The reduction of CO2 emissions in transport and power generation is a key challenge of the current generation. One particular opportunity of CO2 reduction is the introduction of fuels with a smaller CO2 footprint. The combustion characteristics of such fuels are different and require engine settings modification to profit most from these characteristics. The aim of this project is to develop a simulation platform for optimizing the overall engine unit (engine with exhaust gas aftertreatment) of vehicles of all sizes for fuels with different characteristics. Therefore, different diesel like fuels have been tested in a single cylinder research engine to determine their detailed behavior with respect to combustion and emission characteristics, including required particulate filter regeneration energy. The combustion and emission formation process has been modelled and included into a GT Power model of a 6-cylinder heavy-duty engine. The model includes an SCR (selective catalytic reduction) for NOx reduction and a DPF (particulate filter), which requires energy for regeneration depending on the soot oxidation activity of each fuel which was determined through detailed analysis within this project. This model platform enables the comparison of engine efficiency when operating the engine with different fuels, including e.g. benefits from a fuel with lower tendency to form soot. The fuels tested include Hydrotreated Vegetable Oil (HVO), Gas-to-Liquid fuel (GTL) and polyoximethylene dimethylether (OME3-6), which were tested neat and as blends with Diesel or among each other.
The main findings of this project are:
Different liquid diesel-like fuels which can be obtained by renewable sources show different combustion and emission characteristics.
- The fuels have different characteristics when compared to Diesel. For the paraffinic fuels, the ignitability is higher due to the high cetane number. In addition, soot formation is reduced due to absence of aromatic content, which leads to reduced energy consumption for the particulate filter regeneration. The oxygenated fuels inhibit soot formation even more effectively, and the air fuel mixing is faster, resulting in faster diffusion combustion.
The different fuel characteristics result in a potential to optimize diesel internal combustion engines including aftertreatment systems (Diesel particulate filter and NOx SCR catalyst) for operation with fuels of different composition. For the fuels investigated, the following maximum and minimum well-to-wheel CO2 reduction potential was found (well-to-tank in brackets):
- Diesel with 20% HVO: 19.2% / 17.7% (18%)
- HVO: 90.2% / 89.6% (90%)
- Diesel with 7% OME 6.1% / 3.8% (4%)
- 77% HVO with 18% OME and 5% stabilizer: 88.2% / 87.9% (88%)
The approx. 90% CO2 reduction found is dominated by the well-to-tank characteristics of the fuel itself and only small contribution originates from changes in engine operation. For fuels mixtures with small proportions of alternative fuels, the contribution to well-to-wheel improvements from the optimized engine operation is up to 35% of the total improvement observed. The optimization performed also compensates or even overcompensates for the smaller volumetric heating value of the alternative fuels, which otherwise would result in an engine efficiency decrease.
The final results show that engines with a DPF (i.e. used in applications where soot emissions are tightly regulated) can benefit from fuels which are less prone to form soot. After the optimization, in the case of HVO at low load, a higher EGR (exhaust gas recirculation) rate can be applied in comparison to Diesel. The higher EGR rate does not increase DPF regeneration events and results in lower engine-out (before SCR) NOx emissions without a significant penalty in fuel consumption. The reduction in raw NOx reduces urea consumption (more than 50% in this case). This behaviour is even more apparent when the size of the SCR is reduced. As a general rule, it was determined that low sooting fuels are most beneficial when employing small particulate filters (where regeneration frequency is high), an efficient EGR path and a small NOx aftertreatement device.The latter has been demonstrated by an SCR size reduction of 50% which was investigated during the project.
The engine optimization for specific fuel compositions can be conducted at the engine design phase using engine modelling tools which have been developed within this project. Swiss and international engine manufacturers can directly benefit from the project’s findings.