Automotive Engine Performance

Introduction

21-01 Understand how a GDI engine operates.

The introduction of gasoline direct injection (GDI) for use in the production automobile has marked the next evolution of the gasoline engine. Increasing the power by increasing the compression within the engine helps to decrease emissions and increase fuel economy (FIGURE 21-1). To gain these features, the fuel system of the internal combustion engine had to change. The basic four-stroke principles have stayed the same, as well as the combustibility of gasoline fuel. The difference is how the fuel is introduced into the cylinder and how the cylinder reacts to the effects of that event. These type of engines go by various names, such as Volkswagen’s fuel stratified injection (FSI), GM’s spark-ignited direct injection (SIDI), Ford’s EcoBoost engine line, and Mazda’s direct-injection spark ignition (DISI), to name a few.

FIGURE 21-1 The increased efficiency of GDI has been helping to encourage manufactures to adopt this new technology. With government mandates on vehicles’ fuel efficiency, this is the next step in ongoing progression toward more-efficient automobiles.

Description and Operation

The Otto four-stroke cycle is still one of the most widely used combustion processes in production today (FIGURE 21-2). The Miller and Atkinson cycle engines use a version of either variable valve timing (VVT) or boosted intake charge, but still use a form of the Otto four stroke. For most GDI purposes, conventional gasoline engine principles still apply. A GDI engine still has all four strokes of a conventional gasoline engine, but the difference is the way that the fuel is introduced to combustion chamber. In a conventional port fuel-injection system, the fuel is sprayed at the backside of the intake valve. The fuel then stays there until the valve is opened, at which point it is pulled into the combustion chamber during the intake stroke (FIGURE 21-3). This system works in conjunction with the mechanical operations of the engine to sustain combustion.

FIGURE 21-2 The Otto four stroke is used in most internal combustion engines (ICEs) because of its efficiency. This does not change when an engine is using GDI. There are some variants of the Otto four-stroke cycle with the Miller and Atkinson style of engines. Those types of engines use a different valve timing and or forced induction to maintain the gains they provide.

FIGURE 21-3 The port fuel injector is positioned at the backside of the intake valve so that when the fuel is sprayed, it stays in the intake port. This helps to clean and cool the valve. This feature is lost on a GDI engine because the fuel is sprayed in the cylinder.

The GDI engine uses the same Otto four stroke, but there are some inherent features that set it apart from a conventional fuel-injected engine. To increase the power output of an engine, the compression ratio should be increased. Some GDI engines run around 11:1 to 14:1 in compression ratio. Increasing the compression ratio with fuel in the combustion chamber can lead to pre-ignition and/or detonation. As the temperature rises in the combustion chamber, the fuel that is in it can spontaneously combust before the proper time of the event. In a GDI engine, the fuel is not injected into the engine until the piston is near top dead center (TDC). To achieve this feat, the fuel injector is placed inside the combustion chamber so that when it sprays, it sprays directly on the top of the piston (FIGURE 21-4). Along with this relocation, the injection pressure is increased by the high-pressure injection pump so that when it comes out of the injector, it can be sprayed in a fine mist to increase efficient combustion. This type of fuel system process is very similar to a diesel engine’s common rail-injection operation, which allows the gasoline engine to increase power output with little extra fuel input. To overcome these high combustion chamber pressures, the GDI fuel-injection pump creates 500–3,000 psi (35–207 bar) on some models. This pressure is needed to properly inject, atomize, and control the fuel flow into combustion chamber. Fuel is supplied to the high-pressure pump by a low-pressure pump, which is usually mounted in the fuel tank. In some applications, the high-pressure pump acts as the lower pressure pump by using suction to move fuel from the tank to the high-pressure pump. The powertrain control module (PCM) must monitor the inlet pressure and the outlet pressure so that the engine is operating correctly to maximize performance. Along with the changes in fuel systems, the engine’s pistons must be designed to help with fuel dispersion once it is injected into the cylinder (FIGURE 21-5). A conventional piston is usually concerned with piston-to-valve interference, not with how the fuel is atomized within the cylinder. With GDI, the piston design plays an integral part of how the fuel is atomized within the cylinder, so it must be designed to maximize the fuel injection.