Claims:We claim:
1. A method to perform plausibility check of a coolant temperature sensor used with an engine , said method comprising the steps:
- recording initial temperature of coolant temperature as read from said coolant temperature sensor
- starting a first timer when said engine reaches a first operating mode and running the first timer as long as the engine is in first operating mode
- starting a second timer when said engine enters a second operating mode and running the second timer as long as the engine is in second operating mode
- reading the coolant temperature
- checking if first timer count has crossed a first pre-defined value
- checking if the second timer count is below a second pre-defined value
- determining whether the read coolant temperature is erroneous depending upon the first timer count, the second timer count and the coolant temperature

2. A method according to claim 1, wherein said first operating mode is where predefined engine parameters are above or equal to predefined values
3. A method according to claim 1, wherein said second operating mode is where predefined engine parameters are below predefined values
4. A method to perform plausibility check of a coolant temperature sensor used with an engine, said method comprising the steps:
- recording the coolant temperature Tstrt at engine start
- starting a third timer when said engine reaches a first operating mode and running the third timer as long as the engine is in first operating mode
- reading the coolant temperature Tactual once the third timer count reaches a third pre-defined value
- determining whether the read coolant temperature is erroneous depending upon the coolant temperature Tstrt and coolant temperature Tact
5. A controller to perform plausibility check on a coolant temperature sensor, said controller comprising:
- An input interface to read coolant temperature
- A first timer to measure time of engine being in a first operating mode
- A second timer to measure time of engine being in a second operating mode
- An output means to output an indication when read coolant temperature is erroneous, in dependence of the first timer count and the second timer count

6. A controller according to claim 5, said controller further comprising memory, input and output devices.
7. A controller according to claim 5, wherein said input interface comprises input ports

8. A controller according to claim 5, wherein said output means sets error code in memory to indicate faulty coolant temperature sensor

9. A plausibility checking system for an engine, said plausibility checking system comprising :
- a coolant temperature sensor located in a passage of said coolant
- a controller adapted to detect engine firing mode and engine cooling mode; said controller further adapted to read coolant temperature and determine whether the coolant temperature is valid in dependence of the engine mode and the duration of the engine mode
, Description:Field of the invention:
[0001] This invention relates to the field of plausibility check for coolant temperature sensors.

Background of the invention:
[0002] In the vehicles the control units continuously monitor the engine coolant temperature using a temperature sensor. The coolant temperature value is used by the control units to control the operations of the engine. As the coolant temperature decides engine operating parameters, the coolant temperature sensor needs to be providing accurate reading. It is required that the reading of the temperature sensor is checked for plausibility at least once in a driving cycle.
[0003] The prior art US 6200021 B1 discloses an apparatus to detect abnormal behavior in a coolant temperature sensor. After the engine is started, the control unit calculates, over time, an engine heat generation quantity, an estimated coolant temperature on the basis of various engine operating parameters. After the elapse of a predetermined time, the control unit compares the estimated coolant temperature and the coolant temperature actually detected by the temperature sensor. Based on the relationship between the two, the control unit determines whether there is an abnormality in the engine temperature sensor

Brief description of the accompanying drawings:
[0004] An embodiment of the disclosure is described with reference to the following accompanying drawing,
[0005] Fig. 1 illustrates method 1 to perform plausibility check on a coolant temperature sensor
[0006] Fig. 2 illustrates method 2 to perform plausibility check on a coolant temperature sensor

