Method of efficiently determining pressure-based combustion parameters for an IC engine
View Patent ↗Pressure-based combustion parameters of an IC engine are determined based on engine cylinder pressure measurements and crank angle dependent cylinder volume data, where the pressure in a given engine cylinder is periodically sampled at scheduled crank angles to provide a measurement resolution that varies over the combustion cycle for that cylinder, and the cylinder volume data is retrieved from a table of pre-calculated data. The combustion parameter calculations are customized to suit the variable resolution pressure data, and the crank angles used to retrieve the pre-calculated cylinder volume data are offset from the scheduled crank angles to compensate for misalignment of the pressure measurements relative to the scheduled engine crank angles.
1. A method of determining a pressure-based combustion parameter for a cylinder of an IC engine, comprising the steps of:
pre-calculating engine crank angle dependent volume data for said cylinder and storing said volume data as a function of engine crank angle for later retrieval;
measuring pressure in said cylinder at scheduled engine crank angles during a combustion cycle of said engine;
retrieving stored volume data for said cylinder based on said scheduled engine crank angles;
calculating the pressure-based combustion parameter using the measured pressures and the retrieved cylinder volume data; and
calibrating the scheduled engine crank angles such that a resolution of the pressure measurements has relatively low resolution of pressure measurement during intake and exhaust strokes of the combustion cycle, and relatively high resolution of pressure measurement during the compression and power strokes of the combustion cycle.
2. The method of claim 1 , where the pre-calculated engine crank angle dependent volume data includes a volume of said cylinder and a rate of change in volume of said cylinder.
3. The method of claim 1 , including the steps of:
determining an offset corresponding to a misalignment of the pressure measurements relative to respective scheduled engine crank angles; and
retrieving the stored volume data for said cylinder based on said scheduled engine crank angles, as modified by the determined offset.
4. The method of claim 3 , where the engine has multiple cylinders, and offsets are individually determined for each cylinder.
5. The method of claim 3 , where said pressure-based combustion parameter is a mean effective pressure in said cylinder, calculated according to:
1
V
d
∑
P
i
[
ⅆ
V
ⅆ
θ
]
j
spacing
j
where θ i is a scheduled crank angle, V d is a displaced volume of said cylinder, P i is a cylinder pressure measured at scheduled crank angle θ i ,
⌊
ⅆ
V
ⅆ
θ
⌋
j
is the retrieved cylinder volume data for a crank angle (θ i +offset), and spacing j is a crank angle separation between successive cylinder pressure measurements.
6. The method of claim 3 , where said pressure-based combustion parameter is a net heat release energy in said cylinder, calculated according to:
Q
net
=
∑
⌊
[
γ
γ
-
1
]
P
i
[
ⅆ
V
ⅆ
θ
]
j
spacing
j
+
[
1
γ
-
1
]
V
j
P
i
+
1
-
P
i
-
1
2
⌋
where γ is a ratio of specific heats c p and c v , θ i is a scheduled crank angle, P i is a cylinder pressure measured at scheduled crank angle θ i ,
⌊
ⅆ
V
ⅆ
θ
⌋
j
and V j are the retrieved cylinder volume data for a crank angle (θ i +offset), spacing j is a crank angle separation between successive cylinder pressure measurements, and [(P i+1 −P i−1 )/2] is an average change in measured cylinder pressure over an interval centered about the scheduled crank angle θ i .