Controlling Emissions in Diesel Engines!
The diesel engines have an excellent reputation for fuel efficiency, reliability and durability. The diesel engines having very high thermal efficiencies produce lower CO and HC emissions compared to gasoline engines. However diesel engines produce higher PM and NOx emissions. Diesel emissions in the form of black smoke is a public nuisance apart from its health impacts.
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There are two types of diesel engines- indirect and direct injection. In an indirect injection (IDI) diesel engine, the fuel is injected into a pre-chamber (generally located in the cylinder head) where ignition occurs and the combustion then spreads to the main combustion chamber in the cylinder. Indirect injection diesel technology is mainly used for small size, high speed applications such as passenger cars and multi utility vehicles, where low noise and high performance are important.
In a direct injection (DI) engine, the diesel fuel is sprayed directly into and ignited in the combustion chamber of the engine. These engines are used in light and heavy commercial vehicles and some multi utility vehicles and give higher power output and better fuel economy, but they are considerably noisier.
Developments in reducing noise and improving performance have led to the use of direct injection engines in some of the cars in the recent years. Common Rail Direct Injection (CRDI) cars have become popular even in the high end market in India.
Diesel engines meeting current U.S and Euro norms are smokeless, have better fuel efficiency, less noisy and emit significantly less pollutants than equivalent diesel engines manufactured in India. Engine variables with the greatest effect on diesel emission rates are the combustion process, air-fuel ratio, rate of air-fuel mixing, compression ratio, the temperature and composition of the charge in the cylinder, fuel injection pressure and timing and combustion chamber design.
Diesel Combustion Process:
During the compression stroke, a diesel engine compresses only air. The process of compression heats the air to about 700- 900°C which is well above the self-ignition temperature of diesel fuel. Near the end of the compression stroke, liquid fuel is injected into the combustion chamber under tremendous pressure through a number of small orifices in the tip of the injection nozzle.
As diesel fuel is injected into the combustion chamber just before the piston reaches the top dead center, the periphery of each droplet comes in contact with the hot compressed air already present in the combustion chamber. After a brief period known as the ignition delay, the fuel-air mixture ignites.
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The fuel-air mixture formed during the ignition delay period burns very rapidly, causing a rapid rise in the cylinder pressure pushing down the pistons and generating power. In diesel engines, the rate of emissions are largely determined by the conditions during the combustion process.
Air-Fuel Ratio:
In diesel engines, the ratio of air to fuel in the combustion chamber has a significant effect on emission rates of particulate matter and hydro carbons. In diesel engines, the fuel and air mix just before burning, hence a substantial amount of excess air is needed to ensure complete combustion of the fuel within the time allowed by the power stroke. Hence diesel engines operate with excess air. At lower air-fuel ratios, less oxygen is available for soot oxidation, so soot or black smoke emissions increase dramatically at lower air-fuel ratios.
In naturally aspirated diesel engines, the maximum power output is normally smoke-limited, that is, limited by the amount of fuel that can be injected without exceeding the smoke limit. Maximum fuel setting on naturally aspirated engines represent a compromise between smoke emissions and power output. Where diesel smoke is regulated, this compromise must result in smoke opacity below the regulated limit.
Naturally aspirated diesel engines operate with an air-fuel ratio of about 20:1. Naturally aspirated engines are highly sensitive to changes in air-fuel ratio, producing excessive particulates and visible smoke if they are over-fueled or if the air quantity is restricted or reduced during operation.
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Operating with choked air cleaner or operating at higher altitudes reduces air-fuel ratio and results in increased black smoke. In naturally aspirated engines with 20:1 air fuel ratio, the combustion system needs to be well tuned and any small changes in injection timing, load conditions etc., results in excessive particulates and visible smoke.
Naturally aspirated engines at full load have an air -fuel ratio of about 20:1. At this air-fuel ratio, it is difficult to achieve the stringent emission standards. In countries enforcing stringent emission standards (equivalent to Euro 3 and beyond), naturally aspirated engines will slowly disappear from the market, because higher air- fuel ratios are required to take care of the stringent emission norms. The air fuel ratio can be improved by compressing the intake air to suitable pressure. Turbo chargers are used to compress the intake air and increase the air-fuel ratio.
