II-C Power & Energy Systems Dennis Buckmaster dbuckmas@purdue.edu https://engineering.purdue.edu/~dbuckmas/ OUTLINE • Internal combustion engines • Hydraulic power circuits • Mechanical power transmission • Electrical circuit analysis (briefly) References • Engineering Principles of Agricultural Machinery, 2nd ed. 2006. Srivastava, Goering, Rohrbach, Buckmaster. ASABE. • Off-Road Vehicle Engineering Principles. 2003. Goering, Stone, Smith, Turnquist. ASABE. Other good sources • Fluid Power Circuits and Controls: Fundamentals and Applications. 2002. Cundiff. CRC Press. • Machine Design for Mobile and Industrial Applications. 1999. Krutz, Schueller, Claar. SAE. Engines • Power and Efficiencies • Thermodynamics • Performance Engine Power Flows Power & Efficiencies • Fuel equivalent Pfe,kW = (HgkJ/kg∙ṁf,kg/h)/3600 [Hg = 45,000 kJ/kg for No. 2 diesel] • Indicated Pi,kW = pime,kPaDe,lNe,rpm/120000 • Brake Pb,kW = 2πTNmNe,rpm/60000 • Friction Pf = Pi-Pb Power & Efficiencies • Indicated Thermal Eit = Pi/Pfe • Mechanical Em = Pb/Pi • Overall (brake thermal) Ebt = Pb/Pfe = Eit*Em • Brake Specific Fuel Consumption BSFC= ṁf,kg/h/Pb,kW Dual Cycle Related equations • Compression ratio = r r = V1/V2 • Displacement De,l = (V1-V2)*(# cylinders) = π(borecm)2(strokecm)*(# cyl)/4000 • Ideal gas p1V1/T1 = P2V2/T2 • Polytropic compression or expansion p2/p1 = rn [n = 1 (isothermal) to 1.4 (adiabatic), about 1.3 during compression & power strokes] Related equations • Air intake ṁa,kg/h = .03De,lNe,rpmρa,kg/cu mηv,decimal From Stoichiometry (fuel chemistry) • A/F = air to fuel mass ratio = 15:1 for cetane What is the displacement of a 6 cylinder engine having a 116 mm bore and 120 mm stroke? For this same engine (7.6 l displacement, 2200 rpm rated speed), what is the air consumption if it is naturally aspirated and has a volumetric efficiency of 85%? Assume a typical day with air density of 1.15 kg/m3. With a stoichiometric air to fuel ratio based on cetane, at what rate could fuel theoretically be burned? Consider the this same (595 Nm, 137 kW @ 2200 rpm) engine which has a high idle speed of 2400 rpm and a torque reserve of 30%; peak torque occurs at 1300 rpm. Sketch the torque and power curves (versus engine speed). Torque (Nm) Power (kW) Speed (rpm) A quick problem … • Diesel engine generating 60 kW at 2300 rpm • Q: torque available 17 Alternative fuels What has to be similar? • Self Ignition Temperature • Energy density • Flow characteristics • Stoichiometric A/F ratio Power Hydraulics • Principles • Pumps, motors • Cylinders About Pressure • 14.7 psia STP (approx __ in Hg) • Gage is relative to atmospheric • Absolute is what it says … absolute & relative to perfect vacuum • What causes oil to enter a pump? • Typical pressures: – Pneumatic system – Off-road hydraulic systems 20 Liquids Have no Shape of their own 21 Liquids are Practically Incompressible 22 Pascal’s Law • Pressure Exerted on a Confined Fluid is Transmitted Undiminished in All Directions and Acts With Equal Force on Equal Areas and at Right Angles to Them. 23 Application Principles 1 lb (.45kg) Force 10 lbs (4.5kg) 10 sq in (6.5cm2) Piston Area 1 sq in (.65cm2) Piston Area 1 psi (6.9kpa) 24 Hydraulic “lever” 25 Types of Hydraulic Systems Open Center Closed Center The control valve that regulates the flow from the pump determines if system is open or closed. Do not confuse Hydraulics with the “Closed Loop” of the Power Train. (Hydro) 26 ClosedOpen Center Center Hydraulics Flow in Neutral Trapped Oil 27 Extend 28 Retract 29 Neutral Again 30 Pumps Pump Inefficiency • Leakage: you get less flow from a pump than simple theory suggests. – Increases with larger pressure difference • Friction: it takes some torque to turn a pump even if there is no pressure rise – Is more of a factor at low pressures Efficiency of pumps & motors • Em – mechanical efficiency < 1 due to friction, flow resistance • Ev – volumetric efficiency < 1 due to leakage • Eo =overall efficiency = Em * Ev • Eo = Power out/power in Qgpm = Dcu in/rev Nrpm /231 Flow Speed Tinlb = Dcu in/rev ∆Ppsi /(2π) Torque Required Pressure Rise Theoretical pump Effect of leakage Flow Relief valve or pressure compensator Pressure Constant power curve Flow Pressure Php = Ppsi Qgpm/1714 Example pump problems 1a. If a pump turns at 2000 rpm with a displacement of 3 in3/rev, theoretically, how much flow is created? 