With rising fuel price and more stringent emission legislation, recent researches are focus on high efficiency and low emissions technologies in the field of internal combustion engines. In order to simultaneously reduce soot and NOx emissions while achieving high thermal efficiency, many strategies have been proposed in compression ignition (CI) engine. The low temperature premixed combustion is employed by most of the strategies, and the ultimate goal is to achieve Homogeneous Charge Compression Ignition (HCCI) combustion with near zero NOx and soot emissions [1]. However, the controllability of the heat release rate and the ignition timing is the challenge of the HCCI concept.
In order to address these problems, dual fuel combustion concept was utilized by many researchers. With port injection of a low-reactivity fuel combining direct in-cylinder injection of a high-reactivity fuel, the combustion phasing and duration can be flexibly controlled through reactivity gradient [2-4]. A higher octane number (ON) indicates a higher resistance to auto-ignition, which in turn can effectively extend the upper load limit of the dual fuel engine without using too much EGR. From this point of view, natural gas with large proven reserves and high ON is the best choice of the port injection fuel.
Natural gas is a fossil fuel, and it is not renewable. However, methane (which is the main constituent of natural gas) can be produced in renewable manners. Nature gas is a clean-burning alternative fuel for vehicles with a significant potential for reducing smoke emissions. Recently, natural gas dual fuel dual fuel engines have drawn a lot of attention. The experiments showed that the US 2010 heavy-duty NOx and soot emissions regulations can be easily met without after-treatment, while achieving greater than 50% net indicated thermal efficiency [5]. Under dual fuel conditions, the NOx emissions can be significantly reduced in comparison to normal diesel engine conditions [6,7]. Smoke or particulate emissions of the dual fuel engine are very low and even undetectable [8]. Unburned HC (hydrocarbon) emissions of dual-fuel engines are obviously higher than that of the diesel engine at part loads. At high engine loads, unburned HC emissions are comparable to those of the diesel fuel operations [9]. CO emissions tend start at a comparatively higher value at low loads, and gradually approach normal diesel engine levels with increasing equivalence ratio [10].
However, it is not well known how to optimize the exhaust emission characteristics of natural gas engines through combinations of the different performance parameters (such as compression ratio, premixed equivalence ratio and pilot injection timing for dual-fuel CI engines). And rare studies have mentioned the methane emission characteristics. Additional investigation is needed to optimize the dual-fuel CI engine performances operated with natural gas. Studies of varying pilot fuel amounts and injection timing at different engine loads are required in order to optimize emissions characteristics.
The objective of this study is to investigate the emissions characteristics of a CNG/diesel dual fuel engine under maximum diesel fuel substitution ratio conditions with optimized pilot fuel amounts and injection timing. The methane emission characteristics are studied and the energy loss ratios of the unburned methane were obtained.
2 Experimental apparatus and setup
2.1 Test engine
The original engine was a multi cylinder, turbocharged and common rail direct injection diesel engine manufactured by WEICHAI POWER in China. A summary of the engine's specification is listed in Table 1.
2.2 Dual fuel conversion method
The dual fuel engine conversion and gas fueling system is shown in figure 1. The original electronic control unit (ECU) was retained. A fully functional dual fuel ECU was designed and built in Tsinghua University using copyrighted software developed in-house. The dual fuel ECU was designed to control diesel fuel injection pulse width, start of injection and CNG flow rate, while monitoring engine temperatures and pressures for safety and data logging purposes. The dual fuel ECU is designed to switch the engine immediately to full diesel operation in the event of an irregularity. Under dual fuel running conditions, the diesel and CNG injections were controlled by the dual fuel ECU, and the diesel injection command of the original ECU is applied to the dummy load which simulates the solenoid injector.
The gas fueling system consisted of high pressure natural gas bottles, a shut off valve, a high pressure filter, a natural gas flow meter, a coolant heated pressure regulator. After the regulator, natural gas was decompressed from 18 MPa to 0.8 MPa. The amount of natural gas was changed through controlling the fuel injection duration of four SP010A CNG injectors mounted on the gas rail. The gas rail pressure and fuel temperature was also recorded. A simple venturi type gas mixer valve was installed at a distance of ten pipe diameters upstream of the inlet manifold to ensure complete mixing of the air and fuel was achieved.
