Megasquirt-3 MS3 Exhaust Gas Temperature (EGT)

Megasquirt 3 - Exhaust Gas Temperature (EGT)

Introduction

The EGT system in Megasquirt-3 allows inputs from EGT sensors to be datalogged by serial or SDcard. It optionally allows the engine to be shutdown if EGT exceeds a set temperature. If an EGT probe is fitted to each exhaust port it can be used in conjunction with per-cylinder fuel trim to balance cylinders to account for air flow and injector flow differences. EGT is used extensively in piston driven aircraft and much useful information can be found from aircraft literature and then applied to your non-aero engine.
(EGT differences between cylinders can be significant at idle, evidence shows that V8s often run cold on the four corner cylinders and hot on the inner four. One engine required over 10% fuel trim to balance these air-flow differences.)

Hardware

To use the EGT system an EGT-probe (usually a K-type thermocouple) is installed in the exhaust manifold. When installing an EGT per cylinder, it is typical to install ~50mm from the head.
EGT probe

The output from a thermocouple is a very small voltage and a thermocouple amplifier is required. Typical options for this are:
- an external thermocouple amplifier that provides a 0-5V analogue output that is then connected to one of the Megasquirt-3 analogue inputs.
- an external thermocouple amplifier box that connects to Megasquirt-3 via CAN.
- a DIY option that builds an amplifier into the Megasquirt-3. A sample circuit is shown here and here

Settings

No. EGT inputs - specify the number of channels you have connected
Calibration - the calibration depends on the thermocouple amplifier in use. Consult the documentation that came with it.
Data Capture - here you specify the local or remote analogue channels that the EGT inputs are connected to. For remote CAN inputs, see the CAN section for configuration.
The Actions section allows you to specify action if EGT gets too high.
Warn temperature - above this temperature the code will take action. The warning output selected on the AFR Safety system is turned on.
Add fuel - specifies a raw pulsewidth in milliseconds of additional fuel. This could be used to help protect the engine if EGT is too high.
Add to - Normally this is set to "Both". When using staged fuel outputs or dual table you may want to specify a particular channel.
Shutdown engine> - if you want the engine to be shutdown if EGT is too high.
Time > warn temp - if any of the EGT stays above the warn temperature for more that this time the engine is shutdown.
Max temperature - if any of the EGTs exceeds this temperature then the engine is shutdown immediately.
See the AFR Safety system for shutdown and resume settings.

The temperatures, pulsewidth and time presented are purely examples. It is very important that you review your datalogs and think carefully before specifying these settings.
With correct settings, EGT overtemp is likely to detect a failing injector causing a lean mixture on one cylinder or an overall problem such as a faulty pump or regulator causing everything to run lean.

If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray and Ken Culver.