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What is Variable Injection Timing (VIT) - Theory & Practice

Dimitris Tsapis
January 8, 2022
What is Variable Injection Timing? Why do you need it? And most importantly, how can you use VIT to improve the performance of your ship's engine?

Variable injection timing is a process often linked with engine efficiency and fuel economy. But it’s much more than that. In this article, we look at the basics of VIT, how it works, and why you should know all about it. 

By the end of this post, every yacht engineer will at least ponder on the importance of VIT and become aware of its practical applications. Let’s delve in!


What is Variable Injection Timing?

Variable injection timing, often referred to as VIT, is a term that describes the process of adjusting the timing of starting fuel injection. More specifically, it refers to the mechanism which either advances or retards fuel into the injection pump. There are multiple ways to go about this process, and engineers should be familiar with the methods. Variable injection timing works by:

  • Adjusting the location of the barrel to better accommodate the plunger
  • Using a fuel pump that features two plungers instead of one
  • Changing the position of the fuel cam
  • Using specifically designed pumps (e.g. sulzer fuel pump)

Understanding the working principles of VIT

Engineers can choose to use VIT to achieve maximum combustion pressure in the engine (Pmax) at a lower load (±85%) to lower the consumption of fuel. This, in turn, makes the engine’s energy expenditure lower, and thus more efficient combustion.

The best way to compare the benefits of an engine with VIT is to compare it with the engine’s performance without VIT. As you can see in the diagram below, adjusting the injection timing of the fuel inside the delivery pump leads to an increased level of pressure within the engine.

variable injection timing principles

The red line indicates Pmax for VIT fuel pumps. The dotted red line shows the same pump’s performance without using VIT. The blue and orange lines indicate the VIT Index and Fuel Index respectively.

The exponential performance growth observed with variable injection timing starts approximately at an engine load of 40%. It is at this point that the fuel injection is gradually increasing the Pmax. As soon as the engine load reaches ±85%, the value of Pmax will correspond to 100% engine load, as seen above. Maximum combustion is thus attained at a partial load. This has several benefits which we discuss below. 

Why is Variable Injection Timing needed?

Variable injection timing is needed because:

  • Older engines (based on usage or built) tend to lose combustion pressure and temperature at a low load and speed. That leads to lower peak pressure, lower efficiency at low loads, and increased fuel consumption.
  • VIT can help engines achieve maximum combustion pressure during part-load operation (as described above) which adds to fuel economy, engine efficiency, and sustainability.
  • Maximum pressure can be sustained for longer periods of time due to the part-load, which does not overload the engine.

It is also important to look at VIT through the lens of the fuel injection system as a whole, the goal of which is to provide fuel to the engine’s cylinders. VIT (advancing or retarding an engine’s timing) determines how and when the fuel is delivered, which in turn affects the engine’s performance, sound, and emissions. 

For example, retarding the injection timing of fuel can help an engineer repair a smoking problem within the engine, or lower the high fuel cost over longer periods of time. 

Let’s delve a little further into the first point mentioned above: the age and build of your engine. The older your engine, the higher the chances you might need to adjust injection timing. Over long periods of time, the injection pump can present problems such as:

  • High temperatures in the engine
  • Higher fuel expenses
  • Smoke during acceleration and startup
  • Problems during startup

All these issues negatively affect the (long-term) performance of your engine. Therefore, by making the necessary adjustments in the fuel pump using variable injection timing, you can bring your engine’s performance back to its prime days and make sure it works well many years in the future.

VIT in practical application

In the previous chapters we illustrated how the engine responds to VIT-adjusted fuel usage, and the benefits it can offer. The actual steps you need to follow to make the necessary adjustments often differ depending on the engine, and it is thus difficult to give a one-size-fits-all response.

The best way to understand how VIT works for your vessel, and how to make adjustments depending on the situation, is by reading the manufacturer’s manuals. The manufacturer will offer advice on VIT depending on the make and model of your engine, help you determine the default settings, and improve your maintenance work through baseline performance metrics.

We have found a great resource that describes how VIT works, as well as the difference between VIT and super VIT. Keep in mind, however, that the narrator is not a native English speaker, which makes the video somewhat harder to understand:


VIT on ships - Points to keep in mind

If you watched the video above, you might have some questions. In this chapter, we answer some of them by pointing out a list of best practices. Marine engineers should be aware of these points to ensure the proper operation of engines with VIT-equipped fuel pumps.

Ensure moving parts are not blocked

The fuel injection pump works based on the movement of several components, such as the eccentric suction shaft and valves that control the spill. It is the job of the engineer to ensure that both components operate properly to avoid any breakdown. This is best done by scheduling recurring maintenance tasks.

Checking the load indicator

The engineer must manually compare the correspondence between the:

  • Position of the load indicator in the setting plate;
  • Value of the load indicator of the local maneuvering stand;
  • And the value of the load indicator at remote control with VIT set at 0.

If the engineer finds any deviation between the three values, it is important that they are first corrected before adjusting the fuel pump timing.

Examine the actuator setting & stroke

With VIT set at 0, check the actuator setting by placing a distance sleeve between the blocking unit and the regulating lever of the suction valve. Then, briefly move the variable injection timing to its highest advance and lowest retard level, noting down the values in the load indicator. Additionally, check the actuator stroke at the remote control system and note any deviations found (both for setting and stroke). The best way to determine a deviation is to compare your values with those recommended in the manufacturer’s manual.

Refitting VIT after maintenance

In order to refit the VIT after performing regular maintenance tasks, make sure to check the alignment and clearance between the linkage and the stop plate, while the cylinder is fully retracted. Until clearance is attained, the stop plate cannot engage in operation.

Maintenance of electrical connections

If your ship’s VIT operates electronically, make sure you regularly review all cable connections between the Variable Injection Timing terminal and the connecting box.

Checking the pneumatic cylinder

On certain occasions, the pneumatic cylinder may come with default mechanical stoppage which needs to be checked for hamming. This may come in handy in case you need to move the pneumatic cylinder manually (e.g. in case of failure of the automated positioning system).

Adjustments during the run-in period 

Make sure you disconnect or turn off the VIT when the diesel engine is in its running-in period. Such periods occur occasionally due to the replacement of several components found in the engine.

In case a unit is cut off from a running engine

In case you witness an operational main engine with one of the units cut off (for any reason), set the VIT to 0 or switch it off completely to avoid an uneven load distribution inside the engine.

Wrapping up

Variable Injection Timing (or VIT) is a process often utilized to improve the efficiency and performance of the ship’s engine, all while improving fuel economy. The chapters above should give you a basic idea of the processes you need to be aware of, as well as the practices you need to follow.

In short, VIT helps the engine achieve maximum combustion using a partial engine load. Advancing or retarding the fuel at engine startup can help you solve many issues that can affect the performance of your vessel in the long term. Hence, it is a process every engineer should familiarize themselves with.
If you wish to learn more about the process, consider reading the following study. The abstract of the document should give you a good idea about its contents.