Introduction to Thermal Management

COOLING SYSTEM

A system, which controls the engine temperature, is known as a cooling system.

NECESSITY OF COOLING SYSTEM

The cooling system is provided in the IC engine for the following reasons:

  • The temperature of the burning gases in the engine cylinder reaches up to 1500 to 2000°C, which is above the melting point of the material of the cylinder body and head of the engine. (Platinum, a metal which has one of the highest melting points, melts at 1750 °C, iron at 1530°C and aluminium at 657°C.) Therefore, if the heat is not dissipated, it would result in the failure of the cylinder material.
  • Due to very high temperatures, the film of the lubricating oil will get oxidized, thus producing carbon deposits on the surface. This will result in piston seizure.
  • Due to overheating, large temperature differences may lead to a distortion of the engine components due to the thermal stresses set up. This makes it necessary for, the temperature variation to be kept to a minimum.
  • Higher temperatures also lower the volumetric efficiency of the engine.

REQUIREMENTS OF EFFICIENT COOLING SYSTEM

The two main requirements of an efficient cooling system are:

  • It must be capable of removing only about 30% of the heat generated in the combustion chamber. Too much removal of heat lowers the thermal efficiency of the engine.
  • It should remove heat at a fast rate when the engine is hot. During the starting of the engine, the cooling should be very slow so that the different working parts reach their operating temperatures in a short time.

TYPES OF COOLING SYSTEM

There are two types of cooling systems:

  • Air cooling system and
  • Water-cooling system

AIR COOLING SYSTEM

In this type of cooling system, the heat, which is conducted to the outer parts of the engine, is radiated and conducted away by the stream of air, which is obtained from the atmosphere. In order to have efficient cooling by means of air, providing fins around the cylinder and cylinder head increases the contact area. The fins are metallic ridges, which are formed during the casting of the cylinder and cylinder head The amount of heat carried off by the air-cooling depends upon the following factors:

  • The total area of the fin surfaces,
  • The velocity and amount of the cooling air and
  • The temperature of the fins and of the cooling air.

Air-cooling is mostly tractors of less horsepower, motorcycles, scooters, small cars and small aircraft engines where the forward motion of the machine gives good velocity to cool the engine. Air-cooling is also provided in some small industrial engines. In this system, individual cylinders are generally employed to provide ample cooling area by providing fins. A blower is used to provide air.

Advantages of Air-Cooled Engines

Air-cooled engines have the following advantages:

  • Its design of the air-cooled engine is simple.
  • It is lighter in weight than water-cooled engines due to the absence of water jackets, radiator, circulating pump and the weight of the cooling water.
  • It is cheaper to manufacture.
  • It needs less care and maintenance.
  • This system of cooling is particularly advantageous where there are extreme climatic conditions in the arctic or where there is a scarcity of water as in deserts.
  • No risk of damage from frosts, such as cracking of cylinder jackets or radiator water tubes.

WATER COOLING SYSTEM

It serves two purposes in the working of an engine

  • It takes away the excessive heat generated in the engine and saves it from overheating.
  • It keeps the engine at working temperature for efficient and economical working.

This cooling system has four types of systems:

  • Direct or non-return system,
  • Thermo-Syphon system,
  • Hopper system and
  • Pump/forced circulation system.

Though the present tractor has a forced circulation system, it is still worthwhile to get acquainted with the other three systems.

The indirect liquid cooling systems for electric vehicles and the conventional internal combustion engine (ICE) cooling system are very similar: both circulate coolant throughout a series of metal pipes to transfer heat away from the battery pack or engine. Therefore, coolant requirements for indirect liquid cooling systems will be very similar to traditional ICE coolants.

99% of the coolant is a commodity such as a glycol or a poly-glycol, but the 1% additive package is what separates good from great engine protection and performance. When circulating a liquid coolant throughout metal piping, it is important to protect against corrosion to protect vehicle safety and performance.

Metal is very unstable, so it naturally wants to react with other elements by losing electrons to move to a more stable state. Corrosion happens because impurities in the coolant liquid have a positive charge on them, so they interact with the metal pipes and strip away some of the surfaces. Additive packages can be blended with antifreeze to form a coolant that protects against rust, scale, and corrosion. The additive packages used in ICE vehicles contain corrosion inhibitors to protect the many types of metals found in cooling systems, such as the pipes, gaskets, connections, radiator, etc. The American Society for Testing and Materials maintains standards that coolants must meet for protection against the corrosion of different metal types. What is currently known about corrosion prevention in internal combustion engine cooling systems can be easily applied to the indirect liquid cooling system in electric vehicles.

WATER COOLED V/S AIR COOLED

Batteries have to be pampered. That’s because batteries simply perform better when they don’t have to deal with extreme heat or cold. When it comes to batteries for electric vehicles, so-called thermal management systems that ensure batteries operate within a certain temperature range will be crucial to helping electric cars drive greater distances for a longer period of time.

