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2.1 Speed Density
1. Engine speed (1000 RPM) and manifold pressure from the MAP sensor (38 kPa) are read by the ECU. From these
values, it obtains a number from the base fuel map (32).
2. The ECU then looks at the reading from the Air Temperature Sensor and may modify the fuel value. In this case it adds
.8% more fuel.
3. The ECU looks at the reading from the Engine Coolant Temperature Sensor and may further modify the value. In this
case it is at 100% (operating temperature) so it does not add or subtract more fuel.
4. The ECU will look and see if the engine is accelerating based on the Throttle Position Sensor. In this case the engine
speed is not changing.
5. From these values, it will calculate the amount of fuel that the user has programmed to deliver which is called the
pulsewidth. If the engine is closed loop, it will modify this value to maintain an air/fuel ratio of 14.7:1.
The following flowchart (Figure 1) illustrates this example:
BASE FUEL MAP
VALUE
100.8%
1000 RPM 38 kPa
32
47
°
F
AMOUNT OF FUEL
INJECTED
(Pulse Width)
3.1 ms
ENGINE
SPEED
MANIFOLD
PRESSURE
(MAP Sensor)
MANIFOLD AIR
TEMPERATURE
MODIFIER
(Air Temp Sensor)
ACCELERATION
ENRICHMENT
(change in TPS and
MAP Sensor)
COOLANT
TEMPERATURE
MODIFIER
(Water Temp Sensor)
100.0%
0.82v
CLOSED LOOP
FEEDBACK TO
MAINTAIN 14.7:1
AIR/FUEL RATIO
(Oxygen Sensor)
CLOSED LOOP
OR
OPEN LOOP?
AMOUNT OF FUEL
INJECTED
(Pulse Width)
3.8 ms
Open Loop Closed Loop
180
°
F
100.0%
No Change
Sensor Data
Input
ECU Calculations
and Adjustments
Key:
Figure 1 Engine at Operating Temperature; Speed Density ECU Strategy Flowchart