Introduction to Pheidippides
Pheidippides will be the University of Nevada, Reno’s entry into the 2014 SAE Supermileage Competition. The competition provides mechanical engineering students with a realistic and practical design project that involves the development and construction of an extremely high mileage vehicle. The single-person vehicles are powered by small, air cooled, four-cycle engines that are generously provided by Briggs and Stratton. During the competition, the vehicles run six laps around a 2.6 km (1.6 mile) oval test track to determine kilometers per liter (miles per gallon) ratings. Each vehicle must be equipped with steering geometry capable of executing a maximum inside radius turn of 15.2 m and be able to traverse a 30.5 m slalom section in less than 15 seconds. In addition to these dynamic events, each team must submit both written and verbal design reports. The team that earns the highest combined mileage rating plus design segment points is declared the winner of the competition. More information about this competition can be found at: http://students.sae.org/cds/supermileage/
In today’s society, conservation has become paramount for any engineering design. This is especially true when it comes to the design of a vehicle and the consumption of its fuel. The Supermileage Vehicle competition is designed specifically to test college students in their ability to design and build small vehicles with the emphasis on getting the highest efficiency in terms of fuel consumption. Last year, a team designed and manufactured Pheidippides, a Supermileage Vehicle (SMV), and competed against several colleges.
The design problem for Team Ouroboros is to take Pheidippides and make it as fuel efficient as possible. This could have been accomplished in several ways, but it was determined that the best efficiency gain would be obtained through the modification of the engine. As it stands, the engine is a 3.5 horsepower Briggs and Stratton which was supplied by the competition and is stock from the factory. The design is built for power, not efficiency and its compression ratio is a low 6:1. By modifying the engine and increasing its compression ratio, fuel efficiency could be significantly increased.
Aside from the engine, the design for the exhaust is also an issue. Currently the exhaust is re-entering the cabin area and is causing both a safety issue and reduced engine efficiency due to the exhaust re-entering the engine.
The Objective of the team is to increase the miles per gallon (mpg) of Pheidippides from its current 167mpg to a range between 500-1000mpg. This will be accomplished through the conversion of the engine from a Flat Head design to an Overhead Valve (OHV) head design. The engine modifications will increase the compression to approximately 10:1 and thus reduce its fuel consumption.
Flat Head/Side Valve (left) versus Overhead Valve (right) arrangement
An adapter plate will need to be manufactured that will allow the new head to be attached to the crank case as the mounting holes are in a different configuration. Because the adapter plate will add half an inch to the overall height, a new piston rod will need to be manufactured which will allow the piston to reach proper height.
Also, the rerouting of the exhaust system will increase engine efficiency and driver safety by removing the issue of exhaust re-entering the cabin. The design for the exhaust will be completed once the engine has been modified and then mounted to the vehicle.
As the directive of the Supermileage Competition is use the least amount of fuel to complete the course, fuel efficiency is key. Though there are many factors that contribute to lower fuel use, such as the type of wheels used and the profile of the vehicle, the team decided to focus on modifying the engine as it would give the best gains in terms of fuel efficiency. For this purpose, the changes that were made to Pheidippides were narrowed down to the engine, intake, and exhaust.
To meet the objective of increasing fuel efficiency for Pheidippides, the engine head was modified. As it stands, the stock engine was a flat head and gave the engine a compression ratio of 6:1 whereas the new OHV head increased the ratio to approximately 10:1. This increase allowed the engine to get more power with less fuel. However, the new head was made for a different engine and required that an adapter plate be made to allow it to be mounted properly. This addition of the adapter presented problems such as the increased volume of the combustion chamber and the possibility of the piston being caught between the engine and the adapter. These issues were resolved in two parts. First, the piston shaft was bored out and a sleeve was added to prevent the piston catching. Second, the connector rod was lengthened to allow the piston to reach the proper height.
For the proof of concept, initial prototypes of the adapter plate and the new connecting rod were modeled in SolidWorks and were fabricated using a 3D printer, allowing their dimensions to be tested before the final prototypes were manufactured out of aluminum. The adapter plate needed to be have a properly aligned bolt pattern, have the correct number of bolt holes, have appropriately sized holes to allow the pushrods to pass through, and be strong enough to allow the new head and stock engine to be connected securely. The final 3D printed adapter plate showed that the new head would be able to be connected to the stock engine.
