Using Engine as Torsional Shaker for Vehicle Sensitivity Refinement at Idle Conditions

Vehicle idle quality has become an increasing quality concern for automobile manufacturers because of its impact on customer satisfaction. There are two factors that critical to vehicle idle quality, the engine excitation force and vehicle sensitivity (transfer function). To better understand the contribution to the idle quality from these two factors and carry out well-planned improvement measures, a quick and easy way to measure vehicle sensitivity at idle conditions is desired.

There are several different ways to get vehicle sensitivity at idle conditions. A typical way is to use CAE. One of the biggest advantages using CAE is that it can separate vehicle sensitivities to different forcing inputs. As always, the CAE results need to be validated before being fully utilized. Another way to get vehicle sensitivity is through impact test using impact hammer or shaker. However this method doesn't include the mount preload due to engine firing torque [3, 4, & 5].

In this paper the author has proposed an alternative method to measure the vehicle sensitivity at idle conditions by using engine as a torsional shaker through disabling one of the cylinders. Since there is consistently one cylinder not firing, the engine will produce higher 0.5^th order combustion torque (also higher 1.0^th and 1.5^th order due to harmonicas) than the normal operating engine. These higher than normal ½ order combustion torques will excite the P/T rigid body modes and other vehicle modes that are critical to vehicle idle quality. By sweeping the engine speed in the rpm range allowed, the engine effectively works as a torsional shaker with sinusoid input. The corresponding vehicle sensitivity to engine combustion input (roll input) can be generated by normalizing the vibrations measured at seat track and steering wheel with the engine combustion torques (measured directly or indirectly through flywheel acceleration) at given order and given speed. One of advantages of this method is that it includes the engine mount preload in the measurement.