Internal Combustion Engines Introduction : 2

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Internal Combustion Engines

IntroductionLecture 2

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Outline

• In this lecture we learn about:– Basic Engine Components– The Four-Stroke Engine Cycle– The Two-Stroke Engine Cycle

• We will also discuss the differences – and similarities – between both cycles.

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Engine Components• Block:

– Body of engine, made of cast iron or Aluminium. Covered with water jacket or cooling fins.

• Cylinder:– The part of the engine

block in which pistons reciprocate. Its walls have highly polished hard surfaces, to ensure smooth movement of the pistons.

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Engine Components• Combustion

Chamber:– The space in the

cylinder between the head and the piston face. It’s size changes continuously during operation.

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Engine Components• Piston:

– Cylindrical mass transferring the pressure forces in the combustion chamber to the crankshaft. The top surface is called: the crown, which can either be flat or of a concave shape. The sides of the piston are known as the skirt.

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Engine Components• Piston rings:

– Compression rings: • Metal rings that seal

between the piston and the cylinder walls to prevent the high pressure gases from escaping into the crankcase.

– Oil ring:• Assisting in lubrication

and scraping excess oil.

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Engine Components• Crankshaft:

– Rotating shaft that transfers the engine’s work. The reciprocating movement of the pistons is converted to a rotational movement.

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Engine Components• Connecting rod:

– Rod connecting the piston with the rotating crankshaft.

• Crankcase:– Part of the block

surrounding the crankshaft. The oil pan can make part of it.

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Engine Components• Camshaft:

– Rotating shaft used for opening valves at the proper time of the cycle. Driven off the crankshaft by a belt or chain (timing belt/chain).

• Push rod:– A rod linking the

valves in the cylinder head with the camshaft.

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Engine Components• Cylinder head:

– Made of cast iron or Aluminium. Containing the spark plug, the fuel injectors and most cases the valves.

• Valves:– Allow flow in and out of the

cylinder. Poppet valves -which are spring loaded-are most common. Should close perfectly on the surface called valve seat.

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Engine Components• Intake manifold:

– Piping system carrying fresh air to the engine cylinders. Fuel is added either through injectors or with the aid of a carburettor.

• Exhaust manifold:– Piping system carrying

away exhaust gases.

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Engine Components

• Spark plug: – For initiation of combustion in S.I. Engines.

• Carburettor: – Fuel metering device in old S.I. Engines.

• Flywheel: – Rotating mass mounted on the crankshaft to

store energy and smoothen the engine operation

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Engine Components

• Radiator: – Liquid/Air heat exchanger to remove heat

from the engine coolant.• Fan:

– To increase air flow through the radiator.• Fuel pump• Oil pump• Water pump

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Engine Components

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Basic Engine Cycles• Basic definitions:

– Top Dead Centre (TDC): position of the piston when it stops at the furthest point from the crankshaft.

– Bottom Dead Centre (BDC): position of the piston when it stops at the nearest point to the crankshaft.

– Stroke: distance travelled by piston between TDC and BDC or vice-versa.

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The Four Stroke Engine Cycle• First stroke: (Induction stroke)

– Intake valve: open– Exhaust valve: closed– Piston travelling down from TDC to BDC– Volume of the combustion chamber increases

causing the pressure to drop (0.7-0.9 bar).– This drives the flow of air charge into the cylinder

(naturally aspirated engines) and can be assisted with a supercharger or a turbocharger.

– In S.I. Engines fuel is introduced with the air charge by injectors or with the aid of a carburettor.

– In C.I. Engines no fuel is added.

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The Four Stroke Engine Cycle• Second stroke: (Compression stroke)

– Intake valve: closed– Exhaust valve: closed– Piston travelling up from BDC to TDC– Volume of the combustion chamber decreases

compressing the A/F mixture (air only in C.I. engines).– This increases the pressure and temperature inside

the chamber (8-14 bar). – In S.I. Engines, the spark plug is fired towards the end

of the stroke to start combustion. – In C.I. Engines, fuel is injected instead and self ignites

after mixing with the compressed air. Pressures are much higher than those experienced in S.I. Engines.

