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1、2002-01-2743 High Efficiency and Low Emissions from a Port-Injected Engine with Neat Alcohol Fuels Matthew Brusstar, Mark Stuhldreher, David Swain and William Pidgeon U. S. Environmental Protection Agency Copyright 2002 Society of Automotive Engineers, Inc. ABSTRACT Ongoing work with methanol- and e
2、thanol-fueled engines at the EPAs National Vehicle and Fuel Emissions Laboratory has demonstrated improved brake thermal efficiencies over the baseline diesel engine and low steady state NOx, HC and CO, along with inherently low PM emissions. In addition, the engine is expected to have significant s
3、ystem cost advantages compared with a similar diesel, mainly by virtue of its low-pressure port fuel injection (PFI) system. While recognizing the considerable challenge associated with cold start, the alcohol-fueled engine nonetheless offers the advantages of being a more efficient, cleaner alterna
4、tive to gasoline and diesel engines. The unique EPA engine used for this work is a turbocharged, PFI spark-ignited 1.9L, 4-cylinder engine with 19.5:1 compression ratio. The engine operates unthrottled using stoichiometric fueling from full power to near idle conditions, using exhaust gas recirculat
5、ion (EGR) and intake manifold pressure to modulate engine load. As a result, the engine, operating on methanol fuel, demonstrates better than 40% brake thermal efficiency from 6.5 to 15 bar BMEP at speeds ranging from 1200 to 3500 rpm, while achieving low steady state emissions using conventional af
6、tertreatment strategies. Similar emissions levels were realized with ethanol fuel, but with slightly higher BSFC due to reduced spark authority at this compression ratio. These characteristics make the engine attractive for hybrid vehicle applications, for which it was initially developed, yet the s
7、ignificant expansion of the high-efficiency islands suggest that it may have broader appeal to conventional powertrain systems. With further refinement, this clean, more efficient and less expensive alternative to todays petroleum-based IC engines should be considered as a bridging technology to the
8、 possible future of hydrogen as a transportation fuel. INTRODUCTION Alternative fuels, especially alcohol fuels, offer potential to mitigate national security and economic concerns over fuel supplies as well as environmental concerns over tailpipe emissions and resource sustainability. As a result,
9、there has been continuing interest in alternative fuels, heightened recently over proposed legislation that would mandate increases in the use of renewable transportation fuels. Over the last thirty years of automotive research, a variety of alcohol fuelsprimarily methanol, ethanol and blends with h
10、ydrocarbon fuels have demonstrated improved emissions of oxides of nitrogen (NOx) and particulate matter (PM) as well as moderately improved brake thermal efficiency 1-4. Despite this, infrastructure barriers as well as technical challenges, notably cold starting, have limited the widespread use of
11、neat alcohol-fueled vehicles. The benefits and challenges of neat alcohol fuels in PFI applications have been demonstrated in numerous earlier works. Benefits such as higher efficiency and specific power and lower emissions may be realized with alcohols: their high octane number gives the ability to
12、 operate at higher compression ratio without preignition 5; their greater latent heat of vaporization gives a higher charge density 1-3, 6; and their higher laminar flame speed allows them to be run with leaner, or more dilute, air/fuel mixtures 7. In addition, alcohols generally give lower fuel hea
13、t release rates, resulting in lower NOx emissions and reduced combustion noise 2. The engine described in the present work uses these inherent advantages of alcohol fuels as the basis for its design and control, thereby enabling attainment of efficiency levels exceeding that of the diesel, with low
14、emissions. One of the main challenges with neat alcohol fuels is cold start emissions, especially in PFI engines 8. In such applications, the low vapor pressure and low cetane number must be overcome with higher-energy ignition systems or higher compression ratio 9. Further, the increased wetting of
15、 the intake manifold, cylinder walls and spark plugs must be addressed in the design of the combustion chamber and in the control of transient fueling during startup 8, 10. Because of these issues, earlier works with PFI SI methanol engines commonly report starting problems below ambient temperature
16、s of about 10 oC 8, 11. With extended periods of cold cranking (i.e., 60 seconds or more), successful starting has been achieved at ambient temperatures as low as 6.5 oC 12. However, these studies were generally performed with lower compression ratio engines, derived from their gasoline counterparts
17、, which are therefore not necessarily optimized for use with neat alcohol fuels. The ongoing work at EPA with high-compression ratio single cylinder PFI SI engines 13, for example, demonstrates the ability to fire on neat methanol during relatively brief cranking at higher speeds, at temperatures as
