A new high-bandwidth transient dynamometer has been designed and is being built for application to a single-cylinder research engine. The goal of this system is to accurately reproduce all of the dynamic, instantaneous torques that this single-cylinder would experience if it were in a multi-cylinder engine. This is accomplished by calculating these instantaneous torques in real time using a detailed dynamic model of the engine, and applying them by means of a unique high-bandwidth transient dynamometer system. This hardware-in-the-loop system would both extend the operational envelope far beyond what is possible with current hardware, and would now allow the single-cylinder engine to replicate the rapid transients that are experienced in multi-cylinder operation. This may well open up many new areas of research in the transient operational regime, while maintaining the open accessibility and attractive attributes of the single-cylinder engine.