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The Four Stroke Engine Cycle

• Combustion:– In S.I. Engines, combustion starts just before

reaching TDC and lasts for a very short time ending just after TDC. (i.e. combustion can be assumed to take place at constant volume).

– In C.I. Engines, combustion takes place at a slightly longer time (i.e. constant pressure).

– As a result of the combustion process, the temperature and pressure increase to peak values (up to 60 bar).

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The Four Stroke Engine Cycle• Third stroke: (Expansion/Power stroke)

– Intake valve: closed– Exhaust valve: closed (later opened)– Piston is pushed down from TDC to BDC. – Volume of the combustion chamber increases with

the expansion of the gases, decreasing the pressure as a result (around 2-4 bar).

– The exhaust valve is opened late during the stroke before reaching the BDC.

– Due to the pressure difference, most of the exhaust gases escape through the exhaust valve (exhaust blowdown).

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The Four Stroke Engine Cycle

• Fourth stroke: (Exhaust stroke)– Intake valve: closed– Exhaust valve: opened– Piston is moving up from BDC to TDC.– The remaining exhaust gases at nearly

atmospheric pressure are pushed out through the exhaust valve as a result.

– Towards reaching the TDC, the intake valve starts opening and the cycle is repeated.

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The Four Stroke Engine Cycle

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The Four Stroke Engine Cycle

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The Four Stroke Engine Cycle

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The Two Stroke Engine Cycle• First stroke: (Compression stroke)

– The piston moves up towards the TDC closing the exhaust and transfer ports and as a result compressing the trapped charge.

– In S.I. Engines a spark is fired near the end of the stroke to start combustion.

– In C.I. Engines fuel is instead injected to start combustion.

– At the same time a fresh charge is being introduced through a non return inlet valve.

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The Two Stroke Engine Cycle

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The Two Stroke Engine Cycle• Second stroke: (Power stroke)

– The piston is driven down towards BDC due to the high pressure resulting from combustion.

– At about 75° before BDC, the exhaust port opens causing most of the exhaust gases to be blown down.

– As the piston keeps moving towards the BDC, the transfer port is also uncovered.

– The fresh charge pushes much of the remaining exhaust gases taking their place in the cylinder (scavenging).

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The Two Stroke Engine Cycle

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The Two Stroke Engine Cycle• Discussion:

– The two stroke engine is usually smaller (1:1.7), lighter and therefore cheaper.

– Simpler, containing less parts, and thus requires less maintenance.

– Runs more smoothly and thus requires a smaller flywheel.

– For a given size at a given speed, the two stroke engine is more powerful than the four stroke engine since it has twice as many power strokes per unit time.

• (1 power cycle → 1 rev. → 2 strokes), in comparison with: • (1 power cycle → 2 rev. → 4 strokes).

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The Two Stroke Engine Cycle– The main disadvantage of it is that its

efficiency is usually lower resulting in bad fuel economy.

– The trick is to ensure that scavenging is efficient:

• Adequate transfer of the fresh charge into the cylinder

• Minimum residual exhaust gases remaining in the cylinder

• Minimum direct loss of the fresh charge through escaping via the exhaust port.

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The Two Stroke Engine Cycle

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The Two Stroke Engine Cycle– One way of overcoming this problem is by

using supercharging or turbocharging. – Increasing the charge air pressure ensures

proper scavenging.– This is further enhanced with the introduction

of direct injection instead of carburetion.

– Another disadvantage is the increased risk of overheating and thermal failure.

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The Two Stroke Engine Cycle– The application of the two stroke cycle in passenger

car automotive applications is very limited.

– Two stroke engines are most common for small applications (<1000 cc) such as motorcycles where simplicity, light weight and high power are more important than fuel economy.

– They are also common for very large applications (>100 litres) such as marine and stationary engines. As the engine size increases, the efficiency increases since cooling, clearances and auxiliary losses diminish.