Evaluating the Benefits of On-Board Measurement of Ambient Humidity Part-1: Effect on Spark Timing and Combustion Efficiency 2016-01-1067

Engine Mapping is usually performed under nominal conditions which include a humidity level of 8 g/Kg. Customers driving at different conditions (which may range from 1 g/Kg in colder and dry climates and up to 35 g/Kg as in tropical climates) may experience less-than-optimal engine combustion which results in reduced onroad fuel economy. Humidity has an EGR-equivalent effect, and measuring it will correct the spark timing, mainly at Maximum Brake Torque (MBT) and borderline conditions, and claim back some of those losses.

This paper aims at quantifying the small fuel economy benefits associated with on-board humidity measurement for certain customer use cases at high humidity conditions. Dyno data was collected for a Ford 2.3L GTDI engine at three speed load points, and intake air humidity was varied between 20% and 80% relative humidity. The effect of humidity compensation on spark timing, combustion phasing, knock, and consequently on overall engine efficiency was analyzed. Moreover, the interaction between knock sensor correction, inferred octane adaptation strategy, and humidity sensor feedback was assessed to evaluate the on-road benefit of humidity in real world conditions. The results show small benefit (less than 0.3%) at low load conditions when the spark timing is governed by MBT spark. At higher loads, the benefit can go up to 7%, however this benefit can be attained using a knock sensor feeding an adaptive knock strategy, which can account for the octane effect of ambient humidity.