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Cold start control strategy for a two-stroke spark ignition diesel-fuelled engine with air-assisted direct injection
Accepted Manuscript Cold Start Control Strategy for a Two-stroke Spark Ignition Diesel-fuelled Engine with Air-assisted Direct Injection Rui Liu, Minxiang Wei, Haiqing Yang PII: DOI: Reference:
S1359-4311(16)31289-3 http://dx.doi.org/10.1016/j.applthermaleng.2016.07.148 ATE 8746
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
15 October 2015 20 June 2016 21 July 2016
Please cite this article as: R. Liu, M. Wei, H. Yang, Cold Start Control Strategy for a Two-stroke Spark Ignition Diesel-fuelled Engine with Air-assisted Direct Injection, Applied Thermal Engineering (2016), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2016.07.148
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Cold Start Control Strategy for a Two-stroke Spark Ignition Diesel-fuelled Engine with Air-assisted Direct Injection Rui Liu, * Minxiang Wei, Haiqing Yang College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China Abstract: The control strategies of cold start were investigated on a two-stroke spark ignition diesel-fuelled engine with air-assisted direct injection. The successful cold start of the engine at 5°C ambient temperature verified the control strategy. Bench tests at ambient temperature (9 °C) were performed to study and quantify the control strategies according to the engine dynamic characteristics, including the engine speed, cylinder pressure and exhaust emission. The ignition timing, ignition energy, injection timing and amount of fuel injected were quantitatively analysed using the cold-start control strategy. An ignition timing of 30-40 °CA bTDC is suggested to reduce the transition time from the cranking process to the start-up process. The saturated ignition energy favours the cold start performance with a moderate to low fluctuation of the engine speed from the start-up process to the warm-up process. A proper amount of fuel injection can improve the dynamic performance of the start-up and warm-up processes. A rapid cold start with short cranking was employed at an injection timing of approximately 50°CA bTDC, and moderately advancing the injection timing was found to accelerate the warm-up process.
Keywords: cold start; two-stroke spark ignition engine; heavy fuels; air-assisted direct injection; control strategy 1. Introduction * Corresponding author. Tel./fax: +86 025 8489 4596. E-mail address: [email protected] (Rui Liu). 1
Heavy fuels are usually used in compression ignition (CI) engines, which are known to have poor power-to-weight ratios compared to spark ignition (SI) gasoline engines. Considering the high volatility, low viscosity and storage/transportation problems of gasoline, the military requirements for a single-fuel policy to cover multi-type vehicles and equipments are significantly intensified in many countries. In recent years, high performance SI reciprocating engines capable of using heavy fuels (i.e., kerosene and diesel fuels) have been widely applied for the ground fleet and unmanned aerial vehicles (UAV) in the military [1-3]. The demand for multi-fuel SI engines in non-military applications (e.g., outboards, motorcycles, and snowmobiles) is also increasing due to safety concerns [4-6]. An engine cold start refers to starting an engine with an inner chamber temperature that is the same as the ambient temperature that occurred from the last stop. SI gasoline engines have no difficulty achieving a successful cold start as they have a carburettor, port injection and direct injection (DI). In particular, the high atomization of DI is more helpful for rapidly starting the engine. Compared to the physicochemical properties of gasoline, heavy fuels show lower volatility, higher viscosity and lower evaporation. It is difficult to start heavy fuels engines equipped with carburettors or port injection systems without auxiliary preheating or aided ignition, which require additional energy sources [7, 8]. It is worth noting that the low vaporized mass of the injected fuel easily causes pollution issues during the cold start of non-DISI heavy fuel engines. Hence, it is desirable to use DI to achieve excellent fuel evaporation and reduce the fuel mass injected during the cold start compared to traditional heavy fuel engines. The high pressure fuel DI is usually used in common rail CI diesel engines and SI gasoline engines, which are equipped with high pressure fuel pumps and have high application costs. By contrast, the low pressure air-assisted direct injection (AADI) system used in automobiles or motorcycles is fuelled with normal low pressure fuel and 2
compressed air. The fuel is injected into a premixed chamber filled with compressed air. The mixture of fuel and air is directly injected into the combustion chamber by an air-assisted injector. The sauter mean diameter (SMD) of the fuel spray can even reach approximately 5 μm. Hence, The AADI has been taken into account and applied in high specific power output two-stroke DISI engines [9]. To improve the SI engine cold start performances with regard to the fuel demand and exhaust emissions, most work has focused on engines that use gasoline or liquefied petroleum gas (LPG). Some studies have also focused on methanol or ethanol fuelled engines [10-14]; however, little work has been reported on SI heavy fuel engines. Singh et al. [4] compared the cold starts using JP-5/8 and gasoline on a two-stroke multi-fuel SI engine. The amount of fuel required for starting the engine at cold temperatures differed significantly. Compared to gasoline, for starting with JP-5/8, the initial injection amount was heavily compensated and increased at lower temperatures. The fuel was injected very late and the ignition was very early, thus having a stratified combustion start. Homogenous combustion mode was also tried but was only successful with gasoline and ethanol fuel. Suhy et al. [15] studied the feasibility of burning kerosene in a port injection two-stroke SI engine with a vortex pneumatic atomizer to reduce the droplet size. The method of heating the atomization air to overcome the cold starting problem with kerosene fuel was developed, and the engine would not start if the atomizer air temperature was lower than 115 °C. Higher atomizer air temperatures and lower pressures and flow rates were required to start the engine. Groenewegen et al. [16,17] discussed the results of the continuation of experimentation in utilizing heavy fuels, such as JP-8 and D2 diesel, to satisfy the necessary power requirements of a four-stroke SI engine. The engine, which used an air-assisted port fuel injection system, was originally designed to run on gasoline. A cold start was not achieved with the 3
air-assisted injector, and the engine was started using gasoline and then switched to the heavy fuels via a three-way valve. Cathcart et al. [18] applied the AADI technology to SI engines and conducted cold start experiments on a V6 two-stroke engine using JP5 fuel at low ambient temperatures. Due to the excellent atomization produced by the AADI, the starting time test results showed that a start time of approximately 4 sec was achieved at -10 °C, decreasing to 1 sec at a temperature of approximately 20 °C. The AADI has shown a significant advantage in addressing cold start problems in heavy fuels engines; however, the relevant control strategies for realizing rapid cold starts have rarely been reported. Utley et al. [19] adopted a double injection strategy combined with a multifunctional high power ignition system at soaked temperatures from +25 °C to -10 °C and improved the cold start performance of a four-stroke SI ethanol fuelled engine with AADI. Yu et al. [20] conducted the cold start control strategy on a turbocharged direct injection gasoline engine. The results showed that fuel injection was the key parameter, the influence of which was greater than the ignition advance angle of the cold start process. Qi et al. [21] designed a corresponding control strategy to study the DI gasoline engine cold start characteristics and basic requirements, and conducted a validation test of the proposed control strategy. Wang et al. [22] proposed the injection and ignition timing control strategy for a gasoline engine with AADI, which mainly introduced the calculation method of timing control. All of the above works in the literature paid attention to solving or improving the cold start of SI gasoline, LPG, and methanol engines. However, the control strategies of cold start for diesel-fuelled SI engines with AADI have not been completely reported or investigated. The cold start performance of the engine is strongly related to the match of the fuel-air injection and ignition. A reasonable match will facilitate the fuel-air mixture preparation in cylinder. As shown in Fig. 1, the combustion control in the cylinder of the 4
AADI SI engine is based on the sequence of injection and ignition control. The injection control consists of fuel injection and the fuel-air mixture injection, which specifies the amount of fuel and injection timing. Ignition control includes the ignition energy and ignition timing. These parameters have considerable effects on the combustion and emission of the diesel-fuelled SI engine during a cold start. Therefore, it is necessary to investigate the corresponding control strategies to ensure the success of a rapid cold start and reveal the governing laws of these parameters. The objective of this paper is to investigate the control strategies of cold starts for an AADI diesel-fuelled engine. Bench tests were performed to study and quantify the designed control strategies according to the engine dynamic characteristics, including the engine speed, cylinder pressure and exhaust emissions. The ignition timing, ignition energy, amount of fuel injected and fuel-air mixture injection timing were quantitatively analysed using the cold-start control strategies. 2. Cold Start Control Strategy 2.1. Ignition Control Ignition timing control during a cold start is different from the steady-state condition. A constant ignition timing angle based on the cylinder temperature is usually used until the successful cold start. For a two-stroke SI engine with AADI, the ignition timing has a strong relationship with the fuel injection timing. It is notable that an early ignition timing may cause the ignition timing angle to overlap with the angle phase of the fuel-air mixture injection. Additionally, a late ignition timing will delay the combustion phase in the cylinder, thereby reducing the engine work ability of a poor cold start. In this paper, ignition timing control is mainly based on the variations of the engine speed of the start state. The cylinder temperature has an important influence on the fuel viscosity and volatility, so the initial ignition timing angle requires 5
quantitative correction before starting the engine, as represented by Eq. (1):
ig ig (n) ig (Tcl )
(1)
where θig denotes the ignition timing angle and θig(n) represents the interpolation value from a preset look-up table, which is based on the engine speed n. Δθig(Tcl) is the corrected value of the ignition timing angle, and Tcl refers to the cylinder temperature. The ignition energy at the cold start should be improved as much as possible to start the engine smoothly while ensuring that the ignition system remains stable. Inductive ignition generally releases a high discharge energy of approximately 50-100 mJ. A dual-spark plug inductive ignition system is applied in this paper. The ignition energy was controlled by the charge pulse width of the primary coil. It is noted that the 12 V battery terminal voltage decreases to approximately 9 V during the primary period of the cold start. To ensure that the ignition energy is maintained at the target level, the charge pulse width must be corrected with the value of the battery voltage. Equations (2) to (5) give the ignition energy correction calculation basis. As the battery voltage decreases, the charge pulse width could be appropriately increased to make the charge current reach the target value.
