Cobb Custom Features: GT-R Engine Safety Features




GT-R Engine Safety Features 
 
 

4/25/2016



GT-R Fuel Pressure & Safety Features


Overview

The Fuel Pressure monitoring and Safety features are two independently enabled and adjustable features. They can be used in conjunction with one another, as well as the Fuel Pressure Differential system, or if an external fuel pressure sensor is not equipped, you can just add in an engine safety.

Fuel Pressure

This provides the ability to utilize one of the pre-throttle TIP (Throttle Inlet Pressure) sensors for a 0-5V input. This input can be calibrated by the tuner to be used as a Fuel Pressure value within the ECU. This value can then also be data logged.

Safety

This feature provides logic capable of reducing the torque request when a specific set of calibrations for conditions have been exceeded. This allows the tuner to define the safe operating conditions for the engine, and to instantly reduce torque (engine load) if those conditions are not met. The condition will be reset when the driver returns the Accelerator Pedal Position (APP) to below 5%.

It is important to remember these two features can operate completely independently of one another. One does not need to have the Safety feature enabled in order to have Fuel Pressure as a data monitor within the ECU. Likewise, one does not need to have the Fuel Pressure feature enabled in order to use the Safety feature to protect the engine.

However, if you want Fuel Pressure to be a safety condition to protect the engine, then BOTH Fuel Pressure and Safety features need to be enabled.




Fuel Pressure Feature Details

The GT-R has two TIP (Throttle Inlet Pressure) sensors before the throttle body, one for each bank of cylinders. We refer to their bank locations as either Left Hand (LHD) or Right Hand (RHD). For reference, the Left Hand sensor would be on the driver's side of a North American GT-R.

In order to add a new input to the ECU for fuel pressure reading, we re-purpose one of these MAP sensor inputs to become a Fuel Pressure input. Either side may be used, but the ECU must be calibrated accordingly so it knows what side to use as Fuel Pressure. The side that is still connected to the MAP sensor will continue to be utilized as normal, and the ECU is automatically configured to use the single bank input for both banks internally. For example, if you configure the ECU to use the LH input for fuel pressure, when you log the pre-throttle boost pressure you will see the same pressure on both banks as measured by the RH MAP sensor – not a value generated by the fuel pressure sensor.

In order to calibrate the Fuel Pressure sensor input, the same method as a MAP sensor is used – a scalar (multiplier) and offset. The calibration scale for fuel pressure sensor input is approximately 10 bar.
Please note there is currently no additional logic that will alter the engine's calibration based on fuel pressure value. Currently the only fuel pressure related logic within the ECU is the ability to immediately reduce torque if fuel pressure drops below a calibrated threshold (requires that the Safety Feature is enabled).

A new data logging parameter is added with this feature, called Fuel Pressure. If the feature is enabled, the value will show in absolute pressure units (PSI, KPA). If the feature is not enabled, this value will display as 0.




Fuel Pressure Tables

Fuel Pressure Calibration Tables

 


Fuel Pressure Sensor Enable

Table: (Fuel Pressure – Sensor Enable)

This is used to enable the Fuel Pressure feature allowing you to view fuel pressure as an engine monitor. A value of 1 will enable the function and activate the associated Fuel Pressure tables and logic. A value of 0 will disable the function.





Fuel Pressure Input – RH or LH

Table: (Fuel Pressure – Use Left or Right Boost)

This option allows the tuner to select which pre-throttle MAP sensor is used for the Fuel Pressure sensor (input). When a value of 0 is entered, the ECU will use the Left Hand (LHD) sensor as input for Fuel Pressure. Set to a value of 1, the ECU will use the Right Hand (RHD) sensor as input for Fuel Pressure. In either case, the ECU will automatically handle the fact that only a single sensor is used for MAP input so no additional changes need to be made by the tuner.





Fuel Pressure Sensor – Gradient

Table: (Fuel Pressure - Gradient)

The scalar used to calibrate the fuel pressure sensor. The conversion from sensor input voltage to pressure units. This works using the same principle as used for MAP sensors.


Calculating Fuel Pressure Gradient

Figuring out your Fuel Pressure Sensor’s Gradient is a pretty simple task.  First you need to determine the operating voltage range for the sensor.  While you would expect sensors to go from 0v at the bottom, most will actually start at a higher voltage like .5v in order for the ECU to see a minimum voltage level and be able to detect faults like wiring or sensor issues.  For the sake of making things easy we’ll use some pretty standard voltage levels and say our sensor operates from 0.5v-4.5v. 