18、 low as 0 oC. Earlier work at EPA with alcohol-fueled multi-cylinder engines examined both PFI and DI configurations, generally demonstrating improvements in fuel economy and power, as well as promising cold start emissions. Initial work with a methanol-fueled PFI SI engine 14, 15 yielded fuel econo
19、my and emissions that were similar to, but not significantly better than, the baseline gasoline engine. Cold starting was not addressed in that early program, but was instead examined in a follow-on project with a turbocharged, DI, glow-plug-ignited, stratified charge engine 16, based on earlier wor
20、ks showing good cold start performance down as low as 29 oC 17. This project was largely successful, demonstrating good startability and driveability down to 29 oC, and producing low FTP emissions of NOx (0.3 g/mi), HC (0.01 g/mi), CO (0.2 g/mi), PM (0.02 g/mi) and aldehydes (0.002 g/mi) while runni
21、ng lean with a single-stage oxidation catalyst. The measured fuel economy was between 7%- 22% better than the baseline gasoline engine, but still slightly lower than the turbocharged diesel. The ongoing PFI alcohol work presented here builds on our earlier experience, and demonstrates better steady
22、state efficiency than the baseline diesel and low emissions with conventional aftertreatment systems, at a significantly lower cost than the diesel. The engine described below runs unthrottled over most of its load range, much like the diesel, but operates with high EGR dilution ratios at stoichiome
23、tric fueling, rather than lean and stratified. This strategy takes advantage of the favorable dilute flammability limits of alcohol fuels to operate with lower pumping losses, and uses the high levels of EGR to control knock at high compression ratio. As a result, the engine demonstrates its potenti
24、al as an efficient, lower cost, renewable fuels alternative to the diesel. EXPERIMENTAL SETUP The research described below is being conducted under EPAs Clean Automotive Technology Program, in order to demonstrate feasibility of cleaner, more efficient technologies. The primary focus of the work is
25、on methanol fuel, since it represents the limiting case of oxygenated fuels, at 50% oxygen by mass. Also, its physical properties lend some performance advantages over other alcohols, discussed below. For comparison, however, brake thermal efficiency data with ethanol fuel is also given below, demon
26、strating similar benefits. ENGINE AND TEST DESCRIPTION The engine designed for this work is derived from the 1.9L Volkswagen TDI automotive diesel engine, modified suitably to accommodate port fuel injectors and spark plugs. The stock inlet ports give a swirl ratio of about 2.0, a factor that has be
27、en demonstrated to reduce the tendency for knock 18. Knock was further reduced by modifying the stock combustion chamber to eliminate potential preignition sites. A range of compression ratios from 17:1 to 22:1 were tested in this engine with methanol fuel, although the results reported below were c
28、onducted at a nominal compression ratio of 19.5:1. Intake manifold pressure was maintained with a variable geometry turbocharger, which, in turn, also varied the exhaust backpressure on the engine. EGR was metered from the low-pressure side of the turbine to the low- pressure side of the compressor,
29、 using a variable backpressure device in the exhaust. The EGR temperature was reduced with a stock Volkswagen water-to-air cooler before the compressor, and the EGR and fresh air were cooled after the compressor with a stock air-to-air intercooler. Together, these compact heat exchangers were able t
30、o maintain intake manifold temperatures in the vicinity of 30 oC. At least four different types of port fuel injectors were evaluated for measured engine brake thermal efficiency as well as spray characteristics with methanol, verified with high-speed planar laser imaging. The best- atomizing inject
31、ors among the group were racing-style, 36 lb/hr, 12-hole port fuel injectors manufactured by Holley, operating at 4 bar rail pressure. For best startup and transient performance, the injector tip was targeted at the back of the intake valve, from a distance of approximately 80 mm. The ignition syste
32、m consisted of a production Toyota coil with a Champion dual electrode, recessed gap spark plug. High load operation, with a combination of high cylinder pressures and smaller spark advance, placed great demand on both the plugs and coils. Together with higher corrosive properties of methanol, spark
33、 plug durability was somewhat of an issue in this testing, as had been witnessed in earlier works 9. Table 1: EPA alcohol engine specifications Engine Type4 cyl., 4-stroke Combustion TypePFI, SI Displacement1.9L Valves per cylinder2 Bore79.5 mm Stroke95.6 mm Compression Ratio19.5:1 IVO-344 o ATDC* I
