"IMPROVED HEAVY DUTY GAS TURBINE ENGINE"
By H. E. Helms
Chief Project Engineer
Industrial Gas Turbine
Detroit Diesel Allison Division
Genera! Motors Corporation
Indianapolis, Indiana 46206
Prepared for: NAlO/CCMS Fourth International Symposium on Automotive Propulsion Systems
Washington, D.C., April 17-22, 1977
Reference Contract: "Improved Heavy Duty Gas Turbine Engine" – National Aeronautical and Space Administration (NASA) Contract No. NAS 3-20064
NASA Lewis Research Center
Mr. D. Beremond, Technical Manager
Mr. W. Goette, Technical Manager
Mr. M. Krasner, Chief, Transportation Propulsion Division
Program Sponsors: United States Energy Research and Development Administration, Washington, D.C. 20545
Mr. George Thur, Chief Heat Engine Systems Branch, Energy Conservation
An improved heavy duty gas turbine engine program plan has resulted from a study contract issued by NASA Lewis Research Center, Cleveland, Ohio using funds supplied by the Energy Research and Development Administration (ERDA).
Program objectives are to demonstrate improved fuel economy [(213 mg/W-h (0. 35) specific fuel consumption by 1981], conformance to current and projected Federal noise and emission standards and to demonstrate a commercially viable gas turbine engine.
Study results show that increased turbine rotor and regenerator inlet temperatures, using ceramic materials contribute the greatest amount to achieving fuel economy goals. Furthermore, improved component efficiencies (for the compressor, gasifier turbine, power turbine and regenerator disks) can also show significant gains in fuel economy.
Fuel saved in a 500,000 mile engine life, risk levels involved in development, and engine related life cycle costs for fleets (100 units) of trucks and buses were used as criteria to select work goals for the planned program.
Gas turbine engines offer an alternate power plant selection for trucks, buses and other heavy duty engine applications. The diesel engine now is the prime powerplant in these heavy duty vehicles and has established an impressive record for reliability, economy of operation and flexibility of operation. The gas turbine must also achieve a similar level of performance, durability and economy to become a complimentary power plant for heavy duty applications.
An industrial gas turbine (IGT) (Figure 1) has been under development at General Motors Corporation for more than fifteen years and is rapidly approaching a stage of production readiness.
This gas turbine engine is being developed in two power sizes:
- GT-404 series engine rated at 300hp (224kW) at 29. 4°C (85°F) and 500ft (152) altitude.
- GT-505 series engine is rated at 390hp (291kW).
A third member of this turbine engine family is planned, the 605, to be rated at 465hp (347kW).
Several million miles of bus, truck, boat and other field service has now been accumulated on the GT-404/505 all metal engines (See installations in Figure 3). Valuable lessons on performance, durability and operating costs have been learned as introduction to production of this family of engines is considered.
Recent emphasis on fuel economy has produced improvements in fuel consumption for all types of highway and off-highway vehicles. Gasoline and diesel engines have incorporated new features leading to improvements in fuel economy, and the IGT 404/505 gas turbine engines must also be improved to meet target fuel economies in-the late 1970's and the 1980's. This consideration has led to the "Improved Heavy Duty Gas Turbine Engine" program initiated with NASA Lewis Research Center, Cleveland, and the Energy Research and Development Administration (ERDA) in 1976.
This Phase I study program has three definite objectives:
- Improve specific fuel consumption (SFC) from 274 mg/W-h (0.45) to 213 mg/W-h (0. 35) by the year 1981
- Satisfy current and projected government noise and emission regulations
- Develop a commercially viable engine for use in trucks and buses
The suggested method to achieve improved fuel economy in the IGT is to increase turbine and regenerator inlet temperatures and improve component efficiencies of the compressor, turbines, and regenerator. The increased turbine and regenerator temperatures are to be achieved by the introduction of ceramic materials (non-cooled). An inherent advantage of the gas turbine is lower noise and emission characteristics than diesel and gasoline engines.
Meeting current and projected noise and emission standards should be easily achieved. Demonstrating a commercially viable engine can only be achieved by future demonstrations of required performance, durability and costs. This first study phase has the objective of quantifying levels of improvement to be pursued, to plan technical activities, and establish a feasible program schedule to achieve these improvements.
