Energy & Fuels 2008, 22, 2991–2996
2991
Effects of an In-Cylinder Active Thermo-Atmosphere Environment on Diesel Engine Combustion Characteristics and Emissions Xingcai Lu,* Libin Ji, Junjun Ma, Chen Huang, and Zhen Huang School of Mechanical and Power Engineering, Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiaotong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed March 17, 2008. ReVised Manuscript ReceiVed May 1, 2008
This work investigated the effects of an in-cylinder active thermo-atmosphere environment (ATAE) on the diesel engine combustion and emissions in a single-cylinder diesel engine. Port-fuel injection of n-heptane was used to prepare the lean fuel/air mixture, and active radicals and heat release were found during the low-temperature reaction (LTR) and high-temperature reaction (HTR). The premixed ratio of n-heptane was used to denote the ATAE intensity. The effects of ATAE intensity and fuel delivery angle of directly injected diesel fuel on the combustion and emissions were evaluated. The experimental results reveal that, as the premixed ratio of n-heptane increases, both the maximum values of the heat release rate (HRR) in the LTR and HTR increase, which means that the ATAE intensity increases. With the increase of the ATAE intensity, the peak value of the diffusion combustion decreases and NOx emissions and smoke opacity reduce substantially at first and attain to the lowest levels at a certain point. Once the ATAE intensity exceeds this critical value, NOx emissions begin to increase monotonously but the smoke opacity increases to a peak point and then begins to further decrease. Overall, under those operating conditions, the optimized ATAE intensity is about 20-30%.
1. Introduction It is well-known that the homogeneous charge compression ignition (HCCI) combustion process is capable of providing both high diesel-like efficiencies and very low NOx and particulate emissions. HCCI is a feature with auto-ignition of a leaner homogeneous fuel/air mixture. This combustion concept was proposed at the end of the 1970s, while it was widely researched until the middle of the 1990s because diesel engines have been faced with increasingly stringent emission legislation. In the past decade, there have been substantial efforts to develop diesel fuel HCCI combustion worldwide. The main problems that hinder the realization of HCCI combustion are the difficulties in vaporization of diesel fuel and the lack of a combustion phase control method. On the basis of these reasons, many methods have been proposed, including PREDIC,1 NADI,2 PCI,3 and MK combustion concepts.4 In these combustion systems, the homogeneous fuel/air mixture is obtained by the impingent between the fuel sprays or using the strong air motion to mix with the diesel fuel. With the development of common rail technology, it is possible to prepare the homogeneous fuel/air mixture with the multiple-pulse injection in one cycle using a conventional commercial injection system. Then, the diesel fuel HCCI combustion based on the * To whom correspondence should be addressed. Telephone: +86-2134206039. Fax: +86-21-34206139. E-mail:
[email protected]. (1) Takeda, Y.; Keiichi, N.; Keiichi, N. Emission characteristics of premixed lean diesel combustion with extremely early staged fuel injection. SAE 961163. (2) Walter, B.; Gatellier, B. Development of the high power NADITM concept using dual mode diesel combustion to achieve zero NOx and particulate emissions. SAE 2002-01-1744. (3) Iwabuchi, Y.; Kawai, K.; Shoji, T.; Takeda, Y. Trial of new concept diesel combustion systemsPremixed compression-ignited combustion. SAE 1999-01-0185. (4) Kimura, S.; Aoki, O.; Kitahara, Y.; Aiyoshizawa, E. Ultra-clean combustion technology combining a low-temperature and premixed combustion concept for meeting future emission standards. SAE 2001-01-0200.
common rail system was widely investigated, and the effects of the injection times, pulse width of injection, injection timing, pulse-injected fuel mass, and injection nozzle angle on the combustion characteristics, emissions, and operating ranges were evaluated.5–7 Shi et al. prepared the mixture by the injection of the diesel fuel directly into the cylinder at near intake top dead center (TDC) and controls the HCCI combustion by adjusting the valve overlap to obtain a higher internal exhaust gas recirculation (EGR).8 Kook et al.9 used the two-stage injection strategy to control the diesel-fueled HCCI combustion. An extremely early direct injection (DI), named as the main injection, using a common rail fuel system was applied to form a premixed charge. This injection was followed by the other direct injection, named as the second injection, executed near TDC to trigger the combustion. Moreover, port fuel injection of diesel fuel, gasoline, n-heptane, and a combination with the main injection of diesel fuel near the TDC have also been studied by many people.10–13 Diesel-fueled HCCI combustion has the advantages of reducing NOx emission by spontaneous ignition at multiple (5) Helmantel, A.; Denbratt, I. HCCI operation of a passenger car common rail DI diesel engine with early injection of conventional diesel fuel. SAE 2004-01-0935. (6) Buchwald, R.; Brauer, M.; Blechstein, A.; Sommer, A.; Kahrstedt, J. Adaption of injection system parameters to homogeneous diesel combustion. SAE 2004-01-0936. (7) Su, W. H.; Liu, B.; Wang, H.; Huang, H. Z. Effects of multi-injection mode on diesel homogeneous charge compression ignition combustion. Transactions of the ASME. J. Eng. Gas Turbines Power 2007, 129 (1), 230–238. (8) Shi, L.; Deng, K. Y.; Cui, Y. Study of diesel-fuelled homogeneous charge compression ignition combustion by in-cylinder early fuel injection and negative valve overlap. Proc. Inst. Mech. Eng., Part D 2005, 219 (10), 1193–1201. (9) Kook, S.; Bae, C. Combustion control using two-stage diesel fuel injection in a single-cylinder PCCI engine. SAE 2004-01-0938. (10) Inagaki, K.; Fuyuto, T.; Nishikawa, K.; Nakakita, K. Dual-fuel PCI combustion controlled by in-cylinder stratification of ignitability. SAE 200601-0028.
