Questions: Energy Efficiency in Engineering Systems
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
A three-stage system has efficiencies of 80%, 50%, and 90%. What is the overall system efficiency?
A73.3%
B36%
C220%
D50%
Overall efficiency = 0.80 × 0.50 × 0.90 = 0.36 = 36%. The efficiencies multiply, and the result is always less than the lowest individual efficiency. The 50% stage is the bottleneck limiting overall performance.
Question 2 True / False
In the system above, improving the 90% stage to 95% would have a larger impact than improving the 50% stage to 55%.
TTrue
FFalse
Answer: False
Improving 50% to 55%: new overall = 0.80 × 0.55 × 0.90 = 39.6% (a 10% relative improvement). Improving 90% to 95%: new overall = 0.80 × 0.50 × 0.95 = 38% (a 5.6% relative improvement). Improving the weakest link has the bigger impact.
Question 3 Short Answer
Where does the 'lost' energy go in a gasoline car that is only 20-25% efficient?
Think about your answer, then reveal below.
Model answer: About 60-65% is lost as heat in the exhaust gases and through the radiator (waste heat from the combustion process). Another 10-15% is lost to friction (in the engine, transmission, and tires) and aerodynamic drag. All losses ultimately become thermal energy -- the car is essentially a heater that also moves.
Conservation of energy means all 100% of the fuel's chemical energy must go somewhere. Only 20-25% moves the car. The rest heats the environment. This is why improving engine efficiency is so important for fuel economy and emissions reduction.