Research
Humans have been highly dependent on Internal Combustion Engines in the fields of transport and energy generation since their inception. Over the past century, these marvelous machines have developed on all fronts, especially their performance and emissions. In the past few years, pollution from these engines has attracted a lot of heat, especially the soot and NOx pollution from diesel engines. The main challenge in reducing these emissions in conventional combustion methods is the difference in the formation chemistry of these two pollutants, which doesn't allow us to reduce one without enhancing the other.Â
The present research in this field is mainly focused on developing advanced combustion technologies that are cleaner and more efficient. Low-Temperature Combustion is an idea to burn the fuel so that the mixtures are lean and in-cylinder temperatures are low. This is hard to control as this concept gives rise to excessive heat release, knock, and combustion instability.
Prominently 4 different LTC strategies have been researched. Homogeneous Charge Compression Ignition (HCCI), Partially Premixed Compression Ignition (PCCI) and Reactivity Control Compression Ignition (RCCI) and Gasoline Compression Ignition (GCI). All these strategies have positives and negatives but the most important tool that an engine researcher or tuner has is the control of fuel injection parameters. The parameters include injection pressure, timing, quantity, injector design (nozzle size, number etc.), Injection strategy (single, dual multiple etc.) and also fuel properties (which can be altered when using alternate fuels, blends etc.). But as we all know it is close to impossible to see and understand what is happening inside a running engine, researchers have come up with a Constant Volume Combustion Chamber (CVCC) where engine conditions can be generated and maintained. These CVCC's have large optical access allowing us to see the spray propagation, it's mixing with air and the subsequent ignition and combustion.
In the CVCC, the required high ambient pressure and temperature conditions are generated by pre-combustion. Here lean fuel-air mixture is made from high-pressure cylinders. Required mixture composition is achieved by governing the partial pressures of the constituents. Once proper mixing is achieved, a spark plug ignites the mixture (premixed combustion). Initially, a steep rise of pressure in the chamber is observed. As the pressure drops in the combustion chamber and reaches the required ambient pressure, fuel spray to be investigated is injected. A quartz window in the chamber provides optical access for the optical diagnostic techniques like Schlieren, Mie combustion imaging etc.
Schlieren technique works on the same principle. When a light beam passes through the test section, the denser part deflects the light beam shifting the image in the image plane. The Schlieren method measures the first derivative of the density of the fluid and uses a knife-edge for cut-off, as shown in Fig. The position of the knife-edge affects the image on the screen, and it is usually positioned perpendicular to the nozzle, so that the density variation along the length of the spray is visualized.
Mie-Scattering is a popular method to visualize the liquid droplets of a fuel spray using the principle of light scattering. Mie-Scattering is observed when a photon incident on a particle/droplet is having a diameter similar to or greater than the wavelength of the incident light photon. The intensity of scattering is dependent on the size of the particle/droplet and the wavelength and frequency of the incident beam. The test area is illuminated using a High-Intensity Discharge (HID) lamp, and the images are taken using a high-speed camera. It is important to note that the camera is positioned perpendicular to the incoming light beam. Mie-Scattering signal is proportional to the square of the particle/droplet diameter. Interference problem arises in dense regions and hence is used at a distance from the nozzle end. Mie-Scattering is usually used along with Schlieren because it cannot visualize spray vapors. It is a powerful tool to visualize the liquid droplet in a dense fuel spray.