10.1021/ef8001907 CCC: $40.75 2008 American Chemical Society Published on Web 06/28/2008
2992 Energy & Fuels, Vol. 22, No. 5, 2008
Lu et al.
Table 1. Specifications of the Single-Cylinder Engine bore (mm) × stroke (mm) displacement (L) compression ratio inlet valve open inlet valve close exhaust valve open exhaust valve close needle open pressure (MPa) nozzle number and diameter (mm) fuel delivery advance angle
98 × 105 0.782 18.5 16 °CA BTDC 52 °CA ABDC 66 °CA BBDC 12 °CA ATDC 24 5 × 0.24 7 °CA BTDC
Table 2. Accuracies of the Measurements measured parameters
unit
measurement range
accuracy
engine speed engine torque cylinder pressure oil inlet temperature oil outlet temperature coolant inlet temperature coolant outlet temperature CO emission HC emission NOx emission fuel consumption
r/min Nm MPa °C °C °C °C % ppm ppm kg/h
0-10 000 0-200 0-25 0-150 0-150 0-150 0-150 0-4.0 0-10 000 0-4000 0-20
(1 (0.1 (0.0005 (1 (1 (1 (1 (0.01 (1 (1 (0.01
points with a lean premixed mixture, resulting in low combustion temperatures. Particulate matter (PM) emissions can also be reduced by the premixed combustion without fuel-rich zones that characterize the heterogeneous combustion process in the conventional DI diesel engine. The key problems of the dieselfueled HCCI combustion are the difficulties in vaporization of diesel fuel and the lack of a combustion phase control method. It is well-known that the n-heptane/air mixture is prone to undergo a two-stage heat release, including low-temperature reaction (LTR) and high-temperature reaction (HTR), and produces many active radicals in the cylinder. On the basis of this reason, in this paper, a small amount of n-heptane was injected into the intake port, the lean homogeneous fuel/air mixture is formed during the intake and compression stroke, and the diesel fuel was directly injected into the cylinder near the TDC. The authors hope to improve the vaporization, mixing, combustion, and emissions of the diesel fuel by the active thermo-atmosphere environment (ATAE), which was produced from the n-heptane chemical reaction. In the experiments, the ATAE intensity was adjusted by changing the n-heptane quantities. Also, the influence of the injection timing of the main fuel on the diesel engine combustion with the ATAE was evaluated.
Figure 1. Experimental system.
Figure 2. Effects of the ATAE intensity on the in-cylinder pressure curves and heat-release histories.
2. Experimental Section A single-cylinder, four-stroke, water-cooled, natural-aspirated diesel engine was employed in the test, and the engine speed was fixed at 1800 rpm The detailed specifications are shown in Table 1. The measured parameters and their accuracy are summarized in Table 2. Premixed n-heptane was injected into the intake pipe by an electronic fuel injector at the location of approximately 0.35 m upstream of the intake port, so that the leaner homogeneous n-heptane/air mixture could be formed during the intake and compression strokes. Near the TDC, the diesel fuel was directly Figure 3. HRR2 max and θ2 max as a function of the premixed ratio. (11) Kim, D. S.; Kim, M. Y.; Lee, C. S. Effect of premixed gasoline fuel on the combustion characteristics of compression ignition engine. Energy Fuels 2004, 18, 1213–1219. (12) Simescu, S.; Fiveland, S. B.; Dodge, L. G. An experimental investigation of PCCI-DI combustion and emissions in a heavy-duty diesel engine. SAE 2003-01-0345. (13) Odaka, M.; Suzuki, H.; Koike, N.; Ishii, H. Search for optimizing control method of homogeneous charge diesel combustion. SAE 1999-010184.
injected into the combustion chamber by the original diesel engine injection system. In this work, the injection amount of the premixed fuel is controlled by an additional universal ECU, which is synchronized with an engine encoder and various sensors. The experimental system is shown in Figure 1. The cylinder pressure was measured by a pressure transducer (Kistler model 6125A). The charge output from this transducer was
In-Cylinder ATAE on a Diesel Engine
Energy & Fuels, Vol. 22, No. 5, 2008 2993 by an analyzer (AVL Digas 4000). Smoke opacity was measured by a smoke meter (AVL 439).
3. Definitions of the Combustion Parameters
Figure 4. HRR3 max versus ATAE intensity under various overall equivalence ratios.
