 Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 105 Numerical Study of the Operation of Motorcycles Covering the Urban Dynamometer Driving Schedule Albert Boretti * Department of Mechanical and Aerospace Engineering, Benjamin M. Statler College of Engineering and Mineral Resources, West Virginia University, Morgantown, WV 26506, USA Received 02 Sept ember 2016; received in revised form 24 Oct ober 2016; accept ed 25 Oct ober 2016 Abstract It is shown, for the most challenging case of a cruiser mo- torcycle of low weight-specific and displacement-specific power and torque, that the tuning for better top end perfo r- mances is irrelevant for the operation over the driving schedule used for certification. During the certification test, the engine only operates in the low speeds and loads portion of the map. It is concluded that any statement about motorcycles’ pollution and fuel consumption should be only based on the measure- ment of their regulated emissions through proper chassis d y- namometer tests, possibly redefining the driving schedule to better represent real driving conditions. Ke ywor ds : motorcycles, pollutant e missions, driv ing cycles , afterma rket tuners , real driving conditions 1. Introduction The actual operation of motorcycles covering emission and fuel econo my cert ification cycles has been brought back to the attention of lawma kers, original equip ment manufacturers and the general public by the recent ban of Harley-Dav idson (HD) aftermarket Electronic Control Units (ECU) tuners in the Un ited States of A merica (US). The use of afterma rket ECU tuners does not necessarily translate in worse regulated pollutant e missions as othe r- wise a lleged by the US Environ mental Protection Agency (EPA). Actually, these devices are mo re likely not re levant to any claim concerning emissions. The ECU tuners are simple corrections of the fuel in- jection para meters to deliver air -to-fuel (AFR) ratio that may increase throttle response, power/torque output and overall ride ability. Their target is mostly the steady Wide Open Throttle (WOT) operation of the engine, i.e. the high load operation, as well as the high speed operation, plus the sharp accelerations. 5 years. They are not supposed to satisfy the emission rules of the time they were certified after the 5 years. Therefore, it does not ma ke any sense to discuss the po l- lutant emissions of motorcycles older than 5 years with or without ECU tuners fitted. Ho wever, the retuning of an old engine may in principle offe r the opportunity to introduce some improve ments rather than declines in performances, power and torque, as well as fuel econo my and pollutant emissions, as the old factory calibration does not neces- sarily represent the best choice of engine controlling p a- rameter on a specific motorcycle more than 5 years old. According to EPA rules, new motorcycles are tested on a driving cycle , where the engine delivers the power needed for the motorcycle to fo llo w a low velocity schedule with everything but sharp accelerations and everything but high speeds. Hence, engines able to deliver much larger power and torque outputs operate signifi- cantly far fro m high load and high speed during the cert i- fication cycle , changing their load and speed much slower than what they could do, and reaching top power and torque outputs very far from their theoretical maximu m. For a gasoline fue led motorcycle having a three -way catalytic converter (TW C), port fuel injection and an oxygen sensor feed back to the ECU, any change of the controlling para meters returning a c loser to stoichiometric air-fuel-ratio is not e xpected to translate in any worsening of the e missions. Only operating the engine richer for increased power and torque output at higher loads and speeds may have pollutant emissions and fuel economy downfalls in these operating points. Fig. 1 p resents the typical e fficiency map of a catalytic converter. The emission reduction of a typical port fue l injected, ho mogeneous charge, and gasoline engine is based on the effic ient operation of the TWC that require a close to stoichiometry air-to-fuel ratio. Th is is obtained by operating the fuel injectors to deliver a stoichio metric mixtu re as