Eig N p E p
(2)
E p Lp I p 2 2
(3)
In the above, Eig denotes the ignition energy calculated from the number of spark plugs Np, the energy conversion efficiency of the ignition coil η, and the storage energy of the primary coil Ep. According to Eq. (3), Ep is calculated by using the inductance value Lp and the charge current of the primary coil Ip:
6
0 t t 0 I p U p 1 exp( t ) / R p I f t f t Tch t t0 Tch (U p )
t0 t t f (4)
(5)
where t0 denotes the start charging time of the primary current and t denotes the end charging time. tf represents the time of the primary current Ip reaching the saturation charging current If. Up refers to the battery voltage, τ is the time constant, and Rp is the resistance value of the primary coil. Lp, τ, and Rp are constant values for the ignition coil. Thus, the charge pulse width Tch is calculated by t0 and t, which are related to the battery voltage Up. Figure 2 illustrates the variations of the charge pulse width with Up. The figure gives the corrected values of 2 ms, 4 ms, and 6 ms, which will be applied for cold start investigations in sections 3 and 4. 2.2. Injection Control Injection control of AADI has a great influence on engine cold start. An appropriate injection quantity of the fuel-air mixture and injection timing allow for an ignited rich mixture to form around the spark plug. The injected mixture quantity determines the total concentration in the cylinder. An overly lean mixture around the spark plug is difficult to ignite, and the ignition energy requirements are higher. For mixtures that are too rich, even if they are ignited, misfire due to incomplete combustion may easily occur with the increasing engine speed. Additionally, the injection timing of the fuel-air mixture affects the mixture concentration distribution around the spark plug at the ignition timing. At a fixed ignition timing angle, an early fuel-air injection may form a lean mixture due to the scavenging loss or fuel condensing on the wall and the piston top surface at a low cylinder temperature. A late injection will lead to less mixing and evaporation time of the fuel-air, inducing condensation of the mixture on the spark plug electrode. Both 7
cases will lead to insufficient combustion in the cylinder, which could easily lead to engine flameout. Similar to ignition control, the injection control of AADI is also related to the engine speed, the cylinder temperature and the battery voltage during a cold start. According to this, the correction strategies of the fuel injection pulse width and injection timing could be illustrated as Eq. (6) and (7):
Tin Tin ( , n) * f (Tcl ) Tin (U p )
(6)
where Tin denotes the fuel injection pulse width. Tin(α,n) is the interpolation value from a two-dimensional loop-up table, which is based on the throttle valve position α and the engine speed n. f(Tcl) refers to the fuel enrichment factor calculated from the cylinder temperature, and ΔTin(Up) is the corrected value determined by the battery voltage Up. The applied voltage to the fuel injector will affect the opening rate of injectors. If the voltage is low, the injection on signal will be longer to compensate. Figure 3 shows the corrected value of the injection pulse width with Up.
in in ( , n) in (Tcl )
(7)
In Eq. (7), θin denotes the injection timing angle, which is calculated according to the injection timing angle θin(α,n), and the corrected value Δθin(Tcl) is based on the cylinder temperature. 2.3. Cold Start Control Strategy Design According to the peak cranking speed, the start-up target speed and the fuel enrichment factor after start-up, the cold start process is divided into four processes that consist of the cranking process, the start-up process, the transient process after start-up, and the warm-up process. The peak cranking speed is the maximum engine speed allowed by the start motor. The start-up target speed indicates whether the engine enters into the transient process after start-up. Figure 4 gives the cold start control strategy of this paper. The cranking motor advances the engine into the cranking process following a powering electronic 8
control system. The fuel enrichment factor is the calculated interpolation result fst from the look-up table, which is based on the cylinder temperature. The ignition timing angle θig used is the value θ1. Once the engine is accelerated to a speed higher than the value of the peak cranking speed; it can be determined that the engine is fired. The ignition timing angle θig used is the value θ2. Under this condition, fst maintains the same value. When the engine speed reaches the preset value of the start-up target speed, the engine starts successfully. The fuel enrichment factor calculated by fast and fcyl starts to decrease automatically based on the current cylinder temperature with the same ignition strategy. The engine enters the warm-up condition once fast falls to 1. The fuel injection pulse width is corrected by fcyl, and the ignition timing angle is calculated from the look-up table. Figure 5 gives the predicted fuel enrichment factors at temperatures of 0 °C to 90 °C. At lower temperatures, the initial injection amount according to fst is heavily compensated. The fuel enrichment factor fst is not linear with temperature, while fast and fcyl almost show the linear trends. Figure 6 gives the predicted look-up table of the ignition timing angles below 3000 rpm, the values used are based on the CDI (capacitor discharge ignition) system of the prototype engine. f: fuel enrichment factor; fst: fuel enrichment factor of the cranking process and start-up process; fast: fuel enrichment factor of the transient process following the start-up; fcyl: fuel enrichment factor of the warm-up process; θig: ignition timing angle; θ1: ignition timing angle of the cranking process; θ2: ignition timing angle as the engine is fired; θmap: ignition timing angle based on the MAP table; n: engine speed.
3. Experimental Setup and Strategy Verification 3.1. Experimental Setup The cold start test bench for the SI AADI diesel-fuelled engine includes a 12 V battery power supply, the data acquisition system, the electronic control unit, and the control computer. The engine specifications are 9
listed in Table 1. Figure 7 shows the setup installed in the test cell. The air-assisted injectors integrated in the designed fuel-air rail are mounted on the cylinder heads. The computer records the data from the acquisition system in real-time, which mainly includes temperatures and pressures. The engine parameter calibration software is used to preset different control parameters downloaded to the engine electronic control unit. It also records the data of the engine speed, injection parameters, ignition parameters and some other parameters. 3.2. Control Strategy Verification Engine cold start tests were conducted to verify the control strategy of the cold start proposed in subsection 2.3 at the ambient temperature (5 °C). The peak cranking speed was approximately 800 rpm. The throttle valve position was 6%. The value of fst was 4. Figure 8 shows the MAP table of the fuel injection pulse width values used for the verification test. Figure 9 shows the MAP table of injection timing angles. In the cold start test, θin was set at a constant value of 50 °CA bTDC, without using the MAP table, to start the engine. The fuel pulse width and charge pulse width are expressed in μs. Figure 10 shows the related engine parameter histories of the cold start. It can be observed that the variations of the ignition and injection parameters were consistent with the designed control strategy. After the motor started to advance the engine into the cranking process, the fuel pulse width and charge pulse width were corrected and the ignition timing angle was set to 10 °CA bTDC. The engine entered the start-up process as the cranking speed exceeded 800 rpm. Then the ignition timing angle was set to 30 °CA bTDC, and the cylinder head temperature did not change significantly after this process. The engine entered into the transition process after the start-up once the engine speed was higher than 1500 rpm. Then the fuel pulse width decreased at a certain slope, and the cylinder head temperature increased significantly. Finally, the engine entered the 10
warm-up process at the end of the automatic decreasing process of the fuel pulse width. Thus, the designed control strategy was verified, as it resulted in a successful cold start at this ambient temperature. 4. Effects of the Key Parameters on Cold Start Performance The key parameters of the control strategies and their effects on the cold start of the SI AADI diesel-fuelled engine are quantitatively assessed in this section. The ignition timing, ignition energy, amount of fuel injected, and injection timing of the cold start process have been individually investigated. In the tests, the FAD was 2 ms, the throttle valve position was locked at 5%, the ambient temperature was 9 °C, the atmospheric pressure was 100.6 kPa, and the battery voltage was 12.4 V. To concisely describe the investigations, the notations of θig, Tch, Qfuel, and θin are, respectively, used to represent the ignition timing angle, the charge pulse width, the amount of fuel injected, and the injection timing angle. 4.1. Effects of the Ignition Timing Angle on Engine Cold Start Figure 11 shows the engine speed, cylinder pressure and cylinder head temperature during cold start under different ignition timing strategies, with an initial Tch = 6ms, θin = 50 °CA bTDC, and Qfuel = 56.6 mg. The ignition timing angle θig was set as θ1, equal to 10 °CA bTDC, with a cranking speed of less than 800 rpm. With the engine speed exceeding 800 rpm, the ignition timing angle θig was set as θ2, with values of 10 °CA bTDC, 20 °CA bTDC, 30 °CA bTDC and 40 °CA bTDC. It can be observed that with higher ignition timing angles, the duration of the cranking process is shorter and the engine speed is higher. The maximum combustion pressure Pmax of the first apparent fire at 40 °CA bTDC is 46% higher than that of 10 °CA bTDC. The speed increases at the ignition timing angle of 30 °CA bTDC and 40 °CA bTDC during the start-up process are the fastest to reach 1600 rpm. The ignition timing angle at 10 °CA bTDC has a clear lag in starting the engine, with an engine speed of less than 1400 rpm. The overall observations above 11