1b. If the same pump is 95% volumetrically efficient (5% leakage), how much flow is created? Example pump problems 2a. If 8 gpm is required and the pump is to turn at 1750 rpm, what displacement is theoretically needed? 2b. If the same pump will really be is 90% volumetrically efficient (10% leakage), what is the smallest pump to choose? Example pump problems 3a. A 7 in3/rev pump is to generate 3000 psi pressure rise; how much torque will it theoretically take to turn the pump? 3b. If the same pump is 91% mechanically efficient (9% friction & drag), how much torque must the prime mover deliver? Example motor problem If a motor with 2 in3/rev displacement and 90% mechanical and 92% volumetric efficiencies receives 13 gpm at 2000 psi … a. How much fluid power is received? b. What is it’s overall efficiency? c. How fast will it turn? d. How much torque will be generated? Cylinders Force balance on piston assembly: P1 * A1 P2 * A2 Fexternal Example cylinder problem • 3000 psi system • 2” bore cylinder • Extends 24 inches in 10 seconds • Q: max force generated • max work done • power used • flow required 43 • Tractor source with 2500 psi Example cylinder and 13 gpm available problem • Return pressure “tax” of 500 psi • Cylinder with 3” bore, 1.5” rod diameters • Q1: How much force will the cylinder generate? • Q2: How long will it take to extend 12 inches? Power Transmission Transmissions transform power a torque for speed tradeoff Gears Planetary Gear Sets Belt & Chain Drives • Speed ratio determined by sprocket teeth or belt sheave diameter ratio FIRST GEAR FIRST GEAR First gear speeds … if … Input shaft: 1000 rpm Main countershaft: 1000 (22/61) = 360 rpm Ratio = input speed/output speed = 1000/360 = 2.78 Ratio = output teeth/input teeth = 61/22 = 2.78 Secondary countershaft: 360 rpm (41/42) = 351 rpm Output shaft: 351 rpm (14/45) = 109 rpm RATIO: input speed/output speed = 1000/109 = 9.2 Product of output teeth/input teeth = (61/22)(42/41)(45/14) = 9.2 Example gear problem • If 50 kW @ 2400 rpm drives a pinion gear with 30 teeth and the meshing gear has 90 teeth (assume 98% efficiency)… • Q1: What is the speed of the output shaft? • Q2: How much power leaves the output shaft? • Q3: How much torque leaves the output shaft? Example planetary If the sun of a planetary gear set turns at 1000 rpm, what gear problem speed of the ring would result in a still planet carrier? Teeth on gears are sun: 20 and ring: 100. Example belt problem If a belt drive from a 1750 rpm electric motor is to transmit 5 hp to a driven shaft at 500 rpm and the small sheave has a pitch diameter of 4” … Q1: What should the pitch diameter of the other pulley be? Q2: Which shaft gets the small sheave? Q3: How much torque does the driven shaft receive? Php = Tft-lbNrpm/5252 Electricity Voltage = Current * Resistance Vvolts = Iamps * Rohms V I R Power = voltage times current PWatts = Vvolts*Iamps Three Types of Circuits Series Same current, voltage divided Parallel Same voltage, current divided Series / Parallel + 12 v. - Example 12 V DC A 12 V DC solenoid a hydraulic valve has a 5 amp problem fuse in its circuit. Q1: What resistance would you expect to measure as you troubleshoot its condition? Q2: How much electrical power does it consume? Example 12 V DC Q1: Identify specifications for a relay of a 12 V DC lighting problem circuit on a mobile machine if the circuit has four 60W lamps. Q2: Would the lamps be wired in series or parallel? Good luck on the PE Exam! • My email address: dbuckmas@purdue.edu • My web page: https://engineering.purdue.edu/~dbuckmas/ Note … ASABE members can access ASABE texts electronically at: http://elibrary.asabe.org/toc.asp