The Diesel engine has now become a vital component of the transport sector, in view of its performance in terms of efficiency and therefore CO2 emissions some 25 % less than a traditional gasoline engine, its main competitor.However, the introduction of more and more stringent regulations on engine emissions (NOx, PM) requires complex after-treatment systems and combustion strategies to decrease pollutant emissions (regeneration strategies, injection strategies, …) with some penalty in fuel consumption. It becomes necessary to find new ways to improve the Diesel efficiency in order to maintain its inherent advantage.In the present work, we are looking for strategies and technologies to reduce Diesel engine fuel consumption. Based on the observation that large Diesel engines have a better efficiency than the smaller ones, a detailed thermodynamic combustion analysis of one Heavy Duty (HD) engine and two Passenger car (PC) engines is performed to understand these differences.A thermodynamic split of losses method is developed and used to compare these three combustion systems in order to find ways of improvement. Despite the different size of HD and PC engines, results are obtained on similar operating points. A detailed analysis of the differences between the HD and PC engine is then performed.Finally, the use of the 0D modelling platform AMESim to simulate PC engine operating points with unconventional operating parameters borrowed to HD (Burnt Mass Fraction, AFR, …) is relevant to quantify and discuss efficiency gains potentially achievable by PC.