The diesel engine is still one of the most common and most efficient mobile energy converters. Nevertheless, it is troubled by many problems, one of them being nozzle coking. This is not a new problem; however, due to the introduction of more advanced injection systems and a more diverse fuel matrix, including biofuels, the problem has become more complex. The nozzle holes are also much narrower today than when the problem first appeared and are therefore more sensitive to coking.
Two CEC sanctioned coking tests exist for diesel engines, but no universally accepted test for heavy duty engines. In this paper, tests have been performed with B10 doped with 1 ppm zinc on a single cylinder engine, based on a heavy duty engine, with the purpose to develop a simple accelerated coking test. To have relevance to real usage, the test was based on real engine load points from a high power Euro V engine calibration. The coking propensity was studied in an engine speed sweep at max load. Based on this, a repeatable, convenient, single load point, 6 hour test with a one hour soak time in the middle, that managed to produce significant coking, was established. The average nozzle temperature was measured to around 255 °C with a thermocouple instrumented injector. Coking was evaluated based on the measured power loss during the tests and validated in a flow rig were the nozzle was disassembled from the injector and the flow was measured separately before and after the coking tests to isolate the effects of nozzle coking.
Since the start of the tests are is of major importance, to condition the engine is very important. Running the engine on half load was concluded not to have significant effect on nozzle coking. It was also found that overnight engine soak lead to on average an increase in power output of around 1.2 % and that shorter soak periods did not significantly influence the deposit build up.