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engineandecu101 [2009/02/26 12:29]
twdorris
engineandecu101 [2010/02/26 13:12]
twdorris
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 The bottom line is that you need accurate airflow readings to the ECU if you want the ECU to inject accurate fuel. The bottom line is that you need accurate airflow readings to the ECU if you want the ECU to inject accurate fuel.
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 ===== Fuel flow ===== ===== Fuel flow =====
-From the amount of air the ECU is told is going into the engine, the ECU calculates an amount of fuel required to maintain a 14.7:1 A/F ratio using stock injectors at stock fuel pressure. ​ This is where things can get a little tricky to keep straight in your head.  The ECU is coded from the factory to account for the configuration the engine came with from the factory. ​ That is, stock injectors and stock fuel pressure. ​ On a stock 2G DSM, that's 450 cc/min injectors at 43.5psi base fuel pressure. ​ On a stock 1G manual DSM, that's 450 cc/min injectors at 37psi. ​ Stock 1G automatics came with 390s at 43.5psi.  This determines the flow rate of fuel into the engine. ​ From that flow rate assumption, the ECU can calculate how long to hold the injectors open to get a specific flow rate required for the air it believes is coming into the engine.+From the amount of air the ECU is told is going into the engine, the ECU calculates an amount of fuel required to maintain a 14.7:1 A/F ratio using stock injectors at stock fuel pressure. ​ This is where things can get a little tricky to keep straight in your head.  The ECU is coded from the factory to account for the configuration the engine came with from the factory. ​ That is, stock injectors and stock fuel pressure. ​ On a stock 2G DSM, that's 450 cc/min injectors at 42.6psi base fuel pressure. ​ On a stock 1G manual DSM, that's 450 cc/min injectors at 37psi. ​ Stock 1G automatics came with 390s at 42.6psi.  This determines the flow rate of fuel into the engine. ​ From that flow rate assumption, the ECU can calculate how long to hold the injectors open to get a specific flow rate required for the air it believes is coming into the engine.
  
 So, you have two things now that the ECU needs to have accurate information on.  First, it needs accurate airflow to calculate fuel flow requirements and second, it needs to know if you've changed anything that affects fuel flow into the engine. ​ If you change base fuel pressure or you swap in different injectors, you have to adjust the ECU's fuel flow calculations to take this into account. So, you have two things now that the ECU needs to have accurate information on.  First, it needs accurate airflow to calculate fuel flow requirements and second, it needs to know if you've changed anything that affects fuel flow into the engine. ​ If you change base fuel pressure or you swap in different injectors, you have to adjust the ECU's fuel flow calculations to take this into account.
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 At some point, air and fuel is going into the engine and getting all mixed up and pressurized by the piston going up on the compression stroke. ​ The ECU then schedules the ignition of that mixture. ​ The precise point at which the ignition takes places is very critical. ​ Firing too early (advanced too far) can cause very bad things to happen as cylinder pressures increase dramatically beyond design limitations and stuff starts to break. ​ Firing too late (too retarded) generally just causes poor performance. ​ So the trick is finding the happy middle ground. At some point, air and fuel is going into the engine and getting all mixed up and pressurized by the piston going up on the compression stroke. ​ The ECU then schedules the ignition of that mixture. ​ The precise point at which the ignition takes places is very critical. ​ Firing too early (advanced too far) can cause very bad things to happen as cylinder pressures increase dramatically beyond design limitations and stuff starts to break. ​ Firing too late (too retarded) generally just causes poor performance. ​ So the trick is finding the happy middle ground.
  
-The ECU from the factory has a couple big tables it uses to look up basic ignition advance values. ​ Those values, just like the A/F ratio values for fuel, are tweaked a bit based on operating conditions. ​ But the bulk of ignition advance figures comes straight from the ECU's timing tables. ​ As engine speed (RPM) increases, timing needs to increase as well.  "​Why"​ is a long story, but take it for granted that it does.  As engine load (cylinder pressure) increases, timing needs to decrease. ​ So these ignition tables are laid out with RPM on one axis and engine load on the other. ​ The result looks something like the following.+The ECU from the factory has a couple big tables it uses to look up basic ignition advance values. ​ Those values, just like the A/F ratio values for fuel, are tweaked a bit based on operating conditions. ​ But the bulk of ignition advance figures comes straight from the ECU's timing tables. ​ As engine speed (RPM) increases, timing needs to increase as well.  "​Why"​ is a long story, but take it for granted that it does.  As engine load (cylinder pressure) increases, timing needs to decrease. ​ So these ignition tables are laid out with RPM on one axis and engine load on the other. ​ The result looks something like the following.  You can see that as load increases, timing decreases and as engine speed (RPM) increases, timing increases.
  
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engineandecu101.txt ยท Last modified: 2024/03/15 11:16 (external edit)