Exhaust gases were sampled in a Horiba MEXA-7100DEGR exhaust gas analyzer. Total hydrocarbon (THC) was analyzed with a flame ionization detector (FID), CO was analyzed with a non-dispersive infrared analyzer (NDIR), and NOx was analyzed with a chemiluminescent detector (CLD). THC, CO and NOx emissions were the average values of the acquired data on line at each steady state operating condition. Smoke emissions were sampled with an AVL 439 smoke meter and expressed in opacity value of 0~100%. An ONOSOKKO FZ2100 coriolis mass flow meter was used to measure diesel fuel mass flow rate. The airflow rate was measured with a SENSYFLOW hot-film air flow meter. The CNG flow rate was measured by a BROOKS gas flow meter.
2.3 Test Fuels
The engine is fueled with commercial 0# diesel fuel and natural gas obtained from the local distribution network in Beijing City. The detailed specifications of the two fuels are listed in Table 2
2.4 Test Method and Uncertainty
Before test, the engine was warmed-up in normal diesel mode until the coolant and the lubricating oil temperatures reached 75±5°C. All tests were conducted at ambient temperature of approx. 23°C, and the charged air temperature after the intercooler is controlled below 40°C. First, the universal characteristics of the original engine were tested with pure diesel fuel. The engine speed range is 800-2200r/min and the speed interval is 200r/min. At each tested engine speed, eight engine loads was selected:12.5%ã€25%ã€37.5%ã€50%ã€62.5%ã€75%ã€87.5%ã€100%, respectively. Then, the dual fuel engine universal performance map was calibrated based on the next two principles. Firstly, at each calibration point, the premixed natural gas is increased to the maximum diesel fuel substitution ratio. Secondly, the injection timing of the pilot diesel fuel is adjusted with load to achieve the MBT (Maximum Brake Torque). Finally, the engine emissions characteristics were investigated based on the ESC 13-step test with pure diesel and dual fuel operation modes. Engine emissions of CO, NOx, THC, CH4, and smoke were measured through different operating modes.
In each tested condition, the variation of speed was controlled within ±0.5% r/min, the controlled precision of torque was ±1%. The fuel consumption uncertainty was less than 1%. The fluctuation of the air flow rate is less than 2%. The relative experimental error is less than 2% for the CO emission, 1% for smoke opacity, 3% for the THC and NOx emissions.
3. Results and Discussions
This section presents engine emissions characteristics that were studied experimentally through different operating modes. Experiments were executed at three engine speeds of 1320r/min, 1627 r/min and 1933r/min, which refer to ESC test engine speed nA, nB and nC. The power percentages were set to be 25%, 50%, 75% and 100%. At each test condition, experiments were carried out with pure diesel and dual fuel modes. Comparison parameters for different operation modes were conducted.
3.1 Fuel substitution ratio
Figure 2 gives the diesel fuel substitution ratios under dual fuel operation mode. To present the substitution percentage of the diesel fuel, the following expression is used.
Where is the original diesel injection rate, is the diesel fuel injection rate under dual fuel mode. It can be seen that the diesel fuel substitution ratio is over than 90% when the load percentage is below 62.5%. And at the full load conditions, the substitution ratio is around 55%.
3.2 excess air ratio and normalized fuel injection rate
The excess air ratio significantly influences the combustion process and the emission characters. Figure 3 illustrates the premixed and total excess air ratios, which are calculated with the following expressions:
Where: is the actual air mass, is the stoichiometric air/fuel ratio of the premixed CNG，is the total stoichiometric air/fuel ratio of the premixed CNG and diesel.
It can be seen that the total excess air ratio is decreased with the increase of BMEP. However, the premixed excess air ratio decreased first and then it increase. That is because the increasing rate of inlet air is lower than the increasing rate of premixed CNG at low to medium loads. At high loads, the increasing rate of inlet air is higher due to the high boost effect.
The quantities of the premixed CNG, pilot diesel and inlet air were normalized by the quantities of the 25% load as shown in figure 4. When the load increases from 25% to 50%, there are moderate increases of the premixed CNG and pilot diesel. However, when the load is over than 50%, the increasing rate of the pilot diesel quantity is obvious.
