16-09-2016, 03:37 PM
COMBUSTION CHAMBER OPTIMIZATION FOR IMPROVING PERFORMANCE AND EMISSION ON LPG LEAN BURN S.I ENGINE
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ABSTRACT
Lean operation is an attractive operational method to increase thermal efficiency and to decrease exhaust emissions and fuel consumption rate. Gaseous fuels as clean, economical and abundant fuels can improve the lean operating limits. Liquefied petroleum gas (LPG) is one of the members of natural gases and declared as the cleaner fuel.
Lean operation of homogeneous-charge spark-ignited engines reduces peak combustion temperatures, thereby reducing NOx emissions. Lean operation is normally restricted by the air-fuel ratio above which ignition is impossible, or combustion is incomplete. Operation under lean conditions also reduces the mixture burning rate, which can lead to increased spark advance and lower thermal efficiency.
Experiments were conducted to assess the relative effects of swirl (by providing swirl grooves on the piston crown) and squish (SJCC and D-shape cavity entrance) on the performance, emission and combustion characteristics of a lean burn engine operating on liquefied petroleum gas (LPG) at a compression ratio of 11 under 25% and 100% throttle opening conditions. The SJCC configuration at 100% throttle opening resulted in improved thermal efficiency and reduced HC emission, cyclic variations, ignition delay & combustion duration as compared to swirl groove piston and D-shape cavity entrance. The lean misfire limit was extended and there was no increase in the NO level at any given power output. At 25% throttle with high squish, under lean mixture conditions, combustion is even better than the SJCC configuration.
PROJECT EXECUTIVE SUMMERY
The C.I air cooled Kirloskar engine was modified to S.I engine and was coupled to an eddy current dynamometer and LPG was supplied from a cylinder into the venture of the gas carburetor through a pressure regulator, orifice meter, surge tank and a needle valve. The pressure drop across the orifice meter was measured with a micro-manometer to calculate the gas flow rate. Arrangements were made to measure the temperature and pressure of LPG before it enters into the orifice meter. Air flow rate was measured by using a flow meter. Pressure-crank angle data was acquired on a personal computer using a flush mounted piezoelectric pressure transducer. This data was processed by software to find combustion parameters. Arrangement was made for measuring the spark timing with the help of a stroboscope. A NDIR (non-dispersive infrared) gas analyzer was used for the measurement of HC & CO in the exhaust. A CLD (chemiluminiscence device) analyzer which works on the chemiluminiscence principle was used for measuring the NO concentration in the engine exhaust. In this project different piston shapes of 11 compression ratio were produced by using two piece piston concept The tests were conducted by using 5 different types of piston design (Dog Dish, SJCC, Swirl groove, and D-Shape piston), test conducted for finding out the MBT spark timing at different air fuel ratios and then performance test was conducted for five pistons at 25 % throttle and 100% throttle by using MBT spark timing. Emission and Lean misfire limits were found out by considering above parameters
CHAPTER 1: INTRODUCTION
To meet future regulations for stringent emissions, LPG (Liquefied Petroleum Gas) fueled spark ignition engines are being used (1). A Stochiometric LPG fueled engine has limited applications due to high exhaust gas temperatures and a lower thermal efficiency. However, the lean burn technology implemented to overcome these difficulties.
Before 1970’s a very few experimental works were made to study on the lean combustion technology in SI engines. This technology was studied first during 1908 to demonstrate the advantages of higher thermal efficiency (2). Later on the need for emission control and fuel economy improvement became evident and hence the lean combustion technology shows to offer the lower emissions, higher thermal efficiency and also improves the fuel economy (3).
The principal benefits of this operating technique are a reduction in greenhouse gas emissions and NOx emissions. Lean operation is normally restricted by the air-fuel ratio above which combustion is incomplete. A disadvantage of lean operation is that the burning rate can be significantly lower than with Stochiometric combustion. The reduction in burning rate increases the overall combustion duration and also leads to low flame velocities(4). For a successful implementation of the lean burn technology to decrease the exhaust emissions and increase the thermal efficiency, burn rate enhancement is necessary. A number of design variables such as spark plug location, intake port configuration and combustion chamber shape have been shown to influence the burn rate. Among these, “squish-jet” motion by using combustion chamber shape is having greater influence on the burn rate (5).
POLLUTANTS THERE FORMATION AND HEALTH EFFECT
1.1.1 Formation of carbon monoxide (CO) and there effects
The formation of CO is an intermediate step in the hydrocarbon oxidation process leading to the final product of CO2
RH R RO2 RCHO RCO CO
Where, R represents the hydrocarbon radical. The CO formed is then oxidized at a slower rate to CO2 by the reaction”
CO+OH CO2+H
The fuel oxidation rate depends on the available oxygen concentrations, the temperature of the gases and the time left for the reaction to take place that is on the engine speed.
The main parameter governing CO emissions is the fuel air ratio of the carbureted mixture, on a rich mixture, CO concentration increases steadily with the fuel/air ratio and the lack of oxygen causes incomplete combustion of water gas reaction:
CO+H2O H2+CO2
For A temperature of about 1600k to 1700k The freezing of reaction the reaction at these temperature corresponding to an equilibrium constant:
k=([CO][H2O])/([CO][2H2])
In lean mixture, the CO concentrations are low and vary only slightly with the fuel/air ratio. This could be due to incomplete oxidation during the expansion phase of the HC desorbed from the deposits, the oil film or the crevices of the combustion chamber. However, due to heterogeneity of the mixture, i.e. local oxygen deficiencies, temperature levels or residence times that is insufficient to complete the combustion in the form of CO2 can causes CO emissions. This could occur at low and at maximum loads at high speed.
