During the fall semester of my senior year as a Mechanical Engineering undergraduate at UC Berkeley, I became involved with the Super Mileage Vehicle (SMV) team. Two of the other team members (Kevin Ciocia and Christopher Floren) along with myself designed and built a new small displacement, fuel-injected engine as our senior design project. The Super Mileage Vehicle team designs and builds a small, lightweight single passenger vehicle which is designed to obtain the maximum possible fuel economy. After the end of the spring semester, Cal’s Super Mileage Vehicle team goes to a competition in Michigan to compete against other college teams. The Cal SMV website can be viewed at http://smv.berkeley.edu/.
Jump to: Case Modifications Oil System Fuel Injection External Timing Flywheel Final Design Report (download zip file)
The SuperMileage competition provides a Briggs & Stratton (B&S) lawn edger engine as a powerplant. In order to retain a level of fairness in the competition, entries are required to retain at least the case of the B&S engine that they are provided. The B&S engine has a number of serious disadvantages. It displaces approximately 150cc’s, which is more than is necessary to power the car. It is a flathead engine with the intake and exhaust valves in the engine block, which are actuated with a nylon camshaft. There is no pressurized lubrication system, and the crank rides on main bushings instead of bearings. The ignition is a fixed-advance magneto system, and the fuel is delivered through a basic downdraft carburetor.
The B&S is based on archaic technology. The last flathead used in automobiles was probably the Jeep 134ci L-head, which ended its service in the DJ-3A in 1964. (See the 1943 Willys MB rebuild page for the L-head.) Our main objective was to retain the B&S case, while reducing the displacement, replacing the carburetor with a fuel injection system, and adding a better lubrication system. We needed to achieve these objectives in a span of approximately three months, while spending as little money as possible and utilizing our relatively limited resources.
Our initial plan was in large part Kevin’s brainchild. It involved installing a thick aluminum sleeve into the B&S case, and adapting the crankshaft, piston, and head of the venerable OHC Honda 50cc engine used in the Z-50, XR-50, and CRF-50. Kevin initially did a good deal of work on his own, but the most recent successful version of the engine didn’t come about until Kevin, Chris, and I all worked together on the project.
Case Modifications
The first parts Kevin and Chris made for the engine were the sleeve and bearing supports for the Honda crank. The first sleeving attempt failed due to the wrong interference, and a lack of support inside the case. The picture below shows the case with the sleeve, both bearing supports, and the Honda crank. The side cover holds one bearing support, and the other teardrop shaped bearing support bolts to the inside of the case. All aluminum parts are out of 6061-T6. The sleeve is likely going to get a Nikasil plating eventually, but for the time being, we have been running it as-is.
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Oil System
One of the dead-ends Kevin ran into with his first version of the engine was that it seized up soon after it was fired. This was due in part to an inappropriate piston clearance. However, I felt that the failure was more due to the lack of a pressurized oil system. Having helped a friend modify his XR-50 engine, I was somewhat familiar with the engine’s oil system, which fed through the cam, connecting rod big end bearing, and squirted up onto the bottom of the piston and onto the cylinder walls.
I set out to design and build the pressurized oil system. This involved adapting the Honda trochoidal oil pump with oil lines and a drive system, and feeding the head and crankshaft. The external electric oil pump drive concept was used to reduce the mechanical load on the engine. Since I did most of the work on this section, I have the most to say about it, but that’s not to say that any of the other sections were less important.
All lines were 1/4” copper tubing, commonly used for refrigeration applications. Fittings were 1/8” NPT to 1/4” single flare. Flow restrictions in the original Honda engine were duplicated.
The head adapter was relatively easy. All that was necessary was a plate that bolted to the side of the head, and had a tapped bore for an oil line. For the head oil return, the stock return passage was tapped for a 1/4" NPT adapter.
Head oil feed adapter, oil return fitting (with hose removed)
In its original application, the crankshaft is fed through a centrifugal clutch, along the axis of rotation of the crank. This was not practical for our application because we would be mounting the flywheel on that side of the crank, and didn’t have room to have a bulky oiling adapter. After a few measurements/calculations, I determined that we could utilize a bore that already existed in the side of the crank, which was intended to oil the sleeve bearing for the main drive gear. The bore in the end of the crank was plugged with a pressed-in aluminum rod. The crank oil adapter consisted of a body that bolted to the side of the engine case, with two high-pressure rotary seals. The oil was fed through a line attached to the body, and through a passage between the pressure seals.
Crankshaft oil feeder adapter
By far the most time consuming part of the oil system was the oil pump adapter. It went through a series of relatively serious design changes, and the final assembly was much larger than we wanted, but it worked. The oil pump adapter had to have proper oil passages and a drive system. The first objective was achieved by taking almost 30 measurements of the oil pump body, and basing the design of the adapter on the locations of the oil input and output. The input and output ports on the adapter which fed the oil pump were cut with a 1/8" and 3/16" endmill (sequentially). It goes without saying that those operations took quite a long time on a 2-axis CNC.
