Formula SAE

Engine Controls / Final Drive

Thomas DuCourant

Outline

Introduction

Engine and Controls

Engine ControlsEngine SelectionDesign Challenges

Reliable sensorsFinding Parts

Design Solutions

Final DriveBackgroundFinal Drive SelectionDesign Challenges

Oil leaksWeight

Design Solutions

Conclusion

References

IntroductionFormula SAE is annual competition that takes place every year. Competing

schools and universities throughout the world enter mini formula cars into the competition. The event is a comprehensive design competition that judges the cars on ingenuity, presentation, manufacturing and performance characteristics. Each entry must comply with all FSAE rules and regulations. These rules can be found on the Formula SAE’s official web site. The competition gives students a year to design, fabricate and tune their formula style car. The end result is a project that challenges students to use their engineering skills is a realistic and practical way. This senor project is based on these regulations, so many of the geometric design dimensions are constrained to comply with these rules. The ultimate goal for this project is produces a car that can compete and win in this formula SAE competition. This report focuses on the design, fabrication and assembly of the power train, and specific design challenges that had to be solved to produce a reliable product.

Engine and Controls

Engine ControlsMany of the parts used in this project are recycled from previous years. The cost

to replace some of these parts would be too great for each team to gather from sponsors every year. One such part is the MoteC, which is an engine management computer. It receives electrical signals from various sensors and in turn controls the engine’s fuel mixture and ignition timing.

One configuration mode in which the MoteC operates is the sequential fuel injection mode. Sequential fuel injection works by allowing the fuel injector’s pulse to coincide with engine valve timing. As the intake valve opens to draw in a cycle of air, a fuel injector sprays an atomized shot of fuel directly into the engine’s combustion chamber. This cycle repeats every 2 revolutions in a four-stroke engine for each cylinder. To maintain proper stoichiometry the CPU meters each injector’s pulse width. The CPU, or MoteC in the case of FSAE, is constantly adjusting these pulse widths, as different demands are required of the engine. An array of engine sensors allows the computer to know how the engine is performing at every fraction of a millisecond. These sensors include engine coolant temperature, intake air temperature, manifold absolute pressure, throttle positioning, oxygen, crank shaft angle, and cam shaft angle sensors to name a few.

Two engine sensors are critical to sequential fuel injection operation, the cam and crank angle sensors. The cam (SYNC) sensor tells the computer the exact position of the camshaft. The camshaft opens and closes the exhaust and intakes valve. The Honda motor has two over-head camshafts. One is designated for the intake valve, while the other controls the exhaust valves. The crankshaft (REF) sensor tells the computer what angle the crank is making, which corresponds the position of the engine’s pistons. If the computer does not receive constant signals from these sensors, the engine cannot function properly.

Engine SelectionThe 600cc Honda F2 CBR engine has been the workhorse of University of Utah’s

Formula SAE cars for the last several years. The high performance sport-bike engine produces 100HP @ 12,000 and 47ft-lbs of torque @ 10,500rpm. Compared to other engines of the same size and year, our motor produces virtually the same power and torque outputs. The new 2005 engines are capable of marginally higher outputs of about 10 to 20 HP, however they have electronically control fuel injection systems. Despite the power increases of the newer motor. The team decided to continue using the same engine form previous years. The reasons for this are simple. There are three Honda F-2 engines available for this project, which require no additional funding. To obtain a newer engine we would have to purchase an entire bike, that would costs thousands. Secondly, The power increases are marginal. We should be able to make up for this power differential by converting the stock carbureted fuel system to fuel injection. If we could choose any motor we wanted, we obviously would choose the Yamaha YZF-R6 motor that produces 123 HP stock. Interestingly, this is the motor Cornell University, last year’s FSAE winner, used for their car. If we only had their funding.

Figure 1. Cornell University’s turbo setup. Note the Yamaha engine behind it.

Picture courtesy of Josh Smith

Table A. The engine specification used to select the engine

Year Make ModelEngine

Size (cc)

Bore x Stroke

(mmxmm)Power

HP Torque (ft-lbs)Compressio

n Ratio1994 Honda CBR F2 600 65x45.2 100@12000rpm 47@10500 12:011993 Suzuki RF-600R 600 65x45.2 100@11500rpm 47.4@9500rpm NA1994 Yamaha FZR 600 62x49.6 96@11500rpm 65.7@9500rpm 12:011994 Kawasaki ZZR 600 64x46.6 100@11500rpm 47.2@9500rpm NA2005 Honda CBR F4 600 67x42.5 110@12500rpm 47.9@10500rpm 12:012005 Yamaha YZF-R6 600 65.5x44.5 123@13000rpm 50.5@12000rpm 12.4:01

Design Challenges

Reliable SensorsThe goal of the team is to optimize the MoteC’s operation. To do this the team

must configure the MoteC to run in a sequential fuel injection mode, which is the most efficient mode. In past years the team has faced problems the SYNC and REF signals and the computer would report an error. Because these signals are crucial to optimizing the operation of the engine, a reliable system must be implemented.

