|
+
+ DIESEL ENGINE CONTROL +If you are new to the idea of playing with ECU maps. Read this guide before you start. +Do not play with ECU maps until you understand what they do. Mistakes can be expensive! ++ A BRIEF GUIDE TO ECU CONTROL OF A DIESEL ENGINE. ++ To control a modern diesel engine you need to control: +Fuel injected quantity (IQ) +Fuel injection timing (SOI) +Fuel injection duration ++ In order to control fuel injection you must know how much air is flowing into the engine and you must know the engine speed. ++ So you have 5 factors very closely linked. +1. Mass of Air Flowing (MAF) +2. Fuel Injected quantity +3. Fuel injection timing +4. Fuel injection duration +5. Engine speed (rpm) ++ The following guide based on a 1.9 tdi pd VAG engine. +This engine has a capacity of 1.9 litres or 1900 cubic centimetres (cm3). +The exact figure is actually 1897 cm3. +The engine has FOUR cylinders so each cylinder is 474 cm3. (1897/ 4) +All four cylinders are identical so we only need to deal with one. +If one cylinder has a volume of 474 cm3, the maximum amount of air & fuel it can hold is 474 cm3. ++ Fuel request (Drivers wish). ++ The electronic accelerator pedal sends a signal to the ECU showing how much the driver is pushing the pedal down. +The measurement is usually a percentage. +0 % is no push so the engine idles. 100 % is full or Wide Open Throttle. ++ So + at 0 % throttle the injectors must give a fixed QUANTITY of fuel, for a + fixed DURATION starting at a fixed TIME. This results in a pre-set idle + speed. e.g. 900 rpm. +So the idle speed has been Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING. ++ At + 100 % throttle the injectors must give a fixed quantity of fuel, for a +fixed duration starting at a fixed time. So 100 % throttle has been +Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING. +This + means that every other percentage from 1 % to 99 % will also need to be + Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING. ++ How much fuel should be Injected? ++ The + ECU knows how much fuel to inject because it knows how much air is in +the cylinder. So we need to know about AIR before we decide about FUEL. ++ If our cylinder has a volume of 474 cm3, the maximum amount of air & fuel it can hold is 474 cm3. +If we ignore fuel for a moment, it means the maximum amount of air that the cylinder can hold is 474 cm3 +If air was a liquid, life would be easy. The amount of a liquid that fits into 474 cm3 is 474 cm3. +Air is a gas and so you can fit different amounts of air into the same space. ++ So how much air fits into 475 cm3 ? +This is determined by the density of the air and the density of the air depends on the surrounding temperature and pressure. +The density of air at sea level and on a warm day is between 1mg/cm3 and 1.2 mg/cm3. +Let’s assumes air density is 1.0 mg/cm3. ++ So our 474 cm3 cylinder will hold 474 x 1.0 mg of air, which is 474 mg of air. +So every stroke of one piston will suck in 474 mg of air. This is referred to as 474 mg/stroke. ++ So now we know how much air is in our cylinder (474 mg. We can inject some fuel. ++ Injecting fuel (IQ) ++ Our cylinder holds 474 mg of air. +Diesel + burns at maximum efficiency at roughly 14.6 mg of air to 1 mg of fuel. +So 474 mg of air can efficiently burn 32.5 mg of diesel fuel. (474 / +14.6) ++ So we inject 32.5 mg of fuel and off we go. Not really. ++ This doesn’t mean the injectors inject 32.5 mg of fuel per stroke (mg/stroke). +32.5 mg/stroke is the ideal maximum, assuming a normal air supply (EGR shut) +If + the injectors inject more than 32.5 mg/stroke, some of the fuel won’t +burn properly and will come out of the engine as black smoke. (This is +often described as the smoke limit). ++ The injectors can inject any amount of fuel less than 32.5 mg/stroke and that’s what they do. +At idle the injectors may be injecting as little as 6.0 mg/stroke. +To make the engine speed rise the INJECTION QUANTITY is increased ++ The injection quantity is controlled by a map in the ECU often called Drivers Wish. ++ At + idle the accelerator pedal will be set at 0 %, so no EXTRA injection +will occur because idle speed is controlled by an idle speed map, not +the Drivers Wish map. +When fully pressed down (wide open throttle. WOT) the accelerator pedal will be 100 %. +So the ecu receives a signal varying between 0 % and 100%. +If + you apply 30 % accelerator pedal the ecu consults the built in DRIVERS +WISH MAP checks the required INJECTION QUANTITY and injects that amount. ++ SIMPLE. ++ Unfortunately this is not simple because diesel engines don’t really measure how much fuel they inject. ++ Fuel injection is very complicated these days, so this is a very simple explanation. ++ Imagine a fuel injector is like a doctor’s syringe loaded with 100 mg of fuel. +The + driver presses the accelerator pedal and WISHES for 30 %. The ecu +consults the DRIVERS WISH MAP and decides to inject 16 mg of fuel. +SIMPLE. ++ BUT +When do you inject the fuel and how long will the injection take? ++ Engine designers measure time in degrees of rotation of the CRANKSHAFT. That is why you hear people referring to engine timing. +The + ideal point to inject the fuel is generally taken as Top Dead +Center.(TDC). This is the point when both valves are usually shut and +the air has been squashed to its maximum. +TDC + is often referred to as Degrees Before Top Dead Center (BTDC) or +Degrees After Top Dead Center. They are both the same thing, just +opposites of each other. +So 4°BTDC is the same as -4° ATDC. (Only BTDC is used here) ++ Injecting + 16 mg of fuel will take time (DURATION) and because the piston is going + up and down, you need a START OF INJECTION point. ++ Lets assume 2 mg of fuel takes 1 degree of crankshaft rotation (°CR) to inject. +Assuming that the injection best time is 0°BTDC and 16mg will take 8°CR to inject. (DURATION) +Injection will need to start at 8°BTDC instead of at 0°BTDC. +Start of Injection (SOI) has to be 8 degrees BTDC so that all the fuel has been injected by 0°BTDC. ++ So the ecu needs maps to decide on; +INJECTION QUANTITY as requested by the accelerator position. +INJECTION DURATION as calculated from the injection quantity +INJECTION START (SOI) as calculated from the injection quantity. ++ Assuming + that the engine is in perfect condition, the maps for Injection +Quantity, Injection Duration and Start of Injection will be accurate. +So + a precise amount of fuel will be injected for the correct amount of +time (DURATION), starting exactly on time.(Start Of Injection) ++ The + ecu can be sure of this because the crankshaft and camshaft sensors +give precise details of the piston positions. These measurements end up +on the dashboard as engine speed measured in Revolutions Per Minute +(rpm). ++ Below is a graph showing the amount of fuel (IQ) being injected into a cylinder to cause the engine rpm to rise. ++  + On this graph you can see that; +At idle (850 rpm) the ECU is injecting 23mg of fuel. +32mg IQ raises engine speed to 1050 rpm. +45mg IQ raises engine speed to 1400 rpm. +52mg IQ raised engine speed to 1900 rpm. +The ECU could carry on injecting 52mg to make engine speed rise higher (As per the added RED line) +BUT it doesn’t. +52mg IQ raises engine speed to 2550 rpm but after that IQ is reduced by the ECU. +50mg IQ is used to achieve engine speed 3200 rpm. +48mg IQ is used to achieve engine speed 3750 rpm. +44mg IQ is used to achieve engine speed 4200 rpm. +38mg IQ is used to achieve engine speed 4550 rpm ++ So the ECU is LIMITING IQ as the engine speed rises. Why is IQ being limited? ++ The amount of fuel being injected (IQ) must be limited for a number of reasons. ++ 1. We can only inject fuel if we have enough air to allow it to burn. +This is the Air supply limit known as MAF limit or smoke limit. (See smoke map) ++ 2. We only want to produce engine power in a smooth controlled manner to protect the clutch, gearbox etc. +This is the Torque limit. (See Torque map) ++ Lets look at limiting factors. ++ AIR SUPPLY CONTROL ++ We have established that our 474 cm3 cylinder will hold 474 x 1.0 mg of air, which is 474 mg of air. +So every stroke of one piston will suck in 474 mg of air. ++ The ECU can measure the Mass of Air Flowing thanks to a Mass Air Flow (MAF) sensor. +This + sensor constantly monitors the air flow and sends a signal to the ECU. +Typically the air flow varies from 0 to 1000 mg/stroke. The MAF needs to read above the normal atmospheric value of 474mg because of the addition of a turbocharger. ++ The MAF sensor plays a big role in EGR function but I won’t go into that here. +So + our smoke map or IQ limit by MAF is based on information coming from +the MAF sensor on the engine. Obviously the MAF sensor must be in good +condition and give accurate results or the ECU will make the wrong IQ +calculations. ++ CONTROLLING AIR PRESSURE – TURBOCHARGERS. ++ Air pressure and temperature vary depending on where you live in the world, weather etc. +The following assumes an air temperature