Archive for category c. Valve change considerations
Often times forced induction is used in internal combustion motors to increase performance by artificially increasing compression ratios. Superchargers; either crank driven or exhaust driven turbochargers; feed the motor positive air pressure, drastically increasing volumetric efficiency. Unfortunately, superchargers are limited in that they add a parasitic load to the motor, requiring substantial amounts of energy to be used in their operation. Also, their size and weight makes them difficult to use in applications where packaging and total weight are important concerns, such as on a motorcycle.
Certain valve considerations need to be addressed due to their relevance to this project. Modern 4 stroke motors often regulate their air intake and exhaust through the use of poppet valves. Figure B3 shows one of these valves.
Figure B3: Poppet valve
This valve can be considered an active valve, as it requires an external actuator for control. Camshafts are used to open these valves by depressing them. They offer high levels of control, however, due to force being required operate them, can rob up to 10% of the produced power of the engine.
Common in two stroke motors are reed valves. These reed valves, as seen in figure B4, are passive valves, self-actuating.
Figure B4: Reed valve
This one way valve is actuated via air pressure differential, allowing flow in the direction of the arrows, only when pressure is greater before the valve then after. Reed valves often see use in 2 stroke motors where intake and exhaust occur with every stroke of the motor. When extra control is not needed, they make for a far more simple option than the entire valve train that’s required with the use of poppet valves.
For high performance motorcycle applications, where packaging needs prevent the use of external forced induction systems, but an increase in performance is desired, an internal supercharger is proposed. This system would convert one of two cylinders from a normal four stroke Otto cycle to an air pump. For future application, this air pump would feed pressurized air to the adjoining cylinder. To operate, valving changes must be made to the compressor cylinder, allowing intake to occur every rotation of the crankshaft. Based on earlier stated equations, mass flow rate through this cylinder will double. As displacement of the motor for compression is now reduced by ½, mass flow rate returns to the original value, however, thermal efficiency increases. As such, overall performance can increase, without a significant increase in weight or size by keeping everything internal to the motor, utilizing already present components.
Three prominent possibilities were evaluated for conversion of the valve train for this application. The motor used for conversion, as seen in figure E1, is a 487cc parallel twin from a 1996 Suzuki GS500E motorcycle.
Figure E1: 487cc parallel twin engine
First, custom camshafts were considered. Figure E2 shows the stock camshafts.
Figure E2: Stock GS500E camshafts
The proposed design was to create new camshafts where the lobes were modified in such a way to allow opening of the intake and exhaust valves twice as often. This option would be the simplest and most efficient, as cam timing could be perfected for the application, however, due to the incredibly high strength of steel needed for camshafts, fabrication requires specialized equipment and high costs. Quotes received from various camshaft fabricators were nearly $2000 for the pair.
Second, a replacement head was considered. A replacement head, on a split cylinder (v twin or opposed twin motor configuration) would be a simple solution, allowing for increases in efficiency, compression parameters, and simple fabrication. However, due to the availability presented, a parallel twin configuration was chosen. A new head design would require a far more complicated design to ensure normal operation of the unmodified cylinder, making costs much higher and fabrication vastly more difficult.
The third, and selected, solution was to remove the stock valves all together and replace with reed valves mounted externally to the motor. While not offering the same level of performance and ease of packaging as the previous two options, externally mounting reed valves is a lower cost alternative utilizing passive valves that come at a much lower cost. Off the shelf valves are utilized, residing in machined reed cages attached to the intake and exhaust ports.