Study on the effects of nitro-based fuel additive on diesel engine performance and emissions

. Through adding nitro-based fuel additive into diesel fuel,the influences of additive on engine performance, gaseous emission


Introduction
In order to satisfy the increasing stringent legislation, the emissions of diesels have to be further decreased, which promoted companies and researchers to realize diesel emission control by developing advanced technologies and measures, such as ultra high pressure fuel injection system, exhaust gas recirculation (EGR), after treatment technologies, new combustion concepts, and fuel additive [1,2,3,4,5,6] .
In recent years, researchers have continuously studied fuel additive as a means of decreasing diesel emission, particularly metal based additives which can decrease emission and consumption of hydrocarbon fuels [7,8,9,10] .According to the literature review, combustion can be more complete and efficient in gas turbines and diesel engines after adding metal base additives, which can enhance power and fuel economy and decrease emissions of CO, HC and particulate.However, the metallic-based additives have been studied and used to decrease particulate emissions of diesel engines and the particulate formation in exhaust systems and cylinders.The metal based additive can decrease emissions because metals react with water vapour to serve as oxidation catalyst or generate hydroxyl radicals, so that the oxidation temperature is decreased and particle burnout is increased [11] .In the meantime, researchers pay increasing attention to the oxygenated fuel additives because they can decrease the emission of particulate matter (PM) dramatically while not leading to the emissions of carbon moNOxide(CO) and unburned hydrocarbon (UHC).Researchers have explored the influence of cetane number, oxygen concentration and additive types on diesel performance and emissions by experiments, and they have drawn a common conclusion that the increment of oxygen concentration can decrease PM emission, particularly at high load.More oxygen can be introduced more effectively by fuel with oxygenated additives [12,13,14] .Soot precursors are consumed by oxygen atoms in the formation of hydroxyl radicals, leading to a slight increase of NOx and a decrease of soot emission [15] .
In addition to the metal and oxygenated additives mentioned above, a nitro based fuel additive has attracted extensive attention.MAZ as a new type of fuel additive has received worldwide attention, which has low water solubility, is biodegradable and contains no metallic substances making it environmentally friendly, its main components are a combination of nitroparaffins.It consists of 40-65% nitropropane , 15-25% nitroethane, 15-25% nitromethane, 5-15% toluene .Boums from University of California has studied the influence of MAZ additives on fuel economy and emissions of gasoline engines and vehicles.The results show that the engine power, economy and THC, CO and NOx emissions have been improved by adding MAZ additive compared with benchmark gasoline [16] .The engine bench test and motorcycle driving experiment were carried out in South China University of Technology , finding that adding MAZ additive can not only obviously decrease fuel consumption rate and pollutant emission, but also improve the cleanliness of engine piston and cylinder liner surface [17] .There are studies focusing on the influence of adding MAZ additives to ethanol gasoline (E10) on automobile fuel consumption and emission in Beijing Institute of Technology [18] .The results show that the use of ethanol gasoline containing MAZ additives can reduce the emission of harmful pollutants and the fuel consumption under variable operating conditions and constant speed.
Previous research mainly focuses on the influence of nitro-bsed additives on gasoline engine performance and emission characteristics, while the research on diesel engines is scarce.Moreover, the mechanism of reducing the emission and improving the efficiency by nitro additive has not been thoroughly studied.This paper studied the effects of adding nitrobased additive to diesel fuel on gaseous emission, exhaust smoke, particle number concentration and engine performance on a test bench, and explored the functioning mechanism and combustion characteristics through the constant volume combustion vessel.Finally, chemical reaction kinetics is used for further analysis.

Experimental apparatus and methods
Experiments were carried out on a turbocharged inter-cooled heavy duty direct injection diesel engine equipped with selective catalytic reduction(SCR) after treatment system and  The combustion characteristics of diesel fuel after adding MAZ were tested and analyzed through a high pressure constant volume combustion chamber.The system is composed of high pressure common rail fuel injection system, high pressure constant volume combustion vessel, high speed camera, data synchronization and acquisition system, as shown in Figure 2. The maximum diesel injection pressure is 160 MPa, and the compressed air in the combustion chamber can be heated to 1000 K under the back pressure of 3 MPa.

