Questions and Answers on Starting Systems

Below are few conceptual questions and answers on starting systems.

Q1) What is starting of an engine?  

Ans- Starting of an engine is the process of overcoming the rest inertia of the engine and bringing the engine to a stage where it gains the ability to burn fuel(Firing RPM). For CI engines, Firing RPM is the minimum rpm at which when the engine is run, the air is compressed sufficiently to achieve SIT of fuel used.

Q2) What is the scope during starting an engine?

Ans- The scope is to start engine on no load or minimum load.

Q3) What are the checks before starting an engine?

Ans- Engine Availability:-

     – Do physical inspection of the engine. Check for leaks in pipelines, any physical damage, etc.

     – Check L.O parameters. Make sure there is sufficient L.O in L.O Tank.

     – Do the above step for Fuel oil.

     – Make sure the filters and strainers are in good condition.

     – Ensure the free rotation of the engine crankshaft by turning gear.

     – Check if there is any moisture/oil content in the cylinder unit.

     – Check if the engine is preheated to avoid excess thermal loading when starting.

     Other Machinery Availability:-

     – Check the propeller clearance.

     – Check all the ancillary equipments required for engine operation is ready.

     Command:-

     – Make sure the bridge has given command to start the engine.

Q4) What does starting an engine initially require?

Ans- The starting of engine initially requires a stored energy source to overcome the rest inertia of engine.

The sources can be:-

     – Battery Operated Motor

     – Compressed Air

     – Manual power(Decompressing valve to be used)

Q5) What are the threshold values of cranking?

Ans- The minimum threshold is the firing rpm putting the engine to a stage where it can burn fuel. The upper threshold is governed by the scope of cranking. The rpm should be low enough to prevent the loading of the engine. A typical example of a 4 stroke engine with an idling rpm of 720 is 200.

Q6) How do you achieve cranking?

Ans- We achieve cranking by using a stored energy source to overcome rest inertia. The engine crankshaft is rotated by stored energy source, either by rotating the crankshaft directly or by initiating reciprocating motion of Piston.

Q7) State the methods of cranking?

Ans- Battery Operated Motor, Compressed Air Injection, Manual power(Turning the engine c/shaft by hand power after using the decompressing device).

Q8) What is reserved energy source?

Ans- The reserved energy source depends on the type of system used which of-course is dependent on the type of engine and its cranking requirements.

Q9) How is reserved energy source restored?

Ans-        

Q10) How is cranking executed for most energy sources?

Ans- In most of the systems, the starting mechanism directly rotates the crankshaft.

The battery-operated motor is coupled to the engine crankshaft via a drive mechanism engaging with the flywheel (when the signal is applied). The Motor rotates the crankshaft leading to the engine starting. After starting the engine, the motor shaft gets decoupled and the motor stops (due to no start signal by the user). (Due to the return spring arrangement)

In the case of manual starting, the crankshaft of the engine is rotated by manual arm power. The decompressing valve is operated and the crankshaft is rotated. Upon reaching a good enough speed for burning fuel, the decompressing device is shut and the engine fires on fuel and starts.

In the case of Compressed Air Injection, Pressurized air is sent to individual units as per the firing order. The force acting on the top of the piston due to air pressure breaks the inertia and moves the piston which further rotates the crankshaft. Upon achieving firing rpm, fuel is injected and the engine operates on fuel. Starting Air is then shut.

Q11) How is compressed air source used for cranking?

Ans- Air is compressed by reciprocating compressors and stored in air bottles. When starting signal is applied, the compressed air flows in the main air start valve, pilot valve, starting air manifold, starting air distributor. Turning gear interlock valve is a safety device which allows compressed air to flow in the line only when the turning gear is disengaged to prevent engine starting on turning gear. The starting air distributor is responsible for the timing of starting air injection. When the pilot valve of any unit is operated by starting air cam, the pilot air signal is sent to main air start valve leading to its actuation. The main start air valve operates and compressed starting air gushes in the combustion chamber pushing the piston down and breaking the inertia. After some time, when firing rpm is achieved, fuel is injected and combustion of fuel takes place. Engine is now considered as started. Starting Air is then shut.

Q12) State the advantage of air motor cranking over air injection cranking?

