Chapter 6: Ric Engines - Personal Webspace For Qmul

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(1) TYPES OF COMBUSTION ENGINES Chapter 6: R.I.C Engines (1-A) EXTERNAL COMBUSTION ENGINE R.I.C. ENGINES stands for: A steam turbine engine obtains thermal energy from pressurized steam RECIPROCATING and converts it into rotary motion. As it is driven by steam from a boiler INTERNAL which is outside the engine it is said to be EXTERNAL. COMBUSTION ENGINES. R.I.C. is by far the most common type of engine for road vehicles. Main aims. Understanding of the: (1-B) INTERNAL COMBUSTION ENGINE The combustion takes place within the engine, in the so called combustion - Structure of an RIC engine chamber, i.e. is INTERNAL. This kind of engines are commonly used for - Engine operation (2-stroke and 4-stroke engines) vehicles propulsion, as for example the two-stroke and four-stroke piston engines. Another example is the gas turbines, used when very high - Engine power (Morse test) power-to-weight ratios are required, e.g. jet aircraft or large ships. - Fuel combustion (2) RECIPROCATING (continuation) (2) RECIPROCATING Burning fuel (COMBUSTION) inside the cylinder provides the energy to Combustion inside the cylinders produces LINEAR IN-OUT motion of the pistons. The reciprocating mechanism converts this to CIRCULAR motion. This mechanism consists of • A connecting rod • A crank CYLINDER HEAD CYLINDER PISTON turn the shaft. The sequence for a complete cycle is: 1) Intake 2) Compression 3) Ignition 4) Expansion 5) Exhaustion 1) Fuel and air in the correct ratio enters the cylinder. 2) The fuel-air mixture is compressed. CONNECTING ROD CRANKSHAFT CRANK 3) An electrical spark is used to ignite the compressed mixture. 4) Expansion of the combustion products forces the piston away from the cylinder heard. 5) The exhausted combustion products must clear from the piston in order to make way for a fresh fuel-air charge. (3)TWO- STROKE ENGINES (3-A) HOW THEY WORK 2-stroke engines have holes (ports) through which the fuel mixture enters (INLET) and the exhaust gases leave (OULET) as shown in the figure. With the piston near the bottom of its stroke the ports are uncovered. FUEL MIXTURE INLET PORT EXHAUST GAS OULET PORT An air pump forces in a fresh charge With the piston at the top of its stroke the mixture is ignited. CONNECTING ROD of fuel-air mixture. This forces out the exhaust gas. The shape of the piston crown assists this movement. CRANK 1 (3-B) 2-STROKE ENGINES: DISADVANTAGES Rotary momentum from the flywheel and the forces from the other cylinders continues the rotation. The pushes the piston towards the cylinder head. • Reduction in stroke length due to the ports. • Power is lost driving the air pump. The cycle is now complete and is repeated by ignition with the piston at • Only a short time is available to clear out the exhaust gases. the top of its stroke this cycle has taken 2 strokes. • Pollution level is comparatively high. Hence the 2- stroke engines are no longer used for cars. COMPRESSION IGNITION (4) FOUR – STROKE ENGINES a) SUCTION STROKE INLET OPEN The cycle starts with the piston at the top of the cylinder. This position is called Top Dead Centre. INLET CLOSED There are 4 strokes of the piston per cycle: As a) Suction stroke (DOWN) the piston comes down b) Compression stroke (UP) the inlet valve opens. Fuel – air mixture c) Expansion stroke (DOWN) enter. d) Exhaust stroke (UP) There are valves on the inlet and outlet. c) EXPANSION STROKE b) COMPRESSION STROKE Both valves remain closed as combustion forces the piston down. INLET OPEN The inlet closes as the piston moves up. The fuel-air gas INLET CLOSED d) EXHAUST STROKE Near the Bottom Dead Centre the outlet valve opens. As the piston rises the exhausted gas is ejected. is compressed. SUMMARY Ignition occurs at the top of this stroke. SUCTION COMPRESSION EXPANSION EXHAUST 2 (5) OTHER NOTES ABOUT R.I.C. ENGINES (5-b) MULTI – CYLINDER ENGINES (5-a) PISTON RINGS Each cylinder only produces energy on 1 stroke out of 2 (in the 2-stroke The pistons have been shown very simply in the engine diagrams. In engine) or 4 (in the 4-stroke engine). The more cylinders an engine has, reality they are fitted with piston rings to: the more continuous the production of energy. The use of a flywheel also helps smooth out the fluctuation. • Prevent lubricating oil passing the piston