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Iowa Department of Natural Resources Biofuel Blend Testing Project

Executive Summary

Biodiesels are often marketed as being cleaner than regular diesel for emissions. Emission test results depend on the biodiesel blend, but laboratory tests suggest that biodiesels decrease particulate matter, carbon monoxide, hydrocarbons, and air toxins when compared to regular diesel. Results for oxides of nitrogen (NOx) have been less conclusive.

Tests have not evaluated the commonly available ranges of biodiesel blends in the laboratory. Additionally, little information is available from on-road studies, so the effectiveness of using biodiesels to reduce actual emissions is unknown. A more complex relationship exists between engine operation and the rate of emission production than is typically evaluated using engine or chassis dynamometer tests. On-road emissions can vary dramatically since emissions are correlated to engine mode and activity such as idling, acceleration, deceleration, and operation against a grade produce higher emissions than more stable engine operating modes. Since these modes are not well captured in a laboratory environment, understanding on-road relationships is critical in evaluating the emissions reductions that may be possible with biodiesels. More tests and quantifications of the effects of different blends on engine and vehicle performance are required to promote widespread use of biodiesel.

The objective of this research was to conduct on-road and laboratory tests to compare the emission impacts of different blends of biodiesel to regular diesel fuel under different operating conditions. The team conducted engine dynamometer tests as well as on-road tests that utilized a portable emissions monitoring system that was used to instrument transit buses. Regular diesel and different blends of biodiesel were evaluated during on-road engine operation by instrumenting three in-use transit buses, from the CyRide system in Ames, Iowa, along existing transit routes. Evaluation of transit buses was selected for this study rather than heavy-duty trucks because transit buses have a regular route. This way, emissions for each of the biodiesel blends can be compared across the same operating conditions.

Summary for On-road Tests

The three different types of diesel and biodiesel were evaluated in three in-service transit buses using a portable emissions monitor (B-0, B-10, and B-20). Two buses, Bus 973 and 971, fall into the 1998-2003 diesel engine emissions standard time frame.  Data were collected for the two buses during spring-like conditions (April and May 2008 with cooler temperatures).  The third bus, Bus 997, falls into the 2004-2006 diesel engine emissions standard time frame and data was collected during summer conditions (June and July 2008 with hot and humid conditions and regular air conditioning  use). 

Simple comparison of the three fuels for each pollutant of interest for each bus were made by mode (idle, steady state, acceleration, deceleration) and speed range.  Averages are in g/s. Results for Bus 973 indicates that average NOx emissions were generally lower for B-10 than for B-0 but higher for B-20.  Mixed results were found for Bus 971 with NOx emissions higher for some speed ranges and modes for B-10 and B-20 than for B-0 but emissions were lower in some cases.  Average NOx emissions were usually higher for both B-10 and B-20 than for B-0 for all modes and speed ranges for Bus 997.

Average HC emissions were lower for B-10 and B-20 than for Bus 973 for all modes and speed ranges and for Bus 971 except HC emissions during deceleration.  HC emissions for B-20 were lower for Bus 997 than for B-0 but HC emissions for B-10 were higher than for B-0. Carbon monoxide emissions were lower for both B-10 and B-20 than for B-0 for all modes and speed ranges for Bus 973 and 997.  However, while B-10 CO emissions were lower than B-0, B-20 emissions were higher than B-0 for Bus 971. 

Results for carbon dioxide were mixed for Bus 973.  Average CO2 emissions were similar or slightly higher for both biodiesel blends than for regular diesel for idling, steady state, and deceleration while they were slightly lower in most cases for acceleration.  CO2 emissions were generally lower for B-10 than for B-0 but were higher for B-20 for idling, steady state, and acceleration while results were mixed for deceleration for Bus 971.  CO2 emissions were similar for Bus 997 as for Bus 973 with similar or slightly higher average emissions for B-10 and B-20 than for B-0 during idling, steady state, and deceleration while results were inconclusive for deceleration.  PM emissions were much higher for B-10 than for B-0 for Bus 973 and Bus 997 for all modes and speed ranges while B-20 PM emissions were similar or slightly higher.  For Bus 971, the two biodiesel blends resulted in significantly lower PM emissions than B-0 for all modes and speed ranges. 

