Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.
All compression-ignition engines (e.g., diesel engines), and many spark-ignition engines (i.e., petrol (gasoline) engines, such as Otto or Wankel), use fuel injection of one kind or another. Mass-produced diesel engines for passenger cars (such as the Mercedes-Benz OM 138) became available in the late 1930s and early 1940s, being the first fuel-injected engines for passenger car use. In passenger car petrol engines, fuel injection was introduced in the early 1950s and gradually gained prevalence until it had largely replaced carburettors by the early 1990s. The primary difference between carburetion and fuel injection is that fuel injection atomizes the fuel through a small nozzle under high pressure. In contrast, carburetion relies on suction created by intake air accelerated through a Venturi tube to draw fuel into the airstream.
The term "fuel injection" is vague and encompasses various distinct systems with fundamentally different functional principles. The only commonality among fuel injection systems is the absence of carburetion.
There are two main functional principles of mixture formation systems for internal combustion engines: "internal" and "external." A fuel injection system that uses external mixture formation is called a manifold injection system. There exist two types of manifold injection systems: multi-point (or "port") and single-point (or "throttle body") injection.
Internal mixture formation systems can be separated into several varieties of "direct" and "indirect" injection; the most common is common rail injection, a form of direct injection. The term "electronic fuel injection" refers to any fuel injection system controlled by an engine control unit.
System functions
Pressurising fuel Fuel injection sprays pressurised fuel into the engine. Therefore a device to pressurise the fuel is needed, such as a fuel pump.
Metering fuel The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to deliver it.
Several early mechanical injection systems used relatively sophisticated helix-controlled injection pump(s) that both metered fuel and created injection pressure. Since the 1980s, electronic systems have been used to control fuel metering. More recent systems use an electronic engine control unit, which meters the fuel and controls the ignition timing and various other engine functions.
Injecting fuel The fuel injector is essentially a spray nozzle that delivers fuel to the engine during the final stage of fuel delivery. The injector is located in the combustion chamber, inlet manifold, orless commonlythe throttle body.
Fuel injectors that also control the metering, are called "injection valves", while injectors that perform all three functions are called "unit injectors".
Direct injection systems Direct injection means that the fuel is injected into the main combustion chamber of each cylinder. As air and fuel are mixed only inside the combustion chamber, air alone is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual).
Fuel is injected directly into the combustion chamber either with a blast of air or hydraulically; the former was rendered obsolete in automotive engines in the early 20th century by the invention of the precombustion chamber.
Typically, hydraulic direct injection systems spray fuel into the air inside the cylinder, or combustion chamber. Direct injection can be achieved with a conventional helix-controlled injection pump, unit injectors, or a sophisticated common-rail injection system. The last is the most common system in modern automotive engines.
Direct injection for petrol engines
During the 20th century, most petrol engines used either a carburettor or indirect fuel injection. Use of direct injection in petrol engines has become increasingly common in the 21st century.
Common-rail injection systems
In a common-rail system, fuel from the fuel tank is supplied to a common header (called the accumulator), and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator has a high-pressure relief valve to maintain pressure and return the excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve.Helmut Tschöke, Klaus Mollenhauer, Rudolf Maier (ed.): Handbuch Dieselmotoren, 8th edition, Springer, Wiesbaden 2018, , p. 289 Third-generation common-rail diesels use piezoelectric injectors for increased precision, with fuel pressures up to .Helmut Tschöke, Klaus Mollenhauer, Rudolf Maier (ed.): Handbuch Dieselmotoren, 8th edition, Springer, Wiesbaden 2018, , p. 1000
The types of common-rail systems include air-guided injectionRichard van Basshuysen (ed.): Ottomotor mit Direkteinspritzung und Direkteinblasung: Ottokraftstoffe, Erdgas, Methan, Wasserstoff, 4th edition, Springer, Wiesbaden 2017, , p. 62 and spray-guided injection.
Unit injector systems
Used by diesel engines, these systems include: Pumpe-DüseHelmut Tschöke, Klaus Mollenhauer, Rudolf Maier (ed.): Handbuch Dieselmotoren, 8th edition, Springer, Wiesbaden 2018, , p. 295 Pump-rail-nozzle system
Helix-controlled pump systems This injection method was previously used in many diesel engines. Types of systems include: Lanova direct injectionHelmut Hütten: Motoren. Technik, Praxis, Geschichte. Motorbuchverlag, Stuttgart 1982, Afterchamber injection G-System (sphere combustion chamber)Hellmut Droscha (ed.): Leistung und Weg – Zur Geschichte des MAN-Nutzfahrzeugbaus, Springer, Berlin/Heidelberg 1991, . p. 429 Gardner system (hemisphere combustion chamber) Saurer system (torus combustion chamber) Flat piston (combustion chamber between piston and head)
Air-blast injection systems
Other systems The M-System, used in some diesel engines from the 1960s to the 1980s, sprayed the fuel onto the walls of the combustion chamber,Hellmut Droscha (ed.): Leistung und Weg – Zur Geschichte des MAN-Nutzfahrzeugbaus, Springer, Berlin/Heidelberg 1991, . p. 433 as opposed to most other direct-injection systems which spray the fuel into the middle of the chamber.
