1. Introduction

The necessity to change from an economy that is largely based on non-renewable fossil fuels as raw materials to one that is based on renewable sources is a worldwide challenge. Two fundamental drivers lead this challenge: the first is a growing world dem and for crude oil, as a result of the rapidly growing economies of China and other BRIC countries coupled with political instability in many areas of the world in which the major crude oil exporters are situated; the second is the need to reduce net emissions and environmental impact of carbon dioxide and other greenhouse gases(GHG)associated with the consumption of fossil fuels and thereby the environmental footprint of fuels and chemicals production. In this scenario, biodiesel from biomass and other non-food feed-stocks can be considered the primary alternative to diesel fuel due to its properties close to diesel fuel and its miscibility with diesel in any percentage [1,2]. Recently, it is growing the interest in the use of alcohols as a viable alternative either single or blended with conventional based fuels. In particular, the first four aliphatic alcohols(methanol, ethanol, propanol and butanol)are of interest as fuels because they can be synthesized biologically [3,4]. Further, they have physical and chemical properties such as less carbon and sulphur content, and more oxygen than traditional fossil-based fuels [5], this allows them to be applied in current engines. As one of the primary alcohol fuels, butanol has more advantages than methanol and ethanol as an alternative fuel for internal combustion engines. In fact, butanol has a lower auto-ignition temperature than methanol and ethanol, so it can be ignited easier when burned in diesel engines. Besides, butanol has lower volatility and higher energy density than ethanol and methanol. In addition, butanol has lower water adsorption and corro sive properties. Butanol can be produced by fermentation of biomass, such as algae, corn, and other plant materials containing cellulose that could not be used for food and would otherwise go to waste [6,7]. Recent use of genetically enhanced bacteria has increased the fermentation process productivity, even if more work is necessary to make butanol production economically viable [8,9]. The butanol cost is still an important disadvantage, but the benefits of the blending with diesel are likewise important. If compared with other alcohols, as methanol and ethanol, butanol has better solubility with diesel thus avoiding the use of additives, viscosity similar to diesel fuel and has the ability to be used in conventional internal combustion engines without the need for modification [10,11,12]. This allows the electronic control high pressure common rail fuel injection system to realize a flexible and precise control of fuel injection [13,14,15].

It is known that large reduction of emissions can be achieved by applying multiple injection strategies [16]. Soot emission can be reduced by raising the injection pressure to accelerate fuel evaporation through production of smaller fuel droplets, enhance fuel‑air mixing and create faster combustion rates. Moreover, by injecting a small amount of fuel as pilot before the main injection, it is possible to reduce combustion noise by limiting the amount of premixed combustion. The strategy leads to a decrease in-cylinder pressure rise rate, and thus the reduction of the combustion noise. The effect of pilot injection on the exhaust emissions is linked to the injection timing. A long dwell time between pilot and main injection reduces particulate emissions with higher efficiency at high load. On the other h and , at medium-low load, a small dwell time allowed to reduce premixed burn fraction and NOx. Regarding the use of butanol as fuel for compression ignition engines, previous investigations [11,17] showed that butanol, blended with diesel until 40 % in volume(BU40), increased combustion pressure and accelerated burning rate, while having little impact on maximum power and torque output. Moreover, butanol-diesel blends slightly increased BSFC(Brake Specific Fuel Consumption)but attained a higher thermal efficiency than the neat diesel fuel. The effects of butanol addition to diesel fuel on emissions were strongly influenced by EGR and loads. In general, HC emissions of blends were higher than the neat diesel, especially under low-loads. In these conditions, as expected, CO emissions increased while NOx emissions decreased as butanol blending ratio increased. Under high-load conditions, on the contrary, CO emissions decreased but NOx emissions increased. Moreover, butanol-diesel blends significantly reduce soot emissions in all conditions, and the reduction is higher at increasing blending ratio. In conclusion, butanol is a promising biofuel, which can be used conveniently and advantageously up to high blending ratio with diesel fuel(BU40), both from the viewpoints of thermal efficiency and soot emission.

Even if scientific literature reports numerous experimental investigations to evaluate the effects of butanol-diesel blends on performance, exhaust emissions and combustion behaviour for different engine operating conditions, more data are required to better underst and the basis physical and chemical phenomena related to the direct injection and the spray combustion process. For example, an improved knowledge of butanol effects on multi-injection spray combustion processes is useful to reach the fuel flexibility and NOx-PM trade off optimization. Moreover, these experiments provide fundamental support for kinetic chemical modelling and 1D—3D simulation codes.

