October 2021

28 AFTERMARKET OCTOBER 2021 TECHNICAL www.aftermarketonline.net the combustion process forms an expansion of gasses within the cylinder. Remember, both gasoline and diesel are heat engines. This expansion is converted into kinetic energy, a mass energy transfer driving the piston down the cylinder. This energy is not uniform with crankshaft rotation, therefore producing vibration. The vibration created is unique to engine cylinder layout, i.e. the angle of cylinder displacement, no of cylinders and firing order, under or oversize displacement, not to mention the many new tech systems, dual mass flywheel, active vibration control and cylinder select. Each engine variant will produce a unique combustion vibration profile if combustion across all cylinders is both smooth and uniform, and it should be. Then you will get a smooth and uniform mass energy transfer from each combustion event. The result of which will be an extremely low E1 crankshaft vibration profile, E1 represents crankshaft mass vibration. Rotation E2 represents combustion mass vibration; Reciprocation. A 3-directional accelerometer is used, blue equals fore and aft mass. red equals vertical mass, i.e., piston and con rod with combustion effects. Green equals lateral mass. Fig.5. is a Pico image from a Volkswagen Golf 1.4, with a normal NVH profile. Note the extremely low crankshaft mass displacement. Normal combustion shows high vertical mass displacement due to combustion red.Fig.6. is another Pico image, this time showing the Golf 1.4 with a dead cylinder? Note the high mass energy transfer to the crankshaft E1, due to rotation imbalance. Note the increase of mass transfer to the crank due to a dead cylinder. Fig.7. is another Pico image, this time showing a Audi S1 with a normal NVH profile, low E1 & normal E2 profile. Meanwhile, Fig.8. shows a Pico image of the Audi S1 with pre-ignition, normal E1, with erratic broad band ignition mass transfer E2 across all three channels. These were taken from the same vehicle, same drive cycle, with pinking on load; Note the erratic combustion mass. The NVH software is synchronised with the combustion orders set up in the initial engine configuration. My thoughts on this method focus on the simplicity of the setup which can monitor the most complex of engine layouts in real time when driven on the highway. Combustion anomalies often occur infrequently only when driven under load. The magnetic accelerometer was fitted to the driver’s seat frame. If you want to know more, NVH diagnostic principles will be my chosen subject at Autoinform Live at Wolverhampton from 6-7 November. Hope to see you there. www.aftermarketonline.net Fig.5 Fig.6 Fig.7 Fig.8 This energy is not uniform with crankshaft rotation... ”