المحركات الحرارية - الحرارة .. الفيزياء

Heat Engines - Heat .. Physics

Heat Engines

A heat engine is a machine that converts thermal energy into chemical energy. The engine derives all the heat it needs from the combustion of fuel stored for mechanical energy. The table shows us the energy density present in several types of fuel and in energy storage devices, and it is expressed as the amount of energy present in a fuel or device with a mass of one kilogram (see p. 31).

(The four-stroke cycle and the pressure / volume graph in both the Otto and Diesel cycles).

The first condition that must be met in a heat engine is that it be highly efficient. It is known that the total efficiency or thermal efficiency in an engine is the result of dividing the work produced by it by the amount of heat entering it, i.e. C / 5. In addition, the heat engine is supposed to be inexpensive, lightweight, low-noise, and cause only a minimum of pollution.

Heat engines operate on the basis of a single cycle, which is a set of procedures that are repeated between cycles. If we assume that the amount of heat absorbed by the engine, during its number of cycles, at a maximum temperature D1 is 15, and the amount of heat it gives off at a minimum temperature D1 is 25, the work produced by it becomes 15 - 25. (See the first law of thermodynamics, noting that there is no change in its internal energy) and the efficiency becomes as follows: H / 5 1 = (15 - 25) / 15 = 1 - 25/15, and in order to obtain maximum efficiency, you should be reduced and enlarged to the greatest possible extent. Since you are proportional to D, the higher the temperature in the engine, the greater its overall efficiency. The oil-powered engine is characterized by being built on the Otto cycle (named after Nicola Otto 1832 - 1891). The diagram shows the ideal Otto cycle, which shows the change in pressure value in the engine in relation to volume, during the different strokes (stages) of the piston and the corresponding valves. This cycle is not applied in practice, but it illustrates all the principles used in the engine, and consists of six operations that include four strokes of the piston, and for this reason it is known as a four-stroke cycle or a four-stroke cycle.

In the process between 1-2 the piston descends drawing air and oil gas with it through the open inlet valve, then rises during the process 2-3 compressing this gas - in a way that is supposed to be ideal, i.e. without any loss or gain of heat (suppressed). Then the spark plug in the process 3-4 detonates the compressed gas, generating a quantity of heat that causes the pressure in the cylinder to rise violently from full to low before the piston starts moving. The process 4-5 is called the working stroke, during which the expanding gas pushes the piston down, and when it reaches the end of its stroke the exhaust valve 5-2 opens and the pressure value decreases until it becomes equal to the atmospheric pressure value D, and in the last stroke 2-1 the exhaust gas is expelled out of the cylinder. (Comparison of the cost, weight and pollution of different heat engines)

The maximum efficiency of the Otto cycle is, in theory, about 67%, and is based on a compression ratio (i.e. the ratio of the pressure immediately preceding combustion to the atmospheric pressure H2/H1) of 9 to 1.

It is very difficult to achieve a higher compression ratio due to the tendency of oil gas to ignite before the spark occurs (pre-ignition). As for the diesel engine based on the Diesel cycle (named after Rudolf Diesel, 1858-1913), the air is compressed in a static manner during the 2-3 stroke until the temperature becomes sufficient to ignite the oil sprayed into the cylinder. There is no need for a spark here. Then the combustion starts at constant pressure and the piston moves down during this process - represented by the horizontal line 3-4 on the graph instead of the vertical line in the Otto cycle. Despite the use of compression ratios in a diesel engine that reach a rate of 4 or 15 to 1, the maximum efficiency in it does not exceed 64%. However, the efficiency of diesel engines and oil engines is practically less than this amount.

Diesel engines and Otto engines suffer from disadvantages resulting from their inability to exceed a certain combustion temperature and their limited compression ratios. They all cause pollution due to intermittent and incomplete combustion inside them.
As for the Wankel engine, which uses a rotating piston, it has many advantages related to weight and smooth operation. However, like conventional engines, it also causes pollution and suffers from not exceeding certain thermal limits.

Steam turbines are not subject to these restrictions - their compression ratio is 30 to 1, and their maximum operating temperature reaches 1200 degrees.

Since combustion is continuous, the resulting pollution is very low. Air enters the turbine through a rotary compressor, then passes into a chamber where it is heated by burning fuel.

The hot gas then expands and rotates the turbine, which then produces power and operates the compressor, which often rotates around the same shaft.

Fuel or energy storage device

Material: Converts matter into liquid hydrogen energy

Oil / diesel oil
Lead-acid battery
Flywheel
Spring

Air is compressed in two batches in a Wankel engine with two rotors.