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| Know Your Car : Fuel Injection System |
The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements, meet fuel economy requirements and protect engines against abuse. And there are dozens of other requirements to meet as well.
The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other.
Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more.
Pulse Width = Base Pulse Width (rpm, Load) x (Factor A) x (Factor B)
|
| rpm |
Load |
| |
1 |
2 |
3 |
4 |
5 |
| 1000 |
1 |
2 |
3 |
4 |
5 |
| 2000 |
2 |
4 |
6 |
8 |
10 |
| 3000 |
3 |
6 |
9 |
12 |
15 |
| 4000 |
4 |
8 |
12 |
16 |
20 |
|
In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. This is a function of engine speed (rpm) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2000 rpm and load is 4. We find the number at the intersection of 2000 and 4, which is 8 milliseconds.
|
| A |
Factor A |
|
B |
Factor B |
| 0 |
1.2 |
|
0 |
1.0 |
| 25 |
1.1 |
|
1 |
1.0 |
| 50 |
1.0 |
|
2 |
1.0 |
| 75 |
0.9 |
|
3 |
1.0 |
| 100 |
0.8 |
|
4 |
0.75 |
|
In the next examples, A and B are parameters that come from sensors. Let's say that A is the coolant temperature and B is the oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the lookup tables tell us that Factor A = 0.8 and Factor B = 1.0.
|
So, since we know that base pulse width is a function of load and rpm, and that pulse width = (Base Pulse Width) x (Factor A) x (Factor B), the overall pulse width in our example equals :
8 x 0.8 x 1.0 = 6.4 milliseconds
From this example, you can see how the control system makes adjustments. With parameter B as the level of oxygen in the exhaust, the lookup table for B is the point at which there is (according to engine designers) too much oxygen in the exhaust. Accordingly, the ECU cuts back on the fuel.
Real control systems may have more than 100 parameters, each with its own lookup table. Some of the parameters even change over time in order to compensate for changes in the performance of engine components like the catalytic converter. And depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.
Performance Chips
This leads us to our discussion of performance chips. Now that we understand a little bit about how the control algorithms in the ECU work, we can understand what performance chipmakers do to get more power out of the engine.
Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in the ECU that holds all the lookup tables. The performance chip replaces this chip. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions.
For instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark timing (there are lookup tables for that, too). Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the carmakers are, they use more aggressive settings in the fuel maps of their performance chips.
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