Test fuels are D100 (pure diesel) and D80B20. The main properties of D100 and D80B20 are tested and displayed in Table 1. The cetane number, lower heating value, kinematic viscosity and density of test fuels are measured in accordance with the standards of ASTMD 613, ASTMD 240, ASTMD 445 and ASTMD 1298. The cetane number is measured and obtained through the method of calibration and interpolation. The lower heating value is measured using an oxygen bomb calorimeter. The kinematic viscosity is measured using a thermostatic capillary. The density is measured with a density meter (according to the principle of Archimedes). The uncertainties from measurement methods and processes could be neglected because the instruments, the measurement standard were fixed and the experimenters are also the same group. Further, the average of 5 repeated experiments is used as the final result, which can minimize the uncertainties. N-butanol has the low viscosity and distillation temperature, which improve the fluidity and volatility. Blending n-butanol is helpful to improve the spray atomization quality. However, the cetane number of n-butanol is only 16, resulting in a poor ignitability of D80B20. The LHV of D80B20 is 40.6 MJ/kg which is lower than 42.5 MJ/kg of D100.

The combustion and emission performances of test fuels are studied on a common rail diesel engine with EGR system. Table 2 gives parameters and specifications of the test engine. Fig. 1 shows the arrangement for test system. Injection pressures and injection timings are controlled through the BOSCH injection system. An electric dynamo meter (CAMA CW160B) is used to control the engine working torque and speed. The peak torque is achieved between 1600 and 1800 r/min. In this research, 1800 r/min is chosen. The torques of 50, 150, 200, 250, 300 and 350 Nm are chosen to represent different engine loads and the corresponding brake mean effective pressures (BMEP) are 0.14, 0.42, 0.56, 0.70, 0.84, and 0.98 MPa, respectively. The initial injection strategy includes pilot injection and main injection. A piezo-electric type pressure sensor (6052A, Kistler) used to measure the in-cylinder pressure is installed on the top of the first cylinder. And then the pressure signal is amplified and analyzed by a charge amplifier (5019B, Kistler) and a combustion analyzer (KIBOX, Kistler), respectively. A magnetoelectric crankshaft position sensor with the resolution of 0.1 degree crank angle (°CA) is applied to record the crank angle position. In each measurement, 100 consecutive cycles are recorded and averaged with the KiBox-Cockpit software. NO_X emissions are measured by MAHA MET6.X with volume concentrations and UFPs emissions are measured by an aerodynamic particle sizer spectrometer (TSI SMPS-3936). The range and accuracy of the test apparatus are shown in Table 3. The engine is warmed up under idling condition until the coolant temperature and the lubrication oil temperature satisfy with the requirement. When the test fuel is changed, the engine operates for 5 min to ensure previous fuel in fuel line and rail pipe have been consumed. In each condition, the engine operates stably for 3 min.

Based on the in-cylinder pressure, heat release rate and in-cylinder combustion temperature for diesel engines are calculated by a simplified apparent heat release model [27,28]. The heat release rate and temperature in cylinder can be calculated by Eqs. (1) and (2), number geometric mean diameter (NGMD) and volume geometric mean diameter (VGMD) are adopted for a typical diameter analysis of UFPs and the calculation are listed in Eqs. (3)–(7). The meaning represented by the letters in each equation is detailed in reference [10]. (1)dQdφ=kk−1pdVdφ+1k−1Vdpdφ (2)dTdφ=1mcv(dQdφ−pdVdφ) (3)NGMD=exp⁡[∑lun×ln⁡DpNC] (4)VGMD=exp⁡[∑luv×ln⁡DpVC] (5)NC=∑lun (6)VC=∑luv (7)n=ctQ×ϕη

In this study, the start of injection (SOI) is defined as the crank angle corresponding to the maximum driving current of the high pressure injector. The start of combustion (SOC) is defined as the zero point of heat release rate from the negative to positive and the end of combustion (EOC) is the zero point of heat release rate from the positive to negative. The ID is the period between SOI and SOC, and the CD is the period between SOC and EOC. Table 4 the specific parameters of test fuels. Table 5 lists the SOCs and EOCs of D100 and D80B20 under various engine loads. The IDs and CDs of test fuels are displayed in Fig. 2. The SOCs of D80B20 are always later than those of D100 for lower cetane number and worse ignitability. And thereby the IDs of D80B20 are longer than those of D100. Obviously, the ID decreases and the CD increases with engine load from Fig. 2. It can be also observed that the gap of IDs or CDs between D80B20 and D100 get smaller as engine load increases. The gap of SOC and EOC decreases as the load increases among fuels. This indicates that the reactivity differences do not play a significant role at high load cases since the ambient temperature and pressure are high enough at these high load cases. At 0.14 MPa BMEP low load, the ID of D80B20 is 10.5 °CA, which is 1.08 °CA longer than D100. The temperature and pressure in cylinder increase as engine load increases. And at 0.98 MPa BMEP, the ID of D80B20 is 0.64 °CA longer than that of D100.

