Heavy-duty natural gas engines available today are typically derived from diesel engines. The biggest discrepancy in thermal efficiency between a natural gas engine and its diesel counterpart comes at low loads. This is particularly true for a lean-burn throttle-controlled refuse hauler. Field data shows that a refuse hauler operates at low speeds for the majority of the time, averaging between 3 to 7 miles per hour. As a result, many developers focus primarily on the improvement of thermal efficiency at light loads and low speeds. One way to improve efficiency at light loads is through the use of a late intake valve closing (IVC) technique. With the increase in electronic and hydraulic control technologies, the potential benefits of late IVC with unthrottled control are realizable in production engines. At the present, it is still not practical to use complete unthrottled control with late IVC for a lean-burn natural gas engine over the entire speed and load range due to complexities in the valve actuation system and the control algorithm.The objective of this paper is to report how an engine cycle simulation program was used to predict the obtainable benefits of using a simple late IVC system in conjunction with throttled control. It was found through modeling that late IVC with throttled control could improve engine thermal efficiency up to 15 percent at 15 percent loads. The relation between the intake valve timing and the throttle position is not a simple linear relation. This kind of behavior suggests that it would be expensive and time intensive to optimize the relationship between the intake valve timing and the throttle position using actual engine testing methods. Rather, it would be more efficient and cost-effective to make full use of cycle simulation tools to determine the best late intake valve timing and the appropriate throttle position before any engine tests are conducted.