The following article will guide you about how to design vehicles for controlling emissions.
Introduction:
Vehicle technology improvements can dramatically reduce pollutant emissions and improve fuel efficiency. Changes in engine technology can achieve very large reductions in pollutant emissions. Such changes are most effective and cost effective when incorporated in new vehicles. The most common approach to incorporating such changes has been through the establishment of vehicle emission standards.
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The establishment of emission standards has been a major impetus for research and development of engine designs and pollution control technologies. Major advances in this area have come from engine modifications, exhaust after treatment devices (catalytic converters and trap oxidizers), on-board canisters, on-board diagnostics etc.
Major reduction in vehicle pollutant emissions are possible through changes in technology at relatively low cost, and, in many cases with a net savings in life cycle cost as a result of better fuel efficiency and reduced maintenance requirements.
During the first 100 years of development of internal combustion engine, main emphasis was on improvement of power, torque characteristics, reliability and durability. After 1973 oil crisis, emphasis shifted to fuel economy. From 1990, the stringent emission regulations are playing dominant role in shaping the internal combustion engine’s design and technology.
Fundamental changes are happening in internal combustion engine design and technology. The biggest environmental challenge facing the automobile industry is the reduction and if possible total elimination of the pollutant emissions, followed by the need to minimise the fuel consumption and thereby reduce carbon dioxide (CO2) emissions.
Three waves of technology developments and innovations are sweeping through the automotive industry in the world. The first wave of innovation is aimed at increasing the energy efficiency of conventional engines. Better fuel efficiency not only saves fuel, it also reduces emissions, especially CO2 emissions which cause greenhouse effect.
Improvements in the design of combustion chamber, ignition system, electronic fuel injection, increasing the number of valves per cylinder, etc. can substantially improve fuel economy and reduce pollutant emissions.
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The second wave is aimed at reducing pollutant emissions from conventional gasoline and diesel vehicles. They include improvements in combustion processes, in addition to exhaust gas after treatment (viz. catalytic converters, particulate traps, etc.) and use of cleaner burning fuels. These changes are coming in the vehicles manufactured in India in response to the country’s adoption of stringent emission standards.
A third wave of innovations is more radical. It involves the transition away from internal combustion engines to zero-pollution and ultra-low pollution vehicles viz. electric propulsion vehicles and electric hybrid vehicles. These innovations have the potential for the greatest reduction in pollutant emissions and greenhouse gases.
The use of electric propulsion systems-using batteries, fuel cells and electric hybrid vehicles-would improve energy efficiencies by 50 percent or more with much less pollution. These technologies are under commercial trials in many countries including India and their costs are dropping quickly and could become cost-competitive with conventional technologies in a short period.
Compressed Natural Gas (CNG) buses are under commercial operation in Delhi and Mumbai. Battery operated electric vehicles made in India are gradually penetrating into the market. Electric hybrid buses are likely to come on Indian roads shortly. But pricing is central to both, developing and deploying technology. The Government can help and encourage innovation in vehicle technology through subsidies and reforming taxation at Central and State levels.
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In Indian cities, two wheelers are responsible for about 75%, three wheelers for about 5%, cars for about 10% and buses and trucks for about 10% of the total pollution loads. Health effects- wise, fine particulates are the most serious and studies indicate that two and three wheelers and diesel Commercial Vehicles in India contribute to more than 70% of the ambient fine particulates. While two and three wheelers and commercial vehicles are responsible for bulk of the pollutant emissions, their technology in India for reducing emissions needs to be improved.
Most of the car manufacturers in India have foreign collaborations for new technologies. This is however not true for heavy commercial vehicles and two and three wheelers. In contrast to passenger cars, the technology developments for two and three wheelers have to be done with indigenous efforts since ready-made technologies for these vehicles are not available from abroad.
Two and three wheelers are the mode of transport for majority of the population in India in contrast to the Western countries where two wheelers are the mode of entertainment. Hence technology development for these vehicles are of little interest to the western world. The sheer large numbers of two wheelers in India demands that the technology be upgraded to reduce the air pollution in Indian cities.
Technology wise, the commercial vehicle segment in India significantly lags behind U.S, Europe and Japan. Electronic control is absent and the very concept of pollution control devices (catalytic converters, particulate traps, de-Nox catalysts etc.,) has not yet entered the Indian market. The buses and trucks currently used in India have several disadvantages. They are not fuel efficient and confirm to only Euro-1 norms and are not passenger and driver friendly.
