What is tappet clearance, and why it is required ?

It is clearance between the valve stem and the rocker arm contact point.

This clearance is required to accommodate the thermal expansion of the running gear between the camshaft to the valve and to ensure positive closing of the valve at all working condition and temperature.

Often the requirement of tappet clearance is answered as, to maintain correct timing or change in clearance will lead to an early or delayed opening of the inlet or exhaust valves. Even though these are indirect effect of change in tappet clearance value and the one line answer should be to accommodate thermal expansion and ensure positive closing.

Excessive clearance leads to
1. Valve opens late
2. Closes early leading to decrement in scavenging efficiency.
3. Mechanical constraints like damage, noise and impact on the working surface.

Minimal clearance will lead to
1. Open early and closes late
2. Leaking of valve leading to burning of the valve seat and other constraints.
3. Compression pressure will also decrease due to leakage
4. Poor combustion and reduced engine power output.

Clearance should be monitored regularly using feller gauge for excessive or insufficient clearance

Comments

Bilge Injection Valve

Bilge Injection is a valve that enables the engine room bilges to be pumped out directly overboard in the event of an emergency such as flooding. The valve is normally fitted to the end of a branch connection with the main sea water suction line. This enables large main seawater cooling pumps to be used as a bilge pump in an emergency. Emergencies like fire and flooding involve the use of seawater. If there is a fire, seawater is the biggest resource of water available in the sea. Similarly, if it involves flooding of the engine room, cargo spaces or any other place on the ship for that matter; you would again require pumping the sea water out of the ship. In both these cases, you require pumps. There are two valves in close proximity namely main injection valve and bilge injection valve. Both of them have their own independent controls. The diameter of the bilge injection valve is kept nearly 66% of the main valve diameter which draws water directly from the sea th...

Difference Between A, B & C-Class Divisions?

IMO Symbol A Class Division IMO Symbol B Class Division SOLAS has tables for structural fire protection requirement of bulkheads and decks. The requirements depend on the spaces in question and are different for passenger ships and cargo ships. The Administration has required a test of a prototype bulkhead or deck in accordance with the Fire Test Procedures Code to ensure that it meets the above requirements for integrity and temperature rise. Types of Divisions: "A" Class "B" Class "C" Class "A" Class: "A" class divisions are those divisions formed by bulkheads and decks which comply with the following criteria: They are constructed of steel or equivalent material They are suitably stiffened They are constructed as to be capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test. they are insulated with approved non-combustible materials such that the average tempera...

Why do we blow through engine before starting ?

The engine is blown through on air before starting it : a) To Blow out through indicator cock any residual exhaust gas or other products of combustion trapped inside the cylinder after shutting the engine. b) To check if any jacket cooling water from cylinder head or turbocharger or any other source, has leaked while the engine was shut and collected on top of piston. If while blowing through, water comes out of indicator cock or relief valve, we need to investigate and rectify the fault before starting the engine. It is imperative that the engine is blown through before starting. There have been cases where the generator engine has been started from control room without bothering to blow th...

Marine Engineering

Search This Blog