Despite the fact that more than a hundred years have passed since the first design of the compression ignition (CI) engine appeared, its optimal design has not yet been achieved. It is still the subject of constant modernisation in order to meet the new expectations of users in terms of its dynamics, economy, and, in recent months, also ecology. The most effective fulfillment of these requirements is achieved through new solutions of the fuel supply system and electronic control of injection and combustion processes. The publication includes the test results obtained on the basis of two engines. One of them is the single-cylinder CI engine AVL5402, and the other one – a threecylinder CI engine AD3.152. The first one is equipped with the Common Rail fuel supply system, electronically controlled with the selenoid injector. The second engine has a CAV distributor fuel injection pump and traditional, mechanical controlled injectors. The paper demonstrates how these two different structural systems for supplying and controlling engine parameters affect the selected indicators of the injection and combustion process. The influence of diesel fuel (DF) and rapeseed oil (RO) feeding the engine in both different injection and control systems on the unrepeatibility of the injection pressure on the maximum combustion pressures in the engine cylinder and, consequently, non-uniformity of the crankshaft rotational movement of the engine were also pointed out. The continuation of the research in this area seems to be expedient. They can be supplemented with statistical models of these phenomena. The results obtained in this way could be helpful in optimising the design of power supply systems and engine combustion chambers.