In this paper we report the results of experiments done during the transient operation of a single cylinder Cummins NH engine. The data taken include cycle resolved pressure, combustion chamber surface temperatures and ignition delay. The data was taken during a special type of engine operation in which the engine was repeatedly hopped from one load to another. In this way cycle to cycle variations could be averaged out by ensemble averaging individual cycles after the step load change. For analysis of the heat transfer a unique finite difference temperature probe was developed to delineate the 3-D heat transfer effects in place of the standard 1-D assumptions and a new analysis technique was developed to calculate the instantaneous heat flux during the transient.Analysis of the data indicates that the combustion reaches an equivalent steady state condition within 2000 engine cycles after the load change. The exhaust port temperatures, and therefore the exhaust enthalpy of course take much longer to reach their respective steady state values. The heat release rate and peak rate of pressure rise quickly jump (within 4 cycles) to values close to the steady state values and reach their steady state value within 2000 cycles. For cycle simulations we show that the thermal response of the engine can be accurately modelled as an exponential response with two, a long and short, time constants.