Analysis of power performance and economy of engine
In order to compare the influence of MAZ additives for diesel engine power, the test engine operates at the benchmark speed of A, B and C under ESC to compare the difference of total load characteristics of engine burning benchmark fuel and MAZ_1000 fuel.The engine speeds A,B,C shall be declared by the manufacture in accordance with following provisions: The high speed nhi shall be determined by calculating 70% of the declared maximum net power P(n).The highest engine speed where this power valve occurs on the power curve is defined as nhi.The low speed nlo shall be determined by calculating 50% of the declared maximum net power P(n).The highest engine speed where this power valve occurs on the power curve is defined as nlo.The engine speeds A,B and C shall be caculated as follows: Speed A =nlo +25%(nhi-nlo); Speed B =nlo+50%(nhi-nlo); Speed c =nlo +75%(nhi-nlo).The test results show that (as shown in Figure 3) the engine power is slightly improved by 0.5% after adding MAZ into the fuel.

Fig. 3. Full load characteristics of speed points A, B and C.
In order to cover the full speed condition of engine, the full load characteristics were scanned when the engine burned the two kinds of fuels.That is, when the engine is at fullload, the speed is changed from idle speed to rated speed to obtain the external characteristics curve of the engine.The test results are shown in Figure 4.After using MAZ_1000 fuel, the power of engine increased slightly at most speed.Fuel consumption is considered as an important indicator of engine economy.Therefore, the fuel consumption rates at the external characteristic points of the engines burning the two kinds of fuels were compared.The results (Figure 5) indicate that the fuel was saved at most speed points after burning MAZ_1000, except for a little increase in the oil consumption at some low speed point.The minimum fuel consumption rate was reduced from 204.6g/kWh to 202.2g/kWh, and the highest fuel saving rate was about 2%.Based on the universal characteristic curve, the optimal economic zone with the specific fuel consumption under 210g/kWh is obtained, as shown in Figure 6.After adding MAZ, the optimal economic zone of the engine was obviously expanded, and the speed range and load range increased to a certain extent.The covered operating condition area was expanded by about 5%.As mentioned above, MAZ can not only improve fuel combustion quality and combustion velocity, but also shorten combustion duration so as to enhance the maximum explosion pressure of engine.While improving the engine power, it is beneficial to increase the effective thermal efficiency and reduce the fuel consumption rate.

