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Last change 25.04.2024, 18:19:17

Engine control system concepts explained

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  1. Introduction
  2. Technical jargon, nomenclature and buzzwords related to engine tuning that you might have to learn to communicate with your tuner
  3. Common fundamental sensor inputs
  4. Core fundamental outputs
  5. Additional common sensor inputs
  6. Common output examples
  7. ECU fundamentals
  8. Engine control strategies
  9. Idle control
  10. External resources


This page contains resource material written by PNPECU or references other websites with relevant information.

The engine control unit (ECU) or sometimes known as the powertrain control module (PCM) takes in a number of sensors and actuates outputs in order to control the engine. The control in question can vary at any given time and so some outputs are not always active and or they will be changing from one target objective to another.

What sensors are required or are installed can usually be related to the emissions regulations the engine was originally developed for as the more information and well developed the ECU software is the more accurately the outputs can be handled to produce the desired results, be it emissions regulation, drivability, engine performance. All these things can be achieved at the same time for most applications

Technical jargon, nomenclature and buzzwords related to engine tuning that you might have to learn to communicate with your tuner

If you speak to your tuner for more than 10 seconds you will almost certainly hear one of these things spoken about

Load - This usually refers to the amount of air entering the engine and is expressed in mass/cycle or mass/time or manifold pressure usually
Engine speed - This is the literal rotating speed of the engine, half of this speed is the cycles / minute which is the number of times the spark per cylinder happens as well as air enters the engine for a four-stroke engine, this is usually noted in revolutions per minute but sometimes radians per second.
Ignition advance - This is the angular crank shaft position where the spark is meant to happen and start the combustion process. Positive numbers refer to spark happening before the piston reaches the top of the compression stroke while negative numbers refer to spark that happens after the piston reaches the top
Fuel table - This table is where the fuelling numbers are entered to achieve the desired amount of fuel injector at any given load or engine speed. The X axis is usually engine speed and Y axis is the load being used.
Volumetric efficiency - This refers to the amount of air consumed by the engine as a ratio of its engine size. Ie if the engine is 2liters in capacity and it consumes 2liters in a single cycle it´s considered 100% efficient. Most modern systems that use VE also incorporate the manifold pressure into the outgoing calculation and the results are usually pretty flat looking tables.
MBT - Minimum best timing / Minimum best torque - This refers to the least amount of ignition advance required to achieve the most amount of torque producible at a given engine speed and load.
MAP - Manifold absolute pressure - The amount of pressure in the intake manifold
MAP - The second meaning - This is the engine calibration file / tune file and holds all the configuration variables
MAP - The third meaning - This is referring to switching MAPS, which means to make on the fly changes to how the engine operates using a "MAP SWITCH".
Lambda / Air fuel ratio / AFR - This relates to the amount of fuel injected compared to the air ingested by the engine and that can be seen by the Lambda sensor in the exhaust.
Rev limit - This is the highest engine speed the ECU will not allow the engine to go above, fuel and or ignition is shut off at this point and so the engine will stop producing any torque, this is used to manage the life of the engine components.
TPS - Throttle position sensor - refers to the angle of the throttle blade, can be represented in percentages or degrees. Systems are almost exclusively either % or ° and so % system used 0-100% range while angular system tend to use 0-90° range. Throttle bodies usually cannot close to completely 0° blade angle and so the actual closed position is 2-3° or so, this can be measured to retain accuracy in readings
Idle speed - this refers to either the current target idle speed or the current idle speed the engine is at, idle speed is when the ECU is managed to keep the engine from stalling but still keep the engine speed at a point where driving away is easy.
Boost limit - This refers to the maximum manifold pressure that the system has been setup to monitor and when it is crossed it will cut the fuel and or ignition to stop any more pressure from being build and put into the engine, this is to manage the life of the engine components
Fuel cut - This refers to any situation where a fuel injector/s signal is stopped as to stop the engine producing torque and or wasting fuel when the vehicle is coasting without any driver pedal input
Ignition cut - This refers to any situation where the ignition outputs are stopped and is used to stop the engine producing torque that could lead to component failure or manage the engine speed
BSFC - brake specific fuel consumption - This refers to the amount of fuel injected as a ratio of air ingested and the output power. The lower the amount the more fuel efficient the engine is at producing power at a given scenario, this value is not constant at all loads or engine speeds.
Duty cycle / PWM - This refers to the amount an output stays active inside it´s available time period. For instance when an output is being operated at 100Hz or 100 times per second, each of those periods is 10ms long, if the output stays active for 2ms, the duty cycle is 20% an output cannot be above 100% as constantly on is of course the maximum any output can be on for.
Deadtimes / latency offset - This refers to the time delay from when the ECU injection signal is turned on and until actual flow comes out of the injector. The same happens on the other side when the injector is turned off, it will take some time to stop injecting, so collectively the opening delay and closing delay and subsequent flow increase and decrease during those periods are the deadtimes. The target with knowing these is to allow the ECU to demand certain volume flow at any given battery voltage and fuel pressure as those alter these characteristics.
Dwell times - This refers to the time the ignition coil is allowed to charge before it´s stopped and the spark happens.
