Types Of Fuel Cells And Their Efficiency

Non-selective catalytic reduction is mainly used as a three-way catalyst in the automotive industry. This system does not require additional reagents to be injected because the unburnt hydrocarbons are used as a reductant. Though gases must not possess more than 0.5% oxygen. This is why NSCR can only be used with rich combustion mixes. The catalysts used are normally based on platinum. It can only be used for choked mix engines and not for self-combustors like diesel engines.

Multi-point Sequential Gas Injection System

-For vehicles built after 1999 and earlier which had a plastic inlet manifold, this type of kit provides more precise fuelling than an induction.

- Gas Injection System is structured for engines with carburettors or throttle body or mechanical injectors. The system injects LPG as a vapour for each cylinder, very close to the inlet valves. This eliminates any power loss caused by mixer venturi constriction and also eliminates the possibility of spit back. The gas ECU does not require any petrol injector pulses, but calculates gas injector timings from manifold absolute pressure (MAP), RPM and a tuning map.

- This type of system is designed for petrol-injected engines with catalytic. Converter/lambda probe, (given that they have a metal inlet manifold and were manufactured before 1999).

- Direct injection petrol engines are not suitable candidates for single-point induction systems.

Effects on Humans: - Restricts breathing, sore throats.

Effects on Environment: - Smog and also contribute to the formation of acid rain, ozone.

Reduces the flow of oxygen in the blood stream, particularly dangerous to person with heart disease, leading to sudden death.

Effects on Environment: - Harms plant life if high levels of CO exposure for an extended period of time, Contributes to the formation of smog.

Emissions Results From Port Injection Bi- Fuel (Gasoline And Compressed Natural Gas) Demonstrator Vehicles.

Conversion of a diesel engine is different from that of a petrol engine. Diesel always burned - you just have the option of adding LPG or not. Adding LPG helps the diesel to burn more completely. This tends to clean up emissions and increases the power output - allowing the use of a lighter throttle and leading to fuel economy.

Fuel Cell and Motor Control System

This cell is most widely used fuel cell for commercial purposes with an operating efficiency ? approximately 37% to 42%.  The three main elements and operating condition of this fuel cell are as highlighted below:

• Electrolyte - Phosphoric Acid (H₃PO₄)
• Electrodes  - Carbon Paper with a layer of catalyst
• Catalyst – Pt
• Operating temperature – 150 - 205⁰C

It is apparent that Pt catalyst and relatively high temperature is used in this process and therefore, it is considered to be a costly process. In this process, generated electrons would move in the external circuit and the respective protons would run to the cathode. The reaction takes place at the cathode where the free protons (H⁺) and free electrons (oxygen) would react and produce water. Due to high temperature condition, the water will be converted to steam that would be used to generate power in the power station. At this point of time the operating efficient of fuel cell become two times. It can be concluded that the use of fuel cell is appropriate for the automotive industry where output is constant and high power can be generated through the steam.  

This mainly works on the operational principle of proton exchange member fuel cell. It is considered to be highly efficient fuel cell with a potential efficiency of 70%. Initially, this cell was utilized by NASA for their aircraft. Further, the generated water from this fuel cell is used by the astronauts for drinking purposes. Elements used in this fuel cell are as highlighted below (Albert, 2015):

• Pure hydrogen gas
• Pure oxygen gas
• Electrolyte – KOH (water based)

Electrode is double coated by an electro-catalyst layer and another hydrophobic layer. This fuel cell is not suitable for automotive industry because the operating cost and raw material cost is significantly high.

This fuel cell is considered to be efficient and most suitable fuel cell for the automotive industry. It is because the output generated from this cell can be adjusted according to the requirement. Therefore, adjustment in the output can control the driving speed of the respective vehicle. The size of the fuel cell is not much large even though of its high output ranges.

The major issue with this process is that the membrane must be in moist condition at all times. However, at the lower temperature conditions the water gets freeze and it is quite difficult to initiate the process again. CO (from air sources) of this process generally reduces the strength of Pt catalyst which then results in falling output of the fuel cell. Researchers are constantly searching for the technique to overcome this issue.

Solid Oxide Fuel Cell (SOFC)

In the initial stage of mass production in automotive industry solid oxide fuel cell is most widely used. However, they could not be used for the power generation for a traction motor. Further, they are generally used for belt driven alternator especially for the internal combustion motor engine. Therefore, it is said to use SOFC for the alternator because the current alternators are inefficient and mainly depend on rotational speed of the motor. They consume high fuel and hence are not efficient to operate. Therefore, SOFC is used to replace the alternator which then generates high power output that can be utilized to drive more drives. The other imperative aspect related to SOFC is that engine would also run for some time even if the battery is exhausted or the engine is not operating. This would result in high efficiency of engine and the generated heat can be utilized to heat up the passenger components.