Oil pump layout, final adapter design
The complex oil passages were then cut into the block using a Millstar Jr. 2-axis CNC mill. All other oil passages and restrictors were done with appropriately sized drills.
The next, and more difficult part of the oil pump adapter was the drive system. We initially thought that we would be able to run the oil pump directly off of a Mabuchi 540-series motor. We knew the speed range that the oil pump needed to operate within (3000 RPM max), but didn’t know how much power would be required. We found, however, that the lower speed 540’s (0-7000 RPM) didn’t have nearly enough power, and the higher speed 540’s (0-17,000 RPM) were way too fast. The only gearboxes that were readily available were the wrong ratio or far too expensive.
Drawing on my years of experience with taking things apart, I decided that the gearbox out of a cordless driver drill might fit the bill: cheap, fairly durable, and designed for the 540-series motors. I spent a whole $30 on a Coleman Powermate 18V drill, and ditched everything but the gearbox. It was a plastic housing with a triple planetary setup inside. After a few gear calculations, I determined a combination that would give me the ratio I needed. The third planetary was removed and one of the planets was used as an output gear, and the second planetary was left locked in high range. This gave a 5.625:1 ratio that was perfect for the application.
The gearbox adapter is visible in the pictures below bolted to the back of the oil pump adapter. The plastic drill gearbox is behind that (off-white in the photo, brown in the rendering). The original plastic motor plate was replaced with a machined aluminum one that had tabs so it could be bolted to the gearbox adapter. The Johnson Electric motor we used didn’t have an internal cooling fan like the 540-series that came with the drill, so I made a simple bracket to attach a readily available CPU fan to the motor plate.
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Fuel Injection
Kevin did nearly all of the work on the fuel injection system. It was an alpha-N system (inputs: throttle position and engine speed) with an oxygen sensor. The throttle body used a ball valve style throttle valve instead of the traditional butterfly valve to make a higher flow system. The injector was a custom 30cc/min unit built by Mr. Fancy Carol in Japan, who holds the world SuperMileage record. Utilizing aspects of a code written by a past SMV member, Kevin took care of the programming, injector drive circuit, and ignition control.
Throttle body with injector, wireframe drawing
The basic theory behind the injection system was as follows: the alpha-N system uses the throttle position and engine speed to estimate an engine load. Based on this theoretical load, it sets a particular injector (open) time. When the system is in closed loop mode, the controller takes the oxygen sensor’s rich/lean signal and adjusts the theoretical injector time until the desired air/fuel ratio is achieved. Below are a couple of early injector time maps, with RPM and throttle position on the X and Y axes, and injector time on the Z-axis. In each map the plane is the “choke” plane, and the peaks and valleys are the result of adjustments made based on the oxygen sensor signal. Since our load bench testing was limited to a very old twin-screw water pump, which has a good deal of load at “no-load,” the peaks and valleys in the multi-level choke plane map have a space in between them that hasn’t been altered.
An old flat-plane map, and a newer varied-plane map.
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External Timing
In order to convert the Honda head to accommodate an external timing system, a cap was necessary that retained the cam from moving along its axis and held oil in the head. To achieve this, I designed a threaded, sealed cap that fit into the head where the original cam chain cover fit. A large portion of the work on this part was the threading of the cap and the head. Since all three of us were relative beginners in the machine shop, this operation wouldn’t have been possible without the extensive help of our machine shop instructor Gordon Long. Visible in the rightmost picture is the optical encoder and mount which provided the engine speed for the injector control.
The cap design, finished part, and installation, with optical encoder and mount visible.
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Flywheel
Probably the most time-consuming single machining processes for the engine was the flywheel. It needed to mate with the splined end of the crank, and needed to have the proper teeth to mesh with the stock B&S starter motor gear. The new flywheel was sized to match the combined moment of inertia of the Honda flywheel and centrifugal clutch. Chris spent a good 5 or 6 days making this part. Mounted on an indexed rotary vise, the job required taking large cuts with a standard cutter, and then using a specially made tool that was the shape of the teeth, and running it very slowly. Chris probably still hears the slow knocking of this operation in his sleep. It still needs a little bit of fine-tuning, as it has a tendency to eat the plastic starter gears after a while. This is partially due to the relatively sharp teeth on the flywheel, but it is also due to the fact that the starters we have are worn out, and don’t always keep the gear at the end of the shaft, and when it shoots back out, it doesn’t always mesh with the flywheel teeth.
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In addition to designing and building the engine, we were required by our design class to document our design process in two design review reports, two presentations, and a final report. You can download the entire final design report for the project here: Final Design Report (download zip file)