For all these reasons, UV-visible flame emission spectroscopy was applied in the combustion chamber of single cylinder high swirl compression ignition engine equipped with a common rail multi-jets injection system. This optical methodology is a powerful diagnostic because natural emissions from diesel flame consist of both chemiluminescence and soot luminosity [18]. Chemiluminescence is produced by the excitation of the intermediate combustion radicals due to exothermic chemical reactions occurring during the auto-ignition and combustion process [19]; soot luminosity is caused by the emissions from hot soot particles and it is characterized by a continuous broadb and spectrum similar to the Planck black body emission curve [20,21].

All the combustion tests were performed fuelling the optical accessible engine with a commercial diesel(BU00) and a blend of 60 % diesel with 40 % n-butanol(BU40). The combustion process was studied from injection until the late combustion phase, fixing the injection pressure at 70MPa and changing the pilot injection timing. Optical results were correlated with the engine parameters and with exhaust emissions.

2. Experimental Apparatus

The engine used in the work is an external high swirl optically accessible combustion bowl connected to a single cylinder 2-stroke high pressure common rail compression ignition engine. The main engine specifications are reported in Table 1. The external combustion bowl(50 mm in diameter and 30 mm in depth)is suitable to stabilize, at the end of compression stroke, swirl conditions to reproduce the fluid dynamic environment similar to those within a real direct injection diesel engine. The implication of “cylindrical bowl” is related to the peculiar design of the prototype engine that has a large displacement as an air compressor. The main cylinder, connected to the external “swirled bowl” through a tangential duct, allows supply of compressed air to flow to the bowl as the piston approaches TDC. The air flow, coming from the cylinder, is forced within the combustion chamber by means of the tangential duct. In this way, a counter clockwise swirl flow, with the rotation axis about coincident to the symmetry axis of the chamber, is generated. The injector was mounted within this swirled chamber with its axis coincident to the chamber axis; in this way the injected fuel is mixed through a typical interaction with the swirling air flow. The combustion process starts and mainly goes on in the chamber. After a few crank angles the piston moves downward, the flow reverses its motion and the hot gases flow through the tangential duct to the cylinder and finally to the exhaust ports. The combustion chamber provides both a circular optical access(50 mm diameter), on one side of it, used to collect images and a rectangular one(size of 10 x 50 mm)at 90°, outlined on the cylindrical surface of the chamber, used for the laser illumination input. The injection equipment includes a common rail injection system with a solenoid controlled injector located on the opposite side of the circ ular optical access. The nozzle is a micro-sac 7 hole, 0.141 mm diameter, 148° spray angle nozzle. An external roots blower provided an absolute air pressure at the inlet of 0.21 MPa with a peak pressure within the combustion chamber of 4.9 MPa under motored conditions. The pressure in the swirl combustion chamber was measured by a quartz pressure transducer with the assumption of no pressure difference between the swirl and the main cylinder chamber.

Table 1. Specifications of the engine

2-stroke single cylinder engine
Cylindrical Bowl(mmxmm)	50x30
Bore(mm)	150
Stroke(mm)	170
Connecting Rod(mm)	360
Compression ratio	10.1:1
Air supply	Roots Blower
Outlet/Inlet Blower Ratio	2.1
Bosch Injector nozzle	7/0.141/148°

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The injection duration was calibrated for delivering the same mass of fuel by changing the butanol concentration: 30 mg/stroke. Even if the n-butanol density(810 g/dm³)is similar to the Diesel one(832 g/dm³), the other fluid-dynamic properties(e.g. viscosity)could affect the fuel mass injection rate. The injection time was found to be the same for the main injection(990 µs); the pilot injection was set at 420 µs for Butanol blends and 400 µs for pure Diesel. Measurements were performed using the gravimetric method for 1200 consecutive injections, with an accuracy of 0.01 %.

