If somebody running an 4 stroke engine without thermostat, will it harmeful to the engine.

When I begin to heat up a piece of steel with a torch I can see moisture condensing on the metal's surface.
If the cylinder walls are too cool then similar condensation of moisture and some nasty products of combustion occurs.  The result is corrosive wear of the cylinder and piston ring, and maybe some abrasive and other types of wear as well.

I believe this is the source of the frequently heard "most engine wear occurs at start up ."

Coated piston rings (chrome, moly) have many benefits over iron faced rings, including resisting corrosive wear better.

Modern fuel injected engines make major decisions based on engine and coolant temperature.  A cold engine is fed a richer mixture, and running rich with cold cylinder walls is probably not very very good for long term life.

A thermostat closes and almost stops water flow until the engine warms up so that it warms up as quickly as possible.

Once warmed to the set point, the thermostat starts to open, and adjusts itself to maintain the temperature within the set range so long as the radiator has capacity to keep it cool/Reducing Valves.

Until the engine warms up, diluents in the oil (like fuel and water) are not evaporated out. Also the pistons are loose in the bore and bearing clearances are wide. The wide clearances increases impacts as the clearance takes up suddenly as pistons rock in the bore and the loads on bearings changes direction. Excessive tappet clearance can also cause cam and tappet wear. Pistons also wear as the edges of the skirts rather than the full face bear on the bore as the piston rocks excessive. This also wears a barrel shape on the rings.

If the open thermostat caused the water to flow slower, it would not effectively give the water time to cool, as the water would be hotter as it had time to pick up more heat while in the engine. To look at it another way, it passes the water though the radiator faster, but also more often.

We did this a short while ago. It should be easy to find wit the site google feature.

A "short circuit" does not apply to only electronics. In water treatment a "current" that allows water to flow straight through a tank and therefore reduce the contact time with a reagent such as alum for flocculation is known as a short circuit. Baffles have to be added, and sometimes moved to provide a resistance to current flow.

In an engine coolant system with no thermostat causes coolant to flow from the pump, through the front of the intake manifold, to the radiator, and then right back to the pump, often times skipping the entire rear portion of the engine. (Think conventional V-8 designs)  This problem has been reduced by re-engineering engines so that coolant flow traverses the block, and leaves from the cylinder head on the opposite side, but that does not actually eliminate "hot spots", where the coolant flow is allowed to bypass a particular area. When coolant can bypass an area, it is taking a "shorter path" and is the reason for overheating. The idea that the coolant gets through the radiator too fast and causes overheating as another poster responded is incorrect.

"Normal" coolant flow has coolant circulating the block and head and picking up heat until the thermostat opens. Then you should get in effect a "figure 8" with coolant circulating the engine block, as well as the radiator. The thermostat serves as a baffle to prevent all of the coolant from only flowing through the radiator.

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