2 stroke Engines

1.      Explain how a two stroke engine works?

A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft (an up stroke and a down stroke of the piston, compared to twice that number for a four-stroke engine).

2.      What is happening below and above the piston?

In a 2-stroke gasoline engine the events occurring below the piston are: intake of the mixture into the crankcase and its pre-compression. The events occurring above the piston are: compression of the mixture, combustion, and scavenging of exhaust gases.

3.      Explain what is meant, by scavenging when applied to 2 stroke SI engines?

In two stroke engine, each outward stroke of the piston is a power stroke. To achieve the operating cycle, the fresh charge must be supplied to the engine cylinder at a high- enough pressure to displace the burned gases from the previous cycle. Raising the pressure of the intake mixture is done in a separate pump or blower or compressor. The operation of clearing the cylinder of burned gases and filling it with fresh mixture (or air) – the combined intake and exhaust process – is called Scavenging.

4.      Why must roller and ball race bearings be used on 2 stroke SI engine crankshafts?

The two-stroke engine requires a needle roller bearing because the big end has less lubricating oil due to the structure. The needle rollers held in the retainer are inserted into the big end and run on the outer raceway of the big end and the inner raceway of the crankpin. The roller itself receives high stress and also exerts high  stress on the rolling surfaces.

5.      Why are rollers caged in crankshaft bearings?

Cages support high centrifugal and acceleration forces and are suitable for high speeds. They give bearing arrangements with high run out accuracy.

6.      Why must ‘split’ type bearings be used on one piece crankshafts?

A split (two halves) bearing is used on one piece crankshafts because the bearing must be assembled around the journal with the bearing housing being of two parts also, including a cap that holds the assembly together.

7.      Why do we have pins between the piston ring gaps on a 2 stroke?

We have a gap and pin at the centre because when the piston expands and contracts, the pin is used to align the ring at the centre

8.      How does a reed valve work on a two stroke?

Reed valve acts as a one way valve .It lets the fuel and air mixture into the crank case because of the low atmospheric pressure. As the piston travels down, the air pushes the reed valve to close acting as a one way valve.

9.      How do you check a reed valve? 

Remove the reed cage and examine the valves carefully exposing to the sunlight. If the corners are chipped or cracked, then they need to be replaced.

10.  Explain the term ovality and taper and how are they formed?

Ovality: It is a condition where circular object is deformed to an oval shape. Usually, ovality occurs on the Thrust side of a piston.

Taper: Taper means cylinder will be wide enough to accept both piston and rings. As you push piston upward, decreasing taper will squeeze rings to fit cylinder

11.  How does the piston ring seal in the bore?

Because of the gap in the piston ring, the piston ring expands and contracts during the power stroke and the exhaust stroke. During the expansion it can seal in the bore.

12.  What is meant by the terms groove depth, side clearance and end gap on the pistons rings?

Groove Depth: The distance between the cylinder walls with the piston centred and the back of the groove of the ring is known as Groove Depth.

Side Clearance: Clearance between the ring and the ring groove is known as Side Clearance.

End Gap: When fitting new piston rings or breaking them in within an engine, the end gap is a crucial measurement. In order that a ring may be fitted into the "grooves" of the piston, it is not continuous but is broken at one point on its circumference

13.  Explain the difference between a 2 stroke and a 4 stroke engine?

2 stroke engine

4 stroke engine

In a 2-stroke engine, the last three strokes are combined into one. 1.      Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel.) 2.      Power Stroke. The fuel is ignited and drives the piston down. As the piston goes down it not only turns the crankshaft but also pressurizes the fuel/air mixture in the crankcase which is about to be admitted to the piston for the next cycle. Near the bottom of the stroke, an outlet opens and the exhaust gases are released. Even nearer the bottom of the stroke, an inlet opens and new fuel/air rushes in ready for compression.

In a 4-stroke engine the sequence can be written: 1.      Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel). 2.      Power Stroke. The fuel is ignited (by a spark in a gas engine, by being injected into high temperature air in a diesel.) The energy released drives the piston down. This provides the momentum necessary to keep the crankshaft turning and make the other three strokes happen. 3.      Exhaust Stroke. The piston goes up and pushes the burned gases out the exhaust valve. 4.      Intake Stroke. The piston goes down and draws in new air, or fuel/air mixture, ready for the next compression stroke.

14.  What is meant by each of the following terms:

(a)   Mechanical Efficiency

It is the performance of the machine and it is known as a useful power output divided by the total consumed electrical power.

(b)   Thermal Efficiency

It is reciprocal to the actual heat energy stored within the fuel and the power produced in the engine.

(c)    Volumetric Efficiency

It is the percentage of the quantity of the fuel and air which enters the cylinder during induction to the actual capacity of the cylinder under static conditions.

15.  List 2 methods of lubricating the internal parts of 2 stroke S.I. engines?

2 stroke S.I. engines have a dry slump (except diesel).Henceforth, right amount of oil is needed to be mixed according to the specifications of the manufacturer.

OR

Another method is by premixing gasoline with oil.

16.  What happens if the piston gap is too big?

If the piston gap is too big, it can cause a leak in compression resulting lack of power.

17.  What happens if the piston gap is too small?

If the piston gap is too small, it can cease the piston during expansion and contraction or might result in breakage of the piston rings.

18.  How does a 2 stroke diesel engine work? 

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cooling systems

Explain the terms ( Kinetic energy) Conduction,convection and radiation?

CONDUCTION:

Conduction happens between objects at different temperatures which are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature.

CONVECTION:

Convection is the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas.

