What is Valve Overlap?

         Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time.

        Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event. The duration of valve overlap is between 10°-20° of crankshaft rotation, depending on the engine design. The intake valve is opened during the exhaust event just before TDC, initiating the flow of a new charge into the combustion chamber.

Valve Overlap

        As the exhaust gases are evacuated from the combustion chamber, a small but distinct low-pressure area is created on the surface of the piston head. By opening the intake valve earlier than TDC, the charge begins to fill this low-pressure area while exhaust gases exit. The low-pressure area on the head of the piston assists the fresh charge in filling the combustion chamber to its maximum capacity.

Valve Overlap Period

        Valve overlap is designed into the engine and is most useful at higher speeds. At higher speeds, the head start in filling the combustion chamber provides a substantial increase in available power. The amount of time that both valves are open is directly related to engine rpm. The higher the engine rpm, the shorter the amount of time that both valves are open. The degrees of crankshaft rotation when both valves are open do not change. Only the amount of time both valves are open varies with the engine rpm. For example, at idle, the amount of time both valves are open is relatively long compared to top no-load speed.

Other components of Engine

Following are some components of the engines -

1. Flywheel

    The flywheel is a cast iron, aluminum, or zinc disk that is mounted at one end of the crankshaft to provide inertia for the engine. Inertia is the property of matter by which any physical body persists in its state of rest or uniform motion until acted upon by an external force. During the operation of a reciprocating engine, combustion occurs at distinct intervals. the flywheel supplies the inertia required to prevent loss of engine speed and possible stoppage of crankshaft rotation between combustion intervals.

2. Bearings

    The bearing is the component used to reduce friction and to maintain clearance between stationary and rotating components of an engine. Bearings, or bearing surfaces, are located on the crankshaft, connecting rod, and camshaft, and also in the cylinder block. Bearings can be subjected to radial, axial(thrust), or a combination of radial & axial loads. Bearings can be classified as friction or anti-friction bearings.

Bearings & types

3. Carburetor

      This is the Venturi flow device which meters the proper amount of fuel into the airflow using a pressure differential. For many decades it was the basic fuel metering system on all automobile (and other) engines. It is still used on low-cost small engines like lawnmowers but is uncommon on new automobiles.

Carburetor

4. Catalytic converter

        The catalytic converter helps to reduce airborne pollutants that could harm people as well as the environment, turning 90% of harmful emissions into less harmful gasses. A well-maintained catalytic converter improves the efficiency of your vehicle, directly affecting its overall health and performance.

Catalytic Converter

5. Connecting Rod Bearings

        Connecting rod bearings provide the rotating motion of the crankpin within the connecting rod, which transmits cycling loads applied to the piston. Connecting rod bearings are mounted on the Big end of the connecting rod. A bearing consists of two parts.

Bearings

6. Starter

        A starter also known as a self-starter, cranking motor, or starter motor is a device used to rotate (crank) an internal combustion engine to initiate the engine's operation under its own power.

Starter

7. Water Pump
        The water pump, also known as the coolant pump, is the heart of the engine's cooling system. It is the water pump's job to regulate the coolant flow rate and to constantly circulate coolant throughout the engine and cooling system.

8. Supercharger

        The supercharger is a general name for an air compressor used to increase the pressure or density of air entering an engine, providing more oxygen with which to burn fuel. 

Supercharger

9. Turbocharger

        A turbocharger, ordinarily known as a turbo, is a turbine-driven, forced induction device that increases an internal combustion engine's power output by forcing extra compressed air into the combustion chamber.

Turbocharger

10. Radiator

        The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the fluid, which in turn cools the engine. Radiators are also often used to cool automatic transmission fluids, air conditioner refrigerant, intake air, and sometimes to cool motor oil or power steering fluid.

Radiator System Layout

11. Fan

        Most engines have an engine-driven fan to increase airflow through the radiator and through the engine compartment, which increases waste heat removal from the engine. Fans can be driven mechanically or electrically and can run continuously or be used only when needed.