indicate the ignition timing close to injection is more suitable to ignite the mixture on AADI diesel-fuelled engine. The cylinder head temperature increases rapidly with the lager ignition timing angles. The steady temperature at 40 °CA bTDC after 80 seconds increases by 4.1 °C compared to that at 10 °CA bTDC, which results in improving the warm-up process. Figure 12(a) shows the IMEP distributions of the first 120 cycles, and Figure 12(b) shows the COV (coefficient of variation) of IMEP. After the transient process of 80 cycles, the engine attains the steady condition at 30 °CA bTDC and 40 °CA bTDC. However, the engine shows unsteady at 10 °CA bTDC in 120 cycles owing to the heat loss of after-burning, and the corresponding IMEP is distributed in a dispersed manner. The COV of IMEP at 20-30 °CA bTDC shows the lowest. It indicates that, after the engine is fired, extending a reasonable differential angle for ignition and injection can decrease the misfire cycles and make the engine more stable. Figure 12(c) shows the effects of ignition timing on cold start HC emissions. HC emissions at an ignition timing of 40 °CA bTDC is reduced by 34% compared to that of 10 °CA bTDC. In brief, the large ignition timing angle allows for a successful cold start, which indicates that the mixture should be ignited close to fuel injection because of the low volatility of diesel fuel. In addition, due to the low burning velocity of diesel fuel with high viscosity, the large ignition timing angle helps to use the heat released from the fuel burned in the cylinder. This allows the engine speed to be maintained at a high level during the transient process after the start-up and warm-up processes. 4.2. Effects of the Ignition Energy on Engine Cold Start Figure 13 shows the engine speed, the cylinder pressure and cylinder head temperature during cold start using different ignition energies, with an initial Qfuel = 56.6 mg, θin = 50 °CA bTDC, and θ2 = 40 °CA bTDC. The initial charge pulse width was set as 2ms, 4ms and 6ms, corresponding to the measured ignition 12
energies of 15.2 mJ, 39.1 mJ and 54.9 mJ, respectively. It can be observed from Fig. 13(a) that the start-up process occurs earliest with Tch = 6 ms, which indicates higher ignition energy distinctly favours igniting the mixture in cylinder. Pmax of the first apparent fire at 6 ms is 50% higher than that of 2 ms. The measured cylinder temperatures in 80 seconds show higher with the larger ignition energies. Frome the IMEP observed in Fig.14(a), some misfires occur between 30th to 70th cycles with Tch = 2 ms. Compared with that, after the transient process of 80 cycles, the engine attains the steady condition at 6 ms. The COV of IMEP in Fig.14(b) is apparently reduced as the ignition energy is raised to the saturated level. It can be found in Fig.14(c), the HC emissions at 6 ms are reduced by 23% compared with that of 2 ms. Therefore, the inadequate ignition energy causes apparent fluctuations of the engine speed and misfires; it is more difficult to maintain a high engine speed operation state during the warm-up process. On the contrary, the high ignition energy favours less engine fluctuations. The saturated ignition energy during a cold start can significantly improve the combustion performance of the engine with less HC emissions, reduce the time required for reaching the start-up process, and maintain the engine speed as it enters into the warm-up process. 4.3. Effects of the Amount of Fuel injected during the Engine Cold Start Figure 15 shows the engine speed, cylinder pressure and cylinder head temperature using different fuel injection enrichment strategies, with an initial Tch = 6ms, θ2 = 40 °CA bTDC, and θin = 50 °CA bTDC. The IMEP distributions and IMEP-COV of the first 120 cycles are shown in Fig. 16(a) and 16(b). Figure 16(c) shows the cold start HC emissions. fst was set to 2.0, 2.5, 3.0 and 4.0, corresponding to an initial Qfuel of 21.9 mg, 30.6 mg, 39.3 mg and 56.6 mg, respectively. It can be observed that the value of the fuel enrichment factors fst in the cranking process and start-up process exerts a significant influence on the 13
subsequent transient process after the start-up phase. Pmax of the first apparent fire is improved by 72% with the initial amount of fuel increased from 21.9 mg to 56.6 mg. The greater amount of fuel injected, the more combustion energy is produced to shorten the transient process for entering the warm-up process, and the cylinder temperatures at steady conditions show higher. It is noted that approximately 30.6 and 39.3 mg are the most suitable values for the amount of fuel injected, which results in a more stable start process with a lower IMEP-COV. Misfire occurs after the transient period of about 20 cycles at 21.9 mg, and the engine requires at least 100 cycles to achieve the steady condition. As observed from the HC emissions in Fig. 16(c), a fuel amount of 30.6 mg shows the lowest HC emissions. Therefore, according to the results, a larger fst can improve the dynamic cold start performance. A rapid cold start with a sufficient amount of fuel injected results in a high engine speed from the transient process to the warm-up process. However, an insufficient or excessive amount of fuel leads to an increase of the cold start HC emissions. Therefore, the start fuel enrichment factor for fuel injected could be set to a moderate value at the ambient temperature. 4.4. Effects of the Injection Timing Angle on Engine Cold Start Figure 17 shows the engine speed, cylinder pressure and cylinder head temperature during cold start using different injection timing angles, with an initial Qfuel = 56.6 mg, Tch = 6ms, and θ2 = 40 °CA bTDC. The IMEP distributions and IMEP-COV of the first 120 cycles are shown in Fig. 18(a) and Figure 18(b). Figure 18(c) shows the corresponding cold start HC emissions. The injection timing angle θin for the cold start tests was set at 50 °CA bTDC, 60 °CA bTDC, 70 °CA bTDC, and 80 °CA bTDC. From the figures, the injection timing angle at 50 °CA bTDC results in the most rapid cold start with the shortest cranking period. Pmax of the first apparent fire at 80 °CA bTDC is reduced by 26% compared to that of 50 °CA bTDC Advancing the injection timing angle extends the time to start the engine, while the overall variations of 14
cylinder head temperature show an increasing trend. The observations above prove that late injection close to ignition timing helps ignite the mixture on AADI diesel fuelled engine. Otherwise, it will cause the engine firing to be more dependent on the motor cranking process. After the engine is fired, the earlier injection possibly ensures a better mixing of fuel and air to improve the warm-up process. The results show the similar variation trends with the effects of ignition timing. Frome the Fig. 18, the IMEP-COV of the engine at 80 °CA bTDC tends to be more stable once the engine is fired. The engine almost enters into the steady condition after about 45 cycles. However, advancing the injection timing will increase the HC emissions. According to the results, it indicates that the late injection can link the injection process to the ignition timing. It helps finish the mixture injection process after the exhaust port is closed, thereby reducing the scavenging and exhaust loss of the fresh mixture. It is easier to form an ignited mixture around the spark plug to start the engine. However, it is noted that late injection decreases the fuel evaporation time at a certain extent, which results in a relatively lower engine speed during the warm-up condition. Under a low engine block temperature at cold start, the larger injection timing angle extends the residence time of the fuel-air mixture in the cylinder, which allows the fuel-air mixture to adhere to the cylinder wall. Part of the fuel-air mixture may easily flow out before the exhaust port is closed. Both results are not conducive to the formation of the ignited rich mixture around the spark plug. Therefore, the late injection close to ignition timing at the compression stroke helps to realize a successful cold start. Gradually increasing the injection timing angle after the start-up process can extend the duration of the fuel-air mixing/evaporation process and accelerate the warm-up process. 5. Conclusions A cold start control strategy for a two-stroke SI AADI diesel fuel engine was designed and 15