monitored by the e xhaust oxygen sen sor feed-back. Around the stoichiometric point (A/F=14.63), all the three pollutants (HC, CO and NO) are a lmost totally re moved (>95 %).A slightly richer mixture translates in more CO and HC but not NO. A slightly leaner mixture translates in more NO but not CO and HC. The engine operation in super sport, touring and cruiser motorcycles covering the EPA Urban Dyna mo m- eter Driving Schedule (UDDS, 40 CFR Part 86, Appendix I to Part 86 - Dynamo meter Schedules) will be considered in the paper. Cruiser motorcycles are specific models designed with engines having low end specific perfo r- mances, i.e . sma ll d isplacement specific torque and power, small we ight specific torque and power, lo w speed, if compared to touring and obviously super sport bikes. Cru isers have large torques only because of the large dis- placement. *Corresponding aut hor, Email: a.a.boretti@gmail.com Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 106 Copy right © TAETI Copy right © TAETI Copy right © TAETI Copy right © TAETI Cop y right © TAETI Fig . 1 Conversion curves for HC, CO and NO as a funct ion of the a ir/ fuel ratio , for a port fuel injected gasoline engine fitted with a TWC removed (>95 % ) 2. EPA Motorcycles’ emission rules Street motorcycles’ emissions are regulated under section 202 of the Clean Air Act. Bac kground information on emission rules for motorcycles sold in the US may be found in [1, 2]. Table 1 (fro m [1]) su mmarizes the emis- sion limits to be satisfied during chassis dynamo meter testing of the motorcycle. St reet motorcycles’ emissions were regulated by a single unchanging set of standards for all mode l years from 1978-2005. In 2004, EPA established 2 tiers of conventional pollutant exhaust emissions stand- ards. Tier 1 ca me into effect in 2006. In 2010, standards for Class III motorcycles were updated to Tier 2 standards. Only class III motorcycles having a displacement in e xcess of 279 c m 3 are considered here, as the street mo- torcycle market is mostly made by super sport and touring bikes. Scooters are not considered. Highway Motorcycles Exhaust Emission Standards only apply since 1978. Befo re 1978 there we re no e mis- sion standards a motorcycle was requested to comply with. The standards applied first to new gasoline fueled moto r- cycles (since December 31, 1977). Then, later on, the standards were also applied to new, methanol-fueled mo - torcycles (since Dece mber 31, 1989), to new, natural gas-fueled and liquefied petroleu m gas -fueled motorcyc les (since Dece mber 31, 1996) and finally new motorcycles regardless of fuel (since 2006). The table also includes useful life and warranty period. They are e xp ressed in years and kilo meters, and whichever comes first limits the need of co mpliance. The term “useful life” [3] does not mean that a motorcycle must be scrapped or turned over to the government after ce rtain mileage limits are reached. It does not mean that a vehicle is no longer useful or that the vehicle must be scrapped once these limits are reached. The term has no effect on the owners’ ability to ride or keep their motorcycles for as long as they want. The cu r- rent useful life for motorcycles with engines over 279 c m 3 is 5 years or 30,000 kilo meters (about 18,640 miles), whichever first occurs. The test procedures for motorc y- cles fro m M Y 1978 and later are detailed in 40 CFR Part 86 Subpart F. Fig. 2 presents the cycle. This cycle is characterized by low speeds. Fig. 2 UDDS velocity schedule http://www.intechopen.com/books/diesel-engine-combustion-emissions-and-condition-monitoring/nox-storage-and-reduction-for-diesel-engine-exhaust-aftertreatment#F1 Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 107 Cop y right © TAETI Table 1 Emission standards in the US (from [1]) Year Class Engine Size (cm 3 ) HC (g/km) HC + NOx (g/km) CO (g/km) Useful Life Warranty 1978-2005 I 50-169 5.0 - 12.0 5 / 12,000 5 / 12,000 II 170-279 - 5 / 18,000 5 / 18,000 III 280+ - 5 / 30,000 5 / 30,000 2006+ I-A < 50 1.0 1.4 12.0 5 / 6,000 5 / 6,000 I-B 50-169 1.0 1.4 12.0 5 / 12,000 5 / 12,000 II 170-279 1.0 1.4 12.0 5 / 18,000 5 / 18,000 2006-2009 III (T ier 1) 280+ - 1.4 12.0 5 / 30,000 5 / 30,000 2010+ III (T ier 2) 280+ - 0.8 12.0 5 / 30,000 5 / 30,000 3. HD Clean Air Act Settlement The U.S. EPA and the U.S. Depart ment of Justice (DOJ) announced on August 18, 2016 a settle ment with HD co mpanies, that required the companies to stop selling and to buy back and destroy “illegal tuning devices that in-crease air pollution fro m their motorcycles”, and to sell only tuning devices that are cert ified to meet Clean Air Act emissions standards. HD was also requested to pay a $12 million civ il penalty and spend $3 million on a project to mitigate a ir pollution through a project to replace conve n- tional woodstoves with cleaner -burning stoves in loca l communities. EPA alleges that HD vio lated the Clean Air Act by manufacturing and selling about 340,000 devices, known as tuners that “allow users to change how a motorcycle’s engine functions”. According to EPA “these changes can cause the motorcycles to e mit higher a mounts of certain air pollutants than they would in the original configuration that HD certified with EPA”. According to EPA, Since January 2008, HD man u- fac-tured and sold tuners that allow users to modify “cer- tain aspects of a motorcycles’ e missions control system”. According to EPA, these modified settings increase power and performance, but a lso increase the motorcycles’ emissions of hydrocarbons and nitrogen oxides (NOx). The cla im o f vio lations is not based on any chassis dy-namo meter measure ments of the performances of motorcycles not having exceeded the useful life of 5 years or 30,000 km tested first without, and then with the kit fitted, to prove that a specific motorcycle model was not compliant because of the fitting of a specific kit. 4. Street Performance Tuners The Screa min' Eag le Street Performance Tuner is a performance engine manage ment system for e lectronic fuel inject ion (EFI) equipped Ha rley Davidson models [5-7]. The kit utilizes a wide-band o xygen sensor feedback to provide continuous air-to-fuel rat io (AFR) tuning cor- rections based upon riding conditions. The kit is a imed to deliver increased throttle response and torque, improved overall ride ability and performance, as well as a smoother and cooler running engine. In many cases, the kit helps improving fuel econo my, depending upon the bike’s configuration and the set -up of the AFR targets. AFR targets set to richer values than the stock levels to gain performance may result in moderate decrease in fuel econo my. The Street Tuner permits li m- ited tunability within the emissions range to optimize drivability without compro mising e mission, but it is o b- viously intended to work outside the closed loop portion of the engine map where the AFR is ensured to be about stoichiometric for the best operation of the three-way-catalytic converter. Fig. 3 Typical AFR map of a large HD cruiser with a Big V-twins engine Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 108 Copy right © TAETI Copy right © TAETI Copy right © TAETI Copy right © TAETI Cop y right © TAETI A typical tuners fuel map of a large HD c ruiser is provided in [7] and reproduced in Fig. 3. The engine is a Big V-t wins engine (Twin Ca m 96, 96.96 cubic inch or 1,584 c m 3 ).These engines are characterized by much smaller specific torque and power density than the average super sport and touring bikes. Ma ximu m power (but at the wheels, where it is typically 10-15% sma ller than at the crank) is only 68 HP @ 5,000 rp m, while ma ximu m torque (also at the wheel) is 110 N m @ 3,000 rp m. This engine powers motorcycles of 307.5 kg wet we ight including oil and gas. Only above 80% MAP (roughly 50% throttle) and 4,000 rp m, where the engine does not operate during ty p- ical driving cyc les inc luding the cert ification cycle , the AFR is made rich. Diffe rent “stages” of tuning are considered in [7]. The Stage 2 is an upgrade that includes new cams. The Stage 1 is upgrade also requiring e xhaust and air c leaner. Both these upgrades are valid 50-state legal modification. The Air Fue l Map of [7] has the cells in red with the stoichiometric 14.6 AFR in them over the low speed low load portion of the map that is re levant to the emission certification. Both stage 1 and stage 2 tunings do not affect this area. The ECU runs in c losed loop mode looking at the oxygen sensor to satisfy the optima l co mposition of the e xhaust gases for the TWC to reduce the tail p ipe e mis- sions. In [7], the engine works c losed loop to 3,750 rp m and to 80 kPa of manifo ld absolute pressure (MAP). The ECU uses manifo ld pressure from the MAP sensor to determine the actual engine load rather than the throttle. Throttle position does not relate linearly to the MAP sensor reading. 