Although e-mobility is on the rise, modern combustion engines arguably will be around us for the upcoming decades and need to be optimized in terms of efficiency and in particular to stay within the thresholds for particle emissions in exhaust gases imposed by law. One promising approach to tackle these challenges is the increase of injection pressure for gasoline direct injection (GDI) engines. Higher pressure during the injection leads -besides other effects- to smaller fuel droplets in the combustion chamber and ultimately to a more complete combustion of the gasoline. The results are reductions of soot and other particles in the exhaust gas by up to 96%.
However, due to the tribological processes in gasoline lubricated contacts under high local stresses in today’s fuel pumps, an increase of injection pressure provided by these pumps is not possible without fundamental change of used components. Materials that seem suitable in respect to the challenges that come with an increase in pressure, i.e. reduced clearance between piston/liner in the pump and the requirement of excellent tribological properties, are e.g. silicon-based ceramics.
Against this background, model experiments were conducted under reciprocating sliding conditions to investigate the potential use of ceramic materials for media lubricated high-pressure gasoline injection pumps in particular. Tests have been done with two commercially available engineering ceramics, i.e. SSiC and Si3N4. The self-mated tribological experiments were run on a pellet/plate setup, forming a flat-on-flat contact immersed in an isooctane bath under various surrounding atmospheres. With the used test rig, a stroke length of 5 mm in combination with a normal force of 200 N and frequencies between 2.5 and 20 Hz were feasible. Experimental results show a pronounced dependence of friction and wear on the prevalent composition of surrounding atmosphere, especially humidity can alter the tribo-chemical reactions taking place in the contact significantly. Under different conditions, very low coefficients of friction (µ < 0.003) at simultaneous ultra-mild wear rates could be observed for silicon carbide.
Self-paired silicon nitride samples obtained very stable levels of friction (µ ≈ 0.25) which were almost independent of chosen stroke frequency and surrounding atmosphere.