Heat exchangers keep parts other than the A/C system cool and at work.
Without spouting unnecessary scientific terms, let's loosely define the first and second laws of thermodynamics by saying that to begin with, heat must be generated by something at the expense of something else, and follow that by saying heat moves from areas of high concentration to areas of lower concentration.
Furthermore, whenever heat transfer takes place, something always is lost. For example, gasoline is oxidized in a process we call burning. As it burns, it expands. As it expands, it applies force and heat to its immediate environment. And in the controlled environment of a combustion chamber, that force is harnessed and applied in such a way that it can be used to do work. But the conversion of gasoline to power isn't 100 percent efficient, and for that reason, a lot of the energy created is permanently lost, and the matter consumed during energy production can't be recovered. An efficient means of transferring and dissipating wasted heat must be applied, or else the work-producing machinery that benefits from trading matter for energy can be ruined.But heat exchangers don't always protect machinery; sometimes their purpose is more utilitarian, as in the case of the heater core, the A/C evaporator and its condenser. The A/C heat exchangers (working together) act as a heat pump, capturing heat from the cabin area and conveying it to the atmosphere outside the passenger compartment.
Cool Your Engines
To prevent heat damage on liquid cooled engines, coolant is pumped through the engine block and cylinder heads to maintain the integrity of those very expensive components through the process of convection. The spark plugs give up a tremendous amount of heat to the cylinder head, which in turn gives up its heat to the coolant flowing through its passages. And head gaskets have small ports to slow the coolant down on its way through the head so that liquid doesn't travel too fast and absorbs the necessary heat.It is wise to remember that half the width of each fan blade should be visible from above when you're looking straight down at the shroud. If the fan is totally out of sight in the shroud (like when somebody installs a thicker radiator not designed for use in that vehicle) that circumstance can and usually will cause overheating.
The engine that pushed those ancient buggy-like cars Henry Ford built was designed to percolate water out of the block through the cylinder head, up a pipe and into the radiator where the air passing through the radiator would absorb the conveyed engine heat and lower the temperature of the water before allowing it to return to the block. We all know that today's engines are outfitted with highly efficient coolant pumps to keep the liquid flowing.However, we don't often consider the longevity of these robust little spinners, which is fairly amazing when we factor in the hostile environment in which they live and how hard they have to work.
Be that as it may, coolant pumps fail with enough frequency to provide our service bays with a fair amount of cash flow, and in many cases the timing belt or chain has to be removed to facilitate replacement. Engine cooling systems also have thermostats that regulate the temperature to prevent the cooling systems from working too well (cold running engines aren't fuel efficient and tend to make a lot of water and sludge). On most of today's cars, powerful electric or hydraulic fans draw air through the heat exchangers in front of the engine to dissipate the heat when the car isn't moving, and today's fans are PCM controlled. The fan on some late 1990s GM cars doesn't kick in until the coolant is at 228 degrees, as scary as that sounds.
Unlikely Places of Exchange
Air-cooled engines are designed with broad fins cast onto the outside surfaces of high heat components, and they typically have fans and shrouds to channel a high volume of air across the fins (air cooled motorcycle engines are one exception). Because they typically travel at high speeds, they appropriate the natural airflow that comes with vehicle velocity for the purpose of carrying the heat away.Those old Volkswagen Beetle engines are air cooled with fins and a powerful fan mounted in a housing above the engine (driven by the generator), but those engines also have a small upright heat exchanger situated in the fan shroud that cools the oil, and without it, a bug engine will weld its rotating components together in short order.