RADIATION:

Coolant heated from flowing through the engine is fed into the header of the radiator via the inlet and then cools down as it circulates through the tubes to opposite header and cold coolant exits back into the engine via the outlet, and this cycle is repeated. As it circulates through the tubes, the coolant transfers its heat to the tubes which, in turn, transfer the heat to the fins that are present between each row of tubes. The fins then radiate the heat transferred by the tubes to the surrounding air, the process is radiation.

What are the the problems you get with indect and direct cooling?

Indirect cooling

Marine engines use an enclosed cooling system. On the top of the engine there is a small tank that uses both, fresh water and coolant. This fresh water is circulated through the engine and through a heat exchanger. The fresh water, in this system, absorbs the heat of the engine. Raw water is still drawn up through the seacock but only flows through the heat exchanger jacket. This cooler raw water absorbs the heat from the fresh water through the heat exchanger jacket and is then pumped out the exhaust.

Direct cooling

Direct cooling uses a raw water cooling system. It can be fresh or salt water. The waterdrawn through a seacock fitting into the engine , and pumping it through the engine's water jacket and ports by way of a mechanical water pump. The water flows through the engine and directly out the exhaust. This cooler water absorbs heat from the engine to help keep it cool.

What is the puppose of a thermostat?

Thermostat : Thermostat is a device which regulates the flow of water after reaching to the ideal temparature required for the smooth function of engine.Thermostat opens at specified temp acting as one way valve.

Pressure cap

When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled

Why is engines fitted with a core plug, and what is the purpose of a core plug?

Core plug: Core plug is also known as a welsh plug or cold plug. It is designed to take out the sand which the manufacturer's use to mould the block and also in to take out snow in frost conditions.

What is the purpose of a heat exchanger?How does the heat exchanger work?What is the core of the heat exchanger made of?

Fresh water cooling systems, also known as a closed cooling systems, come in several varieties. The most common type utilizes a Heat Exchanger which functions similarly to the radiator in a car. Coolant (antifreeze) is circulated through one side of the heat exchanger where it is cooled by raw water that passes through the other side of the heat exchanger. The engine coolant is then circulated back into the engine. The raw water is expelled out of the boat through the exhaust.

How does the centrifugal circulating water pump work?

A centrifugal pump is a rotodynamic pump that uses a rotating impeller to create flow by the addition of energy to a fluid

How does a raw water pump work?

Raw water cooling systems draw water from outside the boat (seawater or lake water). Water is pumped from the source to the engine block then the engine circulation forces the raw water through the engine block and the water is expelled through the exhaust. Raw water cooling systems are relatively simple and the standard cooling system on most Marine Engines. The raw water pump in most cases is inside the outdrive. On larger engines and inboard engines the raw water pump is located inside the boat and is driven by a v-belt or directly off of the crankshaft. The danger is using salt water as a coolant in your engine. Salt water can be highly corrosive. Running salt water through your engine block and exhaust manifolds will lead to destructive corrosion that is unseen until your engine or exhaust manifolds fail.
marine engines cooled with raw water, especially ones that use salt water, have a shorter life span than marine engines cooled with a closed cooling system

Why is it important to lubicate the impellor when fitting from new?

After the new impellor is in place it is important to lubricate it to prevent wear until water is circulating

When fitting a impellor what should you check before refitting the housing and why?

When refitting an impellor, the gasket has to be intact and in place. Then when bolting down the housing, the housing should be bolted down evenly.

What is the purpose of a oil cooler and how does it work?

An oil cooler works similar to an radiator ,it circulates oil instead of coolant.This takes away the heat by the process of radiation the heat from the oil and help prevent overheating.

What is the processT of a intercooler/aftercooler how does it work?Why do we have Inhibitors & antifreeze?

Most automotive engines are water cooled to remove waste heat although the "water" is actually antifreeze/water mixture and not plain water. The term engine coolant is widely used in the automotive and marine industry which covers its primary function of convective heat transfer for internal combustion engines.Inhibitors used to prevent corrosion

How do you test Antifreeze?

What is cavitation corrosion?

Cavitation Corrosion occurs in areas with high flow rates and rapid pressure changes. This causes gas bubbles to collapse. This gives a high force on the metal surface which can remove oxide and give an active aluminium surface.

Efficient design to avoid large pressure changes or hard surface layers are ways to avoid cavitation corrosion.

What is galvanic corrosion?

Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte.

Describle the terms
Stray Current Corrosion
Stray current corrosion occurs when a metal object becomes anodic with respect to another structure. When the object is anodic to another structure, the surface acts as if it is a sacrificial anode, i.e., the surface is dissolved while electrically protecting the other cathodic structure

Stress corrosion
Stress corrosion is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals.

Corrosion Fatigue

Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress and are exposed to harmful environments during their service life.

Crevice corrosion

Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion.
The damage is normally confined to one metal at localized area within or close to the joining surfaces.

What is Electrolysis?

Electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials.

How do you prevent corrosion?

Why and what is bonding in the marine industry?

What are sacrifical anode made off?

Sacrificial anode made of lead aluminium and zinc.

Where would you place them in the marine industry?

All metal objects exposed to sea water: pipelines, piers

What is the purpose of a water muffle?

Muffler converts harmful carbon monoxide and hydrocarbons to water vapor and carbon dioxide. Some converters also reduce harmful nitrogen oxides. The converter is mounted between the exhaust manifold and the muffler.

Muffler:The muffler serves to quiet the exhaust down to acceptable levels. The combustion process is a series of explosions that create a lot of noise. Most mufflers use baffles to bounce the exhaust around dissipating the energy and quieting the noise