However there is a debate raging, over whether automakers should turn to air or liquid to regulate battery temperature, and the choice has provided fodder for some green car makers to toot their own horns or slam competitors.

Tesla Motors (s TSLA) CEO Elon Musk has derided Nissan’s battery pack, which uses an air cooling system, as “primitive” compared with the sophistication of even Tesla’s first prototype, which uses liquid cooling. As a result, the LEAF pack will have temperatures “all over the place,” claimed Musk, causing it to suffer “huge degradation” in cold environments and basically “shut off” in hot environments. General Motors (s GM) for its Volt and Ford’s (s F) line of hybrids and EVs have also opted to use the liquid for battery temperature regulation. Coda Automotive, meanwhile, uses an air cooling system for its Coda Sedan.

According to Matt Keyser, a senior engineer with the National Renewable Energy Laboratory who focuses on battery thermal management systems, there are advantages to both technologies. Air cooling systems can be less complicated and lower cost, while liquid systems will generally take up less space and let you “drive the battery a little harder,” Keyser explained. Liquid cooling allows the battery to handle a larger “pulse” of power (i.e. more kilowatts, which in electric cars are like horsepower).

Of course, there are trade-offs, too. Among the weaknesses of a liquid cooling system is the potential for fluid to leak (which can cause an electrical short), said Keyser. Maintenance and repair can also be costlier and more complicated for liquid-cooled systems, which require more components and may weigh more than air systems. The battery pack can be “totally isolated” from the liquid by an aluminium shell, however, which ensures that fluid (a 50-50 mix of water and glycol in the Tesla Roadster system) doesn’t “get to the battery itself,” said Keyser.

Air systems, meanwhile, are generally less effective at maintaining a uniform temperature (key for battery longevity) within and between cells in a battery module, and they can’t carry as much heat away from the battery as quickly as a fluid-based system. A similar idea is emerging around liquid cooling for servers in data centres: liquids are much better than air at transferring heat.

Why is that? Think of an empty cup versus a cup filled with liquid, Keyser suggested. The liquid simply has more mass with which to take heat to another medium. Huge amounts of air can be blown through a battery with a fan to help transfer more heat, but this can result in large pressure drops and inefficiency, said Keyer. An air cooling system could in theory work about equivalent to a liquid system, he said, but then the fan would have to be so big that it would need to draw power from the engine (in a hybrid) or run on its own battery (in an electric car).

Keyser emphasized that he still considers air cooling systems a viable option. “I’m trusting OEMs,” he said, to design the thermal management system so that the battery “won’t be abused,” even in the most strenuous drive cycles. To do that, according to Keyser, automakers may opt to “give you slightly less performance so they can use air cooling.”

Alternatively, since a bigger battery pack can deliver a certain amount of power with less current, and therefore less heat, than a smaller pack, automakers could use a slightly larger battery pack with an air cooling system. With plug-in hybrid and all-electric vehicles, said Keyser, liquid cooling appeals in part because “the battery pack is fairly large as it is,” Keyser noted.

For example, the 33.8 kWh battery pack in Coda Automotive’s upcoming electric sedan will weigh about 700 pounds and deliver 100-120 miles of range, Coda Chief Financial Officer Dan Mosher said earlier this year. For GM’s upcoming Chevy Volt, which will have a 16 kWh battery and a gas engine that kicks in after about 40 miles of electric range, the automaker’s executive director of global electrical systems, hybrids, electric vehicles Micky Bly has said that 200 kilograms (a little over 440 pounds), “is the rough estimate within the realm of a few digits for battery weight.”

Put another way, you can get higher performance (e.g. more power) out of a smaller battery pack with pricier liquid cooling than you can get out of an air-cooled pack of the same size. So the decision for car buyers, said Keyser, is ultimate whether you want to “invest your money in a bigger battery, or in the cooling system.”

TYPES OF RADIATORS

  • Two types of car radiators: are available in two types; down-flow versus crossflow radiator. The names refer to how the coolant travels through the radiator core tubes. Downflow radiators rely on gravity to move the coolant from the top tank to the bottom tanks, while cross-flow radiators rely on the characteristic of fluid to seek its own level.
  • Radiator construction: Older radiators were built with a copper inlet and outlet tank and either a copper or aluminium core. However, most modern radiators are built with plastic tanks and an aluminium core. Thin tubes transport the coolant from one tank to the opposite tank. Folded fins fill the space between the tubes and help transfer coolant heat from the tubes to the air passing through the radiator core.
  • Radiator cap placement: In designs where the pressure cap is located on the radiator itself, the cap must always be on the top tank (high pressure) in a downflow radiator. However, the cap on the crossflow radiator can be located on the suction or low-pressure side of the radiator, opposite the upper radiator hose.
  1. Cross Flow radiators

A crossflow radiator features a vertical tank on each side. The coolant moves as it’s pushed along by the pump from the high-pressure (entry) tank as it receives coolant from the engine, through the core to the low-pressure (exit) tank on its way back to the engine.