experimentally verified. The effects of the key control parameters on the engine cold start performance at an ambient temperature (9 °C) were investigated. The conclusions can be summarized as follows: (1) The ignition timing angle has a great effect on cold start. After firing the engine, an ignition timing angle of 30 to 40 °CA bTDC results in an effective transient process after the start-up. The engine speed of the warm-up process is faster and shows higher stability. (2) Using the saturated ignition energy results in the fastest cold start, maintains the engine speed at a high level and shows the least speed fluctuation. (3) A high fuel enrichment ratio improves the dynamic cold start performance. A rapid cold start with a sufficient amount of fuel injected results in a high engine speed from the transient process between the start-up and warm-up processes. An insufficient or excessive amount of fuel injected results in an increase of the cold start HC emissions. (4) The injection timing angle of 50 °CA bTDC used for completing the fuel-air mixture injection process allows for the formation of a combustible mixture to achieve a smooth cold start. After entering the transient process after start-up, modestly advancing the injection timing angle helps extend the time of the fuel-air mixing and evaporation, thus accelerating the warm-up process. References [1] D. Falkowski, D. Abata, P. Cho. The performance of a spark-ignited stratified-charge two stroke engine operating on a kerosene based aviation fuel. SAE Paper 972737, 1997. [2] P. Hooper. Initial development of a multi-fuel stepped piston engine for unmanned aircraft application. Aircraft Engineering and Aerospace Technology 2001, 73. 459-465. [3] B. Duddy, J. Lee, M. Walluk, D. Hallbach. Conversion of a Spark-Ignited Aircraft Engine to JP-8 Heavy 16
Fuel for Use in Unmanned Aerial Vehicles. SAE Paper 2011-01-0145, 2011 [4] R. Singh, R. McChesney. Development of multi-fuel spark ignition engine. SAE Paper 2004-32-00 38, 2004. [5] P. Hooper, T. Al-Shemmeri, M. Goodwin. An experimental and analytical investigation of a multi-fuel stepped piston engine. Applied Thermal Engineering 2012, 48, 32-40. [6] D. Cordon, S. Beyerlein, J. Steciak, M. Cherry. Conversion of a homogeneous charge air-cooled engine for operation on heavy fuels. SAE Paper 2008-32-0023, 2008. [7] S. Tomas. Experimental study of a kerosene fuelled internal combustion engine. M.D. Dissertation, Industrial Energy Systems Laboratory of the Federal Institute of Technology, Lausanne, June 2008. [8] C.F. Wang, M.X. Wei. Research on cold start fuel flow control of aero-piston engine burning kerosene. Journal of Aerospace Power, 2012, 07, 1619-1624. [9] R. Houston, G. Bell, S. Ahern. High specific power output direct injection 2-stroke engine applications. SAE Paper 2005-32-0066, 2005. [10] C.M. Gong, B.Q. Deng, S. Wang, Y. Su, Q. Gao, X.J. Liu. Combustion of a spark-ignition methanol engine during cold start under cycle-by-cycle control. Energy Fuels 2008, 22, 2981-2985. [11] J. Li, C.M. Gong, Y. Su, H.L. Dou, X.J. Liu. Effect of preheating on firing behavior of a spark-ignition methanol-fueled engine during cold start. Energy Fuels 2009, 23, 5394-5400. [12] C.M. Gong, B.Q. Deng, S. Wang, Y. Su, Q. Gao, X.J. Liu. Investigation on firing behavior of the spark-ignition engine fuelled with methanol, liquefied petroleum gas (LPG), and methanol/LPG during cold start. Energy Fuels 2008, 22, 3779-3784. [13] C.M. Gong, S.F. Yan, Y. Su, Z.W. Wang. Effects of fuel injection timing on combustion and emissions 17
of a spark-ignition methanol and methanol/liquefied petroleum gas (LPG) engine during cold start. Energy Fuels 2009, 23, 3536-3542. [14] X. Chen, H. Wang, C.S. Song, W.R. Wang, J. Huang, S.H. Liu, Y.J. Wei. Investigation of the cold-start engine performance at a low temperature for an engine fuelled with alternative fuel. Proceedings of the Institution of Mechanical Engineers, Part D (Journal of Automobile Engineering) 2014, 03, 310-318. [15] P. Suhy, L. Evers, E. Morgan, J.E. Wank. The feasibility of a kerosene fuelled spark ignited two-stroke engine. SAE Paper 911846, 1991. [16] J. Groenewegen, P. Litke, C. Wilson, J. Hoke, S. Sidhu, J. Hoke. The performance and emissions effects of utilizing heavy fuels and biodiesel in a small spark ignition internal combustion engine. 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Orlando, Florida, January 4-7 2011; AIAA, 2011. [17] J. Groenewegen, S. Sidhu, J. Hoke, C. Wilson, P. Litke. The performance and emissions effects of utilizing heavy fuels and algae based biodiesel in a port-fuel-injected small spark ignition internal combustion engine. 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego, California, July 31- August 03 2011; AIAA, 2011. [18] G. Cathcart, G. Dickson, S. Ahern. The application of air-assist direct injection for spark-ignited heavy fuel 2-stroke and 4-stroke engines. SAE paper 2005-32-0065, 2005. [19] T. Utley, S.C. Brewster, A. Tilmouth, S.H. Jin, J.B. Michael. Low Temperature starting on a pure ethanol fuelled direct injection engine. SAE paper 2008-36-0270, 2008. [20] X.M. Yu, X.W. Tan, J. Ma; G.L. Li, W. Dong, W.Q. Qi, F.Y. Cui. Gasoline direct injection engine start control. Journal of Jilin University: Engineering and Technology Edition 2011, 06, 1554-1558. 18
[21] W.Q. Qi, X.M. Yu, Y. Zhang, Z.W. Ma, B.F. Ji, X.F. Zhao. Cold start control strategy of gasoline direct injection engine. Transactions of the Chinese Society for Agricultural Machinery 2012, 07, 7-12. [22] Y.W. Wang, M.X. Wei, H.Q. Yang, R. Liu. Timing control strategies of air-assisted direct injection engine. Journal of Aerospace Power 2014, 03, 2942-2947. Acknowledgments This work was supported by Funding of the Jiangsu Innovation Program for Graduate Education (Grant number: KYLX15_0262) from the Fundamental Research Funds for the Central Universities. Appendix Abbreviations AADI air-assisted direct injection aTDC after top dead centre bTDC
before top dead centre
CA
crank angle
CDI
capacitor discharge ignition
CI
compression ignition
CPW charge pulse width COV coefficient of variation DI
direct injection
DISI direct injection spark ignition EOI
end of injection
FAD fuel to air injection delay 19
HC
hydrocarbon
IMEP indicated mean effective pressure LPG liquefied petroleum gas MPW mixture pulse width SI
spark ignition
SMD sauter mean diameter SOI
start of injection
UAV unmanned aerial vehicles Notation Eig
ignition energy
Ep
storage energy of the primary coil
f
fuel enrichment factor
fast
fuel enrichment factor of the transient process after start-up
fcyl
fuel enrichment factor of the warm-up process
fst
fuel enrichment factor of the cranking process and start-up process
If
saturation charging current of the primary coil
Ip
charging current of the primary coil
Lp
inductance value of the primary coil
Np
number of spark plugs
n
engine speed
Pmax maximum combustion pressure 20
Rp
resistance value of the primary coil
Tch
charge pulse width of the primary coli
Tcl
cylinder temperature
Tin
fuel injection pulse width
t
charging time of the primary current
t0
start charging time of the primary current
tf
end charging time of the primary current
Up
battery voltage
ΔTin
correction value of fuel injection pulse width
Δθig
correction value of the ignition timing angle
Δθin
correction value of the injection timing angle
α
throttle valve position
η
energy conversion efficiency of the ignition coil
θ1
ignition timing angle of the cranking process;
θ2
ignition timing angle during engine firing
θig
ignition timing angle
θin
injection timing angle
θmap
ignition timing angle based on the MAP table
τ
time constant
21
Fig. 1. Sequence of injection and ignition timing. Fig. 2. Corrections of the charge pulse width with different battery voltages. Fig. 3. Corrections of the injection pulse width with different battery voltages. Fig. 4. Cold start control strategy of the AADI SI diesel-fuelled engine. Fig. 5. Fuel enrichment factor at temperatures from 0 °C to 90 °C. Fig. 6. Look-up table of ignition timing angles. Fig. 7. Engine test cell setup. Fig. 8. Fuel injection pulse width. Fig. 9. Injection timing angle. Fig. 10. Parameter variations related to the engine in the cold start condition. Fig. 11(a) Cold start engine speed and cylinder pressure with different ignition times. Fig. 11(b) Cylinder head temperature at different injection times. Fig. 12.(a) Cold start IMEP of first 120 cycles at different ignition times. Fig. 12.(b) Cold start IMEP-COV at different ignition times. Fig. 12.(c) Cold start HC emissions at different ignition times. Fig. 13.(a) Cold start engine speed and cylinder pressure using different ignition energy values. Fig. 13.(b) Cylinder head temperature using different ignition energy values. Fig. 14.(a) Cold start IMEP of first 120 cycles using different ignition energy values. Fig. 14.(b) Cold start IMEP-COV using different ignition energy values. Fig. 14.(c) Cold start HC emissions using different ignition energy values. Fig. 15.(a) Cold start engine speed and cylinder pressure with different amounts of fuel injected. 22
Fig. 15.(b) Cylinder head temperature with different amounts of fuel injected. Fig. 16.(a) Cold start IMEP of first 120 cycles with different amounts of fuel injected. Fig. 16.(b) Cold start IMEP-COV with different amounts of fuel injected. Fig. 16.(c) Cold start HC emissions with different amounts of fuel injected. Fig. 17.(a) Cold start engine speed and cylinder pressure at different injection times. Fig. 17.(b) Cylinder head temperature at different injection times. Fig. 18.(a) Cold start IMEP of first 120 cycles at different injection times. Fig. 18.(b) Cold start IMEP-COV at different injection times. Fig. 18.(c) Cold start HC emissions at different injection times.
23
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Table 1 Engine specifications. Type of engine
Two-stroke, SI engine
Number of cylinders
3
Displacement
939 cm3
Bore
76 mm
Stroke
69 mm
Compression ratio
9.5:1
Scavenging ports opening
118 °CA aTDC
Scavenging ports closing
118 °CA bTDC
Exhaust port opening
85 °CA aTDC
Exhaust port closing
85 °CA bTDC
Intake style
Reed valve
Cooling system
Liquid cooled
24
Highlights Cold start control strategies of an AADI diesel-fuelled engine were evaluated. An ignition timing of 30-40 °CA bTDC with late injection ensured a rapid cold start. Saturated ignition energy favoured cold start performances with a high engine speed. A fuel enrichment factor of 2.0-2.5 at 9 °C resulted in the lowest HC emissions. Moderately advancing the injection timing accelerated the warm-up process.
