Exercise 3. Make up a dialogue of your own based on the one given below.

A.: What are some of the causes of loss of injection air pres­sure?

В.: Leaky, broken or stuck compressor valves, injection air valve sticking open, leaky spray air valve stem packing, leaky joints in air piping, loss of cooling water.

A.: How can a start be had if there is no compressor avail­able?

В.: Procure a bottle of CO₂ gas and connect it to the starting air line. This gas is non-explosive and safe to use for this purpose.

A.: What are some of the causes of failure of an engine to start firing after it begins to turn on air?

В.: Fuel pumps not discharging oil, incorrect timing of fuel valves, low compression, injection air pressure too low and cylin­ders too cold.

A.: What are some of the causes of cracked cylinder heads?

В.: Unequal heating, due to poor design, air pockets in jacket, insufficient cooling water and overloading.

A.: How does wear on main bearings cause crank shaft to break?

В.: By throwing the shaft out of line and causing it to be bent in alternate directions twice in every revolution.

A.: Give the valve timing for the average large, 4-cycle engine.

В.: Injection valve opens 5 degrees before top center, injection valve closes 49 degrees past top center, exhaust valve opens 140 degrees past top center, air inlet valve opens 15 degrees before top center, exhaust valve closes 10 degrees past top center, and air inlet closes 214 degrees past top center. When in use, the air starting valve opens at top center and closes at 128 degrees past top center.

A.: What is the most important valve timing?

В.: The time of opening of the injection valve. A small change in this time may seriously affect the operation of the engine.

A.: When setting valves how to determine when the injection valve opens?

В.: Keep moving the cam roller with the fingers until it makes contact hard enough to bind, turn injection air into the valve and listen for the hiss of air through the indicator cock when the valve starts to open, or use a micrometer dial indicator to note when the valve stem starts to lift.

Lesson 5. STARTING DIFFICULTIES

Failure of the engine to start on air may be due to one or more of the following causes: starting air pressure too low, incorrect timing of starting air valves, one or more starting air valves stuck, compression relief gear out of order, stop valve in air line closed, or, if starting after working on the engine, the jacking gear may still be engaged.

Operating troubles in general. Water in fuel oil. Water may get into the fuel oil by leakage through defective riveting of tanks, through alternate use of tanks for fuel oil and water ballast, or the fuel oil as delivered into the tanks may contain considerable moisture that will settle out. The degree of seriousness of the resulting troubles will vary with the amount of water in the oil, but among the possibilities are cracked heads and pistons, burned out exhaust valves, low compression due to wear of rings and liners, and irregular running of the engines.

Improperly refined oil. Fuel oil must, during the refining process, be treated with sulphuric acid and this acid must later be neutralized with soda. If this neutralizing agent is not thoroughly washed out of the oil, its presence cannot be detected without chemical analysis nor is it removed by the filtering and straining methods ordinarily used on shipboard. When the engine is opened up after running on this insufficiently washed oil the entire surface of the combustion spaces in the cylinders will be found to have a coating of gritty material which looks and feels like sand but which is mostly sodium sulphate. It does not cause any immediate derangement of the engine but it does cause considerable wear of piston rings and cylinder liners. When such deposits are found the only remedy is to stop using the fuel from that particular refinery.

Loss of power or slowing down of engine. When this occurs the first possibility that should be investigated is hot bearings. Other causes are failure of fuel to one or more cylinders, derangement of valves of valve gear or a fall in cooling water temperature. A decided reduction of the sea water temperature will often be experienced when entering a river from the sea, passing out of the Gulf Stream or leaving tropical waters. When this occurs the cooling water supply should be cut down until the jacket water temperature is up to normal. A distinct loss in efficiency sometimes results from the idea that the jacket water should always be kept as cold as possible.

Cracked cylinders and cylinder heads. Cracks may result from unequal heating due to poor design, bad castings, air pockets in jackets, lack of cooling water and overloading. Modern practice in design and construction has greatly reduced the probability of cracking from the first two causes and watchfulness on the part of the engineers eliminates the other causes, so that cracked heads and cylinders are not nearly so common as they were in the early days of Diesel engineering.

Cracked crank shafts. During the early development period cracked crank shafts were so common that it was considered one of the inevitable hazards associated with Diesel engine operation. This trouble has become very rare, however, in modern practice.

Vibration. The amount of vibration of an engine and of the ship's hull in which it is installed will depend on how well the reciprocating and rotating masses in the engine are balanced and the position of the engine relative to a nodal point in the hull. These are both matters over which the operating engineer has no control. Normally Diesel engines, especially in the large sizes, run with very little vibration but it sometimes happens that an engine will have a critical speed, a speed at which the twisting impulses imparted to the crank shaft by the pressure acting on the pistons coincide with the natural period of vibration of the crank shaft. When the engine operates at this speed violent vibration will occur that may result in loosening nuts, shaking off small fittings, breaking pipes and even breaking of the crank shaft, if continued long enough.