Intake Manifold or Port Injection:

With fuel injection in the intake manifold or at the port sufficient time is available for the fuel to vaporize and mix with air and high degree of mixture homogeneity is achieved.  As the  boiling point of diesel fuel is high the fuel  vaporization may be enhanced  by heating of intake manifold. Autoignition occurs due to compression.  In this approach, very low NO_x   down up to 1/100th of the conventional diesel operation are possible.  Smoke is very low, but higher HC and CO emissions are obtained as considerable fraction of lean homogeneous mixture may not burn or burns only partially.
Intake charge heating although, improves fuel evaporation and charge homogenization, but results in early ignition causing high rate of pressure rise and noisy combustion. If no intake heating is used, some fuel may remain in liquid state until late in the compression stroke causing locally over rich mixtures resulting in formation of soot. EGR has been employed along with intake mixture heating to retard autoignition and control rate of combustion. Use of very lean mixtures also results in retard of ignition but it limits imep to very low values that can be obtained without knocking combustion. Use of a lower engine compression ratio to retard ignition results in loss of engine efficiency. Thus, intake manifold fuel injection is not considered a practical approach for HCCI operation due to its negative effects on fuel efficiency and hydrocarbon emissions and low knock limited value of imep

Early In-Cylinder Injection:

This approach has two advantages:

(i) Higher temperatures and density compared to intake air are available. Hence, External heating of air is not necessary and,

(ii) One injection system can be used for early injection for HCCI operation and also for the conventional diesel combustion operation at higher loads.

 Sufficient time is available when the fuel is injected early in the compression stroke to fully vaporize and form homogeneous mixture with air at the time of ignition.
One of the problems faced in practice is that the fuel is injected early during compression when the density of air is low. It results in long length of spray penetration and the liquid fuel may impinge on the combustion chamber and cylinder walls. To prevent fuel impingement redesigning of the injection system may be necessary. Some of the approaches used are:

• Injector in the centre of the cylinder  with more number (  numbering up to 30) of smaller holes
• Use of two side injectors opposite to each other so that spray travel distance is larger compared to the central injector.
• Multiple injection events with small quantities in each injection. A multiple injection strategy using common rail injection system is shown in Fig 7.11. The first injection is made around 90º btdc followed by equally spaced several injection pulses. It allows each fuel pulse to mix well with air. The main injection is made at tdc.