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Indirect injection
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Indirect injection in an internal combustion engine is fuel injection where fuel is not directly injected into the combustion chamber.
Gasoline engines equipped with indirect injection systems, wherein a fuel injector delivers the fuel at some point before the intake valve, have mostly fallen out of favor to direct injection. However, certain manufacturers such as Volkswagen, Toyota and Ford have developed a 'dual injection' system, combining direct injectors with port (indirect) injectors, combining the benefits of both types of fuel injection. Direct injection allows the fuel to be precisely metered into the combustion chamber under high pressure which can lead to greater power and fuel efficiency. The issue with direct injection is that it typically leads to greater amounts of particulate matter and with the fuel no longer contacting the intake valves, carbon can accumulate on the intake valves over time. Adding indirect injection keeps fuel spraying on the intake valves, reducing or eliminating the carbon accumulation on intake valves and in low load conditions, indirect injection allows for better fuel-air mixing. This system is mainly used in higher cost models due to the added expense and complexity.
Port injection refers to the spraying of the fuel onto the back of the intake valve, which speeds its evaporation.[1]
An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, either a prechamber or swirl chamber, 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.
Gasoline engines
[edit]An advantage of indirect injection gasoline engines versus direct injection gasoline engines is that deposits on intake valves from the crankcase ventilation system are washed by the fuel.[2] Indirect injection engines also tend to produce lower amounts of particulate matter compared to direct injection engines as the fuel and air are more uniformly mixed.
Diesel engines
[edit]Overview
[edit]The purpose of the divided combustion chamber is to speed the combustion process, and to increase power output by increasing engine speed.[3] The addition of a prechamber 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 engine (piston crown, head, valves, injector, prechamber, etc.) design and allows the use of 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.[4] 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.[citation needed]
Classification of indirect combustion chambers
[edit]Swirl chamber
[edit]
Swirl chambers are spherical cavities 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.[5] After combustion, the products return through the same throat to the main cylinder at much higher velocity, so more heat is lost to the walls of the passage. This type of chamber finds application in engines in which fuel control and engine stability are more important than fuel economy. These are also called Ricardo chambers, named after the inventor, Sir Harry Ricardo.[6][7]
Precombustion chamber
[edit]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.[8]
Air cell chamber
[edit]
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 its top dead centre (TDC) the majority of the charge mass is contained in the cavity and air cell.[citation needed]
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.[citation needed]
Air cell chambers are commonly named Lanova air chambers.[9] The Lanova combustion system was developed by the Lanova company, which was founded in 1929 by Franz Lang, Gotthard Wielich and Albert Wielich.[10]
In the US, the Lanova system was used by Mack Trucks. An example is the Mack-Lanova ED diesel engine fitted to the Mack NR truck.
Advantages of indirect injection combustion chambers
[edit]- 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, especially for gasoline engines. 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 differed 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.
- Alternative fuels like bio-diesel and waste vegetable oil are less likely to damage the fuel system in an indirect-injection diesel engine, as high injection pressures are not needed. In direct-injection engines (especially modern engines using high-pressure common rail fuel systems), keeping fuel filters in good condition is more critical as debris can damage the pumps and injectors when waste vegetable oil or waste engine oil are used.
Disadvantages
[edit]- Fuel efficiency with diesel engines is lower than with direct injection as the larger exposed areas tend to dissipate more heat and the air moving through the ports tending to increase pressure drops. However, using higher compression ratios will somewhat negate this inefficiency.
- Glow plugs are needed for a cold engine start on diesel engines; many indirect injection diesel engines cannot start at all in cold weather without glow plugs.
- Because the heat and pressure of combustion is applied to a very small area 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, supercharging, 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, although advancements in manufacturing techniques have allowed manufacturers to largely mitigate the effects of uneven expansion, allowing for indirect injection diesels to use turbocharging.
- Indirect engines are often much noisier than direct injection common-rail engines.
- Starting fluid ("ether") often cannot be used in an indirect injection diesel engine as the glow plugs greatly increase the risk of preignition compared to direct injection diesels.
See also
[edit]References
[edit]- ^ Kerr, Jim. "Direct vs. port injection". The Chronicle Herald. Retrieved 28 June 2016.
- ^ Smith, Scott; Guinther, Gregory (2016-10-17). "Formation of Intake Valve Deposits in Gasoline Direct Injection Engines". SAE International Journal of Fuels and Lubricants. 9 (3): 558–566. doi:10.4271/2016-01-2252. ISSN 1946-3960.
- ^ Stone, Richard. "An introduction to ICE", Palgrace Macmillan, 1999, p. 224
- ^ Two-stroke engine
- ^ Electromechanical Prime Movers: Electric Motors. Macmillan International Higher Education. 18 June 1971. pp. 21–. ISBN 978-1-349-01182-7.[permanent dead link]
- ^ "Sir Harry Ricardo". oldengine.org. Archived from the original on 17 October 2010. Retrieved 8 January 2017.
- ^ Dempsey, P. (1995). Troubleshooting and Repairing Diesel Engines. TAB Books. p. 127. ISBN 9780070163485. Retrieved 8 January 2017.
- ^ Dempsey, Paul (2007). Troubleshooting and Repair of Diesel Engines. McGraw Hill Professional. ISBN 9780071595186. Retrieved 2 December 2017.
- ^ Dempsey, P. (1995). Troubleshooting and Repairing Diesel Engines. TAB Books. p. 128. ISBN 9780070163485. Retrieved 8 January 2017.
- ^ "The Lanova Combustion System". The Commercial Motor. 6 January 1933. Retrieved 11 November 2017.
Indirect injection
View on Grokipediafrom Grokipedia
Indirect injection in an internal combustion engine is fuel injection where fuel is not directly injected into the combustion chamber. In diesel engines, indirect injection (IDI) typically involves spraying fuel into a pre-combustion chamber or swirl chamber connected to the main cylinder combustion chamber via a narrow passage.[1] This arrangement generates high turbulence and swirl motion during compression to enhance air-fuel mixing and atomization, with initial combustion occurring in the auxiliary chamber before the burning mixture expands into the main chamber for complete combustion.[2] The system typically requires a glow plug in the pre-chamber to aid cold starts by heating the air, and it operates at compression ratios of around 20:1 to 23:1.[1] In gasoline engines, indirect injection refers to port fuel injection, where fuel is injected into the intake port upstream of the intake valve.[3]
There are two main variants of indirect injection in diesel engines: the pre-chamber design, where the auxiliary chamber occupies 20% to 40% of the total clearance volume and promotes partial combustion buildup under high pressure, and the swirl chamber design, which uses 50% to 70% of the volume to create a tangential air vortex for improved fuel dispersion.[2] Fuel delivery is achieved through a high-pressure injection pump, lines, and a multi-hole nozzle aimed into the auxiliary chamber, with injection pressures typically ranging from 10 to 20 MPa.[4] These systems were developed to address challenges in early diesel engine designs, such as poor fuel atomization in direct injection setups, and have been applied in both naturally aspirated and turbocharged configurations.[4]
These comparisons are representative under mid-load conditions and highlight indirect injection's efficiency penalties offset by favorable NOx trade-offs in diesels.[38][41]
Fundamentals
Definition and Basic Principles
Indirect injection, also known as indirect fuel injection, is a method of fuel delivery in internal combustion engines where fuel is introduced into a secondary chamber, pre-chamber, or the intake port rather than directly into the main combustion chamber. This approach allows the fuel to mix with air in a controlled manner before combustion, with specifics varying by engine type: in gasoline engines, fuel mixes with air in the intake port during the intake stroke for vaporization and homogeneous mixture formation; in diesel engines, fuel is injected into a pre-chamber during the compression stroke for atomization and mixing with compressed air.[4][1] The basic principles of indirect injection revolve around enhancing the air-fuel mixture quality to improve combustion efficiency. Fuel is atomized into fine droplets by the injector; in gasoline systems, these vaporize and mix with incoming air during intake, forming a primarily homogeneous mixture, while in diesel indirect injection (IDI), droplets mix with hot compressed air in the pre-chamber for ignition without full prior vaporization. This process is influenced by fuel spray patterns, where the injector's nozzle geometry determines the spray angle and penetration depth, ensuring even distribution without impinging on cylinder walls. Ignition timing in indirect injection systems is typically dependent on the engine cycle, with spark or compression ignition occurring after the mixture has been prepared in the secondary location, reducing issues like incomplete combustion. Key components in indirect injection include the fuel injectors, which are electronically or mechanically controlled to meter precise fuel quantities; pre-chambers or auxiliary volumes that facilitate initial mixing in diesel systems; and intake manifolds, particularly for port injection variants in gasoline engines, which direct the fuel-air mixture toward the cylinder. These elements work together to optimize the delivery process under varying load conditions. Understanding indirect injection requires a brief overview of the combustion process in internal combustion engines, governed by thermodynamic cycles such as the Otto cycle for spark-ignition engines and the Diesel cycle for compression-ignition engines. In both, combustion involves the rapid oxidation of fuel in the presence of air, converting chemical energy into thermal energy that drives piston motion; indirect injection supports this by preparing the mixture outside the main chamber to achieve more controlled heat release and reduced emissions compared to unmixed states.Comparison with Direct Injection
Indirect injection systems differ from direct injection primarily in their fuel delivery and air-fuel mixing strategies. In indirect injection, fuel is introduced into an auxiliary pre-chamber (diesel) or the intake port (gasoline), with premixing occurring in the intake for gasoline port injection before entering the cylinder, or mixing and initial combustion in the pre-chamber for diesel IDI before propagation to the main chamber; whereas direct injection sprays fuel directly into the cylinder for in-cylinder atomization and mixing.[4] This leads to distinct injector placements: indirect systems position injectors in the intake manifold (gasoline) or pre-chamber (diesel), while direct injection requires high-pressure injectors integrated into the cylinder head to withstand elevated combustion conditions. Performance characteristics also diverge notably. Indirect injection operates with lower peak cylinder pressures in the main chamber due to the staged combustion process in diesel IDI, which mitigates mechanical stress; in gasoline applications, the homogeneous premixed charge from port injection heightens knocking risk. In contrast, direct injection supports stratified charge operation, where fuel is concentrated near the spark plug for leaner overall mixtures, enhancing throttle response and reducing fuel consumption under part-load scenarios. Efficiency trends favor direct injection, with indirect systems typically achieving thermal efficiencies of approximately 30-35% across gasoline and diesel configurations, compared to 35-40% for direct injection in similar engines; this gap arises from direct injection's reduced pumping losses, improved combustion control, and compatibility with higher compression ratios.[1] The following table highlights additional comparative aspects:| Aspect | Indirect Injection | Direct Injection |
|---|---|---|
| Injection Timing | Primarily during intake (gasoline) or in pre-chamber compression (diesel) | Flexible: intake, compression, or power stroke |
| Mixing Quality | Homogeneous premixing in intake (gasoline) or initial mixing in auxiliary space (diesel) | Superior in-cylinder control, enabling stratified charges |
| Complexity | Lower, with moderate-pressure components | Higher, requiring precise high-pressure fuel systems |
| Cost | More affordable production and simpler maintenance | Elevated due to advanced injectors and calibration needs |
Applications in Gasoline Engines
Operating Mechanism
