December 4, 2002
Standup Engine Vs. Laydown Engine
(Editor’s Note: We asked Cat’s Greg Spaulding to explain why Arctic Cat reversed the flow of coolant on Cat sleds with laydown engines. Here’s his explanation.)
1. Purpose of change to reverse flow on laydown engines.
If it is known that the purpose of cooling the engine by liquid is primarily to maintain some consistent temperatures of the cylinder and cylinder heads and not allow the heat generated by the running engine to exceed some limit for safe operation, we then understand the purpose of liquid cooling.
There are many portions of an engine that require this cooling, but for this discussion, we will eliminate all but the two primary components we are trying to cool. These two components are the cylinder and cylinder head. The coolants will move through these components in some pre-determined direction. The flow can either go to the cylinder first and then through the cylinder head last–as in most conventional 2-stroke engines–or the path could be reversed to flow through the cylinder head first and then down through the cylinder afterwards. The latter situation was chosen for the laydown engine.
Since the engine’s combustion process is what generates the heat we want to carry away with coolant, we should look at where this heat generating process is occurring. It occurs at the upper half of the cylinder and the cylinder head combustion chamber. With this in mind, it would seem logical to concentrate on cooling these areas first, with the coolest water, and then cooling the lower half of the cylinder, which is not the primary heat generating area, afterwards. This reverse flow design does not pre-heat the coolant prior to arriving at the primary heat source as in the conventional flow path. Heat removal from the primary heat source is then the most efficient.
2. Description of water flow path. Laydown reverse flow compared to conventional flow.
In a conventional coolant flow system, the coolant enters the engine at the water pump inlet. The water pump then forces the water into the upper half of the crankcase. It is then moved through the crankcase upper half and enters the bottom of the cylinders. It moves through the cylinders upward and enters the cylinder heads. It exits the cylinder head and enters the thermostat where its temperatures are controlled prior to re-entering the cooling, or heat exchanger, portion of the system.
The reverse flow system incorporated on the laydown engine functions in this manner. The coolant arrives at the thermostat first. If its temperature has reached the desired point set by the thermostat, it then enters the water pump. Upon entering the water pump, it is pressurized and moves directly out of the pump and through a hose leading to the cylinder head. It flows through the cylinder head and enters the upper portion of the cylinder. It then moves through the cylinder from top to bottom, and upon leaving the cylinder, it enters the upper half of the crankcase flowing downward and exiting the bottom half of the crankcase and then returning to the heat exchanger system.
If the water entering the thermostat is colder than the thermostat set point, the two-stage thermostat will close off the primary cooling exchange path and open up a bypass path for the coolant to flow. This bypass path redirects coolant out of the pump to the cylinder head, through the cylinders, out the crankcase and directly back in the water pump where it repeats this flow path. This allows a small volume of coolant to heat up quickly until its temperature opens the thermostat, thus closing the bypass flow to return to the normal heat exchanger path.