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.