Table 4

BMEP	EGR rate	Inlet pressure (kPa)		Inlet temperature (°C)		Injection pressure (MPa)		Exhaust temperature (°C)		λ
		D100	D80B20	D100	D80B20	D100	D80B20	D100	D80B20	D100	D80B20
0.14 MPa	0	102.9	103.6	43.5	51	57.8	58.8	161	180	5.11	5.09
	10%	103	103.5	43.6	51	57.7	58.7	164	180	5.21	5.01
	20%	102.9	103.4	43.8	50.9	57.5	58.7	165	178	4.93	4.89
	30%	102.9	103.5	43.7	50.8	57.4	58.7	166	177	3.06	4.72
	40%	103.1	103.3	45.4	50.8	57.4	58.7	168	174	4.75	4.62
0.42 MPa	0	115	113.3	46	50.8	75.2	73.6	213	220	2.94	3.02
0.56 MPa	0	122	121	48.7	51.5	80.7	82.4	273	255	2.46	2.59
	10%	123	122.9	48.9	52.2	80.6	82.4	277	273	2.55	2.50
	20%	122.8	124	49.2	52.8	80.4	82.6	280	285	2.51	2.43
	30%	122.8	123.6	49.3	53.1	80.2	82.8	280	288	2.44	2.37
	40%	123.2	124	49	53	79.9	82.4	280	289	2.40	2.32
0.70 MPa	0	133.8	131.5	48.8	52.8	89.3	89.9	298	300	2.27	2.27
0.84 MPa	0	141.6	139.5	49.5	53.7	97	97.4	325	333	2.08	2.1
0.98 MPa	0	151.7	152.3	50.6	54.9	101.8	103.6	356	360	1.96	1.9
	10%	152.3	153.2	51.8	55.7	101.7	103.6	364	374	1.9	1.86
	20%	153.2	154.3	52.7	56	101.6	103.8	369	377	1.85	1.8
	30%	154.4	155	52.4	55.5	101.5	103.5	373	377	1.81	1.76
	40%	155	155.6	52.3	55.4	102.1	103.3	372	380	1.76	1.71

Fig. 3 shows the pressure, temperature and heat release rate in cylinder of D100 and D80B20 at 0.14, 0.56, 0.98 MPa BMEP. Table 6 lists the maximum heat release rates (MHHRs) and MTs under different engine loads. It can be seen from Fig. 3a that the MHRR of D80B20 is significantly higher than D100 under low load. On the one hand, the pressure and temperature in cylinder are relatively low and the influence of low cetane number on ignition delay is obvious under low engine load. And long ID of D80B20 makes a large amount of the fuel/air mixture. On the other hand, the low viscosity and high volatility of D80B20 improves the spray quality and mixture uniformity. The spray particle size is mainly related to the viscosity and surface tension of the fuel. The lower the viscosity and surface tension, the easier the fuel spray breaks into smaller droplets [17]. Wu et al. [15] found that the air entrainment around the blend enhanced and the spray in front became more disorder after mixing n-butanol. In addition, the color of spray head was shallower than basic fuel, which suggested that spray particle size became smaller and the atomization was more uniform with n-butanol blending [15]. Accordingly, a large and uniform fuel/air mixture is beneficial to speed up the fuel combustion. Therefore, the premixed combustion strengthens and the MHRR increases with n-butanol addition. At 0.14 MPa BMEP, the MHRR of D80B20 is 37.09 J/°CA, increased by 2.4 J/°CA compared to diesel. Further, the MHRR gap between D80B20 and D100 gradually narrows with engine load. And when BMEP ≥ 0.84 MPa, D80B20 has lower MHRRs than D100. As the load increases, the pressure and temperature in the cylinder increase gradually. In high in-cylinder thermal condition, the influence of fuel reactivity becomes less crucial and thereby the gap of ID between D100 and D80B20 becomes small. Meanwhile, the fraction of diffusion combustion becomes larger at high engine load cases. The high oxygen content of D80B20 can release a large number of oxygen atoms and active free radicals during the stage of diffusion combustion, which can speed up combustion speed and promote combustion completeness [26]. However, the LHV of the n-butanol is lower than that of diesel. As a result, under medium and high loads, the MHRR of D80B20 is lower than D100. Meanwhile, D80B20 has a higher heat release rate than D100 in the stage of diffusion combustion, which can be seen from Fig. 3c. It can also clearly been observed in Fig. 2 that the CD of D80B20 is always shorter than D100, which indicates the combustion heat release of D80B20 is more concentrated. Therefore, D80B20 has the high maximum temperatures under all conditions.

