Bài nầy sẽ do anh Đinh Trường Hân CN19, cùng một số kỹ sư khác trong chương trình, thuyết trình tại buổi hội thảo của Society of Automotive Engineers vào đầu tháng 11 năm 2005 sắp tới tại Chicago, Il - http://www.sae.org/technical/papers/2005-01-3641
Anh ĐTHan, một Program Director của USPS ở Washington DC, cũng đã thuyết trình đề tài tương tự tại Hội Nghị về Năng Lượng hồi đầu tháng 4 vừa qua - http://www.newenglandroundtable.org/Arc ... iption.pdf
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Sau đây là bài sẽ được thuyết trình (hình ảnh và các bảng tóm tắt sẽ được bổ túc sau):
05CV-125
Operating Experience and Tear-Down Analysis for Engines Operated on Biodiesel Blends (B20)
Richard Fraer and Han Dinh
United States Postal Service
Kenneth Proc and Robert L. McCormick
National Renewable Energy Laboratory-U.S. Department of Energy
Kevin Chandler
Battelle
Copyright © 2005 SAE International
ABSTRACT
Biodiesel has been used to reduce petroleum consumption and certain pollutant emissions. B20, a 20% blend of biodiesel with 80% petroleum diesel, has become the most common blend used in the United States.
Little quantitative information is available on the impact of biodiesel on engine operating costs and durability. In this study, eight engines and fuel systems were removed from trucks that had operated on B20 and diesel. These include four Ford cargo vans and four Mack tractors (two each running on diesel and B20). The engines and fuel system components were disassembled, inspected, and then evaluated to compare the wear characteristics after a four-year period of operation.
Additionally the vehicle case history, including mileage accumulation, fuel use, and maintenance costs was documented. The results indicate that for the Ford vehicles there was little difference in operational and maintenance costs between the diesel and biodiesel blend fueled groups. No differences in wear or other issues were noted in the engine tear down. The Mack tractors operated on B20 exhibited higher fuel and engine related maintenance costs, primarily because of higher frequency of fuel filter replacement. Additionally, both Mack engines operating on B20 required fuel injector replacement at some point during the study period. Neither of the Mack engines used on diesel required injector replacement. Finally, a build up of sludge was noted around the rocker assemblies for the Mack B20 engines. The sludge contained high levels of sodium leading to the speculation that it may have been caused by accumulation of soaps from out-of-specification biodiesel in the engine oil. Both Mack and Ford engines used similar pump-line nozzle fuel injection systems, but a much larger volume of fuel was recirculated in the larger Mack engines. This factor, along with duty cycle and engine loading, may account for the difference in performance of the two engine types examined here with B20.
INTRODUCTION
Biodiesel is an oxygenated fuel or blending component made from vegetable oils, waste cooking-oil, or animal fats by reaction of the triglyceride fats with methanol to form methyl esters via transesterification. Biodiesel or biodiesel blends are used by fleets and other vehicle users to reduce petroleum consumption and pollutant emissions. Significant reduction in particulate matter (PM), carbon monoxide (CO) and hydrocarbon (HC) can be achieved. [1]. Life cycle analysis indicates that biodiesel is highly renewable and its use, therefore, produces real reductions in petroleum consumption and carbon dioxide emissions [2].
The Energy Policy Act of 1992 (EPAct) as amended in 1996, added biodiesel to the list of approved alternative fuels (compressed natural gas, electric, etc.). EPAct requires that 75% of new vehicle purchases by certain federal, state, and alternative fuel provider fleets be alternative fueled vehicles. As a recognized alternative fuel, any vehicle certified to run on B100 could qualify under the alternative fuel vehicle purchase provisions of EPAct, but it does not appear that any vehicles meeting this requirement are available today. With the signing of The Energy Conservation Reauthorization Act of 1998, EPAct was amended to allow qualified fleets to use B20 in existing vehicles to generate alternative fuel vehicle purchase credits, with some limitations. This has created significant B20 use by government and alternative fuel provider fleets.