25
S1359-4311(16)31289-3 http://dx.doi.org/10.1016/j.applthermaleng.2016.07.148 ATE 8746
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
15 October 2015 20 June 2016 21 July 2016
Please cite this article as: R. Liu, M. Wei, H. Yang, Cold Start Control Strategy for a Two-stroke Spark Ignition Diesel-fuelled Engine with Air-assisted Direct Injection, Applied Thermal Engineering (2016), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2016.07.148
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cold Start Control Strategy for a Two-stroke Spark Ignition Diesel-fuelled Engine with Air-assisted Direct Injection Rui Liu, * Minxiang Wei, Haiqing Yang College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China Abstract: The control strategies of cold start were investigated on a two-stroke spark ignition diesel-fuelled engine with air-assisted direct injection. The successful cold start of the engine at 5°C ambient temperature verified the control strategy. Bench tests at ambient temperature (9 °C) were performed to study and quantify the control strategies according to the engine dynamic characteristics, including the engine speed, cylinder pressure and exhaust emission. The ignition timing, ignition energy, injection timing and amount of fuel injected were quantitatively analysed using the cold-start control strategy. An ignition timing of 30-40 °CA bTDC is suggested to reduce the transition time from the cranking process to the start-up process. The saturated ignition energy favours the cold start performance with a moderate to low fluctuation of the engine speed from the start-up process to the warm-up process. A proper amount of fuel injection can improve the dynamic performance of the start-up and warm-up processes. A rapid cold start with short cranking was employed at an injection timing of approximately 50°CA bTDC, and moderately advancing the injection timing was found to accelerate the warm-up process.
Keywords: cold start; two-stroke spark ignition engine; heavy fuels; air-assisted direct injection; control strategy 1. Introduction * Corresponding author. Tel./fax: +86 025 8489 4596. E-mail address: [email protected] (Rui Liu). 1
Heavy fuels are usually used in compression ignition (CI) engines, which are known to have poor power-to-weight ratios compared to spark ignition (SI) gasoline engines. Considering the high volatility, low viscosity and storage/transportation problems of gasoline, the military requirements for a single-fuel policy to cover multi-type vehicles and equipments are significantly intensified in many countries. In recent years, high performance SI reciprocating engines capable of using heavy fuels (i.e., kerosene and diesel fuels) have been widely applied for the ground fleet and unmanned aerial vehicles (UAV) in the military [1-3]. The demand for multi-fuel SI engines in non-military applications (e.g., outboards, motorcycles, and snowmobiles) is also increasing due to safety concerns [4-6]. An engine cold start refers to starting an engine with an inner chamber temperature that is the same as the ambient temperature that occurred from the last stop. SI gasoline engines have no difficulty achieving a successful cold start as they have a carburettor, port injection and direct injection (DI). In particular, the high atomization of DI is more helpful for rapidly starting the engine. Compared to the physicochemical properties of gasoline, heavy fuels show lower volatility, higher viscosity and lower evaporation. It is difficult to start heavy fuels engines equipped with carburettors or port injection systems without auxiliary preheating or aided ignition, which require additional energy sources [7, 8]. It is worth noting that the low vaporized mass of the injected fuel easily causes pollution issues during the cold start of non-DISI heavy fuel engines. Hence, it is desirable to use DI to achieve excellent fuel evaporation and reduce the fuel mass injected during the cold start compared to traditional heavy fuel engines. The high pressure fuel DI is usually used in common rail CI diesel engines and SI gasoline engines, which are equipped with high pressure fuel pumps and have high application costs. By contrast, the low pressure air-assisted direct injection (AADI) system used in automobiles or motorcycles is fuelled with normal low pressure fuel and 2
compressed air. The fuel is injected into a premixed chamber filled with compressed air. The mixture of fuel and air is directly injected into the combustion chamber by an air-assisted injector. The sauter mean diameter (SMD) of the fuel spray can even reach approximately 5 μm. Hence, The AADI has been taken into account and applied in high specific power output two-stroke DISI engines [9]. To improve the SI engine cold start performances with regard to the fuel demand and exhaust emissions, most work has focused on engines that use gasoline or liquefied petroleum gas (LPG). Some studies have also focused on methanol or ethanol fuelled engines [10-14]; however, little work has been reported on SI heavy fuel engines. Singh et al. [4] compared the cold starts using JP-5/8 and gasoline on a two-stroke multi-fuel SI engine. The amount of fuel required for starting the engine at cold temperatures differed significantly. Compared to gasoline, for starting with JP-5/8, the initial injection amount was heavily compensated and increased at lower temperatures. The fuel was injected very late and the ignition was very early, thus having a stratified combustion start. Homogenous combustion mode was also tried but was only successful with gasoline and ethanol fuel. Suhy et al. [15] studied the feasibility of burning kerosene in a port injection two-stroke SI engine with a vortex pneumatic atomizer to reduce the droplet size. The method of heating the atomization air to overcome the cold starting problem with kerosene fuel was developed, and the engine would not start if the atomizer air temperature was lower than 115 °C. Higher atomizer air temperatures and lower pressures and flow rates were required to start the engine. Groenewegen et al. [16,17] discussed the results of the continuation of experimentation in utilizing heavy fuels, such as JP-8 and D2 diesel, to satisfy the necessary power requirements of a four-stroke SI engine. The engine, which used an air-assisted port fuel injection system, was originally designed to run on gasoline. A cold start was not achieved with the 3
air-assisted injector, and the engine was started using gasoline and then switched to the heavy fuels via a three-way valve. Cathcart et al. [18] applied the AADI technology to SI engines and conducted cold start experiments on a V6 two-stroke engine using JP5 fuel at low ambient temperatures. Due to the excellent atomization produced by the AADI, the starting time test results showed that a start time of approximately 4 sec was achieved at -10 °C, decreasing to 1 sec at a temperature of approximately 20 °C. The AADI has shown a significant advantage in addressing cold start problems in heavy fuels engines; however, the relevant control strategies for realizing rapid cold starts have rarely been reported. Utley et al. [19] adopted a double injection strategy combined with a multifunctional high power ignition system at soaked temperatures from +25 °C to -10 °C and improved the cold start performance of a four-stroke SI ethanol fuelled engine with AADI. Yu et al. [20] conducted the cold start control strategy on a turbocharged direct injection gasoline engine. The results showed that fuel injection was the key parameter, the influence of which was greater than the ignition advance angle of the cold start process. Qi et al. [21] designed a corresponding control strategy to study the DI gasoline engine cold start characteristics and basic requirements, and conducted a validation test of the proposed control strategy. Wang et al. [22] proposed the injection and ignition timing control strategy for a gasoline engine with AADI, which mainly introduced the calculation method of timing control. All of the above works in the literature paid attention to solving or improving the cold start of SI gasoline, LPG, and methanol engines. However, the control strategies of cold start for diesel-fuelled SI engines with AADI have not been completely reported or investigated. The cold start performance of the engine is strongly related to the match of the fuel-air injection and ignition. A reasonable match will facilitate the fuel-air mixture preparation in cylinder. As shown in Fig. 1, the combustion control in the cylinder of the 4
AADI SI engine is based on the sequence of injection and ignition control. The injection control consists of fuel injection and the fuel-air mixture injection, which specifies the amount of fuel and injection timing. Ignition control includes the ignition energy and ignition timing. These parameters have considerable effects on the combustion and emission of the diesel-fuelled SI engine during a cold start. Therefore, it is necessary to investigate the corresponding control strategies to ensure the success of a rapid cold start and reveal the governing laws of these parameters. The objective of this paper is to investigate the control strategies of cold starts for an AADI diesel-fuelled engine. Bench tests were performed to study and quantify the designed control strategies according to the engine dynamic characteristics, including the engine speed, cylinder pressure and exhaust emissions. The ignition timing, ignition energy, amount of fuel injected and fuel-air mixture injection timing were quantitatively analysed using the cold-start control strategies. 2. Cold Start Control Strategy 2.1. Ignition Control Ignition timing control during a cold start is different from the steady-state condition. A constant ignition timing angle based on the cylinder temperature is usually used until the successful cold start. For a two-stroke SI engine with AADI, the ignition timing has a strong relationship with the fuel injection timing. It is notable that an early ignition timing may cause the ignition timing angle to overlap with the angle phase of the fuel-air mixture injection. Additionally, a late ignition timing will delay the combustion phase in the cylinder, thereby reducing the engine work ability of a poor cold start. In this paper, ignition timing control is mainly based on the variations of the engine speed of the start state. The cylinder temperature has an important influence on the fuel viscosity and volatility, so the initial ignition timing angle requires 5
quantitative correction before starting the engine, as represented by Eq. (1):
ig ig (n) ig (Tcl )
(1)
where θig denotes the ignition timing angle and θig(n) represents the interpolation value from a preset look-up table, which is based on the engine speed n. Δθig(Tcl) is the corrected value of the ignition timing angle, and Tcl refers to the cylinder temperature. The ignition energy at the cold start should be improved as much as possible to start the engine smoothly while ensuring that the ignition system remains stable. Inductive ignition generally releases a high discharge energy of approximately 50-100 mJ. A dual-spark plug inductive ignition system is applied in this paper. The ignition energy was controlled by the charge pulse width of the primary coil. It is noted that the 12 V battery terminal voltage decreases to approximately 9 V during the primary period of the cold start. To ensure that the ignition energy is maintained at the target level, the charge pulse width must be corrected with the value of the battery voltage. Equations (2) to (5) give the ignition energy correction calculation basis. As the battery voltage decreases, the charge pulse width could be appropriately increased to make the charge current reach the target value.