Cooling Fan

12. Fuel Pump

        The fuel pump is located at the fuel tank and is designed to carry the required quantity of fuel from the tank to the engine at the required pressure.

Fuel Pump

13. Fuel Injector

        A pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI engines. On SI engines, fuel injectors are located at the intake valve ports on multipoint port injector systems and upstream at the intake manifold inlet on throttle body injector systems. In a few SI engines, injectors spray directly into the combustion chamber.

14. Glow plug

        The glow plug is a small electrical resistance heater mounted inside the combustion chamber of many CI engines and is used to preheat the chamber enough so that combustion will occur when first starting a cold engine. The glow plug is turned off after the engine is started.

Glow Plug

15. Head Gasket

        The gasket serves as a sealant between the engine block and head where they bolt together. They are usually made in a sandwich construction of metal and composite materials. Some engines use liquid head gaskets.

Head gasket

References:

1. Engineering Fundamentals of IC Engine - Willard W. Pulkrabek

2. http://www.substech.com/dokuwiki/doku.php?id=bearings_in_internal_combustion_engines

3. wikipedia.com

Advantages & disadvantages of Two-stroke cycle over Four-stroke cycle engines

Advantages:

1) The two-stroke cycle engine gives one working stroke for each revolution of the crankshaft. Hence theoretically the power developed for the same engine speed and cylinder volume is twice that of the four-stroke cycle engine, which gives only one working stroke for every two revolutions of the crankshaft. However, in practice, because of poor scavenging, only 50-60% extra power is developed.

2) Due to one working stroke for each revolution of the crankshaft, the turning moment on the crankshaft is more uniform. Therefore, a two-stroke engine requires a lighter flywheel. 

3) The two-stroke engine is simpler in construction. The design of its ports is much simpler and their maintenance easier than that of the valve mechanism. 

4) The power required to overcome the frictional resistance of the suction and exhaust strokes is saved, resulting in some economy of fuel. 

5) Owing to the absence of the cam, camshaft, rockers, etc. of the valve mechanism, the mechanical efficiency is higher.

6) The two-stroke engine gives fewer oscillations. 

7) For the same power, a two-stroke engine is more compact and requires less space than a four-stroke cycle engine. This makes it more suitable for use in small machines and motorcycles. 

8) A two-stroke engine is lighter in weight for the same power and speed especially when the crankcase compression is used. 

9) Due to its simpler design, it requires fewer spare parts. 

10) A two-stroke cycle engine can be easily reversed if it is of the valveless type. 

Disadvantages: 

1) The scavenging is not very efficient in a two-stroke engine, the dilution of the charges takes place which results in poor thermal efficiency. 

2) The two-stroke spark-ignition engines do not have a separate lubrication system and normally, lubricating oil is mixed with the fuel. This is not as effective as the lubrication of a four-stroke engine. Therefore, the parts of the two-stroke engine are subjected to greater wear and tear. 

3) In a spark-ignition two-stroke engine, some of the fuel passes directly to the exhaust. Hence, the fuel consumption per horsepower is comparatively higher. 

4) With heavy loads a two-stroke engine gets heated up due to the excessive heat produced. At the same time, the running of the engine is riot very smooth at light loads.

5) It consumes more lubricating oil because of the greater amount of heat generated. 

6) Since the ports remain open during the upward stroke, the actual compression starts only after both the inlet and exhaust ports have been closed. Hence, the compression ratio of this engine is lower than that of a four-stroke engine of the same dimensions. As the efficiency of an engine is directly proportional to its compression ratio, the efficiency of a two-stroke cycle engine is lower than that of a four-stroke cycle engine of the same size.

Construction and Working of Two Stroke Petrol Engine

       A two-stroke cycle engine is an internal combustion engine that utilizes two distinct piston strokes to complete one operating cycle of the engine. The crankshaft turns only one revolution for each complete operating cycle, providing twice as many power strokes in the same number of crankshaft rotations as a four-stroke cycle engine. The valving system in a two-stroke cycle engine requires fewer parts, making it lighter in weight than a four-stroke cycle engine. Weight reduction is especially desirable in applications such as chainsaws, leaf blowers, and other hand-held outdoor power equipment.

