Patent ID: 8849592

Claim:
A fuel-injection condition detector applied to a fuel injection system in which a fuel injector injects a fuel accumulated in an accumulator, the fuel-injection condition detector comprising: a fuel pressure sensor detecting a fuel pressure in a fuel supply passage from the accumulator to an injection port of the fuel injector, the fuel pressure sensor generating a pressure waveform comprising a plurality of fuel pressure values; an approximate means for approximating a descent pressure waveform or an ascent pressure waveform to a straight line, the descent pressure waveform and the ascent pressure waveform consisting of fuel pressure actual values being a part of a pressure waveform detected by the fuel pressure sensor, the descent pressure waveform representing a decreasing fuel pressure due to an opening of the injection port, the ascent pressure waveform representing an increasing fuel pressure due to a closing of the injection port; and an injection condition estimating means for estimating a fuel-injection condition based on the straight line approximated by the approximate means, wherein the approximate means includes: a first approximate means for approximating fuel pressure actual values representing the descent pressure waveform or the ascent pressure waveform into a least-squares approximate line by least-squares method; a weighting means for applying a weight to the fuel pressure actual values, the weight being a coefficient in proportion to a difference of the fuel pressure actual value and the fuel pressure value on the least-square approximate line of a same time on the fuel pressure actual value detected, the weight being set greater as the difference is larger; and a second approximate means for approximating the fuel pressure estimate value having the weight into a least-squares approximate estimate line by the least-squares method.