MEEG 346 Thermal Laboratory

Transcription

1 MEEG 346 Termal Laboratory Experiment #4: Te Four-Stroke Combustion Engine Introduction Small engines are used in many applications. Tey are used to power motorcycles, lawnmowers, go-carts, and even small portable electric generators suc as te one used in tis lab. Tey are not very dierent rom automobile engines at all. In act, te only dierence is te size, te way in wic te engine cools itsel, and te number o cylinders. An automobile engine, being larger and producing a larger amount o energy, as a water cooling system tat pumps water troug cannels in te engine block wile most small engines are air-cooled and ave a an tat orces air across te inned engine block. Tese engines run on te our-stroke cycle, wic describes ow it converts te combustion energy into mecanical energy. Figure 1: Four-Stroke Cycle (Reerence 1) In te igure above, look at te intake stroke irst. It sows te piston pulling in an airuel mixture rom outside te cylinder until it reaces te bottom dead center position (BDC). It ten travels back up te cylinder to compress te air-uel mixture on te 1

2 compression stroke until it reaces te top dead center position (TDC). At tis point, te spark plug ignites te mixture, sending te piston back down to BDC as te combusting gases expand on te power stroke. Finally, te piston travels back up to TDC wile it orces te combustion products out o te cylinder and te cycle continues. Figure is a scematic drawing sowing te basic parts o a ourstroke engine. It sows ow te piston converts te linear up and down motion to rotational motion in te cranksat. Te cranksat spins te transmission in a veicle or in our case, it spins te sat o te generator to produce electric energy. Te generator is similar to an electric motor tat is driven in reverse to Figure : Four-Stroke Engine (Reerence 3) covert mecanical energy into electrical energy instead o te oter way around. Objective To sow tat in tis energy transerring system, as in all systems, te energy is conserved. Te eiciency o te system will also be determined and compared to te amount o pollutant emissions in te exaust. Engine perormance will be evaluated as a unction o load and air-uel ratio.

3 Teoretical Considerations Part 1: Conservation o Energy Conservation o energy means tat te energy into te system is equal to te energy out. In te case o te combustion engine, te energy in is te uel power into te engine and te energy out is te mecanical power out and te eat transerred out. Figure 3: Energy Transer Te energy balance equation is as ollows. o o + W + n ( + ) r = Q + W + Q in in r out out n p ( o + o ) p Were n(r) and n(p) are te molar low rates o te combustion reactants and products respectively. Te s are te molar entalpy o ormation at te standard conditions, te molar entalpy at te present condition, and te molar entalpy at te standard conditions respectively. A standard condition reers to 5 degrees C and 1 atm. 3

4 Te work and eat transer into te system is equal to zero so te equation reduces to: o o o o n ( ) r = Q + W + n ( + ) p r + out out p Te unbalanced stoiciometric equation or tis combustion reaction is as ollows. CH + ( O N ) 4 CO + H O N Te engine is actually burning natural gas, wic is a combination o metane, propane, and a small amount o nitrogen. For tis portion o te lab assume tat te gas is pure metane. Te work output o te engine can be directly obtained rom te generator load, wic is a set o large resistors tat consume te electrical energy produced. Te resistors are set up in our pairs o two. Eac pair consumes about 1000 W o electricity. Te actual work output can be calculated by using te andeld Wattmeter and multiplying it by te number o resistor pairs in te circuit. Te eat transer out o te engine is a very complex eat transer problem. Most o te eat is transerring out troug te exaust. Te remaining eat is transerred rom te inned engine block to te open air wit te elp o te cooling an. But te engine is mostly made out o steel and cast iron wic as a very ig termal conductivity so eat travels in every direction rom te combustion camber. It is not a bad assumption toug to say tat te eat transers rom te engine block uniormly rom te inned surace alone. Te next complication comes rom te engine block geometry. By a quick examination, it can be seen tat te ins are not exactly symmetrical and tat te engine block is not exactly square or circular in cross-section. To simpliy te calculations, te in lengts ave been 4

5 averaged togeter and te cross-section o te engine block is assumed to be circular. Tis simpliication can be visualized in te igure below. Figure 4: Fin Approximation (Rigt side rom Reerence 4.) Te total eat transer can now be calculated using te equation below. 1 L c = L + t Were A = π ( r c r ) A 1 t = NA + πr1 ( H Nt) 1 r c = r + t N is te number o ins. 1/ 3 / Next, determine L were k Al is te termal conductivity o te in c k Al Lct material (aluminum), and is te convection eat transer coeicient. Use igure 3.19 o your text (Incropera and Dewitt, 4 t Ed., Figure 3.19) to determine te in eiciency, η(). H = cm r 1 = 7.00 cm r = 9.53 cm t = 0.5 cm N = 11 ins NA Q = A T out t 1 (1 η ) b At ( T ) For te geometry o te engine block, use te ollowing values: 5