In the Phase I study program many factors were considered. Utilizing existing engines and capitalizing on the availability of spare parts was important. This would promote early acceptance for production of new ceramic components and minimize program cost. Selection of work to be performed had to be established recognizing time, funding, and feasibility of technical accomplishment. The final selection of work had to be based upon reduced fuel consumption and life cycle costs (engine related) for a customer of the gas turbine engines. Evaluation of these life cycle costs involves assumptions relative to fabricability, reliability and costs of ceramic parts which introduce an unknown 'risk' to the final assessments made in the study.
ANALYSIS PERFORMED AND RESULTS OF STUDY PROGRAM
Approach to Study
The approach to establishing a viable program was to make design assessments of candidate engine improvements, evaluate reductions in fuel consumption, assign production and maintenance costs for candidate engines and establish fuel saved and life cycle costs for a fleet of buses or trucks for each level of engine improvement. If fuel saved was significant and life cycle costs improved, the individual developments would be worthwhile and new components should receive serious consideration for early introduction to production engines.
Assessment of Candidate Improvements
An assessment of sensitivity to change in component efficiencies, engine pressure losses, engine heat rejection, cooling air requirements, engine operating temperature, regenerator inlet temperature, engine pressure ratio and leakages was conducted. Table 1 summarizes the sensitivities compiled showing the change in parameter required to obtain 1% improvement in specific fuel consumption. Having these sensitivities, technical analysis was applied to establish which parameters could be improved with development and improved materials.
Two basic routes for improvement were selected as follows:
• Improved component efficiencies
• Higher turbine inlet temperature (through use of ceramic materials)
Changing pressure ratio would not be effective because lower pressure ratio leads to compromised part power engine operation. Only very small improvement in heat and power losses relative to the current engine can be achieved. Small reductions in cooling air flows and leakages will be strived for although increasing turbine inlet temperature will make it difficult to keep cooling air flows at the same level as in the current engine. Pressure losses are largely a function of overall engine geometry (turns, expansions, heat exchanger matrix and inlet and exhaust) and it is desirable to maintain maximum geometric similarity with the current engine because of costs and time to accomplish changes in the basic block, covers and gearbox.
Specific components selected for improvement were the compressor, turbine (compressor drive), the power turbine and the regenerator. It was known that increasing turbine inlet temperature would require lower engine airflow for a given engine horsepower output. Further, lower engine airflow leads to smaller compressor and turbine sizes with the associated loss in component efficiency from decreasing size.
Figure 4 shows a projection of loss in compressor, turbine (compressor drive) and power turbine efficiencies with decreasing airflow. Also shown are the projected improvements achievable in these components with a rigorous development program. Two to three years will be required to fully develop these advanced technology components.
Figure 5 shows a regenerator effectiveness plot. Improved effectiveness is achieved by decreasing airflow while maintaining the same frontal airflow area in the current regenerator matrix. The lower curve represents the current metal matrix, the center curve is a current thin wall (0.0889 mm [0.0035 in.] to 0. 1143 mm [o. 0045 in.]) triangular ceramic (AS) matrix, and the upper curve represents a thin wall rectangular matrix (projected to require two to three years to develop) which has the potential of both higher effectiveness and lower matrix pressure drop.
A prime benefit achieved in the change from metal to ceramic regenerators is the increased temperature capability of the ceramic. The current metal regenerator is limited to a maximum steady state temperature of 774°C (1425°F) and a transient temperature of 913°C (1675°F). The ceramic regenerator can achieve a 982 °F (1800°F) steady state temperature and a transient temperature of 1093°C (2000°F). This increased temperature capability is particularly important at part power operation of the engine since the best specific fuel consumption is achieved by maintaining turbine inlet temperature constant over the power range of the engine and letting the regenerator inlet temperature increase as power level decreases. In the current engine, turbine inlet temperature is reduced starting at approximately 80% power (to accommodate the 774°C [1425°F] metal limit), whereas the ceramic regenerator with a 982°C (1800°F) capability will not require reduced part power turbine inlet temperature until a level of near 1260°C (2300°F) is obtained. In addition, the lower conduction loss of the ceramic provides a favorable part power effectiveness.
In summary, improvements in component efficiencies and regenerator effectiveness can substantially improve the engine fuel consumption and should be vigorously worked on in the proposed program.