In indirect injection for gasoline engines, also referred to as port fuel injection (PFI), fuel is sprayed into the intake port or manifold upstream of the intake valve, allowing it to mix with incoming air prior to entering the combustion chamber.[5] This process contrasts with direct injection by promoting a homogeneous air-fuel mixture formation outside the cylinder, which enhances combustion uniformity.[6] Two primary configurations exist: throttle body injection (TBI), utilizing a single injector at the throttle body to distribute fuel to all cylinders via the intake manifold, and multi-point injection (MPI), employing dedicated injectors at each cylinder's intake port for individualized fuel delivery and improved distribution.[7] Upon injection, the liquid fuel atomizes into fine droplets that evaporate in the airstream, producing an evaporative cooling effect that reduces intake charge temperature, thereby increasing air density and engine volumetric efficiency.[8] The air-fuel mixing occurs primarily during the intake stroke, as the throttle-controlled airflow draws the vaporized fuel through the open intake valve into the cylinder, yielding a well-homogenized charge for spark ignition.[6] Injector timing is precisely synchronized with intake valve events, often initiating when the valve is closed to facilitate fuel film formation on port walls, which promotes complete evaporation and prevents liquid fuel carryover into the cylinder.[9] Port fuel injection operates at typical pressures of 3 to 5 bar, generated by an electric fuel pump to ensure adequate atomization without excessive energy demands.[10] This system forms the core of electronic fuel injection (EFI) architectures, which gained widespread adoption in gasoline engines from the 1980s onward, leveraging engine control units to modulate injection based on sensors for throttle position, airflow, and load.[11] In a standard PFI setup, fuel flows from the tank via a low-pressure supply pump to a rail pressurized at 3-5 bar, where electronically actuated solenoid injectors meter and spray it sequentially into each intake port, integrating with the airflow en route to the cylinder for optimal mixing.[6]Historical Development and Modern Use
Indirect injection in gasoline engines, also known as port fuel injection (PFI), evolved from the limitations of carburetor systems prevalent before the 1970s, which struggled to deliver precise fuel-air mixtures under varying conditions. Early mechanical fuel injection systems appeared in the late 1950s, such as Chevrolet's Rochester unit on the 1957 Corvette, but adoption was limited due to complexity and cost. The breakthrough came with electronic fuel injection, exemplified by Bosch's D-Jetronic system introduced in 1967 on the Volkswagen Type 3, which used sensors and electronic controls for more accurate metering. This was followed by the mechanical continuous injection K-Jetronic system in 1973, initially applied to Mercedes-Benz and Porsche vehicles, marking a shift toward reliable multi-point port injection.[12][13][14] The widespread adoption of indirect injection accelerated in the 1980s, driven by stringent U.S. emissions and fuel economy regulations. The Clean Air Act Amendments of 1970 mandated significant reductions in hydrocarbon and carbon monoxide emissions, requiring a 90% cut by 1975 model year, which carburetors could not achieve efficiently. Concurrently, the Corporate Average Fuel Economy (CAFE) standards, enacted in 1975 under the Energy Policy and Conservation Act, raised passenger car efficiency requirements from 18 mpg in 1978 to 27.5 mpg by 1985, compelling automakers to adopt electronic port injection for better atomization and control. By the mid-1980s, multi-point PFI became standard in most new gasoline vehicles, replacing carburetors and enabling compliance with these mandates while improving drivability.[15][16] In modern applications, indirect injection remains prevalent in naturally aspirated gasoline engines, particularly in cost-sensitive markets and smaller vehicles where simplicity and lower system pressures suffice. Dual-injection systems, combining PFI with direct injection (GDI), have emerged since the 2000s to optimize performance across operating conditions; Toyota's D-4S technology, introduced on the 2007 Lexus GS, uses port injection for low-load efficiency and direct injection for high-power needs, later expanding to models like the 2012 Scion FR-S and 2015 Highlander. However, PFI's market share has declined due to GDI's superior fuel efficiency gains—up to 15-20% better under partial loads—driven by ongoing CAFE tightening and global emissions rules, with GDI comprising about 55% of new U.S. gasoline vehicle sales by the early 2020s compared to 40% for traditional PFI. Indirect injection persists in hybrid and entry-level applications, comprising roughly 70% of global gasoline engines when including dual systems, especially in developing regions prioritizing affordability over peak efficiency.[17][18][19]Applications in Diesel Engines
Overview and Historical Context
