Osama,

The response of an externally mounted sensor rather depends on the mass of the engine block and whatever it is made of (steel, Al, Mg, etc).

I *can* imagine that a pair (or more) of temperature sensors, buried at different depths from the pistons, would be able to detect anomalously high temperature gradients associated with pre-failure piston wear.

One might combine these data with acoustic data arising from under-lubricated sliding.

Osama

The answer to this is dependant on many variables, including such things as the type and extent of the failure.

The principal factors determining the response of the sensors are the thermal conductivity of the engine material and the distance between the heat source and the sensor.

In a modern engine, with a silicon temperature sensor mounted within 2 centimetres of a coolant chamber on an aluminium engine then the response time can be seconds.

However all the other factors in engine design, including the fact that most engines are designed for gradual failure in the event of a problem may mean that the engine will not overheat until some time (maybe hours) after the cause of the failure.

As an example - some 4 years ago I drove a GM / Vauxhall car for over a 50km following a hose rupture, albeit at a slow speed.

Thanks gentlemen for your elaborated response, may I extend the question as follows......

If you are going the equip your car with an "audible engine overheating alarm" (a must-ware that manufacturer for some good or bad reason has decided to limit it to a stupid "silent" dial or gauge on the car's dashboard)....what is the best location to mount the external temperature sensor so that the response time of heat sensing is fast enough for alarming purpose?

Osama

2 points.

1 The 'best' location is exactly where the engine manufacturer puts the existing temperature sensor. Adding a new feature to an existing engine is quite a difficult activity. All the key locations are in heavy use.. Perhaps you may consider using the output from the existing sensor to feed your amplification / alarm unit. Otherwise, on the external casing, close to the top of a piston and as close as possible to a coolant chamber.

2 If you are more concerned with providing a warning about coolant system failure as early as possible, then a heat sensor is not the best choice. As modern cooling systems are pressurised to achieve high efficiencies, then a pressure sensor will advise at the instant a failure occurs, long before there is any chance of damage to the engine. This may be a better opportunity as you can fit a pressure sensor away from the engine, on any part of the coolant system.

Thanks Ian, appreciating your very focused response. The second point is really a good and valid one. Do you agree that a pressure sensor alone can't generate any response in case of severe coolant leakage?! Apart from this situation, yes a pressure sensor would act as an early alarm. The problem with pressure sensor is that you have to intervene the pressurized cooling system which is not an easy task. For temperature sensor, a flush mounting can be easily managed

Osama

Have a look at this article

https://www.autosportslabs.com/how-to-add-a-coolant-pressure-sensor-and-avoid-a-rookie-mistake/

(You will have to copy and paste that link into your browser - I can't seem to paste a hyperlink into this editor)

This isn't an academic article but it may answer some of your concerns.

There are actually many coolant pressure systems in use - industrial engines, heavy goods vehicles and most ships use them because of the inherent delays in using a heat sensor approach to a critical engine function. The pressure sensors range from about US$20 upwards and can be fitted by unclipping a hose and installing them in place.

Yes, I do agree that there is a level of intervention required but depending on your engine, this usually can be done on an upper hose when the engine is powered down - meaning you don't have to drain the system.

However if a temperature sensor is your preferred option then I agree that a surface mounted temperature gauge can be installed. You can get sensors that are bonded in place using a heat conductive cyanoacrylate and which can fit into very tight areas.

Remote, infra-red sensors monitoring a specific area of the engine can also work very well. I used a handheld IR meter/gauge to test this out on my car engine this morning and it was quite accurate and responsive as the car warmed from -2C to over +104C

Thanks Ian, your answers were really helpful