Two-Stroke Working Cycle

1. Ignition/Power: 

       The ignition/power stroke occurs while the piston progress toward TDC and the compressed charge in the cylinder is ignited. During this time, the crankcase has already loaded with a fresh air-fuel mixture. When the charge is ignited, the expansion of hot combustion gases forces the piston toward BDC. Piston motion is transferred from the piston through the connecting rod, causing the crankshaft to rotate. When the piston moves near BDC, the exhaust port is opened and exhaust gases are discharged through the side of the cylinder.

2. Exhaust/Intake: 

      The exhaust/intake stroke occurs as the piston continues moving toward BDC. The intake port opens and the air-fuel mixture is routed inside the cylinder. The shape of the piston head helps to divert the incoming air-fuel mixture to the top of the cylinder. This prevents the incoming air-fuel mixture from passing across the top of the piston and out the exhaust port without burning. Diversion of the air-fuel mixture also helps when removing or scavenging exhaust gases.

3. Compression:

     The compression stroke occurs when the piston is at BDC, exhaust gases have been discharged, and the cylinder is filled with a new charge. The compression stroke begins as the piston starts moving toward TDC. The piston closes the intake and exhaust ports, trapping the charge in the cylinder. The piston functions as a slide valve, exposing the intake and exhaust ports as it moves in the cylinder. The charge is compressed as the piston continues to move toward TDC. Piston movement toward TDC causes the more air-fuel mixture to be drawn into the crankcase. When the piston reaches TDC, the engine has completed one full operating cycle.

Applications of Two-Stroke Engine

• Two-stroke engines are preferred when mechanical simplicity, lightweight, and high power-to-weight ratio are design priorities.
• They are lubricated by the traditional method of mixing oil into the fuel, they can be worked within any orientation as they do not have a reservoir dependent on gravity. This makes them desirable for their use in handheld tools such as chainsaws.
• Two-stroke engines are found in small scales propulsion applications such as motorcycles, Mopeds, and dirt bikes.

Characteristics of Two-Stroke Engine

• Fewer moving parts
• Less weight for comparable output
• Higher fuel consumption
• More noise
• Higher operating speed and temperature
• Smaller size for comparable output
• Greater exhaust emissions

See Advantages & disadvantages of Two-stroke cycles over Four-stroke cycles engine HERE

References:

1. IC Engine by Brody Walker
2. Wikipedia 

3. byjus.com

Construction and Working of Diesel (CI) Engine

 4-Stroke Diesel (CI) Engine-

Working cycles of CI Engine

• Construction: -

            In a diesel engine, fuel injectors are present. The function of the fuel injector is to inject the fuel inside the cylinder. The diesel engine uses only air during suction stroke. Here, fuel is injected into the chamber at the end of the compression stroke. Since ignition in this engine is due to the high temperature of compressed air so, this engine is also called Compression Ignition Engine.

Cycles of 4-Stroke CI Engine

• Working: -

1. Suction Stroke:- 

            During suction stroke, the inlet valve is open and the exhaust is closed. When the piston moves from TDC to BDC. the low pressure is created inside the cylinder. Thus, fresh air is getting sucked into the cylinder. At the end of the suction stroke inlet valve closes. This completes 180 deg of the crankshaft.

2. Compression Stroke: -

                During the compression stroke, both inlet and exhaust valves remain closed. When the piston start moves from BDC to TDC, the fresh air inside the cylinder is compressed up to clearance volume. Due to this compression pressure and temperature of air get increased. The rise in pressure and temperature depends upon the compression ratio, which is 14 to 22 in diesel engines. This completes the rotation of 360 deg of the crankshaft.

3. Power/ Expansion Stroke: -

                The power stroke is generated when both inlet and exhaust valves are kept closed. When the piston reaches almost TDC, the fuel is injected in the form of fine spray into the engine through the fuel injector. At this moment the temperature of compressed air is sufficient to ignite the fuel. This leads to high temperatures and high-pressure gases drive the piston downwards BDC. Thus, power is obtained by the expansion of the product of combustion. This completes the rotation of 540 deg of the crankshaft.

