×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Temperature rise a motor after shutdown

Temperature rise a motor after shutdown

Temperature rise a motor after shutdown

(OP)
In a forum I'm a part of there were a couple of people saying it would be okay to drive without a radiator fan in traffic as long as you turn the engine off at each light to keep it from overheating.  While I understand their intuitive thoughts on this I've known engines to actually spike in temperature a bit after being turned off.  Even though you're not generating any more heat through combustion, you've also lost the forced convective cooling of the coolant system.  Since the heat that has already been generated has no where to go it ends up increasing the core temperature of the motor itself.  I've witnessed this both in a car that I had many years ago that I drove for awhile without a radiator fan and in an electric motor that I did tested with thermocouples under load.  

I've been trying to find more data about (and perhaps a name for) this phenomenon and since I've been having difficulty finding any type of source I wanted to confirm I'm not crazy as well. :)  Thanks for the help!

RE: Temperature rise a motor after shutdown

What you see is true - no crazyness detected smile

There is no specific phrase which I know of but try googling things like "Thermal Lag", "Thermal Over-run", "Thermal Peaking". There are likely some industry-specific terms.

This phenomenon is why car radiator fans keep running for a few minutes in hot weather after a hard drive when you switch off , get out and walk away. They are designed to do it - I noticed French cars are very keen on doing this.

gwolf

RE: Temperature rise a motor after shutdown

The core temperature does not increase after engine shutdown. The hotest parts of the engine are the cylinder heads and pistons and they are hottest while the engine is running. These are much hotter than boiling water. When you shut the engine off you lose water circulation and the water in contact with the hot engine parts will get considerably hotter than when there was circulation. This water will boil and push water out of the radiator cap.

If you turn the heater on in the passenger compartment that will usually prevent overheating of an idling engine.

RE: Temperature rise a motor after shutdown

Yes, but the temperature increase you see is registered by whatever temperature gage is installed - and these are almost universally WATER temperature, not block temperature.

For a water cooled engine to work the water temperature inside the block - at steady-state operating conditions - must be lower than the jacket temperature.  It it wasn't there wouldn't be any heat transfer.

The water gets lower by passing through the water-to-air heat exchanger commonly referred to as a "radiator".

When you shut the engine off you also shut the water pump off.

Heat from the jacket still transfers into the water, but because the water is no longer circulating it can't reject the the heat, so it has to get hotter.  The block meanwhile is getting cooler.

Eventually the water and block temperatures equillibrate, and the both will start to fall.

The total enthalpy in the engine/coolant system can only decrease after the the engine is shut down, but the temperatures of different parts of the system may increase.

RE: Temperature rise a motor after shutdown

The heat from the (usually heavy cast iron) exhaust manifolds is usually the problem on shutdown, especially after high -load operation.  This heat soaks back into the cylinder head and, depending on relative thermal inertias, could heat the head (or parts of it) above its usual operating temperature (this is also what cokes the oil in your turbo bearings).  It can also boil the coolant locally, causing hot spots and warping of components.

I had a Honda with a faulty fan switch and I drove it around for quite a while before I realized it was broken because I don't idle my vehicles long.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources