The use of rapeseed oil (RSO) in diesel engine results in a significant reduction of NOx emissions but the soot that is emitted from the combustion of RSO are several orders of magnitude higher compared to that of diesel. In this study an attempt has been made to reduce the soot that is emitted from the combustion of RSO in diesel engine to take advantage of their lower NOx emissions. This was achieved by using blends of RSO, and the soot emission from the blend of 30% RSO was reduced to diesel equivalent levels of soot by varying the fuel injection parameters. By operating RSO blend under diesel equivalent levels of soot a further reduction in NOx emission was achieved.
In this investigation a 4 cylinder, 16 valve high speed direct injection diesel engine was used. The engine is fully instrumented with sensors and devices which enables us to control the engine torque, speed, injection parameters, in cylinder pressure, emissions and fuel consumption. Detailed schematic of the experimental setup is shown in figure 1. The engine control software allows to control and change engine parameters such as injection pressure, injection timing and EGR rate on a real time basis. The in-cylinder pressures were measured using a Kistler pressure transducer and the LabView software. The gaseous emissions (CO, THC, NOx) were measured using a HoribaMexa gas analyzer. The smoke number was measured using an AVL smoke meter and the fuel consumption of diesel fuel was measured using an AVL fuel consumption meter, however the fuel consumption of RSO was measured using a burette metering management.
As can be seen in figure 1 the engine fuel system is modified to use RSO. Modifications include additional tank, couple of 2-way valves and in-line heater/cooler. A temperature controller is used to regulate the temperature of the heater/cooler to maintain uniform temperature of the fuel. The temperature of the RSO was measured at two points in the fuel pipe line. The temperature T2 was measured at the upstream of the injector and it was maintained at 70 °C during all runs for RSO. To avoid problems with cold flow and cold start, the engine was started with diesel and then switched to RSO and similarly before shutting down, the engine was switched back to diesel fuel.
The results of soot and NOx emissions for pure
RSO and their blends in diesel fuel are described
in [
RSO NOx RSO soot
Diesel NOx Diesel soot 1200 1000
800 m p p, 600 x O400 N 200 0 0
20 40 60 80
Percentage of RSO in blend
100
Fig. 2: The soot and NOx emissions for RSO, their
blends and diesel (summarized from [
It can be seen that the emissions of NOx decreases with increasing concentration of RSO in the blend and on the contrary the smoke number increase with higher concentration of RSO in the blend. By using a blend of 50% RSO in diesel the 0.8 0.6 0.4 0.2 0 r e b m u n e k o m S
NOx emissions are lowered by 18% and the soot emissions increases by 355% compared to that of diesel. Similarly by using a blend of 30% RSO in diesel the NOx emissions are lowered by 12% and the soot emissions increases by 122% compared to that of diesel.
In order to utilize the advantage of lower NOx emissions of 50% and 30% blends of RSO, different fuel injection strategies were tried to reduce their soot levels to that of diesel equivalent soot when the engine is operated under standard engine conditions (injection pressure of 800bar, injection timing of 9deg bTDC, 0% of EGR, engine speed of 2000 rpm and engine load of 2.7 bar BMEP).
Due to the limitations of the experimental operating conditions diesel equivalent soot was not achieved for 50% RSO but it was possible to reduce the soot from the blend of 30% RSO to diesel equivalent soot as shown in figure 3.
Diesel 800 bar 30% RSO 800 bar 30% RSO 1100 bar 30% RSO 1200 bar 30% RSO 1000 bar 30% RSO 1200 bar 0.24 diesel level injection timing 800-1200 bar retarded injection timing 0.22 0.2 0.18 0.16 0.14 0.12 0.1 r e b m u n e k o m S 12 10 8 6 4 2 Injection timing, CAD bTDC 0
Diesel equivalent level of soot emissions for 30% RSO was achieved by increasing the injection pressure from 800 bar to 1200 bar and by retarding injection timing from 9 deg bTDC to 3 deg bTDC. As could be seen in figure 4, column A corresponds to standard engine operating conditions and column B corresponds to operating conditions for diesel equivalent soot for 30% RSO (at 1200 bar and 3 deg bTDC, 0% EGR, 2000rpm, 2.7 bar BMEP). Higher injection pressure causes better atomization and formation of smaller fuel droplets and at the same time higher entrainment of warm air on to the liquid core enhances vaporization of fuel droplets, which effectively lowers soot emissions. % ,% 5 250 ,C FSBC-5 200 ,THO ,x 150 ,C inNO-15 100 inSN eg-25 50 eg an an h-35 0 h C C Fig. 4: Percentage change in exhaust emissions for 30% RSO: Column A (with respect to diesel fuel under standard engine operating conditions) and Column B (with respect to operating conditions at diesel equivalent soot levels).
By retarding the injection timing the combustion is shifted towards the expansion stroke and the peak cylinder pressures are lower, which eventually reduce the global in-cylinder temperature and thereby by lowering the soot formation rates. Under the conditions of diesel equivalent soot the NOx emissions are further reduced by an order of 22 % compared to diesel fuel under standard engine operating conditions. Generally NOx emissions decrease with retarded injection timing (lower in cylinder pressure and temperature) on the contrary an increase in the injection pressure leads to better atomization, better entrainment, higher in-cylinder pressure, higher in-cylinder temperature and higher NOx. In this case the effect of retarded injection timing overrides the contributions from increased injection pressure which eventually results in an efficient reduction of NOx emissions. The other gaseous emissions such as CO and THC measured under diesel equivalent soot conditions are still higher compared to that of diesel under standard engine operating conditions, however these emissions can be easily oxidized by using an oxidizing catalyst.
Other interesting combustion characteristics such as ignition delay, heat release, combustion duration, premixed and diffusion phase combustion under the standard and diesel equivalent soot levels, and their contributions to NOx and soot emissions will be presented.
In this work the advantage of lower NOx emissions for 30% blend of RSO in diesel was further explored to reduce their soot emissions to diesel equivalent levels through different fuel injection strategies. The main findings are summarized as: by using pure RSO and their blends the NOx emissions are lowered compared to that of diesel fuel. diesel equivalent level of soot emissions for 30% blend of RSO were achieved by simultaneously retardation the injection timing and by increase the injection pressure. reduction of NOx emissions by about 22% was achieved for 30% blend of RSO under the operating conditions of diesel equivalent soot. 30% RSO blend can be successfully used in DI diesel engine with low levels of NOx emissions the same and even lower soot emissions compared to that of diesel.