Eig N p E p
(2)
E p Lp I p 2 2
(3)
In the above, Eig denotes the ignition energy calculated from the number of spark plugs Np, the energy conversion efficiency of the ignition coil η, and the storage energy of the primary coil Ep. According to Eq. (3), Ep is calculated by using the inductance value Lp and the charge current of the primary coil Ip:
6
0 t t 0 I p U p 1 exp( t ) / R p I f t f t Tch t t0 Tch (U p )
t0 t t f (4)
(5)
where t0 denotes the start charging time of the primary current and t denotes the end charging time. tf represents the time of the primary current Ip reaching the saturation charging current If. Up refers to the battery voltage, τ is the time constant, and Rp is the resistance value of the primary coil. Lp, τ, and Rp are constant values for the ignition coil. Thus, the charge pulse width Tch is calculated by t0 and t, which are related to the battery voltage Up. Figure 2 illustrates the variations of the charge pulse width with Up. The figure gives the corrected values of 2 ms, 4 ms, and 6 ms, which will be applied for cold start investigations in sections 3 and 4. 2.2. Injection Control Injection control of AADI has a great influence on engine cold start. An appropriate injection quantity of the fuel-air mixture and injection timing allow for an ignited rich mixture to form around the spark plug. The injected mixture quantity determines the total concentration in the cylinder. An overly lean mixture around the spark plug is difficult to ignite, and the ignition energy requirements are higher. For mixtures that are too rich, even if they are ignited, misfire due to incomplete combustion may easily occur with the increasing engine speed. Additionally, the injection timing of the fuel-air mixture affects the mixture concentration distribution around the spark plug at the ignition timing. At a fixed ignition timing angle, an early fuel-air injection may form a lean mixture due to the scavenging loss or fuel condensing on the wall and the piston top surface at a low cylinder temperature. A late injection will lead to less mixing and evaporation time of the fuel-air, inducing condensation of the mixture on the spark plug electrode. Both 7
cases will lead to insufficient combustion in the cylinder, which could easily lead to engine flameout. Similar to ignition control, the injection control of AADI is also related to the engine speed, the cylinder temperature and the battery voltage during a cold start. According to this, the correction strategies of the fuel injection pulse width and injection timing could be illustrated as Eq. (6) and (7):
Tin Tin ( , n) * f (Tcl ) Tin (U p )
(6)
where Tin denotes the fuel injection pulse width. Tin(α,n) is the interpolation value from a two-dimensional loop-up table, which is based on the throttle valve position α and the engine speed n. f(Tcl) refers to the fuel enrichment factor calculated from the cylinder temperature, and ΔTin(Up) is the corrected value determined by the battery voltage Up. The applied voltage to the fuel injector will affect the opening rate of injectors. If the voltage is low, the injection on signal will be longer to compensate. Figure 3 shows the corrected value of the injection pulse width with Up.
in in ( , n) in (Tcl )
(7)
In Eq. (7), θin denotes the injection timing angle, which is calculated according to the injection timing angle θin(α,n), and the corrected value Δθin(Tcl) is based on the cylinder temperature. 2.3. Cold Start Control Strategy Design According to the peak cranking speed, the start-up target speed and the fuel enrichment factor after start-up, the cold start process is divided into four processes that consist of the cranking process, the start-up process, the transient process after start-up, and the warm-up process. The peak cranking speed is the maximum engine speed allowed by the start motor. The start-up target speed indicates whether the engine enters into the transient process after start-up. Figure 4 gives the cold start control strategy of this paper. The cranking motor advances the engine into the cranking process following a powering electronic 8
control system. The fuel enrichment factor is the calculated interpolation result fst from the look-up table, which is based on the cylinder temperature. The ignition timing angle θig used is the value θ1. Once the engine is accelerated to a speed higher than the value of the peak cranking speed; it can be determined that the engine is fired. The ignition timing angle θig used is the value θ2. Under this condition, fst maintains the same value. When the engine speed reaches the preset value of the start-up target speed, the engine starts successfully. The fuel enrichment factor calculated by fast and fcyl starts to decrease automatically based on the current cylinder temperature with the same ignition strategy. The engine enters the warm-up condition once fast falls to 1. The fuel injection pulse width is corrected by fcyl, and the ignition timing angle is calculated from the look-up table. Figure 5 gives the predicted fuel enrichment factors at temperatures of 0 °C to 90 °C. At lower temperatures, the initial injection amount according to fst is heavily compensated. The fuel enrichment factor fst is not linear with temperature, while fast and fcyl almost show the linear trends. Figure 6 gives the predicted look-up table of the ignition timing angles below 3000 rpm, the values used are based on the CDI (capacitor discharge ignition) system of the prototype engine. f: fuel enrichment factor; fst: fuel enrichment factor of the cranking process and start-up process; fast: fuel enrichment factor of the transient process following the start-up; fcyl: fuel enrichment factor of the warm-up process; θig: ignition timing angle; θ1: ignition timing angle of the cranking process; θ2: ignition timing angle as the engine is fired; θmap: ignition timing angle based on the MAP table; n: engine speed.
3. Experimental Setup and Strategy Verification 3.1. Experimental Setup The cold start test bench for the SI AADI diesel-fuelled engine includes a 12 V battery power supply, the data acquisition system, the electronic control unit, and the control computer. The engine specifications are 9
listed in Table 1. Figure 7 shows the setup installed in the test cell. The air-assisted injectors integrated in the designed fuel-air rail are mounted on the cylinder heads. The computer records the data from the acquisition system in real-time, which mainly includes temperatures and pressures. The engine parameter calibration software is used to preset different control parameters downloaded to the engine electronic control unit. It also records the data of the engine speed, injection parameters, ignition parameters and some other parameters. 3.2. Control Strategy Verification Engine cold start tests were conducted to verify the control strategy of the cold start proposed in subsection 2.3 at the ambient temperature (5 °C). The peak cranking speed was approximately 800 rpm. The throttle valve position was 6%. The value of fst was 4. Figure 8 shows the MAP table of the fuel injection pulse width values used for the verification test. Figure 9 shows the MAP table of injection timing angles. In the cold start test, θin was set at a constant value of 50 °CA bTDC, without using the MAP table, to start the engine. The fuel pulse width and charge pulse width are expressed in μs. Figure 10 shows the related engine parameter histories of the cold start. It can be observed that the variations of the ignition and injection parameters were consistent with the designed control strategy. After the motor started to advance the engine into the cranking process, the fuel pulse width and charge pulse width were corrected and the ignition timing angle was set to 10 °CA bTDC. The engine entered the start-up process as the cranking speed exceeded 800 rpm. Then the ignition timing angle was set to 30 °CA bTDC, and the cylinder head temperature did not change significantly after this process. The engine entered into the transition process after the start-up once the engine speed was higher than 1500 rpm. Then the fuel pulse width decreased at a certain slope, and the cylinder head temperature increased significantly. Finally, the engine entered the 10
warm-up process at the end of the automatic decreasing process of the fuel pulse width. Thus, the designed control strategy was verified, as it resulted in a successful cold start at this ambient temperature. 4. Effects of the Key Parameters on Cold Start Performance The key parameters of the control strategies and their effects on the cold start of the SI AADI diesel-fuelled engine are quantitatively assessed in this section. The ignition timing, ignition energy, amount of fuel injected, and injection timing of the cold start process have been individually investigated. In the tests, the FAD was 2 ms, the throttle valve position was locked at 5%, the ambient temperature was 9 °C, the atmospheric pressure was 100.6 kPa, and the battery voltage was 12.4 V. To concisely describe the investigations, the notations of θig, Tch, Qfuel, and θin are, respectively, used to represent the ignition timing angle, the charge pulse width, the amount of fuel injected, and the injection timing angle. 4.1. Effects of the Ignition Timing Angle on Engine Cold Start Figure 11 shows the engine speed, cylinder pressure and cylinder head temperature during cold start under different ignition timing strategies, with an initial Tch = 6ms, θin = 50 °CA bTDC, and Qfuel = 56.6 mg. The ignition timing angle θig was set as θ1, equal to 10 °CA bTDC, with a cranking speed of less than 800 rpm. With the engine speed exceeding 800 rpm, the ignition timing angle θig was set as θ2, with values of 10 °CA bTDC, 20 °CA bTDC, 30 °CA bTDC and 40 °CA bTDC. It can be observed that with higher ignition timing angles, the duration of the cranking process is shorter and the engine speed is higher. The maximum combustion pressure Pmax of the first apparent fire at 40 °CA bTDC is 46% higher than that of 10 °CA bTDC. The speed increases at the ignition timing angle of 30 °CA bTDC and 40 °CA bTDC during the start-up process are the fastest to reach 1600 rpm. The ignition timing angle at 10 °CA bTDC has a clear lag in starting the engine, with an engine speed of less than 1400 rpm. The overall observations above 11