However, B20 operational data are not readily available from the engine and fuel system manufacturers or fleet users. In fact, engine and vehicle manufacturers have expressed concern about the limited information available on the effects of biodiesel and biodiesel blends on extended engine durability and operating costs. Fleets and other potential users also lack technical in-use data to support decisions about the use of biodiesel blends in their vehicles. This lack of information is a major barrier to B20 market penetration and acceptance.
OBJECTIVE
This work will provide quantitative in-use data on the operating cost and engine durability impacts of B20 when used in Postal Service vehicles. This was accomplished through teardown analysis of engines and fuel systems removed from these trucks, and comparison of wear characteristics to standard diesel trucks operated in the same service. Vehicle case histories, including mileage accumulation, fuel use, and maintenance costs, were also prepared.
PREVIOUS STUDIES
Malcosky and Wald [3] reported on roughly ten months of experience in operation of 5 dump truck/snow plows on B20, with a control group operating on conventional diesel. A total of 60,000 miles of B20 operation had been accumulated at the time of this report. A significant focus of the study was on maintaining fuel quality by using new or clean fuel storage tanks and on procedures for blending biodiesel to produce homogenous 20% blends. No significant differences in performance or maintenance costs were identified for the B20 vehicles versus the controls.
Bickel and Strebig [4] reported on a two-year field trial of B20 use in road maintenance vehicles in Minnesota. Nearly 25,000 gallons of B20 were consumed over roughly 135,000 miles of operation during the trial and the B20 vehicles exhibited the same average fuel economy (in miles per gallon) as the diesel control vehicles. Oil analysis was conducted at 5000-mile intervals and indicated that no unusual engine wear or fuel dilution was occurring. Inspection of engine fuel systems at the conclusion of the trial showed no signs of unusual wear or deposits.
Kearney and Benton [5] reported on a one-year B20 demonstration at Scott Air Force Base in Illinois. Fuel quality over the course of the demonstration was monitored. Military specifications limited solids content to 9.46 mg/quart via a procedure similar to ASTM D6217. Several loads of B20 exceeded this limit by a significant amount, many containing visually observable solids. Nevertheless, no filter plugging or other operational problems were reported.
Humberg and coworkers [6] recently surveyed the experience of state DOT agencies with B20. Roughly half of the states using B20 reported some fuel filter plugging issues in excess of those encountered for petroleum diesel. Many states also reported small declines in fuel economy. No noticeable changes in fuel pump and fuel injector durability, or unusual impacts on engine oil analyses were reported.
Chase and coworkers [7] described operation of a heavy-duty line-haul truck on a blend of 50% biodiesel and 50% No. 2 diesel for over 200,000 miles. Extensive inspection and analysis of the engine at the conclusion of the study showed no excessive wear and tests indicated no injector degradation.
While most of these prior studies report fuel use, mileage, and fuel economy, only one included detailed analysis of engine wear following biodiesel blend use and none discuss maintenance costs quantitatively. In the present study we endeavor to take a more detailed look at the impacts of B20 on engine wear and engine operability.
UNITED STATES POSTAL SERVICE (USPS) BIODIESEL BLEND USE
The USPS operates the largest and most visible civilian vehicle fleet in the world – over 212,000 vehicles ranging from heavy-duty trucks to minivans and passenger cars [8]. The Postal Service also currently maintains the largest alternative fuel vehicle (AFV) fleet in the United States which includes ethanol, natural gas, propane, electric and biodiesel fueled vehicles.
B20 has been used to demonstrate the Postal Service’s commitment to foreign fossil fuel independence and to the environment as well as to meet the requirements of EPAct. Starting from 1999, EPAct requires that 75% of newly acquired vehicles for federal fleets and others must be able to operate on alternative fuels.
As a federal agency, the Postal Service is required to meet EPAct. The Biodiesel Fuel Use Credit Rule of 2001 allows the USPS to use B20 to fulfill the EPAct requirements. One biodiesel fuel use credit, which is counted as one AFV acquisition, is allocated for each purchase of 450 gallons of neat (100%) biodiesel or 2,250 gallons of B20. Because of the increased use of biodiesel, the Postal Service has met or exceeded its EPAct requirements in the last eight years, often by ten percent or more.