Status of Vehicle Technology in India:
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In the last two decades, the face of automobile industry in India has changed significantly. From being a protected and relatively small market, it has become a major international market with almost all the international giants opening production facilities in the country. The Indian automobile sector is dominated by two and three wheelers, followed by passenger cars and the commercial vehicles.
The restrictive policies of the Indian government did not allow foreign players to set up shop in India before mid-1980s. The opening of the economy in 1980s led to significant investments and new manufacturers in passenger car segment. The delicensing of the auto industry in 1993 opened the flood gates letting international auto makers into the country. Most of the passenger car manufacturers in India have foreign collaborations for new technologies.
Based on the Supreme Court of India’s decision, gasoline cars complying to Euro-2 norms are to be used in National Capital Region of Delhi from 1 April 2000. Now all the passenger cars manufactured in India are Euro-2 compliant. Today when the European cars are meeting Euro 4 norms, the Indian cars are meeting only Euro-2 norms. The reason given by the car manufacturers is that the required fuel is not available for stringent Euro-3 and Euro-4 norms.
Most of the car manufacturers in India have technologies to manufacture Euro-3 and Euro-4 norm compliant vehicles. Mashelkar Committee has recommended a road map according to which Euro3 compliant cars are to be operated in major Indian cities from 1 January 2005 and Euro-4 compliant cars from 1-January 2010.
India is one of the largest producers of two and three wheelers in the World. They constitute more than 80% of the sales of vehicles in India. The population of two stroke engine powered two and three wheelers in India is very large, although the number of four stroke vehicles is growing rapidly.
Two stroke engines are widely used in India basically due to their lower production and maintenance cost. Their inherent disadvantages are high specific fuel consumption and high hydrocarbon and particulate emissions. Hydro carbon emissions from two-stroke engines are high because a significant part of the air-fuel mixture escapes unburned into the exhaust.
Particulate emissions from two-stroke vehicles are also excessive because engine oil is mixed with the fuel, which recondenses into oil particles in the exhaust. Hydro carbon emissions from a single two-stroke two wheeler can exceed those from three uncontrolled passenger cars and particulate matter emissions can exceed those from a heavy duty diesel bus or a truck.
The emissions from two-stroke two wheelers can be effectively controlled by substituting a four-stroke engine or an advanced two-stroke design that uses fuel injection at a cost of about U.S $60 to U.S $80 per vehicle. This change also reduces fuel consumption by 30 to 40 percent. Further control of emissions from two and three wheelers can be achieved with catalytic converters. According to Mashelkar Committee, Bharat stage 3 two and three wheelers are to be on roads from 1 January 2008.
The commercial vehicle industry Comprises of two parts- medium and heavy duty vehicles (buses and trucks) and light commercial vehicles (LCVS). Till 1970s, the market for commercial vehicles was small and did not show any buoyancy. In the commercial vehicle segment there are four manufacturers-two have collaboration only for engines and the other two for complete vehicles.
The commercial vehicle segment in India is a diesel driven sector, with the recent addition of some compressed natural gas (CNG) buses in the National Capital Region (NCR) of Delhi based on the orders of the Supreme Court of India. Chassis of most of the Indian buses are same or similar to those of trucks.
Passenger bus/truck bodies remain technologically primitive. They are built by ill-organised road side small body builders without adequate facilities. Standards and specifications for bus bodies do not exist. There is a good case for weight reduction, both for chassis and bodies. The bus/truck body should be built by the vehicle manufacturers themselves or the design and technology should be licensed to subsidiary body builders.
The most significant emissions from diesel-fueled vehicles are the particulates and the oxides of nitrogen. Diesel smoke is a visible public nuisance in many Indian cities. Particulate matter and hydro carbon emissions can be substantially reduced through the use of low-sulphur diesel and fitment of an oxidation catalyst and a particulate trap.
But due to non-availability of low -sulphur diesel and particulate traps, the particulate emissions from diesel vehicles in India are very high. Most of the diesel commercial vehicles in India are without turbo chargers, exhaust gas recirculation and particulate traps and are manufactured only to Euro-1 standards.
Design and Technology Improvements in Four-Stroke Gasoline Engines Used on Passenger Cars:
The principal pollutant emissions from four-stroke gasoline engines include unburnt hydro carbons, carbon monoxide and oxides of nitrogen in the exhaust and HC vapours in the evaporative emissions. The important parameters affecting emissions in 4-stroke gasoline fueled vehicles are, air-fuel ratio, ignition timing, turbulence in the combustion chamber and temperature and composition of the charge mix. Of these, the most important factor is the air-fuel ratio.