Back pressure - The amount of positive pressure seen in the exhaust manifold or exhaust system, this information can be used to produce a load input and or to monitor the exhaust system and how much of a restriction it is causing.
EGT / Exhaust gas temperature - This is usually a K-type thermocouple capable of measuring above 1000°C and is used to compare cylinder combustion temperatures as well as monitor altering conditions from known healthy conditions, various valuable information can be derived from having individual sensors per cylinder, but most often users have a single EGT that looks at the average of all the cylinders.
Knock / Pre-detenation - Both are an indication that abnormal combustion has taken place and added significantly higher loads on to the rotating assembly inside the engine, prolonged knock will cause damage to components and eventually engine failure, pre-detonation refers to the combustion starting before the actual spark angle, this is caused by a component inside the cylinder chamber getting to a temperature high enough to ignite the fuel as the piston is compressing the mixture and so the ignition advance has been artificially altered which usually results in engine failure quickly, if the ignition spark also happens the two flame fronts from these two spark events can cause accelerated combustion and so the peak cylinder pressure can be multiples higher than under normal considerations. Rods and pistons can break from this cylinder pressure inside just one occurrence depending on the situation.
Coolant temp - this refers to the average temperature of the coolant flowing through the engine behind the thermostat if one is fitted. Depending on what the temperature is different amounts of fuel must be injected to produce the desired lambda target.
Air temp - This refers to the temperature of the air going into the engine near the throttle body, the density of air is related to the temperature and so fuelling must be adjusted based on the temperature. Air temperature also can require that ignition advance is moved as not to cause knocking and engine failure
CANBUS - This refers to a two-wire communications method and protocol between two or more devices to share information between them in a network. CAN bus is getting more preferred as the information is digital and can be checksum verified at the recipient device to contain valid information that the receiver can then safely carry out changes to any of its outputs. It´s also popular as many devices can be listening to one transmitting device, such as the ECU sending out engine speed to the transmission controller (TCU) or instrument cluster wher each device used this information for a different purpose.
Closed loop / Open loop - Open loop refers to a control system that has no feedback on the results based on its previous output and so cannot take any measures to alter the next output to drive to a target. Closed loop systems work specifically on a pre-defined target/s and so can adjust the output signal as required to achieve the target despite the system mechanics and functionality changing over time or due to some mechanical issue.
Pedal position sensor / Accelerator pedal - The pedal position sensor is required in a electric throttle body application as there will be no direct connection between the driver's food and the throttle body at the engine. A pedal sensor will have to seperated circuits and so if one of the wires and or circuits fail the ECU can see the discrepancies in the two signals and conclude that one of the signals is faulty, it will then stop the system as it may not be completely clear which one is faulty and it cannot risk opening the throttle if infact the driver is requesting it to be closed. The throttle body on the engine also has two signals and if one fails the system will stop in the same manner. There are numerous other monitoring situations possible that will stop the system as it will have concluded that 100% safe operation of the throttle cannot be maintained and so much avoid any dangerous situations at all costs.
60-2 / 36-1 / 6+1 etc, these kinds of numbers refer to the type of crank/cam trigger system an engine will have as the ECU will need to know it to be able to decode the incoming ON OFF signals into crank angle positions so that it can perform accurate and repeatable applications of ignition timing and fuel timing.
Cold start / Warmup - When an engine is cold some of the fuel after getting sprayed into the port will not vaporize and may not get ingested by the engine, so the ECU must counter this by injecting even more fuel to make sure that the amount of fuel required makes it into the cylinder to be combusted.
Low side output / pull to ground - When the output of the ECU provides a circuit path to ground, the other side of the actuator will need to be wired to 12V+
High side output / push to Battery - When the output of the ECU provides a circuit path to battery voltage, the other side of the actuator will need to be wired to ground.
Pull-up - When through a resistor a signal line is connected to 5v, when there is no signal on the line the voltage measured is 5v.
Pull-down - When through a resistor a signal line is connected to ground, when there is no signal on the line the voltage measured is 0v.
Voltage divider - A combination of two or more resistors that will produce a varying output voltage based on either both varying resistances or one of them is varying, this is how a thermistor circuit works.
ADC - Analog to digital converter, this circuit reads the input voltage and gives it a digital value representation based on the depth of the ADC as a function of the bits it´s rated at. For instance, an 8bit ADC that can take in 0-5v would be able to break the 5v range into 0.019v increments for each bit. While a 12bit ADC would be able to divide it into 4095 segments and so each bit represents just 0.00122v.
H-bridge - The H bridge is able to provide 12v or ground on the same wire depending on which direction it wants to turn a motor, these can be normal DC motors or encased motors such as those in throttle bodies.
Lean / Rich - Lean refers to the engine having less fuel burned than what the stoichiometric air fuel ratio is for that air fuel mixture, rich refers to there being excess fuel in comparison. Lean doesn´t always refer to the stoichiometric air fuel ratio though, often lean is simply a term used to mean "leaner than was planned", for instance if the target lambda was 0.8 and the resultant mixture was 0.85 then that is often referred to as lean, same with rich.
misfire - Fuel based misfires are the results of not enough fuel - lean misfire, or to much fuel - rich misfire, ignition based misfires refer to when the ignition system was unable to get the spark ignited and no combustion took place.