The Postal Service is one of the first federal agencies to use B20. This was initiated about five years ago in Manhattan, New York; Detroit, Michigan; St. Louis, Missouri and Miami, Florida. In some locations, the fuel supply comes from a mobile refueler. The mobile refueler eliminates the need of owning and maintaining fuel tanks. Biodiesel and biodiesel blends have been known to promote microbial growth in fuel storage tanks under certain conditions.
In 2004, The Postal Service had a total of 929 cargo vans, truck tractors, spotter tractors and step vans utilizing B20 as fuel. The B20 consumption between 2000 and 2004 is shown in Figure 1 [9]. In 2004 the B20 consumption declined as a result of a fuel filter-plugging problem at the Miami location.
Figure 1. Fiscal Year 2000-2004 USPS B20 Consumption in Gallons
The cost of B20 has been generally higher than conventional diesel, which limits the use of B20 in many locations. A new biodiesel tax incentive signed by President Bush became effective on January 1, 2005 and has great potential to reduce the price difference, thus promoting the expansion of biodiesel blend use without placing a financial burden on the local fleet managers.
APPROACH
With the planned replacement of the cargo vans in 2004 and truck tractors in 2005, the USPS had a unique opportunity to examine these vehicles before they were removed from the fleet and sold to the public. Typically, these vehicles are replaced when age and maintenance costs exceed established replacement criteria such as 8 years and 360,000 miles.
The engines and fuel system components were disassembled, inspected, and then evaluated to compare the wear characteristics after several years of operation. Additional work was performed to document the vehicle case history, including mileage accumulation, fuel use, and maintenance costs.
VEHICLE SELECTION
Selection of heavy trucks within the USPS was focused on the Miami Vehicle Maintenance Facility (VMF) because of its use of B20 for all diesel equipment from approximately October 1999 through November 2003. The next criterion was to consider heavy trucks that were either close to the end of their useful life or near an engine rebuild threshold, so that the study trucks could have the engines removed and disassembled for detailed study.
The USPS identified two potential fleet candidates – (1) MY 1993 Ford 9-ton cargo vans and (2) MY 1996 Mack tractors (Figures 2 and 3). The 1996 Mack truck tractor is used in daily operations to transport trailers of mail and equipment to and from Processing and Distribution Centers (P&DCs). The 1993 Ford cargo van is used to transport large volumes of mail from mailer’s plants and P&DCs to post offices and air mail facilities. For the candidate Ford trucks, they were close to the end of their useful life. The Mack trucks were nearing a scheduled engine rebuild.
Figure 2. 1993 Ford 9-Ton Cargo Van
Figure 3. 1996 Mack Tractor
Limitations of funding for the planned testing and engine teardowns determined that this work could include four of the Ford trucks and four of the Mack trucks. The planned work included a split of two of each type of truck selected from the Miami VMF fleet. The two control vehicles for each type of truck were located at a VMF nearby that operated exclusively on diesel fuel. This selection of a baseline comparison diesel truck also included the need to have a similar duty cycle and climate in the area of operation. This was resolved by selecting the two diesel only Mack trucks from the Ft. Lauderdale VMF and the two diesel only Ford trucks from the Tampa VMF.
TEARDOWN PROCEDURE
Roush Industries performed engine removal and rebuild, disassembly, inspection, evaluation and rebuild (Figure 4). The Bosch fuel injection pumps were tested, disassembled, inspected and rebuilt by a fuel systems facility (Superior Turbo & Injection, Detroit, MI), approved and recommended by Robert Bosch North America. Internal parts were compared for wear, breakage or other damage as well as the accumulation of dirt, sludge and carbon deposits in an effort to uncover any effects which could be attributable to the type of fuel used. The essential results of the evaluation are summarized in the following four general areas that are representative of the overall condition of the engines:
• Cylinder heads and combustion chambers
• Cylinder block and crankcase
• Lubrication system
• Fuel system
Figure 4. Teardown Inspection at Roush
VEHICLE MAINTENANCE COMPARISON
The general approach for the vehicle maintenance cost comparison analysis was to collect all work orders available for each of the eight study trucks. The focus was the maintenance costs for each truck that were related to the fuel system or engine of the trucks. A few systems related to the engine were also included in this focus: air intake, cooling, and exhaust. The labor costs were held to a constant $50 per hour and the parts costs were not corrected for inflation; however, the data analyzed and presented here was from a similar time period for each truck.
