Natural Gas in Automotive Engines - Essay Example

Natural Gas in Automotive Engines Insert Insert Insert Content Page Internal combustion engine ---------------------------------------------------------------- 3 2. Natural Gas Fuel Overview ---------------------------------------------------------------- 4 3. How Engine Works ---------------------------------------------------------------------------7 4. Technical Problems in Using Gaseous Fuels ----------------------------------------- 8 i. Internal Combustion Engine ----------------------------------------------------- 8 ii. Spark Ignition Engine -------------------------------------------------------------- 8 iii. Technical Problem in Compression Ignition Engine--------------------- 9 5. Environmental Characteristics of Automotive Engine Emissions ------------- 10 6. Emission Control----------------------------------------------------------------------------- 11 i. Catalytic Converters---------------------------------------------------------------12 ii. Pulse-air System-------------------------------------------------------------------12 iii. Exhaust Gas Recirculation (EGR) System-------------------------------- 12 iv. Positive Crankcase Ventilation System------------------------------------12 v. Evaporation Emission Control (EVAP) System---------------------------12 7. References ------------------------------------------------------------------------------------- 13 1. Internal combustion engine Internal combustion engine are of different types like reciprocating or rotary, spark or combustion ignition and two and four stroke engine. i. Basic process Four strokes is the most common so we will discuss the basic process used in this type, usually found in lawn movers automobiles etc. There is an operation which every four stroke internal combustion engines have four basic steps that repeat with every two revolutions of the engine Intake: Combustible mixtures are emplaced in the combustion chamber. Compression: The mixtures are placed under pressure Combustion/Expansion: The mixture is burnt, almost invariably adeflagration, although a few systems involvedetonation. The hot mixture is expanded, pressing on and moving parts of the engine and performing useful work. Exhaust: The cooled combustion products are exhausted into the atmosphere. Many engines overlap these steps in time; jet engines do all steps simultaneously at different parts of the engines. The main idea behind a gasoline car engine is of convertinggasolineinto motion so that car can move. Presently, the easiest way of creating motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is generally known as internal combustion engine as combustion takes place internally. It is further divided into two types; spark ignition and compression ignition engine. Spark Ignition Engine is referred to the internal combustion engine in which fuel-air mixture is ignited with aspark within a cylinder. It is different from compression-ignitionengines, where only heat from compression can ignites the mixture. Spark-ignition engines can be eithertwo-strokeorfour-stroke. However, these terms are not preferred, since spark-ignition engines can (and increasingly are) run on fuels other thangasoline, such asautogas(LPG), methanol,ethanol,compressed natural gas(CNG),hydrogen, and (in drag racing)nitromethane. A four-stroke spark-ignition engine is anOtto cycleengine. Until recently, a major distinction between spark-ignition and compression-ignition engines has been where the fuel is mixed - spark-ignition engines mix fuel outside the cylinders and compression-ignition engines mix fuel inside the cylinders. However, both two-stroke and four-stroke spark-ignition engines are increasingly being designed withgasoline direct injection(GDi), eliminating this distinction between the two systems. 2. Natural Gas Fuel Overview: Natural gas is further divided into two types when talking in terms of transportation. CNG (Compressed Natural Gas) LNG (Liquefied Natural Gas) Compressed Natural Gas(CNG) is afossil fuelsubstitute forgasoline(petrol),diesel, or propanefuel. Although its combustion does producegreenhouse gases, it is a more environmentally clean alternative to those fuels; a Compressed Natural Gas(CNG) is afossil fuelsubstitute forgasoline(petrol),diesel, orpropanefuel. CNG is made by compressingnatural gas(which is mainly composed of methane [CH4], to less than 1% of its volume atstandard atmospheric pressure. It is stored and distributed in hard containers, at a normal pressure of 200-220bar(2900-3200 psi), usually in cylindrical or spherical shapes. It is much safer than other fuels in the event of aspill(natural gas is lighter than air, and disperses quickly when released). The advantages of CNG include: Flexibility and ease of use: The basic engine characteristics of a vehicle are retained while converting it to run on CNG. The vehicle therefore is capable of running either on Petrol or CNG at the flick of a switch on its dashboard. Low maintenance cost: CNG engines give longer life service and comparatively low maintenance cost. Economic benefit: The cost of CNG is almost a third of the cost of Petrol in terms of calorific value resulting in substantial saving in fuel cost, and investment on the CNG kit is paid back in a short period. Less Dangerous: If there is a leakage the fuel will dissipate harmlessly in the air eliminating the danger of any sort of blast. Environment friendly: The use of CNG as a fuel reduces vehicular exhaust emissions significantly. Carbon Monoxide emissions are reduced by 70 to 90% and Hydrocarbon emissions by 40 to 60% as compared to vehicles that use the conventional fuel - Petrol. Carbon Dioxide emissions, a cause for global warming, are also reduced significantly by 10%. Liquefied natural gasorLNGisnatural gas(Predominantlymethane, CH4) that has been converted temporarily to liquid form for ease of storage or transport. Liquefied natural gas takes up about 