Gasoline direct injection (GDI) (also known as petrol direct injection, direct petrol injection, spark-ignited direct injection (SIDI) and fuel-stratified injection (FSI)), is a form of fuel injection employed in modern two-stroke and four-stroke gasoline engines. The gasoline is highly pressurized, and injected via a common rail fuel line directly into the combustion chamber of each cylinder, as opposed to conventional multipoint fuel injection that injects fuel into the intake tract or cylinder port. Directly injecting fuel into the combustion chamber requires high-pressure injection, whereas low pressure is used injecting into the intake tract or cylinder port.
Theory of operation
The major advantages of a GDI engine are increased fuel efficiency and high power output. Emissions levels can also be more accurately controlled with the GDI system. GDI engine operates into two modes 1) overall lean equivalence ratio composition during low load and low speed operation. 2) Homogeneous stoichiometric mode at higher loads and at all loads and higher speed. At medium load region charge is lean or stoichiometric. The combustion systems are classified into air guided, wall guided and spray guided system.
The engine management system continually chooses among three combustion modes: ultra lean burn, stoichiometric, and full power output. Each mode is characterized by the air-fuel ratio. The stoichiometric air-fuel ratio for gasoline is 14.7:1 by weight (mass), but ultra lean mode can involve ratios as high as 65:1 (or even higher in some engines, for very limited periods). These mixtures are much leaner than in a conventional engine and reduce fuel consumption considerably.
- Ultra lean burn or stratified charge mode is used for light-load running conditions, at constant or reducing road speeds, where no acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke. The combustion takes place in a cavity on the piston's surface which has a toroidal or an ovoidal shape, and is placed either in the center (for central injector), or displaced to one side of the piston that is closer to the injector. The cavity creates the swirl effect so that the small amount of air-fuel mixture is optimally placed near the spark plug. This stratified charge is surrounded mostly by air and residual gases, which keeps the fuel and the flame away from the cylinder walls. Decreased combustion temperature allows for lowest emissions and heat losses and increases air quantity by reducing dilation, which delivers additional power. This technique enables the use of ultra-lean mixtures that would be impossible with carburetors or conventional fuel injection.
- Stoichiometric mode is used for moderate load conditions. Fuel is injected during the intake stroke, creating a homogeneous fuel-air mixture in the cylinder. From the stoichiometric ratio, an optimum burn results in a clean exhaust emission, further cleaned by the catalytic converter.
- Full power mode is used for rapid acceleration and heavy loads (as when climbing a hill). The air-fuel mixture is homogeneous and the ratio is slightly richer than stoichiometric, which helps prevent pinging. The fuel is injected during the intake stroke.
It is also possible to inject fuel more than once during a single cycle. After the first fuel charge has been ignited, it is possible to add fuel as the piston descends. The benefits are more power and economy, However, certain octane fuels have caused exhaust valve erosion.
Direct injection may also be accompanied by other engine technologies such as turbocharging or supercharging, variable valve timing (VVT) or continuous variable cam phasing, and tuned/multi path or variable length intake manifolding (VLIM, or VIM). Water injection or (more commonly) exhaust gas recirculation (EGR) may help reduce the high nitrogen oxides (NOx) emissions that can result from burning ultra lean mixtures; modern turbocharged engines use continuous cam phasing in place of EGR.
Tuning up an early generation FSI power plant to generate higher power is difficult, since the only time it is possible to inject fuel is during the induction phase. Conventional injection engines can inject throughout the 4-stroke sequence, as the injector squirts onto the back of a closed valve. A direct injection engine, where the injector injects directly into the cylinder, is limited to the intake stroke of the piston. As the RPM increases, the time available to inject fuel decreases. Newer FSI systems that have sufficient fuel pressure to inject even late in compression phase do not suffer to the same extent. However, they do not inject during the exhaust cycle because doing so would waste fuel. Hence, all other factors being equal, an FSI engine needs higher-capacity injectors to achieve the same power as a conventional engine. Some engines overcome this limitation as well as contamination issues by using both direct injection and multiport fuel injection, including Toyota 2GR-FSE V6 and Volkswagen Group TSI Engines.
The first Otto cycle engine direct injection system was designed by German engineer Otto Mader. It was used for a Junkers airplane engine in 1916. Initially, Junkers planned developing an aviation Diesel engine, because Diesel engines were deemed more efficient and less prone to catching fire than Otto cycle engines. Due to the German ministry of war demanding aircraft engines running on either benzene or petrol, Junkers modified their design to use the Otto cycle rather than the Diesel cycle. Being a two-stroke engine, the design had crankcase scavenging, which would result in engine misfire destroying the engine. Therefore, Mader developed a direct injection system to overcome this problem.
