High fuel efficiency, low ownership/ maintenance cost and favorable driving climate are the major reasons for the increasing demand for low-power commuter motorcycles and scooters, particularly in developing countries like India, Brazil and China. Noise Vibration and Harshness (NVH) has now become a new subject for the battle between competing manufacturers in attracting customers. Valvetrain noise is quite significant in the engines of these cost gasoline vehicles as they don't incorporate a Hydraulic Lash Adjuster (HLA) to keep the manufacturing costs less.
The aim of this study was to understand how the cam ramp velocity and height affects the noise generated by the engine and what effect they have on its performance.For this study, a small scooter gasoline engine with an Over Head Camshaft (OHC) and a rocker arrangement with a roller-follower was considered. A commercially available numerical code was used to simulate the kinematic and dynamic behaviour of the valvetrain system. Uniaxial accelerometers were mounted on the head of the engine cylinder to measure the vibration due to tappet impact on the valve stem.
For the base engine, clearly audible tappet noise was observed and the same was captured through the accelerometer as high amplitude vibrations. To reduce the tappet noise without increasing the torque requirement of the camshaft, two sets of profiles Proposal 1(P1) and Proposal 2 (P2) were designed. Only the ramp regions were changed keeping the 1mm valvelift timing the same as that of the base cam. These proposals were tested on the engine at two conditions (i.e.) cold idle & hot idle. The two proposals provided good reduction in tappet noise compared to the base design. But on the downside the power and the torque output of the engine was reduced, which in-turn would result in poor acceleration and drivability of the vehicle. Therefore, eventhough these proposals helped reducing the tappet noise they were decided not to be used as they degraded the performance of the vehicle.
Based on understanding gained from the previous two profiles, profile Proposal 3 (P3) was designed which not only helped in eliminating audible tappet noise but also achieved performance targets set by the base cam.
A methodology in which simple sensors such as uniaxial accelerometers can be effectively used to eliminate tappet noise is explained. This paper would help the reader to have a holistic understanding of how the ramps of cam alters not only the NVH of engine, but also how significant it is deciding its performance characteristics.