and entering the combustion chamber. (5-c) COUNTER – BALANCE WEIGHTS • Prevent combustion gasses leaking out around the piston. As the crankshaft rotates about the main axis there are out-of-balance forces and moments. These could distort the crankshaft and overload the bearings. Weight at the end of the crank-web are used to cancel out these forces and moments. (6) PRESSURE – VOLUME DIAGRAM Inside a cylinder the volume and pressure follows a cycle. For a 4 – stroke During the suction stroke the pressure is slightly below atmospheric. engine this is shown below. This “SUCKS IN” the new fuel-air charge. X indicates ignition. CO MP RE SS IO N IO taken as the zero datum. NS PA atmospheric pressure is EX In this diagram P R E S S U R E During the exhaust stroke the pressure is slightly above atmospheric. This “PUSHES OUT” the exhaust gases. N EXHAUST 1 atm The upper EXPANSION-COMPRESSION loop gives a value of SUCCION positive work (provides energy). The lower SUCTION – EXHAUST (0,0) SWEPT VOLUME VOLUME loop is called the pumping loop. This gives the negative work (consumes energy). CLEARANCE VOLUME (7) CHARACTERISTIC POWERS IN THE ENGINE (7-a) INDICATED POWER (8) MORSE TEST The Morse test is a method of finding the indicated power of a multi-cylinder engine. It uses a machine that can apply an adjustable load to the engine. This is the power actually developed in the cylinders. Step 1) Running the engine in this machine gives the total BRAKE POWER from all the cylinders. (7-b) BRAKE POWER Step 2) One cylinder is cut out by stopping the fuel supply or ignition for that cylinder. This causes a reduction in the engine speed. This is the useful power output of the engine. Losses occur due to • Friction between the piston and the cylinder walls. • Friction in the bearings at the ends of the connecting rod. Step 3) The load is adjusted to restore the original speed. Step 4) The piston in the non-firing cylinder is still moving and producing loses. What has been removed is the INDICATED POWER of that cylinder. This is equal to the difference between the two measured brake powers. Step 5) Back to step 1. Repeating the test for the other cylinders one-at-atime gives the total indicated power. MECHANICAL EFFICIENCY = BRAKE POWER INDICATED POWER 3 (9) QUESTION (10) FUEL An engine has a brake power of 25.3 kW at 1200 r.p.m. with all 4 cylinders operating. This decreases to 17.5 kW at 1200 r.p.m. when one cylinder is cut out. Assuming the losses in all 4 cylinders are the same calculate the engine´s a) Indicated power b) Mechanical efficiency The fuel for I.C. engines is liquid hydrocarbon – i.e. a compound of carbon and hydrogen. Its properties are given by 5 important numbers indicated below. SOLUTION 2. FLASH POINT: The temperature at which the fuel gives off inflammable gas. This is important for storage. Brake power 4 cylinders = 25.3 kW Brake power 3 cylinders = 17.5 kW 3. IGNITION TEMPERATURE: The lowest temperature at which the fuel will ignite without help from a spark. Indicated power of 1 cylinder = 25.3 – 17.5 1.CALORIFIC VALUE: The maximum energy that can be obtained from a given quantity of fuel. = 7.8 kW Indicated power of 4 cylinders = 4 x 7.8 = 31.2 kW 4. OCTANE NUMBER: Indicates the fuel's tendency to detonate. 5. CETANE NUMBER: Indicates the fuel's suitability for a compression ignition engine. Mechanical efficiency = 25.3 / 31.2 = 81.1 % (11) ENERGY TRANSFER The hydrogen in the fuel burns to form water. The carbon burns to form carbon dioxide CO2 or carbon monoxide CO. CO indicates incomplete combustion. (12) POLLUTION The shape of the combustion chamber and the time available for combustion affect the degree of combustion. Too little or too much air causes inefficiency. 1 kg of carbon burnt to CO2 gives 33.7 MJ of energy. 1 kg of carbon burnt to CO gives 10.5 MJ. The engine should be designed to minimise CO production even though CO2 is a greenhouse gas. When a car starts from cold on a cold day, steam can be seen condensing A catalytic converter is an oxydising device. It: • Converts any CO into CO2 • Burns any fuel that has passed through the engine. from the exhaust. Energy is lost as the exhaust products heat the atmosphere. Unburnt fuel is just as bad as CO2 for the “greenhouse” effect. Heat and steam is still lost to the atmosphere when the day is warm. It is just less visible then. (end of chapter 6) 4