HIGHER TURBINE INLET TEMPERATURE
The current engine turbine inlet temperature is 1002°F (1835°F). Table 1 shows that a 12°C (22°F) increase will give a 1% improvement in specific fuel consumption at this level of turbine inlet temperature. This is not a linear function, but increasing temperature does provide improved specific fuel consumption over a considerable range of temperature. Changing this parameter yields the greatest single improvement in specific fuel consumption so long as it is not necessary to change another parameter (such as cooling air from the compressor).
The incorporation of ceramic materials in the high temperature components offers the means for achieving the improved specific fuel consumption. Silicon carbide and silicon nitride are the two leading candidate ceramic materials for the high temperature vanes, blades, combustor, turbine inlet plenum and the stationary turbine tip shroud rings.
Analysis of metal high temperature parts was performed to establish limiting turbine inlet temperatures which render the metal no longer usable. In metal components it was found that either chemical stability (oxidation, sulfidation), creep strength or tensile strength were a limiting factors at given operating temperatures.
Figure 6 shows limits for components plotted against gasifier turbine inlet temperature, regenerator inlet temperature and power turbine inlet temperature. The metal regenerator, gasifier turbine inlet vanes and tip shrouds are the initial metal components that should be replaced with ceramic materials.
Table 2 shows the turbine inlet temperatures at which other metal components must be replaced.
Figure 7 shows these ceramic components in the IGT engine.
Table 2 identifies logical engine development temperature levels for advancing to the target specific fuel consumption levels. These engine temperature levels (1038°C [l900°F], 1132°C [2070°F], 1204°C [2200°F] andl271°C [2500 °F]) were used in the balance of the study to calculate engine performance, design of components and for cost analysis.
IMPROVED ENGINE PERFORMANCE LEVELS
Initial planning specified use of the current engine with the introduction of ceramic components and improved component efficiencies as they became available. Assessment of component efficiency improvements suggest a two to three year development period for compressor, turbine and regenerator efficiency development. A thin wall ceramic regenerator can be introduced into the current engine along with ceramic vanes, tip shrouds, turbine blades and a ceramic turbine inlet plenum. Thus, the current engine can be used for development at turbine inlet temperatures to 1132°C (2070°F). (At this temperature level, the power turbine must be replaced because of limiting blade creep strength and turbine aerodynamic loading.) This permits two to three years development before a significant flow path change can be accomplished. A new compressor and turbine flow path would then be introduced with improved component efficiencies at the 1204°C (2200 °F) gas turbine operating temperature and this technology would be applicable to 1371°C (2500°F). This timing dictated that a technology discontinuity occur between the 1132°C (2070°F) and 1204°C (2200°F) engines.
For comparison of fuel savings and life cycle costs, the engine horsepower selected for engine sizing was 300hp (224kW), the same as the 404-3 engine. Performance calculations were then made using the preceding turbine inlet temperatures, component efficiency improvements and constant horsepower.
Table 3 shows the airflow for constant horsepower, specific fuel consumption at the various turbine inlet temperatures, and other engine component parameters. It is noted that the target 213 mg/W-h (0. 35) specific fuel consumption is achieved at a-temperature level between 1204°C (2200°F) and 1371 °C (2500°F).
Figure 8 shows how specific fuel consumption varies for the baseline engine and the increased cycle operating temperatures over a complete operating power range from 0 to 300hp (224kW). It is noted that the ceramic regenerator temperature limit yields a much flatter specific fuel consumption curve in the 50% (150hp [112kW]) to 100% (300hp [224kW]) power range where most heavy duty engine usage occurs. Also, it is worthy of note that the turbine is very stable to low power levels and yields a torque-speed curve that is ideal for many off-design operating conditions.
Figure 9 shows this torque characteristic (compared to a diesel engine) which experience has shown to be very valuable on icy roads, wet roads, for use on grades and for selecting a less complex transmission when compared to a diesel engine.
