VALVE TIMING AND IGNITION ISSUES IN FUEL SYSTEM FOR COMPRESSED NATURAL GAS DIRECT INJECTION (CNGDI) Yusoff Ali, Zailani Muhammad and Muthana I -Khamas Department of Mechanical and Materials Engineering, Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia.
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ABSTRACT The fuel system is the most significant feature that determines the combustion efficiency and emission performance. The fuel system must deliver fuel and air for proper combustion in the engine for reliability. The valve timing determines when the fuel should be directly injected into the combustion chamber at the right time. If fuel is injected during the exhaust valve not yet fully closed, the fuel will flow into exhaust port and cause poor emissions. If fuel is injected during the intake valve still opening, the incoming air is displaced by gas fuel. This will reduces volumetric and combustion efficiency. Spark plug needs to ignite the combustible air/fuel mixture at exactly the right time to ensure the engine produce maximum power and minimum pollution. If the spark occurred at the wrong time, power will fall and fuel consumption and emissions can increase. Therefore, the fuel system also should deliver the fuel with better spray characteristics at proper pressure and temperature to accelerate the air/fuel mixing and produce ignitable mixture around spark plug. Keywords: Keywords: compressed natural gas, direct injection, valve timing and ignition. INTRODUCTION With the increasing concerns of energy security and the stringent enforcement on air pollution control and regulations of clean air quality, much effort has been focused on the development of alternatives for crude oil fuels. One of the promising solutions to this problem is utilization of compressed natural gas (CNG). But introducing natural gas into carbureted or port injection engine has disadvantage of low volumetric efficiency and difficulty operating with lean mixture. Therefore, direct injection strategies is used to overcome these problems and to achieve both higher thermal efficiency and lower emissions. In CNGDI engine, fuel system plays a main role in determining combustion efficiency and combustion products. Fuel system must deliver fuel to injector at the proper time and pressure with better spray characteristics to accelerate the air/fuel mixing and produce ignitable mixture around spark plug gap. Injection timing is very important parameter for realizing better combustion in CNGDI, since injection timing determines the formation of a combustible mixture, the combustion stability, the
lean combustion limit and the combustion products. Injection timing has a very close relation with valve timing and ignition timing. Therefore, determination of injection timing depends on the timing of intake and exhaust valve open and close and the timing of ignition. VALVE TIMING Valve timing, ignition timing and injection timing has a very close interaction between each other. Interruption or improper setting one of these timing can affects the other timing. Therefore, these timing should be set properly to ensure the combustion in the engine is efficient and optimum (in terms of power, torque, combustion stability, fuel consumption and emissions) to ensure the engine could run smoothly. smoothly. Traditionally, valve timing has been designed to optimize operation at high engine speed and WOT operating conditions. Controlling valve timing can improve the torque and power and reduce fuel consumption and emissions. Under low engine speeds, closing the intake valve at a proper time will increase the volumetric efficiency of the gas exchange process. An early intake valve closing time will cause the fresh air to expand till BDC and therefore its temperature at the commencement of the compression stroke will be lower. As a result, lower amount of NO x but higher amount HC are expected to be emitted [1]. A way of controlling the load while improving the fuel economy was suggested by Ma [2]. Ma has concluded that a late intake valve closing is a practical concept applicable to engines with two intake valves per cylinder where the intake valves can be phased relative to each other to extend the total intake-opening period. In this CNGDI engine, the intake and exhaust valve timing is fixed as shown in Figure 1. The exhaust valve is opened and closed at 45 0 before BDC and 10 0 after TDC respectively. The exhaust valve has to be opened at earlier time to maximize blow down and provide proper scavenging of the residual exhaust gases during the overlap period. The overlap period is a period when both the intake and exhaust valves are open at the same time. Opening the exhaust valve later actually reduces power output because of the increased volume of exhaust gases that must be purged from the cylinder. This means that there are greater pumping losses required to evacuate the cylinder. cylinder.
Getting the relatively small volume of natural gas into the combustion chamber is much easier than getting the large volume of air needed to react with the natural gas. Therefore, the intake valve should be opened early before the piston slightly reaches TDC in exhaust stroke. This gives some advantages such as the incoming air can helps to push or scavenge the combustion products and unburned natural gas out through exhaust manifold. But if the intake valve is opened too early, there is a greater amount of residual exhaust gas still left inside combustion chamber and the reversion of this exhaust gas into the intake manifold will be occurred. The exhaust gas will enter into intake manifold and interrupt the incoming air from flowing into combustion chamber. In carbureted or port injection engine, this problem become worse. The exhaust gas will ignite the fuel-air mixing and causing explosion inside intake manifold.