Table 7 displays the MHRRs and MTs of the test fuels with EGR at 0.56 and 0.98 MPa BMEP. With the increasing of EGR ratio, the MT decreases obviously.

The NO_X emissions of D100 and D80B20 at 1800 r/min are shown in Fig. 4. It can be observed D80B20 has higher NO_X emissions than D100. Previous studies have shown that NO is the mainly composition of NO_X exhausted by diesel engine and meanwhile N₂ and O₂ can react and then produce NO easily when the temperature in cylinder is higher than 1500°C [29,30]. Accordingly, MT has a great influence on NO_X emissions [31]. The MTs of D80B20 are higher than those of D100 and thereby the NO_X emissions of D80B20 are higher than D100. Fig. 5 displays the influence of EGR ratio on NO_X emissions at 0.56 and 0.98 MPa. With the increasing of EGR ratio, the NO_X emissions of D100 and D80B20 both significantly decrease mostly due to the decreased MTs. As load increases, the reduction degree of NO_X emissions with EGR ratio increases. The NO_X emission of D80B20 is always higher than that of D100, irrespective using EGR or not.

PM is one of the harmful exhaust pollutants emitted by diesel engines. 2.5∼220 nm particles are defined as UFPs in this study. It has proved that more than 90% total PM number concentrations are UFPs. Nucleation mode particle (NCMP), aitken mode particle (AKMP) and accumulation mode particle (ACMP) are included in UFPs emitted by diesel engines. The diameter of NCMP is less than 50 nm, AKMP is between 50 and 100 nm and ACMP is between 100 and 220 nm [10]. The particles with a diameter in the range of 5∼100 nm are mainly composed of volatile organic compounds and sulfates [32]. And the particles with a diameter greater than 100 nm are mainly composed of accumulated soot particles and substances attached to the surface [32]. There are differences in the deposition characteristics of particles with different diameters in the human body. Particles with small diameters, especially nuclear particles, are more likely to penetrate into the human respiratory system and endanger human health [3]. Fig. 6 shows the number concentrations (NCs) and volume concentration (VCs) of UFPs under various loads. The diameter distributions of UFPs are displayed in Fig. 7. At 0.14 MPa BMEP, the low thermal condition is not conducive to fuel complete combustion and thereby the UFPs emissions are high. At 0.42 MPa BMEP, although the thermal condition in cylinder has improved, the amount of fuel injection increased. At this time, the quality of the fuel atomization in cylinder is worse than that of 0.14 MPa BMEP. Therefore, the UFPs emission at 0.42 MPa BMEP is higher than that of 0.14 MPa BMEP. The NCMPs and AKMPs of two fuels have a very large proportion in NCs while a very small proportion in VCs. The lower viscosity and distillation temperature of n-butanol making D80B20 has higher spray atomization quality and more uniform mixture than D100. In addition, the chemical mechanism of neat butanol is different from the n-butanol/alkane fuel blends. The oxidation of n-butanol starts with H abstraction in the alpha carbon due to the lowest BDE. And due to the reaction R2915, it forms stable butyraldehyde rather than QOOH radicals, degenerates the branching production of OH [33]. N-butanol addictive can greatly reduce the UFPs emissions, and both the VCs and NCs reduced accordingly. At 0.70 MPa BMEP, the VCs and NCs of D80B20 decrease by 46.4% and 31.1% compared with D100.

Fig. 8 exhibits the effects of EGR ratios on NCs and VCs of UFPs at 0.14, 0.56 and 0.98 MPa BMEP. Fig. 9 displays the diameter distribution of UFPs under various EGR ratio. The NCs and VCs of UFPs increase with EGR ratio due to the worse combustion quality. At 0.14 MPa BMEP and 0.56 MPa BMEP, the NCs and VCs of D80B20 are at a low level when the EGR ratio is less than 30%. At 0.98 MPa BMEP, the UFPs emissions increase significantly with the increasing of EGR ratio. Further, D80B20 with 10% EGR ratio has a lower UFPs emissions than D100 without EGR. Table 8 gives the VGMDs and NGMDs of UFPs. The VGMDs and NGMDs of D80B20 are smaller than those of D100. Additionally, the NGMDs and VGMDs of increase with EGR ratio. For example, the NGMDs of D80B20 increases from 58.8 to 84.2 nm as EGR ratio increase from 0% to 40% at 0.98 MPa BMEP.