MAKE SURE TO USE THE APPROPRIATE VALUES FOR THE SENSOR YOU USE.

Range = Highest Output Voltage – Lowest Output Voltage

Once we know the range of the operation we need to see how wide it is, so you’ll subtract the high pressure voltage from the low pressure voltage, so in our example sweep of 0.5v-4.5v we have an overall range of 4 volts.

Next we need to convert our pressure from PSI into KPA so our ECU knows how to handle the values.  Many common sensors operate at 100psi or 150psi but this would apply for any operating pressures.  For the sake of keeping things easy we’ll assume a 100psi sensor is being used.  You can use any sort of conversion on the web, the table below, or just multiply the pressure in PSI by 6.89476 in order to convert into KPA (Kilopascals) so at 100psi we have 689.476kpa.

Now that we have our pressure in KPA and our voltage range, we simply divide the pressure range by the voltage range to get our gradient. So

Gradient: Pressure (kpa) / Range (Voltage) =Gradient in KPA/V

Or in our example: 689.476KPA / 4v = 172.369 KPA/V




Fuel Pressure Sensor – Offset

Table: (Fuel Pressure – Sensor offset)

This table is used to calibrate the fuel pressure sensor. This works using the same principle as used for MAP sensors.


Calculating Fuel Pressure Offset

Since most pressure sensors do not read zero at 0.5v we need to add an offset into the equation as well. 

Much like setting a scale to 0 before you step on.  Since many sensors like our example one will show voltage at what should be no pressure (For example our sensor is reading 0.5v at 0 KPA) we need to tell the computer to offset in order to read appropriately.  We do that by finding out what the KPA reading would be at the lowest output voltage, and adding the opposite (in many cases negative) of that value.  We do that by multiplying the lowest voltage output by the gradient

Offset: Low End Voltage Output x Gradient = Offset Value

So in our 0.5v-4.5v sensor example it would be

0.5v x 172.369 KPA/V = 86.1845 KPA

Once it is in the table, the software uses that value to zero things out at 0.5v, that way the computer can offset correctly, (so it becomes -86.1845KPA) to make sure that

0.5 v = 0 KPA



Fuel Pressure Sensor – Minimum Voltage

Table: (Fuel Pressure – Min. Sensor Voltage)

This table is used to calibrate the minimum acceptable voltage reading at the pressure sensor. If fuel pressure sensor voltage drops below this threshold, engine torque will be immediately reduced.





Fuel Pressure Sensor – Maximum Voltage

Table: (Fuel Pressure – Max. Sensor Voltage)

This table is used to calibrate the maximum acceptable voltage reading at the pressure sensor. If fuel pressure sensor voltage is above this threshold, engine torque will be immediately reduced.





Safety - Min. Fuel Pressure

Table: (Engine Safety – Fuel Pressure Safety Minimum)

Only active if Safety Feature Enabled

This table is used to protect the engine in the event the fuel pressure drops below a calibrated minimum threshold. The minimum fuel pressure is based on boost pressure. If fuel pressures drop below this threshold, engine torque may be immediately reduced.





Safety Feature Details

In an effort to prevent engine damage, the safety feature uses multiple conditions that if met can immediately reduce engine load. The requested engine torque is reduced by modifying the accelerator pedal position (APP) input to the throttle table lookups, effectively reducing engine load.

All the threshold tables listed in the Safety section are active when the Safety Feature is enabled.

NOTE: If a safety feature is not wanted, the corresponding table data should be set to values that cannot be exceeded.

After a safety condition has been met (Engine Protect Mode), the system will return to normal operation immediately once conditions have returned below the threshold(s) AND the user has returned the accelerator pedal to the re-enable threshold (lift their foot off the accelerator pedal). If the condition exceeds a safety threshold after returning to normal, it will once again enter the torque reduction mode (Engine Protect Mode).




Safety Feature Calibration tables


Safety Enable

Table: (Engine Safety – Master Enable)

This is the engine safety master switch used to enable the global safety feature set. A value of 1 will enable the function. A value of 0 will disable the function. This enables the STFT, Knock, Coolant Temp., and Oil Temp. safety features.




Safety Status Clear – Throttle Position

Table: (Engine safety – Min APP Safety Limp Clear)

This is the minimum throttle position (APP) that must be met (less than this value) in order for the safety feature to clear a failure condition. Once the fault state drops below the error threshold and APP is returned below its threshold, the engine will run as normal assuming no other safety conditions exceeded.