around 20 °C and an air pressure of 1000 millibar (mbar). ++ Let’s assume our engine cylinder is receiving air at 1000 mbar pressure and 20 °C temperature at a rate of 474 mg/stroke. (To keep things simple, I am assuming no EGR is involved) ++ A + typical turbocharger boost value adds an extra 1000 to 1500 mbar of air + pressure. So a typical turbo boost pressure graph against rpm will run +from 1000 mbar (no boost) up to 2500 mbar max boost. (That’s an extra +1500 mbar boost) ++ The + extra air pressure means extra air so if we have 474 mg of air at 1000 +mbar we can have 948 mg of air at 2000 mbar. (2 x 474) So with twice as +much pressure we have twice as much air in the cylinder and can burn +twice as much fuel at the same efficiency as before. +This results in the engine developing more power. ++ The + engine ECU needs to know the boosted air pressure so the engine has a +boost pressure sensor (manifold absolute pressure (MAP)sensor.). +The ECU also needs to know the air temperature so the engine has an Intake Air Temperature (IAT) sensor. +The + desired Boost for engine speed and IQ is controlled by the Boost map. +This map tells the ECU how much boost is required for a specified engine + speed and IQ. ++ So we stamp on accelerator, get maximum IQ, maximum boost and off we go. Not really. ++ The + turbocharger doesn’t instantly change its turbine speed and boost +pressure. It needs to spin up to speed. So the ECU needs to allow for +the ‘spin up’ time. +Once + up to speed the turbocharger will give maximum boost as per the boost +map, which is fine for acceleration but most drivers don’t accelerate +all the time, they cruise. So at 70 mph on the motorway the engine may +have 2500 rpm thanks to high gearing. +IQ may have dropped to 32mg/stroke so the air needed is less than during acceleration so we don’t need lots of boost. +So the ECU needs to be able to control boost levels and make decisions about them. ++ + Turbocharger boost control. ++ As + explained earlier the turbocharger needs to be controlled because the +engine design and fuelling maps assume a certain BOOST level under +certain engine speed and IQ conditions. +The + engine ECU therefore uses the boost pressure sensor (MAP sensor.) and +Intake Air Temperature (IAT) sensor to gather data about current boost +conditions. ++ These sensors allow the ECU to compare current boost pressure with boost pressure maps stored in the ECU. +(The ECU also has a Single Value Boost Limiter (SVBL) which acts like an emergency cut off for boost.) +The turbo boost map controls the boost level inline with the required IQ. ++ The actual control of boost is via an electrical signal that controls the opening of a vacuum valve called the N75 valve. +The engine ECU varies this electrical signal to vary the amount of opening of this valve. +The + ECU contains a map for N75 Duty cycle. The map ensures that the correct + amount of boost is available as set by the Boost map. ++ Boost control has a limiting map known as the boost limit map. This map is to protect the turbocharger. It is based on the measurement of atmospheric air pressure. +Remember we decided to think of air pressure as 1000 mbar at 20 °C. +If the atmospheric air pressure and temperature never change we won’t need a boost limit map. +In + real life, air temperature changes all the time and atmospheric +pressure changes with the weather and when we drive up and down +mountains so our cars will need a boost limit map to protect the +turbocharger and stop the ECU raising IQ when the turbocharger can’t +provide enough air. (Like when you drive up a mountain…If you do +If the boost stays outside the range of the boost limiter for too long, the ECU will switch the boost OFF. (Limp mode) ++ The + engine ECU also contains a Single Value Boost Limiter just in case the +turbo control fails. The turbo will be switched off if the actual boost +goes above the Single Value Boost Limiter. (SVBL). ++ Hopefully the above information will give you a clue about your turbocharged diesel engine and how it works. +Lots + of things in the ECU are inter-linked and changing one ECU map can have + unexpected effects on other maps so it is vital that you think before +you act. +The most basic changes in the ECU mapping will require changes to +1. Drivers wish map +2. IQ limit by MAF (smoke map) +3. Boost map ++ If in doubt…Don’t do it. +Think before you make changes. +Mistakes can be expensive and even dangerous. + |
+
++1. Diesel engine control +
+
+
+
+
+
+
+
+
+
+
+