The first post-World War I example of direct gasoline injection was on the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines used the ultra lean burn principle and injected the fuel in the end of the compression stroke and then ignited it with a spark plug, it was often started on gasoline and then switched over to run on diesel or kerosene. The Hesselman engine was a low compression design constructed to run on heavy fuel oils.
Direct gasoline injection was applied during the Second World War to almost all higher-output production aircraft powerplants made in Germany (the widely used BMW 801 radial, and the popular inverted inline V12 Daimler-Benz DB 601, DB 603 and DB 605, along with the similar Junkers Jumo 210G, Jumo 211 and Jumo 213, starting as early as 1937 for both the Jumo 210G and DB 601), the Soviet Union's (Shvetsov ASh-82FN radial, 1943, Chemical Automatics Design Bureau - KB Khimavtomatika) and the US (Wright R-3350 Duplex Cyclone radial, 1944).
The first automotive direct injection system used to run on gasoline was developed under Hans Scherenberg’s leadership, and was introduced by Goliath and Gutbrod in 1952 to power some of their two-stroke cars. This system made by Bosch was basically a high-pressure diesel direct-injection pump with an intake throttle valve set up. (Diesels only change the amount of fuel injected to vary output; there is no throttle.) It used a normal gasoline fuel pump, to provide fuel to a mechanically driven injection pump, which had separate plungers per injector to deliver a very high injection pressure directly into the combustion chamber. The two-stroke vehicles showed very good performance and up to 30% less fuel consumption over the carburetor version, primarily under low engine loads. The cars enjoyed an extra benefit as the injection system also metered lubricant into the engine from a dedicated oil tank, obviating the need for owners to mix their own two-stroke fuel blend. A portion of the oil was combined with fuel in the injection pump to lubricate the cylinders and piston rings, the rest was ported to the air intake to lubricate the crankcase. But the cars were expensive and difficult to start when the engine was warm due to vapor locks. Also, very few people knew about direct injection, and the injection pumps needed frequent adjustment. Branded repair shops and Bosch services became overloaded, and many cars were converted to carburetor. These two-stroke engines were soon superseded by four-strokes.
The 1955 Mercedes-Benz 300SL is the first passenger car with a direct injected four-stroke Otto cycle engine. The Bosch fuel injectors were placed into the bores on the cylinder wall used by the spark plugs in other Mercedes-Benz six-cylinder engines (the spark plugs were relocated to the cylinder head). Later, more mainstream applications of fuel injection favored the less-expensive indirect injection methods.
Research was conducted in the early 1970s with the backing of American Motors Corporation (AMC) to develop a Straticharge Continuous Fuel-Injection (SCFI) system. The conventional spark ignited internal combustion AMC straight-6 engine was modified with a redesigned cylinder head. The system incorporated a mechanical device that automatically responded to the engine's airflow and loading conditions with two separate fuel-control pressures supplied to two sets of continuous-flow injectors. Flexibility was designed into the SCFI system for trimming it to a particular engine. Prototype "straticharge" engine road testing was performed using a 1973 AMC Hornet, but the mechanical fuel controls had teething problems.
The Ford Motor Company developed a stratified-charge engine in the late 1970s called "PROCO" (programmed combustion) using a unique high-pressure pump and direct injectors. At least one hundred and fifteen (115) Crown Victoria cars were built at Ford's Atlanta Assembly in Hapeville, Georgia using a PROCO V8 engine. The project was canceled for several reasons: electronic controls, a key element, were in their infancy; pump and injector costs were extremely high; and lean combustion produced nitrogen oxides in excess of near future United States Environmental Protection Agency (EPA) limits. Also, the PROCO system was being launched in the late 1970s, a time of the second "gas crisis" in the US, which drove fuel costs higher. PROCO had been initially developed for Ford's 460 Cubic inch V8 engine line, later applied to the 351, and eventually the 302. Because the extreme fuel cost spike, Ford was unsure of the future market for V8 engines, and chose not to commit to such an expensive technology in unstable times.
In 1996 gasoline direct injection reappeared in the automotive market. Mitsubishi was the first with a GDI engine in the Japanese market with its Galant/Legnum's 4G93 1.8 L inline-four. It was subsequently brought to Europe in 1997 in the Carisma. It also developed the first six-cylinder GDI powerplant, the 6G74 3.5 L V6, in 1997. Mitsubishi applied this technology widely, producing over one million GDI engines in four families by 2001. Although in use for many years, on September 11, 2001 MMC claimed a trademark for the acronym 'GDI' (with an uppercase final "I").
