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Environmental Science and Pollution Research

, Volume 25, Issue 15, pp 14674–14689 | Cite as

Effect of compression ratio, nozzle opening pressure, engine load, and butanol addition on nanoparticle emissions from a non-road diesel engine

  • Rakesh Kumar MauryaEmail author
  • Mohit Raj Saxena
  • Piyush Rai
  • Aashish Bhardwaj
Research Article

Abstract

Currently, diesel engines are more preferred over gasoline engines due to their higher torque output and fuel economy. However, diesel engines confront major challenge of meeting the future stringent emission norms (especially soot particle emissions) while maintaining the same fuel economy. In this study, nanosize range soot particle emission characteristics of a stationary (non-road) diesel engine have been experimentally investigated. Experiments are conducted at a constant speed of 1500 rpm for three compression ratios and nozzle opening pressures at different engine loads. In-cylinder pressure history for 2000 consecutive engine cycles is recorded and averaged data is used for analysis of combustion characteristics. An electrical mobility-based fast particle sizer is used for analyzing particle size and mass distributions of engine exhaust particles at different test conditions. Soot particle distribution from 5 to 1000 nm was recorded. Results show that total particle concentration decreases with an increase in engine operating loads. Moreover, the addition of butanol in the diesel fuel leads to the reduction in soot particle concentration. Regression analysis was also conducted to derive a correlation between combustion parameters and particle number emissions for different compression ratios. Regression analysis shows a strong correlation between cylinder pressure-based combustion parameters and particle number emission.

Keywords

Diesel engine Combustion Particle emissions Butanol Heat release rate Compression ratio 

Introduction

Combustion of fossil fuels in internal combustion (IC) engines emits high levels of regulated and unregulated emissions. Diesel engines are typically used in vehicles as well as non-road heavy-duty applications due to their higher power output and thermal efficiency. However, diesel engines emit high levels of NO x and particulate matter (PM) (Heywood 2017). Several after-treatment devices (such as lean NO x trap and particulate traps) are commonly used to meet the stringent emission legislation limits. These after-treatment devices also have a penalty on fuel conversion efficiency of the engine (Johnson 2009). Particulate trap is often used in heavy-duty vehicles for the reduction of PM emission which increases the engine fuel consumption due to the combined effect of energy required for the trap regeneration and back pressure generated in the exhaust manifold (Stamatelos 1997; Johnson 2009). PM emitted from diesel engines has received increased focus since it was designated a toxic air contaminant. In California, it is estimated that diesel PM accounts for 70% of the known carcinogenic risk by inhalation of all air toxics (Ayala and Herner 2005). Several epidemiological studies have shown that atmospheric PM has undesirable human health effects (increased mortality and morbidity). These studies presented the relationship between PM and impairment of the cardiopulmonary function (Burnett et al. 1995; MacNee and Donaldson 2000). Few studies also suggested that the number concentration of ultrafine particles (particles with aerodynamic diameters < 100 nm) may have an important contribution to the adverse health effects (Peters et al. 1997; Ibald-Mulli et al. 2002). Due to evidence of the adverse health effect of the ultrafine particles engine manufacturer, researchers and regulatory agencies are investigating PM sizes and number concentration in diesel engine exhaust.

Various studies have been carried out to investigate the different aspect of particle emissions from diesel engines. Lu et al. (Lu et al. 2012) investigated the nanostructure and size of particles at different operating conditions using high-resolution TEM (transmission electron microscope). They found that particle size and its structure is dependent on engine operating condition. Larger primary particles of irregular shape were formed at higher engine load while particles of a disordered structure formed at lower engine load. Yehliu et al. (Yehliu et al. 2013) experimentally investigated the effect of combustion phasing and engine operating conditions on reactivity and nanostructure of soot generated from a turbocharged diesel engine. The study summarized that higher engine speed is related to more reactive soot and higher volatile organic fraction (VOF) at constant engine torque. Another study investigated the effect of multiple-injection strategy on particle emission from the automotive diesel engine (Li et al. 2015). They showed that pilot-main injection strategy premixed combustion is reduced and diffusion combustion occurs more in comparison to single main injection strategy. This combustion strategy promotes the formation of soot nuclei and results in a significant increase of number and mass of particles with the diameter of 100 nm. Agarwal et al. (2013) investigated the impact of fuel injection parameters on performance, combustion, and emission characteristics of the single-cylinder research engine. Experiments were carried out at 2500 rpm with 500- and 1000-bar injection pressure and the various start of injection (SOI). Their results indicated that with increasing the fuel injection pressure, the concentration of particles reduces at all engine load conditions. Moreover, the concentration of particles can be reduced by advancing the fuel injection timing at higher fuel injection pressure (Agarwal et al. 2013).

Various studies were also conducted to investigate the effect of different fuels (fumigated/blended with diesel) on particle emissions from the diesel engine (Rounce et al. 2012; Ye and Boehman 2012; Zhang et al. 2013; Surawski et al. 2014; Dhar and Agarwal 2015). Rounce et al. (2012) conducted the comparative study to investigate the particles and hydrocarbon emissions from diesel and biodiesel-fueled single-cylinder engine with and without after-treatment devices. They found lower unburned hydrocarbon, PM, and carbon monoxide (CO) emissions using biodiesel while higher nitrogen oxides (NO x ) emission obtained with biodiesel in comparison to diesel. Results also show that biodiesel produced lower solid particles and slightly higher liquid particles in comparison to diesel and diesel particulate filter (DPF) can trap around 99% solid particles (Rounce et al. 2012). Another study experimentally investigated the effect of injection strategy and biodiesel on particle emissions from multi-cylinder CRDI diesel engine with variable geometry turbocharger (VGT) at different load conditions (Ye and Boehman 2012). Their results revealed that particle emission reduces while NO x emission increased with higher injection pressure for neat diesel and biodiesel blends. Zhang et al. (Zhang et al. 2013) investigated the effect of fumigated methanol on particle emissions from multi-cylinder automotive direct injection diesel engine. Results showed that particle mass and concentration decreases from moderate to higher operating load and number of accumulation mode particles reduces with fumigated methanol. LPG-fumigated automotive diesel engine was also investigated for performance and gaseous and particle emissions, and results show that larger particles were emitted with LPG fumigation as compared to diesel (Surawski et al. 2014). Dhar and Agarwal (2015) investigated the effect of biodiesel on particle emissions from an automotive engine at a different speed and load conditions. Their results showed that particle concentration increases with engine speed and highest particle concentration obtained with 100% Karanja biodiesel.