The engine and fuel maintenance cost data was used to compare the diesel only and B20 trucks to determine differences that might have been caused by the use of B20. This maintenance data was also used to investigate issues identified in the engine teardown analysis.
VEHICLE INFORMATION
The USPS purchased the Mack tractors and Ford cargo vans in large block purchases of approximately 900 of the Mack tractors and 2400 Ford trucks.
Table 1 provides specific information about the eight study trucks including vehicle identification number (VIN), engine make/model, and engine serial number (if available). The vehicle mileage at teardown indicates the mileage at which the engines were removed from the trucks for detailed study as described later in this paper. The engine number in the teardown is a cross-reference for identifying which truck is which in the detailed engine teardown discussions.
Table 2 provides some general specification information for the study trucks and engines.
Table 1. USPS Study Truck Information
- USPS Truck Number VIN Vehicle Mileage at Teardown Engine Make/Model Engine Serial Number Engine Number in Teardown
Miami VMF, 1996 Mack MR688P Trucks, B20
6610170 1M1K194Y0VM009162 364,514 Mack E7-300 Mack #1
6610131 1M1K194Y1VM009123 395,584 Mack E7-300 Mack #2
Ft. Lauderdale VMF, 1996 Mack MR688P Trucks, Diesel
6610209 1M1K194Y6VM009201 384,464 Mack E7-300 Mack #3
6610191 1M1K194Y8VM009183 339,916* Mack E7-300 Mack #4
Miami VMF, 1993 Ford 9 Ton Trucks, B20
1891990 1FDXH81A4DVA17292 343,185 Ford 7.8L VZ114881 Ford #1
1892111 1FDXH81A8PVA21037 377,272 Ford 7.8L VZ114984 Ford #2
Tampa VMF, 1993 Ford 9 Ton Trucks, Diesel
1892158 1FDXH81A6PVA23644 351,967 Ford 7.8L VZ115168 Ford #3
1892193 1FDXH81AXPVA25350 347,803 Ford 7.8L VZ116236 Ford #4
Table 2. General Vehicle Specifications
- Vehicle Item 1996 Mack 1993 Ford
Truck Manufacturer/Model Mack MR688P Ford CF-8000
Gross Vehicle Weight Rating 33,000 lbs 33,000 lbs
Curb Weight 13,180 lbs 14,980 lbs
Engine Mack E7-300 Ford 7.8L
Power Rating 300 hp @ 1,950 rpm
1,160 ft-lb @ 1,200 rpm 210 hp @ 2,300 rpm
600 ft-lb @ 1,500 rpm
Displacement 12L 7.8L
Number of Cylinders 6 6
Compression Ratio 16.5:1 17.5:1
PERIODS OF OPERATION
The Ford truck fleet was placed into service starting in early 1993 in USPS service across the country. The South Florida District, which includes the Miami and Ft. Lauderdale VMFs, had 13 of the 9-ton Ford trucks in operation up to the start of this study. The South Florida District also has 43 of the Mack tractors starting operation in late 1996. As mentioned earlier, the USPS Miami VMF started use of biodiesel in a blend of 20 percent biodiesel and 80 percent diesel fuel in October 1999 and used that biodiesel blend almost exclusively until November 2003. Over 1 million gallons of B20 were consumed at Miami over this 4-year period.
In regards to maintenance data, the USPS VMFs generally keep at least 2 years of paper maintenance records on hand. The 2-year limitation is done to control space required to store these paper records. However, at the Ft. Lauderdale VMF, the number of vehicles is smaller and the work orders for the entire life of the study trucks were kept and collected for this study. At the Tampa VMF, only 2 years of work orders were available for this study. At the Miami VMF, there were 2 years of work order records for the Ford trucks. NREL and USPS had been collaborating for some time before the extent of this study was known and the Mack trucks were already of interest. More than 4 years of the Mack study truck work orders were collected during this collaboration between NREL and USPS. B20 use began before the collection of maintenance data for this study, and the data collection periods are detailed in Table 3.
