Turbocharging Technology and EGR In recent years, to enhance the thermal efficiency and power output of diesel engines, high-pressure direct injection has become a common practice. This technique allows for higher torque at lower engine speeds, making it more suitable for small-displacement engines compared to gasoline counterparts. The improved fuel economy during low-speed cruising and the ability to maintain performance at high speeds make turbocharged diesel engines an attractive option. Variable geometry turbochargers (VGT) help achieve a flatter torque curve, reduce black smoke at low speeds, and improve acceleration. Additionally, some manufacturers have introduced dual-stage turbocharging systems to address air intake limitations during rapid acceleration. Looking ahead, increasing both injection pressure and EGR rates will be crucial for meeting stricter emission standards. EGR remains one of the most effective methods for reducing NOx emissions, and its importance is expected to grow in the future.

Common-Rail Injection Technology Among the latest advancements in diesel technology, the common-rail injection system stands out as one of the most effective. It has seen significant improvements in maximum injection pressure and the shortest injection intervals. Fuel is injected in amounts smaller than a single drop of eye drops, reaching pressures of nearly 2,000 atmospheres. This precise and ultra-fast injection process has revolutionized diesel engine performance. Recently, the actuation of injectors has shifted from electromagnetic to piezoelectric type, doubling the responsiveness of control valves. In the near future, injectors may feature a booster piston that further increases injection pressure, potentially exceeding 2,000 atmospheres. These developments are set to push the boundaries of diesel engine efficiency and performance.

Engine Combustion Technology Uniform premixed compression ignition (HCCI) has been researched for over a decade but still faces challenges in high-load applications. It is mainly used in low-load conditions where exhaust gas temperatures are low and aftertreatment systems are less effective. As a result, HCCI is often considered a dual-mode operation—combining traditional diesel combustion at high loads with a very low load near idle. This transition can also be managed through common-rail systems. By adjusting the injection timing closer to top dead center, even if the mixture isn't perfectly uniform, the combustion process is more accurately described as "premixed combustion" rather than HCCI. Moreover, in low-load operations, even with EGR limiting NOx emissions, particulate matter (PM) levels remain low, giving this mode an advantage over unburned hydrocarbons and carbon monoxide emissions.

Exhaust After-Treatment Technology While technologies like EGR and common-rail injection play a role in reducing emissions, they alone are not sufficient to meet Japan’s new long-term emissions standards for both NOx and PM. Exhaust after-treatment systems will take on a more critical role. Particulate matter is addressed using Diesel Particulate Filters (DPF), while Selective Catalytic Reduction (SCR) or Nitrogen Storage Reactor (NSR) technologies are being developed to reduce NOx emissions effectively. These systems are essential for achieving compliance with increasingly stringent environmental regulations.

Diesel Engine Hybrids Although most city vehicles currently use gasoline engines, the growing need for better fuel efficiency has made diesel hybrid vehicles an appealing solution. The latest diesel hybrid buses have demonstrated remarkable improvements in fuel consumption and emissions reduction. There is strong potential for similar benefits to be applied to passenger cars in the near future, making diesel hybrids a promising direction for sustainable transportation.

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