6 Te convection eat transer coeicient () can be calculated by measuring te air velocity, calculating te Reynolds Number, and using te Nusselt number relation below. Nu = D k air = Re max (Note :Te expression at te let is or air, wit Pr = 0.7) Were ρ airu D max ( s + t) S T Re max = and umax = u µ air ( s + t) + s( D D) Te equations above were derived or use wit lows passing inned tubes and can be depicted in te igure below. Te equation above relating u(max) and u( ) is or cases wen tere is more tan one inned tube present. In te case o te engine block, it is being approximated as one inned tube. Tis causes te and S(T) terms to go to ininity. Tereore, te equation reduces to u(max) = Figure 5: Flow Passed Finned Tubes. (Reerence 5) u( ). Te variables tat are not already listed above are as ollows: D = *r 1 D() = *r Re = Reynolds Number Nu = Nusselt Number ρ = air density µ = kinematic viscosity o air 6

7 Part : Emissions and Eiciency Te eiciency or tis system can be calculated wit te ollowing equation: η overall = HHV Wout m uel Were HHV is te iger eating value o te uel. Tere are many dierent components in te exaust o an engine. Tey are mainly carbon dioxide and water but also includes carbon monoxide, oxygen, ydrocarbons (unburned uel), and small amounts o NOx and SOx. Since te amounts o SOx and NOx are minute, tey will be ignored or tis experiment. Wen te air-uel mixture entering te combustion camber is lean (ig air, low uel) tere is an excess amount o oxygen in te exaust. Wen te engine is running ric (low air, ig uel), CO and ydrocarbons are present in te exaust. CO and ydrocarbons are bad or te environment and are deadly i inaled. Running an engine at optimum eiciency is a struggle between increasing te uel and tereore te power and keeping te armul emissions low. Te eiciency o te engine will increase wit increasing uel until all o te available oxygen is used up, and ten it will decrease wit increasing uel. Te actual amount o ydrocarbons will not be measured but note tat wen CO is present, so are te ydrocarbons. 7

8 Example: Assuming an air-uel ratio o, say 15, te combustion equation is (note tat tere are 3.76 parts o N or 1 part o O in air): CH O N CO CO * y z + a + b 4 were x, y, z, a, and b are unknowns. a and b may be obtained using te values rom te O and CO sensors. Te remaining quantities may be calculated by noting tat: x + y/ + z + a/ = 3.15 (balancing O ) x + a = 1 (balancing C) y + b = (balancing H) Procedure 1. Wit te engine warmed up, turn on te irst resistor pair and wait or steady state. Te velocity o te cooling air can be measured wit te andeld anemometer at tis point. Also record te ambient room temperature.. Take mass low readings o te uel and te air, temperature readings o te engine block base and te exaust, and a power reading wit te andeld Wattmeter. 3. Turn on te second resistor pair and repeat step ater waiting or steady state. Repeat wit tree resistor pairs and ten our. 4. Wit two resistors loading te engine and te intake valve all te way open, take readings o te O and CO content in te exaust, a mass low reading o te uel, and a power reading. (tis is te lean setting) 5. Repeat step 4 at te ric setting. Set te engine ric by slowly adjusting te intake valve until te oxygen display reads zero. 6. Te optimum air-uel ratio lies somewere between tese two points. Adjust te intake valve to some point between te lean and ric settings and repeat step 4. H O N x O H 8

9 Analysis 1. Calculate all o te quantities in te energy balance equation or all our trials.. Construct a cart o trial number vs. power and include a line or input power and one or output power. Does te energy appear to be conserved? Are tere any increasing or decreasing discrepancies as te power demand is increased? 3. Calculate te eiciency o te system at te optimum, lean, and ric settings o te air-uel ratio. Construct a plot sowing te eiciency o te system vs. air-uel ratio. Wic setting yields te most eicient system? 4. Construct a cart sowing O and CO content in te exaust vs. air-uel ratio. From te cart, determine te optimum air/uel ratio. 5. Explain te sources o error in tis experiment. Reerences 1. Micael A. Boles and Yunus A. Cengel, Termodynamics: An Engineer Approac, McGraw-Hill Companies, Inc., USA, Marsall Brian, How Stu Works, 3. M. David Burgardt, James A. Harbac, Engineering Termodynamics, HarperCollins College Publisers, USA, David P. DeWitt and Frank P. Incropera, Introduction to Heat Transer, Jon Wiley & Sons, Inc., USA, Eric C. Guyer, Handbook o Applied Termal Design, McGraw-Hill Companies, Inc., USA,