Detailed description of the embodiments:
[0007] Typically any engine cooling system comprises an enclosure or a passage around the engine through which the coolant is circulated. The coolant helps in taking away some of the heat generated by the engine. A temperature sensor is located in the passage in the coolant to measure the coolant temperature. The temperature sensor provides coolant temperature value to the control unit. The control unit continuously monitors the coolant temperature and uses the same to control the engine operations. The coolant temperature needs to be checked for plausibility at least once in a driving cycle. The driving cycle is defined as the time between the cranking of the engine and the engine is shut off. The shutting off of the engine indicates that the engine is not running. The terms coolant temperature and coolant temperature value refer to the reading of the temperature sensor and these terms may be used interchangeably. The coolant temperature sensor may be referred just as sensor or as temperature sensor in this document.
[0008] There are two methods to perform the plausibility check. Shown in fig. 1 is a method to perform plausibility check according to method 1.
[0009] Method 1:
[0010] In step S1, the present coolant temperature Tinitial is read and stored. The expected coolant temperature Texp is set. The Texp may be predefined and may be a fixed value. Based on the Tinitial and Texp, the timer 1 and timer 2 are initialized.
[0011] In step 2, the engine operating parameters are monitored to check whether the engine entered into a first operating mode. The first operating mode is also referred as engine firing mode. The engine firing mode is defined where certain engine operating parameters have reached predefined values. Some of the engine operating parameters may be engine RPM, engine torque, fuel injection quantity etc. These parameters are pre-stored in a controller which is performing the plausibility check. The controller monitors the engine parameters to check whether the engine reached engine firing mode.
[0012] In step 3, once the engine firing mode is entered by the engine, a first counter is started or resumed if it was frozen earlier. The first counter is also referred as Timer 1. The first timer keeps counting till the engine is in firing mode. Once the engine goes out of firing mode, the timer 1 is frozen. When the engine again enters the firing mode, the timer 1 starts running from where it was frozen. When the engine enters firing mode, the timer 2 is frozen. By frozen, it means the count is held at the present value and may restart from the held count, later in time.
[0013] In step 4, the controller waits till the engine enters the cooling mode. The engine cooling mode is the one where the engine is not in firing mode, i.e. the engine parameters are below the predefined values which are defined for firing mode with additional engine parameters like vehicle speed, accelerator pedal position and the like.
[0014] In step 5, once the engine cooling mode is detected, the timer 1 is frozen and timer 2 is started or resumed if frozen earlier.
[0015] In step 6, it is checked whether timer 1 exceeded a first pre-defined value. If yes, then step 7 is executed. If timer 1 has not exceeded, then step 2 is executed.
[0016] In step 7, when the timer 1 count exceeds the first pre-defined value, it is checked whether the timer 2 count is below a second predefined value. If the timer 2 count is not below a second predefined value, then the plausibility check is discarded. This is because, the engine has run for more time in cooling mode than the second predefined value. This will influence the coolant temperature and may lead to erroneous plausibility check. After discarding the plausibility check, the present coolant temperature is stored and the test is restarted with step1. If in step 7, the timer 2 value is below a second predefined value, then step 8 is executed.
[0017] In step 8, the actual coolant temperature Tact is read.
[0018] In step 9, the read coolant temperature Tact is compared against the expected coolant temperature Texp. If the read coolant temperature Tact is less than the expected coolant temperature Texp, then the read coolant temperature Tact is declared as erroneous. In this scenario, the coolant temperature sensor may be faulty and needs to be investigated further. If Tact is equal or greater than Texp, then the coolant temperature sensor is declared as functioning well and the Tact reading is considered valid.
[0019] Thus in method 1, using Timer 1, Timer 2 and the coolant temperatures sensor, the plausibility check on the temperature sensor is performed.
[0020] In method 1, the sensor reading is valid when the engine is in firing mode for a first pre-defined time or more and the engine is in cooling mode for a time lesser than a second pre-defined time. If the engine remains in cooling mode for more than the second pre-defined time and the duration of engine firing mode is less than the first pre-defined time, then the check is discarded and fresh plausibility check is started. This way one of the embodiments of the inventions, proposes a more accurate plausibility check.
[0021] Shown in fig. 2 is method 2 to perform plausibility check on the coolant temperature sensor.

[0022] Method 2:
[0023] Step 1, after the engine is started, the coolant temperature is read and stored as Tstrt.
[0024] Step 2, the engine operating parameters are monitored to check whether the engine has entered into firing mode. If the engine enters into firing mode, execute step 3.
[0025] Step 3, coolant temperature Tact is read.
[0026] Step 4, check whether Tact is equal or greater than Tstrt. If yes execute step 5. Else execute step 3.
[0027] Step 5: Start timer 3.
[0028] Step 6: Wait till timer 3 count exceeds a third pre-defined value.
[0029] Step 7, check whether the Tact is higher than Tstrt by a pre-defined threshold.
[0030] If Tact is higher than Tstrt by a pre-defined threshold, then the coolant temperature is valid, else the coolant temperature is erroneous. In this scenario, the coolant temperature sensor may be faulty and needs to be investigated further.
[0031] A controller or a control unit performs the plausibility check on the sensor. The controller typically comprises a microcontroller, input and output interfaces, timers etc. The controller may also include an output means either to set an error code in the memory once the sensor is determined as faulty. The output means may also generate a visual indication when the sensor reading is determined erroneous. All the timers explained above are part of the controller. All the predefined values and thresholds for timers and for engine operating modes like engine firing and engine cooling are stored in the controller.
[0032] The invention has the advantage that the plausibility check is more reliable as the sensor readings are monitored in engine firing mode as well as engine cooling mode before performing the plausibility check. This will increase the accuracy of the plausibility check.
[0033] The invention also proposes a plausibility checking system for an engine, the plausibility checking system comprising :a coolant temperature sensor located in a passage of said coolant;
[0034] a controller adapted to detect engine firing mode and engine cooling mode; said controller further adapted to read coolant temperature and determine whether the coolant temperature is valid in dependence of the engine mode and the duration of the engine mode. The controller checks the durations of the engine modes and determines the validity of the temperature read from sensor.