During the spectroscopic investigations, the radiative emissions from the combustion chamber were focused by a 78 mm focal length, f/3.8 UV Nikon objective onto the micrometer controlled entrance slit of a spectrometer(Acton SP2150)with 150 mm focal length and 600 groove/mm grating. The central wavelength of the grating was fixed at 300 and 400 nm. From the grating the radiations were detected by an intensified CCD camera(PI-MAX3-Princeton Instruments). The camera has an array size of 1024 x 1024 pixels with a pixel size of 13 x 13 μm and 16-bit dynamic range digitization at 100 kHz. The exposure time was fixed at 42 μs and the dwell time between two consecutive acquisitions was set at 62 μs.

Spectroscopic investigations were carried out in the central region of the combustion chamber, divided in 8 locations. Figure 1 shows the sketches of the experimental apparatus for the optical investigations. To enhance signal to noise ratio the 64 spectra of each location were averaged. The flame emission spectra were corrected for the optical setup efficiency using a deuterium lamp with a highly uniform full spectrum. The wavelength calibration was performed using a mercury lamp. The time evolution of combustion products was evaluated from spectroscopic investigations using a post-processing procedure. For each chemical species, with well-resolvable narrow emission b and s, the height of the b and was evaluated after the subtraction of emission background and other species contribution. Software, developed in Labview environment, allowed simultaneous evaluation of the emissions of the selected compounds and species for each spectrum and time. OH and soot emissions were calculated as average on all the spectra. The optical set-up was used also for UV-visible digital imaging of the spray combustion process to support the spectroscopic investigations. In particular, the central wavelength of the spectrometer was fixed at 0(in this way the device worked as a mirror) and the input slit was opened(3 mm width). The exposure time and the acquisition timing were maintained unchanged.

Figure 1. Sketches of the experimental apparatus for the optical investigations. The locations for the spectroscopic investigations are number labelled.

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Combustion tests were carried out using three blends. The baseline fuel was the European low sulphur(10 ppm)commercial diesel with a cetane number of 52(BU00). Moreover, blends of butanol(20 %)—diesel(80 %)(by vol.)(BU20) and butanol(40 %)—diesel(60 %)(BU40)were tested. The main properties of the fuels are reported in Table 2.

Table 2. Fuel properties.

	BU00	BU20	BU40
Density @15 °C(kg/m3)ASTM D4052	840	830	826
Cetane number ISO4264	52	44	36
Net heat value(MJ/kg)	42.5	41	36.9
Carbon content(%)ASTM D5291	87	80.2	75.19
Hydrogen content(%)ASTM D5291	12.6	15.4	16.8
Oxygen content(%)ASTM D5291	-	4.32	8.64
IBP(°C)ASTM D86	160	139	117
Distillation 50% vol.(°C)ASTM D86	280	250	226
Distillation 90% vol.(°C)ASTM D86	338	328	325

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All the experimental data were collected using the AVL INDICOM driven by an optical encoder with 0.1 crank angle degree resolution. The pressure in the swirl chamber was measured by a quartz pressure transducer AVL QC34C. Results of the in-cylinder pressure were computed over 300 consecutive engine cycles. Exhaust gaseous emissions were acquired by AVL DiGas 4000 analyser for NOx, the Smoke Meter AVL 415S was used for FSN measurements. The accuracy of the acquired quantities was 10ppm for NOx and 0.1 % for smoke.

3. Results and Discussion

3.1. Engine parameters and exhaust measurements

Engine tests were carried out at 500 rpm; 30 mg ± 1% of fuel was injected at the pressure of 70 MPa setting a double injection(pilot and main). In the present paper, the SOI indicates the start of the energizing current to the solenoid injector. Because of the hydraulic and electronic delay a shift between the timing of the energizing current to the solenoid(SOI) and the fuel delivery from the injector nozzles(FSOI)is produced; this delay was estimated in 340 µs from the measurement of the instantaneous fuel flow rate.

Preliminary investigations were performed to evaluate the effect of the pilot injection timing at fixed main injection timing for reference diesel fuel. The start of main injection was set at 11 and 3 CAD BTDC while the start of pilot injection was swept to obtain a pilot-main dwell time of 8, 6 and 4 CAD for both main injections. Figures 2-3 show the exhaust measurements(smoke and NO_x). In Figure 4 the engine combustion work versus pilot-main injection dwell crank angle is plotted. The engine combustion work(ECW)was evaluated as the percentage variation of the area under the pressure signal in the fired(Af) and motored condition(Am)[22].

Figure 2. Engine emissions of smoke(FSN)vs Pilot-Main dwell time for BU00.

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Figure 3. Engine emissions of NOx vs Pilot-Main dwell time for BU00.

