Getting “the word” out to the HEV technician about thermal control
HEVs in general
A Hybrid Electric Vehicle (HEV) is one in which a combination of a liquid hydrocarbon powered engine and electric motor are used for propulsion. These vehicles are being developed by most major car manufacturers in response to the realization that there will be less petroleum in the future. Reserves are declining and the world be facing what is known as “peak oil”, where the rate of petroleum discovery will be less than the rate of consumption. At some point, there is the real possibility that people will have to rely upon electricity as the sole source of vehicle propulsion. This is to say nothing about the second greatest source of air pollution coming from vehicles . For now, however, we need to consider HEVs.
The integrity of an HEV depends upon the integrity of two of the most critical systems, the electric power generation and storage system as well as the thermal control system. It is essential that one understands major principles underscoring the operation of these systems and maintains them properly. There are many types and designs of HEVs, so it is more productive to consider the general thermodynamic principles governing heat and power generation and the means available for thermal regulation. Different vehicles will have different configurations, so it is impractical to direct our attention to specific cases. However, our modularized approach, starting with a discussion of theory, components, and system types will allow appropriate application to specific cases.
We have here only a survey of what is involved in considering thermal regulation in HEVs. A detailed coverage would involve numerous courses in thermodynamics, mechanical design, and mathematics, to name a few areas. This document is directed to well trained HEV supervisory technicians but there are parts that anyone all the way down the repair chain, including ordinary drivers can derive benefit.
It may be noted here that one can get confused or misdirected by over complicating matters. Often, dropping back a few paces and re-examining fundamental principles will lead one directly to a solution. This does not trivialized thermodynamics, as calculation of heat generation and flow can be just as complicated as nuclear physics, as just about any meteorologist will testify. Modeling is very complicated.
“Vehicle” means cars, trucks, and any other means of conveyance that is self-propelled and using electric or a combination of fossil fuel-electric combination of energy source, although this article does not make them our focus, nevertheless, our discussion will apply to motorcycles and scooters.
Several types of HEV exist:
– Fuel Cell-based
– Gas/Diesel-Electric hybrids
Inasmuch as plug-in vehicles don’t use gas and rely solely upon battery power, the battery packs, they are much larger than those found in gasoline-electric hybrids. The greater the range one wants the vehicle to have, the greater size the battery pack will be, and more weight, the greater stress there will be on each moving component of that vehicle, due to friction.
Fuel Cell-based HEV
Fuel cells are a special type of battery, and fuel cell HEVs run both with a hydrocarbon-fueled motor (gas, diesel, or other hydrocarbon) motor and an electric motor. The fuel tank contains the liquid fuel for the hydrocarbon-fueled motor, and the fuel cell is the energy supply for the electric motor.
The hydrocarbon part of the car is a conventional system, except that it is linked to the electrically powered one. The electric system is powered by a battery, and that is charged either by an alternator running off the gasoline engine or by a wall charger, or a combination of both, as in a plug-in hybrid vehicle (PHV). Regardless of the system, thermal control is an issue, although it is more of a problem in gasoline-based engines and vehicles carrying heavy loads than in other cars.
Review of general thermodynamic theory
Before dashing off to read about particulars of systems, it is best to think about an algorithmic approach to heat transfer and regulation issues in HEVs – or anything else, for that matter. We establish some general principles and theories and then apply them. In particular it is useful to review how heat transfer is affected by materials and environment. Then, we can apply this knowledge to particular situations in controlling it.
Heat and its sources
Heat, itself is one of the five types of energy, the others being mechanical, chemical, nuclear, and electrical. Heat is the motion of molecules or atoms. Mechanical energy is defined as work, the sum of force in a specific direction . Chemical energy is that which results from the changes in the associations of atoms in molecules that are bonded together. Nuclear energy comes from forces within the atom. Electrical energy is the flow of electrons.
First, we must know the difference between heat and temperature. Heat, simply put, is the energy created by the movement of atoms and molecules. The only situation in which there is no heat is when neither atoms nor molecules move. However, this is only a theoretical situation, as atoms always are in a constant state of motion, even in the depths of space. Thermal images will confirm this . Temperature is the measurement of average heat in a space or thing. The heat in a small steel ball bearing of two mm in diameter may be the same as that of the Empire State building; size is irrelevant. In theory, one measures the heat of each molecule, sums it, and divides by the number of molecules to get the temperature. The average of 1645/35 = 47 = 141/3, meaning if the total heat content of the first is 35 and there are 1645 molecules, the temperature would be 47. For the second situation, if the total heat content is measured is 141 and there are three molecules, the temperature still is 47. Heat results from the speed of the molecules, which results in more energy. However, we must consider why they move. Energy is not self-generated. Something imparts energy to the molecules, and this comes from the atoms, themselves.
Temperature is measured in three ways: Fahrenheit, Celsius, and Kelvin. A quick glance at SI units is in order. “SI units” means “International System” . We say “SI”, as this is an abbreviation for the French, Le Système International d’Unités . Note that the conversion formula between Fahrenheit and Celsius is: C/F-32 = 5/9.
Heat energy is measured in Joules, named after James Prescott Joule (1818–1889), an English physicist. Other units of heat are the British Thermal Unit (BTU) and the calorie. The former is known by dieticians and weight watchers, and the latter, BTUs, are familiar to those installing thermal control systems. However, the scientific community talks more in terms of joules, and this is the units that we will discuss at present.
Want to read more about thermal control? Continue reading here.
(Source: Text by Jeremy Horne, Ph.D.)
INTERESTED IN LEARNING MORE ABOUT THIS TOPIC?
Thermal Management for Electric Vehicles/HEV
Don’t miss the “International Conference Thermal Management for Electric Vehicles/HEV”, taking place 27-29 June 2011 at the Rhein-Main Hotel in Darmstadt, Germany.
References (All links accessed 10 May 2011)
 http://www.nrdc.org/globalWarming/f101.asp  http://academicearth.org/lectures/work-and-mechanical-energy  http://coolcosmos.ipac.caltech.edu/cosmic_classroom/light_lessons/thermal/heat.html  http://physics.nist.gov/cuu/Units/