MECHANICAL ENGINEERING 8 UNIT 8 HEAT ENGINES HEAT

MECHANICAL ENGINEERING 8 UNIT 8 HEAT ENGINES

HEAT ENGINES � Heat engines use heat to produce mechanical energy, and they are found everywhere � They are divided into two categories: external combustion engines and internal combustion engines � Until recently, external combustion engines were widely used in factories, trains and ships, but now they have been replaced by diesel engines

INTERNAL COMBUSTION ENGINES � I. gasoline / petrol engines these engines consist of 2 parts: a} upper engine → the cylinder head - which includes the carburetor, the spark plugs, the valve springs, the valve guides and the valve, and the cylinder block - which includes the cylinders, the piston rings, the distributor and the camshaft b} lower engine → the connecting rods, the crankshaft, the flywheel and the oil sump Can you give the Greek word for each part?

BASIC OPERATION � In the cylinder block, the pistons inside the cylinders move up and down. Each piston has 3 rings, which fill the gap between the piston and the cylinder wall � In the lower engine, the connecting rods turn the crankshaft and it rotates. This motion is transferred outside the engine to the wheels through the flywheel. At the bottom of the engine is the oil sump, which contains the oil pumped inside the engine. It helps to lubricate the engine and to reduce friction.

ADDITIONAL INFORMATION � When the oil flows through the engine, it absorbs heat produced by combustion. This helps to cool the engine. The oil sump has a thin casing of steel or aluminium, and is large enough to release the absorbed heat to the air around � The piston rings prevent the pressure of combustion from escaping the piston, so no power is lost. They are made of cast iron or hardened steel � The camshaft is driven by the crankshaft through a chain or rubber belt. It helps to drive the distributor

� The flywheel is bolted to the crankshaft and connected to the clutch. On the outside it has gear teeth connected to the starter motor. In this way it can turn, so as to rotate the crankshaft and start the engine � The valves are made from heat-resisting material, and their number and position among engine types. The greater their number, the more efficient the engine is. � The gudgeon pin allows the piston to move up and down. � The pistons move about 100 times per second. So, they must be strong and light: usually they are made

THE 4 -STROKE ENGINE 1. Compression stroke � Both valves are closed � The piston moves up compressing the fuel / air mixture in the combustion chamber � The heat produced vaporizes the fuel / air mixture 2. Exhaust stroke � The inlet valve is closed � The exhaust valve opens and the piston rises to expel the burnt gases from the cylinder � The exhaust valve closes and the inlet valve opens again to let new fuel / air mixture enter the cylinder

3. Induction stroke � The inlet valve opens and the piston moves down, taking fuel and air from the carburetor � The fuel / air mixture enters the cylinder � The inlet valve closes again � The outlet valve remains closed all this time 4. Power stroke � Both valves are closed � Before the piston reaches the Top Dead Center (the top of its stroke), a spark ignites the fuel / air mixture and we have combustion � The burning gases drive the piston down again and rotate the crankshaft half a turn

FUEL IGNITION SYSTEM � It is part of the engine’s electrical equipment. It produces sparks that ignite the fuel / air mixture in the cylinders. It includes the ignition coil, the distributor and the spark plugs � Ignition coil: it is a transformer that converts the 12 V DC current produced by the battery into AC current (over 10, 000 V) � This conversion is made by the contact breaker, a switch that produces electric pulses. When its contacts are open, high-voltage current is generated in the ignition coil. Its points open and close many times very quickly (about 10 million times every 1000 miles)

� The high-tension current is sent back to the distributor through a cable with plastic insulation. Inside the distributor, the rotor arm distributes the current to every spark plug. � The spark plugs convert the high-voltage pulses produced in the coil ignition system into sparks inside the combustion chamber.

CARBURETOR � All carburetors operate on the venturi effect: the air in the engine passes through the carburetor barrel (the main body of the carburetor). At some point, the diameter of the barrel is reduced; this narrow part is called ‘ the venturi’ When the air reaches the venturi, it speeds up and creates a vacuum. This vacuum draws petrol and, as petrol mixes with the air, it is broken down into little drops The car’s speed depends on how much fuel / air mixture is drawn into the engine

CARBURETOR TYPES 1. Fixed jet carburetor � It needs many different jets and circuits to give the engine the correct fuel / air mixture ratio for certain running conditions of the engine. It is very complicated 2. Variable choke carburetor � It has only one jet. It gives the correct fuel / air mixture ratio by changing the size of the venturi

3. Multiple carburetors � The engine has two or more carburetors, so all cylinders receive the same amount of fuel and the engine is more efficient 4. Compound twin-stroke carburetor � It has two or more barrels side by side, and it also has two carburetors

ELECTRONIC FUEL INJECTION � Cars with such systems have injectors which spray fuel directly into the cylinder � The fuel is broken down into drops that mix with the air � The injectors inject fuel for short repeated periods of time (intermittent system) , or give fuel to the engine continuously (continuous injection system) � The duration of injection and the amount of fuel are controlled by an electronic control unit, which monitors the functions of the engine and gathers information. Such cars also have an electrically controlled fuel pump and a fuel-pressure regulator

INTERNAL COMBUSTION ENGINES � II. diesel engines they are used in transport and in electric power plants � Their difference from petrol engines is that they use different fuel (diesel oil), so they operate differently � They let in only air during the induction stroke. This air is compressed by the piston and its temperature becomes very high. In the combustion stroke, a spray of fuel is injected directly into the cylinder. It is automatically ignited, and we have combustion. Diesel engines have no carburetors, distributors or spark plugs � Disadvantages: high initial cost, noise, slow acceleration, low maximum speed � Advantages: efficient, low fuel consumption

SOURCES Coursebook for Mechanical Engineering Technicians www. google. com