1/600th the volume of natural gas at a stove burner tip. It isodorless,colorless,non-toxicandnon-corrosive. Hazards include flammability, freezing andasphyxia. The liquefication process involves removal of certain components, such as dust,helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is thencondensedinto a liquid at close to atmospheric pressure (Maximum Transport Pressure set around 25kPa(3.6psi)) by cooling it to approximately 163C (260F). Theenergy densityof LNG is 60% of that ofdiesel fuel. The advantages of LNG include: Cleaner: LNG has one carbon atom. Additionally, unlike diesel, gaseous fuels do not suffer the risk of spillage or potential discharge into waterways Noise level: LNG engines can achieve a reduction of up to 50% in noise levels compared to diesel engines. Convenient to store: LNG is stored at much lower pressures (typically 700kPa) than CNG (typically 25,200kPa). It has around 3.5 times the fuel density of CNG. LNG's fuel storage mass is also 800-1000kg lighter. Vehicle operators can therefore achieve much greater haulage ranges using LNG. Quick filling: Fill times for LNG vehicles are typically at the same rate as diesel or faster. Fewer emission: Natural gas fuel specifically combust at fewer emission as compare to gasoline or diesel. 3. How Engine Works: Mechanical work is possible after gas engine converts the natural gaseous chemical energy. For instance if we study energy balance we will find out that, 100,000 Btu's into energy will need app. 31,000 Btu's for shaft work, 35000 Btu's of jacket water and lube oil which is 45% recoverable, 28,000 Btu's for exhaust and 6,000 Btu's to cover up the radiation losses. The energy balance also tells us the reason about significant heat recovery and how it adds to the performance of engine. Both jacket and exhaust can recover the usable heat. Hot water from jacket water and low pressure steam from exhaust can be obtained. Natural gas engine are naturally aspirated or turbo charged but air is drawn into the engines at atmospheric pressure; if it's naturally aspirated engines. Exhaust is used to turbocharge the engines for driving a small turbofan that compressed the air intake. Part of the air stream from the ducted fan passes through the gas turbine core, providing oxygen to burn fuel to create power. However, most of the air flow bypasses the engine core, and is accelerated by the fan blades in much the same manner as apropeller. 4. Technical Problems in Using Gaseous Fuels In i. Internal Combustion Engine: The significant factors for the performance and the emission of internal combustion engines are good ignition, optimum combustion rate, high knock resistance and a sufficient energy content of a fuel mixture. Moreover, the properties which are important in relation to the engine performance, includes density, heating value, stoichiometric air- fuel ratio and knock resistance. The main characteristic includes: High energy density of fuel leads to high power to weight ratio, especially when combusting with atmospheric oxygen Heat engines use working fluids which is the simplest of all energy conversion methods Following are the technical problems in combustion ignition engine Large gas composition variation can have important effects on engine performance and efficiency on fixed gas composition. The driving range for this gaseous fuel powered vehicle is also limited because its energy content per volume is relatively low as a result that it is in gaseous state. Therefore, it has to take a larger volume of fuel on board, so it needs a larger size tank and thick material to withstand the high pressure. This extra weight of the material leads to the heavier weight and thus a higher usage of fuel and payload loss. CNG vehicle consumes 0 to 10% more energy than a gasoline vehicle. Duel fuel refers to ability of engine's reaction to combination f natural gas and diesel; majority is reacted to gas while diesel is just used for the combustion. This combination allows retention to diesel compression ratio along with its economy and substitution of diesel fuel with less expensive, learner burning and natural gas. Maximum power drop is when switching to natural gas. The quality of gas can also affect the gas engine; gas with high water content can affect the performance. It works with the natural gas which has significant propane quantities which can cause problems such as poor drivability or even engine storage. The lubricating oil which is carried through the pipelines of the compressor can harm the engine. CNG can condensed the lubricant and create operational problems while lack of lubricant in LNG vehicles can create problems. ii. Spark Ignition Engine: High knock resistance of methane cannot be fully utilized in the bi-fuel engines result in creating the knocking sound in the engine. Detonation and pre ignition are two common problems faced in spark ignition engines. Detonation is the uncontrollable fuel burning in the chamber while pre ignition can cause by burning from other source before the plug is fired. If the engine is dedicated to natural gas it will increase the compression rate, valve timings and ignition setting can be optimized and cause power loss to some extend can be reduced. Methane is easily formed as a homogenous mixture, which is good for combustion. But it is hard to oxidize and can cause quick quenching of flame and can be dangerous. When the engine convert the running of liquid fuel to gaseous fuel it will result in power loss up to 10 %. Rapid erosion of spark plug electrodes is also a technical problem, and if it is damaged which is a very timely thing to replace (app 3000 hours). iii. Technical Problem in Compression Ignition Engine: Major problem of these types of engines are heavy duty automotive gas engines which are of converted diesel engines and also are related mostly to thermal control of the engine and the control of NOx emission. So when the conversion is required we need to cylinder block modification, which will result in the reduction of compression ratio. Producing maximum Braque can cause problem in the longevity and reliability. This will further cause higher stress at increase RPM necessary for getting maximum performance on gaseous fluid. It will result into a mechanical breakdown or reduce lifetime of components making it a faulty engine. Huge amount of carbon mono oxide can cause incomplete combustion, if the vehicles are not able to run on its optimum speed. 