Finding PartsThe design team’s decision to use a rear sub-frame, which transfers the rear

external forces through the engine to the front monocoque body, makes the engine a stressed member. All the parts that would allow for this rear sub-frame set-up had to be located or manufactured. I was able to go through the tedious process of finding most the parts used on formula cars from previous years to make this possible. This set-up is only made possible by the use of “side plates”, which bolt directly to each sides of the engine. The side plates become the backbone of the rear of the car. The engine, rear A-arms, coil springs/ shock assembly, final drive, differential, main hoop/sub- frame assembly, radiator and other miscellaneous components are connected to these side plates.

I created a mock-up of the engine, side plates, final drive, differential, axles, and CV joints. To give my fellow team mates a visual reference of how the aft portion of the car will function.

Figure 2. The side plates connect the F-2 motor. The side plates, along with the entire rear suspension of the car, support the final drive.

Picture courtesy of Kathy Allman

Figure 3. The rear engine sub-assembly complete with suspension and final drive. Note how the side plates along with the engine block support the weight of the entire engine

Picture courtesy of 2003 U of U FSAE team

Design Solutions Cam Sensors

To optimize the power produced by the 600cc Honda F2 motor the cam sensor must operate reliably. Pervious teams have had problems with the cam sensor requiring them to compromise on fuel injection and ignition system configurations. I discovered that they attempted to uses sensor (MD HALL 437) to generate a SYNC signal. According to MoteC’s own schematics this sensor is not recommended for anything other then wheel speed. While the past teams had problems with the cam sensor, their REF signal from the crank sensor was reliable. They employed a small hall-effect sensor (M 3025 SS13 DTM), which is recommended for cam and crank sensors. The simple solution is to use the same sensor for camshaft. It should produce a reliable SYNC signal. The only problem is designing a fixture that will allow us to use this sensor on the camshaft. I have created a fixture and trigger wheel that should accomplish this task. This design makes the sensor’s trigger wheel external rather internal were it is exposed to engine oil. The motor oil will be filled with fine metal particles from the engine bearings and clutch. These particles collect on the sensor’s magnetic face and interrupt the SYNC signal. By making the trigger wheel external the cam sensor will not be exposed to this contaminants. Also, the sensor to trigger wheel gap (.012 to .025 inches) will be adjusted accurately.

Gear Box/ Final DriveBackground

To drive the racecar forward, a mechanism is required to interface the wheels and the output shaft of the gearbox. The gearbox contains six forward gears that are integrated inside a single engine/gear block. The motorcycle from which the engine came employed a chain drive, which is the industry standard for most sport bikes. This chain/ sprocket set constitutes the final drive of the motorcycle. Its drive ratio is calibrated specifically for the rear wheel (17’’) of a motorcycle. It is not uncommon to see motorcycles that use shaft or belt driven final drives. Our racecar does not have a rear swing arm or a single rear wheel, so the gearbox has to be coupled in a different

configuration. Although FSAE cars use shaft, chain, belt, or gear driven final drives, the gearbox must ultimately be coupled to a rear differential. The differential transfers power to the axles, which in turn transfers power to each rear wheel.

Final Drive SelectionThe decision to choose a particular drive mechanism comes down to several key

questions. Which setup is more efficient? How difficult is it to manufacture or obtain each drive? How easily is it to install on the car? Lastly, what is the weight differential between each design choice?

The efficiency of a drive system is defined as the ratio of the output over the input power. V-belts have transmission efficiency of about 95-98% efficiency. After the belt has installed is begins to wear and begins to slip. As this happens its efficiency decreases.

A synchronous belt is made of rubber and steel chords similar to a v-belt. Unlike v-belts it has molded rubber cogs that prevent it from slipping. The efficiency of this drive is maintained at about 98%.

Similar to a synchronous belt a roller chain and sprocket setup has an efficiency of about 98%. A roller chain and sprocket drive experiences an interesting phenomena, during operation. As the rollers engages and disengages the sprocket at constant angular velocity, they impart a small jerk. This is known as chordal action, which causes a variation or ripple in the output velocity. In addition to this chain, belt, sprocket, and pulley setups cogged or otherwise all create lateral vibration to a certain degree. This is due to the constant velocity to angular acceleration change that a belt or chain experiences every time it passes though the pulley or sprocket. Lastly these setups require substantial structure support. As the sprockets or pulleys transfer power there is a tendency for them to be drawn or pulled together. The 2004 team experienced this, when they employed a roller chain drive as their final drive. The end result was a broken out section of their carbon fiber sub-frame to which the rear sprocket was mounted. The engine produces a strong tension force in the chain when the car is in operation.

