Injection Parameters Effect on the Perfom1ance of Compressed Natural Gas Direct Injection (CNG-DI) Engine under Lean Stratified Conditions

Natural gas has been identified as an alternative to crude oil fuels such as gasoline and diesel. Natural gas utilization as an automotive fuel is yet to be fully used to its optimum because most of the vehicles converted to natural gas are still using port injection or carburetors. Natural gas can be used either in compressed (CNG) or liquefied (LNG) fom1. Compressed natural gas (CNG) has huge potential for improving the them1al efficiency of spark ignited (SI) engines due to combustion-specific properties such as high knock resistance and extreme stratification capabilities for lean air/fuel ratio. The main drawback or disadvantage of using natural gas in the engine is that its perfom1ance drops compared to gasoline or diesel engine. For current passenger car standard applications a power drop of approximately I 0% is noticed by use of CNG. The prime source of perfom1ance drop is because of lower volumetric efficiency, lower energy density and longer combustion duration of natural gas. Reduced volumetric efficiency of induction system is being widely studied to optimize for the losses causing this diminution. This drawback can be compensated by direct injection of CNG straight into the combustion chamber, and therefore giving way to utilize the maximum benefits from using CNG as automotive fuel. Combustion of natural gas is cleaner i.e. lower exhaust emissions, also because of its higher octane number, the engines can be designed with higher compression ratios, hence increasing the them1al efficiency. Direct injection systems for natural gas engme are expected to solve the problem of lower volumetric efficiency. Optimization of injection parameters is required for the optimum outcome of natural gas engine. Besides favorable engine out emissions, the engine concept operates without power loss and with absolute low fuel consumption. The "Direct CNG Injection" may be a highly attractive solution for automotive propulsion systems. The following research is carried out on a dedicated 4-stroke natural gas spark ignition engine with a compression ratio of 14. A centre direct injection system is used, where the injector is placed at the centre of cylinder head with spark plug offset by 6mm. Engine is being tested for idle and partload conditions using homogeneous and stratified pistons. Injection parameters such as injection timing and injector spray angle are investigated while keeping the injection pressure constant, to find out the effect of injection parameters on CNG-01 engine. Ignition timing is adjusted to obtain the maximum brake torque (MBT). Results for both stoichiometric and stratified charges at idle and partload conditions are compared to examine the features of both operations under these conditions. The experimental results are categorized based on each injection parameter. Firstly, injection timing from early injection (300 degree BTDC) to late injection (80 degree BTDC) is investigated for stoichiometric conditions. For stratified operation the injection starts after the closing of air intake valve at 132 BTDC. The injection is delayed further till the limit for each RPM as set by the ECU (Engine Control Unit). Injection timing effect for idle and partloads is compared for stoichiometric and lean stratified operations, for injection timings starting after the closing of intake valve. Engine speed is limited from 2000 to 5000 RPM. Injection pressure is kept constant at 18 bars for both stoichiometric and stratified operations. Injection pressure is affecting the fuel delivery rate. Lower injection pressure needs longer injection duration to deliver the required fuel to the engine. Two injectors with different injection angle are investigated, 30 deg (NAI) and 70 deg (W AI). Both injectors have their distinctive characteristics which can be applied on certain engine operational conditions. Lean stratified operation proved to be better for lower engine speeds while having overall lower brake specific fuel consumption over a wide range. Lower performance at higher engine speeds is due to less time for mixture formation, lower fuel content and excessive stratification. Wide angle injector (W AI) proves to be giving better performance than narrow angle injector (NAI) at lean conditions. Faster mixing rate of W AI might be responsible for such behavior. Nitrogen oxides (NOx) emission for lean stratified operation is higher at lower engine speeds which indicate higher temperatures of combustion, for all injection timings except the lean limit where it is lesser. Unburned hydrocarbons are slightly higher than stoichiometric and tend to increase with the engine speed. Higher cycle to cycle variation, mixture fom1ation, excessive stratification and bulk quenching are the reasons for such behaviors. Carbon monoxide (CO) emissions for lean strati tied operation are quite lower compared to stoichiometric for all injection timings