Heat transfer losses in the swirl chamber, throttling losses at the connecting passage and combustion delay in the main chamber are considered as the three factors influencing the thermal efficiency of IDI diesel engines. This paper suggests a thermodynamic model, in which three idealized diesel engines including no passage throttling engine, adiabatic diesel engine for swirl chamber and DI diesel engine are assumed, to isolate heat transfer losses, throttling losses and combustion delay in IDI diesel engines. The Second Law analysis is carried out by the thermodynamic state parameters calculated by the cycle simulation of engines based on the First Law. The effects of heat transfer losses in the swirl chamber, throttling losses at the connecting passage and combustion delay in the main chamber on the irreversibilities and availability losses during the engine cycle are analysed in detail. The relative influences among the three losses are also investigated.The results of First Law analysis indicate that heat transfer losses in the swirl chamber at low load conditions and combustion delay in the main chamber at full load conditions are the main factors impairing the fuel economy of IDI diesel engines. However, the results of further analysis of the Second Law indicate that passage throttling is a key factor affecting the fuel economy of IDI diesel engines at full load conditions.On the basis of thermodynamic analysis, a modified design of connecting passage is made on a single cylinder IDI diesel engine. The modified connecting passage has different inclination angles at both sides of the passage, and reduces throttling losses at the connecting passage, shortens combustion delay and combustion period in the main chamber, and hence reduces the engine fuel consumption and smoke emission.