REDESIGN OF THE INTAKE CAMS OF A FORMULA STUDENT RACING CAR

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1 FISITA2010-SC-P-24 REDESIGN OF THE INTAKE CAMS OF A FORMULA STUDENT RACING CAR Sándor, Vass Budapest University of Technology and Economics, Hungary KEYWORDS valvetrain, camshaft, cam, Formula Student, engine ABSTRACT - According to the FSAE and Formula Student rules, only a car fitted with an engine not more than 610cm 3 of displacement can take part in the contest and also the intake manifold should be fitted with a 20mm diameter restrictor. This restrictor changes the whole operation of the engine, spoiling the torque characteristics and the fuel consumption. That is why the working parameters must be changed, and the gas exchange process must be optimised. One of the best and most effective changes is redesigning the valve timing and displacement, adjusting it to the new circumstances. In this work, after building the model of the engine in a simulation programme it turned out, that the volumetric efficiency of the cylinders drops down to 45% because the original opening time of the intake valves is too wide. The valve opened too early, when the pressure was still high enough to make the exhaust gas flow into the intake runner, than it got again in the cylinder when the piston sucked it back. The valve closed too late when the piston was far in compression phase and the piston pushed the intake gas out of the cylinder. This caused torque and power drop on a wide revolution range and could not be solved with optimised intake runner length. That is why a new cam profile was created based on the engine simulations, which had a shorter open interval and safe enough to work in the engine without making damages in the cylinder head. The simulations showed, that the new cam profile improved not only torque in the mid rpm range, but highered the maximum power too. This was caused by decreased loss of gasexchange and the higher volumetric efficiency. 1. Introduction According to the competition rules mentioned above in the abstract, Formula Student teams have to use a 20mm diameter restrictor in the intake system, just after the throttle valve. The purpose of this restrictor is to keep the power lower in race cars, providing more safety on the racetracks. In spite of this, with major changes in the engine, it is possible to obtain nearly as much torque and power as the original. One of the best way to adjust the operation of the engine to new circumstances is to change the valvetrain system. Valve opening and closing times have one of the greatest effect on the gas exchange process, this way on the torque characteristics. In order to examine this closer we have to build a simulation model based on the properties of the engine. Our engine comes from a 2005 Yamaha R6 motorbike. It is a four stroke, four cylinder in line engine and has a 599cm 3 total displacement. It has four valves per cylinder and double overhead camshafts (DOHC) actuating valves directly. These camshafts are driven by chain just as the oil pump. The fuel injection system is sequential, we applied programmable engine control electronics to adjust the ignition and injection to the new circumstances.

2 2. Original conditions and characteristics We used a program named GT Power from GT Suite 6.1 to simulate the operation of the engine. After building up the model, an intake and exhaust runner length tuning was made last year, and examining the results it was clear that there remained much reserve in the engine. The first diagram shows the brake torque with the original valve lift diagram. Diagram 1: original brake torque As it can be clearly seen in the diagram, there is a big hole at 5000 RPM and the maximum stands only 2000 RPM higher at 7000 RPM, which makes the car hard to drive. The aim was to make ideal conditions from 6000 to RPM where engine operates the most, but this caused that if the pilot changed gear too early, he got straight into the torque minimum loosing precious seconds on the racetrack. Diagram 2: volumetric efficiency of the engine

3 The volumetric efficiency curve follows the torque of course. We can see that it drops down below 45% around 5000 RPM and climbs higher than 105% around 8000 RPM. The good gas exchange process between 6500 and RPM informs about the proper intake and exhaust lengths, but please let me present what is happening behind it. The third diagram shows us the mass flow rate of an intake valve at RPM. Diagram 3: mass flow rate of the intake valve at RPM This is the place where the engine gave its max power, and we can still see backflows at the beginning and the end of the valve lift. Now let me show the mass flow rate at 5000 RPM. Diagram 4: mass flow rate at 5000 RPM

