FIGURE Q21.4 shows the pV diagram of a heat engine. During which stage or stages is (a) heat added to the gas,(b) heat removed from the gas, (c) work done on the gas, and (d) work done by the gas?
(a) Heat added to gas
(b) Heat removed from gas
(c) Work done on gas
(d) Work done by gas
A heat engine is a device that turns heat into work by extracting heat from a reservoir (hot body) and using it to perform some task. A small amount of heat is ejected into the sink (cold body). There will be some waste in the form of heat with this system. There are various types of heat engines, with the Carnot engine having the highest efficiency.
Stage 1: Fixed volume but temperature increases, hence heat is added.
Stage 2: Isothermal process
Stage 3: Heat is removed by doing work on the gas.
A Carnot heat engine and an ordinary refrigerator with coefficient of performance operate between reservoirs at and . The work done by the Carnot heat engine drives the refrigerator. If the heat engine extracts of energy from the hot reservoir, how much energy does the refrigerator exhaust to the hot reservoir?
There has long been an interest in using the vast quantities of thermal energy in the oceans to run heat engines. A heat engine needs a temperature difference, a hot side and a cold side. Conveniently, the ocean surface waters are warmer than the deep ocean waters. Suppose you build a floating power plant in the tropics where the surface water temperature is . This would be the hot reservoir of the engine. For the cold reservoir, water would be pumped up from the ocean bottom where it is always . What is the maximum possible efficiency of such a power plant?
The FIGURE shows the Diesel cycle. It is similar to the Otto cycle (see Problem), but there are two important differences. First, the fuel is not admitted until the air is fully compressed at point . Because of the high temperature at the end of adiabatic compression, the fuel begins to burn spontaneously. (There are no spark plugs in a diesel engine!) Second, combustion takes place more slowly, with fuel continuing to be injected. This makes the ignition stage a constant-pressure process. The cycle shown, for one cylinder of a diesel engine, has a displacement of and a compression ratio These are typical values for a diesel truck. The engine operates with intake air at and pressure. The quantity of fuel injected into the cylinder has a heat of combustion of .
. Find , , and at each of the four corners of the cycle. Display your results in a table.
. What is the network done by the cylinder during one full cycle?
. What is the thermal efficiency of this engine?
. What is the power output in kW and horsepower of an eight-cylinder diesel engine running at ?
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