Instrumented Direct Injection Cylinder Head for the ...
Instrumented Direct Injection Cylinder Head for the University of Idaho Clean Snowmobile Team
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Prepared by:
Nick Harker, Peter Britanyak, and Christopher Tockey
Team: Heads Under Pressure
Mechanical Engineering Department
University of Idaho
Prepared for:
Dr. Karen DenBraven
Mechanical Engineering Department
University of Idaho
Table of Contents
Executive Summary 2
1. Introduction 3
1. Background 3
2. Problem Definition 3
3. Previous Work 3
4. Objectives 4
5. Methods 4
6. Project Overview 4
2. Concept Development/Cylinder Head Specifications 5
1. Squish Band 5
2. Fuel Delivery 6
3. Ignition 6
4. Pressure Transducer 7
5. Chamber Geometry 7
6. Two Piece Design 8
2.7. Coolant Passages/Temperature Sensor 8
2.8. Weight 8
3. Detail Design/Manufacturing 9
1. Polaris Cylinder Head 9
2. Ski-Doo Cylinder Head 9
3. Manufacturing 10
4. Product Evaluation 11
1. Testing/Engine Tuning 11
2. Summary of Results 11
5. Economic Analysis 13
1. Engineering Cost 13
2. Cost of Production 13
6. Conclusion and Recommendations 14
Appendices
A. Polaris DI Cylinder Head Drawing Package
B. Ski-Doo DI Cylinder Head Drawing Package
C. Computational Analysis
D. Manufacturing Steps
E. Casting Study
F. Gantt Chart
G. Resumes
Executive Summary
This report describes the direct injection (DI) cylinder head designed to reduce toxic exhaust emissions and increase fuel economy of a two-stroke snowmobile engine. The DI cylinder head was designed and fabricated for University of Idaho Clean Snowmobile Challenge (UICSC) Team. Gasoline direct injection significantly increases two-stroke engine performance, but this DI cylinder head further reduces emissions and increases fuel economy through the use of improved cylinder geometry and spark plug location. This design is also capable of acquiring in-cylinder pressure data with the use of the appropriate equipment.
Test results show that the implementation of the DI cylinder head on a Ski-Doo 600 H.O. engine resulted in an exhaust emission score that passes the 2012 EPA emission standard and is close to passing the 2012 National Park Service (NPS) emission standard. Figure 10 shows a comparison of the Ski-Doo DI two-stroke engine emissions (UIDI 600) with average production two-stroke and four-stroke emissions. This comparison shows the reduction of exhaust emissions over traditional two-stroke engines and how this technology compares to four-stroke engines.
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1. Introduction
1.1. Background
Snowmobiling offers a great opportunity for winter recreation and exploration. Traditionally snowmobiles have had high levels of harmful exhaust emissions. They are often ridden in environmentally sensitive areas such as Yellowstone National Park where the adverse effects of snowmobiles can be drastic. The snowmobiles negative impact prompted the SAE and other interested parties to begin the Clean Snowmobile Challenge in 2000 to encourage the production of clean snowmobiles [1].
The University of Idaho Clean Snowmobile Challenge team is designing and building a snowmobile with reduced harmful exhaust emissions to compete in the 2007 Society of Automotive Engineers (SAE) Clean Snowmobile Challenge. This collegiate design competition focuses on developing snowmobiles that meet future EPA emission standards.
The University of Idaho has competed at the Clean Snowmobile Challenge since 2001. The first sled built for the competition consisted of a BMW four-stroke motorcycle engine adapted to fit into an Arctic Cat snowmobile chassis. This snowmobile placed first in both 2002 and 2003 and set standards for EPA emissions of snowmobiles. While the BMW snowmobile was clean and quiet, it was heavy and had poor handling. In response to this, the UICSC team began the development of a clean, efficient two-stroke engine. Two-stroke engines are traditionally used in snowmobiles because of their high power to weight ratio. By developing the two stroke engine, emissions can be reduced while gaining fuel economy and maintaining performance.
1.2. Problem Definition
Many participants of the Clean Snowmobile Challenge use four-stroke engines in their snowmobiles because of their characteristic low emissions. The two-stroke engine offers a higher power to weight ratio than four-stroke engines. For this reason UICSC has chosen to develop a clean two-stroke engine. In two-stroke engines, the simplistic design results in poor fuel economy, poor low-load operation, and high exhaust emissions [2]. This is due to the way air/fuel mixture is introduced into the cylinder through the scavenging process [3]. During this process, the intake and exhaust ports are open at the same time and significant amounts of air/fuel mixture are lost, or short-circuited, out of the exhaust. This raw fuel exiting the engine significantly increases toxic emissions and reduces fuel economy.