Table 3. Mileage and Fuel Economy
- Study Group Data Period Total Data Mileage Average Monthly Mileage per Truck General Fuel Economy Range
Miami Mack B20 8/00-9/04 349,874 3,683 5.0-5.5 mpg
Ft. Lauderdale Mack Diesel 8/00-9/04 422,260 4,223
Miami Ford B20 9/02-9/04 146,201 2,924 7.0-9.0 mpg
Tampa Ford Diesel 9/02-9/04 132,749 2,553
FUEL USE AND MILEAGE ACCUMULATION
Table 3 also provides a summary of mileage accumulation during the data collection period for each group of two study trucks. For the Mack trucks, the Miami VMF trucks had lower average monthly mileage than the Mack trucks at Ft. Lauderdale; however, the averages are close enough to indicate that they were used generally in the same way. The tractors in the South Florida District are used for bulk movement of mail and packages to and from distribution centers in the area. The Ford trucks had average monthly mileage that was similar with the Miami trucks having a slightly higher average than the trucks at the Tampa VMF.
According to information provided by the VMFs, the Mack trucks had a fuel economy range that stayed close to 5.0 mpg and the Ford trucks had a fuel economy that ranged from 7.0 up to 9.0 mpg. These fuel economies are estimated ranges because the USPS generally does not track fuel usage by vehicle. The Miami VMF reported that there was no discernable difference in fuel economy during B20 use.
MACK TRUCK RESULTS
In examining the areas of the cylinder block and crankcase as well as the lubrication system, no significant differences were found between the B20 and diesel engines. The main bearings exhibited normal wear for the accumulated mileage on the respective vehicles. The No.1 and 2 B20 engines showed some bearing wear into the copper linings (Figure 5), and the No. 3 and 4 diesel engines showed typical “hen-tracking” and “orange peel” (Figure 6). The crankpin bearings also exhibited normal wear with some wear into the copper lining on the upper inserts, except for the No. 4 diesel engine, which had lower accumulated mileage.
Figure 5. Mack No. 1 B20 Main Bearings
Figure 6. Mack No. 3 Diesel Main Bearings
Minor differences were seen in the oil pump pressure relief valve pistons. The relief valve pistons in the No. 1 and 2 B20 engines showed scuffing and were not suitable for reuse (Figure 7). The No. 3 and 4 diesel relief valve pistons had only minor blemishes and could be reused Figure
![Cool 8)](./images/smilies/icon_cool.gif)
Figure 7. Mack No. 1 B20 Oil Pump Pressure Relief Valve Piston
Figure 8. Mack No. 3 Diesel Oil Pump Pressure Relief Valve Piston
CYLINDER HEADS AND COMBUSTION CHAMBERS
No differences were seen upon the inspection of pistons, intake, or exhaust valves. All components exhibited similar and expected amounts of carbon buildup on the combustion sides. However, the cylinder heads of the No. 1 and 2 B20 engines contained a heavy amount of sludge on the valve deck around the rocker assemblies (Figure 9).
Figure 9. Mack No. 1 B20 Valve Deck
The valve deck sludge was thick and gel like when compared to the oily residue found in the diesel engine heads. Figure 10 illustrates the difference between the substances found. The B20 sludge forms a pile on the sample card, where the diesel just leaves a stain. Looking closer at the differences between the sludge found in the No. 1 B20 engine head and the No. 2 B20 engine head, No. 2 had less accumulation and lighter consistency sludge with none found on the rocker assemblies. Upon inspection of maintenance records, the No. 2 B20 engine had the injectors replaced right after B20 use stopped (November 2003). It would be fair to assume that the rocker assemblies were cleaned as part of the injector replacement and that some effort was made to clean the cylinder head at that time.
Figure 10. Cylinder Head Sludge Comparison (Mack No. 1 B20 vs. Mack No 3. Diesel)
Sludge Analysis
Chemical analysis was performed in an effort to identify the source of this material. This was accomplished using elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP, via ASTM D4951) and results are shown in Table 4. This analysis was conducted on the two sludge samples from the B20 engines, as well as on the engine oil samples from all four engines. Considering that the B20 engines operated on B20 for a four-year period of time, ending in 2003, one would not expect the engine oil samples to exhibit any effects from B20 but to be indicative of engine wear issues with other causes.
