wikiwand.com

Gasoline direct injection - Wikiwand

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, as per manifold injection.
Stratified charge mode has a zone with a higher density of fuel around the spark plug, and a leaner mixture (lower density of fuel) further away from the spark plug.

Homogeneous charge mode

In the homogeneous charge mode, the engine operates on a homogeneous air/fuel mixture ( $\lambda =1$ ), 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 in order to give injected fuel the most time to mix with the air, so that a homogeneous air/fuel mixture is formed.^[5] This mode allows using a conventional three-way catalyst for exhaust gas treatment.^[6]

Compared with manifold injection, the fuel efficiency is only very slightly increased, but the specific power output is better,^[7] which is why the homogeneous mode is useful for so-called engine downsizing.^[6] Most direct-injected passenger car petrol engines use the homogeneous charge mode.^[8]^[9]

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 $\lambda >8$ ,^[10] with mean air-fuel ratios of $\lambda =3...5$ at medium load, and $\lambda =1$ at full load.^[11] 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.^[12]

Since mixtures too lean cannot be ignited with a spark-plug (due to a lack of fuel), the charge needs to be stratified (e. g. a small zone of fuel/air mixture around the spark plug needs to be created).^[13] 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.^[14]

The stratified charge mode (also called "ultra lean-burn" 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 $\lambda =1$ for moderate loads and a richer air-fuel ratio at higher loads.^[15]

In theory, a stratified charge mode can further improve fuel efficiency and reduce exhaust emissions,^[16] 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,^[17] which sometimes require a NOx adsorber in the exhaust system to meet emissions regulations.^[18] The use of NOx adsorbers can require low sulphur fuels, since sulphur prevents NOx adsorbers from functioning properly.^[19] GDI engines with stratified fuel injection can also produce higher quantities of particulate matter than manifold injected engines,^[20] sometimes requiring particulate filters in the exhaust (similar to a diesel particulate filter) in order to meet vehicle emissions regulations.^[21] 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 petrol engine (F5R), which never came with a stratified charge mode,^[22] 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.^[23] Turbocharged Volkswagen engines labelled TFSI and TSI have always used the homogeneous mode.^[24] Like the latter VW engines, newer direct injected petrol engines (from 2017 onwards) 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.^[25]

Injection modes

Common techniques for creating the desired distribution of fuel throughout the combustion chamber are either spray-guided, air-guided, or wall-guided injection. The trend in recent years is towards spray-guided injection, since it currently results in a higher fuel efficiency.

Wall-guided direct injection

In engines with wall-guided injection, the distance between spark plug and injection nozzle is relatively high. In order to get the fuel close to the spark plug, it is sprayed against a swirl cavity on top of the piston (as seen in the picture of the Ford EcoBoost engine on the right), which guides the fuel towards the spark plug. Special swirl or tumble air intake ports aid this process. The injection timing depends upon the piston speed, therefore, at higher piston speeds, the injection timing, and ignition timing need to be advanced very precisely. At low engine temperatures, some parts of the fuel on the relatively cold piston cool down so much, that they cannot combust properly. When switching from low engine load to medium engine load (and thus advancing the injection timing), some parts of the fuel can end up getting injected behind the swirl cavity, also resulting in incomplete combustion.^[26] Engines with wall-guided direct injection can therefore suffer from high hydrocarbon emissions.^[27]

Air-guided direct injection

Like in engines with wall-guided injection, in engines with air-guided injection, the distance between spark plug and injection nozzle is relatively high. However, unlike in wall-guided injection engines, the fuel does not get in contact with (relatively) cold engine parts such as cylinder wall and piston. Instead of spraying the fuel against a swirl cavity, in air-guided injection engines the fuel is guided towards the spark plug solely by the intake air. The intake air must therefore have a special swirl or tumble movement in order to direct the fuel towards the spark plug. This swirl or tumble movement must be retained for a relatively long period of time, so that all of the fuel is getting pushed towards the spark plug. This however reduces the engine's charging efficiency and thus power output. In practice, a combination of air-guided and wall-guided injection is used.^[28] There exists only one engine that only relies on air-guided injection.^[29]

Spray-guided direct injection

In engines with spray-guided direct injection, the distance between spark plug and injection nozzle is relatively low. Both the injection nozzle and spark plug are located in between the cylinder's valves. The fuel is injected during the latter stages of the compression stroke, causing very quick (and inhomogeneous) mixture formation. This results in large fuel stratification gradients, meaning that there is a cloud of fuel with a very low air ratio in its centre, and a very high air ratio at its edges. The fuel can only be ignited in between these two "zones". Ignition takes place almost immediately after injection to increase engine efficiency. The spark plug must be placed in such a way, that it is exactly in the zone where the mixture is ignitable. This means that the production tolerances need to be very low, because only very little misalignment can result in drastic combustion decline. Also, the fuel cools down the spark plug, immediately before it is exposed to combustion heat. Thus, the spark plug needs to be able to withstand thermal shocks very well.^[30] At low piston (and engine) speeds, the relative air/fuel velocity is low, which can cause fuel to not vaporise properly, resulting in a very rich mixture. Rich mixtures do not combust properly, and cause carbon build-up.^[31] At high piston speeds, fuel gets spread further within the cylinder, which can force the ignitable parts of the mixture so far away from the spark plug, that it cannot ignite the air/fuel mixture anymore.^[32]

Companion technologies

Other devices which are used to complement GDI in creating a stratified charge include variable valve timing, variable valve lift, and variable length intake manifold.^[33] Also, exhaust gas recirculation can be used to reduce the high nitrogen oxide (NOx) emissions that can result from the ultra lean combustion.^[34]

Disadvantages

Gasoline direct injection does not have the valve cleaning action that is provided when fuel is introduced to the engine upstream of the cylinder.^[35] In non-GDI engines, the gasoline traveling through the intake port acts as a cleaning agent for contamination, such as atomized oil. The lack of a cleaning action can cause increased carbon deposits in GDI engines. Third party manufacturers sell oil catch tanks which are supposed to prevent or reduce those carbon deposits.

The ability to produce peak power at high engine speeds (RPM) is more limited for GDI, since there is a shorter period of time available to inject the required quantity of fuel. In manifold injection (as well as carburetors and throttle-body fuel injection), fuel can be added to the intake air mixture at any time. However a GDI engine is limited to injecting fuel during the intake and compression phases. This becomes a restriction at high engine speeds (RPM), when the duration of 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 fuel injectors also assist in cleaning carbon deposits from the intake system.

Gasoline does not provide the same level of lubrication for the injector components as diesel, which sometimes becomes a limiting factor in the injection pressures used by GDI engines. The injection pressure of a GDI engine is typically limited to approximately 20 MPa (2.9 ksi), to prevent excessive wear on the injectors.^[36]

Adverse climate and health impacts

While this technology is credited with boosting fuel efficiency and reducing CO₂ emissions, GDI engines produce more black carbon aerosols than traditional port fuel injection engines. A strong absorber of solar radiation, black carbon possesses significant climate-warming properties.^[37]

In a study published in January 2020 in the journal Environmental Science and Technology, a team of researchers 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 a reduction in CO₂. The researchers also believe the shift from traditional port fuel injection (PFI) engines to the use of 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 estimate the annual social cost of these premature deaths at $5.95 billion.^[38]