Aki Tilli

Late post-injection of biofuel blends in an optical diesel engine: Experimental and theoretical discussion on the inevitable wall-wetting effects on oil dilution

In a diesel engine, diesel particulate filter is used to reduce particle matter emissions. Diesel particulate filter requires periodic regenerations under high-temperature conditions in the exhaust pipe in order to oxidize the accumulated soot. A common strategy to produce high exhaust gas temperature is to inject late post-injections after the main injection. However, this practice may dilute the engine oil, causing engine wear. Biofuel addition to petroleum diesel may increase oil dilution even more. This is related to the fuel spray characteristics, the post-injection control and the vaporization process of fuel in engine oil. In this study, spray properties of late post-injection were studied with petroleum diesel and two types of transport biofuel blends containing 30% either fatty acid methyl ester or hydro-treated vegetable oil. Three different late post-injection timings were investigated. Image sequences of the main spray flame as well as the non-combusting late post-injection spray were extracted. In order to verify oil dilution during regeneration cycle and late post-injection, oil samples from six-cylinder test engine were analyzed. According to the present experiments, differences in the spray characteristics are not significant with the tested fuels. However, higher oil dilution rates were observed with fuel blend composed of 30% fatty acid methyl ester. All the studied late post-injection timings were noted to lead to the unwanted cylinder spray/wall interaction and wall-wetting consequently diluting the engine oil. The spray/wall interaction is thoroughly explained by introducing a theoretical/computational framework which characterizes any spray/wall interaction in terms of a phase diagram for any considered operation conditions. The novelty of this study arises from (1) first comparison of fatty acid methyl ester and hydro-treated vegetable oil blends in an optical engine, (2) strong evidence on the phenomena related to post-injection phase in six-cylinder and single-cylinder optical engine configurations and (3) the development of a single-droplet model showing inevitable wall-wetting.

Biofuel blend late post-injection effects on oil dilution and diesel oxidation catalyst performance:

In this article, the effects of different biofuel–diesel blends on engine oil dilution and diesel oxidation catalyst performance during late post-injections were investigated. The engine tests were made with an off-road diesel engine under low load conditions at 1200 r/min engine speed. During the experiments, oil samples were periodically taken from the engine oil and later analyzed. Emissions and temperatures before and after the diesel oxidation catalyst were also measured. The fuels studied were fossil EN590:2013 diesel fuel, 30 vol.% biodiesel (fatty acid methyl ester) and 30 vol.% hydrotreated vegetable oil, which is a paraffinic diesel fuel fulfilling the EN15940 specification. The novelty of the study is based on two parts. First, similar late post-injection tests were run with blends of both hydrotreated vegetable oil and fatty acid methyl ester, giving a rare comparison with the fuels. Second, oil dilution and the fuel exit rates during normal mode without the late post-injections were measured. The results showed the oil dilution and the diesel oxidation catalyst performance to be very similar with regular diesel and hydrotreated vegetable oil blend. With the fatty acid methyl ester blend, increased oil dilution, smaller temperature rise in the diesel oxidation catalyst and higher emissions were measured. This indicates that during diesel particulate filter regeneration by late post-injections, fatty acid methyl ester blends increase fuel consumption and require shorter oil change intervals, while hydrotreated vegetable oil blends require no parameter changes.