In 1998, Toyota's D4 direct injection system first appeared on various Japanese market vehicles equipped with the SZ and NZ engines. Toyota later introduced its D4 system to European markets with the 1AZ-FSE engine found in the 2000 MY onwards Avensis. and US markets in 2005 with the 3GR-FSE engine found in the Lexus GS 300. Toyota's 2GR-FSE V6 first found in the Lexus IS 350 uses a more advanced direct injection system, which combines both direct and indirect injection using two fuel injectors per cylinder, a traditional port fuel injector (low pressure) and a direct fuel injector (high-pressure) in a system known as D4-S. In 2015 Toyota added a self-cleaning cycle idle for the direct fuel injection, the system only operates during idle with a maximum 10-minute cleaning cycle.
In 1999, Renault introduced the 2.0 IDE (Injection Directe Essence), first on the Megane. Rather than following the lean burn approach, Renault's design uses high ratios of exhaust gas recirculation to improve economy at low engine loads, with direct injection allowing the fuel to be concentrated around the spark. Later gasoline direct injection engines have been tuned and marketed for their high performance as well as increased fuel efficiency. PSA Peugeot Citroën, Hyundai, and Volvo entered into a development agreements and licensed Mitsubishi's GDI technology in 1999. The Mitsubishi engines were also produced in the NedCar factory and used in the 1.8 L Carisma and the GDI-powered Volvo S40/V40 models.
In 2000, the Volkswagen Group introduced its gasoline direct injection engine in the Volkswagen Lupo, a 1.4 L inline-four unit, under the product name "Fuel Stratified Injection" (FSI) and "Turbo Fuel Stratified Injection" (TFSI). The technology was adapted from Audi's Le Mans prototype race car R8. Volkswagen Group marques use direct injection in its turbocharged 2.0 L TFSI and naturally aspirated 2.0 L FSI four-cylinder engines. Later, a 1.6 L inline-four unit was introduced in the MY 2002 Volkswagen Golf Mk4/Jetta/Bora, a 1.4L in the MY 2002 Volkswagen Polo Mk4 and a 2.0L in the model year 2003 Audi A4. PSA Peugeot Citroën introduced its first GDi (HPi) engine in 2000 in the Citroën C5 and Peugeot 406. It was a 2.0-liter 16-valve EW10 D unit with 140 hp (104 kW), the system was licensed from Mitsubishi.
In 2003, Ford debuted a 1.8 L Duratec SCi naturally aspirated engine for the Mondeo. Ford introduced its first European Ford engine to use direct injection technology in 2001, badged SCi (Smart Charge injection) for Direct-Injection-Spark-Ignition (DISI). The range will include some turbocharged derivatives, including the 1.0 L, three-cylinder turbocharged unit showcased at the 2002 Geneva Show.
In 2003, BMW introduced a low-pressure gasoline direct injection N73 V12. This initial BMW setup could not enter lean-burn mode, but the company introduced its second-generation High Precision Injection (HPI) system on the new turbocharged N54 straight-6 in 2006, which used high-pressure injectors. This system surpasses many others with a wider envelope of lean-burn time, increasing overall efficiency. PSA is cooperating with BMW on a new line of engines that made its first appearance in the 2007 MINI Cooper S. Honda released their own direct injection system on the Stream sold in Japan. Honda's fuel injector is placed directly atop the cylinder at a 90-degree angle rather than a slanted angle.
In 2003, General Motors released a 155 hp (116 kW) version of the 2.2 L Ecotec for the Opel/Vauxhall Vectra and Signum. Several direct injected versions of the Ecotec engine have been introduced, using the SIDI (Spark Ignition Direct Injection) moniker: in 2006, a 2.0 L turbocharged Ecotec LNF using a Gen II block for the Pontiac Solstice GXP and the Saturn Sky Red Line; in 2010, a Gen II block 2.4 L Ecotec LAF; and in 2012, a 2.5 L Ecotec LCV and 2.0 L turbocharged Ecotec LTG in a Gen III block. In 2018 the Corvette ZR1 (C7) introduced GM's twin port and direct injector system.
In 2004 Isuzu produced the first GDi engine sold in a mainstream American vehicle, standard on the 2004 Axiom and optional on the 2004 Rodeo. Isuzu claimed the benefit of GDi is that the vaporizing fuel has a cooling effect, allowing a higher compression ratio (10.3:1 versus 9.1:1) that boosts output by 20 hp (15 kW), and that 0-to-60 mph times drop from 8.9 to just 7.5 seconds, with the quarter-mile being cut from 16.5 to 15.8 seconds.
