First law of Thermodynamics examples [Mechanical engineering]

# First law of Thermodynamics examples

Examples of First Law of Thermo

 Energy Exploration Pages
Digressions & Further Explanations Section
How much energy is stored in fuel? It depends on the fuel. There's all kinds of fuel. Diesel fuel, gasoline (or petrol as it's called in some countries), methanol in race cars, jet fuel in jets, propane in fork lifts and backyard grills, and some engines burn a gas called natural gas which is mostly made of methane.

It is fairly straight forward with a liquid fuel like diesel to measure the energy content in a sample. A little bit of the fuel is burned in something called a "bomb calorimeter" and the energy released by combustion is measured by the increase in temperature of a surrounding water bath that absorbs the energy.

This is generally called the Heating Value of the fuel. Fuels like diesel and gasoline actually vary quite a bit in composition being generally a hodge-podge of different hydrocarbon molecules. When measuring fuel consumption in an engine lab or doing an energy balance like the one shown in the picture above, it is very important to have an accurate heating value of the fuel being burned, so it is typical to take frequent samples, each of which are sent to the lab to be analyzed. If you don't have an accurate heating value then your value of fuel energy into the engine won't be accurate.

"Energy In" Equals "Energy Out" plus "Energy Stored":
Steady State means the engine is running at a steady constant load and constant speed and is not warming up or cooling down. In this steady state condition the energy flows are also steady and constant.
Sometimes people say the conservation of energy is "Energy In" equals "Energy Out". This is only true for steady state systems. A good example is our engine above. If the engine has just been started after sitting all night and getting cold it will take a while to get warmed up before it reaches steady state. During the warm up period, some of the fuel's energy will go into heating up the cold metal and cold water...

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