1.3. Previous Work
For the past two years the UICSC team has successfully implemented a direct injection system on a Polaris 600cc two-stroke engine. DI sprays fuel directly into the combustion chamber and thereby reduces fuel escaping into the exhaust through short-circuiting. Using the DI system, exhaust emissions are reduced by 50-95% when compared to a stock engine. The two-stroke’s characteristic high power-to-weight ratio is also sustained [2].
In 2004 UICSC began the research and development necessary to adapt a DI system to a two-stroke snowmobile engine. Over the next two years UICSC converted a Polaris 600cc two-stroke engine to direct injection. During the development of the system, UICSC encountered problems with engine tuning and therefore the engine did not operate properly. The Polaris DI system’s poor performance was due to poor combustion chamber geometry and a lack of engine data. In response to this inadequate performance a new DI cylinder head was requested by the UICSC team.
1.4. Objectives
This report describes the design, fabrication, and testing of an instrumented DI cylinder head for the UICSC team. The new DI cylinder head improves upon the previous designs with the integration of in-cylinder pressure transducers and improved combustion chamber shape. The new chamber geometry allows the use of stratified combustion. Stratified combustion ignites the fuel prior to fully mixing with intake air, and thereby uses less fuel and reduces exhaust emissions. The pressure transducers are used to experimentally find in-cylinder pressure versus crank-angle data. In-cylinder pressure can be used to identify cycle-to-cycle variations, peak pressures, identify detonation, calculate the mass fraction of burned fuel, and calculate the heat release of combustion. All of which can be used to optimize combustion and improve engine performance.
1.5. Methods
The design, fabrication, and analysis of this project were based on methods and principles of mechanical and materials engineering taught at the University of Idaho. Solid Works, Master-cam, and TK Solver software programs were used in the design, construction, and analysis of the DI cylinder head. Background knowledge and previous design information has been acquired through technical documents, previous design papers, and literature.
1.6. Project Overview
In response to the inadequate performance of previous DI cylinder heads, our design team designed a new DI cylinder head for Polaris 600cc two-stroke engine from June 2006 to August 2006. In late August the UICSC team was donated a Ski-Doo 600cc H.O. two-stroke engine. The Ski-Doo engine was chosen for the new DI cylinder head because it provides a better platform for the DI system. The DI power, oil, and ignition systems are easier to adapt to the Ski-Doo engine. This meant a redesign of the DI cylinder head to accommodate the Ski-Doo engine’s different dimensions and characteristics.
2. Concept Development/Cylinder Head Specifications
The development of the DI cylinder heads required the design and analysis of various cylinder head and combustion chamber parameters. Squish, fuel delivery, ignition, pressure transducer, and chamber geometry are the parameters that affect the combustion chamber of the DI cylinder head and the overall engine performance. The two piece design, coolant passages, temperature sensor, and weight are all cylinder head parameters that affect engine performance and usability. The following sections describe the specific combustion chamber and cylinder head parameters, their effects, and target values for the Ski-Doo engine.
2.1. Squish Band
The squish band is the inclined surface around the perimeter of the cylinder used to concentrate mixture charge in the center of the cylinder, promoting mixing and vaporization (figure 1). The squish band design is characterized by squish velocity, which is the velocity of mixture being compressed and forced from the edges of the cylinder to the center of the combustion chamber. For proper mid to high load operation the ideal squish velocity is shown to be 15 to 20 m/s [4].
Squish velocity is a function of piston speed, piston location and combustion chamber design. The combustion chamber design parameters affecting squish velocity are squish area, squish diameter, squish height, and clearance (figure 2). To obtain the proper squish velocity these values were analyzed and optimized using the model outlined in Blair’s Design and Simulation of Two-Stroke Engines (Appendix C). The optimal values for the parameters were found to be:
• Squish Diameter – 1.81 in
• Squish Height – 0.1 in
• Squish Area – 39.5% (Found from squish diameter and cylinder bore)
• Clearance – 0.00 in (Same as stock Ski-Doo cylinder head)
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Figure 2: Ski-Doo DI combustion chamber cross section showing final geometrical design parameter values
2.2. Fuel Delivery
The fuel injector is located on the DI cylinder head so that fuel can be sprayed directly into the top of the combustion chamber. At low load and engine speed fuel can be injected after the exhaust ports are closed, which eliminates short circuiting. The fuel injector is angled 5 degrees toward the intake port to promote mixing with the incoming scavenged air and to reduce the amount of raw fuel lost out of the exhaust port. The injector positioned so that the fuel spray is targeted at the center of the piston at top dead center. This is to improve stratified operation by centralizing the charge in the chamber. Fuel spray quality is improved by expanding the dome radius from 0.4 to 0.7 in. This improved near nozzle geometry improves fuel injection efficiency and air entrainment for stratified operation.