4. Exhaust Stroke: - 

                At the end of the power stroke, the exhaust valve opens and the inlet valve remains closed. When the piston moves from BDC to TDC the burnt gases inside the cylinder are thrown out into the atmosphere. The exhaust valve closes at the end of the exhaust stroke. This completes the rotation of 720 deg of the crankshaft.

See Also:-

1. Classification of Diesel Fuel Injection Pumps

2. What is valve overlap?

Construction and Working of 4-Stroke Petrol (SI) Engine

4-Storke SI Engine-

Working cycles of SI engine

• Working Principle: -

        The working cycle of an engine is completed in a 4-stroke of piston i.e. two revolutions of the crankshafts.  Each stroke consists of 180 deg revolution of a crankshaft. Therefore, the cycle consists of 720 deg of crankshaft for 1 cycle. Petrol is used as a fuel for SI engines. These engines work on Otto cycles. Here air & fuel mixture is ignited by producing a spark plug, just before the end of the compression stroke. Therefore, these engines are also called Spark Ignition Engines.

·        Following are the 4-strokes in a petrol engine: -

Cycles of 4-stroke SI Engine 

1. Intake/Suction Stroke: - 

During this stroke, the inlet valve opens and the exhaust valve remains closed. As the piston moves downwards, creates a low-pressure area in the cylinder. Thus, the air-fuel mixture gets sucked into the cylinder. As the piston reaches BDC, the suction valve gets closed completing suction stroke.

2. Compression Stroke: -

During this stroke, the piston starts moving toward TDC from BDC, both valves remain closed. The charge is compressed to clearance volume. Just before the piston reached TDC i.e. before the end of the compression stroke the mixture is ignited with the help of a spark plug.

Combustion is almost completed when the piston is at TDC producing a temperature rise of about 2000^oC and pressure also considerably increased. This completes one revolution of the crankshaft.

3. Expansion/Power Stroke: -

Both valves remain closed during this stroke. The ignited mixture at high pressure and temperature drives the piston downwards. During this stroke, power is obtained. Both pressure and temperature decrease during expansion.

4. Exhaust Stroke: -

In this stroke, the product of combustion is thrown out from the engine cylinder. The exhaust valve remains open during this stroke and the inlet valve remains closed. The piston is moving from BDC to TDC. The exhaust valve closes at the end of the exhaust stroke and some residual gases remain inside the cylinder.

• What do you mean by Valve Overlap? see HERE

References

1. Wikipedia.com

What is an Engine ? and how engines are classified ?

        A heat engine is a device that converts heat energy into mechanical work. The chemical energy of a fuel is converted into heat energy by combustion of fuel to produce work expansion.

Heat Engine has two types:-

1. External Combustion Engine

   This is an engine, where the combustion of air & fuel takes place outside the cylinder using secondary fuel.

2. Internal Combustion Engine

   This is an engine, where the combustion of air and fuels take place within the cylinder and are used to convert chemical energy to mechanical.