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Figure 4. Percentage engine combustion work vs Pilot-Main dwell time for BU00.

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This parameter may be correlated to the combustion efficiency of the selected fuel injection strategy. Experiments demonstrated that advanced start of main injection induced a strong decrease(>80 %)in the exhaust smoke for all pilot injection timings. On the other h and , the expected increase in the NOx emission was about 50 % and it was matched by an increase in net engine working cycle higher than 40 %. All the injection strategies attained the autoignition of pilot and main before the end of fuel delivery from the nozzle, inducing a mixing controlled combustion regime(MCC). To substantiate this statement, a selection of spray combustion images, detected fixing the SOI of pilot injection at -17 CAD ATDC, is reported in Figure 5. As a consequence of these results, the investigation was focused on the effect of butanol-diesel blends fixing the main injection at 11 CAD BTDC.

Figure 5. Images detected during the BU00 spray combustion for the double strategy, setting the pilot injection 17 CAD BTDC and the main injection at 11 CAD BTDC.

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Following the previous studying approach, Figures 6-8 show smoke and NO_x exhaust emission, and the net engine working cycle percentage for diesel and butanol blends fixing the main injection at 11 CAD BTDC at different pilot-main dwell time. Even if the diesel smoke number resulted already low for diesel, butanol induced a further decrease in soot exhaust emission as butanol blending ratio increased. The effect was expected due to the lower carbon content, the better mixing related to the higher volatility of butanol blends and the presence of oxygen within the butanol molecule. The highest reduction in smoke was obtained setting the dwell time at 6 CAD. The increase in butanol volume fraction produced an appreciable rise in NO_x emissions only for BU40 with a slight effect given by the dwell-time. Regarding the combustion efficiency, the net engine working area increased at higher butanol volume fraction. Again, the increase was influenced by the fuel properties while a minor effect was due to the pilot injection timing.

Figure 6. Engine exhaust emissions of smoke(FSN)vs Pilot-Main dwell time for selected fuels at fixed SOI of main injection(11 CAD BTDC).

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Figure 7. Engine exhaust emissions of NOx vs Pilot-Main dwell time for selected fuels at fixed SOI of main injection(11 CAD BTDC).

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Figure 8. Percentage engine combustion work vs Pilot-Main dwell time for selected fuels at fixed SOI of main injection(11 CAD BTDC).

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3.2. Optical results

To better underst and the fuel injection strategy outcome on the combustion process, natural emission spectroscopy measurements were performed. For brevity, in this work the spectra detected setting the pilot injection at SOI = 15 CAD BTDC are reported. Figure 9 shows the first spectroscopic evidence of combustion process due to the pilot auto-ignition for both fuels. It can be noted that BU40 pilot combustion started around 0.2 CAD after diesel pilot combustion.

Figure 9. Flame emission spectra detected at the autoignition of pilot injection for BU00 and BU40(SOIpilot 15 CAD BTDC; SOImain 11 CAD BTDC).

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For both fuels, the pilot auto-ignition was characterised by the OH emissions due to the A2S+→C2P transitions. Table 3 lists the heads of A-X system with an estimation of their intensities [23]. The most intense b and s are centred around 280 nm and 309 nm. Excited OH radical is representative of the primary combustion zone and it is produced by the radiative chemical reaction CH + O2 → CO + OH [24]. Hydroxyl radicals exist both in the flame front and hot post-combustion gases. Moreover, the relative balance between the two b and s(280 and 309 nm)is frequently used for flame temperature investigations [25]. It should be noted an auto-ignition delay of 0.2 CAD for butanol blend with respect to diesel due to different heating values(Table 2).

Table 3. Heads of the b and s in A 2S+ → C2P transitions. The related intensities are in the brackets [23].

(n’, n”)	0	1	2	3	4
0	306.4(12)	342.8(7)
1	281.1(9)	312.2(9)	318.5(6)
2	260.9(4)	287.5(9)	308.5(10)
3	244.4(1)	267.7(5)	294.5(7)	325.4(4)
4		251.7(2)	275.3(4)	302.2(5)	333.1(4)

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In the Diesel spectra of Figure 9, OH is superimposed on a broadb and chemiluminescence. This was determined by the Vaidya’s b and system of HCO with highest heads from 290 nm to 360 nm and by the tail of Emeleus’ b and s due to excited formaldehyde molecule CH₂O*(350 - 460 nm)[26]. The identification of the Vaidya hydrocarbon flame b and emitters and their formation mechanisms are complex and have been the subject of much reference speculations [27,28]. Among this, it should be noted the reaction of formaldehyde with a free atom or radicals(O, H, OH, or alkyl radical)that induced HCO* formation [Error! Bookmark not defined.].