Indirect injection (IDI) in diesel engines involves injecting fuel into a small pre-chamber connected to the main combustion chamber by a narrow passage, where initial combustion occurs before flames propagate to the primary chamber, enhancing air-fuel mixing and reducing noise compared to direct injection (DI) systems that spray fuel straight into the main chamber.[20] This approach allows for more controlled ignition in high-speed engines, distinguishing it from the original DI concepts by Rudolf Diesel, who patented compression-ignition principles in the 1890s but focused on low-speed, large-scale applications.[21] The commercial development of IDI accelerated in the 1920s and 1930s to enable smoother operation in smaller, faster automotive diesels, with British engineer Sir Harry Ricardo patenting the influential Comet swirl chamber design in 1931, which became widely adopted for its efficiency in pre-chamber mixing.[21] IDI engines typically operate at higher compression ratios of 18:1 to 23:1 to ensure auto-ignition in the pre-chamber, and they often incorporate glow plugs to provide additional heat for reliable cold starts, as the divided chamber design can hinder initial combustion in low temperatures. By the 1970s and 1980s, IDI dominated passenger car diesel applications in Europe, offering quieter performance and easier adaptation to mechanical fuel systems.[21] The peak use of IDI occurred through the 1970s to 1990s, particularly in light-duty vehicles, but stricter emissions regulations, such as the European Union's Euro 3 standards introduced in 2000, drove a transition to DI systems for better fuel efficiency and lower particulate and NOx outputs, with Fiat pioneering automotive DI in 1988 and widespread adoption following in the 1990s via electronic controls.[21] This shift marked the decline of IDI in new production, though legacy systems persisted in some markets into the early 2000s.[22]Swirl Chamber Systems
Swirl chamber systems in indirect injection diesel engines employ a divided combustion chamber design, consisting of a small auxiliary chamber, often hemispherical or bulb-shaped, integrated into the cylinder head. This swirl chamber is connected to the main combustion chamber by a narrow, tangentially oriented throat that facilitates controlled gas flow. Tangential intake ports or inherent chamber geometry generate intense rotational air motion, or swirl, as compressed air enters the chamber during the intake and compression strokes, promoting rapid fuel-air mixing. Fuel is injected directly into the swirl chamber through a dedicated nozzle, typically positioned to align with the swirling air path.[23] In operation, the compression stroke forces air from the main chamber into the swirl chamber, where the tangential throat induces high-velocity swirl, elevating air temperature to ignition levels without requiring excessively high overall compression ratios. Near top dead center, diesel fuel is sprayed into the hot, turbulent environment of the swirl chamber, where it atomizes, mixes with the swirling air, and auto-ignites rapidly. The ensuing combustion generates high-pressure flames and gases that jet through the throat into the main chamber, igniting the remaining air charge and completing the power stroke. This staged combustion process leverages swirl-induced turbulence to enhance mixing and reduce ignition delay compared to direct injection systems.[23] The swirl chamber design was pioneered by engineer Sir Harry Ricardo in the late 1910s and 1920s, culminating in the influential Comet series of combustion chambers that enabled practical high-speed diesel engines for automotive and industrial use. Notable applications include the Mazda 626 passenger car diesel engine in the 1980s, which utilized a Ricardo-inspired swirl chamber for reliable performance in compact vehicles. High swirl ratios, typically ranging from 5 to 10, intensify turbulence within the chamber, optimizing combustion efficiency and flame propagation. A key advantage of swirl chamber systems is their delivery of superior low-speed torque, attributed to the effective initiation of combustion under lean conditions and the resulting high cylinder pressures at partial loads.[24][25]Precombustion Chamber Systems
