Cal Super Mileage Vehicle Engine
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, Calís 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/.
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 150ccís, 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 Kevinís 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 didnít come about until Kevin, Chris, and I all worked
together on the project.
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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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 engineís 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
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 thatís not to say that any of the other
sections were less important.
All lines were 1/4Ē
copper tubing, commonly used for refrigeration applications. Fittings were 1/8Ē
NPT to 1/4Ē single flare. Flow restrictions in the original Honda engine were
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 didnít 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 didnít know how much power would be required. We
found, however, that the lower speed 540ís (0-7000 RPM) didnít have nearly
enough power, and the higher speed 540ís (0-17,000 RPM) were way too fast. The
only gearboxes that were readily available were the wrong ratio or far too
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 didnít 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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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 sensorís 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 ďchokeĒ 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 ďno-load,Ē the peaks
and valleys in the multi-level choke plane map have a space in between them
that hasnít been altered.
An old flat-plane map, and a newer varied-plane map.
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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 wouldnít 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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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 donít always keep the gear at the end of the
shaft, and when it shoots back out, it doesnít always mesh with the flywheel
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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)