2.3. Ignition
The initiation of combustion in the DI system is accomplished using the NGK spark plug ZFR7F. The DI cylinder head was designed to accommodate the dimensions of this spark plug. The spark plug was located on the exhaust side of the cylinder to promote combustion. It has been shown that there is greater chance of an ignitable mixture at this location [2]. The depth, angle, and height of the spark plug in the chamber were optimized to promote stratified combustion. All of these parameters affect the spark location which for efficient stratified combustion is required to penetrate the fuel spray and be mid height in the chamber. The spark plug design parameters are as follows:
• Electrode Height – 0.6 in
• Spark Plug Angle – 22˚
• Electrode/Fuel Cone Spacing – 0.02 in
2.4. Pressure Transducer
The pressure transducer the chosen for use in the DI cylinder head was the Kistler 6052C (Figure 3). This transducer was chosen for its reputation in industry for quality in-cylinder pressure measurement and the availability of supporting equipment. The pressure transducer has various options for placement into the combustion chamber. It can be implanted into the sparkplug, in a mounting sleeve passing through the water jackets or directly threaded into the combustion chamber. The sparkplug mounted sensor was tried in the past, but issues were found in removing the sparkplug without damaging the sensor. The sleeved design would pose problems in a two piece head design. Our DI cylinder head was designed so that the pressure transducer could be directly threaded into a boss on the combustion chamber. This design simplifies the DI head while improving reliability and ease of use.
2.5. Chamber Geometry
The combustion chamber has improved geometry for fluid flows within the chamber which improves charge mixing, combustion quality, and reduces toxic exhaust emissions. This is shown through the increased squish radius and expanded dome radius (figure 2).
• Squish Radius – 0.25in (previous design 0.04in)
• Dome Radius – 0.7in (previous design 0.4in)
To produce the comparable power as stock while only modifying the cylinder head, the compression ratio of the engine must remain the same as stock. The power output of the engine was not designed to be exactly the same due to small power variations found in modifying the fuel delivery system and chamber geometry. Trapped compression ratio is typically calculated with the simple equation:
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The trapped compression ratios of the Polaris and Ski-Doo engines were calculated using TK solver software. Calculating the clearance volume gave the necessary combustion chamber volume of 26.2 cc. This volume provides a stock trapped compression ratio of 6.5:1. This necessary combustion chamber volume dictated the value for the dome height of 0.96 in.
2.6. Two Piece Design
The DI cylinder head was designed in two pieces, the head and cap, to allow for the machining of the coolant passages and water jackets. This required the intricate design of the parting surface and seals. To seal the water jacket O-rings were used around the head perimeter, injector bosses, and spark plug bosses. This method of sealing the water jacketing was proven to work in previous designs. The two piece design and O-ring grooves can be seen in the drawing packages of both the Polaris (Appendix A) and Ski-Doo (Appendix B) designs.
2.7. Coolant Passages/Temperature Sensor
The coolant passages from the engine to the head were kept in the stock location to maintain proper engine cooling. The coolant line to the head is located on the intake side of the engine between the two cylinders. This location allows for the coolant line to run directly down to clean up the appearance of the engine package. The temperature sensor is located in the rear wall of the cylinder head (figure 4). This also cleans up the appearance of the engine package. When installed in the snowmobile chassis the temperature sensor and coolant line cannot easily be seen in the engine compartment.
2.8. Weight
To retain or improve performance of the UICSC team snowmobile it is desirable to both maintain or increase power and maintain or decrease weight. During the design of the DI cylinder head, weight was reduced from the previous design. This was accomplished by reducing overbuild and decreasing the height of the head from 1.73 in to 1.53 in. The height reduction also reduces coolant volume in the DI cylinder head.
• Weight (dry) – 3.835 lb (previous design 4.285 lb)
3. Detail Design/Manufacturing
3.1. Polaris Cylinder Head
The original Polaris combustion chamber was designed with a symmetrical revolved dome. While the symmetrical design was simple, the proper sparkplug to fuel spray distance was unobtainable. For stratified operation the spark plug must penetrate the fuel spray. Due to the large dome radius (0.7 in) the spark plug was not long enough to reach the fuel spray.
To obtain the proper spark plug to fuel spray distance three different combustion chamber alterations were designed. The first iteration involved using a boss to locate the spark plug further into the combustion chamber. The boss eliminates the symmetry of the combustion chamber and is therefore harder to manufacture because a tool cannot be created to cut the combustion chamber. The second iteration accomplishes proper spark plug to fuel spray distance by creating a flat area on the side of the combustion chamber. This design improves fluid flows from the boss design but would still be complex to manufacture.
