Question

Why does diesel engine initiate 2 stage fuel injection?

What is the reason for that?

Answer 1

Main purpose of the fuel injection system is to deliver fuel into the cylinders of an engine =

1. Fuel must be injected at the proper time, that is, the injection timing must be controlled

2. The correct amount of fuel must be delivered to meet power requirement, that is, injection metering must be controlled.

3. fuel atomization and vapourisation.

While conventional fuel injection systems employ a single injection event for every engine cycle, newer systems can use multiple injection events.The main injection event provides the bulk of the fuel for the engine cycle. One or more injections before the main injection, pre-injections, provide a small amount of fuel before the main injection event. Pre-injections can also be referred to as pilot injection. Some refer to a pre-injection that occurs a relatively long time before the main injection as a pilot and one that occurs a relatively short time before the main injection as a pre-injection. Injections after the main injections, post-injections, can occur immediately after the main injection (close post-injection) or a relatively long time after the main injection (late post-injection). Post-injections are sometimes called after-injections. While there is considerable variation in terminology, a close post-injection will be referred to as a post-injection and a late post-injection as an after-injection.

The term split injection is occasionally used to refer to multiple injection strategies where a main injection is split into two smaller injections of approximately equal size or into a smaller pre-injection followed by a main injection.

Unintended post-injections can occur in some fuel injection systems when the nozzle momentarily re-opens after closing. These are sometimes referred to as secondary injections.

Due to Doble injection =

• Different injection strategies produce different mean cylinder pressures. Larger first injection and higher fuel injection pressure increase mean cylinder pressure and engine efficiency.
• Larger first injection results in an increase in engine load and a decrease in exhaust gas temperature.
• Late main injection results in a decrease in engine load and an increase in exhaust gas temperature.
• When the interval between injections is longer than 2.5ms, exhaust gas temperature decreases. This result might be connected with incomplete combustion and higher exhaust CO.
• Increase in engine speed results in an increase in exhaust gas temperature and an increase in engine load. At higher engine speed, the location of the main injection has a more significant effect on engine load.
• Higher fuel injection pressure results in a slight increase in engine load and a slight decrease in exhaust gas temperature

This study investigated double injection in diesel engines as a tool for gaining better control of combustion and to decrease combustion emissions. With this system, a variety of injection strategies under low to medium loads with different engine speeds and different fuel injection pressures were tested and characterized. The main findings and observations from the experiments are:

• Exhaust NOx is correlated with the location of the second injection.
• Exhaust soot formation is correlated with the location of the second injection.
• Exhaust HC is correlated with the location of the second injection.
• Exhaust CO is correlated with the location of the first injection.
• When time between injections is shorter then 1.4 ms, the injections combust together (ignition delay time of the second injection is very small).
• When time between injections is longer than 1.4 ms, each injection ignites and combusts independently.
• When time between injections is larger than 1.4 ms, NOx and soot emissions decrease and CO and HC increase with light load. With medium load, CO and HC shows no change. However, when time between injections is longer than 2 ms under medium loads, CO and HC increase.
• Very early first injection (time between injections larger than 2 ms) shows a decrease in exhaust NOx, a decrease in exhaust soot, and an increase in CO and HC. Under these conditions an assumption was made regarding partial combustion of the first injection which results in high CO concentration at the end of the first combustion. This CO affects the second combustion as a radical inhibitor and increases ignition delay time. This may increase mixing and push the second combustion closer to premix combustion, which has lower tendency to create soot. This result suggests that proper multiple injection strategy may affect the kinetics of combustion and create different derivatives of the fuel, which may change combustion chemistry and improve combustion control.

Two stage Inection maintain combustion with Good Engine performance and Efficiency and minimum amount of exhaust soot, due to the fact that emission exhaust after-treatment represents the greater challenge.