This section of Biofuels for Transport provides details of previous work that has been done on the comparison of regular fossil diesel to biodiesel. Mostly this work has focused on tailpipe emissions as opposed to engine mechanical performance and this is reflected in this section.

There are a number of ways in which a comparison between conventional road fuels and bio-road fuels can be made. A direct comparison can be made on the basis of the fuels’ calorific value. However, it is commonly felt that in some situations it is more convenient and appropriate to make comparisons on the basis of the distance travelled by road vehicles using each fuel type. However, the disadvantage of this method is that it depends on the relative performance of road vehicles using biodiesel and fossil diesel.

Most of the research that has been carried out on engine performance with alternative fuels has been concerned with the resultant tailpipe emissions. The varying factors that come into effect in these instances are type of road vehicle and driving conditions (urban, motorway, rural, etc.). These factors are subsequently specified as standardised tests so that the results produced are meaningful, the test conditions can be easily reproduced, and the test results are comparable. As performance varies with engine design and vehicle technology, it is widely accepted that comparative results are likely to vary over time.

A number of studies have been conducted investigating the comparison of tailpipe emissions from conventional diesel and biodiesel. The test results shown below are from a study conducted by G.S Hitchcock et al. in 1998 in the UK. The comparison generally shows how using biodiesel marginally reduces carbon monoxide (CO), hydrocarbon (HC) and particulate matter (PM) emissions. Net carbon dioxide (CO2) is effectively reduced to zero, as are sulphur dioxide (SO2) emissions. However, by using biodiesel, nitrogen oxide (NOx) emissions are slightly higher. In this study, it was not very explicit about the type of diesel but by the figures it was judged to have been Low Sulphur Diesel (LSD) that was used. If this was the case it could be assumed that such a marked reduction in sulphur dioxide emissions shown here, would be less due to the widespread use of Ultra Low Sulphur Diesel (ULSD) in the UK today.

Sample Of Tailpipe Emissions From Road Vehicles Using Biodiesel And Conventional Diesel For The UK (G.S Hitchcock et al. 1998)

Another study was conducted by M. Kaltschmitt et al. in Germany in 1997 for a car using biodiesel and conventional diesel, the results of which are shown below. For this study it was Low Sulphur diesel that was used and the results show similar levels of CO, HC, NOx and N20 emissions for both conventional diesel and biodiesel. For particulate matter, biodiesel emits marginally less and for SO2 there is a larger decrease. As before, net CO2 is reduced to zero by using biodiesel.

Sample of tailpipe emissions from road vehicles using biodiesel and conventional diesel for Germany (M. Kaltschmitt et al. 1997)

In both the UK and German studies it is Low Sulphur Diesel that is used and so this makes them fairly out-of-date, as ULSD is now the main DERV fuel in use in the UK. However, a study was conducted in Australia by Beer et al.which aimed to compare biodiesel and ULSD by adjusting results from a variety of tests and other studies of alterative road transport fuels. These results are shown in Table 3 and judging from the results it is evident that for buses using biodiesel; CO, HC, NOx and PM are actually higher compared with ULSD. SO2 emissions are not provided and net CO2 is again nil.

Sample Of Tailpipe Emissions From Road Vehicles Using Biodiesel And ULSD For Australia (T. Beer et al. 2002)

In the same study by Beer et al., substantial variations were experienced in the tailpipe emissions of the same type of vehicle under the same test conditions (see table below). An offered explanation of this variation is that apart from CO2, it is only trace amounts of pollutants being measured.

Sample Of Variability Of Tailpipe Emissions In Road Vehicle Tests (T. Beer et al. 2002)

Even though it is only trace amounts of pollutants being produced (with the exception of CO2), it is known that PM emissions have a link with respiratory diseases in humans. This is where the suggestion arises that biodiesel could be put to good use in sensitive areas such city centres or just to maintain a pristine environment such as in national parks. This is possibly one of the reasons why some countries have adopted biodiesel use for buses and taxis. Another advantage over conventional diesel, which these tests results don’t show, is the fact that biodiesel is non-toxic and biodegradable.

In extensive tests comparing biodiesel with conventional diesel, bus trials were conducted in Austria by Graz University (T. Sams et al). In the test, Low Sulphur Diesel was used as the conventional fuel and the test took place over three years on two city buses. In the table below it can be seen that emissions of CO are 20% lower than conventional diesel. Tailpipe emissions of SOx are reduced by almost 100% while particulate matter is reduced by almost 40% by using biodiesel. VOC’s are reduced by approximately a third. Emissions of NOx from biodiesel can be reduced substantially by altering the timing of injection. T. Sams et al. stated that, with reference to the buses used in their study, NOx emissions could be reduced 23% compared with fossil DERV by advancing injection timing.

Sample Of Emissions From Austrian Bus Trials Relative To Low Sulphur Fossil Diesel (T. Sams et al. 1996)

In the US, tests were carried out by the Southwest Research Institute (Sharp) in 1997 using a 5.9L Cummins pick-up truck as its chosen vehicle. In this study a blend of fossil diesel (20%) and biodiesel (80%) was used. It was found that by just using the blended biodiesel, substantial decreases in the trace substances can be attained. VOC emissions fell by nearly 30%, benzene decreased by 78%, PAH’s dropped 35% and butadiene decreased 85%.

It is difficult to make assumptions about the effects of long-term use of biodiesel, as there is only limited experience of its use. Beer et al. and Hitchcock et al. both reported problems of the softening or failure of rubber engine components but this trouble can be avoided by the replacing of selective components with more compatible materials.

From these previous studies of the topic of biodiesel performance versus conventional diesel performance, it is still difficult to come to any definite conclusion. However, generally speaking almost all the previous studies describe how biodiesel does reduce the majority of its tailpipe emissions when compared with conventional diesel. The problem we found with the previous studies was the variation in their results. This was unsurprising as each tests used a different form of transport. In fact, it would be difficult to come up with a more diverse set of vehicle types, engine design, technical modifications and driving conditions for the comparing of biodiesel with fossil diesel. We feel that this is directly responsible for the lack of clarity on the subject of biodiesel versus fossil diesel and it will be hard to make the issue any more transparent by offering more test results completed under new conditions and circumstances. However, it is feasible to say that every study returned similar results as far as biodiesel claiming carbon neutrality. This is possible because the fuel crop grown to make the biodiesel absorbs CO2 and balances the CO2 produced when the biodiesel is eventually burned.

The term ‘carbon neutral’ is a misnomer in this situation as fossil fuels are consumed in the production of biodiesel so this does upset the carbon balance. It is possible to measure this discrepancy in the balance with regards to its comparison with conventional diesel but it is necessary to use life cycle assessment.