According to our early investigation,14 n-heptane HCCI combustion will undergo LTR and HTR before 10 °CA BTDC and produce many active radicals and heat release. This means that the n-heptane HCCI combustion has reacted almost completely before the diesel fuel was injected into the cylinder. In theory, the premixed n-heptane quantities determined the heat release amount and the radical concentrations. Then, the intensity of the ATAE, R, can be defined as follows: R)
Figure 5. Tmax versus ATAE intensity under various overall equivalence ratios.
˙1Hu1 m × 100% m ˙2Hu2 + m ˙1Hu1
(1)
where m ˙ 1 and m ˙ 2 are the fuel supply rates of n-heptane and diesel fuel and Hu1 and Hu2 denote the lower heating values of n-heptane and diesel fuel, respectively. To analyze the effects of the ATAE on the diesel engine combustion and emissions, some combustion parameters are defined as follows: HRR2 max and θ2 max are the maximum heat release rate and its corresponding crank angle during the HTR, HRR3 max and θ3 max are the maximum heat release rate and its corresponding crank angle during the diffusion combustion, and Tmax and Pmax are the peak values of the temperature and pressure histories. 4. Results and Discussion
Figure 6. Maximum pressure rise rate and its corresponding crank angle as function of ATAE intensity under various overall equivalence ratios.
converted to an amplified voltage using a charge amplifier (Kistler model 5015). Pressure data were recorded using high-speed memory (Yokogawa GP-I). At each test point, the 1440 pulses per rotation (4 pulses per crank angle) from a shaft encoder on the engine crankshaft were used as the data acquisition clocking pulses to acquire the cylinder pressure data. In each operating condition, the cylinder pressures recorded at each crank angle were averaged over 50 consecutive cycles for the experiment. For all data presented, the 0 °CA was defined as TDC at the compression stroke. According to the averaged in-cylinder gas pressure, the heat release curve at each operating point could be calculated by a zero-dimension combustion model. CO, HC, and NOx emissions were measured
Figure 2 gives the effects of the ATAE, which was produced from the chemical reaction of the premixed n-heptane on the in-cylinder gas pressure and the heat release rate. For a fixed overall equivalence ratio, with the increase of the premixed ratio of n-heptane, the ignition timing of the LTR and HTR advance and both the maximum values of the LTR and HTR increase obviously. This means that the intensity of the ATAE increases as the premixed ratio increases, while the ATAE intensity plays a moderate effect on the ignition timing of the diffusion combustion. This point can be attributed to the following reasons: For a certain overall equivalence ratio, increasing the ATAE intensity will need to decrease the diesel fuel quantity. Then, the amount of diesel fuel was reduced as well as the incylinder turbulent kinetic energy production associated with the diesel spray, possible resulting in deteriorated mixing of the DI fuel and decreasing of the local fuel concentrations. As a result, it may offset the improvement of ignition, which was induced by the ATAE. Figure 3 displays the peak value of the heat-release rate during the HTR and its corresponding crank angle as function of the premixed ratio. It is obviously that the HRR2 max increases as the premixed ratio and overall equivalence ratio increase. Also, θ2 max is found to be advanced for a larger premixed ratio and overall equivalence ratio. These tendencies are mainly due to the n-heptane concentrations and in-cylinder gas temperature dominated by the HRR2 max and θ2 max. Figure 4 shows the HRR3 max as a function of the ATAE intensity. For a specific overall equivalence ratio, HRR3 max decreases as the ATAE intensity increases. For a constant ATAE intensity, HRR3 max also decreases as the overall equivalence ratio increases. This point can be explained as follows: In(14) Lu, X. C.; Chen, W.; Huang, Z. A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 1: The basic characteristics of HCCI combustion. Fuel 2005, 84, 1074–1083.
2994 Energy & Fuels, Vol. 22, No. 5, 2008
Lu et al.
Figure 7. Effects of the ATAE intensity on the engine emissions.
Figure 8. Comparison of the engine emission with ATAE to the traditional DICI engine and HCCI engine.
cylinder n-heptane concentrations will increase absolutely in the above two conditions. The increase of the ATAE intensity leads to the shorter ignition delay of the diesel fuel and resulted in a slightly faster initiation of heat release. On the other hand, as the premixed fuel was increased, the amount of diesel fuel was reduced as well as the in-cylinder diesel fuel concentration. As a result, these factors prevent the sharp increase of heat release in the premixed combustion region of the conventional diesel combustion. Figure 5 illustrates the effects of the ATAE intensity on the maximum in-cylinder gas temperature. For a constant overall equivalence ratio, the increase of ATAE intensity improved the diesel ignition and combustion and then leads to the increase of Tmax. Figure 6 shows the maximum pressure rise rate and its corresponding crank angle versus the ATAE intensity under the different overall equivalence ratios. It is obvious that the maximum pressure rise rate increases rapidly as the ATAE intensity and overall equivalence ratio increase. In most condi-
tions, it should be noted that the maximum pressure rise rate does not occur in the diffusion combustion of the diesel fuel but in the HTR of the n-heptane. This point can be verified from the bottom panel of Figure 6. Under lower ATAE intensity (
