Narrow angle direct injection is an approach which has the possibility of paring down the power limitation in the HCCI engines. The aim of this method is the usage of highly premixed fuel air charge combustion at low and medium loads while conventional diesel combustion at high and full loads. Using this approach the system can interchange between the two combustion modes.
In conventional diesel engines, fuel injected towards periphery during combustion initiation, but in NADI the fuel is injected at the centre. Narrow angle injection prevents the fuel deposition on combustion chamber walls, thus lowering the emissions due to unburnt fuel particles.
Injecting fuel early aids in the formation of homogeneous charge but too early injection results in poor evaporation of fuel and leads to piston bowl spray issues. This ultimately results in combustion occurring earlier and hence a negative work being done during the compression stroke.
The main objective of NADI is to determine best combustion chamber design suitable for narrow injection and create a homogeneous mixture. The piston bowl is modified in such a way that there is more area under the spray to encourage fuel-air mixing using the injection spray velocity.
NADI chamber is designed with compression ratio of 14:1 to provide the same power as the conventional diesel engine. Results indicate that HCCI combustion is favoured with the narrowest spray cone angle.
Another advantage of NADI is the possibility of managing early injections, as a result multiple stage injection can be tested efficiently and its effect on the performance can be validated. The multiple stage injection leads to a reduction of CO emission by half and noise reduction by 4dB compared to the single injection. HC emission persists which have to be treated in order to meet prescribed standards. The intake pressure is lower and well-suited with turbocharger performance. The advantage of multiple stage injection is the extension of HCCI range without any compromise in the fuel economy.
Distinction between NADI combustion mode compared to conventional diesel combustion include:
Spray angle is narrow thus it limits the linear impingement
Piston bowl designed for guiding the fuel
Reduced compression ratio
Multiple stage injection
Effects of NADI on engine performance:
Nox and particulate matter: Nox emission is 100 times lower than conventional diesel engine and particulate matter is 10 times lesser.
HC and CO emission: HC and CO emissions is at the same level compared to conventional gasoline engines, also exhaust gas temperature is quite low.
Combustion noise: Due to early injection the noise level increases substantially with engine speed and load.