Gasoline direct injection
Gasoline direct injection, also known as petrol direct injection, is a fuel injection system for internal combustion engines that run on gasoline, which injects fuel directly into the combustion chamber. This is distinct from manifold injection systems that inject fuel into the intake manifold, which mixes with the incoming airstream before reaching the combustion chamber.
The use of GDI can help increase engine efficiency and specific power output as well as reduce exhaust emissions from vehicles.
The first engine to use GDI to reach production was the Swedish Hesselman engine, which was a low-compression multi-fuel spark ignition engine which was more efficient than traditional carburated Gasoline engine, the engine could however be run on diesel, kerosene, ethanol and tar oil and because they were cheaper than gasoline was usually only used for starting it, it was introduced in 1925 and utilized by truck and heavy equipment manufacturers in Sweden and stationary engine and heavy vehicle manufacturers in the US throughout the 1940s. The first mass-produced GDI engine to use Bosch's mechanical fuel injection system was the DB601 V12 for the Messerschmitt Bf109 in 1936. The utilization of the Bosch mechanical fuel-injection system in a DI configuration, enabled the Germans to use extremely high compression ratios and very high pressure forced induction, to produce massive power reliably using extremely low quality gasoline with an octane rating of only 87. In addition to which the fighters engine did not stall in a negative G turn, like carburetor engines like the Rolls-Royce Merlin. Conventionally-charged engines not only required the use of 100-200 octane av-gas to achieve the same levels of power as the DB601 and the later DB605, but fighters using the Merlin and other carburetor solutions were vulnerable to being tactics that forced the pilot of an allied fighter into a negative-G turn to avoid being shot down. The subsequent total loss of engine power could lead to total loss of the aircraft in this situation. Rolls-Royce was able to fix this issue only very late in the war, circa late 1943. Several German cars used a Bosch mechanical GDI system in the 1950s. However, technology usage remained rare until 1996 when Mitsubishi introduced an electronic GDI system for its mass-produced vehicles. Since then, GDI has been adopted by the automotive industry, increasing in the United States from 2.3% of production for model year 2008 vehicles to approximately 50% for model year 2016.
Operating principle
Charge modes
The 'charge mode' of a direct-injected engine refers to how the fuel is distributed throughout the combustion chamber:- 'Homogeneous charge mode' has the fuel mixed evenly with the air throughout the combustion chamber, in the manner of manifold injection.
- Stratified charge mode has a zone with a higher fuel density around the spark plug, and a leaner mixture further away from the spark plug.
Homogeneous charge mode
Compared with manifold injection, the fuel efficiency is only very slightly increased, but the specific power output is better, which is why the homogeneous mode is useful for so-called engine downsizing. Most direct-injected passenger car gasoline engines use the homogeneous charge mode.
Stratified charge mode
The stratified charge mode creates a small zone of fuel/air mixture around the spark plug, which is surrounded by air in the rest of the cylinder. This results in less fuel being injected into the cylinder, leading to very high overall air-fuel ratios of, with mean air-fuel ratios of at medium load, and at full load. Ideally, the throttle valve remains open as much as possible to avoid throttling losses. The torque is then set solely by means of quality torque controlling, meaning that only the amount of injected fuel, but not the amount of intake air is manipulated in order to set the engine's torque. Stratified charge mode also keeps the flame away from the cylinder walls, reducing the thermal losses.Since mixtures too lean cannot be ignited with a spark-plug, the charge needs to be stratified. To achieve such a charge, a stratified charge engine injects the fuel during the latter stages of the compression stroke. A "swirl cavity" in the top of the piston is often used to direct the fuel into the zone surrounding the spark plug. This technique enables the use of ultra-lean mixtures that would be impossible with carburetors or conventional manifold fuel injection.
The stratified charge mode is used at low loads, in order to reduce fuel consumption and exhaust emissions. However, the stratified charge mode is disabled for higher loads, with the engine switching to the homogeneous mode with a stoichiometric air-fuel ratio of for moderate loads and a richer air-fuel ratio at higher loads.
In theory, a stratified charge mode can further improve fuel efficiency and reduce exhaust emissions, however, in practice, the stratified charge concept has not proved to have significant efficiency advantages over a conventional homogeneous charge concept, but due to its inherent lean burn, more nitrogen oxides are formed, which sometimes require a NOx adsorber in the exhaust system to meet emissions regulations. The use of NOx adsorbers can require low sulfur fuels, since sulfur prevents NOx adsorbers from functioning properly. GDI engines with stratified fuel injection can also produce higher quantities of particulate matter than manifold injected engines, sometimes requiring particulate filters in the exhaust in order to meet vehicle emissions regulations. Therefore, several European car manufacturers have abandoned the stratified charge concept or never used it in the first place, such as the 2000 Renault 2.0 IDE gasoline engine, which never came with a stratified charge mode, or the 2009 BMW N55 and 2017 Mercedes-Benz M256 engines dropping the stratified charge mode used by their predecessors. The Volkswagen Group had used fuel stratified injection in naturally aspirated engines labelled FSI, however, these engines have received an engine control unit update to disable the stratified charge mode. Turbocharged Volkswagen engines labelled TFSI and TSI have always used the homogeneous mode. Like the latter VW engines, newer direct injected gasoline engines usually also use the more conventional homogeneous charge mode, in conjunction with variable valve timing, to obtain good efficiency. Stratified charge concepts have mostly been abandoned.