diesel vehicles (OECD, 2007).
Diesel Passenger Vehicle Pollutants’ Control Technologies
Exhaust gas recirculation system. During the recycling of exhausts, part of them is returned to the inlet and fed to the combustion space. Because the exhaust gases contain very little oxygen, the maximum combustion temperature is reduced, and therefore, the maximum combustion pressure is also minimized. This results in the reduction of the emission of nitrogen oxides. The exhaust recirculation takes place via the Exhaust Gas Recirculation (EGR) or otherwise EGR valve. The amount of recycled exhausts depends on the engine speed, the injection quantity, the intake air mass, the intake air temperature and the air pressure.
EGR systems are currently divided into two categories, systems with cooling of the EGR and exhaust control recirculation and systems to control both in the cooling of the EGR and the exhaust recirculation (Roychowdhury, Chak, & Banerjee, 2006).
Selective catalytic reduction. Selective Catalytic Reduction (SCR) is an even more effective technology to reduce nitrogen oxide emissions from diesel engines. In this method a special catalytic converter, which processes the diesel engine exhausts and reduces nitrogen oxide emissions, is used. In SCR, before the entry of the diesel exhausts to the SCR catalyst, NH3 or urea is sprayed so that NH3 react with NO and NO2. Consequently, harmless N2 and H2O are produced (Roychowdhury, Chak, & Banerjee, 2006).
Nitrogen oxide storage catalysts. Nitrogen oxide storage catalysts (NSCs), unlike catalysts which convert during their entire period of operation, nitrogen oxides into N2, store nitrogen oxides under lean mixture conditions and convert them to N2 under rich mixture conditions. This way, NSCs can achieve up to 90% reduction in nitrogen oxides. Their major drawback is that they are extremely sensitive to sulfur and require the use of diesel fuel with a low content of sulfur (Heck, Farrauto, & Gulati, 2012).
NSCs can be combined with smoke traps, leading to a system known as the DPNR (Diesel particulate and NOx Reduction), which simultaneously reduces solid particles (PMs) and nitrogen oxides in the exhausts of diesel engines (Heck, Farrauto, & Gulati, 2012).
Diesel particulate filters and catalyzed particulate filters. The widespread types of particulate filters are the Diesel Particulate Filter (DPF) when referring to an isolated, independent future, and the Catalyzed Particulate Filter (CPF) when it comes to a filter along with oxidization catalyst incorporated into a single device (Heck, Farrauto, & Gulati, 2012).
In particular, in a DPF, the oxidization catalyst and the particulate filter are separated, while in the CPF the two components are embedded in a shell. The DPF and CPF filter out smoke particles inside the exhausts using a porous filter. The filter walls may be composed of porous materials, which are usually made up of fibers or powder. The fibers or powder is composed of ceramic. The classic ceramic types are cordierite and silicon carbide (Heck, Farrauto, & Gulati, 2012).
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