• Anode & Cathode -  Ceramic
• Electrolyte  phase  - Solid
• Operating temperature  -  700 to 1000°C
• Single stage operation (built-in reformer)
• Efficiency  - 40-45%

Alkaline Fuel Cell

It is noteworthy that in this fuel cell, catalyst is not used and therefore, the cost of this cell is not high. Hence, it can be preferable fuel cell to replace the alternator. It is apparent that significant high temperature is used which then reduces the probability of poisoning of the electrodes from CO. High heat is generated from this fuel cell however, when this is used in place of alternator then the generated heat amount is comparatively very less and can easily be transmitted to the atmospheres from vehicle.

It is also a type of PEM fuel cell or modified PEM fuel cell in which liquid methanol is taken into consideration in place of hydrogen gas. Redox reaction takes place in fuel cell where fuel is oxidized at anode and then the oxygen from outside sources would reduce at cathode. Further, the liquid methanol is the source of hydrogen gas for the fuel cell. Aqueous liquid methanol is fed to fuel cell where the electrodes are coated with a layer of Pt catalyst. Further, this cell produces CO₂ & H₂O at the out stream which is difficult to handle. The methanol and water is fed which then produces hydrogen gas at the negatively charged electrode. After generation of hydrogen, carbon is the residual which then reacts with oxygen and forms CO₂. The electrons would move into external circuit and then combine with the protons. After the combination of electrons and protons, water would form that is removed at the outlet of the fuel cell. It is noteworthy that in traditional PEM fuel cell, reformer is used. However, in this process there is no need to use reformer and hence, the cost of this cell is lower than PEM fuel cell. Also, it is easy to store and handle liquid methanol as compared with the hydrogen gas. The construction of fuel cell is easy and significant amount of energy can be produced for long run. However, in terms of overall performance, the efficiency of PEM fuel cell is higher than direct methanol fuel cell (Albert, 2015).

Molten fuel cell is not generally used in automotive industry, while they are used in the factories where high power is required.

• Molten carbonate salt is the electrolyte used in fuel cell.
• The operating temperature is 600 to 650⁰
• Anode – Alloy (sintered nickel powder combined with the Cr)
• Cathode – Alloy ( porous nickel oxide combined with Li)

The drawback of using fuel cell is that if the operating temperature falls, then the overall output of the fuel cell is reduced by 15%. CO₂ is the by-product of this process which needs to recirculate in the fuel cell and the generated water would be collected at the cell outlay. It is essential to note that the maximum amount of fed hydrogen is combined with the recirculating CO₂. However, it is not the case and there is a possibility that unused hydrogen remains at the one end of the membrane. Researchers are involved in the design of the membrane in order to ensure that unused hydrogen would return back to the membrane to react with the CO_2.

There are two main motor control system are used in the automotive industry.

1. Current controlled fuel cell system
2. Throttle controlled fuel cell system

From the above figure it can be seen that the driver is taking command with the help of acceleration pedal position which is then injected into the respective “electric drive train.” As a result, the acceleration would cause generation of current. The generated current from the respective fuel cell and the two feedback loops would be the parameter to find the total acceptation of vehicle. The current would create significant disturbance to the stack voltage and to form polarization curve. Further, this current would works as a “control variable” for fuel processor system. This would ensure that the system would receive sufficient fuel and would complete the fuel utilization efficiently.                 

Proton Exchange Membrane (PEM) Fuel Cell

In throttle controlled fuel cell system, the process is exactly the same but the control variable is the acceleration pedal position for the fuel processor. Hence, the work of generated current is to create significant disturbance for the cell stack voltage and to adjust the hydrogen amount mainly based on the control variable. In this case, throttling control system is used on which the position of acceleration pedal is known as the throttling position of the system which would drive the fuel train and provide the requisite amount of fuel to the system. It has been found that throttling system is inefficient as compared with the current system because throttling system reduces the fuel economy. Further, current system is having more fuel economy and can easily satisfy the need of “second-by- second” motor current requirement. As a result, necessary torque requirement would also be satisfied. Further, in throttling system, high hydrogen flow is pre-requisite in the acceleration period irrespective of the fact that at a time a significant portion of hydrogen is utilized by the fuel cell.

It can be assumed that a combined mode of current and throttling control system is the best possible way to resolve the unexpected issues in the fuel cell and to maintain the fuel economy. This system would be considered to be best suitable method as compared with the other techniques.

• There is no need to install additional units of hardware. It is because this system can be used for large sized reformer and stack system or a system called hybrid FCV design.
• No need to worry about maintaining fuel economy.
• No need to installed large sized hardware or units. It is because this system can improve the overall reformer time.  