After the auto-ignition timing the spectra evolved with similar features; moreover, in about 0.5 CAD, butanol-blend reduced the gap of intensity. As shown in Figure 10(left), for both fuels at 3.4 CAD ASOI pilot, OH and HCO emissions are added to a weak blue continuum. This emission is due to the chemiluminescence accompanying the recombination of CO and O [28]: CO(C¹S+)+O(³P)→CO₂(C¹S+g)+hn. It occurred between 250 and 800 nm with a maximum intensity around 400 nm. The broad emission is also featured by the CO and CO₂⁺ b and systems present on the whole range of wavelengths. After around 0.5 CAD, the speed of flame propagation doubled in the emission intensity(Figure 10 right); in particular, the increase in the burned mass fraction and local temperature gave rise to an increase in OH emissions and consequently in the radical concentration. It should be noted that CO₂* evidence resulted unchanged, due to the fast transition to the ground state that induced the increase in neutral mole cules [28]. For diesel, at 3.9 CAD ASOI pilot, the spectral evidence of a blue flame emission due to carbonaceous structures and soot nano-precursors was observed [29,30]. After the ignition of main injection(but during the fuel delivery period)the flame spreads across the combustion chamber. Spectra were characterised by a strong soot emission featured by a broadb and signal that increased with the wavelength like a blackbody curve, as shown in Figure 11 left [20,30]. Soot emission and thus soot concentration was lower for BU40 than diesel. The relative intensity between the two fuels remained unchanged during the combustion process, as shown in the right plot of Figure 11. Moreover, BU00 showed comparable emission intensity of the two OH b and systems at 310 nm and 280 nm due to(0, 0) and (1, 0)of the A-X system, while for BU40 only the emission due to diagonal transition was well resolved. This optical evidence persisted until the late combustion phase in which the oxidation mechanism reduced the soot and highlighted the OH emission. The effect is well observed in Figure 12, which shows the spectra detected at 12.7 CAD ATDC(27.7 ASOI pilot)for both fuels. The relative emission intensity of OH at 280 and 310 nm is correlated to the adiabatic flame temperature. As expected, it is lower for BU40. Even if this contributed to reduce the NO formation rate, the significantly higher oxygen availability of BU40 induced local conditions nearer to stoichiometric compared to the neat diesel fuel operation. This works towards higher NOx liability in agreement with exhaust data.

Figure 10. Flame emission spectra detected at 3.4 CAD(left) and 3.9 CAD(right)after pilot injection for BU00 and BU40(SOIpilot 15 CAD BTDC; SOImain 11 CAD BTDC).

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Figure 11. Flame emission spectra detected 5.6 CAD(left) and 18.4 CAD(right)after pilot injection for BU00 and BU40(SOIpilot 15 CAD BTDC; SOImain 11 CAD BTDC).

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Figure 12. Flame emission spectra detected 27.7 CAD after pilot injection for BU00 and BU40(SOIpilot 15 CAD BTDC; SOImain 11 CAD BTDC).

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In order to better underst and the effect of fuel and injection strategy on the combustion process, the soot and OH behaviours were evaluated. In particular, OH(0, 0)emission was measured as height of the 310 nm b and system after the subtraction of the emission background, evaluated as the mean value between the emissions measured at 300 nm and 320 nm. Soot emission was evaluated as mean intensity at 500 nm using data obtained fixing 400 nm as central wavelength. OH and soot evolutions for SOIpilot 15 CAD BTDC are plotted in Figure 13.

Figure 13. Time evolution of the OH(left) and soot(right)emission for both fuels(SOIpilot 15 CAD BTDC; SOImain 11 CAD BTDC).

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OH radical in Figure 13(left)progressed following three phases: the first one was due to the pilot ignition and it can be observed for around 5 CAD(first dashed line)after the pilot injection; at this time the main injection started and OH was generated by the ignition of the fuel delivered by the main injection involved with the mixed controlled combustion. In this phase, as shown in the right plot of Figure 13, the soot concentration began to rise up to the end of the main injection. Afterwards, starting from 14 CAD(second dashed line)after the pilot injection, the oxygenated species, including OH radicals, acted to quickly reduce the soot concentration. It is also evident that there is significant difference in soot emission intensity given by the BU40 with respect to the diesel fuel.

