Comparison of Real World Emissions from B20 and ULSD fuels for Transit buses

Vinay Kumar V NerellaDepartment of Civil Engineering, University of Toledo, Toledo, OH

Objective

Abstract

This study is conducted to compare the real-world on-road vehicular emissions of ultra low sulfur diesel (ULSD) fuel with B20 biodiesel (80% ULSD fuel and 20% methyl ester biofuel) fuel for Toledo Area Regional Transit (TARTA) buses. The tailpipe emissions were collected during their normal duty cycle using a Portable Emission Measurement System (PEMS). The pollutants that are measured in this study are carbon monoxide, nitrogen oxide, sulfur dioxide, nitrogen dioxide, and carbon dioxide. A total of 12 buses with 6 buses running on ULSD fuel and 6 running on B20 biodiesel fuel were tested and the testing was carried out for one day on B20 biodiesel bus and for one day on ULSD bus. The average pollutant concentrations of both the fuels were measured and analyzed. The test results show that B20 fueled buses emitted a lower concentration of all the monitored pollutants expect for CO2, which showed a 3% increase as compared to ULSD fueled buses. NO concentrations were observed to reduce by 7%. SO2 and NO2 for B20 fuel showed nearly 9% and 4% reductions respectively. NOX concentration was observed to reduce by 3% for B20 fueled buses.

• The main objective of this study is to compare the real-world on-road exhaust emission variation of transit buses, running on ULSD and B20 biodiesel fuels, during their regular service with passengers on-board.

Introduction

Experimental Design

BusesTail pipe emissions depend on the vehicle characteristics like vehicle make, model, engine type and size. Toledo Area Regional Transit Authority (TARTA) runs over 180 buses with nine different fleets. TARTA has characterized its buses into different fleets based on the engine type and the year of manufacture. Different types of fleets available are 100, 200, 300, 400, 500, 600, 700, 900, and 1000. Among these fleets only two fleets (300 and 500 series) of buses are running on both B20 and ULSD fuels. The 12 buses tested in this study are selected from 500 fleet buses. These 500 series buses contain MBE900 Mercedes Benz engine and they are medium duty buses.All the buses were allowed to run on the selected fuel for more than 6 months in order to avoid any discrepancies that can arise because of change in fuel. To which bus is to be tested will be decided one day before the testing date and the instrument will be installed on the day of testing, around 30 minutes prior to the scheduled bus time, in the TARTA garage. For each of the 12 buses, the bus number, and mileage are listed in Table 1.InstrumentationThe portable emission measurement system (PEMS) used for collecting the emission data is Testo350XL. It measures the exhaust emissions with six gas sensors: O2, CO2, CO, SO2, NO and NO2 with calculated NOX, and a temperature sensor. The set-up was installed on the rear seats of the bus powered from the bus electrical system. The instrument measures the exhaust concentrations from tail pipe by clamping the probe of the instrument to the tail pipe. The instrument was set-up for 1 minute concentration measurement and was connected to lap-top to download the data simultaneously. The collected data is transferred to the Excel sheet. Continuous power supply to the instrument and to the laptop has to be maintained in order to avoid discrepancies during the time of data collection. The power was supplied to the instruments from the vehicle power control panel present at the rear of the bus.Routes and testingReal time emission data is collected when the selected bus goes out on its regular specified route. Route number 20 is selected for this study as it was convenient for the test protocol. The total testing time for this study was 15 minutes starting from TARTA garage and ending at Franklin mall. On the day of testing the equipment was installed on the bus in the garage. It takes around 15-20 minutes for installing and removal of the set-up from the bus.The testing is carried out in such a way that one bus was tested for each day and the driver remained same for the complete study. Everyday the testing used to start at 10:31 A.M. at TARTA garage and end at Franklin Mall Park at 10:46 A.M.

Results and Discussion

Out of the twelve buses selected for testing, problem has occurred during data collection for two buses, one with B20 fuel and one with ULSD fuel. Hence the comparison of emissions has been conducted for ten buses, five with B20 fuel and five with ULSD fuel. Both the maintenance staff and the drivers were very cooperative but occasionally passengers were concerned about the safety.On-road emissions were monitored for around 15 minutes (first 15 minutes of the route) for all 10 buses (5 B20 + 5 ULSD fuels) from 500 series. Comparisons of real-world on-road emission concentrations are provided in the Figure 1. The total 15 minute average concentration of each pollutant was measured for all the buses and the comparison was done individually for each pollutant.

