WHAT REALLY HAPPENS WHEN AN ENGINE “COLD SEIZES”
OK….. The, infamous, “COLD SEIZE”.. This has always been a big concern related to the 2 stroke engine. BUT.. What is REALLY happening when your engine experiences a cold seize?
Is it caused by a mad rush of colder “coolant” passing through the hotter cylinder??
Is it caused by you not letting your engine “warm up”?
Well… Let’s dive into this “mystery” and look at what is happening.
1st off.. we need to touch on a LITTLE bit of physics and the laws of thermodynamics. We will simplify these principles so as not to bore anybody and then get right into the “meat and potatoes” of what is happening internally.
There are several methods for heat to transfer anywhere. These methods are well known and can be proven very easy. Let’s list these methods:
1)Conduction: Direct contact of 2 or more objects at different temps.
2)Convection: Heating of a gas or liquidà Like your forced air home furnace.
3)Radiation: Infrared electromagnetic radiationà Space heaters for example.
One can make a case for methods 1 and 2 being utilized in a liquid cooled 2- stroke internal combustion engine.
OK, One very important concept to remember à Heat will always transfer from HOT to COLD. Heat will never seek the “Hotter” path but, always the “Cooler” path.
The piston/cylinder area is the heat source and will be the hottest whenever the engine is running. The cooling system will ALWAYS be cooler in temp. ALWAYS!
Your Coolant temperature will always be equal to or colder than the piston/cylinder temperature. This is important to remember!
Heat will always be in the form of energy. Heat is energy! It is little molecules dancing around (vibrating) and in motion.
OK.. The 1st law of Thermodynamics is a MAJOR player here. It is VERY simple and , I Promise, we will not dive into anymore physics after this!
1st Law of Thermodynamics: (robbed from Wikipedia)
This states that energy can be neither created nor destroyed. However, energy can change forms, and energy can flow from one place to another. The total energy of an isolated system remains the same.
A quantity of heat that flows from a hot body to a cold one can be expressed as an amount of energy being transferred from the hot body to the cold one.
So, simply stated, when one object cools, another object in its general vicinity heats as a result. Heat (energy) does NOT just disappear! It is transferred to another object. Equilibrium is the goal.
WHEW!! Now that is all out of the way…let’s relate this to our engine.
OK, We have a piston, crank, and cylinder as a heat source.
We have a cooling system comprised of heat exchangers, anti-freeze, water, pump, hoses, and sometimes even a fan trying to REMOVE heat from the engine’s heat source.
We KNOW that heat always transfers from hot to cold. That this transfer can not create or remove energy and that the TOTAL heat energy present in this “SYSTEM” will remain the sameànone lost and none gained.
Now.. finally… we are at the “COLD SEIZE” Topic!!
One widely held belief is that a Cold Seize is the result of “mad” rushing cold coolant contacting the HOT, running, engine and the result is this cold coolant is shrinking the cylinder walls in a manner such that the hotter piston will immediately fuse itself to the cylinder walls. This is VERY unlikely!!
WHY?? Well.. see what we talked about above.
Let’s look at a few similar scenarios..
Example #1: You are in Antarctica and you build a fire in the frozen tundra and snow. The outside temp is -30 F BELOW ZERO and the temperature of the fire is 1800 F. The fire is giving off heat energy and you and the surrounding air are absorbing it and getting “hotter”. NOW.. you decide to construct an igloo around the fire. You KNOW that the igloo is at least 32 F, if it were not it would not be frozen solid!
Now, as a result of this igloo, the air inside is warmer and you are warmer… BUT the FIRE is still at the SAME 1800 F Temperature. You have surrounded/encompassed the fire with a structure that is at least 32 F yet the fire is still burning at the same temperature.
Sound like anything else? Say… an engine (fire) and a cooling system (igloo). Point being, just because you surround a heat source with a cooling source does not make the heat source “colder”.
Example #2: You have a stove burning a gas flame. You have a large empty pot on the stove. The pot’s walls are getting very hot. You now dump cold water into the pot. Did the flame get colder because of this? NO. Did the pot’s walls get colder? Yes, slightly, but most of the heat went directly into the water. Remember, heat always travels from hot to cold. The cold water did not cool the pot’s walls, the cold water absorbed the heat energy from the pot.