Emission characteristics of gaseous pollutants
In order to explore the difference of engine emission characteristics when using standard diesel and MAZ diesel fuel, the universal characteristics of engine were tested by using two kinds of fuel, and the exhaust pollutant concentration was measured under each working condition.In the process of testing universal characteristics, the urea injection system is kept closed to obtain the NOx emission in the engine cylinder.The data were recorded at each operating point after the temperature was stable at the frequency of 10 Hz.The arithmetic average of 60 seconds is the final result.In order to comprehensively compare the emission pollutants and engine performance, the specific emission values of exhaust pollutants at each operating point were calculated according to Formula 1.According to formula 2, the weight ratio emission values in all operating conditions are calculated to represent the overall emission level of the engine.
where, Epoint is the specific emission value at current operating points.E represents the exhaust pollutant concentration value of the current working condition.P indicates the engine power at the current operating condition.
where, E_spec is the weighted specific emission values under all operating conditions.The weighted specific emission values of CO2, CO, THC and NOx at universal characteristic operating point are calculated respectively, and the results are shown in Table 2.When the engine burns MAZ diesel, the overall emissions of major gaseous pollutants were reduced to some extent on the whole.CO2 is the main greenhouse gas, and it is mainly generated from the combustion of fuel.After adding MAZ, the combustion quality of diesel was improved, and the specific fuel consumption was decreased, leading to lower content of CO2 in exhaust pipe.CO mainly comes from incomplete combustion of fuel oil, and THC mainly comes from diesel fuel which is not involved in combustion or incompletely combusted.Because of the combustion characteristics of MAZ diesel fuel, the entrainment effect enhancement optimizes the mixing of the mixed gas, and reduces the concentration zone and the dilute zone.More free radicals increase combustion efficiency, and the fuel combustion is more complete, which directly decrease the proportion of CO and THC zones.Because of the importance of NOx emission reduction, the emission characteristics of diesel engine were particularly analyzed.Figure 7 shows the equal discharge line of NOx generated when the engine burnt the two kinds of fuels.It can be found that the low NOx emission area is extended within the main operating range of the engine after adding MAZ.The NOx specific emission ratio decreased by 5.7% under each operating condition.
NOx is mainly formed by the reaction of N2 and O2 in the air entering the cylinder at high temperature, and its formation rate is restricted by three factors, namely, temperature, oxygen content and reaction time.For most working conditions, although the MAZ diesel combustion velocity is faster, the maximum temperature in the cylinder is slightly higher.However, since the mixture concentration in the cylinder is more uniform and combustion is more complete, the probability that high temperature oxygen enrichment region occurs decreases.The total oxygen content is low in the combustion zone at the end of combustion, which hinders the formation of NOx.In addition, because of higher thermal efficiency, more heat is converted to mechanical energy, but the actual high temperature duration in the cylinder is relatively short.Although the MAZ component has been introduced with N atom, its content is very low, which cannot be the main factor affecting the NOx emission.Therefore, based on the above factors, the amount of NOx generated in the engine cylinder decreased after adding MAZ.
In order to measure MAZ diesel characteristics based on emission regulation, after recovering the function of the urea injection system, the ESC cycle was run and ESC cycle was operated to calculate the gas pollutant specific emission results of the whole cycle, as shown in Table 3.Similar to the universal characteristics, the specific emissions of CO2, CO and THC decreased to some extent when the MAZ_1000 fuel was burnt in the ESC cycle.After recovering urea injection, although the NOx emission at each operating points increased slightly (Fig. 8), the NOx weighted emissions of the whole cycle still decreased by more than 2%.Fig. 8. NOx concentration at various operating points of ESC.

Analysis of emission characteristics of smoke and particulate matter
When the ESC cycle was operated, the transient smoke intensity at each operating point was measured based on the AVL439 opaque smoke meter.Figure 9 shows the mean value of opacity smoke of ESC under each working condition.The average smoke intensity of most operating points showed a downward trend, with a maximum drop of 55%.  Figure 10 shows the weighted particle mass and particle number specific emissions in ESC when the engine burns the two kinds of fuels.It can be found that the variation trends of particle quality and smoke consistency are consistent.After adding MAZ, the mass of the recycled particles can be reduced by about 15% because the main components of MAZ have strong polarity.These components can be adsorbed on the surface of particles in cylinder and prevent the polymerization of nuclear particles into polymeric particles.Therefore, in the process of stroke, the average particle size of particles in combustion is smaller in cylinder, and the dispersion is more even.However, some small particles are not involved in combustion, and the decrease in the number of large particles can hardly offset the increase in the number of small particles.On the whole, the number of particles increased by less than 1% after adding MAZ.The increase in the number of particles caused by MAZ can be basically ignored.

