The  objective of the paper is to improve the diesel engine operation by decreasing the brake energetic specific consumption, the cyclic variability and pollutant emissions. This goal can be achieved fuelling the engine with liquefied petroleum gas (LPG). A good correlation between the investigated regimens and the engine adjustments must be realised in order to limit the maximum pressure and smoke level, knock and rough engine functioning. A particular objective of this paper is to reduce the nitrogen oxides emission, which represents an important problem for a diesel engine. The correlation between the LPG quantity and the working regimen allows obtaining an improve in the functioning of the engine. In this way, by fuelling the engine with LPG is assured a low cyclic variability for relatively high substitute ratios. This paper also presents a nitrogen monoxide formation model for the case of LPG fuelling. To fuel the engine with LPG was utilised the diesel-gas method. This method can be also applied for engines which are already in service and consist in gaseous LPG injection into the intake manifold of the engine. Using the diesel-gas method, in the intake manifold of the engine is injected LPG in gaseous aggregation state and the air-LPG homogeneous mixture is ignited by the flame appeared in the diesel fuel sprays. The investigated regimen was 55% engine load and 3500 rpm.