The list of technical jargon could be made into a book and will be added to this list over time

Common fundamental sensor inputs

Crank position - A variable reluctor sensor or Hall effect sensor that the ECU uses to pick up trigger teeth or slots on a trigger wheel.
Coolant temperature - Usually a thermistor that alters it´s resistance with temperature and so the measured voltage changes
Intake air temperature - Usually a thermistor that alters it´s resistance with temperature and so the measured voltage changes, on some occasions users have installed -k-type thermoucouples which produce their own voltage but also require a controller to amplify the signal so the ECUs circuits can read it with enough resolution.
Throttle position - The throttle angle represented as percentages or angle.
Intake manifold pressure - The measured absolute pressure inside the intake manifold, depending on the system represented by millibar, kilopaskals or pounds per square inch.

This may seem like a lot fewer sensors than new engines or modern engines come with, but these inputs are what the bulk of the engine operations revolve around which is to determine the amount of air mass coming in and how to operate the injectors to provide the correct amount of fuel mass to produce the desired target air fuel ratio

Core fundamental outputs

Ignition output/s - Either a TTL output/logic level signal/smart coil signal, which means when it´s ON the output is at 5v/12v, when it´s off it´s at 0v or a Low side signal through a IGBT, the IGBT when activated will provide a path to ground for current to flow to the coil. Both coils are wired to 12V as well.

Injection output/s - For port fuel injectors it´s practically universally always low side outputs through MOSFETs, Two main types of injectors exist, low impedance and high impedance, the high impedance ones only require the ground path provided by the ECU as their current draw is around 1A, while low impedance injectors can be controlled with specific circuitry that initially provides full current of about 5A to open the injector rapidly and then maintains a much lower average current to keep it open, or the mosfet is pulsed at around 10kHz after the initial opening period.