Indirect injection systems Manifold injection
Manifold injection systems are common in petrol-fuelled engines such as Otto and Wankel engines. In a manifold injection system, air and fuel are mixed outside the combustion chamber, then sucked into the engine. The main types of manifold injection systems are "multi-point" and "single-point" injection.
These systems use either a "continuous" or an "intermittent" injection design. In a continuous injection system, fuel flows at all times from the fuel injectors, but at a variable flow rate. The most common automotive continuous-injection system is the multi-point Bosch K-Jetronic system, introduced in 1974 and used by various car manufacturers until the mid-1990s. Intermittent injection systems can be sequential, in which injection is timed to coincide with each cylinder's intake stroke; batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke; simultaneous, in which fuel is injected at the same time to all the cylinders; or cylinder-individual, in which the engine control unit can adjust the injection for each cylinder individually.Konrad Reif (ed.): Ottomotor-Management, 4th edition, Springer, Wiesbaden 2014, , p. 107
Multi-point injection
Multi-point injection (also called 'port injection') injects fuel into the intake ports just upstream of each cylinder's intake valve, rather than at a central point within an intake manifold. Typically, multi-point injected systems use multiple fuel injectors, but some systems, such as GM's central port injection system, use tubes with poppet valves fed by a central injector instead of multiple injectors.1997 Chevrolet Truck Service Manual, page 6A-24, drawing, item (3) Central Sequential Muliport injector.
Single-point injection
Single-point injection (also called 'throttle-body injection') uses one injector in a throttle body mounted similarly to a carburettor on an intake manifold. As in a carburetted induction system, the fuel is mixed with the air before entering the intake manifold.Kurt Lohner, Herbert Müller (auth): Gemischbildung und Verbrennung im Ottomotor, in Hans List (ed.): Die Verbrennungskraftmaschine, Band 6, Springer, Wien 1967, , p. 64 Single-point injection was a relatively low-cost way for automakers to reduce exhaust emissions to comply with tightening regulations while providing better "driveability" (easy starting, smooth running, no engine stuttering) than could be obtained with a carburettor. Many of the carburettor's supporting components—such as the air filter, intake manifold, and fuel line routing—could be used with few or no changes. This postponed the redesign and tooling costs of these components. Single-point injection was used extensively on American-made passenger cars and light trucks during 1980–1995, and in some European cars in the early and mid-1990s. In the US, the G10 engine in the 2000 Chevrolet Metro became the last engine available on an American-sold vehicle to use throttle body injection.
Diesel engines In indirect-injected diesel engines (as well as Akroyd engines), there are two combustion chambers: the main combustion chamber, and a pre-chamber (also called an ante-chamber) that is connected to the main one. The fuel is injected only into the pre-chamber (where it begins to combust), and not directly into the main combustion chamber. Therefore, this principle is called indirect injection. There exist several slightly different indirect injection systems that have similar characteristics.Olaf von Fersen (ed.): Ein Jahrhundert Automobiltechnik. Personenwagen, VDI-Verlag, Düsseldorf 1986, . p. 273
Types of indirect injection used by diesel engines include: Precombustion-chamber injection Air-cell chamber injection
Hot-bulb injection
History
1870s–1930s: early systems
In 1872, George Bailey Brayton obtained a patent on an internal combustion engine that used a pneumatic fuel injection system, also invented by Brayton: air-blast injection.Sass In 1894, Rudolf Diesel copied Brayton's air-blast injection system for the diesel engine, but also improved it.Sass He increased the air blast pressure from to .Sass In the meantime, the first manifold injection system was designed by Johannes Spiel in 1884, while working at Hallesche Maschinenfabrik in Germany.Richard van Basshuysen (ed.): Ottomotor mit Direkteinspritzung und Direkteinblasung: Ottokraftstoffe, Erdgas, Methan, Wasserstoff, 4th edition, Springer, Wiesbaden 2017, , p. 6
In 1891, the British Herbert-Akroyd oil engine became the first engine to use a pressurised fuel injection system. This design, called a hot-bulb engine used a 'jerk pump' to dispense fuel oil at high pressure to an injector. Another development in early diesel engines was the pre-combustion chamber, which was invented in 1919 by Prosper l'Orange to avoid the drawbacks of air-blast injection systems. The pre-combustion chamber made it feasible to produce engines in size suitable for automobiles and MAN Truck & Bus presented the first direct-injected diesel engine for trucks in 1924.von Fersen (ed.), p. 130 Higher pressure diesel injection pumps were introduced by Bosch in 1927.