Precombustion chamber systems in indirect injection diesel engines utilize a compact auxiliary chamber integrated into the cylinder head, typically occupying 20-30% of the total clearance volume.[26] This prechamber contains the fuel injector for direct delivery of diesel fuel, along with a glow plug to facilitate cold-start ignition and a diffuser pin that scatters the fuel spray for improved atomization and mixing with compressed air. The design fosters a rich fuel-air mixture formation within the prechamber, contrasting with the leaner mixture in the main combustion chamber, and features multiple narrow orifices connecting the two spaces to enable controlled flame propagation. Shaped piston crowns in the main chamber further enhance turbulence for efficient burning of the incoming charge.[27] In operation, fuel is injected into the prechamber toward the end of the compression stroke, where high temperatures from air compression—often exceeding 500°C—promote rapid autoignition and partial combustion of the rich mixture. The ensuing pressure buildup expels a high-velocity, torch-like jet of flames and hot gases through the orifices into the main chamber, igniting the stratified lean charge and completing combustion across the cylinder volume. Injection timing integrates with valve events through centrifugal advance mechanisms that adjust delivery earlier at higher engine speeds, ensuring optimal ignition across load conditions while exhaust and intake valves follow standard four-stroke sequencing.[27][28] These systems gained prominence in Mercedes-Benz OM-series engines from the 1950s onward, such as the OM 636 used in early post-war vehicles, offering quieter operation and reliable performance in passenger car applications until the shift to direct injection in the 1990s. The localized high temperatures in the prechamber reduce ignition delay—often by 20-30% relative to direct injection setups—by minimizing the physical and chemical delay periods through enhanced fuel vaporization and initial mixing. However, the intense, stratified burning in the prechamber can produce hotter local spots, contributing to elevated NOx formation in those regions despite overall lower peak temperatures in the main chamber.[27][29]Air Cell Chamber Systems
Air cell chamber systems represent a specific configuration of indirect injection in diesel engines, characterized by a distinct auxiliary air cell separated from the main combustion chamber by a narrow connecting passage. In this design, the air cell is typically machined into the cylinder head or piston, serving as a confined space for initial fuel-air mixing and ignition. Fuel is injected directly into the air cell rather than the main chamber, allowing for higher local compression ratios within the smaller volume. The air cell generally accounts for 5 to 15 percent of the total clearance volume, optimizing the balance between initial combustion intensity and overall engine efficiency.[30] During the compression stroke, as the piston ascends, intake air is forced through the narrow passage into the air cell, where it undergoes further compression and reaches ignition temperatures. Upon fuel injection into the air cell, autoignition occurs due to the elevated temperatures and pressures, initiating combustion in this isolated volume. The ensuing pressure surge propels the ignited mixture back through the passage into the main chamber as a high-velocity jet, enhancing turbulence and promoting rapid flame propagation across the larger space. This staged combustion process reduces peak pressures in the main chamber compared to direct injection, contributing to smoother operation and lower noise levels. Notably, air cell systems exhibit lower heat losses than other indirect injection variants, such as swirl chambers, because the initial combustion is confined to a smaller, adiabatically hotter region with minimal surface area exposure.[25] The geometry of the connecting passage plays a critical role in system performance, influencing the velocity and direction of the gas jet, which in turn affects flame speed and combustion completeness. Passages aligned with the main chamber's swirl motion can accelerate flame propagation and reduce unburned hydrocarbons, while misaligned designs may impede mixing and increase emissions. Developed in the late 1920s, the Lanova air cell system—pioneered by the Lanova company in Germany—exemplifies early adoption, with licenses granted to engine manufacturers in the 1930s for applications in trucks and industrial machinery. Despite these innovations, air cell chambers have become less prevalent in contemporary diesel engines due to their mechanical complexity, including the need for precise machining of the cell and passage to withstand repeated thermal cycling.[31][32]Advantages and Disadvantages