indicate the ignition timing close to injection is more suitable to ignite the mixture on AADI diesel-fuelled engine. The cylinder head temperature increases rapidly with the lager ignition timing angles. The steady temperature at 40 °CA bTDC after 80 seconds increases by 4.1 °C compared to that at 10 °CA bTDC, which results in improving the warm-up process. Figure 12(a) shows the IMEP distributions of the first 120 cycles, and Figure 12(b) shows the COV (coefficient of variation) of IMEP. After the transient process of 80 cycles, the engine attains the steady condition at 30 °CA bTDC and 40 °CA bTDC. However, the engine shows unsteady at 10 °CA bTDC in 120 cycles owing to the heat loss of after-burning, and the corresponding IMEP is distributed in a dispersed manner. The COV of IMEP at 20-30 °CA bTDC shows the lowest. It indicates that, after the engine is fired, extending a reasonable differential angle for ignition and injection can decrease the misfire cycles and make the engine more stable. Figure 12(c) shows the effects of ignition timing on cold start HC emissions. HC emissions at an ignition timing of 40 °CA bTDC is reduced by 34% compared to that of 10 °CA bTDC. In brief, the large ignition timing angle allows for a successful cold start, which indicates that the mixture should be ignited close to fuel injection because of the low volatility of diesel fuel. In addition, due to the low burning velocity of diesel fuel with high viscosity, the large ignition timing angle helps to use the heat released from the fuel burned in the cylinder. This allows the engine speed to be maintained at a high level during the transient process after the start-up and warm-up processes. 4.2. Effects of the Ignition Energy on Engine Cold Start Figure 13 shows the engine speed, the cylinder pressure and cylinder head temperature during cold start using different ignition energies, with an initial Qfuel = 56.6 mg, θin = 50 °CA bTDC, and θ2 = 40 °CA bTDC. The initial charge pulse width was set as 2ms, 4ms and 6ms, corresponding to the measured ignition 12
energies of 15.2 mJ, 39.1 mJ and 54.9 mJ, respectively. It can be observed from Fig. 13(a) that the start-up process occurs earliest with Tch = 6 ms, which indicates higher ignition energy distinctly favours igniting the mixture in cylinder. Pmax of the first apparent fire at 6 ms is 50% higher than that of 2 ms. The measured cylinder temperatures in 80 seconds show higher with the larger ignition energies. Frome the IMEP observed in Fig.14(a), some misfires occur between 30th to 70th cycles with Tch = 2 ms. Compared with that, after the transient process of 80 cycles, the engine attains the steady condition at 6 ms. The COV of IMEP in Fig.14(b) is apparently reduced as the ignition energy is raised to the saturated level. It can be found in Fig.14(c), the HC emissions at 6 ms are reduced by 23% compared with that of 2 ms. Therefore, the inadequate ignition energy causes apparent fluctuations of the engine speed and misfires; it is more difficult to maintain a high engine speed operation state during the warm-up process. On the contrary, the high ignition energy favours less engine fluctuations. The saturated ignition energy during a cold start can significantly improve the combustion performance of the engine with less HC emissions, reduce the time required for reaching the start-up process, and maintain the engine speed as it enters into the warm-up process. 4.3. Effects of the Amount of Fuel injected during the Engine Cold Start Figure 15 shows the engine speed, cylinder pressure and cylinder head temperature using different fuel injection enrichment strategies, with an initial Tch = 6ms, θ2 = 40 °CA bTDC, and θin = 50 °CA bTDC. The IMEP distributions and IMEP-COV of the first 120 cycles are shown in Fig. 16(a) and 16(b). Figure 16(c) shows the cold start HC emissions. fst was set to 2.0, 2.5, 3.0 and 4.0, corresponding to an initial Qfuel of 21.9 mg, 30.6 mg, 39.3 mg and 56.6 mg, respectively. It can be observed that the value of the fuel enrichment factors fst in the cranking process and start-up process exerts a significant influence on the 13
subsequent transient process after the start-up phase. Pmax of the first apparent fire is improved by 72% with the initial amount of fuel increased from 21.9 mg to 56.6 mg. The greater amount of fuel injected, the more combustion energy is produced to shorten the transient process for entering the warm-up process, and the cylinder temperatures at steady conditions show higher. It is noted that approximately 30.6 and 39.3 mg are the most suitable values for the amount of fuel injected, which results in a more stable start process with a lower IMEP-COV. Misfire occurs after the transient period of about 20 cycles at 21.9 mg, and the engine requires at least 100 cycles to achieve the steady condition. As observed from the HC emissions in Fig. 16(c), a fuel amount of 30.6 mg shows the lowest HC emissions. Therefore, according to the results, a larger fst can improve the dynamic cold start performance. A rapid cold start with a sufficient amount of fuel injected results in a high engine speed from the transient process to the warm-up process. However, an insufficient or excessive amount of fuel leads to an increase of the cold start HC emissions. Therefore, the start fuel enrichment factor for fuel injected could be set to a moderate value at the ambient temperature. 4.4. Effects of the Injection Timing Angle on Engine Cold Start Figure 17 shows the engine speed, cylinder pressure and cylinder head temperature during cold start using different injection timing angles, with an initial Qfuel = 56.6 mg, Tch = 6ms, and θ2 = 40 °CA bTDC. The IMEP distributions and IMEP-COV of the first 120 cycles are shown in Fig. 18(a) and Figure 18(b). Figure 18(c) shows the corresponding cold start HC emissions. The injection timing angle θin for the cold start tests was set at 50 °CA bTDC, 60 °CA bTDC, 70 °CA bTDC, and 80 °CA bTDC. From the figures, the injection timing angle at 50 °CA bTDC results in the most rapid cold start with the shortest cranking period. Pmax of the first apparent fire at 80 °CA bTDC is reduced by 26% compared to that of 50 °CA bTDC Advancing the injection timing angle extends the time to start the engine, while the overall variations of 14
cylinder head temperature show an increasing trend. The observations above prove that late injection close to ignition timing helps ignite the mixture on AADI diesel fuelled engine. Otherwise, it will cause the engine firing to be more dependent on the motor cranking process. After the engine is fired, the earlier injection possibly ensures a better mixing of fuel and air to improve the warm-up process. The results show the similar variation trends with the effects of ignition timing. Frome the Fig. 18, the IMEP-COV of the engine at 80 °CA bTDC tends to be more stable once the engine is fired. The engine almost enters into the steady condition after about 45 cycles. However, advancing the injection timing will increase the HC emissions. According to the results, it indicates that the late injection can link the injection process to the ignition timing. It helps finish the mixture injection process after the exhaust port is closed, thereby reducing the scavenging and exhaust loss of the fresh mixture. It is easier to form an ignited mixture around the spark plug to start the engine. However, it is noted that late injection decreases the fuel evaporation time at a certain extent, which results in a relatively lower engine speed during the warm-up condition. Under a low engine block temperature at cold start, the larger injection timing angle extends the residence time of the fuel-air mixture in the cylinder, which allows the fuel-air mixture to adhere to the cylinder wall. Part of the fuel-air mixture may easily flow out before the exhaust port is closed. Both results are not conducive to the formation of the ignited rich mixture around the spark plug. Therefore, the late injection close to ignition timing at the compression stroke helps to realize a successful cold start. Gradually increasing the injection timing angle after the start-up process can extend the duration of the fuel-air mixing/evaporation process and accelerate the warm-up process. 5. Conclusions A cold start control strategy for a two-stroke SI AADI diesel fuel engine was designed and 15
experimentally verified. The effects of the key control parameters on the engine cold start performance at an ambient temperature (9 °C) were investigated. The conclusions can be summarized as follows: (1) The ignition timing angle has a great effect on cold start. After firing the engine, an ignition timing angle of 30 to 40 °CA bTDC results in an effective transient process after the start-up. The engine speed of the warm-up process is faster and shows higher stability. (2) Using the saturated ignition energy results in the fastest cold start, maintains the engine speed at a high level and shows the least speed fluctuation. (3) A high fuel enrichment ratio improves the dynamic cold start performance. A rapid cold start with a sufficient amount of fuel injected results in a high engine speed from the transient process between the start-up and warm-up processes. An insufficient or excessive amount of fuel injected results in an increase of the cold start HC emissions. (4) The injection timing angle of 50 °CA bTDC used for completing the fuel-air mixture injection process allows for the formation of a combustible mixture to achieve a smooth cold start. After entering the transient process after start-up, modestly advancing the injection timing angle helps extend the time of the fuel-air mixing and evaporation, thus accelerating the warm-up process. References [1] D. Falkowski, D. Abata, P. Cho. The performance of a spark-ignited stratified-charge two stroke engine operating on a kerosene based aviation fuel. SAE Paper 972737, 1997. [2] P. Hooper. Initial development of a multi-fuel stepped piston engine for unmanned aircraft application. Aircraft Engineering and Aerospace Technology 2001, 73. 459-465. [3] B. Duddy, J. Lee, M. Walluk, D. Hallbach. Conversion of a Spark-Ignited Aircraft Engine to JP-8 Heavy 16
