Theory: Modes of Operation


What makes engine management tricky is that even the best theoretical models fail to accurately represent physical behavior in certain situations.  All engine controllers would use the same logic and procedures for running the engine 100% of the time if there was a perfect engine control strategy.  Instead, most engine management schemes incorporate several different modes of operation in which different sensors dictate fuel and timing requirements.  Also, engine controllers have specific logic dictating when to switch between different modes of operation based on different demands from the driver and different engine conditions.  Some of the most common mistakes made by people starting out (hell, even experienced tuners too) are changing some of the “main” functions in order to try to fix a problem that is being caused by a secondary table or the computer operating outside its normal mode(s).  Better understanding of the various modes of operation will help pinpoint what needs to be changed in a tune.

It would probably be a good idea for you to have read the other articles about Injectors, Speed-Density, Mass Air Flow, and Alpha-N before reading the rest of this.

Some basic vocabulary:

  • ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
  • RPM : Revolutions Per Minute – how fast the motor is spinning
  • MAP : Manifold Absolute Pressure – (usually) the pressure of air entering the motor
  • ECT : Engine Coolant Temperature sensor – sensor used to measure the temperature of coolant circulating through a motor.  Sometimes called different things by different manufacturers.  I will use ECT here
  • IAT : Intake Air Temperature sensor – sensor used to measure the temperature of air entering the motor.  Sometimes called different things by different manufacturers – I will use IAT here.
  • MAF : Usually used as a shorthand for Mass Air Flow Sensor / Meter
  • MAP : Manifold Absolute Pressure Sensor – a sensor that measures the pressure of air in the intake manifold
  • Idle Valve : A electromechanical valve controlled by the ECM that allows air into the engine in order to control engine speed.
  • Displacement : the volume swept by a piston descending from the top to the bottom of the cylinder bore.  More here.
  • AFR : Air Fuel Ratio – the ratio of air to fuel present in a combustible mixture.  Usually stated as a ratio, i.e. 14.7:1 for the stoichiometric AFR for gasoline.  Stoichiometric AFR varies from fuel to fuel.
  • Lambda : similar to AFR, except usually expressed as a number where 1.0 represents a stoichiometric mixture for all fuels.  Lambda and AFR are the same concept expressed in different units.
  • Stoichiometric : a mixture containing the precise amount of oxidants required for complete combustion of all fuel present.  See here or here for more information on chemistry involved.
  • Injector : a special type of solenoid that allows fuel to flow through it when energized (more)
  • Pulsewidth : the length of time the engine computer applies electricity to the injector, or how long the injector is commanded to be open
  • Flow Rate : The amount of fuel an injector flows once open.  These values are typically given in units of cc/min or lbs/hr at a specified fuel pressure. (injector flow rate varies with the square root of fuel pressure.)
  • Latency : the length of time after the injector is turned on before it achieves its linear flow rate.

Goals of Engine Management

Although the answer is somewhat obvious (“make the engine run as well as it can”) it is worth a closer look at what engine management systems try to achieve and why.  Operating optimally normally means one of several things:

  1. Making the most power possible without engine damage happening
  2. Consuming as little fuel as possible in order to make a specific power output (maximizing efficiency)
  3. Minimizing emissions

Most of the time engine management systems aim for more than one of these at once, i.e. Fuel efficiency while minimizing emissions or power and efficiency.  Generally, you cannot have your cake and eat it too when it comes to engine management because the physical conditions required to achieve optimal fuel economy are vastly different than those required to achieve optimal power production.  Minimizing emissions frequently conflicts with BOTH power and economy!

So how do engine management systems deal with the conflicting requirements of economy, emissions and power?  The answer is the title of this section – engine management systems switch between different modes of operation based on input from the driver, measurements from sensors and how they are programmed from the factory.  Sometimes in the course of tuning it is necessary to change not only configuration parameters of an ECM but also how it switches from one mode of operation to another.

Common Modes of Operation

Different ECMs will have different modes of operation and different rules for switching among them.  Many modes of operation exist to service requirements common to all engines, leading to many modes of operation being shared between different engine management implementations:

  • Cranking: This is the first task for an ECM – help an engine transition from being spun by the starter to spinning on its own propelled by combustion.  This might not sound like a very difficult task, but there is a LOT involved!  While cranking, cam and crank sensors need to be monitored so the ECM can determine how fast the motor is spinning and what angle the crankshaft is at in order to provide accurate ignition timing, injectors have to be fired in order to deliver enough fuel to get the engine moving, the ignition system has to deliver sparks at an opportune time to ignite the mixture, (in some cases) the idle valve needs to be opened to allow enough air into the motor to get it running on its own…  And more sometimes!  Combine this with (typically) the lowest operating voltages because the alternator is not providing electrical energy and you have a potentially tricky situation.
  • Startup: Once the engine is spinning under its own power, the fun can start.  There are often special rules that change the behavior of the ECU immediately after the engine starts.  Idling higher to prevent stalling is a common task in startup mode.  Many engines add additional fuel and change timing in order to try to help the engine warm up to desired operating temperature faster.
  • Open Loop: This is a critical mode for the overall operation of the engine.  Open-loop mode is the mode used most often for performance, but it is important all the time.  Open loop operation uses a control strategy like MAF, Speed-Density or Alpha-N to determine fueling and ignition parameters to use to run the engine.  If tuning parameters related to open loop are incorrect, the motor will never run optimally.
  • Closed Loop: This is an important mode for fuel economy and emissions.  In Closed Loop mode, the fueling and ignition values from Open loop are adjusted using feedback from additional sensors (usually Oxygen sensors).  Small imperfections in a tune can be corrected in closed loop, letting the ECU maintain much closer control over operating conditions than is possible with open loop alone.  There are usually limits to how large changes can be made by closed loop, which can lead to diagnostic error codes.  (Too Lean / Too Rich / O2 sensor)
  • Power Enrichment: (aka “PE” mode) This is a subset of Open Loop operation where engine conditions such as AFR and ignition timing are adjusted with the goal of maximizing power.  Frequently, TPS readings close to wide open throttle serve as a trigger for PE mode.
  • Tip-in: Sudden changes are a problem for all control strategies.  When the TPS sensor indicates the throttle has changed quickly enough, the ECU can enter Tip-in mode where
  • Decel Fuel Cut Off (DFCO): When you take your foot off the gas, many ECMs will shut off fuel injectors in order to decrease fuel consumption and help promote engine braking.
  • Dashpot: Many ECUs implement some form of digital dashpot using the Idle Valve.  The idea here is to prevent stalling when the throttle plate closes suddenly by opening the idle valve enough to gradually bring the engine to idle.
  • Idle: At idle, the ECU tries to maintain engine speed while little or no load is placed on the engine.  Idle is often one of the trickiest states to control well.  Usually a mixture of airflow control (via Idle Valve or Drive-by-Wire), spark control and fuel control is used.  Strategies for controlling idle vary immensely among manufacturers.
  • Limiting/Protection: Engines have limits – how fast they can safely spin, how much boost they can handle, how fast the car can safely travel.  Part of the ECM’s job is to monitor engine conditions and take measures before damage occurs.  Frequently, spark or fuel will be cut off until engine conditions return below a pre-set limit.

More to come later on this topic…