This may seem even fewer than what almost all engines have, old and new in terms of outputs, but these are the two core elements in operating a combustion engine aside from all the other things that are required to produce the best results.

The engine produces cylinder pressure / torque thanks to burning fuel inside the engine cylinders while under compression, combustion can take place when all 3 elements required are in place. These are an oxidizer, fuel to burn and heat.

• The engine being a pump consumes air by itself simply by rotating, the amount of opening of the throttle body controls how much air the engine gets.
• The injectors provide the fuel to burn
• The spark plugs / ignition system provides the heat.

Additional common sensor inputs

Camshaft position for one or more camshafts - allows for sequential and timed injection, sequential individual ignition output for multiple individual coils as well as variable camshaft control, some ON / OFF variable camshafts do not require camshaft angle monitoring to achieve the cam movement, but having it aids in diagnostics and detecting faults in the system.
Lambda sensor - provides the ECU information about the residual exhaust gas composition and the ECU can derive what the air fuel ratio was inside the cylinders during combustion, this is crucial in the tuning process as well as closed loop air fuel ratio control after the tuning if wanted.
Fuel pressure - the information is used to determine what the injector flow and deadtime characteristics are at any given time and so the ECU can operate the injector opening duration to manage the amount of volume flow.
Fuel temperature - the information is used to determine the mass of fuel injected for a given injected amount of fuel volume, the ECU adjusted the volume flow to obtain the accurate target fuel mass flow
Fuel tank pressure - as a part of the sealed emissions systems of most modern engines this century, the ECU can monitor pressure buildup of fuel vapors in the fuel tank and allow those vapors to be ingested by the engine and burn them in a managed manner. This is done to avoid a pressurized fuel tank that could release harmfull emissions fuel vapors when the fuel tank cap is removed when more fuel is to be added at a fuel station.

There is a great deal many more sensors available to fit to engines, but this is just an example

Common output examples

Fuel pump control - Fuel pump relay, pulse width modulated signal or CANBUS signal to a pump control module
Main relay control - The ECU when it´s powered and has concluded that no critical errors are evident will turn on the main relay which feeds power to the actuators on the engine, such as injectors, idle control valve, boost solenoid/s and more. Without this relay being active no actuator will function. This on modern vehicles is often handled by a central body module or a power distribution module and so the ECU will communicate to the BCM/PDM that it is ok to turn the main relay on.
Idle control valve - These come in various forms, their function is to manage additional airflow to the engine by bypassing the throttle body. In many modern engines these are no longer installed as electric throttle bodies can provide the same function.
Variable valve solenoid - these come in various forms, their function is to usually manage oil flow to the variable valve timing system which can alter the phasing of the camshaft in comparison to the crankshaft, this can have drastic effect on how well the engine can consume air and so the maximum torque potential, intake cams are often also adjusted to increase the manifold pressure without improving airflow, this has the effect of reducing the engines pumping losses which results in lower emissions and improve fuel economy, exhaust camshafts being phase adjusted can produce internal EGR which allows the engine manufacturer not have to install EGR hardware on their engine while achieving the required emissions requirements.
Boost solenoid - When left unpowered the engine will usually be able to produce the amount of boost as dictated by the spring/s in the wastegate canister, with increasing amount of PWM DC% the wastegate will route more exhaust to the turbine section which will increase the amount of boost produced. By varying the DC%, the ECU is able to reach an exact target as desired by the user at any given time.
Electric throttle body / Drive-by-wire - The ECU will operate the throttle body itself using an H-bridge driver circuit based on driver pedal demand as well as other internal overriding factors depending on the situation, one of those is if the ECU detects a fault or an error in a throttle position signal or driver demand signal it will stop operating the throttle as it is clear that proper control is not possible and so any attempt at control might lead to danger, others can be engine speed limiting, vehicle speed limiting, launch control, rolling launch control, boost control, , anti-lag, gearshift downshift control and many more
Radiator fan - many cars are equipped with electric radiator fans, the control for this was in the past done outside the ecu with temperature switches, but with more demanding engine temperature management this functionality is now most often in the ECU, the ECU can have varying coolant / radiator temperature targets depending on the situation. So the fan control is not always reactive but can be proactive.
Intake manifold flaps/valves - Many engines have flaps or valves that allow the intake manifold to effectively change shape and produce enhanced airflow characteristic abilities in certain scenarios, increasing drivability.

