The useful life of an engine oil may vary considerably; therefore, an
on-board sensor would be helpful in monitoring the oil's condition con
tinuously. In this work, in situ macro and micro oil-condition sensors
have been developed and tested in two sets of engine-oil samples. It
is found that after suppressing the high output current associated wit
h fresh engine oil, the microsensor's output correlates with the total
acid number (TAN) of engine oil. Because an oil's TAN will increase w
ith use, this microsensor has the potential to detect the end of usefu
l oil life and signal the need for an oil change.