Published studies show that formation of particles is dependent on the various engine parameters such as compression ratio, injection pressure and timing, number of injections, and engine speed and load. These studies are mainly conducted on automotive diesel engine at fixed compression ratios. However, relatively lower number of studies also investigated the performance, combustion, and emission characteristics of non-automotive diesel engines (Agarwal and Rajamanoharan 2009; Saravanan et al. 2010; Agarwal and Dhar 2013; Dhar and Agarwal 2014). Among the emission characteristics, previous studies mainly focused on the gaseous emissions (i.e., carbon monoxide, nitrogen oxide, hydrocarbon emissions). Few studies investigated the particle emissions from the non-road diesel engine (Zhang and Balasubramanian 2014a, b; Saxena and Maurya 2016). Zhang and Balasubramanian investigated the effect of butanol addition in the diesel fuel on the particle emissions from a non-road diesel engine (Zhang and Balasubramanian 2014a). Their results indicate that an increase in the butanol percentage leads to increase the fraction of organic carbon in the particles and decreases the particulate mass as well as elemental carbon emissions. Moreover, addition of butanol leads to decrease the total number concentration of volatile as well as non-volatile particles. In another study, the impact of butanol addition in the diesel/biodiesel blend on the particle emission from a non-road diesel engine was investigated (Zhang and Balasubramanian 2014b). Results demonstrated that addition of butanol has potential to reduce the particle geometric mean diameter as well as total particle concentration. However, study summarizes that reduced total particle concentration is due to the reduction in larger-size particles. Additionally, diesel/biodiesel/butanol blend showed reduced total particle-phase polyaromatic hydrocarbons and cytotoxicity of particle extracts. Another study investigated the nanoparticle emissions from a non-road diesel engine fueled with butanol/diesel blends (Saxena and Maurya 2016). It was observed that addition of butanol results in advanced start of combustion and shorter combustion duration in comparison to neat diesel fuel operation. Results also showed that the mass of particles increases with load and maximum reduction in the mass of the particles is observed for 30% butanol/diesel blended fuel. Study summarized that the addition of butanol in the diesel fuel has a potential to reduce the particle emissions. Since very few studies investigated the particle emission from non-road engines (especially mechanical fuel injection system), it is necessary to further explore the different aspects of particle emissions from a non-road diesel engine. To the best of the author’s knowledge, no published study was found on the effect of compression ratio and nozzle opening pressure on particle size and number distribution from a stationary diesel engine. The test engine used in this study is widely used for the small irrigation purpose equipment, small gensets and pumps, especially in India. These engines are mainly equipped with mechanical fuel injection system which injects the fuel in the cylinder typically at low pressure (typically at ~ 200 bars). Fuel injection pressure plays a significant role in the formation of particles during diesel combustion. Currently, particle number concentration emission is not yet included in the emission norms for automotive as well as non-road engines in India. For automotive engines, the particle number emission concentration may be incorporated in the next emission norm in India (Bharat stage VI). From future point of view, stringent legislation norms for non-road diesel engines make this study more relevant. The present study tries to fill the gap in the literature by detailed investigation on particle number emissions from a stationary diesel engine at different compression ratios (16, 17, and 18), nozzle opening pressure (170, 200, and 220 bars), and engine load conditions. This study also explains the correlation between combustion parameter and particle number emissions. Results of this study may be used in the development of this particular kind of engines to meet the future engine norms.

Experimental setup

A four-stroke, single-cylinder, direct injection, variable compression ratio (VCR) diesel engine was used for the present study. The engine was integrated with eddy current dynamometer for torque measurement and control with suitable instrumentation. The specifications of the test engine are given in Table 1. Figure 1 presents the schematic diagram of experimental test rig used for the present study. All the tests are performed at a constant engine speed of 1500 rpm. Experiments are conducted at compression ratio of 16, 17, and 18 at different nozzle opening pressures of 170, 200, and 220 bars. A tilting cylinder block strategy is used for varying the compression ratio of the engine without changing the combustion chamber geometry. In this arrangement, the clearance volume of the combustion chamber can be changed by tilting the cylinder block while the swept volume remains constant. The test engine is equipped with a mechanical fuel injection system which consists of fuel tank, fuel filter, fuel pump, and injector. In this mechanical fuel injection system, the fuel is injected typically at 170 to 220 bars. The range of fuel injection pressure is very limited. Lower fuel injection pressure leads to larger fuel droplet, which results in relatively poor atomization that affect the charge preparation adversely. Thus, it is necessary to investigate the combustion and emissions characteristics for this kind of fuel injection system with lower fuel injection pressure.
Table 1