Ans- Advantages are:-

     – Simpler Controls

     – Normally used for unidirectional engines.

     – No chances of starting air line explosion

     – No main air start valves required i.e. Increased strength of Cylinder head due to reduced holes

     – No starting air distributor required for administering the starting air injection timings

– Normally employed for low powered engines.

– Engine can be cranked even if one cylinder is out of use, without any concerns

Q13) State the advantage of air injection cranking over the rest?

Ans- Less torsional loading on the crankshaft. Easy to execute reversing

Q14) Why a starting air distributor is an integral part of a cam operated engine?

Ans-

The spindle of pilot valves rest on the starting air cam contour surface when starting signal is applied. As the starting air cam rotates, the respective pilot valve operates and sends pilot air to the respective main air start valves. The sequence in which the pilot air actuates the main air start valve is according to the engine firing order. This sequence is imbibed in starting air system by the arrangement of starting air cam and pilot valves mounted on it.

So, Starting air cam and pilot valve which collectively is called starting air distributor, is responsible for starting engine in firing order.

Q15) What is the logic of air injection starting?

Ans- To identify the piston which is past TDC, admit air to it, Continue to admit air to pistons past TDC in sequence to create a momentum of the crankshaft. The air injection system also ensures admitting air to consecutive units in firing order to accelerate the momentum. This is known as overlap. In case one unit piston is missing the engine can still crank.

Q16) What are the safe guards in a starting air system?

Ans- Turning Gear Interlock:- This prevents the air from entering the starting air line if the turning gear is engaged. This is to prevent engine starting on turning gear.

Flame Trap/Arrester :- It is a set of copper tubes which extracts the heat from the leaking combustion gases(if any, due to leaky air start valve) thus mitigating chances of starting air explosions.

Relief valve:- Relieves the components from over pressure.

Starting Synchro:- Starts the engine only when the bridge signal matches the ECR signal.

Q17) Explain a starting air system?

Ans-

(Refer Diagram) Air is compressed by reciprocating compressors and stored in air bottles. The isolating valve needs to be opened in order to allow compressed air to enter the system. Now, the air is blocked by the Turning gear interlock valve. This valve allows the air to flow further only if the turning gear is disengaged. Now, on the application of start signal, this air accumulates on top of the pilot valve leading to engagement of the pilot valve spindle and starting air cam contour. Additionally, this air actuates the automatic valve and the compressed air is available to the pilot valve and starting air manifold. There is a non-return valve to prevent backflow. When at rest, there is at least one pilot valve that is in open condition. The pilot air from this valve is sent to the respective main starting air valve leading to actuation. The engine crankshaft starts rotating and so does the starting air cam. The other pilot valves operate and send pilot air signal to the respective main air start valve. The sequence of sending pilot air to respective valves is governed by the firing order of the engine. The engine picks up rpm gradually. Once it reaches firing rpm, fuel is injected and the combustion of fuel takes place. The engine is now in started condition. The starting air is now shut. The air bottles are charged by the compressors onboard. Flame trap and relief valve are some safety devices inculcated in the design.

Q18) What is the predicted faults in a starting air system?

Ans- Case 1/Engine fails to start on application of start signal:

     Although this can be due to various reasons, but here we will confine to reasons pertaining to starting air system.

     – Turning Gear engaged. This shuts the turning gear interlock valve.

     – Air bottle pressure low. The low pressure air may be incapable of starting the engine.

     – Spindle of pilot valve or main air start valve is stuck.

     – Safety intervention of Starting synchro. The synchro won’t allow the engine to start if the ECR signal is not in unison with Bridge signal.

     – If two or more air start valves are blanked (because they were leaky), then the engine won’t be able to start in a particular crankangle range.

     Case 2/Others:

     – A leaky main start air valve will make the pipe (connecting the start air manifold and main start air valve) red hot.

     – Leaky Start Air manifold will lead to reduced air pressure available for starting engine.

     – Negative change in pressure setting of Relief valve(may be accidently) will prevent engine from starting.

That’s all for this post. Please share your queries, comments, suggestions, corrections, etc. below in the comments. We will be happy to read them.

Authored By:- Cdt. Abhishek Kumar Ojha, TMI