Metal Engines

Metal Internal Combustion Engines Are Very Inefficient:

 

How efficient are engines? Combustion engines are stupefyingly inefficient. Most diesel engines do not even have a thermal efficiency of 50%. Of every gallon of diesel burned by a combustion engine, less than half of the energy generated becomes mechanical energy. That is to say, of the energy produced by the diesel engine in a pickup truck, for example, less than half of the energy produced actually pushes the pickup down the road.

And, gasoline-powered vehicles are even more inefficient, considerably more inefficient.

While it may sound like a vehicle that only converts 50% of the thermal energy it produces during combustion into mechanical energy is extraordinarily inefficient, many vehicles on the road actually waste close to 80% of the energy produced during the combustion of fuel. Gasoline engines often blow more than 80% of the energy produced out the tailpipe or lose that energy to the environment around the engine.

The reasons combustion engines are so inefficient are consequences of the laws of thermodynamics. Thermodynamics determine the thermal efficiency — or inefficiency — of a combustion engine.

“Internal combustion engines produce mechanical work (power) by burning fuel. During the combustion process the fuel is oxidized (burned). This thermodynamic process releases heat which is transformed partly in mechanical energy,” according to X-Engineer.org. But, a great deal of the energy produced is lost. A great deal of the energy produced by a combustion engine is wasted.

While even a short explanation of why combustion engines necessarily require a somewhat lengthy explanation of thermodynamics, a Twitter feed length explanation is easy to understand: the difference in temperature between fuel combustion, the engine, and the air outside the engine determines thermal efficiency — i.e. combustion engine inefficiency.

Additionally, a huge portion of the energy produced by a combustion engine simply blows out the exhaust, again, never becoming mechanical energy.

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[2] Source: https://en.wikipedia.org/wiki/Internal_combustion_engine#Measures_of_engine_performance 

Measures of engine performance

 

Engine types vary greatly in a number of different ways:

Energy efficiency

Once ignited and burnt, the combustion products—hot gases—have more available thermal energy than the original compressed fuel-air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure that can be translated into work by the engine. In a reciprocating engine, the high-pressure gases inside the cylinders drive the engine’s pistons.

Once the available energy has been removed, the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (top dead center, or TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat that is not translated into work is normally considered a waste product and is removed from the engine either by an air or liquid cooling system.

Internal combustion engines are heat engines, and as such their theoretical efficiency can be approximated by idealized thermodynamic cycles. The thermal efficiency of a theoretical cycle cannot exceed that of the Carnot cycle, whose efficiency is determined by the difference between the lower and upper operating temperatures of the engine. The upper operating temperature of an engine is limited by two main factors; the thermal operating limits of the materials, and the auto-ignition resistance of the fuel. All metals and alloys have a thermal operating limit, and there is significant research into ceramic materials that can be made with greater thermal stability and desirable structural properties. Higher thermal stability allows for a greater temperature difference between the lower (ambient) and upper operating temperatures, hence greater thermodynamic efficiency. Also, as the cylinder temperature rises, the engine becomes more prone to auto-ignition. This is caused when the cylinder temperature nears the flash point of the charge. At this point, ignition can spontaneously occur before the spark plug fires, causing excessive cylinder pressures. Auto-ignition can be mitigated by using fuels with high auto-ignition resistance (octane rating), however it still puts an upper bound on the allowable peak cylinder temperature.

The thermodynamic limits assume that the engine is operating under ideal conditions: a frictionless world, ideal gases, perfect insulators, and operation for infinite time. Real world applications introduce complexities that reduce efficiency. For example, a real engine runs best at a specific load, termed its power band. The engine in a car cruising on a highway is usually operating significantly below its ideal load, because it is designed for the higher loads required for rapid acceleration.[citation needed] In addition, factors such as wind resistance reduce overall system efficiency. Engine fuel economy is measured in miles per gallon or in liters per 100 kilometres. The volume of hydrocarbon assumes a standard energy content.

Most iron engines have a thermodynamic limit of 37%. Even when aided with turbochargers and stock efficiency aids, most engines retain an average efficiency of about 18–20%.[38] However, the latest technologies in Formula One engines have seen a boost in thermal efficiency past 50%.[39] There are many inventions aimed at increasing the efficiency of IC engines. In general, practical engines are always compromised by trade-offs between different properties such as efficiency, weight, power, heat, response, exhaust emissions, or noise. Sometimes economy also plays a role in not only the cost of manufacturing the engine itself, but also manufacturing and distributing the fuel. Increasing the engine’s efficiency brings better fuel economy but only if the fuel cost per energy content is the same.

Measures of fuel efficiency and propellant efficiency

For stationary and shaft engines including propeller engines, fuel consumption is measured by calculating the brake specific fuel consumption, which measures the mass flow rate of fuel consumption divided by the power produced.

For internal combustion engines in the form of jet engines, the power output varies drastically with airspeed and a less variable measure is used: thrust specific fuel consumption (TSFC), which is the mass of propellant needed to generate impulses that is measured in either pound force-hour or the grams of propellant needed to generate an impulse that measures one kilonewton-second.

For rockets, TSFC can be used, but typically other equivalent measures are traditionally used, such as specific impulse and effective exhaust velocity.