The ratio of air to fuel in the combustible mixture is a key design parameter for gasoline engines. An air -fuel mixture that has exactly enough air to burn the fuel, with neither air or fuel left over after combustion is popularly known as “stoichiometric” and has an air-fuel ratio of 1.0. Mixtures with more air than fuel are lean and mixtures with more fuel are rich. Engines using lean mixtures are more fuel efficient than those using stoichiometric mixtures. HC, CO and NOx emissions are least during stoichiometric operation.
Modern three-way catalysts used in gasoline engines to control CO, HC and NOx emissions require the air-fuel ratio to be as close to stoichiometry as possible. Though HC and CO are oxidised better during “lean” operation, NOx is reduced only during operation slightly rich of stoichiometry. Hence there exists a very small air- fuel ratio window of operation around stoichiometry where catalyst conversion efficiency is maximum for all the three pollutants (viz., CO, HC and NOx).
To maintain the precise air-fuel ratio required, gasoline cars use exhaust sensors (also known as oxygen sensors) with electronic control systems for feedback control of the air- fuel ratio. The “oxygen sensor” continuously switches between “rich” and “lean” readings. By controlling the fuel system, oxygen sensor maintains stoichiometry.
Because of their superior fuel efficiency and low carbon monoxide emissions, lean burn engines are other-wise an attractive technology (compared to stoichiometric operation), if NOx emissions can be controlled by other methods. Researchers have developed zeolite catalytic materials that reduce nitrogen oxide emissions, using unburned hydro carbons in the exhaust as the reductant.
Although lean nitrogen oxide catalyst (using zeolite) is typically about 50 percent effective-considerably less than a three-way catalyst under stoichiometric conditions-the benefit is still significant. A few Japanese models are using the lean nitrogen oxide catalysts.
Combustion Chamber Design-Turbulence in the Combustion Chamber:
Unburned fuel can be trapped momentarily in crevice volumes (i.e. the space between the cylinder wall and top piston ring) before being subsequently released. Since trapped and re-released fuel can increase engine-out HC emissions, reduction of crevice volumes is beneficial to emission performance. One way to reduce crevice volume is to design pistons with reduced “top land height” (the distance between the top of the piston and the first piston ring).
The most promising technique for reducing combustion temperatures, and thus NOx emissions is to increase the rate of combustion using “fast burn” combustion chamber designs.
The most common approaches for “fast burn” combustion chamber design are:
1. Induce turbulence into the combustion chamber which increases the surface area of the flame front and thereby increase the rate of combustion, and
2. To locate the spark plug in the center of the combustion chamber.
Many engine designs induce turbulence into the combustion chamber (on the top of the piston) by increasing the velocity of the incoming air-fuel mixture and having it enter the chamber in a swirling motion (popularly known as “swirl”). Locating the spark plug in the center of the combustion chamber promotes more thorough combustion and allows the ignition timing to be retarded which reduces NOx formation.
Conventional engines have two valves per cylinder, one for the intake of the air or air-fuel mixture and the other for exhaust of the combustion products. The duration and lift of valve openings is constant regardless of the engine speed. As engine speed increases, the aerodynamic resistance to pumping air in and out of the cylinder for intake and exhaust increases.
By doubling the number of intake and exhaust valves, pumping losses are reduced, improving the volumetric efficiency and useful power output. The two streams of incoming air/air-fuel can be used to achieve greater mixing of air and fuel, increasing combustion efficiency and lowering of emissions especially HC and PM emissions.
The multiple valve design (typically 4 valves per cylinder instead of two) provides many advantages in the gas exchange process and the combustion process of the engine compared to the conventional 2 valve design. The four valve design provides the possibility to fit spark plug/injector closer to the center of the combustion chamber, which decrease the distance of the flame travel inside the combustion chamber.
The experience has shown that engines with 4 valves not only produce higher outputs and better torque, but also improves fuel efficiency by about 10% compared to a 2-valve design. The performance is better especially in part load conditions.
Exhaust Gas Recirculation (EGR):
One of the most effective means of reducing NOx emissions in both gasoline and diesel vehicles is exhaust gas recirculation (EGR). In EGR, a portion of the exhaust is recirculated back to the intake manifold and is drawn into the combustion chamber.
The resulting mixture of fresh air and exhaust products has a lower concentration of oxygen than fresh air alone. The lower concentration of oxygen in the combustion chamber results in lower oxygen pressure, which lowers its propensity to oxidize nitrogen to NO or NO2 during the combustion process.