80 kPa MAP is typically a round 40% throttle. Ref. [7] assumes the OEM AFR table is s ame or very similar of Fig. 3, but may obviously differs outside the 3,750 rp m and 80 kPa area. The refore, tuners are possibly delivering same AFR vs. MAP and speed of the OEM in the low MAP and low speed area of emissions’ control, and then they differ. It is worth to mention that usually steady state AFR maps do not need fuel rich conditions except than a p- proaching WOT conditions, i.e. close to the ma ximu m loads for any speed. The fuel rich mixture at speed e x- ceeding 3,750 rpm any load seems quite questionable. The tuner operates rich everywhere out of th e closed loop area very like ly because the injection system is ev e- rything but effective in delive ring the amount of fuel needed when the throttle opens sharply. Even racing engines these days go rich only approaching WOT cond i- tions at any speed, as even these extre me engines run slightly lean part load to reduce unnecessary fuel co n- sumption. In addition to the AFR map, Ref. [7] a lso provides the bias tables and the Ignition Advance map for the HD Stage 1 and Stage 2 bikes. It is not the object of the paper to enter more in details of the specific tunings, only to show in the next section how the operation of a motorcycle over a driving schedule for e mission certification never utilizes the high loads or high speeds parts of the map that are the ultimat e goal of tuning an engine for mostly imp roving power and torque output. 5. Method Map based computer models are used to investigate the operation of an engine when the motorcycle is covering a driving schedule. Vehicle Driv ing Cycle Simulat ions have been around for many years. Basic solutions of the New-ton’s equation of motion fo r a vehicle following a pre-scribed velocity schedule returns the instantaneous power requested to the engine with a simp lified modelling of transmission losses, aerodynamic and rolling resistance, and vehicle and engine inertia. Trans mission ratios then also return the speed requested to the engine. Interpolating the steady state maps of brake specific fuel consumption or specific e missions, it is then possible to evaluate the fuel consumption and the pollutant emissions on a driving cycle. For cold start, correction curves are needed. For the interested reader, these simulat ions are presented in [12-24]. To simplify, a driving cycle simulator solves the Newton’s equation of motion. If Fp,e is the engine propul- sive force and Fb,f is the friction brake force, it is: Fp,e-Fb,f-Fa-Fr= m∙a (1) with m the mass, a the acceleration, =dv/dt, with v velocity of the motorcycle and t the time , Fa the ae rodynamic drag force, =½∙ρ∙v 2 ∙CD∙A, with ρ a ir density, CD drag coeffi- cient (a lways positive for a retard ing force) and A re fe r- ence area, Fr the rolling resistance force, an e mp irica l function of the speed of the motorcycle. In terms of powers, by multiplying for the speed of the motorcycle, it is then Pp,e -Pb,f = m∙v∙dv/dt +½∙ρ∙v 3 ∙CD∙A+Pr (2) The above propulsive power is computed at the wheel. The power of the engine at the crankshaft Pb is larger than the power at the whee l Pp,e to include the transmission efficiency η. The speed of rotation of the engine is then obtained by the speed of the motorcycle by considering tire radius, gear and gear ratios. The gear is determined by an upshift/downshift strategy. Fro m a ve locity schedule v(t), it is thus possible to compute the instantaneous power Pp,e and Pb,f, and fro m Pp,e, then the power Pb and the speed N that the engine must provide. When m∙v∙dv/dt +½∙ρ∙v 3 ∙CD∙A+Pr ≥0 , equation (2) returns Pp,e with Pb,f=0. When m∙v∙dv/dt +½∙ρ∙v 3 ∙CD∙A+Pr <0, equation (2) returns Pb,f with Pp,e=0. Pb,f represents in this case not only the actual power dissipated in the