This paper presents the investigation of engine optimisation when plastic pyrolysis oil (PPO) is used as the primary fuel of a direct injection diesel engine. Our previous investigation revealed that PPO is a promising fuel; however the results suggested that control parameters should be optimised in order to obtain a better engine performance. In the present work, the injection timing was advanced, and fuel additives were utilised to overcome the issues experienced in the previous work. In addition, spray characteristics of PPO were investigated in comparison with diesel to provide in-depth understanding of the engine behaviour. The experimental results on advanced injection timing (AIT) showed reduced brake thermal efficiency and increased carbon monoxide, unburned hydrocarbons, and nitrogen oxides emissions in comparison to standard injection timing. On the other hand, the addition of fuel additive resulted in higher engine efficiency and lower exhaust emissions. Finally, the spray tests revealed that the spray tip penetration for PPO is faster than diesel. The results suggested that AIT is not a preferable option while fuel additive is a promising solution for long-term use of PPO in diesel engines.
Human developments have been coupled with the evolution of the extraction, use, and disposal of natural resources. The way in which waste is disposed has changed dramatically over the last decades, as have attitudes towards waste reduction, reuse, and recycling, as well as recovering energy from waste. Energy from waste can be recovered through the pyrolysis process which converts the waste into oil and gas. Plastic is a type of waste that is plentiful and can be used effectively due to the high energy content. The conversion products can be used in internal combustion engines to produce power and heat. The effect of plastic pyrolysis oil (PPO) in diesel engines has been studied by various authors mainly in blends with diesel in single cylinder engines [
One of the most important engine parameters is the injection timing. The effect of injection timing (IT) in alternative fuels based on waste plastics has been studied in single cylinder diesel engines and the results are promising. Mani and Nagarajan [
On the other hand, fuel additives are preferable in the case of good quality oil that needs to boost a property such as cetane number or lubricity in order to improve the engine performance. Diethyl ether is an organic compound with high cetane number, which has been used in research as a cetane number improver. Devaraj et al. [
What has not been investigated yet in larger diesel engines is the advanced IT by using PPO in blends with diesel and the use of a fuel additive to improve the engine’s performance when running on PPO. Moreover, the spray characteristics from oil that derives from the pyrolysis of plastics have not been determined yet. Our previous investigation on the use of PPO in a four-cylinder diesel engine revealed that there is longer ignition delay and higher heat release rate (HRR) in comparison with diesel [
The conversion of the waste plastics into oil, gas, and char is taking place in the pyrolysis plant. More specifically, the plant consists of the primary and secondary chambers, where the plastics are purged with carbon dioxide to ensure that no oxygen is transferred into the next chamber which is the conversion chamber. The conversion chamber is maintained at a temperature of 900°C and the plastics are converted into gas and char. Finally, the gas is passed into a condenser, where it is cooled, and pyrolysis oil is separated out. The basic properties of PPO benchmarked with diesel and the test methods which were used to determine them are presented in Table
PPO and diesel properties.
Property | Method | PPO | Diesel |
---|---|---|---|
Density@15°C (kg/l) | ASTM D4052 | 0.9813 | 0.8398 |
Kinematic viscosity@40°C (cSt) | IP 71 | 1.918 | 2.62 |
Flash point (°C) | ASTM D93 | 13 | 59.5 |
Aromatic content (%) | IP 391 | 65.5 | 29.5 |
Acid number (mg KOH/g) | IP 139 | 41 | 0 |
LHV (MJ/kg) | ASTM D240 | 38.300 | 42.900 |
Water content (mg/kg) | ASTM D6304 | 1190 | 65 |
Ash content (wt.%) | IP 391 | 0.166 | <0.001 |
Carbon residue (wt.%) | ASTM D4530 | 4.83 | <0.01 |
Hydrogen content (wt.%) | ASTM D5291 | 8.5 | 13.38 |
Carbon content (wt.%) | ASTM D5291 | 87.9 | 86.57 |
Oxygen content (wt.%) | ASTM D5622 | 3.3 | 0.05 |
Sulphur content (wt.%) | ASTM D5453 | 0.155 | 0.0014 |
Nitrogen content (mg/kg) | ASTM D4629 | 820 | 44 |
Although the precise cetane number of the PPO is not provided in the table, it was clearly observed from our previous investigation that the combustion delay was considerably extended with higher PPO blending ratio, which suggests that PPO has lower cetane number than diesel [
Fuel additive composition.
Compound | Quantity (% wt) |
---|---|
Petroleum naphtha | 29–38 |
2-Ethylhexanol | 16–24.25 |
2-Ethylhexyl nitrate | 7.75–15.5 |
1,2,4-Trimethylbenzene | 7.75–15.5 |
1,3,5-Trimethylbenzene | 0.775–3.875 |
Propylene glycol ether | 0.775–3.875 |
Xylene | 0.8–4 |
Trimethylbenzene | 7.75–15.5 |
1,2,3-Trimethylbenzene | 0.8–4 |
Soy methyl ester | 5 |
The diesel engine that is used to conduct the experiments is a four-cylinder, direct injection, turbocharged water-cooled diesel engine. Figure
Test engine specifications.
Brand | AKSA |
---|---|
Model | A4CRX46TI |
Compression ratio | 17 : 1 |
Displacement | 4.58 l |
Rated power | 68 kW |
Rated speed | 1500 rpm |
Injection pressure | 240 bar |
Bore | 110 mm |
Stroke | 125 mm |
Schematic layout of the experimental setup.