3.3 CO emission characteristics
CO emission is the product of incomplete combustion and it is controlled primarily by the fuel/air equivalence ratio and temperature. High CO have been found to occur in lean regions in the temperature range 800﹤T﹤1500K as well as in over rich mixtures that have insufficient oxygen availability for complete combustion [11]. The amount of OH radicals is particularly important in the oxidation process of CO, and OH concentration increases exponentially with peak charge temperature [12]. A minimum temperature of 1450K was found to be necessary for near complete CO oxidation in lean mixture region. This temperature is also the limited temperature of the flame extinction [13].
Figure 5 illustrates the CO emission characteristics with pure diesel and dual fuel operation mode. It can be seen that the CO emission levels under dual fuel mode are considerably higher than their values corresponding to normal diesel operation conditions, which indicated that there exist some flame extinction regions, even at high load. Therefore, most of the CO emission is from the incomplete oxidation of the premixed CNG. When operating with lean premixed mixtures at light load, most of the energy release comes from the combustion of the pilot diesel and the gaseous fuel entrained within its envelope. The mixture in the peripheral zones of the injection spray is too lean to sustain the flame propagation. Due to this, the local temperature falls and freezes the reactions of CO oxidation. Under high load conditions, the increase of the pilot quantity improves the pilot fuel spray characteristics, which increases the number of ignition centers. And the unburned mixture is compressed to a higher temperature with larger pilot energy release. Moreover, the intake pressure and temperature is higher, which lead to a higher in-cylinder compression temperature. So, more premixed fuel can be oxidized completely. However, cold wall quenching may become the primary source of the high CO emissions. With the increase of the engine speed, less time is left to complete the CO to CO2 reactions before the local temperature below freeze temperature during expansion stroke which increases the CO emissions.
3.4 NOx emission characteristics
NOx is formed in greater quantity with high peak combustion temperatures, sufficiently high oxygen concentrations and long residence time. Significant NO is produced when the local temperature is above 2,000K for mixtures at or below stoichiometric. The variations of NOx emissions for diesel and dual fuel mode at engine loads and speeds are shown in figure 6. NOx emissions for the two operation modes increase significantly when the engine load increases. Dual fuel mode shows greater NOx emission reductions in comparison to diesel mode, which is more obviously at high load. Dual fuel mode averagely reduces NOx emissions by 30% in comparison to diesel mode. That is because most of the fuel is burned under lean premixed conditions which result in lower local temperature. At high load operation conditions, the pilot fuel injection timing is retarded to reduce the risk of knock, which leads to a reduction of the NOx emissions. With the increase of the engine speed, the NOx emissions reduced, that is because there is less residence time for the NO formation.
3.5 THC emission characteristics
The HC formation condition is almost similar to that of the CO. However, the temperature of near complete oxidation of HCs is lower, which has been found to be around 1200K with independence of the original fuel type. Normally, the unburned HC emissions of the natural gas fueled dual-fuel engines are obviously higher than that of the diesel engines at low to intermediate loads [14-16]. This is caused by the unburned natural gas (i.e. mostly methane) surviving to the exhaust. Figure 7 indicates HC concentrations of over than 10000 ppm at low to medium loads, compared with significantly less than 100 ppm in conventional diesel operation conditions. As shown in figure 8, around 90% of the THC emissions were composed by unburned methane. There are three major factors contributing to this phenomenon. Firstly, the gaseous fuel and air mixture being too fuel-lean for combustion, the flame (initiated by the pilot fuel) does not propagate throughout the charge. Secondly, part of the premixed natural gas is forced into the crevices of the piston ring and dead zone of the combustion chamber during the compression strokes, which escapes from combustion. Finally, as the combustion of the natural gas-air mixture usually begins in the expansion stroke, flame quenching may be a contributing factor [15]. The above influences are amplified by the lower flame speed of natural gas, especially at lean premixed conditions. At high load operation conditions with the increase of the pilot quantity, THC emissions reduce significantly which is expected due to a number of contributing factors. These include a greater energy release on ignition, improved pilot spray atomization characteristics, enhanced turbulence intensity caused by the higher injection energy, a larger number of ignition centers and extended pilot fuel envelope, higher rates of heat transfer to the unburned gaseous fuel-air mixture and enhanced compression effect to the unburned premixed mixture [17]. Furthermore, at high load the boost pressure and temperature is higher, which lead to an increased in-cylinder compression temperature. All of these effects reduce the combustion limits of the lean premixed CNG fuel and increase the flame propagation speed. So, more premixed CNG participates in the combustion process. However, the THC concentrations are still higher than 2000 ppm at full load conditions. With the increase of engine speed, there is less residence time for the fuel oxidation, which results in higher THC emissions.
