Detailed and highly time resolved experimental measurements were used to characterize the effects of transient operation on the performance of a light duty diesel engine, and to identify the physical processes responsible for transient-specific combustion behavior. The engine response to transient events varied with the size and type of transition and the combustion strategy used, but the underlying processes were similar in all cases. Differences in the response rate of the fuel and air systems caused large variations in the equivalence ratio of the combustion charge during transient events. For moderate to low load conditions, this was primarily due to the discrepancy between the instantaneous intake air flow rate and the composition of the intake charge caused by storage of exhaust gas in the EGR system. This effect was particularly significant for early injection LTC operation due to higher EGR rates and greater dependence of combustion phasing on intake charge composition.
Individual combustion cycles during transient events were compared to steady state operation at the same speed and load to quantify the differences in physical conditions. The greatest effect on combustion and emissions was due to differences in intake charge composition, which varied significantly between transient and steady state operation. The response time of the common rail pressure also contributed to transient behavior in situations where the target pressure varied with changes in speed or load. During larger load transitions, thermal inertia of the engine system had a significant effect on emissions, particularly UHC, but did not influence the combustion phasing or heat release rate.
The characteristic rates of change of the charge gas, fluid, and physical component temperatures in response to speed or load transitions were much slower than those of other variables such as pressures or flow rates, and were consistent with concurrent variations in engine-out emissions levels. Numerous mechanisms by which thermal inertia could affect emissions formation were identified, including variation of the intake manifold charge gas temperature, in-cylinder heat transfer, and changing physical properties of the fuel.