Class 4 (4th Engineer) MEKM 📅 Jun 2023

Exam Question

(a) Describe VIT of any 2-stroke marine diesel engine by Diagrammatic sketch, naming each component and its function. (8)

(b) Describe following;

(i) What is Effective stroke and Idle stroke of a Fuel Pump? (2)

(ii) What is fuel pump Lead? (2)

(iii) What is Super VIT? (2)

(iv) What is breakpoint for VIT? (2)

Reference Answer

### (a) Variable Injection Timing (VIT) System
#### 1. Purpose of VIT
Variable Injection Timing (VIT) is a mechanism designed to optimize the combustion process in large two-stroke marine diesel engines across varying loads. The primary objectives are:
* **Maintain Constant Maximum Pressure (Pmax)**: At part loads (typically below 85% MCR), the compression pressure and temperature are lower than at full load. This leads to a longer ignition delay period for the injected fuel. Without correction, combustion would occur later in the expansion stroke, resulting in a lower Pmax, reduced thermal efficiency, and consequently, higher Specific Fuel Oil Consumption (SFOC).

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