friction brakes P * b,f , but also the negative power requested to motor the engine at the given speed N (engine brake). Engine performances are typically defined in terms o f power Pb, torque Tb and brake mean effect ive pressure BM EP. The power Pb is proportional to the product of torque Tb and speed N. The BM EP is proportional to the ratio of torque Tb and total displaced volume Vd. Engine data are provided as the wide open throttle torque output Tb vs. speed N, plus the maps of specific fue l consumption and pollutant emissions vs. BM EP and N. Th is way the driving cycle simu lator returns the fuel economy and the pollutant emissions during warmed -up cycles, with e m- pirical penalty functions needed for cold -start cycles. Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 109 Cop y right © TAETI The model simu lates a motorcycle perfo rming a test cycle. The UDDS is considered. The cycle is everything but aggressive, and it is characterized by mostly low speed. In the UDDS cyc le, Fig. 2, only in one of the acceleration, cruise and deceleration schedules it is requested a bike velocity of 90 km/h, and in only 3 other areas the bike reaches a speed above 50 km/h but less than 60 km/h. Figs. 4, 5 and 6 present reference data of BM EP, torque and power vs. engine speed and throttle opening % for the typica l large cruiser considered here, having a lo w displacement specific powe r and torque, lo w ma ximu m speed. The engine is 1,300 c m 3 and it is fitted on a heavy motorcycle of weight 380 kg inc luding the driver during the simulated chassis dynamometer test. Fig. 4 Typical Brake Mean Effective Pressure map of a large cruiser motorcycle Fig. 5 Typical torque map of a large cruiser motorcycle Fig. 6 Typical power output map of a large cruiser motorcycle Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 110 Copy right © TAETI Copy right © TAETI Copy right © TAETI Copy right © TAETI Cop y right © TAETI Table 2 Model parameters Rated Engine Speed 6,500 RPM Upshift 4,000 RPM Downshift 2,000 RPM Ratio of 1 st Gear 9.312 Ratio of Gear 2 6.421 Ratio of Gear 3 4.774 Ratio of Gear 4 3.926 Ratio of Gear 5 3.279 Ratio of Gear 6 2.79 Motorcycle Weight 377 kg Engine Power at Rated Speed 55 kW Tire Rolling Radius 457.2 mm Tire Rolling Resistance Factor 0.0122 Engine Displacement 1,304 cm 3 Engine Inertia 0.05 kg-m 2 Frontal Area 0.2 m 2 Coefficient of Drag 0.6 Wheelbase 2 m Initial Engine Speed 1,500 RPM Initial Gear Number 1 Table 2 presents the relevant model para meters, id le speed, rated engine speed and engine power at rated speed, upshift and downshift speed, that may differ at every gear, ratios of 1 st to 6 th gear (if a 6 gear transmission is consid- ered as it is in this case), motorcycle we ight, tire ro lling radius, tire ro lling resistance factor, engine displacement, engine inertia, frontal a rea, coeffic ient of drag, whee lbase, initial engine speed and gear number. 6. Results and Discussion The engine map BM EP (bra ke mean effect ive pressure) vs. engine speed at different loads is the one of Fig. 4, where the load is expressed in terms of acceleration pos i- tion (AP). Fig. 7 presents the computed operating points, while Fig. 8 presents the computed time d istribution on engine map of the operating points of a large cruiser motorcycle covering the UDDS cyc le. The engine operates below 2.5 bar BM EP and belo w 4,000 rp m over the cycle. Every map point above these values has time d istribution ze ro, i.e . whatever could be the e mission in these points, and this has no effect on the regulated emissions. The most part of the time the engine is idling. Then, when delive ring an output, the engine is always operating well be low 2.5 bar BMEP and 3,750 rpm. Fig. 7 Typical operating points of a large cruiser motorcycle covering the UDDS cycle Fig. 8 Typica l t ime distribution on engine map of the operating points of a large cru iser motorcycle covering the UDDS cycle Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 105 - 112 111 Cop y right © TAETI In terms of performances, today’s super sport, touring and cruiser bikes may have very high specific power and torque densities. As HD does not provide online info r- mat ion about power and torque figures, typical perfo