By circulating spent exhaust gases into the combustion chamber, the air-fuel mixture is diluted, lowering peak combustion temperatures and reducing NOx. A recent development in EGR technology has been the introduction of cooled EGR. Cooler EGR also provides for reduced soot/PM. For compliance with Euro-3 norms, EGR becomes a standard fitment. EGR is considered to be a most cost-effective solution to reduce NOx emissions, both in gasoline and diesel vehicles.
A higher compression ratio increases the thermal efficiency of an engine, improves fuel economy and reduces emissions. Compressions ratios are limited in gasoline engines by the problem of knocking. As unburned air-fuel mixture in the gasoline engine cylinder is compressed, its temperature increases, resulting in self- ignition. This results in knocking and the resultant shock waves can damage the engine or cause it to overheat.
The ability- of a fuel to resist self-ignition is measured by its octane value. Higher the octane value, lower is the self-ignition. Knocking increases with increase in compression ratio. Higher octane value fuel, faster combustion and lean mixtures can allow higher compression ratios resulting in better fuel economy and reduced emissions.
The compression ratio of conventional gasoline engines is between 9 and 11:1 at peak load. Maximum thermal efficiency and fuel economy is obtained when compression ratios are in the range of 13 to 14:1. An engine with variable compression ratio has the potential to be optimized with a higher ratio for best fuel economy at part load conditions (typical of normal driving), while allowing detonation – free full load operation at a lower ratio.
The relationship between the motion of the piston and the combustion of the charge has a major effect on pollutant emissions and engine efficiency. Combustion should be timed, so that most of the combustible mixture burns near or slightly after the piston reaches the top- dead center. Mixture that burns late in the expansion or power stroke do less work on the piston, decreasing fuel efficiency.
Mixture that burn before top dead center increases the compression and work done by the piston. Since combustion takes time to complete, it is necessary to compromise between these two effects. For typical gasoline engines, the spark advance is 20 to 40 degrees crank shaft rotation before the top dead center.
The portion of the air-fuel mixture that burns at or before the top dead centre accounts for a disproportionate share of nitrogen oxide emissions, since the burned gases remain at high temperatures for long periods. To reduce nitrogen oxide emissions, it is common to retard the ignition timing. Similarly PM and HC emissions will be more if the ignition is retarded.
Evaporative and Refueling Emissions and Their Control:
Gasoline is a relatively volatile fuel. Even at normal temperatures, significant gasoline evaporation occurs if gasoline is stored in a vented tank. Gasoline fueled vehicles emit a significant amount of hydrocarbons (HC) as evaporative emissions from their fuel system.
The main sources of evaporative emissions from gasoline engines are:
1. Breathing losses from fuel tanks caused by the expansion and contraction of gas in the tank with changes in air temperature.
2. Hot-soak emissions from the fuel system when a warm engine is turned—off.
3. Running losses from the fuel system during vehicle operation.
4. Resting losses from permeation of plastic and rubber materials in the fuel system.
5. Refueling emissions consist of gasoline vapour displaced from the fuel tank when it is filled.
Evaporative and refueling emissions are strongly affected by volatility. Evaporation emissions also vary depending on the daily temperature and the amount of vapour space in the fuel tank. Evaporative emissions can be reduced by venting the fuel tank and the carburetor to a canister containing activated carbon. This material absorbs volatile emissions from the fuel system when the engine is not running.
When the engine is running, intake air is drawn though the canister, purging it of hydro carbons, which then form part of the fuel mixture fed to the engine. A large canister can also be used to control refueling vapour emissions, or these emissions can be controlled by capturing them through the refueling nozzle and conducting them back to the service station tank.
The most common measures of gasoline volatility is the Reid Vapour Pressure (RVP), which is the vapour pressure measured under standard conditions at an air to liquid ratio of 4:1 and a temperature of 37.8°C. Gasoline volatility is normally adjusted to compensate for variations in ambient temperature. When temperatures are below O°C, gasoline is usually adjusted to an RVP of about 90 K pa (13 ps i) to increase fuel evaporation. Similarly when temperatures are high, RVP of gasoline is adjusted in order to reduce the emissions.
Design and Technology Improvements in Two Stroke Gasoline Engines Used in Two Wheelers:
Technologies to reduce two stroke engine emissions include advanced fuel metering systems, improved scavenging characteristics, combustion chamber modifications, improved ignition systems, improvements in engine lubrication and exhaust after treatment technologies.
Advanced Fuel Metering Systems:
Two stroke engines make a strong case for fuel injection in place of the existing conventional carburetor system to enhance their fuel efficiency and to control their hydrocarbon emissions. The advantage of fuel injection in two-stroke engines are twofold-precise control of air-fuel ratio, and the ability to reduce the loss of fresh charge into the exhaust.