Advantages of Cross Flow Radiator

In modern vehicles with an aerodynamically shaped hood, there simply isn’t enough room to accommodate a tall down-flow radiator. So crossflow radiators are used to accommodate body design. They’re shorter but wider than a comparably sized downflow radiator.

Because the pressure cap can be located on the low-pressure tank, the design prevents premature pressure cap opening caused when a high flow water pump runs at high engine RPMs.

Because cross-flow radiators are wider, they are often cheaper to build, owing to the fact that this design requires fewer tubes and thus fewer weld/solder joints.

If the crossflow radiator can be mounted so the pressure cap is higher than the engine, the radiator is self bleeding. However, if the radiator is placed lower than the engine, a surge tank or bleed line is required.

Disadvantages of Cross Flow Radiator

The biggest disadvantage of a cross-flow radiator is its width, making it a bit more difficult to fit in an engine compartment. Designers must ensure that enough outside air enters the grille area and distributes to the extreme ends of the cross-flow radiator. In addition to the space limitation, cross-flow radiators often have less coolant capacity than a downflow radiator.

Because cross-flow radiator relies on a fluid’s ability to seek its own level, cross-flow radiators often have a higher temperature differential between the inlet and outlet sides. That higher temperature differential can cause joints on the radiator to break and leak or allow air into the system.

  1. Downflow radiators

A downflow radiator features a horizontal top tank and bottom tank. When hot coolant leaves the engine, it enters the top tank and then travels down to the bottom tank via tube passages in the core, pushed by the water pump and aided by gravity.

Along the way, as coolant runs through the core, the fins provide additional surface area for heat transfer to the atmosphere.

Advantages of a downflow radiator

They’re narrower than a comparably sized crossflow radiator. If width is an issue and you have the height, a downflow radiator maybe your best option.

Disadvantages of a downflow radiator

Aside from its taller height, the single biggest disadvantage of a downflow radiator is the fact that the pressure cap is always located on the top tank. This is the hottest part of the radiator with the highest internal pressure. At high engine RPMs, the combination of high heat and high pressure can force the pressure cap to open and bleed pressure and coolant into the recovery tank.

Verdict – Basically, either style works. A builder will usually decide between a crossflow or downflow radiator based on the engine bay’s packaging limitations.

In theory, a crossflow radiator is said to be more efficient since the radiator’s pressure cap is located on the low-pressure side of the radiator, which allows longer high engine speed operation without forcing coolant past the pressure cap. The (usually) larger surface area of a crossflow radiator may also allow the use of increased radiator capacity and cooling surface area.

However, if the vehicle owner desires an original, nostalgic appearance, a downflow may be the only choice. Also, retrofitting a crossflow radiator into an engine bay that was originally designed for a downflow unit may require a certain amount of fabrication time, which may or may not be an issue. In order to gain increased core surface area, creativity comes into play. On the flip side, in many cases, a downflow style may actually transfer heat better than a crossflow.

Single Pass

In a single pass radiator, coolant only crosses the radiator core one time.

  • Fluid enters the inlet.
  • It flows across the core in tubes.
  • It is collected in the opposite side tank.
  • It exits the outlet and flows through the system again.

Single-pass radiators work well in most street applications. For stock and mildly modified engines, they provide adequate cooling.

Replacing a single pass radiator usually requires no modifications. The inlet and outlet will be on opposite sides of the radiator. Also, your stock water pump can handle the flow requirements.

Double Pass

A Double Pass Radiator is divided in half. Coolant crosses the core twice.

  • Fluid enters the inlet.
  • A baffle inside the tank directs incoming fluid across the top half of the core.
  • The fluid collects in the opposite side tank.
  • It flows back across the core on the bottom half.
  • It exits the outlet and flows through the system again.

A double pass radiator is considered an upgrade over a single pass. All else being equal, a double pass radiator can increase cooling efficiency 5-10%. It works well in street and race applications.

Replacing a single pass radiator with a double pass will require some modifications. The inlet and outlet will be on the same side of the radiator. So, custom hoses may be required. Also, a High-Volume Water Pump is recommended.

Triple Pass

A Triple Pass Radiator is exactly what you think. Coolant crosses the core 3 times.

  • Fluid enters the inlet.
  • A baffle inside the tank directs incoming fluid across the top of the core.
  • A baffle in the opposite side tank directs the fluid across the middle of the core.
  • The fluid collects in the opposite side tank.
  • It flows back across the core on the bottom.
  • It exits the outlet and flows through the system again.

A triple pass radiator works best with highly-modified race engines. High horsepower and power adders require more cooling capability. Depending on the specific application, a triple pass radiator can increase cooling efficiency 10-20%. A triple pass radiator is probably overkilled for most street vehicles.

The triple pass design will have the inlet and outlet on opposite sides. A High-Volume Water Pump and a 1-to-1 Pulley are usually required to ensure proper flow.

 

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