5. Environmental Characteristics of Automotive Engine Emissions As the amount of greenhouse gases in the atmosphere increase so do the concerns regarding the possible increase in the global temperatures. The working of the process is that the reradiated solar energy is trapped by the greenhouse gases and this increases the temperature of the earth up to 180o C. Greenhouse gases include carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NO and NO2) more commonly known as NOx, chlorofluorocarbons (CFCs), and hydrocarbons such as CH4. Hydrocarbons and NOx directly influences the overall effect of the greenhouse gases whereas, non-methane hydrocarbons (NMHC) and CO have an indirect influence. Emissions of these gases from motor vehicles have the largest involvement in this and are expected to a further rise because of the growing number of vehicles on the road. The production of these gases occurs due to the combustion of fossil fuels into hydrocarbons, and of oxygen into water and carbon dioxide. Hydrocarbons, CO2 and NOx are major contributors in the automotive engine emissions which account for three quarters of total energy consumption. Or in other words, it is the most important regarding energy consumption. The most dominant contributor, carbon dioxide, is expected to rise from 42 % in the 1990 to 62 % in 2010 in the transport sector alone. The carbon content in the fuel shows the amount of carbon dioxide that will be produced in per unit of energy and the effectiveness of the whole process. Gasoline accounts for highest amount of CO2 production while diesel comes slightly lower as of its lower energy consumption. Liquid petroleum gas (LPG) and natural gas (NGV) has further lower production because of the smaller carbon-hydrogen ratio. Another cause of may be of the data of retrofitted engines compared to the dedicate ones is that it has higher CO2 emissions because the fuel has been converted from gasoline first. Methane is the product of unburnt hydrocarbons present in the atmosphere and unfortunately cannot be converted to gasoline as of its highly stable molecular structure giving it a disadvantage of NGVs. It is also a main component of greenhouse gases. However, presently nitrogen monoxide (NO) is brown in color hence, the haze of brown in heavily polluted areas. It also leads to the production of the ozone in the atmosphere which accounts for the destruction of trees and crops. Nitrogen dioxide (NO2), on the other hand, is responsible for the exposure to breathing risks. Moreover, when combines with sulphur dioxide, it produces acid rain and smog. Emissions of hydrocarbons are not only cancer causing i.e. carcinogenic but also are a chief component of photochemical smog. 6. Emission Control The significant decrease in a gram per mile emissions of greenhouse gases is due to the increased standards of technology. The remarkable advancements in the transport sector of reducing greenhouse gases emissions include carrying out emission reduction regulations and introduction of vehicle emission reduction technology. i. Catalytic Converters These help by cleaning the exhausts of vehicles are composed of catalysts speeding up the process between the pollutants and oxygen in the environment. Nox is converted to nitrogen gas and CO and hydrocarbons are oxidised to water and CO2; a three-way converter. Two-way only deals with CO and hydrocarbons. Catalytic converter works at a high temperature which is a disadvantage because it cannot be used to reduce pollution in cold temperatures where emissions maybe higher. ii. Pulse-air System Solenoid valve and valve of pulse-air raise the temperature f the exhaust gases hence, the time taken for the process to wait for the oxygen sensor and converter to reach the operating temperature is reduced. iii. Exhaust Gas Recirculation (EGR) System Combustion temperatures are reduced by re-circulating the exhaust gases through an EGR valve in the manifold inlet. iv. Positive Crankcase Ventilation System The escaped hydrocarbon gases through the piston rings are drawn back and burned through this system. Hence, reducing the emission of unburnt hydrocarbons and minimising the formation of oil sludge. v. Evaporation Emission Control (EVAP) System EVAP system minimises the emission of unburnt hydrocarbons from the fuel tank into the atmosphere by pulling them back inside and burning them. References: M. Schneider. (2000). Utilization of Natural gas and Biogaz in Gas Engines - requirementsand experiments. Gas Energetic Conference. Szczyrk. Gordon P. Blair. (1999). Design and Simulation of Four-Stroke Engines. Publisher: SAE John B Heywood. (1988). Internal Combustion Engine Fundamentals. Publisher: McGraw-Hill. Mohamed Y.E. Selim. (2004). Effect of engine parameters and gaseous fuel type on the cyclic variability of dual fuel engines. Mechanical Engineering Department, Faculty of Engineering, United Arab Emirates. Qing Ping Zheng, Hui Ming Zhang and De Fu Zhang. (2004). A computational study of combustion in compression ignition natural gas engine with separated chamber. State Key Laboratory of Engines, Tianjin University. Reiter. J. Aaron and Song-Charng Kong. (2005). Demonstration of Compression-Ignition Engine Combustion Using Ammonia in Reducing Greenhouse Gas Emissions. Department of Mechanical Engineering, Iowa State University. www.fuelingthefutrure.org www.cleaner-drive.com www.aircompressor.org www.howstuffworks.com www.altfuels.org Read More