An engine feature maintained in all the engines analyzed is the power transfer unit. This device has several advantages as follows:
- Engine braking capability equal to maximum engine horsepower
- Single shaft operation capability (particularly valuable in gen set installations)
- Power turbine overspeed control
- Lower transient turbine temperature excursions compared to most turbine engines
This power transfer feature is considered to be important to helping the turbine successfully compete against current heavy duty diesel powerplants. All engine performance presented is shown for an SAE standard day rating of 29.4°C (85°F) and 152 (500 ft) altitude. Gas turbines are sensitive to ambient inlet temperatures as shown in Figure 10. Attention is directed to the feature that lower ambient inlet temperatures produce increased power and lower specific fuel consumption for a given turbine engine. The average United States temperature is 13.3°C (56°F).* This lower average ambient air temperature (compared to the SAE Rating temperature) means an average improvement in fuel economy of approximately 5% above the values shown herein.
PERFORMANCE — NOISE AND EMISSIONS
Current and projected regulations on noise and emissions for heavy duty vehicles/engines are shown in Table 4. These regulations are subject to change, but can materially affect the current gasoline and diesel heavy duty power plants if they are not changed. The production diesel engine will find it difficult to meet the 1980 and 1983 California standards without compromising fuel economy. However, it is believed that Federal standards may override state standards and may continue to be much less stringent than state standards.
The noise standards may become restrictive for the diesel with encapsulation a possible solution to the truck noise problem. This will cause increased acquisition and maintenance costs.
Noise and emission values were calculated for each of the four study engine cycles [1O38°C (1900°F), 1132°C (2070°F), 1204°C (2200cF) and 1371 °C (2500°F)]. Because of reduced airflow and lower exhaust velocities, noise was found to be reduced below current turbine engine noise values of 75-77 dBA at 15.2 m (50 ft) sideline measuring locations. The exhaust noise will be reduced by over 5 dBA at the 1371°C (2500°F) turbine inlet temperature. Even the 75 dBA noise at 50 foot (15.2 meter) sideline regulations of some cities and states can be met by these improved turbine engines (with no special noise insulation or encapsulation). It is noted that the normal air inlet filter and regenerator disks are very effective noise suppressors for compressor and turbine noise.
Emission values obtained at each of the improved engine cycle temperatures are shown in Table 5.
These emission levels were obtained for the 13 mode Federal Heavy Duty Diesel Cycle. HC, CO and smoke all reduce at; a result of higher cycle temperatures. The primary pollutant of concern is NOx. This would only be of concern if a Federal Standard of less than 10. 0 is legislated in 1979 and will only very slightly exceed the projected 1980 California standard. Work for pollutant (NOX) reduction will be performed in the proposed program to achieve a minimum NOx margin of 25% to accommodate deviations obtained in production combustors.
VEHICLE CHARACTERISTICS WITH IMPROVED ENGINES
Having performance established for the improved engines, typical truck and bus usage with these power plants was evaluated.
The tractor-trailer selected was a 70,000lb (31,751kg) gross weight unit with 102 square foot (9.5 m2) frontal area, 480 revolutions per mile (0. 2983 tire revolutions per meter), a 4.886 axle ratio and a Fuller RT9509A nine speed transmission.
The bus was a typical Greyhound unit of 36,000lb (16,329kg) gross vehicle weight, 74 square foot (6. 9 m2) frontal area, 497 revolutions per mile (0.3088 tire revolutions per meter), an axle ratio of 4.330 and a DDA HT-740CT four speed automatic transmission.
In each vehicle, appropriate accessory loads and driveline losses were assigned for various vehicle speeds. Each vehicle was then run through the General Motors vehicle computer analysis for typical runs from Los Angeles to Salt Lake City and from Chicago to Boston.
Figure 11 shows the plots of miles per gallon (kilometers per liter) versus rotor turbine inlet temperature for both the truck and the Greyhound bus. Note is made of the discontinuity in the curves between 1132°C (2070°F) and 1204 °C (2200° F) as predicted earlier in the paper. Particular note is made in the improvement made in fuel consumption.
Figure 12 shows fuel used in a typical 500,000 mile (804,672 kilometer) engine life for the baseline engine and fuel savings for the study engines.
The gallons saved for one truck with a 1204°C (2200°F) engine multiplies to 3,060,000 gallons (72, 857 barrels) for a 100 truck fleet. It is believed that fuel savings shown are extremely significant and will have a meaningful impact on conservation of petroleum products.