Fig.1: Intake and exhaust valve t iming diagram But, if the exhaust valve were opened too early, some amount of work available from the expanding exhaust gases would be lost and blown out through the exhaust manifold. This means the engine didn’t take full advantage of the power stroke, thus reducing the work done on the piston. As a result, the engine torque will reduce. Also, there would be less exhaust inertia at TDC because of the decreased mass of exhaust gas available at the end of the exhaust stroke. This reduction in inertia would lead to a smaller force exerted on the incoming intake air during the overlap period and results lower volume of total air in the cylinder. The exhaust valve also should be closed lately to assure that no exhaust gases are left in the cylinder. In CNGDI engine, if the exhaust valve is closed too late, only the incoming fresh air flows out through exhaust manifold during overlap period. But, we should remember that the piston is going down to do sucking process and there is possibility of exhaust gases reentering into the cylinder. In carbureted or port injection engine, if the exhaust valve is closed too late, a little amount of the fuel-air mixing will flow out into exhaust manifold and this results poor fuel economy and emissions. The intake valve open 12 0 BTDC and close 48 0 after BDC. This gives enough time to air to flows inside and fills combustion chamber and helps existing exhaust gases flow out through exhaust manifold (scavenging process) during overlap period. More air enters into combustion chamber results higher volumetric efficiency. Higher volumetric efficiency means more natural gas can be burned in combustion chamber and more energy can be converted to output power.
An early intake valve closing time will not permit enough fresh charge to enter the cylinder, thus reduce the engine torque. Therefore, the intake valve should be closed at a later time to give ample time for the incoming air to fill the combustion chamber. Previous study [3] also shows later closing timing of the intake valve prevents the decay of vertical vortex after intake BDC and consequently the turbulence energy near compression TDC is intensified. The retardation of intake valve closing timing also much more effective than squish flow for increasing the turbulence energy near compression TDC. But, since the closing point of intake valve determines where the cylinder actually begins to build pressure, the maximum pressure and temperature generated during compression stroke will be decreased if the intake valve is closed too late. This can results lower combustion efficiency and lower maximum engine torque. INJECTION TIMING Injection timing has a very close interrelation with the valve timing. Therefore, the setting of injection timing is depends on the both timing of intake and exhaust valve. The injection timing also can be varied depend on the combustion mode, engine load and engine speed. In direction injection engine, natural gas should be injected into combustion chamber when the both intake and exhaust valve is closed. If the natural gas is injected during the intake valve still opening, the space inside the cylinder is replaced by natural gas as shown in Figure 2. Thus, the volume of the incoming air sucks into the cylinder during intake stroke will decrease and this causes the volumetric efficiency also decreases. Conversely, if the natural gas is injected during exhaust valve still opening, a small amount of natural gas will be pushed out through exhaust manifold (short circuit) thus increasing the unburned gaseous fuel emissions. Natural gas also must be injected when the piston at the right position to ensure the piston crown shape can manipulate the spray for better mixing and brings the fuel spray to spark plug.
Fig.2 : Direct injection of natural gas at 4000 rpm For homogenous charge combustion operation engine, the injection timing is set at early of compression stroke to ensure the better fuel/air mixing. For stratified charge operation combustion engine, natural gas is injected at the late of compression stroke with suitable injection timing to enable the engine have stable operation at a very lean overall mixture. Air-fuel ratio near the spark plug should be relatively rich to improve ignitability of overall lean CNG mixtures. This can be achieved by using the piston crown shape to manipulate injected fuel flows to spark plug or by using suitable spray pattern. But, injection at the late of compression stroke needs the higher injection pressure to overcome the increasing cylinder pressure. Injection during later compression stroke also causes CNG fuel has limited time to mix with air. Therefore, the injector must inject CNG fuel with better spray characteristics to accelerate CNG-air mixing. CNG fuel also may be injected into combustion chamber in dual injection timing. Firstly, CNG fuel is injected at the early compression stroke with relatively longer injection duration and lower injection pressure to ensure CNG fuel has adequate time to mix with air. Second injection occurs at the late compression stroke with shorter injection duration and higher injection pressure to improve ignitability by promoting rich air-fuel ratio at spark plug. The amount of injected CNG fuel into combustion chamber depends on the engine load and engine speed as shown in Figure 3. If the engine load or the engine speed increases, the amount of fuel that should be burned to produce work required also slightly increases. The fuel amount or the fuel flow rate can be varied by varying injection duration and pressure. Longer injection duration and higher injection pressure give higher fuel amount and higher fuel flow rate. Therefore, injection duration for high-pressure injection is relatively shorter than low-pressure injection.
Fig.3: Effect of engine speed on injection duration Figure 4 shows the variation of injection timing with engine speed. As the engine speed increases, the natural gas and air will not have enough time for mixing if injection occurs at the end of the compression stroke. Therefore, natural gas should be injected at early of compression stroke. This means injection timing should be advanced to give enough time for air-fuel mixing to complete.
