Post by rick1776 on Apr 18, 2003 5:33:12 GMT -5
Engine mapping: the basics. Everyone talks about it but what is it actually? It is the control of both the ignition timing and the fuel delivery/timing. Why is it needed?
Ignition mapping
Well the rule of thumb is that you want the maximum cylinder pressure (after the fuel has been ignited and is burning) to occur at about 20 degrees after top dead centre (ATDC). Earlier than this, and a lot of the energy is wasted pushing the piston up against the rising cylinder pressure. Later than this you do not maximise the energy release that the rising pressure can deliver onto the piston.
The rate at which the fuel burns is not constant but depends upon a variety of factors. Is the fuel ratio lean or rich? Is the charge density high or low (i.e. is it on full throttle of part throttle?) What are the temperature and atmospheric pressure conditions. How much load is the engine under? What kind of head chamber design are you using etc etc. You also need to know the RPM the engine is doing. The faster the engine is going the earlier (more ignition advance) the fuel needs to be ignited so that maximum pressure is reached at about 20 degrees ATDC. So all these factors are added up and the map selects an appropriate ignition advance. A map can be as simple as a look up table. (i.e. At 12000RPM and max load select 30 degrees BTDC.) Alternatively a map can be smart. It can have the basic look up tables and then have an algorithm that says something like select 30 degrees BTDC, next cycle try 31, then 32, then 33. If engine knock detected at 33 then stay at 32. It will do this at every RPM and every load condition and memorise the optimum ignition setting. If conditions change it will re-learn new settings.
Fuel mapping
Maximum power is usually achieved with a slightly rich mixture, usually about a lambda of 0.9 to 0.95. The fuel map dictates how long the injectors stay open for so that the appropriate amount of fuel is delivered to the engine. The amount of time that they stay open for is either measured in milliseconds or crankshaft degrees. Typically they stay open so that the maximum power is developed. If however you are stuck behind another car and cannot pass you might want to lean the mixture and save fuel so that you can perhaps do an extra 2-3 laps on the same amount of fuel.
Another word you may hear is the term “trim”. This refers to the exact moment the fuel injector comes on and can also be measured in crankshaft degrees. (i.e. At 16000RPM the injector needs to be on for 0.75millisecond and the injector comes on at 32 degrees BTDC. Varying the trim angle has a huge impact on the power a motor makes. (In Australian V8 Supercars varying the trim can produce an extra 30-50 HP.)
So there you have the basics. Traction control I’ll save for another day, but basically it modifies all of the above.
Cheers
Rick1776
Ignition mapping
Well the rule of thumb is that you want the maximum cylinder pressure (after the fuel has been ignited and is burning) to occur at about 20 degrees after top dead centre (ATDC). Earlier than this, and a lot of the energy is wasted pushing the piston up against the rising cylinder pressure. Later than this you do not maximise the energy release that the rising pressure can deliver onto the piston.
The rate at which the fuel burns is not constant but depends upon a variety of factors. Is the fuel ratio lean or rich? Is the charge density high or low (i.e. is it on full throttle of part throttle?) What are the temperature and atmospheric pressure conditions. How much load is the engine under? What kind of head chamber design are you using etc etc. You also need to know the RPM the engine is doing. The faster the engine is going the earlier (more ignition advance) the fuel needs to be ignited so that maximum pressure is reached at about 20 degrees ATDC. So all these factors are added up and the map selects an appropriate ignition advance. A map can be as simple as a look up table. (i.e. At 12000RPM and max load select 30 degrees BTDC.) Alternatively a map can be smart. It can have the basic look up tables and then have an algorithm that says something like select 30 degrees BTDC, next cycle try 31, then 32, then 33. If engine knock detected at 33 then stay at 32. It will do this at every RPM and every load condition and memorise the optimum ignition setting. If conditions change it will re-learn new settings.
Fuel mapping
Maximum power is usually achieved with a slightly rich mixture, usually about a lambda of 0.9 to 0.95. The fuel map dictates how long the injectors stay open for so that the appropriate amount of fuel is delivered to the engine. The amount of time that they stay open for is either measured in milliseconds or crankshaft degrees. Typically they stay open so that the maximum power is developed. If however you are stuck behind another car and cannot pass you might want to lean the mixture and save fuel so that you can perhaps do an extra 2-3 laps on the same amount of fuel.
Another word you may hear is the term “trim”. This refers to the exact moment the fuel injector comes on and can also be measured in crankshaft degrees. (i.e. At 16000RPM the injector needs to be on for 0.75millisecond and the injector comes on at 32 degrees BTDC. Varying the trim angle has a huge impact on the power a motor makes. (In Australian V8 Supercars varying the trim can produce an extra 30-50 HP.)
So there you have the basics. Traction control I’ll save for another day, but basically it modifies all of the above.
Cheers
Rick1776