Test engine specifications

Engine characteristics

Specifications

Make/model

Kirloskar TV1

Maximum power

3.5 KW @1500 rpm

Injection type

Direct injection

Number of cylinder

Single

Cylinder bore/stroke

87.5/110 mm

Connecting rod length

234 mm

Compression ratio

18:1 to 12:1

Total displacement volume

661.45cm3

Fuel injection variation timing

− 27 to − 7 CAD aTDC

Piston bowl shape

Hemisphere

Inlet and exhaust valve diameter

34 mm

Inlet valve open/close

− 364.5/− 144.5 CAD aTDC

Exhaust valve open/close

144.5/364.5 CAD aTDC

Fuel injection pressure range

170–250 bars

Number of nozzle hole

3

Nozzle hole diameter

0.288 mm

Fig. 1

Schematic diagram of experimental setup

Fuel consumption was measured using electronic fuel transmitter installed in the fuel line between the fuel tank and fuel pump of the engine. Cylinder pressure is measured using piezo-electric pressure transducer mounted in the cylinder head. To measure the crank shaft position, crank angle encoder of one crank angle degree (CAD) resolution is used. Average in-cylinder pressure data of 2000 consecutive engine cycles is used for combustion heat release analysis. The rate of heat release (ROHR) is used for calculation of combustion parameters.

Rate of heat release is calculated by using Eq. (1)

$$ \frac{d Q\left(\theta \right)}{d\theta}=\left(\frac{1}{\gamma -1}\right)V\left(\theta \right)\frac{d P\left(\theta \right)}{d\theta}+\left(\frac{\gamma }{\gamma -1}\right)P\left(\theta \right)\frac{d V\left(\theta \right)}{d\theta} $$
(1)

where Q is heat release, P (θ) and V (θ) is pressure and volume as a function of crank angle position θ, and γ is the ratio of specific heat.

Volume as function of crank angle position (θ) is calculated by using engine geometry and presented in Eq. (2)
$$ V\left(\theta \right)={V}_c+\frac{\pi }{4}{B}^2\left(L+R-R\cos \theta -\sqrt{L^2-{R}^2{\sin}^2\theta}\right) $$
(2)

where V c is the clearance volume, L is the length of the connecting rod, R is the radius of the crank, and B is bore of engine cylinder.

Hydrocarbon emission was measured using gaseous emission analyzer (manufacturer: AVL, Austria; model: CDS 450). The equipment is working on the principle of infrared spectroscopy. The details about the accuracy of measurand (hydrocarbon) from the equipment are presented in Table 2. Engine exhaust particle size and particle number distribution were measured by a fast response differential particle spectrometer (manufacturer: Cambustion, UK; model: DMS 500) with measuring particle diameter in the range of 5 to 1000 nm. Exhaust gas samples from the engine exhaust tailpipe passing through the cyclone in primary dilution stage (exhaust gases diluted with hot/warm air). Diluted gases from cyclone orifices pass through the heating line towards the high ratio diluting rotating disc for the second stage of dilution. The optimal dilution ratio controlled by the PC-based user interface and measured particle concentration are corrected for the total applied dilution. Typically, the particles that have diameter (size) less than 50 nm are termed as nucleation mode particles while the particles having diameter between 50 and 1000 nm are termed as accumulation mode particles (Price et al. 2006). In this paper, the nucleation and accumulation of size particles are calculated by fitting the bimodal log normal distribution curve. The total particle number concentration and mass are calculated by using Eqs. (3) and (4) respectively. (No Author, 2015a, b; Saxena and Maurya 2017a).
$$ \mathrm{Total}\ \mathrm{Particle}\ \mathrm{Number}=\frac{1}{16}\sum \limits_{d_{p1}}^{d_{p2}}\frac{dn}{d\kern0.15em \log \left({d}_p\right)} $$
(3)
$$ \mathrm{Mass}\ \left(\upmu \mathrm{g}\right)=2.20\times {10}^{-15}\times {d}_p^{2.65}\ \left(\mathrm{nm}\right) $$
(4)
Table 2

Measuring range and accuracy of gaseous emission analyzer

Equipment

Measurand

Measuring range

Resolution

Accuracy

Gaseous Emission Analyzer (Manufacturer: AVL, Austria; Model: CDS 450)

HC

0 ... 30,000 ppm vol.

≤ 2.000: 1 ppm vol.

> 2.000: 10 ppm vol.

< 2000 ppm vol.: ± 4 ppm vol.

± 3% o.M.

≥ 5000 ppm vol.: ± 5% o.M.

≥ 10,000 ppm vol.: ± 10% o.M.

ppm parts per million

where d p is the diameter of the particle.

A typical emission compliance for this particular kind of engine is presented in Table 3. The existing emission compliance indicates that the total particle number concentration emissions were not considered in the legislation until now. Investigation of particle number concentration for this type of engine makes this study more significant for future legislation norms.
Table 3

Emission compliance of diesel engine for genset application (Indian Emissions Regulations 2017)

Power category

Date of impleme-ntation

NO x (g/kW h)

THC (g/kW h)

NO x  + THC (g/kW h)

CO (g/kW h)

PM (g/kW h)

Smoke (m−1)

At full load

Test cycle

≤ 19 kW

1-7-2003

9.20

1.30

5.0

0.60

0.7

D2-5 mode cycle specified under ISO 8178 Part 4

≤ 19 kW

1-7-2004

9.20

1.30

3.5

0.60

0.7

≤ 19 kW

1-7-2005

9.20

1.30

3.5

0.60

0.7

≤ 19 kW

1-7-2014

  

7.5

3.5

0.30

0.7*

*Smoke shall not exceed above value through the operating load of the test cycle

Results and discussion

In this section, combustion and particle number emission characteristics of a diesel engine at different compression ratios and nozzle opening pressures are presented and discussed. Experiments are conducted at five different engine operating load conditions, i.e., 0, 25, 50, 75, and 100% load for compression ratio 16, 17, and 18. For each test condition, average in-cylinder pressure data of 2000 cycles were used for combustion analysis to reduce cyclic variability in combustion parameters.