In 2005, Mazda began to use their own version of direct-injection in the Mazdaspeed6 and later on the CX-7 sport-utility, and the new Mazdaspeed3 in the US and European market. It is referred to as Direct Injection Spark Ignition (DISI).
In 2006, BMW released the new N54 twin-turbo-charged direct injection inline-six engine for its 335i Coupe and later for the 335i Sedan, 535i series. and the 135i models. Mercedes-Benz released its direct injection system (Charged Gasoline Injection, or "CGI") on the CLS 350 CGI featuring common rail, piezo-electric direct fuel injectors. The CLS 350 CGI offers 292 BHP versus 272 BHP for the CLS 350, with reduced carbon dioxide emissions and improved fuel economy. Audi also released its V8 engine with FSI technology in Audi R8 that can produce 424 BHP with low carbon emission and more fuel economy.
In 2007, GM released the 3.6 L V6 LLT SIDI for the redesigned Cadillac CTS and STS and the Holden Commodore SV6. The 3.6 L has been used in the 2010 Chevy Camaro, a first for this model. In 2010, the 3.0 L LF1 SIDI was introduced.
In 2007, Ford introduced its EcoBoost engine technology designed for a range of vehicles. The engine first appeared in the 2007 Lincoln MKR Concept under the name TwinForce. The EcoBoost family of 4-cylinder and 6-cylinder engines features turbocharging and direct injection technology (GTDI - Gasoline Turbocharged Direct Injection). A 2.0 L version was unveiled in the 2008 Explorer America Concept.
In 2009, Ferrari began selling the front-engine California with a direct injection system, and announced the 458 Italia will also feature a direct injection system, a first for Ferrari mid-rear engine setups. Porsche also began selling the 997 and Cayman equipped with direct injection. Ford produced the new generation Taurus SHO and Flex with a 3.5 L twin-turbo EcoBoost V-6 with direct injection. The Jaguar Land Rover AJ-V8 Gen III 5.0 L engine (introduced in August 2009 for the 2010 model year) features spray-guided direct injection.
In 2013 the Acura RLX came with direct-injection, becoming the first Honda GDI V6.
The 2014 General Motors LT1 (separate from the 1990s era LT1 / LT4 engines), a 6.2 L V8, will use direct injection as well as VVT and variable displacement (cylinder deactivation). The 2014 Hyundai Accent features an aluminum block, I4 GDI engine that produces 138 hp.
In two-stroke engines
The benefits of direct injection are even more pronounced in two-stroke engines, because it eliminates much of the pollution they cause. In all two-strokes other than those with split-single engines or similarly sophisticated arrangements, the exhaust and intake ports are both open at the same time, at the bottom of the piston stroke, for "scavenging". In conventional two-strokes, a portion of the fuel/air mixture entering the cylinder from the crankcase through the intake ports goes directly out, unburned, through the exhaust port. With direct injection, only air (and usually some oil) comes from the crankcase, and fuel is not injected until the piston rises and all ports are closed.
Two types of GDi are used in two-strokes: low-pressure air-assisted, and high-pressure. The former, developed by Orbital Engine Corporation of Australia (now Orbital Corporation) injects a mixture of fuel and compressed air into the combustion chamber. When the air expands it atomizes the fuel. The Orbital system is used in motor scooters manufactured by Aprilia, Piaggio, Peugeot and Kymco, in outboard motors manufactured by Mercury and Tohatsu, and in personal watercraft manufactured by Bombardier Recreational Products.
Lubrication is achieved by injecting oil into the crankcase, resulting in a lower oil consumption (80:1 range) than with the oil mixed with fuel lubrication.
The high-pressure direct injector for two-stroke engines was developed in the early 1990s by Ficht GmbH of Kirchseeon Germany. Outboard Marine Corporation (OMC) licensed the technology in 1995 and introduced it on a production outboard engine in 1996. OMC purchased a controlling interest in Ficht in 1998. Beset by extensive warranty claims for its Ficht outboards and prior and concurrent management-financial problems, OMC declared bankruptcy in December 2000 and the engine manufacturing portion and brands (Evinrude Outboard Motors and Johnson Outboards), including the Ficht technology, were purchased by Bombardier Recreational Products in 2001.
Evinrude introduced the E-Tec system, an improvement to the Ficht fuel injection, in 2003, based on U.S. patent 6,398,511. In 2004, Evinrude received the EPA Clean Air Excellence Award for their outboards utilizing the E-Tec system. The E-Tec system has recently also been adapted for use in performance two-stroke snowmobiles.