The final Polaris combustion chamber design moves the spark plug further into the combustion chamber by completely smoothing the transitions of the second iteration (figure 6). This greatly improves fluid flows in the chamber. This combustion chamber design also meets all desired specifications.
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Figure 6: Polaris combustion chamber iterations 1, 2, & 3 (final).
3.2. Ski-Doo Cylinder Head
The Ski-Doo DI cylinder head incorporates similar parameters as the Polaris design. The cylinder head design had to be altered for requirements of the Ski-Doo 600 H.O. engine. The combustion chamber for the Ski-Doo is similar to that of the Polaris. The perimeter of the cylinder head, coolant passages, coolant return, temperature sensor, cylinder bore, and combustion chamber volume all had to be altered for the Ski-Doo design (figure 7). The combustion chamber volume was reduced from the Polaris volume of 36.9cc to a volume of 26.2 cc. The combustion chamber volume was reduced to obtain the proper trapped compression ratio. This reduction also allowed for proper spark plug to fuel spray distance while using a symmetrical combustion chamber design.
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Figure 8: Polaris (left) and Ski-Doo (right) perimeter comparison.
3.3. Manufacturing
The Ski-Doo DI cylinder head was manufactured in the University of Idaho Mechanical Engineering machine shop by our design team. The fabrication was completed on the HAAS 4-axis CNC mill shown in figure 9. The previous DI cylinder head was also manufactured using the same HAAS mill. The head was milled from two solid blocks of 6061 T6 aluminum. A manufacturing summary is located in Appendix D.
4. Product Evaluation
4.1. Testing/Engine Tuning
The performance of the Ski-Doo DI cylinder head is based on physical specifications and overall engine performance. Following manufacturing, the Ski-Doo DI cylinder head was inspected and all specifications were met sufficiently. It was then installed on the Ski-Doo engine and the other necessary engine modifications were completed for the application of direct injection.
A steady-state engine dynamometer was then used to tune the engine and test for horsepower and torque performance. An exhaust emissions analyzer was also used to tune for the lowest achievable emissions. Pressure transducers were used to see in-cylinder pressure data and monitor cycle-to-cycle variations of the engine, both aid the tuning process.
Due to the use of the DI system, the engine must be programmed with various parameters. Air fuel ratio, injector timing, and spark timing maps were developed for the engine’s range of engine speed, load, and throttle position. These maps were developed to optimize the engines power output while maximizing fuel economy and minimizing harmful exhaust emissions. All engine tuning and emissions measurements were completed by Andrew Findlay, a graduate student at the University of Idaho and member of the UICSC team.
The performance of the DI cylinder head is also based on the ability of the UICSC engine to operate properly while creating manufacturers specified power with exhaust emissions similar to 4-stroke engines. The UICSC engine will be tested at the 2007 SAE Clean Snowmobile Challenge. At the challenge it will compete against many four-stroke engines for lowest toxic exhaust emission levels. This will show if the head is designed properly and if the DI system can produce emissions comparable to a four-stroke engine.
4.2. Summary of Results
Based on the testing procedures outlined above, the Ski-Doo DI cylinder head performed very well. The DI cylinder head did not leak, all of the DI components were compatible, the engine exhaust emissions were low, and no power was lost from the stock engine configuration.
The SAE Clean Snowmobile Challenge compares exhaust emissions using the SAE 5-mode emission test. The SAE 5-mode test measures emissions at each of the engines mode points. The mode points are spread from idle to max torque at different engine speeds and loads. Each mode point has a different weight on the overall score. To meet the 2012 EPA emissions standard the SAE 5-mode score must be above 100. To pass the 2012 NPS emission standard for entrance into national parks the score must be above 170.
The UICSC Ski-Doo DI 600 HO engine scored a 158 on the SAE 5-mode emission test. This score is very high for a two-stroke engine. The cleanest production two-stroke snowmobile, the Ski-Doo semi-direct injection, scored a 120 on the same test. The SAE 5-mode test is based on HC, NOx, and CO power specific exhaust emission quantities. Figure 10 shows a comparison between the UICSC DI engine and production two-stroke and four-stroke engines. This data shows that the Ski-Doo DI cylinder head performed very well and achieved very low emissions for a two-stroke engine.
