Cooling Systems - The Basics
(Adapted from Cat Marine Engine - Application & Installation Guide)
Any combustion engine will generate heat as it converts fuel and gas into energy to power your vessel or machine, or generate electricity. Maintaining the temperature is also an important factor to enable peak performance of the engine and also helps to keep the cost of design and manufacturing down.
There are mainly 2 ways to cool an engine - Air Cooled and Liquid Cooled.
Air cooled is the simplest form of cooling system. This method of cooling does not require a coolant - liquid or gas, that is used to reduce or regulate the temperature of a system. The heat is rejected directly into air with lower temperature (convection).
Air cooled power systems often include fins to the outer surface to provide added surface area, and cooling fans are mounted close to the engine to pull air across the hot surfaces - much like your desktop or laptop cooling fans.
However, air has limited ability to carry heat. This limits how much heat can be rejected from the engine into air. This is a major consideration given the high amount of heat that is generated from an engine, much less an engine that will be running on long hours.
Due to the limitation of air cooled systems, modern high output engines are using liquid cooling systems to manage the higher heat loads during operations.
While liquid cooling system and liquid coolants add complexity to the design and manufacturing, the benefits outweighs as it can greatly increase the heat carrying capacity by approximately 4 times.
Heat from the coolant is then dissipated into the air, and "recycled" back into the cooling system, returning to do its job once more.
Heat Rejection & Energy Balance
Maintaining the heat balance at an optimal is important in an engine - the heat must be removed from the engine at a similar rate it is being produced.
There are a few ways heat energy is being removed:
- Engine work - convert to energy for shaft work
- Surface heat - lost to atmosphere by convection and radiation
- Exhaust heat
- Cooling Circuits - Jacket Water, Aftercoolers, Oil Coolers
It can be expressed in the following equation :
Qin = Wout + Qexh + Qjw + Qac + Qoc + Qsur
Q - denotes heat quantity
W - denotes work done by engine.
- Typical heat rejection path for an internal combustion engine (Source: Cat Assembly & Installation Guide)
Energy Balance Components
Fuel Energy Input
This is the usable energy of the fuel to be consumed by the engine to generate work. It is important to note that the term "Usable" is emphasised as some of the total energy present in the fuel will be unavailable for use by engine. Water vapor will produced as a byproduct of combustion of hydrocarbon fuel, and a portion of the energy is lost to latent heat of vaporization.
This is the power produced by the engine and used by all of the loads connected to the engine. This power is noted as HorsePower (HP)
Jacket water, also known as "cooling jacket" is the passages cast into the engine's cylinder block and cylinder heads where coolants are circulated. Heat from the engine is transferred to this main coolant flow.
This is the heat that escapes to the surroundings from the surface of the engine. There are 2 main ways for this heat transfer
- Convection - where the hot surface of the engine warms the surrounding air
- Radiation - where the engine's surface loses heat to surroundings as infrared energy
Both mechanisms are highly dependent upon the site conditions. Therefore this quantity has the largest variability of all the heat flows listed in the technical data.
Lube Oil a.k.a Oil Cooled
Cooling is the second important function performed by Lube Oil, in addition to its job as a lubricant. Heat is accumulated in the lubricating oil as it is passed through the critical bearing surfaces inside the engine. Certain engine designs specifically use lube oil as a coolant for areas where is it impractical or impossible to design a way to use the jacket water coolant to do the job. Examples are like Turbocharger bearings and piston crown.
This is the heat retained in the exhaust gases as it leaves the engine. This can be such a large amount of the totalheat lost during the engine’s operation that some installations will seek to recover this heat through the use of anexhaust mounted heat exchanger. The amount that can reasonably be captured in this manner depends on the temperature difference between the exhaust and the recovery medium (often another coolant loop) and the time the two spend in the heat exchanger. However, a heat exchanger that provides a longer contact time will increase the exhaust backpressure (which has a direct impact on engine performance), making such a heat exchanger difficult to implement.
Two exhaust heat rejection quantities are often listed on Cat engine technical data
- “Lower Heat Value to 25°C” represents the full heat quantity present for the temperature difference of the exhaust flow from the exhaust stack temperature to an approximated ambient air temperature of 25°C
- “Lower Heat Value to 177°C” represents an estimate of the recoverable heat present for a temperature difference from the exhaust stack temperature to an approximated recovery medium temperature of 177°C
This is the heat transferred in the engine-mounted heat exchanger known as the aftercooler. An aftercooler is present on nearly all turbocharged engines - its main job is the remove the heat added to the incoming combusion air during its compression in the turbocharger.
In this manner the aftercooler simultaneously decreases the air temperature (to limit detonation) and increases the air density (a denser air/fuel charge enables greater powerfrom a given engine displacement).
Some engines may use a two-stage aftercooler, an arrangement that runs the high temperature air first through a heat exchanger stage on the high temperature jacket water cooling circuit, then the same air is passed through a second stage on the lower temperature auxiliary circuit, improving performance of the aftercooler. For such arrangements, two values are displayed in the engine technical data,one for each stage of the aftercooler.
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