Gasoline direct injection

Use of GDI can improve engine efficiency and specific power output, and can also reduce exhaust emissions from vehicles.

The first production engine to use GDI was the Swedish Hesselman engine, a low-compression multi-fuel spark-ignition design that was more efficient than traditional carbureted engines. It could also run on diesel, kerosene, ethanol and tar oil. Because gasoline was more expensive than less-refined fuels, it was usually only used for starting; once running, the engine was switched to a cheaper fuel.

Introduced in 1925, the Hesselman engine was used by truck and heavy equipment manufacturers in Sweden and by stationary engine and heavy vehicle manufacturers in the United States throughout the 1940s. The first mass-produced GDI engine to use Bosch's mechanical fuel injection system (the same type used in pre-chambers in diesel engines) was the DB601 V12 for the Messerschmitt Bf 109 in 1936.

Use of the Bosch mechanical fuel injection system in a direct-injection configuration allowed the Germans to employ extremely high compression ratios and very high-pressure forced induction to produce high power reliably from low-quality gasoline with an octane rating of only 87. In addition, the fighter's engine did not stall in negative‑G turns, unlike carbureted engines such as the Rolls-Royce Merlin.

Conventionally charged engines not only required 100–200 octane avgas to achieve the same power levels as the DB601 and later DB605, but fighters using the Merlin and other carbureted solutions were vulnerable to tactics that forced the pilot into a negative‑G turn to avoid being shot down. The resulting loss of engine power could lead to the aircraft's loss in such situations. Rolls-Royce was able to correct this problem late in the war, around late 1943.

Several German cars used a Bosch mechanical GDI system in the 1950s. However, use of the technology remained rare until 1996, when Mitsubishi introduced an electronic GDI system for its mass-produced vehicles.

Since then, GDI has been adopted widely 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:

• In homogeneous charge mode, the fuel is evenly mixed with air throughout the combustion chamber, similar to manifold injection.
• In stratified charge mode, there is a zone with a higher fuel density around the spark plug and a leaner mixture (lower fuel density) further away from the spark plug.

Homogeneous charge mode

In the homogeneous charge mode, the engine operates on a homogeneous air–fuel mixture (${\displaystyle \lambda =1}$), meaning that there is an (almost) perfect mixture of fuel and air in the cylinder. The fuel is injected at the very beginning of the intake stroke to give it the maximum time to mix with the air, so that a homogeneous mixture is formed. This mode allows use of a conventional three-way catalyst for exhaust gas treatment.

Compared with manifold injection, fuel efficiency is only slightly increased, but the specific power output is better, which is why homogeneous mode is useful for engine downsizing. Most direct-injected passenger car gasoline engines use homogeneous charge mode.

Stratified charge mode

In stratified charge mode, a small zone of fuel–air mixture is created around the spark plug, surrounded by air in the rest of the cylinder. This means less fuel is injected into the cylinder overall, leading to very high air–fuel ratios of ${\displaystyle \lambda >8}$, with mean air–fuel ratios of ${\displaystyle \lambda =\mathrm{3...5}}$ at medium load, and ${\displaystyle \lambda =1}$ at full load. Ideally, the throttle valve remains open as much as possible to avoid throttling losses. Torque is then controlled solely by changing the amount of injected fuel (quality torque control), rather than varying the amount of intake air. Stratified charge mode also keeps the flame away from the cylinder walls, reducing thermal losses.

Since mixtures that are too lean cannot be ignited with a spark plug (due to the lack of fuel), the charge must be stratified, i.e., a small zone of fuel–air mixture must be created around the spark plug. To achieve this, a stratified charge engine injects fuel during the later 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.

Stratified charge mode (also called "ultra lean-burn" mode) is used at low loads to reduce fuel consumption and exhaust emissions. At higher loads, it is disabled, and the engine switches to homogeneous mode with a stoichiometric air–fuel ratio of ${\displaystyle \lambda =1}$ at moderate loads, and a richer mixture at higher loads.

In theory, stratified charge mode can further improve fuel efficiency and reduce exhaust emissions. In practice, however, it has not shown significant efficiency advantages over a conventional homogeneous charge concept. Its inherent lean burn forms higher levels of nitrogen oxides, sometimes requiring a NOx adsorber in the exhaust system to meet emissions regulations. NOx adsorbers can require low-sulfur fuels, since sulfur prevents them 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 (similar to a diesel particulate filter) to meet vehicle emissions regulations. As a result, several European car manufacturers have abandoned the stratified charge concept or never adopted it. For example, the 2000 Renault 2.0 IDE gasoline engine (F5R) never used stratified charge mode. The 2009 BMW N55 and 2017 Mercedes-Benz M256 engines dropped the stratified charge mode used by their predecessors. The Volkswagen Group used fuel stratified injection in naturally aspirated engines labeled FSI, but these engines received an engine control unit update that disabled stratified charge mode. Turbocharged Volkswagen engines labeled TFSI and TSI have always used homogeneous mode. Like these Volkswagen engines, most newer direct-injected gasoline engines (from 2017 onwards) use homogeneous charge mode in conjunction with variable valve timing to achieve good efficiency. Stratified charge concepts have mostly been abandoned.

