Never Open When Hot...

   When was the last time you thought about the radiator cap on your cars cooling system? It took a few years of working with machines for me to appreciate what was going on with that simple device. On the surface, it would seem that a radiator cap is indeed a simple device to keep the coolant in, but on closer inspection it becomes a complex regulator of pressure and temperature required for proper engine operation. It took automotive engineers many iterations and many years of R&D to come up with the system we take for granted today. 

 

   The radiator cap is responsible for venting excess pressurized coolant out to the reservoir to prevent an over-pressure or overheat situation while maintaining internal cooling system pressure to prevent boiling and allow the automatic refill of the system when cooldown occurs. These three seemingly separate functions are performed perfectly now by one device.  

   On most radiator caps, there are two rubber seals, an upper and lower seal. The smaller, lower seal is spring loaded to keep pressure on the coolant in the system as it warms up and expands, while the upper seal is the actual seal out to atmosphere. Between the two sealing areas on the radiator is a port for a small hose connection to the coolant reservoir tank. A tiny metal flap at the very bottom of the cap prevents coolant escape when hot, but allows coolant to flow back in from the reservoir to keep the system full as the engine cools after shutdown.  

   As things heat up, they expand. There are a few exceptions to the rule, but for most materials, heating causes expansion, and the corollary is true as well- as they cool, they shrink or contract The engine and coolant in your vehicle is no different- as the engine warms, the metal and coolant expand and thus increase the pressure inside the cooling system. The temperature increase in a contained system would drive the pressure higher and higher until explosive release occurred by breaking stuff. Having a pressure relief valve on any system that changes temperature or pressure is mandatory for that reason. Preventing hot steam and water explosions is always a bonus... or so I hear. 

    As the coolant heats up inside the engine, the pressure inside the cooling system increases- K we got that part. Some pressure is needed to keep the water from boiling inside the engine- just like using a pressure cooker allows the water temperature to go above boiling point without becoming steam. Keeping water under pressure prevents it from boiling to steam even at well above the normal boiling temperature, so some increase is necessary. Here we have a challenge- keeping just enough pressure inside the engine to prevent spot boiling without allowing over pressure and damage.  

   Later, after the engine is shut off and is cooling down, the pressure drops inside the cooling system because of contraction. As soon as the pressure inside the cooling system falls below outside atmospheric pressure, that difference in pressure pushes coolant back out of the reservoir and into the engine making sure that the system is constantly being refilled at every cooldown cycle. This is generally phrased as: 'The cold engine sucks coolant back in'. 

   As you continue to read my articles, over and over again you'll find that I stick strictly to the laws of physics which state that there is no 'suck' only pressure difference which causes flow. It's air pressure outside the system trying to balance with the lower pressure inside the cooling engine that pushes coolant back up the hose since that is the only path for the pressure to equalize through.

   Any cracks in the rubber seals, failure of springs or jamming of the mechanism can cause catastrophic failure, or at least annoying overheat or coolant loss. Thus it makes sense, does it not, that the radiator cap should be inspected carefully every few months and replaced every 5 years or less- or whenever there is any physical damage or noted change in performance (ie- system never pressurizes, doesn't refill at night...). A 'simple' $8 radiator cap is responsible for your entire engine. Don't neglect it.

   Til Friday, take care of those machines, they'll take care of you.