A map axis is an ordered list of breakpoints — discrete input values at which the map value is directly defined. Between breakpoints, the ECU interpolates. A 2D map (table) has an X axis (e.g. RPM) and a Y axis (e.g. injection quantity or load). The actual operating point is almost always between two breakpoints.
Typical axis units on diesel ECUs: RPM in revolutions per minute (often as integer × factor 0.25 or 0.5), load in mg/stroke (injection quantity per working cycle), pressure in mbar or kPa, temperature in °C or Kelvin (with offset), lambda as a dimensionless factor.
By far the most common interpolation method in automotive ECUs is bilinear interpolation: the ECU calculates the output value as a weighted average of the four surrounding breakpoints. The weighting depends on how close the current operating point is to each breakpoint.
This sounds simple — but has practical consequences: between two widely spaced breakpoints the response is perfectly linear. If a map should represent a non-linear response in a region (e.g. a boost pressure peak at a specific RPM), more breakpoints are needed there. Not understanding this is why a map edit only affects the area immediately around the edited breakpoint.
Axis resolution defines how precisely a map can respond to changes in the operating point. A RPM map with only 8 breakpoints (e.g. 800, 1200, 1600, 2000, 2500, 3000, 3500, 4000 rpm) can only interpolate linearly between 1200 and 1600 rpm — any detail that should be represented there is lost.
Manufacturers deliberately design the breakpoint distribution: more breakpoints where engine behaviour is non-linear (lower RPM band, full-load transition) and fewer where it is sufficiently linear (mid partial load). When remapping it is important to know this resolution before adjusting a value — a value that lies "between" two breakpoints can only be influenced indirectly.
ECUs almost never calculate in physical units internally. Instead, scaled integer values are used: an RPM axis value of 0x1A00 might correspond to 3200 rpm if the scaling factor is 0.5 rpm/LSB. Understanding this scaling is essential — both when editing hex directly and when interpreting A2L/DAMOS description files.
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