Do Diesels Run Cooler Than Gasoline-Power Cars and/or Hybrid-Electric Vehicles?

Okay, for those of you who helped me with my science fair project, daydreaming may occur.  For those who did not help me, listen up, and look.  The popular urban myth is that a diesel engine will run cooler than a gasoline engine and/or a hybrid-electric engine.  I had always been interested in this myth, and wanted to find it out for myself.  I decided to try and figure out if it was true.  It wasn’t.  How did I figure it out?  Google is not the answer.  A science fair project was the answer.  For me.  Read below to find out what my findings tell us!

The hypothesis states:  If a diesel engine idles at 9 degrees Celsius, then it runs cooler than hybrid-electric engines and/or gasoline-power engines.  

Some research has been done to see if this myth is true.  From what I found, in 1998, the EPA, CalTech, and UC San Diego all did similar (if not the same) experiments.  The EPA wanted to know if heat on an engine decreased fuel economy, and CalTech and UC San Diego wanted to know if electric motors ran cooler than diesel engines.

My materials list was relatively short, compared to the other projects.  It included:  

• Texas Instruments Thermocouples and Data Readers
• 2003 Chrysler Town & Country LXi AWD 3.8L V6
• 2003 GMC Sierra 2500 HD 4X4 6.6L Duramax Diesel
• 2013 Toyota Prius Plug-in 1.5L Hybrid-Electric
• Duct Tape

My procedure was a bit more complicated:  To start the experiment, the car had to be measured “cold” – before it had been turned on.  The thermocouples were then attached to the:  Radiator, engine block, and exhaust pipe.  All measurements were taken in degrees Celsius.  Once the measurements had been recorded, the outside temperature was taken (as a good base to compare against).  The car was then started and allowed to run for five minutes without the climate system running.  When the clock hit five minutes, the car was turned off, and the measurements were taken again.  The different temperatures were compared against one another.

The results are what I expected.  I must be a car guru, since I was able to predict this (or at least I read my research!!). Since I can’t draw a table on WordPress, I’ll have to go by car, okay?

1. 2013 Toyota Prius Plug-in:  Engine block cold:  17 degrees Celsius, Engine block heat after five minutes:  21.8 degrees Celsius, Radiator heat cold:  14 degrees Celsius, Radiator heat after five minutes:  13.9 degrees Celsius, Tailpipe heat cold:  15 degrees Celsius, Tailpipe heat after five minutes:  14.1 degrees Celsius, Temperature outside:  16 degrees Celsius.
2. 2003 Chrysler Town & Country*:  Engine block heat cold:  30 degrees Celsius, engine block heat after five minutes:  58 degrees Celsius, Radiator heat cold:  13 degrees Celsius, Radiator heat after five minutes:  14 degrees Celsius, Tailpipe heat cold:  18 degrees Celsius, Tailpipe heat after five minutes:  17 degrees Celsius, Temperature outside:  13 degrees Celsius.
3. 2003 GMC Sierra 2500HD:  Engine block heat cold:  9.2 degrees Celsius, Engine block heat after five minutes:  9.2 degrees Celsius, Radiator heat cold:  8.5 degrees Celsius, Radiator heat after five minutes:  8.7 degrees Celsius, Tailpipe heat cold:  9.0 degrees Celsius, Tailpipe heat after five minutes:  16.2 degrees Celsius, Temperature outside:  15 degrees Celsius.

By crunching the data, one can easily find out that the GMC Sierra 2500HD 4X4 6.6L Duramax Diesel easily runs cooler than any of the other vehicles used in this experiment.  An optimal idling temperature for a diesel engine is 9.2 degrees Celsius.  Both above and below this temperature, the engine will run smoothly.  However, at heat greater than 75 degrees Celsius, the idle quality will start to degrade.  By the heat redline of 104 degrees Celsius, the idle quality of the vehicle will have degraded to such a point that it will crack the engine block, or cause some other form of damage to the engine.

For those of you interested in doing a similar experiment, here are some similar experiments that you can do:  Only hybrid-electric vehicles, only gasoline-power engines, and only diesel engines.  These results can then be compared to the temperature gauge in the vehicle.   A final experiment could be measuring the temperature of the engine from the temperature gauge alone.

*The asterisks by the 2003 Chrysler Town & Country LXi AWD 3.8L V6 are important – the experiment was conducted twice.  The first measurements may have been a bit inaccurate due to the fact that the Town & Country had just been driven for over an hour.  The second experiment was what I expected, as the car had sat overnight.

I didn’t make it to the County finals, but I loved my topic.

Many thanks to Mark Miller for providing all the instrumentation, and to Unc Howie for being such a good sport and not even asking for clarification when I said “I want to do a science experiment on your brand new Prius.”  You guys are the bomb!