34、VC-155 o ATDC* EVO152 o ATDC* EVC341 o ATDC* Bowl Volume18 cc Clearance volume26.4cc Swirl Ratio2.0 InjectorsHolley, 36 lb/hr, 12-hole nozzle Rail Pressure4 bar Spark PlugsChampion recessed gap, dual electrode Turbocharger typeVariable geometry Exhaust Aftertreatment Ford FFV 2-stage, three-way cata
35、lyst *-relative to fired TDC The engine was run with anhydrous chemical-grade methanol and ethanol fuels, and batch chemical analyses were performed to verify the heating value and density. NOx emissions were measured with a chemiluminescent NOx analyzer, while CO emissions were measured with a non-
36、dispersive infrared analyzer. Unburned hydrocarbon (HC) emissions were measured with a heated flame ionization detector calibrated with propane, but corrected separately for response to methanol and ethanol. A two-stage, three-way Ford FFV catalyst was used for exhaust aftertreatment, and was aged a
37、pproximately 10 hours at high, variable load prior to testing. ENGINE CONTROLS DESCRIPTION The engine controller was a Rapid Prototype Engine Control System (RPECS) provided under contract from Southwest Research Institute. The EPA operating strategy was based on three fundamental principles: (1) Hi
38、gh compression ratio, in order to give an expanded dilute operating range; (2) Turbocharging with high levels of EGR, for primary load control and low NOx emissions; (3) Stoichiometric fueling (based on oxygen to fuel), to permit operation with a three-way catalyst. The performance and/or emissions
39、benefits of individual components of this strategy have been demonstrated in earlier works, discussed below. Taken together, however, the present strategy is unique, and presents a path for attaining high levels of efficiency and low emissions in a practical, feasible system. Methanol and ethanol ha
40、ve relatively high octane numbers compared with gasoline; published RON values for methanol and ethanol are between 105-109, compared with about 91-99 for gasoline 19, 20. As a result, they may be run at a much higher compression ratio, thereby yielding higher engine thermal efficiency. Earlier work
41、s with single-cylinder SI methanol engines 5, for example, showed 16% improvement in brake efficiency when raising the compression ratio from 8.0 to 18.0, while still achieving minimum best torque (MBT) spark timing with only light knock. A compression ratio of 19.5:1 was chosen for this work based
42、on earlier experience with a wider range of compression ratios, which showed this to be the best compromise between full spark authority without knock at high load and dilute combustion range at light load. The full spark authority at high load is enabled partly by the relatively high levels of EGR,
43、 which has been shown in earlier works to suppress knock at higher compression ratio 21. Light load stability, meanwhile, is improved by the high compression ratio, which raises the temperature of compression and enhances the already comparatively high flame propagation velocities of the alcohol fue
44、ls. As a result, earlier works 21, 22 have demonstrated the ability to operate satisfactorily with as much of 33%-40% EGR with methanol, even with a relatively low compression ratio of 8-8.5. Using a higher compression ratio, the present work was able to achieve nearly 50% EGR without unacceptable c
45、ycle-to-cycle combustion variability, using a production spark ignition system. The main objective of the engine load control strategy was to exploit the physical properties of the alcohol fuels in order to run unthrottled, and therefore more efficiently, over a relatively wide range of loads. Metha
46、nol-fueled engines using high levels of EGR to modulate load 21- 23 have demonstrated efficiency gains of greater than 10% over throttled engines, while giving considerably lower NOx emissions. Combining variable EGR rates with variable intake manifold pressure allows for a wider range of load contr
47、ol. This strategy has also been shown as an effective means of achieving NOx levels below 1.0 g/kW-hr and peak efficiency around 42% in DI, lean stratified-charge methanol engines 23 and similar improvements in PFI lean burn methanol engines 24. In the present engine, EGR and boost levels are mainta
48、ined to achieve the best NOx and efficiency, and still enabling MBT (or near MBT) spark timing at high loads. Manifold absolute pressure (MAP) was varied between 1.0-1.5 bar, while the maximum dilution level was limited to about 50% EGR. Throttling, meanwhile, was used only to achieve near-idle load
49、s. The engine is controlled to stoichiometric fueling, enabling use of a three-way catalyst for attainment of emissions at the levels required to achieve Federal Tier II LDV standards. Earlier experience operating lean with an oxidation catalyst 16 showed the ability to achieve Tier II-level emissions on a methanol vehicle for all but NOx, pointing to the need for a three-way catalyst. Operating at stoichiometric has the added benefit of enabling a higher specific power than a similar lean, stratified engine. This strategy was successfully employed to achieve the steady state effici