The engine oil typically contains calcium and/or magnesium from the detergent and contains zinc and phosphorus from the ZDDP anti-wear additive, and all of these metals are evident here. The sludge appears to be enriched in calcium and zinc relative to the oils, with zinc and phosphorus present in roughly the correct proportions for ZDDP. Iron in the used oil samples is low, so wear problems are not evident. Iron is concentrating in the valve deck sludge, but these levels are also low and probably not indicative of a wear problem.
Molybdenum is used in some engine oils as an anti-oxidant. Because the B20 and diesel engines were used in different fleets, the absence of molybdenum in the B20 engine oil samples suggests that this fleet employed an oil formulation that did not contain a molybdenum additive. The higher levels of magnesium in the oil from engines Nos. 3 and 4 supports the idea that these are operating on a different oil formulation.
The sludge is clearly enriched in sodium relative to the engine oils. While there may be other potential sources of sodium contamination, a likely source for the B20 engines is a bad batch of biodiesel that contained high levels of sodium from the biodiesel production process. Sodium hydroxide is used as a catalyst in biodiesel manufacture, and if not adequately removed the sodium will form soaps that can cause a variety of operational problems. Thus our current hypothesis for the formation of the valve deck sludge is exposure of the engine to one or more off-specification batches of biodiesel blend containing high levels of sodium.
Sodium soaps have a high boiling point and would concentrate in the engine oil, potentially forming deposits and clogging oil drain passages. If this occurred in the valve deck area engine oil would have a long residence time leading to higher levels of decomposition and deposition of the detergent and anti-wear additives, consistent with the concentration of calcium, zinc, and phosphorus in the sludge.
Table 4. Results of metals analysis by ASTM D4951 for engine oil and valve deck sludge samples.
- Silver Boron Calcium Copper Iron Magnesium Molybdenum Sodium Phosphorus Silicon Zinc
Engine #1 Oil 2 70 3,880 1 13 18 0 0 1,081 0 1,231
Engine #1 Sludge 1 108 5,088 5 150 34 0 55 1,460 35 2,095
Engine #2 Oil 0 51 4,064 2 24 35 0 0 1,091 0 1,293
Engine #2 Sludge 0 158 9,156 6 110 58 0 165 1,759 33 2,408
Engine #3 Oil 6 20 2,933 4 21 302 25 0 1,273 0 1,506
Engine #4 Oil 1 10 2,916 22 43 314 26 0 1,270 0 1,530
FUEL SYSTEMS
The fuel pumps did not show any difference between B20 and diesel engines. All four fuel injection pumps tested within the normal ranges for pressure and flow. Injection pump delivery valves exhibited little wear and were consistent for the four pumps. Fuel injectors, however, did have some notable differences.
Fuel Injector Nozzle Assemblies
All the fuel injector nozzle assemblies were removed and bench tested for leak down rates and then disassembled. The injector nozzles from No. 1 B20 were not within specified leak down limits and required replacement (Figure 11). The No. 3 and 4 injector nozzles tested at the low end of the limits but could be reused. The No. 2 B20 injectors tested well within limits, confirming maintenance records of recent injector replacement (noted above). In comparing fuel injector nozzle replacements from the beginning of the data period (August 2000) till the time teardown was completed, both B20 engines required injector replacement, whereas the diesel engines did not.
MACK MAINTENANCE COMPARISON
Figure 12 shows maintenance costs per vehicle mile for the two study groups of Mack trucks. These data points are a cumulative running average of the engine and fuel system costs per mile of operation. B20 use ended in November 2003. In general, the B20 use Mack trucks at the Miami VMF had slightly higher maintenance costs for the engine and fuel-related systems than the Mack trucks operating only on diesel fuel at Ft. Lauderdale VMF. This was caused by a difference in the fuel system maintenance costs, which were made up of mostly extra fuel filter changes that were a part of unscheduled or roadcall repairs for a “won’t start” report from the driver.
