The radio-frequency (RF) or microwave-based catalyst state determination offers the opportunity to operate an automotive catalyst at its optimal point. This has already been proven for the oxidation state of TWCs, the soot loading state on DPFs/GPFs, and the ammonia storage state of vanadium and zeolite based SCR catalysts. However, the latter has only been demonstrated in laboratory scale with synthetic exhaust using gaseous ammonia.This work presents first results on an engine test bench with a serial-type zeolite-based SCR catalyst, using urea solution and the RF tool to detect the current ammonia loading in real time and to control directly the urea dosing system without any additional sensors. The original catalyst volume was reduced by 50 % to operate deliberately the SCR system under high space velocities and to challenge its function. Stationary conditions and operation points with continuously changing NOx emissions and space velocities were observed. In all tests, high NOx conversion was achieved and the ammonia loading could be detected reproducibly in all states by the RF tool. Furthermore, the RF tool was successfully used with closed-loop control of the urea dosing as a two-point control with and without hysteresis. By varying the controlled ammonia storage window, the operation borders of too high or too low ammonia storage were investigated, and the ideal storage value was found. The performed experiments demonstrate that direct operation on a specific ammonia loading can ensure both maximum NOx conversion and avoid ammonia slip, even with space velocities over 180,000 h-1. Therefore, the control strategy using the RF tool might allow a catalyst volume reduction.