Particle size and number distribution

Particle size-number distribution was measured using differential mobility spectrometer at thermally stable and steady-state condition of the engine at each test point. The sampling of particle size data was done for 1 min at 1-Hz sampling frequency. Average of 60 acquired data points of particle size–number concentration is presented in this study. To ensure the repeatability of trends in particle size distribution, measurement was repeated twice at every test point. Figure 2 shows the particle size distribution and rate of heat release (ROHR) at different engine loads. It can be noticed from Fig. 2 that at no-load conditions, the peak concentration of particles are very high and mostly, particles are in nucleation mode (typically < 50 nm). With increase in engine load, peak particle concentration drastically decreases, and peak concentration of particles shifts from smaller diameter particles (nucleation mode) to larger diameter particles (accumulation mode). In other words, particle size and number distribution curve shift from unimodal to bimodal lognormal curve on increasing the engine load. It can be noticed from Fig. 2 that accumulation mode particles (larger diameter) are higher at higher engine loads. At higher loads, increase in accumulation mode particles is also confirmed by another study on the similar engine (Srivastava et al. 2011). With the increase in engine load, the overall temperature of the combustion chamber increases leading to increase in the rate of agglomeration and decrease in nucleation mode particles. Additionally, at higher engine load, amount of fuel injected into the cylinder increases resulting into more fuel-rich regions, which leads to the formation of soot with polyaromatic hydrocarbons (PAH) and other pyrolysis products forming the core for larger-sized particles (Tree and Svensson 2007; Tao et al. 2009). This trend can also be correlated and explained by premixed-mixing controlled combustion using ROHR curve. It can be observed from ROHR curve that crank angle position for maximum ROHR advances (move closer to TDC position) with an increase in engine load due to decrease in ignition delay and higher combustion temperatures result into higher evaporation rates at higher engine loads. Smaller ignition delays at higher loads lead to higher mixing (diffusion) controlled combustion duration, which leads to the formation of larger-sized particles and lower nucleation mode particles. Low/no-load conditions have lower combustion temperature (lower amount of fuel burned) that leads to higher ignition delay and more premixed phase combustion. This leads to higher nucleation mode particles due to lower agglomeration. Few studies suggested that nucleation mode particles (< 50 nm) are possibly liquid nucleation particles and larger particles consist of solid soot particles agglomerates (Kaiser et al. 2002; Maurya and Agarwal 2011). At higher engine load, mixing controlled combustion phase is larger, and temperature in the chamber is also higher at higher loads. Formation and oxidation rates of soot are governed by Eqs. (5) and (6) provided in a recent study (Cheng et al. 2014).
$$ {K}_{\mathrm{f}}={A}_{\mathrm{f}}\ {\mathrm{P}}^{0.5}\exp \left(-\frac{{\mathrm{E}}_{\mathrm{f}}}{\mathrm{RT}}\right) $$
(5)
$$ {K}_o={X}_{O_2}\ {A}_{\mathrm{o}}{P}^{1.8}\exp \left(-{E}_{\mathrm{o}}/ RT\right) $$
(6)
Fig. 2

Variation of particle size–number distribution and ROHR at different engines

where Kf is the formation rate coefficient; Ko is the oxidation rate coefficient; Af and Ao are the pre-exponential factors; Ef and Eo are the activation energies; XO2 is oxygen molar fraction; R is the specific gas constant; T is the gas temperature.

At higher engine load, high combustion temperature and lower oxygen in spray (mixing controlled combustion) lead to the formation of more solid soot particles, which leads to higher accumulation mode particles. A similar trend is observed for the variation of the engine load at other compression ratios as well (Fig. 2c, d), which justifies the strong correlation between particle emission and premixed-mixing controlled combustion based on ignition delay and combustion phasing.

Most of the particles are in size range of 5–300 nm (Fig. 2). Particles larger than 500-nm size are also emitted in significant number from this engine, and these particles are not typically measured by instruments/researcher (Srivastava et al. 2011). Significant concentrations of particles larger than 500-nm size are possibly due to mechanical fuel injection system (fuel injection at lower injection pressure) which results in poor atomization (larger-size fuel droplets). Larger fuel droplet leads to poor evaporation and mixing, which results in pyrolysis of the richer mixture and larger particles are formed in significant number at higher engine load. High fuel injection pressure technologies (such as common rail direct injection) in modern engines lead to better atomization (due to smaller sauter mean diameter (SMD) droplets) and efficient combustion resulting typically smaller soot particles. Mean droplet diameter is generally used to indicate the atomization characteristics of diesel spray. Typically, sauter mean diameter (SMD) of droplet is used to represent the mean diameter of droplet as it indicates the total surface to volume ratio of the entire spray droplets (Pundir 2010).

Fuel injection pressure (FIP) is a very important parameter, which affects the combustion and emission characteristics of a diesel engine (İçıngür and Altiparmak 2003; Agarwal and Agarwal 2007). The fuel evaporation rate depends on the size of the droplets formed due to the atomization of injected fuel in the cylinder (Hiroyasu and Arai 1990). Sauter mean diameter of fuel spray decreases with increase in injection pressure due to cavitations and turbulence (Siebers 1998; Shervani-Tabar et al. 2013), which leads to faster vaporization of the fuel droplets. Pressure and temperature in the combustion chamber (dependent on compression ratio and engine load) play an important in evaporation of fuel droplets. Spray penetration length is also affected by an increase in FIP. With the increase in spray penetration length, chances of spray impingement increases resulting to poor mixing and higher emissions of unburned hydrocarbons, which also leads to increase in soluble organic fraction (SOF) in the particles and nucleation mode particles.

