Combustion Systems and Fuels Used in Engines—A Short Review

In the coming years, strong measures are planned to reduce emissions from various transportation modes. CO₂ emissions per km are expected to reach 0 g by 2035 [1]. In the short term, i.e., by 2030, the reduction in CO₂ emissions from passenger vehicles is expected to reach 55% (43 g·km⁻¹). In this case, the vehicle should be equipped with EV propulsion or have ICE supplemented with 50% PHEV or FCV propulsion. During homologation, vehicles are subjected to laboratory (NEDC, WLTC) [2,3] and road (RDE) tests [4]. The imminent introduction of the Euro VII requirements in Europe [5] will pose new challenges for vehicle propulsion sources. It is clear that in the course of daily vehicle operation, emissions can be reduced through eco-driving [6]. Current restrictions focus on passenger vehicles and commercial vehicles. Despite the non-optimistic scenario regarding the possibility of the further use of ICE for various means of transportation, there is still hope for their use in heavy and long-distance transportation or work machinery [7].

Adapting the ICE to the subsequent emission requirements entails the modification of the combustion mixture formation process, port/direct injection [8], combined combustion with main and pre-chambers [9,10,11,12], downsizing [13], the retrofitting of after-treatment systems with EC, DOC, FAP, DPF, GPF and SCR [14,15,16,17] and many other design and functional features, such as injectors [18,19]. Modifications to the organization of the combustion process are especially significant. Here, in addition to the standard methods, one can distinguish ATAC [20], CAI/HCCI [21,22], HPDI or RCCI [23].

Reductions in CO₂ emissions can be achieved by using fuels with a lower carbon content than standard fuels [24]. The most commonly used alternative fuels in ICE are LPG [25,26,27,28], CNG [29,30,31] and LNG [26,32,33]. There is also growing interest in H2 [34,35,36]. HVO is being introduced with positive results [37,38], and many lead modern engines are already prepared to run on this fuel. Research is also being conducted on the use of PVO, especially in agricultural machinery [39]. By using additives with PVO, it is possible to obtain FAME [40], which is more resistant to freezing. In agricultural applications and stationary power generators, biogas has a positive effect [41]. Current trends in research on the use of alternative fuels in transportation focus on fuels that are produced not only directly from plants but also from waste. These are mainly syngas [34], GTL [42], biomass-gasifier [43], POMDME [44] and TPO [45] or NH₃ [46]. OME e-fuels [47] and fuel cells [48] are considered the greenest. In parallel with experimental studies, computer simulations are being carried out that account for the formation of the combustible mixture and the combustion process itself with regard to alternative fuels. In this case, it is necessary to determine the characteristic parameters and physical and chemical properties of the fuels or their mixtures [49]. In cognitive terms and as an application in modeling the movement of a vehicle, it is very important to know the full load engine characteristic powered by any fuel. The determination of coefficients describing such characteristics enables their reproduction under modeling conditions [50]. The procedures described in [51] can be used to determine the full load engine characteristics.

Alternative fuels, like any fuels ultimately intended for use in transportation, must meet the requirements outlined in the CAFÉ General Regulations, AMFA [52]. Any action in this regard will, in any case, be determined by CO₂ emissions (CARB-CAR) [53].