Let's see the classification of each-

1. External Combustion Engine

External Combustion Engine Classification

2. Internal Combustion Engine

Internal Combustion Engine Classification 

• Detailed Classifications of IC Engine
  1. According to no. of strokes-
  1. Two-Stroke cycle engine
  2. Four-Stroke cycle engine
  2. According to combustion cycle used-
  1. Otto cycle (constant volume cycle)
  2. Diesel cycle (constant pressure cycle)
  3. According to no. of cylinders-
  1. Single cylinder 
  2. Multi-cylinder
  4. According to the arrangement of cylinders-
  1. Horizontal engine
  2. Vertical engine
  3. V-type engine
  4. Inline engine
  5. Radial engine 
  5. According to ignition method-
  1. Spark ignition 
  2. Compression ignition
  6. According to the cooling method-
  1. Air-cooled
  2. Water-cooled
  7. According to the speed of Engine-
  1. Low-speed engine
  2. Medium-speed engine 
  3. High-speed engine
  8. According to the type of fuel used-
  1. Petrol engine
  2. Diesel engine 
  3. Natural Gas engine (CNG or LPG)
  4. Bio-fuel engine
  5. Light oil (kerosene) engine
  6. Alcohols (methanol or ethanol)
  7. Hydrogen engine 
  9. According to mixture preparation -
  1. Carburetion
  2. Fuel injection into port or intake manifold 
  3. Fuel injection into the engine cylinder
  10. According to the valve or port design & location-
  1. Overhead valve
  2. Underhead valve
  3. Rotary valve
  4. Cross-scavenged porting
  5. Loop-scavenged porting
  6. Uniflow-scavenged
  11. According to the uses of application-
  1. Stationary engine
  2. Portable engine
  3. Marine engine
  4. Automobile engine
  5. Tractor engine
  6. Aero-engine
• Quick Recall for classification of IC Engine-
• S-strokes
• C-combustion 
• C-no. of cylinders
• A-arrangement of cylinder
• I-ignition method
• C-cooling method
• S-speed of engine
• F-fuel used
• M-mixture preparation
• V-valve/port design & location 
• A-application 
• "SCAM ICC FAVS" - remember this

Applications of IC Engines

 What are the applications of IC Engines?

        IC Engine has an advantage since it can provide high power to weight ratio together with excellent fuel energy density. Therefore, these are used in transport in almost all vehicles.

1. Two-stroke cycle engines are used in light and medium-duty automobiles such as mopeds, scooters, tempos, etc.
2. Four-stroke cycle engines are used in heavy-duty automobiles like buses, trucks, delivery vans, etc.
3. Railway traction - Heavy diesel engines are used to run railways.
4. V-type IC engines are used in heavy-duty motorcycles which are designed for the long run.
5. Single-cylinder horizontal engines are used in scooters and mopeds.
6. Inline vertical multi-cylinder engines are used in cars, buses, trucks, etc.
7. In the marine field, IC engines are used to propelling ships.
8. Radial piston engines are used in aircraft propulsion.

9. Engines are also used in heavy earth-moving equipment like scrapers, bulldozers, power shovels.

See the difference between Petrol (SI) engine & Diesel (CI) engine HERE

See the difference between Two-stroke & Four-stroke HERE

See components of IC Engine HERE

See Engine and its classification HERE

What is difference between Petrol (SI) engine & Diesel (CI) engine?

        The difference between petrol engine & diesel engine can be carried out with various parameters as follows

Sr. No.	Petrol Engine (SI)	Diesel Engine (CI)
1. Suction stroke	Petrol engine takes a mixture of air & petrol	A diesel engine takes only air during suction stroke
2. Fuel Mixing	The carburettor is used to mix air & petrol	An injector or atomizer is used to inject fuel at end of the compression stroke.
3. Pressure at end of the compression stroke	Pressure is about 10bar	Pressure is about 35 bar
4. Fuel Ignition	Ignited with the help of Spark Plug	Fuel-injected is sufficient to ignite the compressed air
5. working Cycle	Constant volume (Otto) cycle	Constant Pressure (Diesel) cycle
6. compression ratio CR	The petrol engine has 6 to 12	The diesel engine has 10 to 24
7. Engine Starting	Easy due to low CR	Difficult due to high CR
8. Running cost	High due to the high cost of Petrol	Low due to less cost of diesel
9. Maintenance Cost	Less	High
10. Engine Weight	Lightweight	More weight
11. Thermal Efficiency	About 24%	About 40%
12. Engine speed	High	Low
13. Life span	Less	More
14. Overhauling requirement	Frequent overhauling required	Overhauling is done after a long time
15. Maximum temp inside the cylinder	The temperature reaches up to 1100 deg C	The temperature reaches up to 2600 deg C
16. Application	Scooter, bikes, cars, etc	Trucks, buses, tractors, etc

See the Difference between Two-Stroke & Four-Stroke Engine HERE