According to eq. (4), it is apparent that transmission efficiency would decrease the motor torque which has been supplied to the wheels of the vehicle. Further, the transmission ration would be determined with the help of total reduction (motor speed and wheel speed) along with the decrease in the differential. Moreover, the transmission inertia would be determined with the help of dividing   and addition the inertia for both the cases i.e. inertia of motor side to its side and then inertia of wheel side to its side. The requisite drive train current would be computed with the help of eq. (5). For this computation efficiency map needs to be prepared by converting torque, voltage, and speed into the train current. It is essential to prepare the map at high supply voltage because the motor efficiency is independent with respect to the supply voltage.

Finally, to determine the association between the two variable, motor speed and real vehicle speed eq. (6) needs to be taken into consideration. It is apparent that this equation is having a linear differential term i.e.  by which means the units of the shaft speed would be radians per seconds. The association would depend on the “reform time constant, fuel consumption/ utilization rate, size of the cell and type of system used.”

Two examples of fuels that are clean are LPG and CNG. Natural gas has a supply which has more abundant supply than gasoline and hence tends to be cheaper. The light and medium vehicles use CNG while the heavier commercial vehicles tend to use LNG. A potential issue with the CNG is that safe   storage needs to be ensured. Being lighter than air, CNG tends to disperse in a quick manner if leaked.   The air to gas ratio required for CNG (10:1) is lower in comparison with LPG (25:1).  The flammability of CNG (2-15%) is higher than LPG (2-9.5%).  For internal combustion engines, after petrol and  diesel, the most common fuel  is LPG.

Solid Oxide Fuel Cell (SOFC)

There are two main CNG engine control systems that are used in the automotive industry (Allen, Durell & Heath, 2000).

In Venturi type system, lower cost kit is used which is considered to be older generation system. The basic principle behind the operation of the system is “Venturi effect” in which the fluid /gas would flow across the constricted pipe and in order to create force, a narrow section would be placed on the pipe so that sufficient force and pressure can be created. As a result velocity of the fluid /gas would be increased. As the name suggests, the mixer system has a mixer kit that would be installed easily into the system. However, dilemma is faced regarding the various factors related to the system performance because acceleration would be slugged and mixer can be choked which reduce the overall performance of the engine system. Therefore, it is essential to keep a constant tinkering of the mixer kit.

It is known as CNG conversion kit or sequential injection kit. In this system computerized system is used which would ensure that the necessary amount of CNG would be injected into the engine. Further, only one injector would be used for one engine cylinder to inject requisite CNG. For example “V8 engine would have eight injectors.”  Further, it is noteworthy that this electronic fuel injection system would enhance the performance of the system and would work whether CNG is used or gasoline is used by the engine. Further, it is quite expensive and hard to install. However, this sequential injection engine kit is useful and can be used for long term of operation (Marini, 2012).  

• Multi point (sequential gas injection system)

This system is used for the vehicles which have been made after 1999 and comprises plastic inlet manifold. It has been observed that this system is more efficient and issues sufficient fuelling to the system as compared with the induction.

• Gas injection control system

This type of system is generally useful for the vehicles for which the engines are having either throttling body/ mechanical injectors or carburettors body. They are using LPG vapours which are injected to the cylinders that are located near to the inlet valves. The location and vapour phase of LPG results in reduction of power loss (that can be incurred due to mixer constriction) and spit back. It is noteworthy that using gas ECU does not need separate petrol injector pulse. However, it can compute the exact gas injector timing with the help of MAP, RPM and tuning map.

• Single point (closed loop system – with induction and petrol injection system)

This type of system is generally used when the vehicle is having petrol injected engine along with the catalytic converter. For example: Converter or lambda probe which comprises mental inlet manifold and were made before 1999. Further, when the vehicle is having direct injection petrol engines, then this type of system cannot be used.

• Single point (open loop system – with induction and petrol injection system)

When the vehicle’s engine does not comprise catalytic converter and is having only petrol injected engine, then this system is more suitable.

• Single point (open loop system – with induction and single carburettor system)

This is used when the vehicle does not comprise any fuel management or lambda probe system but has single carburettor system.

• Single point (open loop system – with induction and double carburettor system)

Direct Methanol Fuel Cell

This is used when the vehicle does not comprise any lambda probe system but has double carburettor system.

• Diesel Kit

It can be seen that conversion in diesel engine is entirely different from petrol engine. If LPG is used in the engine, then there is high possibility that diesel would completely burn. As a result the overall output of the engine is increased and therefore, it would be beneficial to use lighter throttling to increase the fuel economy.

The below highlighted figure shows the various parts of the engine that would be used for CNG conversion

Description of parts

1. Fuel tank
2. Regulator  
3. Fuel rail
4. Adaptor
5. Wiring Harness
6. Fuelling computer
7. CNG injector
8. Wiring Harness

The regular connected to the fuel tank reduces the pressure of tank from 3600 psi to 125 psi and then the fuel is injected to the fuel rain (parallel) and finally into the secondary injector that is directly connected with the adapter. Wiring harness is connected with the ECU which would collect the throttling information and directly send to the computer (fuelling). The computer would analyse the data and then finally instruct the CNG injector with the help of wiring harness (Allen, Durell & Heath, 2000).

