Direct fuel injection

Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an 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 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, while a carburettor 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 comprises various distinct systems with fundamentally different functional principles. Typically, the only thing all fuel injection systems have in common is a lack of carburetion. There are two main functional principles of mixture formation systems for internal combustion engines: ''internal'' mixture formation, and ''external'' mixture formation. 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 injection (or port injection) and single-point injection (or throttle body injection). Internal mixture formation systems can be separated into direct and indirect injection systems. There exist several different varieties of both direct and indirect injection systems; the most common internal mixture formation fuel injection system is the common-rail injection system, a direct injection system. The term "electronic fuel injection" refers to any fuel injection system controlled by an engine control unit.

Fundamental considerations

An ideal fuel injection system can precisely provide exactly the right amount of fuel under all engine operating conditions. This typically means precise air–fuel ratio (lambda) control, which allows, for instance: easy engine operation even at low engine temperatures (such as a cold start), good adaptation to a wide range of altitudes, and ambient temperatures, precisely governed engine speeds (including at idle and redline), good fuel efficiency, and the lowest achievable exhaust emissions (which allows emissions control devices, such as a three-way catalytic converter, to function properly).

In practice, an ideal fuel injection system does not exist, but there is a huge variety of different fuel injection systems with certain advantages and disadvantages. Most of these systems were rendered obsolete by the common-rail direct injection system that is, as of 2020, used in many passenger cars. Common-rail injection allows for petrol direct injection, and is even better suited for diesel direct injection. However, common-rail injection is a relatively complex system, which is why in some passenger cars that do not use diesel engines, a multi-point manifold injection system is used instead.

When designing a fuel injection system, a variety of factors has to be taken into consideration, including:
*System cost
*Engine performance and vehicle driveability (ease of starting, smooth running, etc.)
*Exhaust emissions
*Diagnostic provisions and ease of service
* Fuel efficiency
*Reliability
*Ability to run on various fuels

System overview

The fundamental functions of a fuel injection system are described in the following sections. In some systems, a single component performs multiple functions.

Pressurising fuel

Fuel injection operated by spraying pressurised fuel into the engine. Therefore a device to pressurize the fuel (such as a fuel pump) is needed.

Metering of fuel

The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount.

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 the metering of fuel. More recent systems use an electronic engine control unit which meters the fuel, controls the ignition timing and controls various other engine functions.

Injecting of fuel

The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The injector is located in the combustion chamber, inlet manifold or - less commonly - the throttle body.

Fuel injectors which also control the metering are called "injection valves", while injectors which 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. The air and fuel are mixed only inside the combustion chamber. Therefore, only air is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual).

This can be done either with a blast of airRüdiger Teichmann, Günter P. Merker (publisher) or hydraulically, with the latter method being more common in automotive engines. 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 latter is the most common system in modern automotive engines.

Direct injection for petrol engines

During the 20th century, most petrol (gasoline) engines used either a carburetor 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 chamber. 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. Third-generation common rail diesels use piezoelectric injectors for increased precision, with fuel pressures up to .

The types of common-rail systems include ''air-guided injection''Richard 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 injection
* 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 the Otto engine and the Wankel engine. In a manifold injection system, air and fuel are mixed outside the combustion chamber so that a mixture of air and fuel is sucked into the engine. The main types of manifold injections systems are ''multi-point injection'' and ''single-point injection''.

These systems use either a ''continuous injection'' 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 Bosch K-Jetronic system, introduced in 1974 and used until the mid-1990s by various car manufacturers. 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.

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.

Diesel engines

In indirect-injected engines used by 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 and development

1870s – 1920s: 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. In 1894, Rudolf Diesel copied Brayton's air-blast injection system for the diesel engine, but also improved it. He increased the air blast pressure from to . In the meantime, the first manifold injection system was designed by Johannes Spiel in 1884, while working at ''Hallesche Maschinenfabrik'' in Germany.

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.

In 1898, German company Deutz AG started producing four-stroke petrol stationary engines with manifold injection. The 1906 Antoinette 8V aircraft engine (the world's first V8 engine) was another early four-stroke engine that used manifold injection. The first petrol engine with direct injection was a two-stroke aircraft engine designed by Otto Mader in 1916. Another early spark-ignition engine to use direct injection was the 1925 Hesselman engine, designed by Swedish engineer Jonas Hesselman. This engine could run on a variety of fuels (such as oil, kerosene, petrol or diesel oil) and used a stratified charge principle whereby fuel is injected towards the end of the compression stroke, then ignited with a spark plug.

1930s – 1950s: automotive and military aircraft engines

The Cummins ''Model H'' diesel truck engine was introduced in America in 1933. In 1936, the Mercedes-Benz OM 138 diesel engine (using a precombustion chamber) became one of the first fuel-injected engines used in a mass-production passenger car.Olaf von Fersen (ed.): ''Ein Jahrhundert Automobiltechnik. Personenwagen'', VDI-Verlag, Düsseldorf 1986, . p. 274

During World War II, several aircraft piston engines used direct injection systems, such as the European Junkers Jumo 210, Daimler-Benz DB 601, BMW 801, and the Shvetsov ASh-82FN (M-82FN). The German direct-injection systems were based on diesel injection systems used by Bosch, Deckel, Junkers and l'Orange. By around 1943, the Rolls-Royce Merlin and Wright R-3350 had switched from traditional carburettors to single-point fuel-injection systems (called "pressure carburettors" at the time). From 1940, the Mitsubishi Kinsei 60 series engine used a direct-injection system, along with the related Mitsubishi Kasei engine from 1941. In 1943, a low-pressure fuel injection system was added to the Nakajima Homare Model 23 radial engine.