Yamaha also has a high-pressure direct injection (HPDI) system for two-stroke outboards. It differs from the Ficht/E-Tec and Orbital direct injection systems because it uses a separate, belt driven, high-pressure, mechanical fuel pump to generate the pressure necessary for injection in a closed chamber. This is similar to most current 4-stroke automotive designs and fat pump systems.
Husqvarna/KTM is developing a DFI system for their two-stroke Enduro motorcycles. In 2011, Ossa developed the 300i a fuel-injected enduro motorcycle, the company was sold to Gas Gas and the release to market was thwarted by financial problems.
EnviroFit, a non-profit corporation sponsored by Colorado State University, has developed direct injection retrofit kits for two-stroke motorcycles in a project to reduce air pollution in Southeast Asia, using technology developed by Orbital Corporation of Australia. The World Health Organization says air pollution in Southeast Asia and the Pacific causes 537,000 premature deaths each year. The 100-million two-stroke taxis and motorcycles in that part of the world are a major cause.
Gasoline direct injection does not have the valve cleaning action a port fuel injection (PFI) system inherently provides. As contamination such as atomized oil is carried through the intake ports, contamination deposits onto the intake valves, and is left on the valve surface with no intake of gasoline to act as a cleaning agent as a PFI engine would experience. Over time this can result in reduced intake valve clearances, and will also effectively insulate the valve from the valve seat which can significantly affect valve cooling ability. Due to this, Gasoline Direct Injection may reduce engine performance and efficiency as contamination deposits on valve surfaces increase. Likewise, engine lifespan may be adversely affected due to heat accumulation in intake valves due to insulative layers of contamination. In extreme instances, often involving inadequate or failed PCV systems, operational issues and engine failure can occur due to adverse effects of contamination buildup.
Gasoline direct injection requires more engineering effort and therefore causes higher manufacturing costs. Engines solely using stratified injection do not benefit from the fuel saving advantages if they are mostly running under high load, which makes using engines solely relying on stratified injection questionable. In general, series production engines with gasoline direct injection usually burn a homogenous air-fuel-mixture. This causes gas-exchange losses, which are compensated with variable valve timing and lifting.
Compared with manifold injected engines, the exhaust of gasoline direct injected engines differs. When using stratified injection, a regular three-way-catalyst cannot work anymore, because the air-fuel ratio is . As stratified injection causes more nitrogen oxides as well as hydrocarbons, a special exhaust gas treatment system such as an NOx adsorber is required, because solely using special combustion altering measures is insufficient for meeting modern emission standards. Abandoning the stratified injection method and sticking to conventional homogenous air-fuel mixtures is an easy alternative and allows using a standard three-way-catalyst. Stratified injection requires using a fuel with a very low sulphur amount (fuel meeting the EN 228 specification), which is not available worldwide. High sulphur would prevent an NOx absorber from working correctly and thus require regenerating it with hydrocarbons; creating extra-hydrocarbons for this purpose on the other hand would require using more fuel and render the fuel saving advantages of stratified injection useless. Furthermore, stratified injection causes more particulate matter, which requires using a particulate filter.
Gasoline has inferior lubricity compared with fuels usually used for Diesel engines, which is a limiting factor for the injection pressure. Gasoline direct injected engines use an injection pressure of approximately 20 MPa, because higher pressures would cause too much wear. On the other hand, high injection pressure is required to create a fine fuel spray. An improper fuel spray causes "rich zones", and the fuel does not burn correctly. Unburnt fuel can cause carbon building up on the injection nozzle and spark plug.
Amongst other power unit changes, one of the rule changes for the 2014 season was that Direct Injection had been made compulsory, with regulation 5.10.2 stating 'There may only be one direct injector per cylinder and no injectors are permitted upstream of the intake valves or downstream of the exhaust valves.' 
In 2013, research by TÜV NORD found that although gasoline direct injection engines dramatically reduce carbon dioxide emissions, they release about 1,000 times more particles classified by the World Health Organization as harmful than traditional petrol engines and 10 times more than new diesel engines. The release happens because direct injection results in uneven burning of fuel due to uneven mixing of fuel and air (stratification) and because direct-injection engines operate with a higher pressure in their cylinders than older indirect-injection engines.
This pollution can be prevented with a relatively inexpensive particulate filter that can significantly reduce the emissions of particles. However, fitting such a filter is not mandatory yet. As of September 2017, Euro 6 emission regulations limit particle numbers at a maximum of 6×1011 per kilometre over the New European Driving Cycle. Some gasoline direct injection engines might require gasoline particulate filters (GPFs) to meet that standard.
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