•The average CO emissions from the buses fueled with B20 were observed to be reduced. The high oxygen content in biodiesel improves the fuel burning characteristics and thus encouraging for complete combustion. •The CO2 emissions were observed to be high for B20 fueled buses. But, if the complete life cycle of the biodiesel was considered, these higher CO2 emissions were observed to be balanced. Bus # 528 showed the highest CO2 concentration, the ambient temperature at the time of testing was 79oF.•NO concentrations were higher for all the ULSD fueled buses except for bus # 527. Among the five pollutants measured, the emission concentration of nitrogen oxide was observed to be higher.•NO2 concentrations were higher for all the ULSD fueled buses with bus # 525 having the highest concentration level.•SO2 concentrations were higher for all the ULSD fueled buses with bus # 525 & 528 having highest concentration levels. The amount of SO2 in emissions depends on the sulfur content present in the fuel. As biodiesel is free from sulfur, the SO2 emissions from the buses fueled with B20 can be lower than the buses fueled with ULSD fuel.

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Conclsion

Summary

The test results show that B20 fueled buses emitted a lower concentration of all the monitored pollutants expect for CO2, which showed a 3% increase as compared to ULSD fueled buses. NO and SO2 concentrations were observed to reduce by 7% and 9% respectively. NO2 emission concentration does not seem to vary much for both the fuels, but B20 fuel showed 4% reduction. NOX concentration was observed to reduce by 3% for B20 fueled buses. CO emissions from the buses with B20 fuel were reduced by 19%.The exhaust emission study on transit buses analyze the variation of fuels running on both B20 and ULSD fuels, during their regular service with passengers on board. This study suggested that a large number of vehicles are to be tested and engine performance characteristics have to be considered in order to characterize and understand the emissions as the real-world driving conditions cannot be controlled. But obtaining engine characteristics using on-board diagnostic software, when the bus is in its regular service with passengers on-board, creates problem for driver. This is because the software needs a connection, using a cable, from a place below the steering of the bus which can make both driver and the passengers feel inconvenient. The emission comparison showed that the buses running on B20 biodiesel fuel produce lower emission concentrations of CO, NO, SO2, NO2, and NOX. But the concentration of CO2 was observed to increase for the buses operated on B20 biodiesel fuel.

Disclaimer and Acknowledgements

This is a biodiesel project funded by Toledo Area Regional Transit Authority (TARTA).

Conventional diesel engines produce high concentrations of particulate matter and nitrogen oxide (NOX) emissions.1 Most of the transit buses run with conventional diesel engines in over populated and polluted areas. The concern towards cleaner environment has lead to implement strict emission standards to reduce vehicular exhaust emissions in recent years. Several strategies like engine improvements and installation of aftertreatment devices, the use of alternative fuels to replace conventional fuels is considered to be an effective measure to meet the strict emission standards.2 In order to meet these strict emission standards many transit authorities have decide to operate their buses on alternative fuels like ultra-low sulfur diesel fuel (15 ppm sulfur) and bio-diesel blended fuels. This makes the characterization of fuel emissions an important study to develop a methodology to compare the exhaust emissions of different fuels. The interest in using bio-diesel as a substitute for petroleum based diesel fuel has been increased in recent years. It can be produced from renewable sources like vegetable oils or animal fats which reduces the nation’s dependence on foreign oil. This study is conducted to analyze the variation of exhaust emissions from TARTA (Toledo Area Regional Transit Authority) buses running with two different fuels: Ultra-low-sulfur diesel fuel (ULSD); and B20 (a mixture of 80% ULSD fuel and 20 % methyl ester biofuel) biodiesel fuel. TARTA runs over 180 buses everyday becoming a potential source to contribute exhaust emissions in the region of Toledo, OH. Understanding the importance of emission reduction TARTA has adopted the use of alternative fuels including B20 and ULSD in June 2006.