Safety Activate – Min RPM

Table: (Engine Safety – Minimum RPM)

This is the minimum engine RPM that must be exceeded (higher than) in order for the safety feature to be active. This value is used in conjunction with the Min. load threshold. Both must be met for the safety feature to be active. If engine RPM or load is below this threshold, the torque reduction engine protection will NOT occur even if a threshold of the Safety tables has been exceeded.




Safety Activate – Min Load

Table: (Engine safety – Minimum Load)

This is the minimum engine Load that must be exceeded (higher than) in order for the safety feature to be active. This value is used in conjunction with the Min. RPM threshold. Both must be met for the safety feature to be active. If engine RPM or load is below this threshold, the torque reduction engine protection will NOT occur even if a threshold of the Safety tables has been exceeded.




Knock – Max Thresholds

Table: (Engine Safety – Max Knock)

The maximum amount of knock allowed based on boost pressure. If knock exceeds the value referenced in this table, the Engine Protect mode is entered. Once knock conditions are reduced and the APP is returned below its threshold, the engine will run as normal assuming no other safety conditions have been exceeded.
(DBA Ignition strategy example shown)

 




Oil Temp – Max Thresholds

Table: (Engine Safety – Max Oil Temp.)

The maximum amount of oil temperature allowed based on engine load. If the oil temperature exceeds the value referenced in this table, the Engine Protect mode is entered. Once oil temperatures fall below the threshold and APP is returned below its threshold, the engine will run as normal assuming no other safety conditions exceeded.





Coolant Temp – Max Thresholds

Table: (Engine Safety – Max Coolant Temp.)


The maximum amount of engine coolant temperature allowed based on engine load. If the coolant temperature exceeds the value referenced in this table, the Engine Protect mode is entered. Once coolant temperatures fall below the threshold and APP is returned below its threshold, the engine will run as normal assuming no other safety conditions exceeded.





STFT – Max Thresholds

Table: (Engine Safety – Max STFT)


The maximum amount of short term fuel trim allowed based on boost pressure. If the ECU is attempting to positively correct (add fuel) above the thresholds in this table, the engine protect mode is entered. Once the short term fuel trims drop below the threshold and APP is returned below its threshold, the engine will run as normal assuming no other safety conditions exceeded. This function checks both banks for a failure.





Max Accelerator Pedal Pos.

Table: (Engine Safety – Max App. limit)

The maximum allowed Accel. pedal position (APP), based on RPM, used to reference the throttle tables while in Engine Protect Mode. This will effectively reduce the requested engine torque. Even if the driver maintains full throttle during Engine Protect Mode, the actual Accel. pedal Position used by the ECU will not exceed the values in this table.

Tuning Tips: High horsepower vehicles will require low Pedal Position values (less than 10%) to reduce power effectively in the event of a failure.





Data Monitors

Accel. Pedal Pos. - Displays the position of the vehicles accelerator pedal in a percentage value. (95% is equal to WOT)

Accel. Pedal Pos. Hijack (%) - Displays the percentage of commanded accelerator pedal in a percentage value when in a failure state. This value is derived from the Table: (Engine Safety – Max App. limit). This value will match Monitor: Accel Pedal Pos. when not in a failure state.

Fuel PressureWhen Table: (Fuel Pressure – Sensor Enable) is active (value set to 1), the vehicle is equipped with an aftermarket fuel pressure sensor (wired to the appropriate pre throttle MAP sensor) and the sensor calibration data has been correctly populated this will provide a fuel pressure reading in PSI or BAR (depending on your display options settings). 

Limp Status (Safety) This is currently a compound monitor, allowing you to see all failure state causes in a single monitor.

  • 00 = OK
  • 01 = Fuel Pressure Failure
  • 02 = STFT Failure
  • 04 = Oil Temp Failure
  • 08 = Coolant Temp Failure
  • 16 = Knock Failure

Examples:

  • A failure of the fuel pressure system and knock at the same time would produce a value of 17 (1+16)
  • A failure of the cooling system temp and knock at the same time would produce a value of 24 (8+16)
  • A failure of the STFT and oil pressure at the same time would produce a value of 34 (32+2)

Enabled SafetyThis monitor will display 1 when the system is active (All enables set to 1) and conditional thresholds have been met (Table: (Engine Safety – Minimum RPM) and Table: (Engine safety – Minimum Load)). If either condition is not met, the value will remain 0. A value of 1 does not signify a failure; simply the system is active and monitoring conditions.