A turbo charger consists of a centrifugal air compressor mounted on the same shaft as an exhaust gas turbine. By increasing the mass of the air in the cylinder prior to compression, turbo charging improves the maximum power output, fuel economy and reduces emissions. The process of compressing the air increases its temperature.
By cooling the air in an inter-cooler before it enters the cylinder, the density of the air increases, allowing an even greater mass of air into the cylinder. Turbo charging and inter-cooling offer an inexpensive means to simultaneously improve power-weight ratio, fuel economy and control of NOx and PM emissions.
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For effective performance, it is necessary to match turbo charger response characteristics to engine requirements. Because of the inherent mismatch between engine requirements and those of fixed geometry turbo chargers, a number of engine manufacturers are using variable geometry turbo chargers.
In these systems, the turbine nozzles can be adjusted to vary the turbine pressure drop and power level in order to match the engine’s boost pressure requirements. This results in substantial improvements in fuel economy and significant reduction in PM and NOx emissions.
The stringent emission standards now effective in U.S and Europe have eliminated naturally aspirated diesel engines. High Efficiency variable geometry turbo chargers with charge cooling are a standard fitment in diesel vehicles in USA and Europe.
Air-Fuel Mixing:
The rate of mixing between the compressed air in the cylinder and the injected fuel is among the most important factors in determining diesel engine performance and emissions. The mixing rate during the ignition delay period determines how much fuel is burned in the premixed burning phase. The higher the mixing rate, greater the amount of fuel burning in the pre-mixed mode, and higher the noise and NOx emissions.
In the subsequent mixing- controlled combustion phase, the rate of combustion is limited by the mixing rate. The more rapid and complete this mixing, the greater the amount of fuel that burns near the top dead center, the higher the efficiency, and lower the PM emissions.
The primary factors affecting the mixing rate are the fuel injection pressure, the number and size of injection orifices, swirling motion imparted to the air as it enters the cylinder, and air motions generated by combustion chamber geometry during compression. Maintenance problems such as injector tip deposits can also degrade air-fuel mixing and result in greatly increased emissions. This is a common cause of high smoke emissions.
Charge Temperature:
Reducing the temperature of the compressed air charge going into the cylinder has benefits for both PM and NOx emissions. Reducing the charge temperature directly reduces the flame temperature during combustion, and thus helps to reduce NOx emissions.
In addition, colder air is denser, so that (at the same pressure) a greater mass of air can be contained in the same fixed cylinder volume. This increases the air-fuel ratio in the cylinder and thus helps to reduce PM and soot emissions. Charge air cooling can also make possible a significant increase in power output. For this reason, many turbo charged engines incorporate charge air- coolers even in the absence of emission controls.
Charge Composition:
Nitrogen oxide emissions are heavily dependent on flame temperature. By altering the composition of the air charge to increase its specific heat and the concentrations of inert gases, it is possible to decrease the flame temperature significantly. The most common way of accomplishing this is through exhaust gas recirculation (EGR). At moderate loads, EGR has been shown to be capable of reducing NOx emissions by a factor of two or more with little effect on PM emissions.
Studies have indicated that in heavy duty diesel vehicles, significantly low NOx emissions (less than 3 grams/kwh) could be achieved using EGR. Exhaust gas recirculation is considered one of the most promising NOx control technology. None of the diesel vehicles in India currently use EGR. The Indian diesel vehicles will be required to use EGR from 2010 when Euro 4 norms are implemented and NOx emissions are specified for control.
Fuel Injection System in a Diesel Engine:
The fuel injection system in a diesel engine includes the machinery by which the fuel is transferred from the fuel tank to the engine, then injected into the cylinder at the right time for optimal combustion, and in the correct amount to provide the desired power output. The quality, quantity, and timing of fuel injection determines the engine power, fuel economy, and emission characteristics. Hence fuel injection system is one of the most important components of a diesel engine.
The fuel injection system normally consists of a low pressure pump to transfer fuel from the tank to the system, one or more high pressure fuel pumps to create the pressure that actually send the fuel into the cylinder, the injection nozzles through which fuel is injected into the cylinder, and a governor and a fuel metering system. These determines how much fuel is to be injected in each stroke and thus the power output of the engine.