Figure 11. Mack No. 1 B20 Fuel Injector Nozzle Disassembled
Figure 12. Cumulative Fuel and Engine Maintenance Costs per Mile for the Mack Trucks
As the analysis of maintenance results progressed, it became apparent that the two diesel only Mack trucks at the Ft. Lauderdale VMF also had some biodiesel blend operation experience in 2001 (about 6-8 months of operation). During this timeframe, both of the diesel only Mack trucks had multiple maintenance actions for “won’t start” which resulted in having the fuel filters replaced. These unscheduled fuel filter replacements started at the end of 2000 and stopped around the end of July 2001. The Ft. Lauderdale VMF staff reported that they started using B20 at the end of 2000 but quickly stopped using it because of the fuel filter plugging problem.
USPS personnel at the Miami VMF reported that during the B20 use period the Mack trucks had significant problems with the biodiesel blend. It was also noted that none of the other diesel equipment had any significant problems with the biodiesel blend, only the Mack trucks. The problems with the biodiesel blend resulted in repeated fuel filter plugging. Based on the two Mack trucks studied here from the Miami VMF, the significant filter plugging problems did not necessarily affect every one of the Mack tractors. The fuel system for the Mack engine has a larger displacement pump and a greater diesel fuel recirculation volume than other engines in the fleet at that time. This difference was suspected to be the reason for fuel filter plugging experience. Some investigation by USPS into the source of the contaminant was done, but was not conclusive.
The USPS was not using any kind of additive in the diesel fuel or biodiesel blend because of a general policy of not using additives. This policy may need additional investigation with regards to the use of biodiesel blend fuel. The suspected problem with the use of biodiesel blend fuel in this truck type was biocontamination; which in part can be treated with biocidal additives. Note, however, there is no conclusive evidence to support biocontamination as the cause of the filter plugging and operational problems. Additional study should be considered. Another significant contributing factor to this problem is probably the way in which the USPS fuels its trucks. The trucks are filled every night or every other night. This means that for many times, only 5-10 gallons of fuel are used from the fuel tank before it is refilled. The fuel in the on-board fuel tank is left unused for a long period of time and is most likely a contributing factor to the fuel tank sludge buildup.
FORD TRUCK RESULTS
In examining the areas of cylinder heads and combustion chambers as well as the fuel systems, no significant differences were noted in the teardown of the Fords. The cylinder head cavities of all four engines were clean with only a normal oily residue found in each. The pistons in all of the engines were in good condition with some carbon buildup in the bowl area and above the ring lands (Figure 13). The intake and exhaust valves were clean with light carbon deposits on the sealing surfaces and normal buildup on the valve heads.
Figure 13. Ford No. 1 B20 Piston
The fuel injection pumps and nozzle assemblies were removed and bench tested. All four pumps tested well within specified limits for pressure and flow and were suitable for reuse (with new gaskets and seals). All nozzle assemblies also tested well within acceptable limits. Table 5 details the checks performed on the fuel system, typical of all four trucks.
Table 5. Ford No. 1 B20 Fuel System Inspection Checklist
- Part(s) General Condition
Injection pump Pressures well within specified limits. Overall condition good, suitable for cleaning and reconditioning
Fuel lines, high pressure (cylinders 1-6) Condition OK, no issues
Fuel inlet tubes (cyl. 1-6) Condition OK, no issues
Injector nozzles (cyl. 1-6) Condition good, suitable for cleaning and reconditioning
Filters, primary and secondary No issues noted
Fuel supply pump Condition good, suitable for cleaning and reconditioning
FORD NO. 3 DIESEL BEARING FAILURE
Bearings on the No. 3 diesel engine exhibited heavy to severe wear. Main and connecting rod bearings show wear into the copper linings with severe wear at the fourth main journal position resulting in complete bearing failure (Figure 14).
Figure 14. Ford No. 3 Diesel Main Bearing Failure
The lower bearing insert is worn through the copper lining, scoring the crankshaft at the fourth journal position. Severe wear and damage exhibited in the No. 3 diesel engine is indicative of a lack of lubrication. Examination of the maintenance records for this vehicle show that this Ford truck had some problems which may have indicated or contributed to the lubricating problem; however, there is no conclusive evidence as to the cause of the lack of lubrication.
