engineandecu101
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engineandecu101 [2009/02/26 12:24] 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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| ==== Closed loop ==== | ==== Closed loop ==== | ||
| Closed loop mode is explained in more detail on the [[fueltrimupdatepoints|Fuel Trims]] page. Please read this page and really understand how fuel trims play a role in closed loop mode of operation. They are very important numbers when it comes time to dial in a new set of injectors, for example. | Closed loop mode is explained in more detail on the [[fueltrimupdatepoints|Fuel Trims]] page. Please read this page and really understand how fuel trims play a role in closed loop mode of operation. They are very important numbers when it comes time to dial in a new set of injectors, for example. | ||
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| The "closed loop" target of 14.7:1 is called the stoichiometric ratio for gasoline. This ratio is the theoretically "ideal" mixture where all fuel is combined with all the free oxygen exactly. This is the ratio the ECU targets under light load conditions (idle/cruise). But it is inappropriate as load increases. As engine load increases, it generally becomes less tolerant of "lean" mixtures. Trying to run an engine too lean for the operating conditions can cause things to melt or can cause the combustion process to break down and "explode" rather than burn (typically called detonation or knock). So it's important to keep the mixture rich enough to prevent this but lean enough to make decent power. | The "closed loop" target of 14.7:1 is called the stoichiometric ratio for gasoline. This ratio is the theoretically "ideal" mixture where all fuel is combined with all the free oxygen exactly. This is the ratio the ECU targets under light load conditions (idle/cruise). But it is inappropriate as load increases. As engine load increases, it generally becomes less tolerant of "lean" mixtures. Trying to run an engine too lean for the operating conditions can cause things to melt or can cause the combustion process to break down and "explode" rather than burn (typically called detonation or knock). So it's important to keep the mixture rich enough to prevent this but lean enough to make decent power. | ||
| - | So when running "open loop", the ECU does not always target 14.7:1. It targets an A/F ratio that depends on load and RPM. The picture below illustrates what this type of target table looks like. | + | So when running "open loop", the ECU does not always target 14.7:1. It targets an A/F ratio that depends on load and RPM. The picture below illustrates what this type of target table looks like. You can see that as RPM and load increase, the target A/F ratio gets richer and richer. |
| {{ http://www.ecmtuning.com/images/forums/ecmlink101/2gdsmafr3d.gif }} | {{ http://www.ecmtuning.com/images/forums/ecmlink101/2gdsmafr3d.gif }} | ||
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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. |
| {{ http://www.ecmtuning.com/images/forums/ecmlink101/2gdsmtmng3d.gif }} | {{ http://www.ecmtuning.com/images/forums/ecmlink101/2gdsmtmng3d.gif }} | ||
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