3.6 Smoke emission characteristics
Based on the calculation findings of Kitamura et al. [11], a soot formation peninsula is evident at Φ>2 and between temperatures of 1600-2500 K. Particulate or smoke emissions in dual-fuel engines are very low and in some cases undetectable [18]. The smoke emission of the dual fuel mode is significantly lower than that of the diesel engine, as shown in figure 9. Methane, the primary constituent of CNG, has no carbon-carbon bonds with high hydrogen to carbon ratio, which lead to lower sooting tendencies [19]. In addition, the natural gas has enough residence time in traveling from the intake manifold to the combustion chamber to form a well mixed mixture with air prior to combustion. Therefore any emitted particulates are produced during the diffusion combustion process of the pilot diesel fuel in the very fuel rich and high temperature region of the pilot fuel spray core, which is the same with the conventional diesel engines [20]. Some of the soot will be oxidized during the combustion process of the natural gas-air mixture, further lowering the smoke emission levels. However, at high load conditions, with the higher boost pressure and retreated pilot injection timing, the ignition delay time is reduced. So, with the increase of the pilot fuel quantity, more pilot fuel participate in the diffusion combustion process, which result in an obviously increase of the smoke emission.
3.7 Break thermal efficiencies
The break thermal efficiencies of the dual fuel engine and the original diesel engine are compared in Figure 10. The break thermal efficiencies of the original diesel engine are in the range of 37-45% corresponding to full operation conditions. The break thermal efficiencies of the dual fuel engine are much lower than that of the diesel engine at low load (ex. around 25%, at 25% load with the engine speed of 1627r/min). At these part load conditions, the pilot fuel fails to ignite most of the natural gas-air mixture. As shown in the figures, the energy of the unburned CH4 form the exhaust is about 40% of the total input energy at 25% load. At high load conditions, the injection of a larger proportion of the pilot fuel provide more ignition points, so that the oxidation of the natural gas air mixture would be more complete [21]. As can be seen in the figures, the exhaust energy of the unburned CH4 is less than 10% at full load. The break thermal efficiencies of the dual fuel engine also have an obviously improvement with the increase of load (ex. 49.5%, at 100% load with the engine speed of 1627r/min).
4. Conclusions
For CNG/diesel dual fuel engines, the effects of pilot injection timing and fuel substitution ratio are noticeable and significant. An appropriate understanding of these effects is necessary. The CNG/diesel dual fuel engine emissions have been experimentally investigated in this paper with maximum fuel substitution ratio and optimized pilot injection timing. The investigation results indicate the following:
1. The diesel fuel substitution ratio is over than 90% when the load percentage is below 62.5%. And at the full load conditions, the substitution ratio is around 55%.
2. The CO emission levels under dual fuel mode are considerably higher than that under normal diesel operation conditions. Which are caused by the flame quenching of the lean premixed natural gas-air mixture.
3. Due to the low combustion temperature of the lean premixed mixture, dual fuel mode averagely reduces NOx emissions by 30% in comparison to diesel mode.
4. The unburned HC emissions of the natural gas fueled dual fuel engines are obviously higher than that of the diesel engines. And around 90% of the THC emissions were unburned methane.
5. With the premixed nature of the dual fuel mode and molecular structure of the methane, smoke emissions are considerably lower than normal diesel engine.
6. With an optimized boost pressure, pilot injection timing and pilot quantity, break thermal efficiencies of the dual fuel engine can reach a very high level. It reaches 49.5% in this study.
Acknowledgement
This research was supported by the Ministry of Science and Technology of China through the project 2012DFA81190, 2011DFA60650, 2010DFA72760 and 2010DFA72760-202.