Fuel for Use in Unmanned Aerial Vehicles. SAE Paper 2011-01-0145, 2011 [4] R. Singh, R. McChesney. Development of multi-fuel spark ignition engine. SAE Paper 2004-32-00 38, 2004. [5] P. Hooper, T. Al-Shemmeri, M. Goodwin. An experimental and analytical investigation of a multi-fuel stepped piston engine. Applied Thermal Engineering 2012, 48, 32-40. [6] D. Cordon, S. Beyerlein, J. Steciak, M. Cherry. Conversion of a homogeneous charge air-cooled engine for operation on heavy fuels. SAE Paper 2008-32-0023, 2008. [7] S. Tomas. Experimental study of a kerosene fuelled internal combustion engine. M.D. Dissertation, Industrial Energy Systems Laboratory of the Federal Institute of Technology, Lausanne, June 2008. [8] C.F. Wang, M.X. Wei. Research on cold start fuel flow control of aero-piston engine burning kerosene. Journal of Aerospace Power, 2012, 07, 1619-1624. [9] R. Houston, G. Bell, S. Ahern. High specific power output direct injection 2-stroke engine applications. SAE Paper 2005-32-0066, 2005. [10] C.M. Gong, B.Q. Deng, S. Wang, Y. Su, Q. Gao, X.J. Liu. Combustion of a spark-ignition methanol engine during cold start under cycle-by-cycle control. Energy Fuels 2008, 22, 2981-2985. [11] J. Li, C.M. Gong, Y. Su, H.L. Dou, X.J. Liu. Effect of preheating on firing behavior of a spark-ignition methanol-fueled engine during cold start. Energy Fuels 2009, 23, 5394-5400. [12] C.M. Gong, B.Q. Deng, S. Wang, Y. Su, Q. Gao, X.J. Liu. Investigation on firing behavior of the spark-ignition engine fuelled with methanol, liquefied petroleum gas (LPG), and methanol/LPG during cold start. Energy Fuels 2008, 22, 3779-3784. [13] C.M. Gong, S.F. Yan, Y. Su, Z.W. Wang. Effects of fuel injection timing on combustion and emissions 17
of a spark-ignition methanol and methanol/liquefied petroleum gas (LPG) engine during cold start. Energy Fuels 2009, 23, 3536-3542. [14] X. Chen, H. Wang, C.S. Song, W.R. Wang, J. Huang, S.H. Liu, Y.J. Wei. Investigation of the cold-start engine performance at a low temperature for an engine fuelled with alternative fuel. Proceedings of the Institution of Mechanical Engineers, Part D (Journal of Automobile Engineering) 2014, 03, 310-318. [15] P. Suhy, L. Evers, E. Morgan, J.E. Wank. The feasibility of a kerosene fuelled spark ignited two-stroke engine. SAE Paper 911846, 1991. [16] J. Groenewegen, P. Litke, C. Wilson, J. Hoke, S. Sidhu, J. Hoke. The performance and emissions effects of utilizing heavy fuels and biodiesel in a small spark ignition internal combustion engine. 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Orlando, Florida, January 4-7 2011; AIAA, 2011. [17] J. Groenewegen, S. Sidhu, J. Hoke, C. Wilson, P. Litke. The performance and emissions effects of utilizing heavy fuels and algae based biodiesel in a port-fuel-injected small spark ignition internal combustion engine. 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego, California, July 31- August 03 2011; AIAA, 2011. [18] G. Cathcart, G. Dickson, S. Ahern. The application of air-assist direct injection for spark-ignited heavy fuel 2-stroke and 4-stroke engines. SAE paper 2005-32-0065, 2005. [19] T. Utley, S.C. Brewster, A. Tilmouth, S.H. Jin, J.B. Michael. Low Temperature starting on a pure ethanol fuelled direct injection engine. SAE paper 2008-36-0270, 2008. [20] X.M. Yu, X.W. Tan, J. Ma; G.L. Li, W. Dong, W.Q. Qi, F.Y. Cui. Gasoline direct injection engine start control. Journal of Jilin University: Engineering and Technology Edition 2011, 06, 1554-1558. 18
[21] W.Q. Qi, X.M. Yu, Y. Zhang, Z.W. Ma, B.F. Ji, X.F. Zhao. Cold start control strategy of gasoline direct injection engine. Transactions of the Chinese Society for Agricultural Machinery 2012, 07, 7-12. [22] Y.W. Wang, M.X. Wei, H.Q. Yang, R. Liu. Timing control strategies of air-assisted direct injection engine. Journal of Aerospace Power 2014, 03, 2942-2947. Acknowledgments This work was supported by Funding of the Jiangsu Innovation Program for Graduate Education (Grant number: KYLX15_0262) from the Fundamental Research Funds for the Central Universities. Appendix Abbreviations AADI air-assisted direct injection aTDC after top dead centre bTDC
before top dead centre
CA
crank angle
CDI
capacitor discharge ignition
CI
compression ignition
CPW charge pulse width COV coefficient of variation DI
direct injection
DISI direct injection spark ignition EOI
end of injection
FAD fuel to air injection delay 19
HC
hydrocarbon
IMEP indicated mean effective pressure LPG liquefied petroleum gas MPW mixture pulse width SI
spark ignition
SMD sauter mean diameter SOI
start of injection
UAV unmanned aerial vehicles Notation Eig
ignition energy
Ep
storage energy of the primary coil
f
fuel enrichment factor
fast
fuel enrichment factor of the transient process after start-up
fcyl
fuel enrichment factor of the warm-up process
fst
fuel enrichment factor of the cranking process and start-up process
If
saturation charging current of the primary coil
Ip
charging current of the primary coil
Lp
inductance value of the primary coil
Np
number of spark plugs
n
engine speed
Pmax maximum combustion pressure 20
Rp
resistance value of the primary coil
Tch
charge pulse width of the primary coli
Tcl
cylinder temperature
Tin
fuel injection pulse width
t
charging time of the primary current
t0
start charging time of the primary current
tf
end charging time of the primary current
Up
battery voltage
ΔTin
correction value of fuel injection pulse width
Δθig
correction value of the ignition timing angle
Δθin
correction value of the injection timing angle
α
throttle valve position
η
energy conversion efficiency of the ignition coil
θ1
ignition timing angle of the cranking process;
θ2
ignition timing angle during engine firing
θig
ignition timing angle
θin
injection timing angle
θmap
ignition timing angle based on the MAP table
τ
time constant
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Fig. 1. Sequence of injection and ignition timing. Fig. 2. Corrections of the charge pulse width with different battery voltages. Fig. 3. Corrections of the injection pulse width with different battery voltages. Fig. 4. Cold start control strategy of the AADI SI diesel-fuelled engine. Fig. 5. Fuel enrichment factor at temperatures from 0 °C to 90 °C. Fig. 6. Look-up table of ignition timing angles. Fig. 7. Engine test cell setup. Fig. 8. Fuel injection pulse width. Fig. 9. Injection timing angle. Fig. 10. Parameter variations related to the engine in the cold start condition. Fig. 11(a) Cold start engine speed and cylinder pressure with different ignition times. Fig. 11(b) Cylinder head temperature at different injection times. Fig. 12.(a) Cold start IMEP of first 120 cycles at different ignition times. Fig. 12.(b) Cold start IMEP-COV at different ignition times. Fig. 12.(c) Cold start HC emissions at different ignition times. Fig. 13.(a) Cold start engine speed and cylinder pressure using different ignition energy values. Fig. 13.(b) Cylinder head temperature using different ignition energy values. Fig. 14.(a) Cold start IMEP of first 120 cycles using different ignition energy values. Fig. 14.(b) Cold start IMEP-COV using different ignition energy values. Fig. 14.(c) Cold start HC emissions using different ignition energy values. Fig. 15.(a) Cold start engine speed and cylinder pressure with different amounts of fuel injected. 22
Fig. 15.(b) Cylinder head temperature with different amounts of fuel injected. Fig. 16.(a) Cold start IMEP of first 120 cycles with different amounts of fuel injected. Fig. 16.(b) Cold start IMEP-COV with different amounts of fuel injected. Fig. 16.(c) Cold start HC emissions with different amounts of fuel injected. Fig. 17.(a) Cold start engine speed and cylinder pressure at different injection times. Fig. 17.(b) Cylinder head temperature at different injection times. Fig. 18.(a) Cold start IMEP of first 120 cycles at different injection times. Fig. 18.(b) Cold start IMEP-COV at different injection times. Fig. 18.(c) Cold start HC emissions at different injection times.
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Table 1 Engine specifications. Type of engine
Two-stroke, SI engine
Number of cylinders
3
Displacement
939 cm3
Bore
76 mm
Stroke
69 mm
Compression ratio
9.5:1
Scavenging ports opening
118 °CA aTDC
Scavenging ports closing
118 °CA bTDC
Exhaust port opening
85 °CA aTDC
Exhaust port closing
85 °CA bTDC
Intake style
Reed valve
Cooling system
Liquid cooled
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Highlights Cold start control strategies of an AADI diesel-fuelled engine were evaluated. An ignition timing of 30-40 °CA bTDC with late injection ensured a rapid cold start. Saturated ignition energy favoured cold start performances with a high engine speed. A fuel enrichment factor of 2.0-2.5 at 9 °C resulted in the lowest HC emissions. Moderately advancing the injection timing accelerated the warm-up process.
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