Similar trends were detected for the other fuel injection strategies with different pilot injection timings. The increasing in the pilot-main dwell time decreased the OH concentration in the first phase for both fuels, and reduced the difference between the two fuels. This effect was related to the lower pressure and temperature within the combustion chamber, at the pilot injection timing, that influenced the auto-ignition because of the small amount of injected fuel. The heat released by the pilot obviously influenced the main injection combustion; in particular, increasing the dwell time between pilot and main, the auto-ignition of the fuel delivered by the main injection was delayed and the OH formation rate decreased. As a consequence, the soot formation was increased by a simultaneous OH oxidation action. Finally, the OH evolution during the main injection combustion was opposite to that observed for pilot injection combustion. This was due to the lower temperature gradient and lower oxygen concentration in the combustion chamber during the main injection combustion. These effects were more evident for BU40 due to higher oxygen concentration in the fuel. As consequence of these phenomena, soot concentration was lower for BU40, as also confirmed by the exhaust measurements, and the discrepancy between the two fuels decreased with the pilot-main dwell time.

In conclusion, it may be highlighted that the effect of butanol was well marked during the soot formation phase. This occurred because soot formation, caused by high temperature fuel decomposition, mainly takes place in the fuel-rich zone at high temperatures and pressures, specifically within the core region of each fuel spray. If the fuel is partially oxygenated, as in the case of butanol blends, it has the ability to reduce locally fuel-rich regions and limit soot formation, reducing smoke opacity. Moreover, butanol blends are characterized by a lower stoichiometric air‑fuel ratio(less air is needed to achieve stoichiometry and consequently complete combustion), which reduces the existence of fuel-rich regions in the non-uniform fuel‑air mixture. Finally, the absence of aromatic compounds in butanol blends reduces the concentration of soot precursors.

4. Summary

In this paper, the effects of the injection strategy on the combustion process, comparing the baseline diesel to a blend of diesel and n-butanol(40 % by vol.), was experimentally investigated through UV-visible flame emission spectroscopy. Tests were performed within a single cylinder, high swirl compression ignition engine, optically accessible, equipped with a common rail multi-jets injection system. The investigation was carried out setting the start of the main injection at 11 CAD BTDC, for different dwell timings between the pilot and the main injections. The strategy allowed operating under a dominant mixed controlled combustion for both fuels. Spectroscopic investigation allowed following temporal and spatial evolution of the combustion process chemical markers, with particular interest for OH radicals and soot.

At fixed fuel injection strategies, BU40 showed higher combustion efficiency combined with lower soot emission and higher OH concentration(310 nm). This result was due to the oxygen content within the butanol blend and its higher volatility that enhanced mixing and promoted a better homogeneity of the vapour within the combustion chamber.

Regarding the effect of fuel injection strategy, the reduction of pilot-main dwell time, increased the OH(310nm)emission of the burnt fuel delivered during the pilot injection, both for the baseline diesel and the blend. As a result, during the main injection combustion, the difference of OH emission between the baseline diesel and the butanol blend was reduced. Further, the OH emission detected at 280 nm was always higher for BU40 due to lower adiabatic flame temperature of the butanol blend.

Acknowledgements

The authors would like to express their appreciation to Mr. A. Mazzei for the technical support in preparing the equipment and during the experiments.

Nomenclature

ASOI After Start Of Injection

ATDC After Top Dead Center

BRIC Brazil, Russia, India and China

BSFC Brake Specific Fuel Consumption

BTDC Before Top Dead Center

BU00 pure commercial fuel

BU20 Commercial fuel blended with 20% of butanol(vol)

BU40 Commercial fuel blended with 40% of butanol(vol)

CAD Crank Angle Degree

CCD Charge-Coupled Device

ECW Engine Combustion Work

EGR Exhaust Gas Recirculation

FSN Filter Smoke Number

FSOI Fuel Delivery Start Of Injection

GHG Greenhouse Gases

MCC Mixing Controlled Combustion

PM Particulate Matter

SOI Start Of Injection

TDC Top Dead Center

UV Ultraviolet