# What is Engine efficiency: Definition and 47 Discussions

Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-

Internal combustion (gasoline, diesel and gas turbine-Brayton cycle engines) and
External combustion engines (steam piston, steam turbine, and the Stirling cycle engine).Each of these engines has thermal efficiency characteristics that are unique to it.
Engine efficiency, transmission design, and tire design all contribute to a vehicle's fuel efficiency.

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1. ### Heat Engine Efficiency: How Ice Water Affects Performance

Consider mixing ice water with the hot water and vice versa so that the two reservoirs are closer to the same temperature. What will happen to the mechanical/thermodynamic efficiency and ideal Carnot efficiency of a heat engine?
2. ### Thermodynamics problem involving engine efficiency

Q2: An engine has power of 26.5 kW, needs 9 kg coal during an hour for energy. The heat capacity of coal is 7800 cal/g. Define the engine’s efficiency. Qh= mcΔT = 9000x7800xΔT ( stuck) P=26.5kW(is this the power output of the engine in an hour?) If only I could find ΔT, thenI would be able to...
3. ### Increasing Diesel Engine Efficiency with Retractable Plates

In case of diesel cycle thermal efficiency is low, because heat addition takes place at constant pressure.so can we introduce some retractable plate which sits below the piston during end of compression stroke,and retracts back after heat addition,thus providing constant volume heat addition and...
4. ### Heat Engine Efficiency and Entropy

In deriving the Carnot Efficiency, the assumption is made that theoretically most efficient engine will generate no net entropy, meaning that the entropy that enters the system during heat absorption must equal the entropy that leaves the engine during heat rejection. Why is the case? Why would...
5. ### Calculating work done by a Carnot engine

"A Carnot engine operates using a heat source at 500 °C, and a heat sink at room temperature (20 °C). Suppose that as a heat source, you use the combustion of 100 cubic feet of natural gas at room temperature and pressure (e.g. in a fuel cell of some kind). Under ideal conditions, what is the...
6. ### Car randament (engine efficiency)

Diesel and otto car cannot have randament of 100 per cent, practically. But teoretically they could have?
7. ### How to calculate the efficiency of a fuel engine

Homework Statement [/B] Im trying to find the efficiency of a gasoline engine regarded as a control volume. Am I right to use the HV as total energy input into the system? Assuming the enthalpy of formation of the fuel can't be found. I am really wondering if enthalpy of formation equals the...
8. ### Please check my math about this engine efficiency calculation

Hello. I wanted to see how much more efficient a car would be if hydrogen and oxygen (created through electrolysis of water) were injected into the engine alongside gasoline. I got lost when I found out that I would need 241.8 kJ/mol of water to electrolyze it...
9. ### Engine Efficiency: Find ΔW & ΔQh

Homework Statement An ideal gas with Cv = 5 2R, and γ = 1.4 starts at a volume of 1.5m3 , a pressure of 2.0×105Pa ,and a temperature of 300K. It undergoes an isobaric expansion until the volume is V , then undergoes an adiabatic expansion until the volume is 6.0m3 , and finally undergoes an...
10. ### Electrical properties of standard lubrication oil (SAE)

What are the electrical properties of standard lubrication oil (Engine oil)? Where to find? Please give the sources. I have tried my best on the internet.
11. ### Automotive Effect of vibration on engine efficiency

It has been stated in previous threads that engine inefficiencies are caused by: Pumping losses (induction and exhaust) Frictional losses (in bores, bearings, and ancilliaries) Thermal losses to exhaust, coolant (jacket, charge and oil), and surroundings Does engine vibration have an effect on...
12. ### Automotive Why engine efficiency drops for light loads?

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well. Do we know the reasons for that? I am particularly interested...
13. ### Carnot engine efficiency problem

Homework Statement A Carnot engine absorbed 1.0 kJ of heat at 300 K, and exhausted 400 J of heat at the end of the cycle. What is the temperature at the end of the cycle? Homework Equations The efficiency of a Carnot engine is given by the formula Efficiency = 1 – Qc/Qh = 1 –...
14. ### Impact of the engine thermostat temperature setting on engine efficiency

Will a four cycle, internal combustion engine have greater thermal efficiency with a relatively cool jacket water temperature, say 160 degrees F, or a relatively hot water jacket temperature, say 190 degrees F? These specific numbers are only important in as much as they are commonly available...
15. ### Wankel/rotary engine efficiency

I was just wondering how we can find out the efficiency of a rotary engine? Obvious, it is more efficient than a gasoline/diesel engine.For example the RX8 on 1.3 L in 2 rotors get 250 hp which is quite impressive for that capacity,also it works in 4 stroke,and the power to weight ratio is...
16. ### Electric engine for car vs Electric Engine for Drone

Hello, I am trying to find out if there are any differences between how the Electric engine for car works vs how an Electric Engine for Drone works. I am sure the both consist of an Ignition coil, stator and rotor. However I have not been able to find the any schematics for a drones electric...
17. ### Efficiency of Irreversible Engine

I have a small confusion that can an irreversible engine have greater efficiency than reversible engine if both have different source and sink temperatures?I know that under same conditions reversible engines are more efficient but what can be concluded under different conditions like both...
18. ### High temperature insulating material for new engine concept

In the past, reciprocating engine platforms have prevented the use of thermal insulating materials like ceramics due to, among other reasons, abrasion and vibration. One of the problems in the past has been the movement of the piston and piston rings over the cylinder wall where the insulating...
19. ### Thermodynamics - Carnot Cycle-esque question?

Homework Statement A reversible heat engine produces work from the temperature difference that exists between a mass of m = 9 kg of an ideal gas (cv = 716 J/kgK, R = 287 J/kgK) in a rigid container and a heat reservoir at THR = 285 K. The only heat transfer interaction experienced by the...
20. ### Efficiency Calculation for a Stirling Engine Cycle

Homework Statement Figure 2 represents a model for the thermodynamic cycle of the Stirling engine, patented by Scottish clergyman Robert Stirling in 1816. The engine operates by burning fuel externally to warm one of its two cylinders. A xed quantity of inert gas moves cyclically between the...
21. ### Internal combustion engine efficiency

hello we know that there is 42% limitation in the efficiency of internal combustion engine, thermodynamically calculated by Carnot and we can never overcome this can someone explain me in few simple words why this is true? also, does this apply to the energy from internal combustion to...
22. ### Heat engine efficiency could approach 100%?

According to Carnot theorem, the higher temperature of a hot side and the lower temperature of a cold side the higher is the efficiency of a heat engine. Does it mean that in theory it could get anywhere close to 100%? If we take in account that 100% efficiency means it is a perpetuum mobile...
23. ### Rocket engine vs jet engine efficiency

Which are more efficient, Rocket Engines or Jet engines, and why? It would make sense that rocket engines are more efficient because they aren't effected by air pressure but I have been told by some people that actually Jet engines are more efficient so I am confused. If you know of any link...
24. ### Fuels and Engine efficiency Mechanical Engineering

Homework Statement Car (A) C180K WITH A 1.8 litre supercharged spark ignition, Maximum power output 104kw, based upon standard fuel efficiency test a vehicle with this engine would consume 6.1 liters over 100k journey Car (B) C220 CDI with a 2.0 litre turbocharged diesel engine. Maximum...
25. ### Improving IC Engine Efficiency: Reducing Heat Loss Through Cylinder Walls

hi.I would like to know something about the IC engine.I just read an article.It said that 30% efficiency is gone through conduction of heat by the cylinder wall.If it is reason for one of the efficiency loss why can't we build an cylinder coated with a material that is non conduction.So only...
26. ### Heat Engine efficiency Problem

Please help me with the question below with a proper explanation and also suggest me a reference to read for the same. Thank you! A solar collector receiving solar radiation at the rate of 0.6 kW/m2 transforms it to the internal energy of a fluid at an overall efficiency of 50%. The fluid...
27. M

### Why can no other engine be more efficient than a Carnot engine?

Hello, I am having difficulty understanding the paragraph in my textbook explaining why no other engine can be more efficient than a carnot engine. What does it mean? To prove the validity of this theorem, imagine two heat engines operating between the same energy reservoirs. One is a Carnot...
28. ### Basic thermo question: heat engine efficiency

Is it strictly true that an engine will be more efficient if operating in a hotter environment... e.g. your car is more efficient in the summer than the winter? Just going through thermo 1, not a HW question, but it just seems to be true to me because the simple Newton's law of cooling says...
29. ### Does Increasing ICE Engine Efficiency Lower Exhaust Temperatures Proportionally?

So the current estimate is that an ICE engine is about 20%. If we were able to increase the efficiency of a given engine, would that result in a proportional decrease in exhaust temperatures? (Given that driveline loss to friction is constant)
30. ### Carnot engine efficiency * Carnot refrigerator efficiency

Carnot engine efficiency is: \eta_{work} = 1 - \frac{T_c}{T_h} Carnot refrigeration efficiency is: \eta_{cool} = \frac{ T_c }{T_h-T_c} \eta_{cool} = \frac{ 1 }{\frac{T_h}{T_c} - 1} Simple multiplication should give me the efficiency where both the engine and the refrigeration share the...
31. ### Heat engine efficiency calculation

Homework Statement I have an spark ignited engine and after modifying it I have improved the efficiency. What is its efficiency at idle? Fuel consumption at 650 rpm = 9.8 cc/min measured Fuel consumption at 750 rpm for normal engine = 25-30 cc/min estimated for about 5 HP Normal engine...
32. ### What is the thermal efficiency of an ideal gas cycle?