There are many more things a modern engine control unit has to manage on top of managing the fuelling and ignition system/timing

ECU fundamentals

Fuelling - One of the fundamental tasks of the ECU is to manage the amount of fuel mass that is injected at any given time. One of the primary aspects of a modern ECU is to lookup what the users target lambda / air fuel ratio is and then lookup what the current mass airflow is and then determine how long the injectors need to be opened for to flow the appropriate amount of fuel volume to match the target fuel flow mass.

ECU´s do this in many different ways but they all have the same trend and that is it's a mathematical physics model in some parts and empirical results input into lookup tables or curves in other parts to fulfill the mathematical model. Even if the ECU is completely run-on empirical inputs into tables and curves the math in the ECU still works from the underlying physics model, even if the ECU developer never planned to and or the values presented in the ECU are not displayed accurately in terms of their engineering values

To obtain mass air flow there are a number of sensors or combination of sensors that can be used. The most common ones in the aftermarket are the air temperature sensor and manifold pressure sensor, those sensor inputs coupled with a volumetric efficiency table allow the ECU to calculate the mass airflow on a per cylinder bases.

Volumetric efficiency, engine size and current engine speed provides you with volume air flow results. When that result is multiplied with the current air density (which is derived from air temperature) you will have mass air flow results.

This kind of information is usually represented as kg/sec or kg/hr if the ECU provides it. When this information has been calculated then two parts of the air fuel ratio triangle have been discovered and the amount of fuel mass targeted to be injected can be calculated

Air fuel ratio means how many kgs of air you want for a given amount of fuel you want. At for instance 10:1 AFR that means for every 10kg of air coming in, the fueling should be 1kg.

Many ECU´s do not have all the displayed values or run a simplified model, but the proper model would include fuel temperature, fuel pressure, injector flow rate, injector deadtime and fuel density.

Injectors behave very differently if battery voltage and or fuel pressure has changed both in their flow rates as well as the time it takes for them to begin to flow any fuel. Both of these need to be compensated for so that the right amount of mass is injected for all scenarios. Most large injector retailers in the aftermarket will provide this data for use with popular ECUs.

Injector flow rate can be presented as mass flow rate for a specific fluid with a specific density or more usually simply volume flow rate. Gasoline for example is usually around 745kg/m^3 or 745grams/liter, that means for every liter of fuel flowed 745grams of fuel has been injected and so a 100cc/min injector can provide 74.5grams/min mass flow rate for example. These are all the basics of how the ECU determines the amount of fuel to inject. Things become more complicated when the conditions inside the intake manifold are changing rapidly for example when the throttle body opening is quickly changed as the manifold pressure sensor is not quick enough to react to this change

Acceleration enrichment - The rate of change of the throttle is often used to determine when the airflow model is wrong and additional unmeasurable air is coming in, the conditions quickly settle but if not acted upon right away the engine will run lean and if to lean it can misfire and cause a stumble in the acceleration felt by the driver causing very poor drivability. At the same time, it is possible to inject to much fuel and also cause a stumble.

There are a number of ways to determine this and it varies with ECU manufacturers how they tackle this and how comprehensive their strategies are. While some only provide a few variables to adjust some others have alot of varibles which can be very difficult to calibrate due to the complex nature of the strategy.

Engine control strategies

Idle control

Idle control is an engine control strategy in where either and the airflow and ignition timing is adjusted to keep the engine speed to a certain target. The target can vary based on a number of reasons but the main one is coolant temperature. Idle ignition control is a form of torque management as idle advance is rarely at MBT during idle and so the engine is running in-efficiently. MBT timing could be 25° while a 4 valve engine might be idling at 10° advance to maintain its target idle speed.