Fig.4: Variation of injection point with engine speed Previous studies [4] using rapid compression machine (RCM) showed that the injection timing gives a great influence on combustion characteristics. Early injection shows a slower burn in the initial stage and a faster burn in the late stage, which is similar to flame propagation of premixed gas. In contrast to this, the late injection showed a faster burn in the initial stage and a slower burn in the late stage that is similar to diesel combustion. Faster burn at the initial stage is caused by moderately strong turbulence and faster burn at the late stage is caused by moderately proceeding mixture formation.
The study also shows that late injection produces relatively lower NO x at the equivalence ratio greater than 0.5, while CO level is higher for the late injection at the equivalence ratio greater than 0.9 [4]. IGNITION TIMING The ignition processes strongly affect the overall performance in spark ignition engines. Thermal efficiency and the level NO x emission are strongly influenced by ignition timing. Proper ignition timing can offer high engine thermal efficiency, low levels of NO x emission and longer engine operational life. Improper ignition timing can lead to misfire or knock and high level of cycle-to-cycle variation [5]. Cycle-to-cycle variation can lead to fluctuation in engine output and poor drivability. Therefore, spark plug must ignite the natural gas/air mixing at the right time with enough energy to start the combustion in the cylinder. The ignition should occur after injection process complete as shown in Fig.5 and when the combustible mixture is produced around the spark plug gap.
Figure 6: Variation of ignition timing with engine speed and injection timing Previous study on effect of ignition timing to NO x emissions in spark ignition engine by Alasfour [6] shows advancing the ignition timing causes the combustion process to occur near TDC and leads to an increase in the level of NO x. While retarding ignition timing causes the exhaust temperature to increase and engine thermal efficiency to decrease. Ignition timing is controlled by the relationship of the position of the piston to when the spark occurs. If the ignition spark occurs too soon it can actually push against a piston traveling up the cylinder as it is compressing the natural gas/air mixture. This can causes detonation, lost power, much higher combustion temperatures, backfires and early internal engine part failures. If the spark occurs too late, the ignition of the natural gas/air mixture occurs after the piston is traveling back down the cylinder. This is a wasted energy and can results unburned fuel, higher emissions and lower power output.
Figure 5: Timing event diagram When the compressed natural gas/air mixture inside a cylinder is ignited, it takes time for the flame front to reach the piston and for the expanding gases to start pushing it down. The time for the flame front to expand and push the piston is depend on a number of variables such as the mixture strength, volumetric efficiency, compression ratio and combustion chamber shape. As engine speed increases, there is less time to ignite the natural gas/air mixture, so spark plug must starts burning the mixture earlier in order for the flame front to push the piston at the right time. In other words, the ignition timing should be advanced as the engine speed increases as shown in Figure 6. If the ignition timing is advanced too much, the burning mixture will hit the piston as it rises and causes instability movement of the piston thus damages the engine. If the ignition timing is advanced too little, the flame front reaches the piston too late and does not do a good job of pushing the piston down.
The combustion chamber shape and the position of the spark plug also determine the ignition timing setting. The shape of combustion chamber like pent roof and centrally spark plug promote a very fast burning of natural gas/air mixture. Therefore, less advance ignition timing is needed. CONCLUSIONS 1. Variation in exhaust valve timing affect the timing of the intake valve. Therefore, both the exhaust and intake valve timing should be set simultaneously. 2. The early opening of exhaust maximize blow down and provide proper scavenging of the residual exhaust gases during the overlap period. 3. Opening the exhaust valve later reduces power output due to greater pumping losses required to evacuate exhaust gases from the c ylinder. Advancing the exhaust valve timing too much can 4. reduces volumetric efficiency thus reduces engine torque and power. 5. The early opening of intake valve gives some advantages such as the incoming air can helps to push residual exhaust gases out of cylinder.
6.
Advancing the intake valve timing too much is opened too early can causes the reversion of the exhaust gases into the intake manifold and interrupts the incoming air from flowing into combustion chamber. 7. Closing the intake valve later increases volumetric efficiency and the turbulence energy near compression TDC. 8. Injection during intake stroke can reduces volume of incoming air flows into the cylinder, while injection during exhaust stroke can causes short circuit thus increases emissions. 9. The ignition timing should be advanced as the engine speed increases. But advancing too much can causes detonation, lost power, much higher combustion temperatures, backfires and early internal engine part failures. 10. Retarding ignition timing can results unburned fuel, higher emissions and lower power output. Acknowledgement The authors would like to thank the Malaysian Ministry of Science, Technology and Environment for sponsoring this study under IRPA 03-02-02-0058 PR0030/10-02 grant. Abbreviations CNGDI compressed natural gas direct injection WOT wide-open throttle BDC bottom dead center TDC top dead center RCM rapid compression machine REFERENCES 1. E. Sher and T. Bar-Kohany, Optimization of variable valve timing for maximizing performance of an unthrottled SI engine-a theoretical study, Energy 27, 757-775 (2002). 2. T.H. Ma, Effect of variable engine valve timing on fuel economy, SAE paper 880390 (1998).
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