It's no secret that automobile manufacturers are trying to reduce emissions and improve fuel economy, while at the same time maintain or increase engine performance. These challenges have forced manufacturers to push the technological envelope and revisit just about every aspect of the automobile. This includes the development of new methods of fuel injection that optimize every ounce of fuel.
In this article, we explain Gasoline Direct Injection (GDI). Engines equipped with GDI first appeared on the market in 2007, and by 2008 they were installed in 2.3% of all light-duty vehicles produced. Ten years after introduction, GDI was used in about half of all light-duty vehicles produced. Throughout 2020, GDI installations continued to rise, reaching a new high of 55.3%.
Let's discuss the foundations of gasoline direct injection.
What is gasoline direct injection?
At first, multi-port fuel injection (MPI) systems were a significant improvement over the carburetor and throttle body injection systems they replaced. However, a new fuel injection system known as Gasoline Direct Injection (GDI) soon became the successor to MPI systems. If you haven't already, you can learn more about MPI systems in our Fuel Injection 101 article.
GDI differs from MPI in that the fuel is injected directly into the combustion chamber rather than into the intake port. The results are impressive; a 15% decrease in fuel consumption, 5% improvement in engine torque output and reduced emissions. The downside being it costs a manufacturer 10-15% more to equip an engine with GDI versus an MPI system. Most of this cost is associated with the fact that GDI operates at much higher fuel pressures than a MPI system. A typical MPI system operates at pressures of 36-50 psi, whereas the GDI system operates at pressures of up to 2,900 psi.
What are the advantages of GDI?
Because fuel is injected directly into the combustion chamber, several advantages are realized. Fuel delivery into the combustion chamber is no longer tied to valve timing. This allows aggressive valve timing strategies operating independently of the injection event. This can increase torque output and decrease turbo spool-up time on turbocharged engines, improving performance in both instances. GDI can also reduce cold-start hydrocarbon emissions by up to 25% compared to a typical MPI system. Another big advantage is that GDI offers better engine knock (ping) control. This allows engines to operate at higher compression ratios; improving engine efficiency and increasing power output.
Major components in the GDI System
Many GDI components seem similar to those used in MPI systems at first glance. However, they are quite different. We'll discuss each major component in the GDI system:
GDI utilizes two fuel pumps. A low pressure electric fuel pump delivers fuel to a high pressure mechanically driven pump. The high pressure pump is driven off of a dedicated lobe on the engine's camshaft and supplies fuel to the fuel rail where it is distributed to each fuel injector.
The high pressure fuel system is closed-loop. Controlled by the vehicle's on-board computer, it is adjusted for optimum performance based on engine operating parameters. The computer monitors the fuel pressure via a sensor located on the fuel rail. The computer controls a fuel pressure control valve on the high pressure pump to vary the fuel pressure.
GDI fuel injectors are much different than MPI injectors. GDI injectors operate at much higher pressures - as high as 2,900 psi. The tip of the injector is exposed to combustion temperatures and pressures which require a GDI injector to be far more robust than a typical MPI injector.
On an MPI system, fuel and air are drawn into the combustion chamber when the intake valve opens. Although the timing of the injection event is controlled by the on-board computer, the timing of when the fuel enters the combustion chamber is mechanically controlled by the relationship between the camshaft and crankshaft. This is not the case with GDI. Because fuel is injected directly into the combustion chamber, it must be done at the correct time during the intake or compression stroke. As a result, there is less time to inject the necessary fuel for the combustion event, which is why a GDI injector is required to operate at much higher speeds.
Where a typical multi-port injector on-time (pulse width) may be 3 to 20 milli-seconds, a GDI injector on-time is between 400 micro-seconds to 5 milli-seconds. High speed piezo injectors are used to accomplish the task.
The shape and mixture of the air/fuel charge within the combustion chamber is critical on GDI applications. One of three methods is used to develop the geometry of the air/fuel charge.
The Wall Guided method relies on the geometry of the piston and cylinder wall to form the proper air/fuel charge. The piston has contours on its crown to properly mix the injected fuel with the incoming air. On this system the injector is positioned off-center of the combustion chamber (see photo above).
The Charge Air Guided method relies on the incoming air charge to develop the proper air/fuel charge. These systems usually have dynamically controlled intake manifolds to affect the swirl of the incoming air. The injector is strategically placed near the incoming air flow.
The Spray or Jet Guided method relies solely on the injector spray pattern to develop the geometry of the air/fuel distribution. The injector tip is centered in the combustion chamber whereas the spark plug is located off-center at the top of the combustion chamber.
GDI has several different operating modes that previous MPI systems did not utilize. These operating modes are based on engine conditions and driver demand. They are designed to improve fuel economy, performance and keep emission levels to the lowest level possible. Operating modes include: 1) Lean Mode, 2) Stratified and Knock Protection Mode, and 3) Stratified Catalyst Heating Mode.
The on-board computer determines which operating mode should be used and seamlessly switches between them during vehicle operation.
Lean Mode is used during light engine loads when little or no acceleration is required. In this mode, fuel is injected near the end of the compression stroke, just before ignition. This mode is “ultra-lean” and reduces emissions and fuel consumption.
Stratified and Knock Protection mode is used when engine load is high. During this mode two injection events are utilized; one on the intake stroke and one on the compression stroke. Because some of the required fuel is delivered later in the combustion process, engine knock is reduced.
Stratified Catalyst Heating Mode uses two injection events, one during the compression stroke and one just after top dead center of the compression stroke. This mode causes an increase in exhaust gas temperature which heats up the catalytic converter to the optimal temperature for emission reduction.
The ability to freely control injection timing and operate in different modes is the major advantage that GDI has over current MPI systems.
The future of GDI
In December 2021, the Environmental Protection Agency finalized revised national greenhouse gas (GHG) emissions standards for passenger cars and light trucks for Model Years 2023- 2026. The final standards will require significant GHG emissions reductions along with reductions in other criteria pollutants.
As emissions standards and regulations only continue to increase, manufacturers will continue to equip gasoline vehicles with GDI engines to abide by these measures. As the GDI market grows, the aftermarket can anticipate an increase of vehicle owners seeking shops with GDI expertise, parts and know-how.
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