Engines PDF

ENGINES C.I. VS S.I. ENGINE 4-STROKE CYCLE VS 2-STROKE CYCLE MULTI-CYLINDER OPERATION The arc of travel of the crankshaft in degrees between successive explosions in an engine is known as firing interval (FI). 720° 𝐹𝐼 = for a four stroke-cycle engine...

ENGINES C.I. VS S.I. ENGINE 4-STROKE CYCLE VS 2-STROKE CYCLE MULTI-CYLINDER OPERATION The arc of travel of the crankshaft in degrees between successive explosions in an engine is known as firing interval (FI). 720° 𝐹𝐼 = for a four stroke-cycle engine 𝑛𝑜.𝑜𝑓 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟𝑠 360° 𝐹𝐼 = for a two-stroke-cycle engine 𝑛𝑜.𝑜𝑓 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟𝑠 MULTI-CYLINDER OPERATION Crank arrangement for 2-cylinder twin engine: (A) Opposed cranks, (B) twin cranks MULTI-CYLINDER OPERATION Chart showing occurrence of events in a two-cylinder four-stroke-cycle engine with twin crank arrangement. MULTI-CYLINDER OPERATION Chart showing occurrence of events in a two-cylinder four-stroke-cycle engine with opposed crank arrangement. MULTI-CYLINDER OPERATION The order in which the engine cylinders fire, or deliver their power strokes, beginning with cylinder number 1 is called firing order (FO). MULTI-CYLINDER OPERATION The chart below shows the occurrence of events in a four- cylinder four-stroke-cycle engine having a 1-2-4-3 firing order Draw the chart for engine with firing order of 1-3-4-2. ENGINE OPERATING CHARACTERISTICS Bore (d) – diameter of the engine cylinder Stroke (l) – distance the piston travels in one direction Piston displacement (Pd) – volume displaced by the piston in one stroke. 𝜋𝑑2 𝑃𝑑 = ×𝑙 4 Engine displacement (Ed) – the total piston displacement - It provides information to the relative size of the engine and is related to its ability to produce power. 𝐸𝑑 = 𝑃𝑑 × 𝑛 where: n – number of cylinder ENGINE OPERATING CHARACTERISTICS Sample Problem 1: Determine the piston displacement of an engine with the bore of 70mm and stroke of 90mm. ENGINE OPERATING CHARACTERISTICS Sample Problem 2: What is the total piston displacement for a 3” x 4” four stroke cycle engine with 4 cylinders. ENGINE OPERATING CHARACTERISTICS Top dead center (TDC) – top most position to which piston can travel inside cylinder. Bottom dead center (BDC) – bottom most position to which piston can travel inside cylinder. Clearance volume (CV) – the space inside the cylinder when the piston is at top dead center position Cylinder volume – the sum of piston displacement and clearance volume, Vcylinder = Pd + CV  Clearance volume Cylinder Piston volume displacement ENGINE OPERATING CHARACTERISTICS Compression Ratio It is degree to which the fuel mixture is compressed before ignition. The greater the CR, the greater the potential efficiency of engine. It is ratio of cylinder volume to the clearance volume. 𝑃𝑑 +𝐶𝑉 𝐶𝑅 = 𝐶𝑉 Where: 𝑃𝑑 = +1 Pd = piston displacement 𝐶𝑉 CV = clearance volume ENGINE OPERATING CHARACTERISTICS Sample Problem 3: An engine has piston displacement of 38.4 in3. If the clearance volume of the cylinder is 2.5 in3, what is the compression ratio? ENGINE OPERATING CHARACTERISTICS Sample Problem 4: A 4-cylinder 4-stroke cycle engine has a bore diameter of 7.5 cm and stroke bore ratio of 1.3. Determine the piston displacement and the compression ratio if the clearance volume is 45 cu.cm. ENGINE OPERATING CHARACTERISTICS Indicated Horsepower (IHP) It is the power developed in the cylinders of an engine as calculated from the average pressure of the working fluid, the piston area, the stroke, and the number of working strokes per minute. 𝐏𝐋𝐀𝐍𝐧 𝐈𝐇𝐏 = 𝐂𝐅 “P”- “indicated mean effective pressure” or IMEP (psi) “L”- length of the stroke (feet) “A”- area of cylinder (square inch) – πd2/4 “N”- rotational speed (rpm) “n”- number of cylinders “CF” – conversion factor (33000 ft-lb/min = 1 hp) Note: Multiply the equation by ½ if the engine is 4-stroke cycle. ENGINE OPERATING CHARACTERISTICS Indicated Horsepower (IHP) 𝐏𝐋𝐀𝐍𝐧 𝐈𝐇𝐏 = (4SCE) 𝐂𝐅 × 𝟐 𝐏𝐋𝐀𝐍𝐧 𝐈𝐇𝐏 = (2SCE) 𝐂𝐅 ENGINE OPERATING CHARACTERISTICS Brake Horsepower (BHP) It is the actual, measured output of the engine. The "brake" comes in as the machine used to measure the power is often a brake dynamometer. BHP= 2πTN/CF “T” - Torque – measured twist produced by the shaft (ft-lb) T = FLba “F” = force (lb) “Lba” = length of brake arm (ft) “N” – speed of the engine (rpm) “CF” =33000 𝟐𝛑𝐋𝐛𝐚 𝐁𝐫𝐚𝐤𝐞 𝐜𝐨𝐧𝐬𝐭𝐚𝐧𝐭 = in min-hp/lb 𝟑𝟑𝟎𝟎𝟎 ENGINE OPERATING CHARACTERISTICS Brake Horsepower (BHP) Dynamometer are used to determine the belt or brake horsepower of engine. Types can be: Pony-brake dynamometer – brake arm is attached to the crankshaft with the scale at the end. The scale registers the force after applying rotative force. ENGINE OPERATING CHARACTERISTICS Brake Horsepower (BHP) Dynamometer are used to determine the belt or brake horsepower of engine. Types can be: Pony-brake dynamometer – brake arm is attached to the crankshaft with the scale at the end. The scale registers the force after applying rotative force. Electric dynamometer – a more accurate device where the load is supplied with a current. It consists of generating unit, resistor unit, and the control and instrument board. Hydraulic dynamometer – torque action is produced by hydraulic reaction created by fluid in the housing. The load is adjusted and controlled by controlling the temperature and the rate of flow of the fluid. ENGINE OPERATING CHARACTERISTICS Mechanical Efficiency (ME) It is the ratio of the power delivered by a mechanical system to the power supplied to it. 𝑩𝑯𝑷 𝑴𝑬 = × 𝟏𝟎𝟎 𝑰𝑯𝑷 ENGINE OPERATING CHARACTERISTICS Sample Problem 5 If a 4-cylinder, 2-stroke-cycle internal-combustion engine has 150 rpm what is the indicated horsepower of the engine assuming a mean effective pressure of 102 psi? Consider the piston diameter and piston stroke is 4” and 5” ENGINE OPERATING CHARACTERISTICS Sample Problem 6 What is the indicated horsepower of a four-cylinder four- stroke gasoline engine if the engine speed is 1000 rpm. Assuming the mean effective pressure of 80psi. The diameter and piston stroke is 110mm and 140mm. ENGINE OPERATING CHARACTERISTICS Sample Problem 7 A pony-brake having 20 in brake arm length is connected to a tractor pulley rotating at 350 rpm. If the net load on the scale is 50 lb, what is the brake constant and the horsepower developed? ENGINE OPERATING CHARACTERISTICS Sample Problem 6 What is the brake horsepower of the engine with 9.7 indicated horsepower if the mechanical efficiency is 80 percent?