The cabin heater on a VW Bug uses a heat exchanger of this type for cabin comfort in cold weather; a bevy of shrouded fins cast onto the two exhaust pipes leading to the front cylinders carries hot air forward through the shrouded components from the engine cooling fan into the engine compartment when the driver desired cabin heat. While the hot exhaust is traveling rearward toward the muffler, the air that heats the cabin is traveling forward through the exhaust pipe shrouds into the passenger compartment. The two levers down by the hand brake are cabled to open the flaps to channel cabin heat to the floor area or to the windshield.The process of consuming fuel isn't the only heat-producing part of an automobile that needs to be managed. On turbo or supercharged engines, the air-jammer's squeezing of atmosphere produces heat from friction. Superchargers and turbochargers shove their air charge so that it passes through an intercooler en route to the engine, which is like a special radiator. Hot, high pressure air flows through the intercooler, while cooler low pressure air passes through the intercooler fins and greatly reduces the temperature of the air charge, making it denser and more combustion-friendly by the time it reaches the chambers.
Fluids that are moved under pressure by a positive displacement pump (like the power steering pump) must be cooled as well, and for years manufacturers added finned coolers for the power steering fluid. But most designers nowadays just allow the fluid to travel a long way through naked steel or aluminum pipes to cool the power steering fluid en route. Likewise, transmission fluid has to deal with intense heat as it is sheared by the impeller and turbine in the torque converter and is sent to a specially shaped heat exchanger that is usually built into the cool side of the radiator.
Sometimes heat exchange takes place where we aren't even thinking about it. For example, to prevent the boiling of brake fluid where brake fluid is piped through high heat areas (like the exhaust manifold area of an engine compartment), hydraulic brake lines are either coiled in big loops to dissipate heat or have steel wire coiled tightly around them with space between each pass of the wire around the brake line.
Then there is Exhaust Gas Recirculation (EGR), which is an emission control strategy designed to divert a small amount of inert gas from the exhaust system into the airstream, usually right behind the throttle plate so each combustion chamber gets an equal amount of it. Nitrogen and oxygen are the two principal components of our atmosphere, and when combustion heat rises above 2,500 degrees, these two gases bond together to make a variety of deadly gaseous compounds.EGR's inert gas lowers that temperature to reduce NO
As for cooling the recirculated exhaust gas, modern gas burners pipe the inert exhaust gas through a fairly long thin-walled steel tube en route to the EGR valve, and the heat transfer that takes place while it's passing through that tube is all the cooling necessary for the EGR on those platforms. Because diesel engines have even more of a need for EGR than gas burners, they need powerful heat exchangers as the exhaust gases need to drop several hundred degrees (F) of temperature between the exhaust stream and the point at which they enter the manifold.
To accomplish such a meteoric drop in temperature, diesels typically use an EGR cooler with the recirculated exhaust gases flowing through small pipes or flues that travel through engine coolant.
The EGR cooler used on early Powerstrokes almost never failed, but then a supposedly more efficient cooler became the order of their day, and those coolers tended to fail and contaminate the cooling system with oil. The 6.4L Powerstroke uses two of these coolers, and when the horizontal one fails, it just about always floods the engine with coolant in a rod-snapping hydraulic lock that is both terrifying and sickening at the same time.
Packing the Power(stroke)
Speaking of the Powerstroke, the 6.4L has seven heat exchangers and a dedicated cooling system with its own special coolant supply for the fuel system. Because the 6.4L is a common rail system, the fuel needs to be cooled after it has been under so many thousands of pounds of pressure in the rail system. Cummins and Duramax pipe their fuel so that it carries heat from their fuel injector control module, and Duramax runs the fuel through a small radiator-like heat exchanger underneath the floor pan right in front of the fuel tank.
As for the 6.4L, the relationship between the turbo actuator, the PCM and the coolant pump is an interesting one. If, for example, the PCM calls for a change in turbo vane angles and the requested change will raise the actuator temperature too much, the actuator demands that the PCM send it some coolant flow from the fuel cooler pump and the PCM complies. That little rascal knows its limitations and requires heat transfer of its own.
As an addendum, it's important to note that hybrid vehicle batteries perform better cold than they do hot, so a dedicated A/C system works diligently to cool the batteries on those units. Also, there is a dedicated liquid cooling system up front for the electrical components.
Managing heat is and has always been a big deal, particularly on 21st century vehicles, and it goes without saying that a great portion of engineering synergy is directed at creative new ways of managing heat. Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years.
Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at [email protected].