Indirect injection (IDI) systems in diesel engines provide smoother combustion due to the pre-chamber design, which promotes stratified charge formation and reduces peak pressure rise rates, leading to lower noise and vibration levels compared to direct injection (DI) systems.[20] This results in noise reductions of up to 10 dB, making IDI engines preferable for applications where acoustic comfort is prioritized, such as passenger vehicles.[33] Additionally, IDI facilitates easier cold starting, as the smaller pre-chamber retains heat more effectively, aiding ignition in low-temperature conditions without requiring the high pressures needed for DI fuel atomization.[20] Injector requirements are also simpler and less costly, operating at pressures typically around 100-200 bar versus over 2000 bar for modern DI systems, which reduces manufacturing complexity and wear.[34] Despite these benefits, IDI systems exhibit lower thermal efficiency, approximately 2-5% less than DI due to increased heat losses from the larger surface area in the pre-chamber and the energy required to transfer the combustion process to the main chamber. This inefficiency contributes to higher overall emissions of particulate matter (PM) and hydrocarbons (HC), as incomplete combustion in the pre-chamber can lead to unburned fuel residues and poorer oxidation.[35] The added components, such as the pre-chamber and connecting passages, increase engine complexity, weight, and manufacturing costs compared to simpler DI designs.[36] IDI proves advantageous for small-displacement engines in light-duty applications, where quiet operation and reliable starting outweigh efficiency losses, but it underperforms in high-power scenarios due to limited scalability and higher specific fuel consumption under load.[37] Stricter emission standards, such as those introduced in the EU following Euro 4 in 2005, accelerated the phase-out of IDI in passenger cars, as DI systems better accommodated advanced aftertreatment and met reduced PM and HC limits.Performance and Efficiency Considerations
Fuel Efficiency and Emissions
Indirect injection systems in diesel engines typically exhibit higher brake specific fuel consumption (BSFC) compared to direct injection counterparts, with values around 220 g/kWh for indirect diesel injection (IDI) versus approximately 200 g/kWh for direct injection (DI).[38][39] This difference arises from less efficient combustion due to fuel impingement on chamber walls in the pre- or swirl chamber, leading to incomplete burning and heat losses. In gasoline engines, port injection (the indirect method) incurs greater pumping losses than direct injection, as the throttle must remain more closed to maintain air-fuel ratios, increasing the work required to draw in the charge; direct injection mitigates this through in-cylinder fuel evaporation cooling, which allows wider throttle openings.[40] Emissions profiles for indirect injection reveal trade-offs, particularly in diesel applications. IDI systems produce higher carbon monoxide (CO) and hydrocarbon (HC) emissions due to wall wetting in the auxiliary chamber, where fuel contacts cooler surfaces, promoting quenching and incomplete oxidation; these can be 20-50% elevated relative to DI under similar loads. Conversely, IDI yields lower nitrogen oxide (NOx) emissions—often reduced by up to two-thirds compared to DI—owing to lower peak combustion temperatures from the divided chamber design.[41][42] In gasoline port injection, CO and HC tend to be marginally lower than in direct injection without stratification, but particulate matter can increase in the latter without advanced controls. Modern indirect injection engines achieve compliance with stringent standards like Euro 6 through integrated aftertreatment, including exhaust gas recirculation (EGR) for NOx control, which is well-suited to IDI's inherently lower NOx output, and diesel particulate filters (DPF) to address elevated particulates from wall wetting. Dual-injection hybrids, combining port and direct methods in gasoline engines, enhance efficiency by 3-5% on average by optimizing fuel delivery for better atomization and reduced wall impingement, while improving EGR compatibility for emissions management.[43][44][45]| Engine Type | Injection Method | BSFC (g/kWh) | CO/HC Emissions (Relative to DI) | NOx Emissions (Relative to DI) |
|---|---|---|---|---|
| Diesel | Indirect (IDI) | ~220 | Higher (20-50%) | Lower (~66% reduction) |
| Diesel | Direct (DI) | ~200 | Baseline | Baseline |
| Gasoline | Port (Indirect) | ~250 | Lower (~10-20%) | Similar |
| Gasoline | Direct (GDI) | ~240 | Baseline | Similar |