Same with your engine, the coolant removes heat from your cylinder. Since this energy can not be created or destroyed (1st Law of TD) the colder coolant is becoming HOTTER and heating very quickly while the heat source (engine) temp is remaining the same.
NOW, IF you could surround and engine with cold coolant and the coolant did NOT absorb the heat energy (heat up) and remained cold.. you could then have an issue with a cold seize. BUT.. This will NEVER happen! The coolant will always increase in temperature and increase VERY rapidly until the coolant and what it surrounds are near or at the SAME temperature causing ZERO shock to any component. THIS is the REALITY!
OK, We have all heard of this “cold shot” on the Polaris 800 CFI and one can see where the coolant temp gauge will increase after you turn off the engine then after you sit for a bit, fire up the sled, and ride, one will see the coolant temp begin to drop significantly.
What is happening with this?? Simple… When you turn off the engine you have a “system” that never reached equilibrium. You have an engine that is much hotter than the surrounding coolant medium. When you turn off your engine the heat source is no longer active, we KNOW heat ALWAYS transfers from hot to cold. You have a hot engine and a colder coolant medium. The transfer of heat is present and the coolant becomes heated from the radiating engine ALL in attempt to achieve equilibrium. Many will have you believe that your engine is getting HOTTER when this is happening, when, in reality, your engine is getting COOLER (remember the 1st Law of TD).
So, your engine is NOT heat soaking but is actually cooling down.
OK.. So, you decide to fire the engine back up and you notice the coolant temp begin to rapidly drop. You think… OMG… I am shocking my engine with this cold, cold coolant… Well... Remember the 2 examples above and the 1st Law of TD? You are doing nothing of the sort. The heat source (engine) is active again and is MUCH hotter than your coolant. The coolant is being heated by the engine NOT the other way around!
Well.. WHY is the coolant temp lowering? Simple.. while you sat with the engine off, your engine as a “SYSTEM” strived to reach temperature equilibrium (everything is the same temperature). Your cooling system was also doing the same. The hotter heat exchangers and stagnant coolant was trying to reach equilibrium with the cold outside air and snow. They (the heat exchangers and held coolant) are cooling down transferring their heat energy to their surroundings (air and snow) and the result is that their temperature dropped.
NOW Once you fire up the engine ALL stagnant areas now become active and things are all moving again. The heat source is actively heating its surroundings and the water pump is actively pumping the coolant through the entire system. Stagnant becomes Active! The system as a WHOLE is trying to reach temperature equilibrium and temperatures will be changing.
What is surely NOT happening is that your piston is getting COLDER! It is getting HOTTER and no rush of cold coolant passing by is going to change that!
OK It should be clear that it is VERY unlikely to have ANY coolant cause enough rapid cooling of a running engine to seize it… NEAR IMPOSSIBLE! Simply because the "magnitude" of the heat source is simply too great for the ,smaller, cooling source to over-come. It is like a spider vs. your foot!
So.. What does cause an engine to cold seize?
Glad you asked… A cold seizure is the result of the piston expanding too fast with respect to its cylinder.
Your engine is constructed on many different types of materials all housed together and designed to work as one system. All internal engine components, when heated (running engine) will expand to some extent. This expansion is normal and unavoidable. Different components will expand at different rates and will expand to different extents.
When you fire an engine after it has been able to completely cool down to a state where all internal components are at their “NEUTRAL” state.. meaning they are in no state of any expansion due to heat, these components will begin their expansion all over again. It is THIS expansion rate differences that will cause an engine to “Cold Seize”. So, you have an entire engine that is being heated and, as a result, ALL of its internal components are expanding at different rates. This INITIAL expansion is what you need to be mindful of to avoid a cold seize. Once these engine components have reached their full expansion, then the engine should be operating in its design spec and safe to run and run hard. If you choose to fire up a cold engine and immediately pour large amounts of heat into the components (i.e. high rpm) you RISK having these varying expansion rates collide and seize. Make sense??
Cold Seizures have nothing to do with cold coolant passing over a hotter engine!
You, simply, can NOT seize an engine from cold coolant passing over an already "up to temp" running engine!
COLD Seizures DO relate to the varying expansion rates of the different components within your engine. Once these components are at their operating temperature, they will remain there and function as designed.
NO rush of cold coolant would/could adequately change the temperature OR DIMENSIONS of any of these components ,as long as, the "fire" (engine) is still "flaming"