LIFE CYCLE COSTS
Engine drawings were generated for each engine cycle temperature (1038°C [l900°F] , 1132°C [2070oF] , 1204°C [2200°F] and 1371°C [2500°Fj ) with emphasis placed on ceramic components and supporting metal parts. Figure 13 is a general arrangement drawing showing the 1371°C (2500°F) configuration study engine. Sketches of each unique part were made and pricing for ceramic components was performed by the Carborundum Company, Niagara Falls, New York. Costs for ceramic parts to produce 6000 engines per month were supplied.
The Harrison Radiator Division of General Motors Corporation supplied quantity costs for regenerators and seals and Detroit Diesel Allison priced all other engine parts required. These prices were then used to generate engine acquisition and maintenance costs in the life cycle cost study.
Life cycle costs were calculated for both trucks and buses. A 100 tractor truck fleet and a 100 coach bus fleet were chosen for these studies. A 500,000 mile (804,672km) engine life was used. An average of 9,000 miles per month (14,484km per month) was used for the trucks and 16,500 miles per month (26,554km per month) for the buses.
A mature engine schedule of maintenance was applied for each engine. In this maintenance schedule it was assumed that ceramic and metal parts would have the same reliability since no data base is available for ceramic parts in production gas turbine engines. Road applied maintenance included oil and fuel. Shop applied maintenance included predicted items (oil change, oil filter, fuel filter, and other predicted component parts). Sixty percent of shop maintenance was in-frame, forty percent was out-of-frame. Of the 40% out-of-frame, 10% was overhaul and 5% was major repair. Fuel costs were calculated based upon $0. 40 per gallon (current price) and $0.60 per gallon (projected 1980 price).
Data gathered for vehicle performance, maintenance material and labor and engine acquisition costs were then entered into a life cycle cost computer program and actual life cycle costs calculated. The current all metal 404 engine was used as a baseline engine for comparison purposes with the improved engines.
Figure 14 shows a plot of life cycle costs relative to the baseline engine life cycle costs. This figure is for the line haul truck fleet.
Figure 15 is for the Greyhound bus fleet.
Note the effect that $0.40 and $0. 60 fuel has on relative costs. Note the discontinuity between 1132°C (2070°F) and 1204°C (2200°F) caused by the introduction of components with improved efficiencies.
Figure 16 summarizes gains in fuel savings and costs for the study engines.
The most obvious conclusion drawn from the life cycle cost analysis was that only 1% improvement is achieved in going from a 1204°C (2200°F) turbine rotor inlet temperature to a 1371°C (2500°F) turbine rotor inlet temperature. This is caused by fuel cost being the primary cost driver and the fact that so little improvement in specific fuel consumption is achieved in this change in operating temperature level.
It is observed that high risk is introduced by the use of ceramic materials at the 1371°C (2500°F) cycle temperature level. At this temperature level, the single stage gasifier turbine rotor blades, two stages of power turbine rotor blades and all three stator vane rows (one gasifier and two power turbine stages) are made of ceramic materials.
Also, variations in costs of ceramic components can severely impact the acquisition costs and thus total life cycle costs. Analysis was performed where ceramic components were priced 10% to 25% over those levels used in the basic analysis and it was found that total life cycle costs at 1371°C (2500°F) actually increased over the baseline engine costs. Costs are very important and must be closely monitored as engine development proceeds.
(It is acknowledged that ceramic component prices could also be less than the base prices obtained.)
Based upon the low improvement in life cycle costs and high risk with today's technology with ceramic materials in going from 1204°C (2200°F) to 1371°C (2500°F) it was decided that the program recommended would have a maximum cycle temperature that would produce the target specific fuel consumption of 213 mg/W-h (0. 35) at 100% power. It is also recommended that flexibility be maintained in the program to permit re-evaluation of this conclusion as progress is made toward meeting program objectives and experience is gained in working with ceramic flow path components.
PROGRAM PLAN EVOLVED FROM STUDY
Two primary means of improving fuel economy evolved from the study conducted:
1. Improved component efficiencies are feasible and can achieve approximately one-third the specific fuel consumption improvement desired. Improved efficiency in the compressor, both gasifier and power turbines and in the regenerator should receive emphasis in the program.
2. Increased engine cycle and regenerator inlet temperatures are feasible using ceramic materials and can achieve approximately two-thirds the specific fuel consumption improvement desired. This is achieved by increasing turbine rotor inlet temperature from 10C2°C (1835°F) to 1241°C (2265°F) and regenerator inlet temperature from 774°C (1425°F) to 982°C (1800°F).