The parameters in fuel injection system which can affect the combustion process and emissions of a diesel engine are injected fuel pressure, quantity, timing, injection duration, spray distribution in combustion chamber and atomization of the diesel fuel.
Systems offering electronic control of these parameters have been introduced. Some manufacturers are also pursuing technology to vary the rate of fuel injection over the injection period, in order to reduce the amount of fuel burning in the pre mixed combustion phase. Reduction in NOx, noise and maximum cylinder pressures have been demonstrated using this approach.
Fuel Injection and Combustion Timing:
Injection timing plays a very important role in diesel economy and emissions. The timing of beginning and ending of fuel injection has an important effect on diesel emissions and fuel economy. For best fuel economy and lower particulate emissions, it is preferable that combustion begins before the piston reaches top dead center since there is finite delay between the beginning of injection and start of combustion due to ignition delay of the fuel.
The fuel is injected generally 5-15 degrees of the crank shaft rotation before the piston reaches the top dead center. The earlier the fuel is injected, longer the ignition delay. Longer ignition delay provides more time for air and fuel to mix, resulting in increase in maximum temperature and pressure attained in the cylinder. Both of these tend to increase NOx emissions. On the other hand, earlier injection timing tends to reduce PM and HC emissions. This also improves fuel economy.
The end of injection timing also has a major effect upon black smoke emissions-fuel injected after the piston reaches the top dead center burns more slowly, and at a lower temperature, so that resulting soot has no or less time to oxidize during the power stroke resulting in increased PM emissions. For a fixed injection pressure, fuel quantity and orifice size, the end of injection is determined by the timing of the beginning of injection.
The injection timing must compromise between PM emissions and fuel economy on the one hand and NOx emissions on the other. The terms of compromise can be improved to a considerable extent by increasing injection pressure, which increases mixing and advances the end-of injection timing.
Compared to uncontrolled diesel engines, modern diesel engines with emission controls generally have moderately retarded injection timing to reduce NOx, in conjunction with high injection pressures to limit the effect of retarded timing on PM emissions and fuel economy. Great precision in injection timing thus becomes necessary-a change of one degree crank angle can have a significant effect on emissions and fuel economy.
Low Sac/Sacless Nozzles:
The nozzle sac is a small internal space in the tip of the injection nozzle. The nozzle orifices open into the sac so that fuel passing past the needle valve first enters the sac and then sprays out of the orifices. The small amount of fuel remaining in the sac tends to burn or evaporate late in the combustion cycle, resulting in significant PM and HC emissions.
The sac volume can be minimized or eliminated by redesigning the injector nozzle. One of the vehicle manufacturers reported nearly 30 percent reduction in PM emissions through elimination of nozzle sac. The injection nozzle used in modern diesel engines have very low sac volume and shorter spray hole length.
Fuel Injection Pressure:
Higher fuel injection pressures are desirable to improve fuel atomization and fuel-air mixing and to offset the effects of retarded injection timing by increasing the injection rate. Studies indicate marked reduction of PM or smoke emissions as injection pressure is increased.
The current fuel injection systems with a mechanical pump, fuel lines and injectors cannot achieve more than 800bar. European, U.S and Japanese diesel engines are using fuel injection systems with considerably increased injection pressure than used in Indian buses and trucks.
Modern diesel engines using unit injectors and “common rail” injection systems can achieve injection pressures exceeding 1500 bar. Common rail injection systems can control and maintain fuel injection pressures nearly independent of engine speeds and hence produce low emissions and give better fuel economy.
Compression Ratio:
In an internal combustion engine the ratio of the volume of combustion space at the bottom dead center to that at the top dead center is known as compression ratio. Compression ratio is one of the design parameters which has significant influence on engine performance, fuel economy, cold starting and emissions. Diesel engines rely on compression heating to ignite the fuel, so an engine’s compression ratio has an important effect on combustion.
A higher compression ratio results in a higher temperature for the compressed charge, and thus in a shorter ignition delay. Engine fuel economy, cold starting and maximum cylinder pressure are affected by compression ratio. In a diesel engine, thermal efficiency and compression ratio are directly related. Studies indicate that fuel economy improvements of about 8% can be obtained for every unit increase in compression ratio.