The engine was started and run for 30 minutes on diesel to warm-up and stabilise the oil and coolant temperatures and then it was switched on the desired fuel blend and run for 5 minutes before the data acquisition was started. The flow-meter measurements, manifold pressure, temperatures, and exhaust emissions data were taken for a period of five minutes and the average values were calculated. As regards the combustion analysis, 100 consecutive cycles were acquired from the in-cylinder pressure sensor and the average was calculated. In addition, the heat release rate was calculated from (
The spray characteristics tests were carried out on a constant volume, high pressure chamber. The chamber was equipped with two windows on two sides in order to have optical access for the spray visualization. Moreover, the background pressure of the chamber was controlled at 5 bar. The diagram in Figure
Schematic layout of the spray test rig.
The fuel injection system was composed of a fuel tank, a fuel pump that was able to adjust the injection pressure up to 500 bar, an injector, and an electronic control unit (ECU). The signal from the ECU triggers the injector and the high speed camera, achieving synchronization of the spray visualization. The injector used in the experiments was a single-hole solenoid injector. Two different nozzle holes’ diameters of 0.12 mm and 0.18 mm were used in the experiments. Moreover, two different injection pressures of 300 bar and 450 bar were tested at every nozzle diameter size. The reason that the injection pressures were not tested at higher values is because the PPO is going to be used in diesel engines for stationary power and heat generation, which are usually equipped with mechanical injectors.
For the spray macroscopic characteristic investigation (spray tip penetration, spray cone angle, and spray area) a light source was used on one side and a high speed camera on the other. The high speed camera (Dantec Dynamics, Speedsence) was set up to record the spray images with an imaging speed of 60,000 frames per second and resolution of 256 × 256 pixels. In order to ensure the reliability of the results every experiment was repeated five times on each test condition. After that, the images were processed for further analysis of the spray characteristics. A program was written in MATLAB software, where a batch of spray images was able to get analysed at once and provide the spray tip penetration, spray cone angle, and spray area versus the time after the start of injection.
In this section are presented and discussed the experimental results obtained from the engine by running on advanced IT (AIT) with a blend of 75% plastic pyrolysis oil and 25% diesel (PPO 75) at 75% and 100% engine loads which represent 9.47 bar and 12.63 bar of BMEP, respectively. The results of AIT (−23°CA bTDC) are compared with the standard IT (SIT) operation (−18°CA bTDC). The investigation is focused on the combustion characteristics, engine performance, and exhaust emission analysis. Moreover, the spray characteristics of PPO are analysed and compared with diesel.
Figure
Variation of cylinder pressure with crank angle at 75% load.
The heat release rate (HRR) for the diesel and PPO 75 at SIT and AIT is presented in Figure
Variation of heat release rate with crank angle at 75% load.
Figure
Brake thermal efficiency for diesel, PPO 75 SIT, and PPO 75 AIT.
Figure
Normalized values of CO, UHC, and NOx emissions.
It can be noticed from Figure
Figure
Variation of spray tip penetration with time for nozzle 0.12 mm (a) and 0.18 mm (b).
In this section are presented the experimental results obtained from the engine by running on PPO 75 blended with a commercial fuel additive at two different ratios of 1 : 80 and 1 : 40. The composition of the fuel additive is presented in Table
Figure
Variation of cylinder pressure with crank angle at 85% load.
The HRR for diesel and PPO 75 fuel additive blends at 85% load is presented in Figure
Variation of heat release rate with crank angle at 85% load.
Figure
Variation of BTE at 85% load.
Figure
Normalized values of CO, UHC, and NOx emissions.
An experimental investigation was carried out to analyse and understand the combustion, performance, and emission characteristics of a diesel engine running on advanced injection timing and on standard injection timing with the addition of a fuel additive on oil which derives from the pyrolysis of waste plastics. The following conclusions can be drawn from the test results: The engine was able to operate at AIT on PPO 75 at 75% load but with longer ignition delay, higher cylinder peak pressure, and higher heat release rate in comparison with the PPO 75 SIT operation. The addition of the fuel additive reduces the ignition delay period, cylinder peak pressure, and peak heat release rate. As a result, the brake thermal efficiency, CO, UHC, and NOx emissions are all improved. The engine’s thermal efficiency decreases with the AIT and all measured emissions, including CO, UHC, and NOx, increase with AIT. The spray test revealed that the PPO spray has a longer tip penetration, which explained why AIT is not a preferable solution.
The testing results suggest that for both long-term and short-term operation, the AIT is not preferable as the engine performance declines. As regards to the fuel additive engine testing, the results suggest that the use of a dedicated cetane number fuel additive would achieve even better combustion performance (similar to diesel).
75% plastic pyrolysis oil + 25% diesel fuel
Standard injection timing
Advanced injection timing
Before top dead centre
Nitrogen oxides
Particulate matter
Carbon monoxide
Carbon dioxide
Unburned hydrocarbon
Lower heating value
Brake mean effective pressure
Heat release rate
Equivalence ratio
Brake thermal efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors sincerely thank Thermitech Solutions Ltd and UK Engineering and Physical Sciences Research Council (EP/K036548/2) for the support to conduct this research.