The below highlighted figure shows the various parts of the engine that would be used for LPG conversion

LPG conversion kit is manufactured with the help of Al- alloy that enhances the useful life of the kit. The various parts of the kit are shown above. The imperative aspect of this kit is that there is change over switch present on the dash board of the vehicle which can allow changing the mode of the car i.e. either diesel/petrol or gas operating mode. The solenoids are mainly magnetic vales which are manufactured with the copper alloy (brass).

It causes etching in throats and eyes and difficulties in breathing in human beings. From environmental point of view, it contributes to the smog formation and causes acid rains. The emission of NO_X also results in ozone layer depletion (Michigan, 2009).

It causes breathing problems and morbidity in the respiration system and can create severe damages in the capillaries lungs. It adheres on the surface and can damage structures, sculpture, painting and cause corrosion of the metals. It also contributes to the acid rain deposition and high absorbance of solar radiation which decrease the level of visibility.  Further particular maters can be divided into three parts (Peer, 2008):

• Total suspended particulate  - size range  0.1 – 50 microns
• Particular matters 10 – size less than 10 microns
• Particular matters 2.5 – size equal to or less than 2.5 microns  (values as per standard set by EPA)

Reduction of oxygen content in blood cell, high exposure can cause accidental death or severe lung and heart problems. It contributes in smog formation and in acid rain. Further, it has harmful effects on plants.

The DPF principle of operation is different from catalytic converter as it involves particle capture in discontinuous periods segregated by periods of burning of particle. The capture phase involves that there is detainment of solid particulate fractions with high efficiency.  Also, the organics and sulfates exhibit detainment to some degree while the gaseous phase passes on. The main objective of DPF is holding of particles for enough time which causes incineration. The incineration phase is thought to be oxidative, but this is true only for carbonaceous fraction.  Vaporization is also observed for sulfates fraction but some escaping does happen in the form of sulfuric acid. The fraction of ash remains as it is considering the nonvolatile nature along with lack of combustibility (Peer, 2008).

There is a wide effect of any exhaust gas that escapes to the intake of the engine and blends with inducted air. Despite the effect, the central objective is to lower the NO_x emissions. The EGR enables a richer fuel coupled with lower peak combustion temperature.

NSCR is deployed in the form of three-way catalyst. Further, there is no requirement of extra reagent injection as the reductant is unburnt hydrocarbons. NSCR requires combination mixes that are rich and thus have higher oxygen levels than 0.5%. The platinum based catalysts are deployed in case of engines using choked mix but not for diesel engines (Tinley Tech, 2016)  

The basis of this technique is on a uric acid and urea based solution which would be sprayed on the exhaust gas related to the catalyst. The requisite solution has a 5% concentration and on coming in contact with the catalyst, there is breaking of the uric acid along with area which leads to catalyst surface reconditioning. This allows the returning back of catalyst leading to pollution control of NOx. Reduction of gas phase NOx can occur in the pre-catalytic stage even though it is essentially secondary in nature.  This has a comparative advantage over NSCR in terms of reduction of higher NOx. The relevant schematic assembly for SCR exhaust is indicated below.

The installation of catalytic converter was done so as to keep HC and CO emissions in check. The particulate emissions in the form of group would be dependent on particulate fraction as the interaction for different types would essentially differ.  These interactions can be of two types namely the storage effects along with side reactions.  The storage mode may be based on gas phase condensation or adsorption with regards to catalyst surface that would be at cold temperatures. However, if the temperature is very cold, particle conversion from gas happens in the exhaust system.  Also, if the cold conditions increase, white smoke may be formed and a collision of fuel droplets may  happen.

Reference

Albert, M. (2015). Engine Fuels and Emissions APTE6510, Unitec Course Materials and Notes.New Zealand. Published by Unitec.

Allen, J., Durell, E. & Heath, J. (2000). Emissions Results From Port Injection Bi- Fuel (Gasoline And Compressed Natural Gas) Demonstrator Vehicles. Proceedings of the Institute of Mechanical Engineers, December, London, Paper Number C588/005/2000.

Marini Autogas (2012). Retrieved from https://www.mariniautogas.altervista.org/Reducer_System_1.html  

Michigan, D.  (2009) Diesel Exhaust Emission Control..U.S.A: Sae International World Congress Publishers.

Peter, E. (2008). Particulate Emissions from Vehicles. Warrendale, U.S.A: Sae International Publishers.

Tinley Tech (2016). Retrieved from https://tinleytech.co.uk/complete-lpg-guide/converting-to-lpg/which-conversion-kit-is-right-for-my-engine/