r- mance parameters are proposed for other manufacturers. The 998 c m 3 Ya maha YZF R-1 [9], one of the most powerful super sport bikes, has for e xa mp le 200 HP/ liter revving 13,500 rp m. The specific torque is less e xceptional, as the result of the tuning for high speeds , but still 112 N m/ lite r revving 11,500 rp m. The wet we ight including full oil and fue l tank is 199 kg. Th is bike has a top speed of 300 km/h. As an e xa mp le of touring bikes, the 1,298 c m 3 Ya maha FJR1300A [10] has 112 HP/liter revving 8,000 rpm and 106 N m/ liter revving at 7,000 rp m. The wet weight (inc luding full oil and fuel tank) is 289 kg. This bike has a top speed of 245 km/h. Finally, as a typica l cruiser, the 1,304 c m 3 Ya maha XVS1300 Custom [11] has 56 HP/ liter revving 5,500 rp m and 79 N m/ liter rev ving 3,000 rp m. The wet we ight including full oil and fuel tank is 293 kg. This b ike has a top speed of 175 km/h. There fore, in norma l driving correctly accounted for e mission reg u- lations, motorcycles work very far from their potentials. The most part of the motorcycles in the super sport and touring classes are usually mo re performant than the cruisers. They have much larger power and torque to weight ratio, as they are much lighter, and also have much larger displace ment specific powe r and torque. The most part of the super sport and touring motorcycles are there- fore working even fa rther away fro m their h ighest speed and highest load points where they may operate off-stoichio metry during typical driving cycles including the UDDS e mission cycle. The resu lts proposed in the previous section are therefore a worst case scenario. 7. Conclusions It is pure speculation to cla im that ECU tuners can cause the motorcycles to e mit higher a mounts of certain air pollutants than they would in the orig inal cert ified co n- figuration without even mentioning the specific motorc y- cle where the tuners are fitted. In princip le, ECU tuners are not expected to affect any regulated emission. If fitted to motorcycles having e xceeded the useful life , presently defined as 5 years or 30 ,000 kilo meters (about 18,640 miles) whichever first occurs, as these motorcycles are not presently expected to comply with any emission rule, having or no the tuners makes no difference. For new motorcycles, the ECU tuners are expected to modify the AFR only at the higher loads and speeds that are very far fro m the area of operating points that are d e- signed closed loop stoichiometric, to comply with the emission rules properly using the TWC. Old and new motorcycles cannot be claimed a-priori not compliant without providing any evidence of failure to per- form as required by regulation, and obviously they cannot be claimed not compliant if there is no rule to comply with. Any statement about motorcycles’ pollution and fuel consumption should be only based on the measurement of their regulated emissions through proper chassis dynamometer tests. The results emphasize the importance of rea l world driving in motorcycles. The paper shows that the ECU tuners have no effect on the presently regulated pollutants emission, even if modifying the AFR certa inly lead to change in e mission performance of the vehicles. While the ECU tuners may not affect the pollutants emission under well-constraint laboratory certification tests, they certainly change the emissions over real world driving. The paper therefore e mphasizes the importance of the inclusion of real world driving in emission certification tests. The introduction of better e mission certification tests will ultimately translate in superior fuel conversion effi- ciencies of the internal co mbustion engine over the full range of loads and speeds, for e xa mp le also simply adopting jet ignition and direct injection [25], plus the hybridizat ion of the powe r tra in, for e xa mp le with a fly- wheel or a Li-ion battery based kinetic energy recovery system [26]. References [1] “US EPA Light -Duty veh ic les a nd t ruc ks e miss ion standa rds,” h ttps :// www.ep a .gov/ e miss ion -stan da r ds-re fe ren ce -gu ide/ light -duty -v eh ic les -and -t ruc ks - emission-standa rds , retrieved August 31, 2016. 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