Have you wondered, whats is the use for variable injection timing or “VIT” if we always learn and told to avoid early and late ignition ! Why am i asking this question? Variable injection timing is a process to alter ignition timing early or late for affecting engine performance.
We all know early ignition can cause heavy shock load with vibrations leading to engine failure and unnecessary maintenance. On other hand late ignition is like illness for the engine with high exhaust temperature, improper combustion, burning of exhaust valve & sheet, corrosion, shoot and increased pollution.
Knowing the consequence of early and late ignition; why did we imply variable ignition timing in almost all modern engines? We will discuss all such doubts, questions and query along with the operation, its working and use of variable injection timing. Each topic will be explained in details in the following paragraphs in this article.
Q. What is the need for change of Injection Timing?
Traditionally a fuel pump had a flat top; which means fuel is injected in the cylinder at same time irrespective of the load or demand. Modern diesel engine with constant ignition tends to show decrease in peak pressure and thermal efficiency while the power or demand is reduced. This lead to increased fuel consumption with poor engine performance at low load; say 40-60% of MCR.
So, variable injection timing is introduced / installed in the system to alter barrel position with respect to the plunger to obtain maximum peak pressure ( Pmax ) at load as low as 70% of MCR ( Maximum Continuous Rating ). Fuel timing is advanced gradually from 40% of MCR up to 85% of MCR; it is then retarded upto 100% of MCR to maintain ( Pmax ) Constant.
It is being found that by applying ( VIT ) in an engine; one can save up to 5 to 6 gallon of fuel per hour. apart from that, it also helps in proper combustion of different grades of fuel in the engine units.
Bosch Jerk Pump
A conventional marine diesel engine uses bosch jerk pump for fuel ignition. It contains a single acting – cam operated plunger with a helical spring fitted on top for downward movement. The helical spring ensures there is constant contact in-between plunger and its follower with the cam all times.
The plunger is fitted inside the barrel with a helix machined onto it. Both suction and spill ports get open when the plunger moves downward; filling the barrel with the fuel. Now as the plunger starts to move upward; pressure is created once both the parts are closed / covered by the plunger. Fuel injector start to inject fuel into the cylinder at this pressure until the plunger again moves downward opening the suction and spill port.
The quantity of fuel per injection can be altered by rotating the barrel to change the angle of helix with respect to the parts in barrel. The turning movement of the plunger in barrel is achieved by using rack and pinion arrangement. Additional spring loaded non return valve are fitted inside the pump for positive sealing and reduced corrosion.
The injecting timing for a cylinder is altered by changing the effective height of the plunger with respect to the barrel. This can be done either by moving the plunger itself or the barrel in respect. Raising the plunger in respect to the barrel leads to advance injection; while lowering the plunger cause retardation. Advancing and retarding injection timing can effect the engine performance in many ways which you can learn in the below topic.
Effects of Altering Injection Timing
A. Result of advancing fuel injection in a system:
- Gradual Rise in Peak / Maximum pressure.
- Increased fuel efficiency by better specific fuel consumption.
- Decreased Exhaust Temperature
- Improved Thermal Efficiency
- Uneven vibration and shock load
B. Result of retarding fuel injection in a system:
- High / Extremely dangerous exhaust temperature.
- Decreased Peak Pressure.
- Low thermal efficiency
- Heavy corrosion
- After burning
Working Principle of Variable Injection Timing ( VIT )
Injection timing of fuel injector in a marine diesel engine is adjusted by changing relative height of the plunger with respect to spill and suction ports. Injection timing can be retarded by lowering the plunger; while raising results in advance in injection timing. This alteration in plunger position can be done either by moving cam with respect to the shaft or turning cam to its ahead or retarded position on the camshaft.
Employing VIT; We can ensure to get peak pressure from 85% of MCR to 100% of MCR increasing thermal and fuel efficiency of the engine. This in turn positively affects the overall efficiency of the plant or ship. The above image shows the relation of changing peak pressure for the fuel pump under constant and variable injection timing.
From the diagram you can see a gradual increase in the peak pressure from 40% of MCR; reaching the level of maximum peak pressure at 85% of MCR. A straight line thereafter represents the maximum peak pressure of the unit maintained in between 85% of MCR and 100% of MCR. This constant peak pressure is obtained by retarding the injection timing to just maintain the constant maximum peak pressure.
In sulzer RTA engines; variable injection timing is obtained by changing the position of eccentric pivot for the suction and spill ports. This affects the opening and closing of these ports thus achieving the variable injection in the engine.
#NOTE: This Article is produced on special request from ” Rahul Bansal ” on April 28, 2018 7:50 PM. I will look forward for your helpful comment and recommendations to improve this Article ( What’s the need to change Injection Timing? – Variable Injection Timing ).
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