Because diesel engines operate differently from gasoline engines in terms of combustion and emissions, many new technical terms are introduced when discussing diesel vehicles. Models like the Jetta SDI and Bora 1.9 TDI exemplify this trend. These names reflect the specific technologies used in their engines.
SDI stands for "Spark Ignition" (though it's actually a direct injection system without turbocharging). It refers to a naturally aspirated direct injection diesel engine that uses Bosch’s electronic control fuel distribution pump. For instance, the FAW-Volkswagen Jetta 1.9GDF is equipped with a 1.9-liter SDI engine featuring a compression ratio of 19.00. This engine meets Euro III emission standards and achieves a fuel efficiency of 4.6 km per liter at a speed of 90 km/h. In real-world use, drivers typically consume less than 5.5 liters per 100 km.
TDI, on the other hand, stands for "Turbocharged Direct Injection." Compared to SDI, TDI incorporates a turbocharger and uses Bosch’s pump nozzle technology, which allows for much higher injection pressures—up to 1870 bar. An example is the FAW-Volkswagen Polaris 1.9 TDI engine, which also has a compression ratio of 19.00. It features a variable vane turbocharger that can adjust boost pressure at any speed, significantly improving performance over traditional turbo systems. The engine meets Euro III emission standards, consuming about 5.5 liters per 100 km at a constant speed of 90 km/h, while actual consumption by users is usually under 6.3 liters per 100 km.
Pump nozzle injection technology increases the injection pressure, allowing for a more complete mixture of diesel and air, resulting in more thorough combustion. Additionally, the use of an oxidation-type catalytic converter helps reduce carbon monoxide and nitrogen oxide emissions. However, the high-pressure fuel lines experience fluctuating pressure depending on the vehicle's speed, which can lead to engine vibration and increased noise levels.
The high-pressure common rail injection system represents a newer approach, combining direct injection, pre-injection, and electronic control technologies. Unlike earlier systems, it separates the injection pressure from the injection process, allowing precise control of fuel pressure via a common rail. This means the pressure in the high-pressure lines is independent of the engine’s speed. As a result, the system improves fuel combustion, reduces harmful emissions, minimizes noise, and enhances cold start performance. However, the fuel injection pressure in common rail systems is generally lower than in pump nozzle systems, typically reaching around 1600 bar.
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