The first mass-produced petrol direct-injection system was a mechanical injection system developed by Bosch and initially usd in small two-stroke petrol engines. Introduced in the 1950 Goliath GP700 small saloon, it was also added to the Gutbrod Superior engine in 1952. This was essentially a specially lubricated high-pressure diesel direct-injection pump of the type that is governed by the vacuum behind an intake throttle valve. A Bosch mechanical direct-injection system was also used in the straight-eight used in the 1954 Mercedes-Benz W196 Formula One racing car. The first four-stroke direct-injection petrol engine for a passenger car was released the following year, in the Mercedes-Benz 300SL sports car. Later, more mainstream applications of fuel injection favored the less-expensive manifold injection design.

1950s – 1970s: manifold injection for petrol engines

Throughout the 1950s, several manufacturers introduced their manifold injection systems for Otto engines, including General Motors' Rochester Products Division, Bosch, and Lucas Industries. During the 1960s, additional manifold injection systems such as the Hilborn, Kugelfischer, and SPICA systems were introduced.

The first commercial electronically controlled manifold injection system was the Electrojector developed by Bendix and was offered by American Motors Corporation (AMC) in 1957. Initial problems with the Electrojector meant only pre-production cars had it installed so very few cars were sold and none were made available to the public. The EFI system in the Rambler worked well in warm weather, but was difficult to start in cooler temperatures.

Chrysler offered Electrojector on the 1958 Chrysler 300D, DeSoto Adventurer, Dodge D-500, and Plymouth Fury, arguably the first series-production cars equipped with an EFI system. The Electrojector patents were subsequently sold to Bosch, who developed the Electrojector into the Bosch D-Jetronic.
The ''D'' in D-Jetronic stands for ''Druckfühlergesteuert'', German for "pressure-sensor controlled"). The D-Jetronic was first used on the VW 1600TL/E in 1967. This was a speed/density system, using engine speed and intake manifold air density to calculate the "air mass" flow rate and thus fuel requirements.

Bosch superseded the D-Jetronic system with the '' K-Jetronic'' and '' L-Jetronic'' multi-point injection systems for 1973, though some cars (such as the Volvo 164) continued using D-Jetronic for the following several years. The L-Jetronic uses a mechanical airflow meter (L for ''Luft'', German for "air") that produces a signal that is proportional to volume flow rate. This approach required additional sensors to measure the atmospheric pressure and temperature, to calculate mass flow rate with an analogue ECU. L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later. The K-Jetronic (K for ''Kontinuierlich'', German for "continuous") is a fully mechanical system that injects the fuel in a continuous spray with a variable flow rate. It does not have an ECU, instead, it relies on the intake manifold vacuum to actuate its fuel distributor's plunger.Olaf von Fersen (ed.): ''Ein Jahrhundert Automobiltechnik. Personenwagen'', VDI-Verlag, Düsseldorf 1986, . p. 256 The first car equipped with this system was the American 1973 Porsche 911T (F-series).Olaf von Fersen (ed.): ''Ein Jahrhundert Automobiltechnik. Personenwagen'', VDI-Verlag, Düsseldorf 1986, . p. 260

1980s – present: digital systems and gasoline direct injection

The first digital engine management system ( engine control unit) was the Bosch Motronic introduced in 1979. In 1980, Motorola (now NXP Semiconductors) introduced their digital ECU EEC-III. The EEC-III is a single-point injection system.Olaf von Fersen (ed.): ''Ein Jahrhundert Automobiltechnik. Personenwagen'', VDI-Verlag, Düsseldorf 1986, . p. 262

Manifold injection was phased in through the latter 1970s and 80s at an accelerating rate, with the German, French, and U.S. markets leading and the UK and Commonwealth markets lagging somewhat. Since the early 1990s, almost all petrol passenger cars sold in first world markets are equipped with electronic manifold injection. The carburettor remains in use in developing countries where vehicle emissions are unregulated and diagnostic and repair infrastructure is sparse. Fuel injection systems are gradually replacing carburettors in these nations too as they adopt emission regulations conceptually similar to those in force in Europe, Japan, Australia, and North America.

In 1995, Mitsubishi presented the first common-rail petrol direct injection system for passenger cars. It was introduced in the 1997 Mitsubishi 6G74 V6 engine. Subsequently, common-rail direct injection was also introduced in passenger car diesel engines, with the Fiat 1.9 JTD being the first mass-market engine. In the early 2000s, several car manufacturers attempted to use stratified charge concepts in their direct injection petrol engines to reduce fuel consumption. However, the fuel savings proved to be almost unnoticeable and disproportionate to the increased complexity of the exhaust gas treatment systems. Therefore, almost all car manufacturers have switched to a conventional homogeneous mixture in their direct-injected petrol engines since the mid-2010s. In the early 2020s, some car manufacturers used manifold injection, especially in economy cars, but also in some high-performance cars. Ever since 1997, car manufacturers have been using common-rail direct injection for their diesel engines. Only Volkswagen used the Pumpe-Düse system throughout the early 2000s, but they have also been using common-rail direct injection since 2010.

Notes

External links

History of the D Jetronic system

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Engine components
Fuel injection systems