1.1.1.1 Health effects of CO:
Odorless, colorless & poisonous gas.
Caused by incomplete combustion of fuel and air
Most of it comes from motor vehicles
CO causes dizziness & vomiting sensation.
CO reacts with Hb (hemoglobin) in blood to give carboxy-haemoglobin (CO-Hb) which makes Hb unavailable for O2 transport, thus blocking transport of oxygen to heart and brain
Affects mental functions & visibility more severely even at low levels
Accelerates angina (chest pain) coronary artery disease
Known to cause death at high levels of exposure
1.1.2 Formation of NOx and their effects
Nitric oxide and nitrogen dioxide is usually grouped together as NOx in which NO largely predominates. The main source of NO is molecular nitrogen in the air used for the combustion in the engine
There is a temperature distribution cross the chamber due to passage of flame.
Mixture that burns early is compressed to higher temperatures after combustion, as the cylinder pressure continues to rise.
Mixture that burns later is compressed primarily as unburned mixture and ends up after combustion at a lower burned gas temperature.
Zeldovich was the first to suggest the importance of first two reactions and Lavoice added 3rd reaction to the mechanism.
1.1.2.2 Effects causes by NOx
NO reacts with atmospheric O2 and produce NO2, which is an insidious poisonous reddish brown gas.
NOx results from high temperature combustion processes, e.g. cars and utilities
Reacts with moisture in lungs to form Nitric acid.
Affects respiratory systems causing bronchitis, pneumonia and lung inflections.
Visibility reduction.
Contribute to acid rain.
They play a major role in atmospheric reactions
Overall levels unchanged but transportation sources are cleaner
Air/Fuel (A/F) ratio deviation from stoichiometry. Fuel air mixture is too lean to burn. Lower temperature reduces evaporation. Fuel air mixture is too rich to burn resulting in-complete combustion.
Incomplete combustion.
Flame quenching at walls.
Absorption and desorption in lubricating oils and deposits.
Crevices in combustion chamber and piston rings.
Short-circuiting of fresh charge.
1.1.3.1 The sequence of processes involved in the Engine out HC Emissions
1. Storage
2. In-cylinder post-flame oxidation
3. Residual gas retention
4. Exhaust oxidation
1.1.3.2 HC Sources
1. Quench Layers
Quenching contributes to only about 5-10% of total HC. However, bulk quenching or misfire due to operation under dilute or lean conditions can lead to high HC.
Quench layer thickness has been measured and found to be in the range of 0.05 to 0.4mm (thinnest at high load) when using propane as fuel.
Diffusion of HC from the quench layer into the burned gas and subsequent oxidation occurs, especially with smooth clean combustion chamber walls.
2. Crevices
These are narrow volumes present around the surface of the combustion chamber, having high surface-to-volume ratio into which flame will not propagate.
They are present between the piston crown and cylinder liner, along the gasket joints between cylinder head and block, along the seats of the intake and exhaust valves, space around the plug center electrode and between sparkplug threads.
During compression and combustion, these crevice volumes are filled with unburned charge. During expansion, a part of the UBHC-air mixture leaves the crevices and is oxidized by the hot burned gas mixture.
The final contribution of each crevice to the overall HC emissions depends on its volume and location relative to the spark plug and exhaust valve.
3. Lubricant Oil Layer
The presence of lubricating oil in the fuel or on the walls of the combustion chamber is known to result in an increase in exhaust HC levels.
The exhaust HC was primarily unreacted fuel and not oil or oil-derived compounds.
It has been proposed that fuel vapor absorption into and desorption from oil layer so nth walls of the combustion chamber could explain the presence of HC in the exhaust.
.4.Deposits
Deposit buildup on the combustion chamber walls (which occurs in vehicles over several thousand kilometers) is known to increase UBHC emissions.
Deposit buildup rates depend on fuel and operating conditions.
Olefin and aromatic compounds tend to have faster buildup than paraffinic compounds
5. Liquid Fuel and Mixture Preparation– Cold Start
The largest contribution (>90%) to HC emissions from the SI engine during a standard test occurs during the first minute of operation. This is due to the following reasons:
The catalytic converter is not yet warmed up
A substantially larger amount of fuel is injected than the Stochiometric proportion in order to guarantee prompts vaporization and starting.
6. Poor Combustion Quality
Flame extinction in the bulk gas before the flame front reaches the wall is a source of HC emissions under certain engine operating conditions.
1.2 MEASURES TO CONTROL EMISSIONS
1.2.1 Nontechnical control measures:
Good town planning
Congestion charges, area licensing scheme, parking charges, phasing out of vehicles, etc.
Proper traffic pattern & road networking (e.g. Flyovers)
Public awareness (energy efficient purchase, driving habits, car pooling)
Efficient public transport/transit system (Bus Rapid Transit)
Promote cycling/walking/Bicycle Lanes/2WheelerBridges
Proper maintenance (I&M program :Burari)
Prevent Adulteration of fuels
Living with plants.
Framing the specifications of fuel quality in line with emission legislation.
Traffic Management.
Public Awareness
RTO Checks
Supreme Court directive led to implementation of CNG/LPG program in Delhi, which faced high pollution levels, almost 70% from automobiles.
Delhi today boasts of the world’s largest CNG/LPG operated buses and 3 wheelers Fleet > 15,000 taking mini and RTVs together; Delhi breathes cleaner air today. Mumbai has also implemented a major drive to convert taxis and auto rickshaws to use of CNG and LPG.