The present paper describes the conceptual design, simulation, and fabrication of a magnetorheological (MR) fluid variable-valve actuation (WA) system. Two set-ups, one for testing the conceptual design and the other for application of the MR valve on a real engine, have been developed. The test set-up measures the displacement of the valve, which operates by the cam-follower mechanism. The results of valve lifts at different speeds and control currents are presented. To implement the MR fluid VVA system, a four-stroke single-cylinder Kirloskar TV1 vertical diesel engine has been selected. Inlet and exhaust manifolds have been modelled to estimate volumetric efficiency. The equations governing the unsteady air flow in the ducts have been solved using the method of characteristics. A numerical code to simulate the entire gas exchange process has been developed, and the results have been compared with the experimental results. Further, this code has been used to identify the optimal valve lift requirements of the engine at various crankshaft speeds. The quantified requirements obtained have been used for the design of an MR fluid VVA system. A prototype MR VVA system has been developed. Experimental results are reported in the form of maximum valve lift variation with speed and d.c. current. fluid (MRF-336 AG) with an operating range of −40°C to 150°C has been used. India. email: hirani@iitb.ac.in JAUTO408