Direct and Indirect Injection Systems

There are two types of injection systems, direct and indirect.  Indirect injection or (IDI) is when the fuel injection is not directly injected into the combustion chamber.   Instead, fuel goes into a pre-combustion chamber or an air cell to begin the process that then spreads into the main combustion.

With a direct injection system or (DI) the fuel is injected directly into the combustion chamber, usually on the top of the piston which is designed to control the spread of combustion.

IDI systems are primarily from the 80’s and 90’s.  DI is the type of system most engines have today. Below are some more specifics on both types of injection systems.

Direct Injection (DI):

  1. Injection pressure is higher (21 kg/cm^2) in this case as compared to IDI because fuel atomisation solely depends on pressure of injection.
  2. The engines are easier to start in cold weather.
  3. Fuel injection nozzles have multiple holes as high atomisation and mixing is required because of a lack of moving air.
  4. Thermal efficiency is higher as compare to IDI engine.

Indirect Injection (IDI):

There are two designs for an IDI system; a pre-combustion chamber and air cell.

Pre-combustion Chamber:

Fuel is injected into a pre-combustion chamber and initial combustion takes place.  Pressure builds and the fuel is expelled at a high speed into a main combustion chamber.  Because of the high velocity, fuel atomized and mixes with air in the main combustion chamber and combusts.

Air Cell:

Air cell designs have a small chamber in the piston. Pistons compress the air during a stroke and pushes the fuel into the air cell.  As the piston moves down pressure in the cylinder falls causing the flow of hot air from the cell into the combustion chamber. The mix of air and fuel completes the combustion.

The difference between DI and IDI is the layout of the injection systems.   The IDI system has a small swirl chamber above the cylinder, where the fuel is injected, there’s also a glow plug in the chamber needed to help start the engine.  The DI system has the injection nozzle attached to the top of the combustion chamber. The piston usually has a crown shape to create the air swirl.

The IDI has a high rate of air swirl over a wide range of speeds, does not require as much pressure, and has a less chance of blockage due to self-cleaning pintle injectors.

The IDI however has less fuel consumption due to lower thermal efficiency and a higher compression ratio required to start the engine.

The DI system is better with cold starts, has a smaller combustion space, better thermal efficiency and as a result is more economical.

A few disadvantages of the DI system is they tend to be louder, prone to blockages due to smaller injection holes, lower power output, and slower air swirling especially at lower speeds.

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