Naturally, it is desirable to initiate this program using the engine being prepared for production and introduce improvements as soon as possible. Steps to achieve engine fuel consumption improvements as soon as possible are suggested. The first step is to introduce ceramic regenerators, ceramic gasifier turbine vanes and ceramic stationary tip shrouds into the engine and increase turbine rotor inlet temperature to 1038°C (1900°F).
The second step is to introduce additional ceramics into the current engine for the turbine inlet plenum and ceramic gasifier turbine rotor blades and increase turbine rotor inlet temperature to 1132°C (2070°iT) (the current power turbine temperature limit for minimum demonstration testing).
While these two steps are in progress, improved efficiencies for the compressor, turbine and regenerator disks should receive emphasis with rig testing and engine evaluations as practical. Estimates of time required to achieve these steps is two to three years. (This recognizes funding and technical constraints.) The highest risk element is considered to be the rotating gasifier turbine rotor with ceramic blades. It is recommended that an alternate air-cooled rotor be carried as a parallel program in case development problems with the ceramic bladed rotor assembly are not solved in time to meet the proposed schedule.
The third step in the program will be an advanced engine with new, improved compressor, turbine and regenerator components and maximum use of ceramics. This engine would demonstrate the target specific fuel consumption of 213 mg/W-h (0. 35).
Table 6 summarizes the steps for introduction of ceramic components.
Table 7 summarizes the temperature steps, engine airflow, horsepower and specific fuel consumption that will be obtained. Test stand and vehicle demonstrations will be made with each of the last three steps in the program.
Figure 17 shows the predicted specific fuel consumption versus power to be achieved by the recommended engines in the proposed program. The timing required for the third step is recommended to be three years. However, it is desirable to perform the design and initial fabrication of the advanced engine during the third year of the program thus giving a five year overall program.
Figure 18 shows a schedule with various steps identified for reference. On this schedule, the vehicle demonstrations with the improved engines will be made either in a line-haul truck or a bus as appropriate.
This study program has identified a program plan which should meet the objectives of improving specific fuel consumption from 274 mg/W-h (0. 45) to 213 mg/W-h (0. 35) by 1981, and should produce demonstration engines meeting Federal noise and emission regulations.
Sufficient durability running of engines is planned in the program to demonstrate components that can progress to a commercially viable engine for use in trucks and buses.
Technology generated in this program will be applicable to automotive turbines and will represent a significant step forward for commercial gas turbine engines.
The work summarized in this paper has been prepared by a large team of experts at Detroit Diesel Allison Division of General Motors Corporation. These persons have contributed skills in engineering, materials, fabrication, cost analysis, service and maintenance, marketing, accounting and management. A complete listing of contributors would be lengthy, however, the following individuals have been the largest contributors to material presented herein.
Beuford C. Hall Technical Director for Phase I Study Program
Roger C. Dycus Project Direction and Coordination
Samuel R. Thrasher Regenerator Design and Analysis
James R. Wooten Performance Analysis
Terry Knickerbocker Performance Analysis
Carlton Curry Life Cycle Cost Analysis
John Wertz Design Analysis
James Lunsford Design Analysis
John Hayes Design Analysis
David Decker Design Analysis
Dr. Peter Heitman Materials
Acknowledgement is directed to personnel from the Energy Research and Development Administration for their conceptual program planning, efforts to seek and provide program funding and continuous monitoring of program progress.
Specific recognition is directed to:
Mr. John Brogan - Assistant Director, Highway Systems, Transportation Energy Conservation
Mr. Goerge Thur - Chief, Heat Engine Systems Branch Energy Conservation
Mr. Thomas Sebasteyn - Program Engineer, Heat Engine Systems Branch
Mr. Robert Mercure - Program Engineer, Heat Engine Systems Branch
Acknowledgment is given to NASA Lewis Research Center personnel for their assistance in defining the Phase I study work and their technical and administrative assistance in sponsoring this work at Detroit Diesel Allison.
Specific recognition is directed to:
Mr. Mort Krasner Chief - Transportation Propulsion Division
Mr. Don Beremond - NASA Technical Program Manager - "improved Heavy Duty Gas Turbine Engine" Contract No. NAS3-20064
Mr. William Geotte - Technical Program Manager - (Replaced Mr. Beremond 12/1/76)