5. Economic Analysis
5.1. Engineering Cost
The cost to design the DI cylinder head includes design time, modeling time, and analysis time. The time spent designing both the Polaris and Ski-Doo DI cylinder heads are broken down in tables 1 and 2. The Ski-Doo DI cylinder head design required significantly less design time because experience had been gained from the Polaris DI cylinder head design. Overall, the Ski-Doo cost more because it was manufactured. There are also additional costs including equipment and fabrication expenses. Table 3 summarizes these expenses throughout the entire project.
Table 1: Polaris DI cylinder head design time Table 2: Ski-Doo DI cylinder head design time
|Polaris Head: | | |
|Area |Time (hrs) |Cost (@$20/hr) |
|Design |150 |$3,000 |
|Solid Modeling |200 |$4,000 |
|Manufacturing Prep. |40 |$800 |
|Manufacturing |0 |$0 |
| |Total: |$7,800 |
|Ski-Doo Head: | | |
|Area |Time (hrs) |Cost (@$20/hr) |
|Design |25 |$500 |
|Solid Modeling |80 |$1,600 |
|Manufacturing Prep |80 |$1,600 |
|Manufacturing (more than one |250 |$5,000 |
|person's time) | | |
| |Total: |$8,700 |
Table 3: Equipment and fabrication expenses
|Item |Cost |
|Kistler Pressure Transducer Tooling |$372.40 |
|Gasket Material |$13.89 |
|Head Bolts |$65.12 |
|Aluminum Stock |$258.29 |
|Machine Shop Time (~110hrs @ $10/hr) |$1,100.00 |
|Total: |$1,809.70 |
5.2. Cost of Production
The cost to have the DI cylinder head fabricated in a machine shop would vary from the cost to our customer. We machined the cylinder head in 110 hrs where a professional shop could complete this in approximately 80 hrs. Additionally the shop could also write the CNC code in approximately 20 hrs. Assuming a CNC shop rate of $80.00 per hour the DI cylinder total cost would be $8,000. This is the expense for the production of a single DI cylinder head. If mass production is utilized this cost could be significantly reduced. Other manufacturing processes, such as aluminum casting, could also be used to further reduce fabrication cost.
6. Conclusion and Recommendations
The Ski-Doo DI cylinder head significantly reduces exhaust emissions, improves fuel economy, and maintains the power of the engine. The engine’s emissions could be reduced even further through the use of a catalytic converter. This could result in meeting the NPS 2012 emission standard.
The NPS 2012 emission standard could also be met through further two-stroke engine design and development. Significant exhaust emission reductions could be made through port design and optimization. Further improvement could also be made to the DI cylinder head design. Utilizing CFD analysis the combustion chamber design could be improved for emission reduction. High engine speed stratified operation could be improved through the use of two spark plugs per cylinder. Two spark plugs would create two flame fronts to propagate through the combustion chamber and would increase power output of the engine during high speed stratified operation. The use of a dual spark plug DI cylinder head would also create a more stable combustion and would allow for the use of less fuel and therefore better fuel economy and further exhaust emissions reduction.
By meeting the NPS 2012 emission standard the UICSC two-stroke snowmobile would be allowed entrance into national parks such as Yellowstone National Park.
This is why further development of the UICSC two-stoke is needed. Overall, significant improvements can be made through further engine design and CFD analysis.
References
1. Bradbury N., Findlay A., Johnson J., Van Patten E., Den Braven K., “University of Idaho’s Clean Snowmobile Design Using a Direct-Injection Two-Stroke,” SAE paper 2006-32-0050.
2. Bradbury N., Harris T., Schiermeier R., Den Braven K., “University of Idaho’s Clean Snowmobile Design Using a Direct-Injection Two-Stroke with Exhaust Aftertreatment,” SAE Paper 2005-01-3680, Oct., 2005.
3. Heywood J.B., “Internal Combustion Engine Fundamentals”. McGraw Hill, Inc. 1988.
4. Blair G.P. “Design and Simulation of Two-Stroke Engines”. Society of Automotive Engineers, Inc. Warrendale, Pa, 1996.
5. Findlay A., O’Neil R. “Design of a Direct Injection Two-Stroke Engine for Low Emissions Operation.” Design Report, 2004.
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Figure 1: Combustion chamber components
Figure 3: Kistler 6052C Pressure Transducer
Figure 4: Temperature Sensor
Figure 5: Ideal spark plug to fuel cone distance with final Polaris combustion chamber design.
Figure 9: HAAS 4-axis CNC mill
Figure 10: Comparison of UICSC DI, average production 2-stroke, and average production 4-stroke exhaust emissions.
Figure 7: Final Ski-Doo DI cylinder head.
Figure 10: Comparison of UICSC DI, average production 2-stroke, and average production 4-stroke exhaust emissions.
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