In this study, nozzle opening pressure of fuel injector is varied to change the initial fuel injection pressure. Test engine has mechanical inline fuel injection system with the factory setting of nozzle opening pressure 170 bars. Tests are conducted at three different nozzle opening pressures to characterize the particle emissions and find the injection pressure at lower particle emission. The need of optimizing the nozzle opening pressure for the similar engine is established in a published study for utilization of vegetable oil and study had not measured particle emissions (Agarwal and Agarwal 2007). Figure 3 shows the effect of nozzle opening pressure on particle size–number distribution and ROHR at different engine loads. It is observed from Fig. 3 particle emission in nucleation mode is very high at lower engine loads for 200-bar nozzle opening pressure in comparison to 170 and 220 bars.
Fig. 3

Effect of nozzle opening pressure on particle size–number distribution and ROHR for different engine loads

It can be observed from ROHR curve that at 200-bar nozzle opening pressure, maximum ROHR occurs at a retarded position showing higher ignition delay period. Due to late combustion phasing, average combustion chamber temperature is also lower for 200-bar nozzle opening pressure case.

A possible explanation for higher ignition delay at 200-bar nozzle opening pressure is as follows. Spray penetration length is a function of the momentum of fluid injected, and it increases with increase in injection pressure. A study modeled the spray penetration length and the evaporation of the fuel for injection orifice with pressures ranging from 10 to 200 MPa and found a drastic increase in the spray penetration length when injection pressure was increased from 10 to 30 MPa with a hydraulic flip at 120 MPa (Shervani-Tabar et al. 2013). They also showed that with an increase in injection pressure from 10 to 30 MPa, the rate of evaporation also increases significantly. Another study also concluded that excess air coefficient of spray mixture increases with spray penetration distance (Wakuri et al. 1960). From 170- to 200-bar nozzle pressure, the increase in spray penetration length may be more dominant and leads to the formation of the leaner mixture due to increase in excess air coefficient. Increase in ignition delay can be attributed to delay in attaining the proper combustible fuel-air mixture for combustion due to more spray penetration than 170 bars but the comparatively smaller increase in evaporation than 170 bars. When nozzle opening pressure is increased from 200 to 220 bars, the increase in evaporation is much more dominant and leads to better mixing and less ignition delay. A study also reported about 20% decrease in penetration length with better vaporization (Naber and Siebers 1996).

It can also be observed that nucleation mode particles are higher for lower engine load, and accumulation mode particle increases as engine load increases. This trend is similar to the trend explained in Fig. 2.

At 50% engine load, the increase in nozzle opening pressure shifts the peak concentration towards smaller-sized or nucleation mode particles (Fig. 3). This is possibly due to the fact that with an increase in injection pressure, the droplet size decreases leading to the development of sites for smaller-sized particles. Nucleation mode peak particle concentration is higher for 200-bar nozzle opening pressure as discussed at no load. In accumulation mode particles, peak concentration decreases with increase in fuel injection pressure. At full engine load, accumulation mode particle concentration and distribution is almost similar for 200- and 220-bar nozzle opening pressure. It can also be observed from ROHR curve that the ignition delay difference among the three injection pressures is decreasing with increase in the engine load. It can also be noticed from Fig. 3 that peak ROHR in premixed phase is higher for 200-bar nozzle opening pressure in comparison to other injection pressures. The higher gas temperatures at high loads lead to better evaporation at higher loads than lower engine loads. More fuel injected and high temperature lead to decrease in excess air coefficient as compared to low-load injection (Wakuri et al. 1960). This leads to a better combustible mixture and lower ignition delay resulting in advance in the crank angle position of maximum ROHR as compared to lower engine loads. Due to the higher temperatures and better evaporation at higher engine loads, the ROHR increases with increase in ignition delay. At 200-bar nozzle opening pressure, ignition delay is higher, and peak ROHR value is also higher in comparison to other fuel injection pressures at higher engine loads.

The increase in compression ratio of engine leads to higher peak pressure and temperature in the cylinder. Due to increase in fuel conversion efficiency with increase in compression ratio leads to growing trend to implement higher compression ratio in IC engines. Effect of compression ratio on particle size and number distribution and ROHR is presented in Fig. 4.
Fig. 4

Effect of compression ratio on particle size–number distribution and ROHR at different engine load conditions

It can be observed from Fig. 4 that the at low-load condition, peak particle concentration is higher for lower compression ratio and peak particle concentration shifts towards smaller-size particles with increase in compression ratio. Lower engine load and lower compression ratio have higher nucleation mode particles due to lower combustion temperature and late combustion phasing. ROHR curve confirms that lower compression ratio has retarded combustion phasing leading to a lower combustion temperature. At higher compression ratio, the temperature is higher and combustion starts early. Higher temperature and pressure in the cylinder leads to better vaporization and mixing resulting in higher combustion efficiency and lower unburned hydrocarbon. Therefore, nuclei mode particles are lower at higher compression ratio for low engine load case. At higher engine load, the peak concentration of accumulation mode particles is higher, and peak particle concentration decreases with increase in compression ratio. Increase in temperature due to higher combustion increases the soot oxidation resulting to less accumulation mode particles (Cheng et al. 2014). Gas density in the cylinder (near TDC) increases with increase in compression ratio, which leads to significant decrease in spray penetration length (Surawski et al. 2014). Vaporization of fuel droplets also decreases with increase in gas density. The decrease in excess air coefficient is possible, due to a decrease in spray penetration length and decrease in clearance volume with increasing compression ratio. Both these factors lead to increase in fuel-rich zone in spray and hence, decrease in nucleation mode particles. While the higher temperature and pressure also control the number of accumulation mode particles due to increase in soot oxidation.