Analysis of combustion characteristics
The combustion characteristics experiment based on natural luminescence method was carried out by adding two kinds of MAZ_0 and MAZ_1000, respectively.During the test, the injection pressure was 80 MPa, the injection quantity was 17.6mg, the temperature in the constant volume combustion chamber was set to 800K, and the speed of the camera was 10000 Ftps.Each fuel was tested for 3 times and the average was taken as the final result.
Fig. 11 shows the process of flame shape evolution when the two kinds of fuels were burnt respectively.It can be found that when the diesel was mixed with MAZ, the flame emergence was delayed slightly, and the flame extinction time was ahead.That is, the duration of combustion was shortened, and the combustion velocity of fuel was accelerated.In order to quantitatively analyze the influence of MAZ on diesel combustion, the time when the picture of obvious flame was shot by high-speed camera was defined as the starting point of combustion.The ignition delay lasts from the injection to the combustion.The distance from the end of nozzle to flame in combustion process was defined as flame floating length.The duration with cumulative heat release rate from 10% to 90% is determined as the duration of combustion.The results show that the delay period of MAZ_1000 is longer than that of benchmark diesel, extending from 1.98ms to 2.53ms.Therefore, the ignition was delay and the flame lift-off length increased slightly.This also directly increased the penetration distance of the unburned fuel beam and the floating length of the flame (Fig. 12).For the diffusion flame, the flame floating length is the range of the air entrainment.The longer the flame floating length is, the more the air entrainment of the oil bundle is, which is more conducive to the full mixing and combustion of the fuel and air.Therefore, compared with the benchmark fuel, MAZ_1000 has more active groups involving in the reaction and more fully mix with the air.The combustion velocity is faster, the combustion efficiency is higher, and the combustion quality is optimized.
It can be seen from Fig. 13 that the ratio of premixed combustion increases, the peak value of heat release increases, the duration of combustion becomes shorter, the pressure increases slightly in the constant volume chamber and the thermal efficiency increases.In the whole combustion process, the flame floating length of MAZ_1000 has improved to some extent (Fig. 13).The combustion duration was shortened from 3.60ms to 3.13ms, and the combustion rate was increased by about 13.1%.Fig. 13.Pressure and rate of heat release.

Chemical reaction kinetics analysis
In order to explore the mechanism of Nitro-based additives on diesel combustion, based on homogeneous reactor model of software CHEMKIN, the simulated simulation was carried out by using n-heptane instead of diesel and nitromethane instead of MAZ additive.In the low temperature stage, the chemical kinetics between MAZ and diesel has an important effect on the ignition delay.When MAZ and diesel are blended, they build chemical reaction relationship by radical pool, OH is the most important radials in the pool.The free energy of C7H16 combined with OH is about 2.57 E7 cal•mol-1, and the free energy of CH3NO2 combined with OH is about 3.00 E6 cal•mol-1, so CH3NO2 is combined with OH to consume OH , inhibit the combination of C7H16 and OH, and the ignition delay was prolonged.The longer ignition delay led the higher fraction of premixed combustion, which could provide a higher apparent heat release rate peak and a better combustion efficiency.As the temperature rises further, this effect is weakened, releasing enough OH, and burning faster.

Conclusions
The effects of adding nitro-based additive to diesel fuel on gaseous emission, exhaust smoke, particle number concentration and engine performance on a test bench were studied and then combustion characteristics through the constant volume combustion vessel and the functioning mechanism chemical reaction kinetics were further explored, and the main conclusions are as follows: Nitro-based additives not only enhances the power of engine, but also enlarges the working area with economic fuel consumption.The minimum fuel consumption rate was reduced from 204.6g/kWh to 202.2g/kWh, and the highest fuel saving rate was about 2%.After adding MAZ, the mass of the recycled particles can be reduced by about 15%,and the average smoke intensity of most operating points showed a downward trend, with a maximum drop of 55%, the NOx weighted emissions of the whole cycle decreased by more than 2%, while the particle number concentration increased slightly.
Longer ignition delay and lift-off length were observed in diesel mixed with MAZ additive compared with diesel.The longer ignition delay led the higher fraction of premixed combustion, diesel mixed with MAZ additive could provide a higher AHRR peak and a better

Fig. 9 .
Fig. 9. Opaque smoke intensity at each operating point of ESC.

Fig. 10 .
Fig. 10.Specific emission of PM and PN at each ESC cycle.

Fig. 11 .
Fig. 11.Flame development status of two kinds of fuels.

Table 1 .
Main parameters of engine

Table 2 .
Weighted specific emission value of gas pollutant

Table 3 .
Specific emission of gaseous pollutants in ESC cycle.