Experimental investigation of CI engine performance, emissions and combustion using advanced biofuels

There is an ongoing interest in developing new alternative fuels (such as biofuel) for both aviation and road transport sectors to meet increasing energy demand and assist in reducing greenhouse gas (GHG) emissions. The major contribution of this work is to develop an aviation biofuel from a new feedstock and create the best possible biodiesel-diesel blends for the transport sector. This study focuses on improving engine performance and reducing emissions by enhancing combustion efficiency using these newly developed fuels without any modification of the modern engine. The combustion and emissions were closely monitored to evaluate the pollutants formation in a compression ignition (CI) engine. Better performing fuels were identified and their tribological behaviour was also studied to assess their impact on engine life.

A wide range of biofuel feedstocks (over 150 species) was initially investigated to identify the most prospective feedstocks for producing biodiesels. The study eventually identified six prospective feedstocks namely Mandarin peel waste, Crambe, Tamanu, Borage, Waste Avocado flesh and Bush nut for biofuel production. The biofuels were produced in the laboratory from these selected feedstocks. The fatty acid methyl esters (FAMEs) composition and physio–chemical properties of these newly produced biofuels were evaluated using ASTM and EN standards.

The fuel properties of these biodiesels revealed that the properties of the Mandarin biofuel closely fit with the properties of commercial jet fuel with a calorific value of 44.66 MJ/kg (4.3% higher than commercial jet fuel) and a higher flash point of 52 °C. This biofuel has a lower viscosity (about 2.13 mm2/s at minus 20 degree C.) which is desirable and is self–oxygenated and sulphur free. Therefore, it is seen as a prospective new source of aviation biofuel production which is a new finding. This has not been studied earlier.

As an aviation engine was not available, Mandarin aviation biofuel was tested in a lean diesel engine and showed excellent performance and a large reduction in engine emissions. It can achieve reductions of up to 30.0% CO, about 33.5% HC and around 19.2% PM (particulate matter) at full load with variable speed and 33.0% CO, 32.8% HC, 28.5% PM emission reduction at variable load as compared to ultra – low – sulphur diesel (ULSD) by blending 20% with fossil fuel.

Other biodiesel (Crambe, Tamanu, Borage, Avocado, Bush nut) blends (B5 to B20) were also tested in a four stroke diesel engine to evaluate the performance and emission parameters at different operating and load conditions. The results revealed that Avocado biodiesel shows overall better performance (about 0.50% less BP, 0.83% more BSFC, and 0.18% less BTE as compared to ULSD at full load and rated speed) compared to other fuels. However, Crambe, Borage, and Bush nut also show close performance with Avocado biodiesel. Blending up to 20% of this biodiesel can reduce emissions by up to about 50% CO, 27% HC and 36% PM, however it increases NOx emission by about 26% compared to ULSD at full load and rated speed. On the other hand, Tamanu biodiesel blends show poor engine performance though emission reduction is comparable with other biodiesels at the same operating conditions.

For further improvement in engine performance and emission reduction this study developed four mixture blends by combining two biodiesels (totalling 5% at different proportions) and paraffin as an additive at 4% with the remaining 91% being ULSD. The mixture blends are described as ManCr_Pa (Mandarin-Crambe_Paraffin), TaMan_Pa (Tamanu-Mandarin_Paraffin), BoMan_Pa (Borage-Mandarin_Paraffin) and AvBn_Pa (Avocado-Bush nut_Paraffin). The mixture blends show improved performance compared to each B5 blend and significantly reduce emissions like B20 blends due to their improved fuel properties. Among these mixture blends, the Avocado-Bush nut and paraffin (AvBn_Pa) ternary mixture demonstrates comparable performance with ULSD. It reduces about 48.0% CO, 30.0% HC, 40.0% PM emissions compared to ULSD. This equates to about 16.0% CO, 8.7% HC and 28.0% PM more reduction of emissions compared to an Avocado B5 blend. This mixture blend produces about 9% less NOx compared to the B5 blend of Avocado biodiesel. On the other hand, the ManCr_Pa mixture blend reduces about 62% HC emission compared to ULSD with about 12% lower NOx emission.

The advanced combustion analysis was done on the better performing blends (i.e. for ManCr_Pa and AvBn_Pa mixture blends) to evaluate pollutant formation mechanisms during combustion. The results revealed shorter ignition delay and longer combustion duration for AvBn_Pa. This blend also exhibits higher cylinder pressure and higher heat release rate with a longer duration of the diffusion combustion phase. Additionally, a knocking characteristic was identified for ManCr_Pa mixture blend. The tribological characteristics such as friction, wear, lubrication stability and metal surface morphology were also evaluated using high-resolution SEM/EDX microscopy to assess energy savings, engine reliability, and impacts on engine life.

This study revealed an excellent tribological performance of AvBn_Pa blend compared to ULSD with about 21% less friction coefficient at steady state condition, around 19% less wear scar diameter, higher lubrication film stability, as well as less wear debris and metal corrosion. The study concluded that AvBn_Pa blend is the best mixture blend in all aspects of performance considered, namely emission reduction, improved combustion and tribological behaviour for a sustainable environment as well as sustainable engine health for the transport sector.

The study will provide useful information and guidelines to biofuel stakeholders, the transport sector, engine designers, the aviation industry and policy makers involved with newly developed aviation biofuels and other biodiesel usage in a full-scale diesel engine. It will provide new opportunities to future researchers to develop Mandarin aviation biofuel as a commercial aviation fuel. This research will help engine designers to develop more efficient and sustainable engines and to customise newly developed biodiesels for application in the transport sector.