Particle size and surface area distribution

It is necessary to analyze the surface area–size distributions because it is directly related to the toxic potential of particles. Higher total surface area of particles means higher availability of surface area for adsorption of PAHs (polyaromatic hydrocarbons), aldehydes, and ketones, which leads to higher toxicity (Khalek et al. 2000; Matti Maricq 2007; Gupta et al. 2010). The growth of nucleation mode particles is by condensation of sulfuric acid and hydrocarbon (Khalek et al. 2000) while accumulation mode is mostly carbon. The effective surface area interacts with the alveoli (when particles inhaled by a human being) cause a serious health issue. For the same mass, smaller particles have a comparatively higher surface area in comparison to larger particles. Smaller particles also have higher suspension time in the environment and therefore smaller particles are more dangerous to health.

Figure 5 shows particle surface area and size distribution with varying nozzle opening pressure for different engine loads. It can be noticed from Fig. 5 that at higher engine load, peak concentration of accumulation mode particles is higher and peak particle concentration decreases with increase in compression ratio; the surface area and size distribution has comparatively very small value in particle size ranging from 200 to 500 nm due to lower concentration. The surface area concentration starts increasing after 500-nm size due to the emission of larger particles in large number. Surface area concentration is proportional to the number of particles and square of particle diameter. It can be observed that for same particle size, the nozzle opening pressure at 200-bar case has the highest values of surface area concentration due to very high concentration of the number of particles. It can also be noticed that surface area concentration is higher for particles with size > 500 nm in 200-bar case among all the three pressures, which was not evident in particle number distribution.
Fig. 5

Particle surface area and size distribution with varying nozzle opening pressure for different engine loads

It can be observed from Fig. 5b that at higher engine loads, the peak surface area concentration shifts from particle diameter < 50 nm to about 200 nm, which again starts increasing for particles with Dp > 500 nm. This type of conventional diesel engines emits significant concentration of large diameter particles also. At higher engine load, there is a significant increase in surface area of particles with diameter 400 nm > Dp > 50 nm. The peak concentration is greater for higher loads due to more amount of fuel injected at higher engine loads. The increase in peak surface area concentration could also be attributed to the fact that the particle concentration has increased, but the peak has shifted to larger diameter particles. The increase in diameter of particles will increase the surface area and size distribution peaks. Moreover, the peak concentration areas are wide (in terms of particle size range) in higher engine load than that at lower engine loads. The trend shows the contribution in surface area by particle diameter < 50- and > 500-nm sized particles significant for lower loads. It was contributed more by 400 nm > Dp > 200 nm and Dp > 700 nm for higher loads.

Particle size and mass distribution

The investigation of mass-size distribution is important because it helps in finding out the diameter ranges contributing most towards gravimetric particulate emissions. Presently, total particle mass and particle number are important for automotive emission legislation. Larger particles have lower atmospheric retention time and captured in the upper part of the respiration system. Therefore, larger particle seems comparatively less harmful. At lower engine load, nucleation mode particles are more but nucleation mode particles consist mostly of low-density volatile part. Density of particles is a function of particle diameter; hence, larger particles have large density and volume. Therefore, the investigation of the mass distribution of the particles is important.

Figure 6 shows the particle mass and size distribution with varying nozzle opening pressures for different engine loads. It can be observed that particle mass distribution shows a similar trend as that of surface area distribution trend. The mass-size distribution also helps in analyzing the contribution of particular size range towards gravimetric particulate emissions. It can be noticed from Fig. 6 that at lower engine load, particle mass is significantly higher for particle diameter larger than 600 nm. Although at lower engine loads, nucleation mode particles are higher but density is low. Since particle density is a function of diameter, therefore, larger particles have large density and volume. Studies also suggest that nucleation mode particle consists mostly of low-density volatile part (Shi et al. 1999, 2000; Burtscher 2005). This leads to a significant increase in mass beyond 600 nm diameter at lower engine loads.
Fig. 6

Particle mass and size distribution with varying nozzle opening pressure for different engine loads

It can be noticed from Fig. 6b that at higher engine loads, particle diameter in the range of 50–400 nm contributes the majority of the total mass while a significant increase in mass distribution for particle is higher than 700 nm particle diameter. This shift in the trend at higher engine load is due to increase in combustion temperature at higher engine load oxidation of large-size particles increases. Additionally, at higher temperature, the volatile part decreases leading to diminishing of the significance of nucleation mode particle mass at higher engine loads (Burtscher et al. 1998).

Correlation between particle number and particle mass emissions

The relationship between particle number and particle mass with respect to the particle size was discussed in the study, and found that smaller-size particles contribute less to the particle mass and larger-size particles contribute more to the particulate mass (Kittelson 1998). For the same particle mass emission, higher particle number emissions contribute more to the surface area distribution. Higher particle surface area tends to adsorb higher quantity of toxic organic compounds. Therefore, it is important to study the relationship between particle number and particle mass emissions. The present study applied a novel approach to correlate the particle number and particle mass emissions. A similar type of approach was also applied in the previous study (Shukla et al. 2017), where particles are measured up to 560 nm, and thus, larger particles having higher mass concentration are not shown.