Studies have indicated that best results can be achieved by in-cylinder injection which could be mechanical, compressed air assisted or electronically controlled. With precise injection timing using electronic controls, it is possible to reduce the hydrocarbon content of the air that short-circuits the combustion chamber during scavenging. Significant reduction in hydrocarbon and carbon monoxide emissions can be achieved with fuel-injection systems and electronic controls.
Scavenging Control Technologies:
In a two stroke engine, exhaust and intake events overlap. Hence high pressure combustion gases blow into the exhaust manifold before combustion resulting in higher hydrocarbon emissions. Ideally, the fresh charge should be retained in the cylinder, while the spent charge from the last cycle is exhausted. Scavenging control technologies modifies exhaust flow by introducing control valves in the exhaust.
Honda two-stroke two wheelers are equipped with a control valve which not only controls emissions but has significantly improved engine performance. The exhaust charge control resulted in a reduction of hydrocarbon emissions by about 30 per cent and fuel consumption by about 6 percent.
In direct-injection systems, advancing ignition timing at light load reduces hydro carbon emissions by reducing the dispersion of the fuel cloud. This cloud is less likely to contact the walls of the combustion chamber. This reduces the hydro carbons produced by the quenching effects at the combustion chamber walls. With advanced ignition timing, both HC and CO emissions can be reduced, but NOx emissions will increase.
In two-stroke engines, combustion chamber and piston configuration can be improved to induce more turbulent motions during the compression stroke, which minimises the short circuiting and loss of fresh charge which causes increased HC and PM emissions.
Lubricating oil is the major source of particulate matter emissions from two-stroke engines. Since the crank case of a two stroke engine is used for pumping air or the air-fuel mixture into the combustion chamber, it cannot be a lubricating oil reservoir.
Engine oil mixed with gasoline and air enters the cylinder as a mist. As this stream passes through the crank case, lubrication is provided for cylinder walls, crankshaft bearings and connecting rod bearings. The oil mist burns in the combustion chamber. This results in higher particulates emission in two-stroke engines.
In electronic oil injection systems, the metered quantity of oil is directly injected into the intake manifold. Yamaha two-stroke motor cycles use an electronic oil metering system to alter the oil flow to the carburetor according to engine load.
The Yamaha computer-controlled Lubrication system (YCLS) supplies required lubricating oil to the engine according to engine speed using an electronic control unit and a three-way control valve Q. This process has significantly reduced particulate emissions.
What is Good? Two Stroke Two Wheeler or Four Stroke Two Wheeler?
If we consider the emissions from two stroke two wheelers without the catalytic converter, the difference in emissions between two stroke and four stroke are staggering. On an average, the total regulated tail-pipe emissions from a two stroke two wheeler without catalytic converter is two-and a half times that of the four stroke two wheeler.
The CO and HC + NOx emissions from a two stroke two wheeler with catalytic converter is 23% and 38% higher than the CO and HC + NOx emissions from a four stroke two wheeler without a catalytic converter respectively.
Despite having a small engine (less than half of that of the four stroke two wheelers), the average fuel efficiency of the two stroke two wheelers is 15% lower than that of the four stroke two wheelers.
Table 4.1 indicates the comparison of a two stroke two wheeler and a four stroke two wheeler in Indian conditions.
The above analysis clearly shows the over whelming superiority of the four stroke two wheelers over two stroke two wheelers with respect to emissions and fuel efficiency.
Cost Implications of Emission Control Options:
As a result of advance emission standards or regulations, several levels of control technology have been developed.
Table 4.2 summarises the cost implications in adopting new advanced auto technologies for compliance of various Euro norms.
These estimates were made by the Society of Indian Automobile Manufacturers (SIAM) and indicated to the Mashelkar Committee.
In addition to the incremental cost of vehicles, the vehicle manufacturers are required to move from India 2000 to Euro-4 in case of two wheelers and heavy duty diesel buses and trucks and from euro-2 to Euro-4 for cars. The society of Indian Automobile Manufacturers estimates the additional capital investment as Rs 24,026 crores.
Advanced technologies to comply with Euro 3/Euro 4 norms require better fuel specifications. Improved fuel specifications result in increased investment in the refineries and increased cost of the fuel. According to Mashelkar Committee, the investment required in Indian refineries to comply with Euro-4 norms will be about Rs. 20,000 crores.
This results in increased cost of fuels. However adoption of advanced technologies to comply with stringent emissions norms is likely to improve the fuel efficiency. The improved fuel efficiency is likely to compensate the increased fuel costs.