A summary of the results of the statistical model are presented below.  As shown, emissions by Bus by fuel types, pollutant, and mode are presented.  Evidence of difference in emissions means (g/s) was found for all the buses for all the studied pollutants for almost all the compared fuel types and the different driving modes.  However, in some cases differences in estimated means were small.  Number 1 represents the highest estimated mean emissions.  In most cases the results were statistically significant.  So for instance, B-10 had the highest mean NOx emissions (g/s) for Bus 971.  In all cases emissions were highest while the bus was in acceleration mode.

 

Summary of Model Results by Bus and Pollutant for Fuel and Mode

 

 

NOx

HC

CO

CO2

PM

Bus

Ranking

Fuel

Mode

Fuel

Mode

Fuel

Mode

Fuel

Mode

Fuel

Mode

 

1

 B10

Accel 

B0 

Accel  

 B20

Accel 

 B20

Accel 

 B0

Accel  

971

2

 B0

Steady 

 B20

Steady 

 B0

 Steady

 B0

Steady 

 B10

Steady 

 

3

 B20

  Idle

 B10

 Decel

 B10

  Idle

 B10

 Idle 

 B20

 Decel

 

4

 

 Decel

 

 Idle

 

 Decel

 

 Decel

 

 Idle

 

1

 B20

Accel 

B0 

Accel  

 B20

Accel 

 B20

Accel 

 B10

Accel  

973

2

 B0

 Steady

 B10

Steady 

 B0

 Steady

 B0

 Steady

 B20

 Steady

 

3

 B10

  Idle

 B20

 Decel

 B10

  Idle

 B10

  Idle

 B0

 Decel

 

4

 

 Decel

 

 Idle

 

 Decel

 

 Decel

 

 Idle

 

1

 B20

Accel 

B10 

Accel  

 B0

Accel 

 B10

Accel 

 B10

Accel  

977

2

 B0

 Steady

 B0

 Steady

 B10

 Steady

 B20

Steady 

 B20

Steady 

 

3

 B10

  Idle

 B20

 Decel

 B20

  Idle

 B0

 Idle 

 B0

 Idle

 

4

 

 Decel

 

 Idle

 

 Decel

 

 Decel

 

 Decel

 

Results of the descriptive statistics and statistical modeling are fairly consistent.  NOx, HC, CO, emissions for results are generally consistent with what has been reported for biodiesels.  PM emissions were much lower for one bus for B-10 and B-20 which is consistent with other studies but for the other two buses, PM emissions for biodiesels were either higher or similar to those for regular diesel.

Summary for Laboratory Tests

The effects of biodiesel blends on engine performance and exhaust emissions were investigated and verified by the laboratory engine testing. Various engine load conditions that are representative of the operation of the present engine class were tested. Results indicate that increases in NOx and decrease in soot, CO, and HC emissions are obtained by using biodiesel blends.  Engine test results show that the increased NOx emissions using B-10 and B-20 are approximately the same for the three load conditions studied. In general, soot emissions were reduced by using B-10 and B-20. However, soot emissions are approximately the same for three fuels at the 1,200 rpm light load condition, under which the soot emissions are already relatively low and it is hard to distinguish among them. The CO emissions decrease as the biodiesel contents increase. However, a clear trend of declining HC emissions was not observed with increased biodiesel contents. Both B-10 and B-20 produced lower HC emissions than B-0, but B-20 produced higher HC emissions than B-10.

In general, the trends of increasing NOx emissions and decreasing soot, CO, and HC emissions are obtained by using biodiesel blends. There are only a few operating points for which a clear trend is not observed. Although certain trends may be expected, it should be noted that the emission results for various biodiesel blends may vary due to differences in specific engine operating conditions and fuel properties. This study followed the test protocol described above, and the results obtained were consistent throughout the testing. The general effects of biodiesel on engine performance have been observed.

 

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