Figure 7a shows the relationship between particle number and particle mass emission distribution. This approach is used to understand the tendency of particle emissions in the diesel exhaust. Particle number and particle mass for a common particle size were plotted for the measured particle size range 6–1000 nm in increasing order of particle sizes. The larger the size of lobe suggests higher particle numbers as well as mass emissions. The degree of inclination of a specific particle size towards X- or Y-axis indicates its contribution towards particle number and particle mass emissions. If a particle size is closer to Y-axis, this particular particle size contributes more towards particle number emissions. Similarly, if a particular particle size is contributing more towards X-axis (particle mass), it is contributing to particle mass significantly. Few measured points contributing mainly to particle number or mass represent a straight line. The lobe also helps in highlighting the high concentration of small-sized particles and high contribution in mass for large-sized but small in concentration particles with two perpendicular lines.
Fig. 7

Correlation between particle number and particle mass emission

Figure 7b presents constant compression ratio (18) and engine load; the highest concentration of small-sized particles as well as the highest mass of large-sized particles are observed for nozzle opening pressure of 200 bars. The larger lobe for nozzle opening pressure 170 and 200 bars highlights the presence of a wider spectrum of size than nozzle opening pressure 220 bars. At nozzle opening pressure 220 bars, accumulation mode size range decreases due to better oxidation of soot with large diameter.

At lower load, the nucleation mode particles dominate, and the lobe is very close to straight line signifying the very thing spectrum of particle size emitted (Fig. 7c). With increase in engine load, rapid increase in the size range of particles emitted. Larger lobe size suggested an increase in particle number and higher inclination towards X-axis shows more contribution in particle mass. At the highest load, the biggest lobe is observed along with maximum mass of large-sized particles among all the loads.

On increasing the compression ratio, the size of lobe decreases (Fig. 7d), suggesting that particle number and concentration decreases with increase in compression ratio. Increase in compression ratio only affects the size of lobe but not the inclination towards the axis. This leads to a conclusion that with an increase in compression ratio, both the mass and the peak particle concentrations decrease.

Total particle number concentration

Total particle number emission is an important criterion for evaluating the effectiveness of engine operating strategies. Lower total particle number emission into the environment is desirable, and emission regulations also enforce the total emitted particle numbers from engine exhaust. The total particle concentration is the result of the sum of both nucleation and accumulation mode particles. Figure 8 shows the variation of total particle concentration with injection pressure and engine load at different compression ratios. It can be noticed from the figure that at low load conditions, very high total concentration is observed due to the dominance of nucleation mode particles while after 20% engine load, the overall total concentration decreases by order of two. This is due to increase in accumulation mode particles at higher engine loads. The slightly different trend is observed for compression ratio 18 showing the largest area with high concentrations ranging from 1.3 × 109–1.5 × 109 #/cm3.
Fig. 8

Variations of total particle concentration with nozzle opening pressure and engine load for different compression ratios

Figure 9a shows the variation of total nucleation mode particle concentration with engine load and nozzle opening pressure at compression ratio 17. It can be noticed that no-load condition has maximum nucleation mode concentration for all the nozzle opening pressures. The total concentration of nucleation mode particles decreases with increase in engine load at each nozzle opening pressure. The maximum concentration of nucleation mode particles is observed for the case of 200 bars at no-load operating conditions. While at higher engine loads, the nucleation mode–concentration mode decreases by order of two and there is no significant difference observed after 50% load for all test conditions. Possible reasons for the observation are explained in Fig. 3.
Fig. 9

Contours showing variation of a nucleation mode particles and b accumulation mode particles with nozzle opening pressure and engine load

Figure 9b shows the variation of total accumulation mode particle concentration with engine load and nozzle opening pressure for compression ratio 17. Accumulation mode particles are mostly formed in fuel-rich zones where there is a high probability of fuel pyrolysis (Tree and Svensson 2007). At higher engine load conditions, the amount of fuel injected increases and air-fuel ratio decreases considerably in comparison to low-load conditions (lean mixtures). The higher concentration of accumulation mode particle is dominant at low nozzle opening pressure due to large Sauter mean diameter, leading to increased fuel pyrolysis and a liquid core for soot formation. At constant nozzle opening pressure, the increase in load leads to increase in-cylinder temperature and pressure which further exponentially increases the rate of agglomeration. Highest accumulation mode particles are found at full load and lower nozzle opening pressure (Fig. 9b).

Effect of butanol addition on particle number concentration

Due to the crises of depletion of crude oil resources, various alternative fuels were proposed for diesel engine such as different biodiesels, methanol, and butanol. However, these fuels are not commercialized for engine applications due to its limitations (such as carbon deposition on fuel injector tip and instability of fuel blends). In the past few years, engine community researchers intensively investigated the butanol as an alternative fuel for diesel engine due to its inimitable properties (Saxena and Maurya 2016, 2017b; Algayyim et al. 2017; Gürgen et al. 2018). Butanol has higher cetane number in comparison to ethanol and methanol and it is easily miscible with diesel fuel. For understanding the effect of butanol addition in the diesel fuel on the particle number concentration, nucleation and accumulation mode particle concentration are presented with the butanol fraction (%) in diesel fuel for different engine operating load (%) conditions. Figure 10 shows the effect of butanol addition on the nucleation as well as accumulation mode particle concentration for different engine operating load conditions. As discussed in the “Particle size and number distribution” section, Fig. 10 also depicts that the nucleation size particles are higher at lower engine operating load condition while at higher engine operating load, accumulation mode particles are dominant. At higher engine load condition, the mean combustion temperature is higher due to more amount of fuel burned in the combustion chamber. This leads to enhance the rate of agglomeration and results in reduced nucleation size particles (Fig. 10a).
Fig. 10

Effect of butanol addition on the nucleation and accumulation mode particle concentration at CR of 18 and IP of 200 bars

