Indirect injection

In an internal combustion engine, the term indirect injection refers to a fuel injection where fuel is not directly injected into the combustion chamber. Gasoline engines are usually equipped with indirect injection systems, wherein a fuel injector delivers the fuel at some point before the intake valve.

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber. The prechamber is carefully designed to ensure adequate mixing of the atomized fuel with the compression-heated air. This has the effect of slowing the rate of combustion, which tends to reduce audible noise. In addition, it softens the shock of combustion and produces lower stresses on the engine components. The addition of a prechamber, however, increases heat loss to the cooling system and thereby lowers engine efficiency. In an indirect injection system the fuel/air mixing occurs with the air moving fast, and the fuel therefore need only move relatively slowly. This simplifies injector design and allows the use of less tightly toleranced designs which are simpler to manufacture and more reliable. Furthermore achieving the correct gas flow patterns in the swirl chamber is a relatively straightforward task. Direct injection, by contrast, uses slow-moving air and fast-moving fuel; both the design and manufacture of the injectors is more difficult, the optimisation of the in-cylinder gas flow is much more difficult than designing a swirl chamber, and there is much more integration between the design of the injector and that of the engine it is to be used in. It is for this reason that car diesel engines were almost all indirect injection until the ready availability of powerful CFD simulation systems made the adoption of direct injection practical.

Aside from the above advantages, early diesels often employed indirect injection in order to use simple, flat-top pistons, and made the positioning of the early, bulky diesel injectors easier.

Classification of indirect combustion chambers(prechambers)

wirl chamber

It consists of a sperical chamber located in the cylinder head and separated from the engine cylinder by a tangential throat.About 50% of air enters this swirl chamber during compression stroke of the engine producing a swirl.The products after combustion returns through the same throat to the main cylinder at much higher velocity.So more heat loss to walls of the passage takes place.However this loss can be reduced by providing insulation.Such type of chambers finds application in those engines where fuel control and engine stability is more important than fuel economy.

Precombustion chamber

This chamber is located at the cylinder head and is connected to the engine cylinder by small holes. It occupies 40% of the total cylinder volume. During the compression stroke, air from the main cylinder enters the precombustion chamber. At this moment, fuel is injected into the precombustion chamber and combustion begins. Pressure increases and the fuel droplets are forced through the small holes into the main cylinder, resulting in a very good mix of the fuel and air. The bulk of the combustion actually takes place in the main cylinder. This type of combustion chamber has multi-fuel capability because the temperature of the prechamber vaporizes the fuel before the main combustion event occurs.

Air cell chamber

The air cell is a small cylindrical chamber with a hole in one end. It is mounted more or less coaxially with the injector, said axis being parallel to the piston crown, with the injector firing across a small cavity which is open to the cylinder into the hole in the end of the air cell. The air cell is mounted so as to minimise thermal contact with the mass of the head. A pintle injector with a narrow spray pattern is used. At TDC the majority of the charge mass is contained in the cavity and air cell.

When the injector fires the jet of fuel enters the air cell and ignites. This results in a jet of flame shooting back out of the air cell directly into the jet of fuel still issuing from the injector. The heat and turbulence give excellent fuel vaporisation and mixing properties. Also since the majority of the combustion takes place outside the air cell in the cavity, which communicates directly with the cylinder, there is less heat loss involved in transferring the burning charge into the cylinder.

Air cell injection can be considered as a sort of half way stage between fully indirect and fully direct injection, gaining some of the efficiency advantages of direct injection while retaining the simplicity and ease of development of indirect injection.

Advantages of indirect injection combustion chambers

#The injection pressure required is low, therefore making the injector cheaper to produce.
#The injection direction is of less importance
#Indirect injection is much simpler to design and manufacture; much less engine and injector development is required and the injectors themselves are less tightly toleranced and more reliable.

Disadvantages

#Specific fuel consumption is high because of heat loss to large exposed areas and pressure loss due to air motion through the throats.
#Indirect-injection diesel engines are harder to turbocharge than direct-injection engines. Because combustion moves from the pre-combustion chamber to the cylinder through a single throat, one area of the piston is exposed to much greater temperatures than the rest. This uneven heating causes the piston to expand unevenly as combustion temperatures rise. Naturally-aspirated engines experience lower combustion temperatures and this is is not a problem. Turbocharging an engine significantly raises the internal temperatures of the engine. Unless the level of boost from the turbocharger is low, the piston ideally must be cast to a slightly oval shape to compensate for the uneven expansion at high temperatures. As boost pressure rises, this becomes more vital to prevent piston damage or cracking, and the piston's ovoid shape must become more pronounced. At high turbo boost pressures, it becomes easier (and cheaper) to switch to a direct injection system (where temperatures are evenly distributed) than to design and engineer complex pistons for an indirect injection engine.