Indirect injection

The term Indirect injection, in an internal combustion engine, refers to 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.

Overview

The purpose of the divided combustion chamber is to speed up the combustion process, in order to increase the power output by increasing engine speed. The addition of a prechamber, however, increases heat loss to the cooling system and thereby lowers engine efficiency. The engine requires glow plugs for starting. In an indirect injection system the air moves fast, mixing the fuel and air. This simplifies injector design and allows the use of smaller engines and less tightly toleranced designs which are simpler to manufacture and more reliable. 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 air flow is much more difficult than designing a prechamber. There is much more integration between the design of the injector and the engine. 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.

Gallery

Cylinder head of a small Kubota indirect injection diesel engine.

• 
  Top of head
• 
  Piston
• 
  Injector holes
• 
  Cylinder head
• 
  Injection Pump
• 
  Head close up
• 
  Combustion chambers
• 
  Glow plugs

Classification of indirect combustion chambers

Swirl chamber

It consists of a spherical chamber located in the cylinder head and separated from the engine cylinder by a tangential throat. About 50% of the air enters the swirl chamber during the compression stroke of the engine, producing a swirl. After combustion, the products return through the same throat to the main cylinder at much higher velocity. So more heat loss to walls of the passage takes place. This type of chamber finds application in engines in which fuel control and engine stability are more important than fuel economy. These are Ricardo chambers.

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 compromise between indirect and direct injection, gaining some of the efficiency advantages of direct injection while retaining the simplicity and ease of development of indirect injection.

Air cell chambers are commonly named Lanova air chambers.

Advantages of indirect injection combustion chambers

• Smaller diesels can be produced.
• The injection pressure required is low, so the injector is cheaper to produce.
• The injection direction is of less importance.
• Indirect injection is much simpler to design and manufacture; less injector development is required and the injection pressures are low (1500 psi/100 bar versus 5000 psi/345 bar and higher for direct injection)
• The lower stresses that indirect injection imposes on internal components mean that it is possible to produce petrol and indirect injection diesel versions of the same basic engine. At best such types differ only in the cylinder head and the need to fit a distributor and spark plugs in the petrol version whilst fitting an injection pump and injectors to the diesel. Examples include the BMC A-Series and B-Series engines and the Land Rover 2.25/2.5-litre 4-cylinder types. Such designs allow petrol and diesel versions of the same vehicle to be built with minimal design changes between them.
• Higher engine speeds can be reached, since burning continues in the prechamber.

Disadvantages

• Specific fuel consumption is higher than with direct injection because of heat loss due to large exposed areas and pressure loss due to air motion through the throats. This is somewhat offset due to indirect injection having a much higher compression ratio and typically having no emissions equipment.
• Glowplugs are needed for a cold engine start.
• Because the heat and pressure of combustion is applied to one specific point on the piston as it exits the precombustion chamber or swirl chamber, such engines are less suited to high specific power outputs (such as turbocharging or tuning) than direct injection diesels. The increased temperature and pressure on one part of the piston crown causes uneven expansion which can lead to cracking, distortion or other damage due to improper use; use of " starting fluid" (ether) is not recommended in glow plug, indirect injection systems, because explosive knock can occur, causing engine damage.

See also

• Harry Ricardo
• Prosper L'Orange
• Premixed flame