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Simos 18

Revision 2

by llandowtuninguk@gmail.com
Add a sourced overview of Simos 18 torque modelling and engine control

This is a historical revision. Read the current page or return to its history.

Page organisation in this revision
Page type
ECU hub
Subject
ECUs
Parent
Simos
Applies to
No ECU-specific applicability
Tool groups
Not recorded in this older revision
Manufacturers
Not recorded in this older revision

What is Simos 18?

Simos 18 is a family of petrol engine management systems used in some Volkswagen Group applications. As one documented example, Volkswagen identifies Simos 18.1 on the EA888 2.0 TSI CHHB and CHHA engines, with an electronic throttle and a combination of direct and intake-port injection. Other applications and markets can have different hardware. The variant name alone does not identify an exact calibration.

This is a basic orientation to the torque-oriented control described in EcuTek's EA888 Simos 18 guide. It is not a function diagram for every Simos 18 version; map names, units, limits and available functions depend on the particular software and engine.

From pedal request to engine output

  1. Request: The ECU interprets accelerator position and operating conditions as a driver torque request. Other vehicle functions can also request or reduce torque.
  2. Coordinate and limit: Applicable requests, limits and corrections determine the torque the ECU is allowed to produce. A request, an applicable limit and an estimated delivered value are different signals.
  3. Plan the air charge: The calibrated torque and air model relates the desired output to the air charge required at that operating point. The ECU uses air-path targets and controls the available throttle, turbocharger and camshaft actuators to pursue it.
  4. Control combustion: The ECU meters fuel for the air and lambda target, controls ignition and applies any relevant corrections or interventions. The result depends on what the engine and air system actually achieve.

This is a simplified explanation. Limits, corrections and feedback can act in parallel or at different stages, rather than following one fixed sequence.

What is the torque model?

The torque model is the ECU's calibrated estimate of the relationship between engine output, air charge and operating conditions. It is not a direct measurement of crankshaft torque. In EcuTek's EA888 example, a reference air-charge relationship uses an indicated torque target; frictional losses and the effects of ignition and lambda form part of the relationship between torque at different points in the model.

Indicated torque describes the contribution from combustion before mechanical losses. A value described as engine output or clutch torque takes account of losses in a different way. Check the definition of each logged or communicated value: requested, limited, estimated and measured torque need not agree. Gearbox and other vehicle systems may rely on the ECU's torque information, so changing a model simply to make a reported number smaller can make that information less representative.

Air charge, boost and the two control paths

Air charge is the air entering a cylinder for a combustion cycle. It may be shown in milligrams per stroke (mg/stroke). That unit describes air here, not the fuel injection quantity also expressed in mg/stroke on some diesels such as EDC16. Air charge is related to pressure, temperature, engine speed and valve operation; boost pressure alone does not tell the whole story.

EcuTek describes a slow path for air charge, including throttle, turbocharger actuator and camshaft targets, and a fast path including ignition, lambda/fuel control and fuel cut. This distinction helps explain why torque can be reduced quickly while the air system takes longer to respond. It does not mean the paths work independently.

When comparing logs, distinguish requested from actual or estimated air charge, and manifold pressure from pressure upstream of the throttle. Check whether a pressure is absolute or gauge before comparing values.

Fuel, lambda and ignition

Lambda expresses the actual air-to-fuel ratio relative to the chemically balanced, or stoichiometric, ratio for the fuel: 1 is stoichiometric, below 1 is richer and above 1 is leaner. The ECU uses the air estimate and lambda target to control fuelling, with feedback and corrections as applicable. Direct injection, additional port injection and fuel-pressure hardware depend on the application; SSP 522's dual-injection engine is an example, not a guarantee for the whole family.

Ignition timing affects torque and combustion. Knock control and temperature-related corrections can alter the final timing or operating target. A requested value is not proof that the engine achieved it, and a timing correction needs to be understood in the context of fuel, temperature and load.

Limits and protection

Torque, air-charge and pressure limits can constrain the request. Other interventions may depend on temperature, turbocharger conditions, transmission requirements, engine speed or a fault. Which limit is active depends on the software and conditions. Increasing one limit does not by itself ensure the requested air charge or torque is reached; removing a protection can hide a problem or expose components to excessive load.

A practical starting point

  1. Record the vehicle, engine, injection equipment, ECU variant, hardware number and exact software and calibration identifiers. Save a recoverable original read and establish a healthy baseline.
  2. Match any definition to that software. Verify map functions, axes, units and scaling; do not assume a familiar label or address transfers to another version.
  3. Where supported, compare requested and limited torque with the ECU's estimated torque; requested and actual or estimated air charge; manifold and upstream-throttle pressure; lambda target and actual; ignition timing and knock correction; fuel-pressure target and actual; and relevant temperatures. Identify which limit or intervention is active before changing a calibration.

Channel names and availability vary by tool and application. COBB's monitor list gives examples for selected MQB vehicles, not a guarantee that all Simos 18 variants expose the same data.

Family scope

Alientech lists 18.1, 18.2, 18.4, 18.6, 18.7 and 18.10 in its DataLogger coverage. VW_Flash documents a narrower custom-flashing scope.

Start here

Open a variant below, then read Simos 18 tool and file scope. A coverage entry describes that product's stated capability; read the relevant procedure before using a tool.

Sources

Curated reference notes. Applicability follows the cited sources; hardware testing by this wiki is not implied.