FORD MAINTENANCE COMPARISON
Figure 15 shows cumulative maintenance costs for the two study groups of Ford trucks. B20 use ended in November 2003. The general observation here is that the B20 use Ford trucks at the Miami VMF had slightly higher maintenance costs for the engine and fuel-related systems than the Ford trucks operating only on diesel fuel at the Tampa VMF. Both of the Ford truck study groups had a significant number of maintenance actions for the accelerator pedal and throttle problems that caused significant cost. The Miami Ford trucks had more costs associated with this problem and accounted for most of the cost difference between the two study groups. This problem does not appear to be related to the use of B20, and with the additional accelerator costs removed from Miami, the engine and fuel system maintenance costs are nearly identical.
Figure 15. Cumulative Fuel and Engine Maintenance Costs per Mile for the Ford Trucks
FUEL SYSTEMS COMPARISON
The Ford 7.8L engine uses a Bosch P3000 series in-line fuel injection pump. This style of fuel system contains a supply pump to deliver fuel from the tank through a filter to the injection pump. The injection pump feeds high-pressure fuel (up to 1150 bar) via high-pressure lines to the fuel nozzles for injection into the cylinders [10]. A return line brings overflow from the injection pump and fuel from the nozzles back to the fuel tank.
The Mack E7-300 engine uses the same style of Bosch fuel system, but has a larger P7100 series in-line fuel injection pump to supply the larger displacement (12L) engine. As was suggested earlier, the greater fuel circulation volume was suspected to be contributing to the fuel filter plugging evident on the Mack trucks but not on the Fords. Vehicle installation of the fuel system (filter sizing, plumbing) may also be a contributing factor, but fuel system differences alone do not seem to account for all the differences in results between the Ford and Mack trucks operating on B20 (e.g. valve deck sludge accumulation, injector replacement, and higher operational costs for filter replacement in the Macks). Duty cycle and engine operation (loading) may also be responsible for susceptibility to B20 use issues.
CONCLUSIONS AND RECOMMENDATIONS
A few differences were noted in comparing vehicle operation and engine teardown between B20 and petroleum diesel use in the Mack tractors.
These differences are as follows:
• The cylinder heads of the B20 engines contained a heavy amount of sludge around the rocker assemblies that was not found in the diesel engines. Out of specification fuel is the suspected cause.
• Both B20 engines required injector nozzle replacement over the evaluation and teardown period.
• The B20 tractors had higher fuel and engine related maintenance costs due primarily to higher frequency of fuel filter replacements (filter plugging).
These differences, however, were not observed on the Ford cargo vans. The Fords did not have the filter plugging, injector replacement, or sludge accumulation issues noted with the Macks. Differences in fuel and engine system maintenance costs were attributable to non-biodiesel related issues.
Further research and analysis is necessary to determine the susceptibility of different engine and vehicle types to possible operating issues related to the use of B20. In future comparison and evaluation of B20 operation, controlled data on fuel supply (sampling and analysis) and operational characteristics (duty cycle, loading, etc.) are critical, especially when multiple vehicle types are involved. Better control and collection of critical data is possible with evaluations done real-time with continuous monitoring of data collection instead of from a historical perspective (analysis conducted after the operational period).
ACKNOWLEDGMENTS
The authors would like to thank the Engineering, Research and Development Department of the United States Postal Service for their support. Special thanks are extended to Powel Bernhardt and Robert Montebello from Philadelphia Vehicles Category Management Center, Les Machek from Miami VMF, Ray Hushon from Fort Lauderdale VMF, Charles Venator from Tampa VMF and Jim Howell from Roush Industries.
The National Renewable Energy Laboratory’s participation in this project was sponsored by the FreedomCAR and Vehicle Technologies Program, Fuels Technologies Subprogram of the U.S. Department of Energy. The authors wish to thank Ralph Cherillo of Shell Global Solutions for assistance in interpretation of the oil and sludge analysis data.
REFERENCES
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DEFINITIONS, ACRONYMS, ABBREVIATIONS
ASTM: ASTM International
B20: A blend of 20% biodiesel with diesel fuel
EPAct: Energy Policy Act
ICP: Inductively coupled plasma
NREL: National Renewable Energy Laboratory
P&DC: Processing and Distribution Center
USPS: United States Postal Service
VMF: Vehicle Maintenance Facility
ZDDP: Zinc dithiodiphosphate