First (left) and second (center) 3D printed prototypes with final machined prototype (right)
Stock connecting rod (left) and first 3D printed prototype (right)
The new connecting rod needed to be able to replace the stock connecting rod and be able to reciprocate inside the engine without coming into contact with the crankcase, cylinder wall, or any other engine components. Since the connecting rod will be in motion, it needed to be able to move smoothly, fit snugly, and be the proper length so that it would not raise the piston too far and damage the new head. The extended connecting rod allowed the new head to be used to its potential by compensating for the additional cylinder length created by the adapter plate.
Measurements for the adapter plate were made according to the stock engine’s bolt pattern and pushrod locations using a digital vernier caliper with a rated accuracy of 0.001 in and resolution of 0.0005 in. Several iterations of the 3D printed adapter plate prototype were created since the initial measurements were not accurate enough for practical use. In the first models, bolt holes were not the proper size and their locations were misaligned, requiring the measurements to be repeated and corrected until a satisfactory prototype was created.
The connecting rod was based on a SolidWorks model provided by the engine’s manufacturer, Briggs and Stratton. Dimensions from the SolidWorks model were supplemented by measurements made using the vernier caliper. The length of the first 3D printed prototype of the connecting rod turned out to be slightly off, but its dimensions were close enough that it was able to be connected to the engine’s crankshaft and piston. This confirmed the connecting rod’s ability to move the cylinder through a full up and down cycle.
The prototypes for the adapter plate and connecting rod were modeled in SolidWorks and were fabricated using the 3D printer operated by the DeLaMare Library at the University of Nevada, Reno. The final 3D printed prototypes of the adapter plate and connecting rod were installed on the stock engine to prove that they were dimensioned properly.
The adapter plate proof of concept was first attached to the stock crankcase with the actual bolts that would be used for the final aluminum plate. Next, the new OHV head was attached to the opposite side of the plate proof of concept, also with the actual bolts that would be used on the aluminum plate. Two gaskets were fabricated out of copper and installed at the interfaces between the engine block, the adapter plate, and the cylinder head. The connecting rod proof of concept was attached to the crankshaft and the piston and was tested by manually executing one full piston stroke.
Final prototypes for the adapter plate and connecting rod were manufactured out of aluminum in the machine shop. Both components were manufactured out of the same piece of aluminum. The cylinder was bored and sleeved at AIMS Machine Shop. The decision was made to have this work done professionally because the interfaces between the engine, cylinder, sleeve, and adapter plate require precision to function properly.
Before taking the engine to AIMS, the lower copper gasket was attached to the engine with silicone and the adapter plate was securely bolted on top. This was done so that the cylinder and adapter plate diameters would be exactly the same and there would be no gap between the plate and cylinder for the piston rings to catch on. Once the engine had been assembled to this point, it was taken to AIMS Machine Shop where it was bored and sleeved. The top of the adapter plate was also machined flat to create a smooth surface for the upper gasket and head to attach to.
After this machining was completed, the new connecting rod was affixed to the crankshaft and piston. Next, the upper copper gasket was attached to the top of the adapter plate, silicone was applied, and the OHV head was bolted on. To complete the assembly, the new intake and exhaust pipes were attached to the head. A small cover plate was also added and sealed with silicone to cover the old intake and exhaust ports from the stock flat head.
Engine with adapter plate and OHV head attached
Final engine assembly with valve cover and UNR themed paint job
ABOUT THE TEAM
From left to right: Nathan, Ben, Russ, Anthony, and Evelyn
Russell is the current team leader of Ouroboros. He is an undergraduate studying at University of Nevada with the goal of pursuing a graduate degree in Biomechanics/Robotics. He is the head of the Supermileage Vehicle team, and is preparing a new team to build a vehicle from the ground up. In the previous year, he was also the Vice President of American Society of Mechanical Engineers and assisted in reinvigorating the club and getting it recognized officially by ASUN.
Anthony Farkas is a 25 year old software engineer for a local document imaging company, and will graduate in spring with a degree in mechanical engineering. His interests are in thermodynamics and engines.
Ben is a fourth year undergraduate studying at the University of Nevada, Reno. He is preparing to graduate this coming May with a minor in Statistics and a Bachelor of Science in Mechanical Engineering, with interests in pursuing the fields of biomechanics and electromechanical systems. Ben is one of UNR’s Presidential Scholars and an active member in the American Society of Mechanical Engineers, serving as Treasurer and President for the UNR chapter.
Evelyn Jo is a senior at the University of Nevada, Reno majoring in Mechanical engineering and will graduate in Winter 2014. She likes learning and hands-on activities which includes building and taking things apart.
Nathan Neben is a space enthusiast, an ultramarathon runner, and a senior in the Mechanical Engineering program at the University of Nevada, Reno. He likes to spend his free time reading science fiction and fantasy novels, backpacking, and cooking.