It also has been observed from Fig. 10 that with an increase in the butanol fraction in the blended fuel, the particle number concentration decreases. It is attributed to the higher oxygen content of butanol, which leads to better combustion. The decrease in particle number concentration is more obvious for nucleation mode particles at lower engine load conditions. The figure illustrates that for all the test engine load condition and fuel blends, the nucleation size particles are higher in concentration in comparison to accumulation size particles and butanol addition drastically reduces the nucleation mode particle concentration. The correlation between the particle number and particle mass emission for neat diesel and butanol/diesel blends is presented in Fig. 11. The figure reveals that at 0% engine operating load condition, nucleation mode particles are higher in concentration and the lobe is very close to straight line signifying the very thing spectrum of particle size emitted. However, at 100% engine load condition, the rapid increase in the size range of particles emitted is observed. At 100% engine load condition, larger lobe (in comparison to 0% engine load condition) represents the higher particle numbers as well as mass (Fig. 11). At full load conditions, inclination towards X-axis indicates more contribution in particle mass (similar trend is also observed in Fig. 7c for neat diesel fuel (D100)). The figure also reveals that for 10, 20, and 30% butanol/diesel blends (i.e., B10, B20, and B30), smaller lobe size was observed for both the engine operating load condition, which suggests a lesser number of particles with the lower contribution of particles mass.
Fig. 11

Correlation between soot particle number and mass emissions for neat diesel and butanol/diesel blends

Regression analysis

In this section, further investigation is done to find the combustion parameters affecting total particle concentration emissions. Experimental data of total particle concentration and nucleation mode particles is used to find the empirical correlation between combustion parameters that affect particle number emissions. Parameters selected to characterize the particle emission variations are the air-fuel ratio (A/F), the start of combustion (CA10: crank angle position of 10% heat release), crank angle position of maximum cylinder pressure (CAPmax), and maximum cylinder pressure (Pmax). The air-fuel ratio is considered for regression analysis as air-fuel ratio is an important parameter that determines the richness of charge which is responsible for soot formation. The CA10 is taken into account as the start of ignition as it is indirectly related to the total concentration of particles emitted. The CAPmax helps in determining the combustion phasing or indirectly refining the dependence on the premixed and diffusion phase. The CAPmax is also related to the position of maximum temperature reached which affect soot oxidation. Pmax captures the maximum mean gas temperature reached during engine cycle as pressure is directly related to mean gas temperature.

The correlation found to work well with the experimental data is
$$ \mathrm{Particle}\ \mathrm{Concentration}=a+b\left(\raisebox{1ex}{$A$}\!\left/ \!\raisebox{-1ex}{$F$}\right.\right)+c\left({CA}_{10}\right)+d\left({CA}_{P\max}\right)+e\left({P}_{\mathrm{max}}\right) $$
where a, b c, d, and e are constants, which are determined using regression of experimental data. Resulting values of constants found during regression of experimental data are listed in Table 4.
Table 4

Values of emperical constants for calculation of particle concentrations

 

Regression constants

Correlation coefficient

a

b

c

d

e

R 2

Total particle concentration

CR = 16

− 1,512,618,271

− 5,309,675

192,712,242

− 54,527,786

31,255,318

96.74%

CR = 17

− 1,972,499,288

1,454,403

135,713,460

− 20,691,196

35,746,913

89.71%

CR = 18

− 1,983,311,906

3,872,630

55,469,286

34,644,556

6,512,139

97.0%

Nucleation mode particles

CR = 16

5,640,122,587

− 46,897,427

2,262,402,326

− 28,284,358

− 1,424,140,928

98.52%

CR = 17

− 30,726,405,060

− 67,140,136

1,713,993,354

465,897,345

1,209,195,101

82.65%

CR = 18

248,653,943,442

− 294,524,251

− 2,926,845,598

− 4,010,962,820

1,298,486,141

95.0%

Multivariable regression analysis results indicate statistical significance of the relationship between combustion parameters and particle concentration. The values of the coefficients and the correlation coefficient (R2) for total particles and nucleation mode concentration at different compression ratios (CR) are presented in Table 2. It can be observed that the correlation is strong between the selected parameter and particle concentration. It can be summarized that particle emission is dependent on air-fuel ratio, the start of combustion, combustion phasing, and maximum cylinder pressure. These parameters can be used for prediction, control, and model development for particle emission in conventional diesel engines.

Conclusions

The effect of compression ratio and nozzle opening pressure on particle number emissions has been investigated for a stationary diesel engine at a constant speed of 1500 rpm at different engine loads. It was found that for 0% engine load condition, the peak concentration of particles was very high and most of the particles were in the range of nucleation mode size range (typically < 50 nm). With an increase in the engine load, the particle size and number distribution curve shifts from unimodal to bimodal lognormal curve. Results reveal that the particles having diameter larger than 500-nm size were also emitted in significant number for this kind of engine. Nozzle opening pressure and compression ratio has significant effect on the formation of soot particles and particle size number distribution. The peak particle concentration was higher for lower compression ratio and peak particle concentration shifts towards smaller size particles with an increase in compression ratio. Results also indicate that with the addition of butanol in the diesel fuel, the particle number concentration reduces. Particle diameter corresponding to peak surface area concentration increases with increase in engine load. At lower engine load, particle mass was found significantly higher for particle diameter larger than 600 nm and for higher engine loads, particle diameter in the range of 50–400 nm contributes to the majority of the total mass. A Strong correlation was found for particle number emission with air-fuel ratio, the start of combustion, crank angle position for maximum cylinder pressure, and maximum cylinder pressure using regression analysis. It can be summarized that particle number emissions can be significantly reduced by optimal selection of nozzle opening pressure, compression ratio, and butanol addition at particular engine load.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Rakesh Kumar Maurya
    • 1
    Email author
  • Mohit Raj Saxena
    • 1
  • Piyush Rai
    • 1
  • Aashish Bhardwaj
    • 1
  1. 1.Department of Mechanical Engineering, Advanced Engine and Fuel Research LaboratoryIndian Institute of Technology RoparRupnagarIndia

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