4 At this engine speed the backflows are so high that nearly half of the gas aspirated flows back to the intake runner. This is the cause of the torque drop at this speed. This makes clear that the valve opening time is too wide. This wide opening time ensured good gas exchange process providing high power at high engine speeds with the original intake system. Air flow with large mass flow rate has a larger inertia, which helps charging the combustion chamber, because air is still streaming in, when the piston is already in compression phase and moving up. But as the air has a pressure loss flowing through the restrictor, it cuts down the mass flow rate, so the replenishing effect is weaker. Because of the restrictor and the wide valve opening time the intake valve opens when the pressure is above 1 bar in the combustion chamber, so that exhaust gas is flowing in the intake system. Then the piston sucks it back in the intake phase and when the valve is closing deep in the compression phase the piston pushes the fresh air-fuel mixture back, decreasing the volumetric efficiency even more. This small volumetric efficiency - combined with much remaining exhaust gas - causes that big hole in the torque-speed curve. As the air-fuel mixture flows back at every engine speed, it becomes quite obvious that those negative areas in the mass flow rate diagrams have to be cut down. This means a narrower opening time. 3. Cam shapes, valve lifts When it was clear that a new valve lift curve had to be created I had to start with measuring the original attributes of the valvetrain system. First of all, the valve lift curve had to be measured, than the valve spring stiffness of both of the intake springs. Apart from these, I had to measure the mass of every part very carefully, as well as diameters and lengths to build up the simulation model. The spring stiffnesses were very important, because these have a major influence on the dynamics and the Hertz contact pressure of the cam. It was decided that a new cam profile would be the safest and cheapest solution, and it could be achieved by regrinding the original cam profile. Before starting the design, the following aims were set out: Cease the torque hole around 5000 RPM, or push it to lower engine speeds Keep the max torque around 7000 RPM, achieve at least 60Nm Avoid torque drop at high engine speeds, so increase power output Keep valve acceleration under factory values Keep Hertz contact pressure below 900Mpa to shirk chipping off of the cam surface After building up the model in GT Valvetrain, I have firstly examined the original factory results. Because of the inaccuracy of the measurement, the acceleration curve could not be used as a base for comparison with the new valve lift profile. Measurement would have to had been accurate to 0,001mm, which could not be achieved with our tools. The design of the new cam profile had begun with drawing the new valve lift curve. It was made with a spline drawn on definite points, which were defined by myself. The curve was described by the valve lift at every cam angle in one degree steps. After that I loaded the curve in the model and ran the simulation. If the accelerations were satisfactory, I loaded it in the engine simulation model and examined the changes. This way an iteration had began, I repeated this until a good compromise was achieved between the high power output and the larger torque in the lower RPM regions. In the next diagrams I present the valve lift, speed and accelerations.

5 Diagram 5: old and new valve lift and cam radius curves In the diagram 5 you can see the factory valve lift and cam radius curves compared to the new ones. Mind that the new lift is nearly 2 mm less then the original to ensure safety. Only this way the narrower opening time could be achieved. The ramps on both ends of the new curve are not as steep as the original. It was made in order to decrease the opening and closing speed of the valve, so that we can avoid damaging the cylinder head either the valve. Diagram 6: old and new kinematical valve acceleration at RPM In the diagram 6 you can see the second derivative of the valve lift curve. The original one is the red. As mentioned above, the inaccuracy of the measurement made it so billowy. In spite of this the tendency is clear: the new one follows the old, the peak values are less in the old one.

6 Diagram 7: old and new cam profile In the diagram 7 I intended to show that the new cam profile could be regrind from the factory cam shape. The base radius remained the same, so valve clearances will not be disadjusted. Diagram 8 and 9: dynamical valve lift and valve speed at RPM Diagrams 8 and 9 shows us, that the valve follows the lift curve and does not spring off, the valve speed does not exceed the factory values.

7 Diagram 10: dynamical valve acceleration at RPM The dynamical valve acceleration shows us some oscillation where the apex of the cam reaches the lifter. This can be caused by the stick-slip effect, because the oil film extenuates and the contact friction becomes larger. At lower engine speeds this phenomenon ceases. As these diagrams proved us, the valve lift curve meets the dynamical requirements, but we have to see how it works in the engine. 4. Engine simulation results Diagram 11: original and new brake torque

8 According to the torque characteristics, the minimum value increased by 20Nm due to the better volumetric efficiency. However the maximum went up to 8000 RPM and between 6000 and 7500RPM the values decreased. Diagram 12: volumetric efficiency The volumetric efficiency could not reach such a high value as the original curve had, but is more balanced and does not have a point below 0.7. The flow backs nearly disappeared. According to the facts listed above, the aims were achieved. 5. Manufacturing Lastly a few words about how the manufacturing went. Firstly the cam radius values were taken out of the software. These were computed from the valve lift curve for every cam angle. Than I constrained in a sketch every point given, with the help of a 3D drawing software. From this 3D model a CNC hobbing machine could make a templet. The templet was used up by a cam grinding machine which regrinded the factory cams to the new profile. Now the new camshaft has been installed in the engine, soon measurements are going to be taken to validate the engine model.