Heat engines.docx

**Heat engines** Engines are mechanical devices that convert heat energy of fuels into mechanical energy. In external combustion engines (ECE), the fuel is ignited and burned outside a cylinder and the heat energy is applied indirectly to a piston by an intermediate medium, usually water vapor. A...

**Heat engines** Engines are mechanical devices that convert heat energy of fuels into mechanical energy. In external combustion engines (ECE), the fuel is ignited and burned outside a cylinder and the heat energy is applied indirectly to a piston by an intermediate medium, usually water vapor. A steam engine is an external combustion engine. In internal combustion engines (ICE), the fuel is ignited and burned inside a cylinder and the resulting explosion causes an instantaneous application of pressure to a piston. Internal combustion engines have completely replaced the steam engine for all types of agricultural power applications. a. They are used as stationary power sources for milling, grinding, water pumping, generation of electricity, etc. b. They are used as power units of (1) portable farm equipment such as sprayers, and threshers, and (2) self-propelled machines such as tractors, combines and harvesters. A great number of them are mounted on tractors. Tractors can classified into: a. 2-wheel tractor (hand tractor) b. compact 4-wheel tractor c. combination wheel type and track type d. tract type or crawler e. combination wheel type and tract type Main engine components a. cylinder h. piston rings b. cylinder block i. piston pin c. cylinder head j. flywheel d. valves k. crankcase e. piston l. intake manifold f. connecting rod m. exhaust manifold g. crankshaft Physical nomenclature a. bore (d) -- the diameter of the engine cylinder b. stroke (l) -- distance the piston travels in one direction c. piston displacement (P~d~) -- volume displaced by the piston in one stroke d. clearance volume ( C~v~ ) -- the space inside the cylinder when the piston is at top dead center position e. cylinder volume -- the sum of piston displacement and clearance volume, P~d~ + C~v~ f. compression ratio ( C~r~ ) -- ratio of cylinder volume to the clearance volume g. engine efficiency -- a measure of the ability of an engine to convert fuel into useful work Engine cycles and principles of operation In the operation of internal combustion engines, four events repeatedly takes place inside the combustion chamber one after the other in a definite order. These cycle of events are the following: 1. 2. 3. 4. Classification of engines: a. 1. 2. *e* = 1 -- ( *V~C~ / V~B~* ) ^k-1^ \[{( *V~D~* / *V~C~* )^k^ -1} / k (*V~D~* ~/~ *V~C~* -1)\] The figure shows the comparative efficiency values of ideal Otto and Diesel cycles. At first, the efficiency increases rapidly with compression ratio but the rate of increase decreases as compression ratio becomes high. Otto cycle gives a higher theoretical efficiency than Diesel cycle. In actual practice, it is not higher, because the compression ratio of Otto-cycle engine is limited by the fuel characteristics. Diesel engine normally uses higher compression pressures than Otto-cycle engine and, therefore, higher fuel-utilization efficiency. The table below summarizes the differences between spark-ignition and compression- ignition engines. -- -- -- -- -- -- b. 1. Four-stroke cycle engines require four movements of the piston to complete the cycle. 2. Two-stroke cycle engines require two movements of the piston to complete the cycle. **Comparison of 4-stroke cycle and 2-stroke cycle engines** ---------------------------------------------------------------- ---------------------------- ---------------------------- **Characteristics** **4-stroke** **2-stroke** Number of power strokes 1 per 2 rev. of crankshaft 1 per 1 rev. of crankshaft Power small Large (1.5 times) Valve mechanism required absent Construction/design complicated simple Fuel consumption less more (15% more) Durability good good Stability of rpm high with large flywheel low with small flywheel Lubrication independent of oil circuit mixed with fuel Oil consumption little much Carbon deposit inside cylinder and contamination of spark plug little much Self-weight and size heavy and large light and small Market price expensive cheap From the above classification, there are four different types of engines: Four-stroke cycle spark ignition engine Four-stroke cycle compression ignition engine Two-stroke cycle spark ignition engine Two-stroke cycle compression ignition engine