Since idle control is really two separate functions working in tandem to the same goal they will be discussed separately

Airflow control, airflow into the engine can be controlled in a number of ways

• Electronic bypass valve - bypasses the throttle body and allows additional air to enter the engine
  • 2 wire pwm type
  • 3 wire dual solenoid pwm type where one solenoid aims to open the valve and one aims to close it
  • 4+ wire stepper driven valve where the bypass is controlled by a screw thread, the rotation of the screw is controlled by the stepper motor
• Mechanical bypass valve - coolant temperature-controlled valve that allows additional air to enter the engine purely based on coolant temperature, often bolted to the coolant passage back to the water pump and controls the amount of vacuum available to a diaphragm valve that can push on the throttle body to open it, or push on a bypass valve to open it - not ECU adjustable
• Electronic throttle body - the ECU manages how closed the throttle is to manage the total amount of air entering the engine - behavior mimics that of an electronic bypass valve
• Mechanical throttle body - when no valve is fitted the amount of throttle angle opening directly influences the total air entering the engine, to open and the engine tends to hand on the overrun if fuel is active. to closed and the engine will tend to dip down below the target and can stall. not ECU adjustable

In almost all scenarios more pulse width duty cycle given will mean more bypass air which will result in a higher idle speed.

In ECUs you will have either open loop or closed loop control of the idle valve, if you only have open loop, you must fill in a table of values for the given coolant temperature for instance, to reach your target idle speed.

In a closed loop system you still have to fill in the open loop table values and the closed loop will increase or decrease the bypass amount to manage the airflow so that you get the target idle speed, here the idle opening is acting like torque management as more air means more fuel, combined a bigger air fuel mixture going into the engine will create more torque which when there is no load/drag on the engine will increase its engine speed.

Closed loop idle valve control is almost universally managed by a PID loop using all 3 aspects.

Idle control using ignition advance out of the two can be called the fast loop while the airflow control is the slow loop, this is because ignition advance can be adjusted quickly between cylinder firings and so in matter of 30-60ms you can adjust the torque output by a large amount while an idle valve would take a few cycles of the engine to make any measurable change in airflow (100-200ms).

Idle ignition as above mentioned is already set to an inefficient ignition angle in open loop fashion so the engine is not producing the max torque available for the given airflow, if it was then the airflow would be much lower and this does impact throttle response as well as amplifies stalling potential.

Idle ignition advance closed loop in many ECUs is only the P term out of the PID loop, while some ECUs like the BCS LPC incorporates all 3 aspects of the PID loop.

With proper idle control

• Cammed engines can be made smooth
• Smooth engines can be made lumpy

In addition, idle control valves can and have been used in antilag strategies where the closed throttle is bypassed and enough air is bypassed to keep the turbo spinning on the overrun. Although they are usually not large enough to provide highly aggressive antilag.

The general process of tuning an idle control system is to keep idle ignition control to an absolute minimum and adjust the open loop coolant temperature-based table first. When that is complete, idle ignition control is adjusted to maintain smooth and accurate idle speed. Note that it is possible to have too much airflow and so you´ll end up running negative ignition advance. Aim to have the ignition advance around the same as a the stock engine would have or in the middle of the below ranges.

• 2 valve big bore engines - 20-22°
• 2 valve medium bore engines - 17-19°
• 2 valve small bore engines - 13-16°
• 4 valve engines - 7-15°

All the values above are for general guidance only.

The ideal scenario for idle ignition control is that each degree of adjustment in both directions takes away/adds the same amount of torque, this will make it much easier to tune without elaborate adjustment curves. One thing to mention is if you seem to have very high ignition advance it suggests that the airflow is not enough, also this means that you may be at the MBT for this load and so if the idle speed drops there is no reserve torque available from the airflow to counter the effect of stalling.

External resources

VEMS Forum post by Cliff - Very comprehensive and detailed - http://www.vemssupport.com/forum/index.php/topic,97.0.html

Horsepower Academy Youtube channel - tons and tons of videos, use the search function to find what you want to learn about - https://www.youtube.com/c/learntotune/videos

Last change 08.04.2024, 08:52:54

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