Most of the conventional diesel engines have optimal compression ratios of about 12 to 15. To ensure adequate starting ability under cold conditions, better fuel economy and lower emissions, diesel engine designs require a higher compression ratio-in the range of 20. The compression ratio is influenced by the quality of fuel (cetane value in the diesel fuel), combustion chamber shape and design, materials used for pistons, cylinder block etc.
Combustion Chamber Design:
The geometry of the combustion chamber and the air intake port control the air motion in the diesel combustion and thus play an important role in air-fuel mixing and emissions. Combustion chamber design has a significant impact on emissions, fuel economy and reliability. Changes in combustion chamber geometry-such as reentrant lip on the piston bowl-can markedly reduce emissions by improving air-fuel mixing. Swirl is determined mostly by the design of the air intake port.
Optimising the intake port shape for best swirl characteristics has yielded significant benefits in reducing PM emissions. Design changes to reduce crevice volume in diesel engines increases the amount of air availability in the cylinder and significantly reduces emissions. At lower speeds, higher swirl provides better mixing, permitting more fuel to be injected and thus greater torque output at the same smoke level.
Electronic Controls:
The advent of computerized electronic engine control systems has greatly increased the potential flexibility and precision of fuel metering and injection timing controls. By continuously adjusting the fuel injection timing to match a stored “map” of optimal timing vs speed and load, an electronic timing control system can significantly reduce the NOx and PM emissions and improve fuel economy. Potential reduction in PM emissions of upto about 40 percent have been observed using electronic controls for engine management.
Other electronic control features such as cruise control and communication with an electronically controlled transmission helps to reduce fuel consumption and emissions significantly.
Advances in Fuel Injection Systems- Common Rail and Unit Injectors:
Common Rail is the most modern fuel injection system in diesel vehicles. A hydraulic pump generates high pressure – also at low engine speed-in a common rail (accumulator) mounted to the engine block side. Pressure regulation/control is possible between 200-1500 bars. Several new diesel vehicles are now fitted with common rail. Higher pressure provides better atomization.
Solenoid control unit injectors are placed centrally in a four valve cylinder head. High pressure is provided mechanically by cam actualization to individual injectors. High pressure rotary pump with solenoid controls have replaced the traditional rotary pumps.
New generation of fast, reliable and durable solenoids combined with powerful electronic controls enables the latest generation of—unit injectors, and common rail- to work fully integrated in the fueling system. This new “package” gives full flexibility. The advantages are improvements in fuel economy and reduction in emissions.
Engine Oil Control:
Studies indicate that a significant fraction of diesel particulate matter consists of engine lubricating oil derived hydrocarbons and particulates – estimates range from 10 to 50 per cent. Reduced engine oil consumption has been a design goal of diesel engine manufacturers and the current generation of diesel engines consume relatively little engine oil compared to earlier models.
Further reductions in engine oil consumption are possible through attention to roundness of cylinder bore, its surface finish, optimization of piston ring tension and shape, and attention to valve stem seals, turbo charger oil seals, and other possible sources of engine oil loss.
However some engine oil is required to perform its lubricating and corrosion retarding functions in the cylinder. The reduction in diesel fuel sulphur content has reduced the need for corrosion protection. Changes in oil formulation can also help to reduce particulate matter emissions in diesel vehicles by 10 to 20 percent.
Diesel and Water Emulsion:
Diesel/water injection is being used to reduce pollutant emissions in heavy duty diesel vehicles.
There are two ways by which it can be achieved:
1. A stable water-fuel emulsion is prepared and filled into the tank of the vehicle. Packages of additives are used for time and temperature stability of the mixture. The water to fuel ratio is fixed.
2. Water and fuel are available in separate tanks on – board, and injected into the combustion chamber. Injection can be done-with two separate injectors or sequential injection in the same injector. In both the cases, the engine must be adapted. The water -fuel ratio is controlled and mapped with the engine conditions. This is perceived as the most appropriate process for Euro IV norms.
Demonstration with diesel – water emulsion is currently in operation on buses in several cities, including Bangalore in India. Emulsion system seems more appropriate for captive fleets. Studies indicate that roughly one percent water addition reduces 15 to 30 percent NOx, 10 to 50 percent PM and substantial reduction of the smoke. The major disadvantage is the potential corrosion and wear issues.