Injection modes

Common techniques for creating the desired distribution of fuel throughout the combustion chamber are spray-guided, air-guided, and wall-guided injection. A trend is towards spray-guided injection, since it currently offers higher fuel efficiency.

Wall-guided direct injection

In wall-guided injection engines, the distance between the spark plug and the injection nozzle is relatively large. To get the fuel close to the spark plug, it is sprayed into a swirl cavity at the top of the piston (as seen in the picture of the Ford EcoBoost engine), which guides the fuel toward the spark plug. Special swirl or tumble intake ports assist this process. Injection timing depends on piston speed; therefore, at higher piston speeds, injection and ignition timing must be advanced very precisely. At low engine temperatures, some of the fuel that impinges on the relatively cold piston cools down so much that it cannot combust properly. When switching from low engine load to medium engine load (and thus advancing injection timing), some of the fuel may end up being injected behind the swirl cavity, again causing incomplete combustion. Engines with wall-guided direct injection can therefore suffer from high hydrocarbon emissions.

Air-guided direct injection

In air-guided injection systems, as with wall-guided systems, the distance between the spark plug and injection nozzle is relatively large. However, the fuel does not contact relatively cold engine parts, such as the cylinder walls and pistons. Instead of being sprayed against a swirl cavity, the fuel is guided toward the spark plug solely by the intake air. The intake air must therefore have a specific swirl or tumble motion to direct the fuel toward the spark plug, and this motion must be maintained long enough to push all of the fuel into the spark plug region. This requirement reduces the engine's charging efficiency and, in turn, its power output. In practice, a combination of air-guided and wall-guided injection is usually used. Only one engine is known to rely solely on air-guided injection.

Spray-guided direct injection

In engines with spray-guided direct injection, the distance between the spark plug and injection nozzle is relatively small. Both are located between the cylinder's valves. Fuel is injected during the later stages of the compression stroke, resulting in very rapid, inhomogeneous mixture formation. This creates large stratification gradients, with a central region that has a very low air ratio and outer regions with a very high air ratio. The fuel can be ignited only in the intermediate zone between these two regions. Ignition occurs almost immediately after injection, improving engine efficiency. The spark plug must be placed so that it is located precisely in the zone where the mixture is ignitable. As a result, production tolerances must be very tight, since small misalignments can drastically reduce combustion quality. The fuel cools the spark plug immediately before it is exposed to combustion heat, so the spark plug must withstand significant thermal shocks. At low piston (and engine) speeds, the relative air–fuel velocity is low, which can prevent proper vaporization and result in a very rich mixture. Rich mixtures do not combust properly and cause carbon build-up. At high piston speeds, fuel is spread further within the cylinder, which can move the ignitable parts of the mixture so far from the spark plug that it can no longer ignite the mixture.

Companion technologies

Other technologies used to complement GDI in creating a stratified charge include variable valve timing, variable valve lift, and variable length intake manifold. Exhaust gas recirculation can also be used to reduce the high nitrogen oxide (NOx) emissions that can result from ultra-lean combustion.

Disadvantages

Gasoline direct injection does not provide the valve-cleaning action that occurs when fuel is introduced upstream of the cylinder. In non-GDI engines, gasoline traveling through the intake port acts as a cleaning agent for contamination such as atomized oil. The lack of this cleaning action can cause increased carbon deposits in GDI engines. Third-party manufacturers sell oil catch tanks intended to prevent or reduce these deposits.

The ability to produce peak power at high engine speeds (RPM) is more limited in GDI engines, since there is less time available to inject the required fuel quantity. In manifold injection systems (as well as carburetors and throttle-body fuel injection), fuel can be added to the intake air mixture at any time. Still, a GDI engine is limited to injecting fuel during the intake and compression phases. This becomes a restriction at high RPM, when each combustion cycle is shorter. To overcome this limitation, some GDI engines (such as the Toyota 2GR-FSE V6 and Volkswagen EA888 I4 engines) also have a set of manifold fuel injectors to provide additional fuel at high RPM. These manifold injectors also help to clean carbon deposits from the intake system.

Gasoline does not provide the same level of lubrication for injector components as diesel, which can limit the injection pressures used in GDI engines. Injection pressure in a GDI engine is therefore typically limited to approximately 20 MPa (2.9 ksi) to prevent excessive injector wear.

Adverse climate and health impacts

Although GDI technology is credited with boosting fuel efficiency and reducing CO₂ emissions, GDI engines produce more black carbon aerosols than traditional port-fuel-injection engines. Black carbon is a strong absorber of solar radiation and has significant climate-warming properties.

In a study published in January 2020 in the journal Environmental Science and Technology, a research team at the University of Georgia (USA) predicted that the increase in black carbon emissions from GDI-powered vehicles will increase climate warming in urban areas of the U.S. by an amount that significantly exceeds the cooling associated with reduced CO₂ emissions. The researchers also concluded that the shift from traditional port fuel injection (PFI) engines to GDI technology will nearly double the premature mortality rate associated with vehicle emissions, from 855 deaths annually in the United States to 1,599. They estimated the annual social cost of these premature deaths at $5.95 billion.