How to find efficiency

In order to calculate the coefficient of useful action (CPA) divide the useful work Up to the work expended AZ and multiply the result by 100% (efficiency=AP/AZ∙100%). Get the result in percent.

When calculating the efficiency of the heat engine, useful work, consider the mechanical work done by a mechanism. For the spent work take the amount of heat generated by the burned fuel which is a source of energy for the engine.

Example. The average tractive power of the engine of the vehicle is 882 N. At 100 km it consumes 7 kg of gasoline. Determine the efficiency of its engine. First, find a good job. She is the product of force F for a distance S traveled by a body under the influence of AP=F∙S. Determine the quantity of heat allocated at burning 7 kg of gasoline, it will be spent working AZ=Q=q∙m, where q – calorific value of fuel, for gasoline it is equal to 42∙10^6 j/kg and m is the mass of fuel. Engine efficiency is equal to efficiency=(F∙S)/(q∙m)∙100%= (882∙100000)/( 42∙10^6∙7)∙100%=30%.

In the General case, to find the efficiency of any heat engine (internal combustion engine, steam engine, turbine, etc.) where the work is done by the gas has a coefficient of useful action is equal to the difference of heat given to the heater, Q1 and Q2 received by the fridge, find the difference between the warmth of the heater and of the refrigerator, and divide by the warmth of the heater efficiency= (Q1-Q2)/Q1. Here efficiency is measured in units of decimal fraction from 0 to 1 to convert the result into a percentage, multiply it by 100.

To get the efficiency of an ideal heat engine (Carnot), find the ratio of the temperature difference of the heater T1 and cooler T2 temperature the heater efficiency=(T1-T2)/T1. This is the maximum possible efficiency for a particular type of heat engine with given temperatures of the heater and of the refrigerator.

To find the spent motor work as the product of the power at the time of its execution. For example, if the motor of the crane capacity of 3.2 kW lifts a load weighing 800 kg to a height of 3.6 m for 10 s, then its efficiency is equal to the ratio of useful work AP=m∙g∙h, where m is the load mass, g≈10 m/S2 the acceleration of gravity, h is the height to which lifted the load, and spent working AZ=R∙t, where P – motor power t – the time it works. Will receive the formula for definition of efficiency=AP/AZ∙100%=(m∙g∙h)/(R∙t) ∙100%=%=(800∙10∙3,6)/(3200∙10) ∙100%=90%.