The last setup to be considered is the simple gear train. The overall efficiency of gear trains is very high. Usually there is only a 1 to 2% power loss per gear set depending on lubrication, tooth finish, etc. This method is typically the preferred method of power transfer if the distance between rotating input and output shafts is not too large. The biggest draw back of using gear trains is they tend to be heavier that other drive mechanisms.

Table B. The decision matrix used to select a final drive.

Drive Mechanisms Efficiency WeightCost/

Availability Noise VibrationRoller Chain and Sprockets 98% 3 3 4 4Gear Train 99-98% 4 1 2 2V-belts and pulleys <98% 2 3 3 3Synchronous belt 98% 2 4 3 4

A decision matrix was used to help the power train sub-team choose a final drive. Given the parts available, a gear train final drive was on the top of the list. It is the most

efficient means of power transfer, and it produces minimum noise and vibration. The final drive would also required no additional chassis support structure other than an encased housing that bolts directly to the side plate. It can easily be coupled to the torson-gleeson limited slip differential that will be used on the car. This type of differential is the race industry standard. The torson-gleeson differential can also be found the Audi Quattro, which is world renown for its traction and rally sport capabilities. The biggest draw back to the gear train is weight.

Figure 4. Torson-Gleeson limited-slip differential that will be used in this years car.

Picture courtesy of Kathy Allman

Design ChallengesTo make the gear train more competitive the weight issue had to be addressed.

Using a design that had been used in past FSAE teams, the power train sub-team had to exam if any extra weight could be removed. As is, the gears are solid, bulky and heavy.

The second major problem that needed to be addressed what oil leaks. According to FSAE rules the car cannot leak any fluids whatsoever or the car will be disqualified from racing. This was an issue for the 2003 FSAE team. Their final drive design leaked were it was coupled to the engine’s output shaft. Since our design will be a modified version of the 2003 team’s final drive we must come up with a solution.

Design SolutionsGear Weight

To reduce weight and the rotating inertia of gear train a simple solution was devised. Remove as much material from the gear as possible. Because the 2003 team used solid gears, this is possible. Although the gear-face surface area on each tooth must remain the same, the solid webbing of the gears can be reduced. The exact amount will have to be careful calculated, because a small safety factor will undoubtedly be used. The webbing of gears will be trimmed down on a lathe, and next we will use a wire EDM to cut perpendicular sections from the webbing of each gear in our final drive. To minimize weight and maximize strength the geometry of these cuts will be impart determined by examining and benchmarking other gear sets that have similar performance requirements. This will be a main focus point of next semester’s work. Our initial estimation of weight reduction is approximately 20%, when compared to similar modifications done by past Baja teams.

Figure 5. The Honda F-2 motor connected to a gear set final drive.

Picture courtesy of Kathy Allman

Oil LeaksWe have also devised two methods of eliminating final drive oil leaks. The first

is to fabricate a simple breather on the top of the final drive housing to prevent a pressure from build up as the internal parts and lubrication heat up. Secondly, we have determined how the oil leaks pass the oil seal on the 2003 team final drive. Oil was able to completely by-pass the seal and leak through the final drive coupler. The coupler essentially is a sleeve with splines on one half of its outer edge. It mates with the primary gear of the final drive. The other half of the sleeve’s outer edge has a smooth cylindrical surface finish protruding though final drive housing. This smooth surface is where the rubber seal interfaces. The sleeves inter edge mates with the splines of the engine’s output shaft. Through this inter edge of the coupler is precisely were the oil bypasses the oil seal and leaks.

The solution is to plug the inter edge of the coupler with an epoxy. With the addition of the breather, this modification will prevent the oil from being pumped through the coupler, and leaking on to the ground.

ConclusionAfter implementing this design the U of U’s FSAE team stands an excellent

chance of making a good showing at this year’s competition. By using the lessons learned from past FSAE teams, we have been able to improve upon their designs. The fuel delivery system, and final drive will have significantly performance modifications, that should make this year’s power train the best a University of Utah FSAE team has every been able to produce.

References

Design of Machinery Robert L. Norton 3rd edition 2004

Honda Service Manual CBR600F3 Honda Motor Co.,LTD December 1994

http://www.motorbikes.be/en/Honda/1994/CBR%20600%20F(2)R/

http://www.motorbikes.be/en/Honda/2005/CBR%20600%20F(4)i/

http://www.motorbikes.be/en/Suzuki/1993/RF%20600%20R/

http://www.motec.com/

http://www.motorbikes.be/en/Kawasaki/1994/ZZR%20600/