Homework Statement A possible ideal gas cycle operates as follows: (i) From an initial state (p_1,V_1), the gas is cooled at constant pressure to (p_1,V_2); (ii) the gas is heated at constant volume to (p_2,V_2); (iii) the gas expands adiabatically back to (p_1,V_1). Assuming...
33. ### Which Temperature Adjustment Improves Carnot Engine Efficiency More?

Homework Statement Determine which gives the higher efficiency of a Carnot engine, increasing the temperature of the hot reservoir, or decreasing the temperature of the cold reservoir by the same amount. Justify your answer carefully. Homework Equations (TH-TC)/TH)The Attempt at a Solution By...
34. ### Can IC engine efficiency can be increased beyond 60% ?

I found following claim from an inventor: “The limit of IC engine efficiency cannot be normally exceeded due to facts entropy cannot be decreased and 2nd law of thermodynamics (heat cannot flow from cold to hot body). Injecting water in the beginning of expansion stroke can overcome these...
35. ### New 4 stroke engine efficiency

The new issue of the mechanical efficiency of the new 4 stroke engine.What will have the efficiency of the engine, which increases its volume swept by two times slower rotating crankshaft- piston systems ?? Two times slower or inertia four times smaller. Regards Andrew
36. ### Does Revving Closer to Peak Power Improve Fuel Efficiency in Motorcycles?

When I got my motorbike it had a non-standard rear sprocket with two extra teeth. I decided to swap it for a standard one. I expected to get a better overall mpg, but it didn't change noticeably. So... An engine has a red line at 14000 rpm. Peak power is delivered at 12000 rpm. Driving...
37. ### Power & Engine efficiency - Using heat (kelvin)

Homework Statement 1. Homework Statement A power plant taps steam superheated by geothermal energy to 505 K (the temperature of the hot reservoir) and uses the steam to do the work in turning the turbine of an electric generator. The steam is then converted back into water in a condenser...
38. ### Engine efficiency - using heat (kelvin)

Homework Statement A power plant taps steam superheated by geothermal energy to 505 K (the temperature of the hot reservoir) and uses the steam to do the work in turning the turbine of an electric generator. The steam is then converted back into water in a condenser at 323 K (the temperature...
39. ### Engine Efficiency Cycle: 500K-300K, Isothermal Compression & Expansion

process A: isothermal compression at low temperature with an input of work of 83 J process B: constant volume increase in pressure with an energy input by heating of 200 J process C: isothermal expansion at high temperature with work output of 139 J process D: constant volume cooling to the...
40. ### Carnot engine efficiency and exhaust problem

Homework Statement The exhaust temperature of a Carnot heat engine is 121◦C. What is the intake temperature if the efficiency of the engine is 13.4 %? Answer in units of ◦C. Homework Equations e=1-(Tc/Th) The Attempt at a Solution This seems like just a plug and chug problem...
41. ### Finding Efficiency from a P-V Graph

uh...just wondering but how do you find out the efficiency from a P-V graph?
42. ### Thermodynamics - engine efficiency

Homework Statement A reversible engine contains 0.20 mol of ideal monatomic gas, initially at 600 K and confined to 2.0 L. The gas undergoes the following cycle: Isothermal expansion to 4.0 L. Isovolumic cooling to 300 K. Isothermal compression to 2.0 L. Isovolumic heating to 600 K...
43. ### What is the true statement about Carnot engine efficiency?

Homework Statement Which of the following statements about a Carnot engine is true? a) Any Carnot engine has an efficiency of 100% b) Irreversible energies have the greatest possible efficiency. c) Heat can spontaneously flow from a cold reservoir to a hotter reservoir. d) If a process is...
44. ### Another heat engine efficiency problem

A reversible heat engine has an efficiency of 34.2%, removing heat from a hot reservoir and rejecting heat to a cold reservoir at 0°C. If the engine now operates in reverse, how long would it take to freeze 1.10 kg of water at 0°C, if it operates on a power of 183 W? I am getting stuck...
45. ### Calculate Engine Efficiency: 0.490 kJ of Heat for 0.120 kJ of Work

An engine releases 0.490 kJ of heat for every 0.120 kJ of work it does. What is the efficiency of the engine? I am having trouble with this problem. I tried Wnet/ heat input = efficiency 0.120/0.450 = 0.27 but it says it is the wrong answer. (correct answer is 0.21)
46. ### Part B: "150W Engine Efficiency - Heat Given off Per Cycle

An engine with an output of 150W has an efficiency of 25%. It works at 10 cycles/s. A. How much work is done in each cycle? work=power(delta t) = 150 W * 1s = 150 J work =15.0 J per cycle **got part A correct B. How much heat is given off in each cycle? delta...
47. ### Maximizing Heat Engine Efficiency: How to Find the Optimum Performance

how can I find efficiency of heat engine operating between two differenct temperatures?