Calculating Thermal Efficiency of a 4-Process Heat Engine: Otto or Diesel Cycle

In summary, the conversation is about calculating the thermal efficiency of a heat engine with four processes, specifically the Otto or Diesel Cycle. The data provided includes temperature, specific gas constant, specific heat capacity, pressure, and volume for each state. The suggested method is to use air tables to calculate enthalpy/internal energy values and then use the formula n (thermal) = (Qh-Qc)/Qh to determine efficiency. There is also a discussion about calculating work done for an Otto Cycle and the helpful input of Gordan. Overall, the conversation provides useful information for someone new to the subject.
  • #1
EngNoob
38
0
Hi

Could anyone help me with this?

How would i go about calculating the thermal efficiency of a heat engine with four processes?

I am thinking of moddeling it on the Otto or Diesel Cycle.

The data i have is.

Temp @ State/Process 1

Specific Gas Constant

Specific Heat Capacity

and i have

Pressure

Volume

for states one, two, three and four.

How would i got about figuring out the thermal efficiency? do i use a NFEE equation?

Thanks for any help.
 
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  • #2
Although my knowledge is limited on the subject, I assume that if the working fluid is air, then you would need some air tables to look up the enthalpy/internal energy values for each state. Once you have those values you can calculate the net work and the heat input to the engine. Your efficiency then is the ratio of the two.
 
  • #3
See how much heat is transferred at the high temperature (Qh) then see how much heat is transferred at the low temperature (Qc), then use

n (thermal) = (Qh-Qc)/Qh or W/Qh
 
  • #4
Thanks for that, i will have a go using that formula.

However, i need to calculate work done first for a otto cycle, any ideas on that?

I have Volume, Pressure and Exp/Comp Index
 
  • #5
Hi there:

Check out your post on compression and expansion.

The same input material covers this post.

Here are two plots regarding the thermal efficiency of Otto Cycle and Diesel Cycle.

Slide43.GIF


Slide50.GIF


Thanks,

Gordan
 
Last edited:
  • #6
Thanks for that, understand better now, cheers...
 
  • #7
EngNoob:

I am glad to be of some help to you and PF members and/or visitors ...

Thanks,

Gordan
 
  • #8
Hi, I'm new by here.
The information helps.I will see something about jet engines.
But, just to make presence, leave a reply.Ok?
See you later.
 

1. What is a 4-process heat engine?

A 4-process heat engine is a type of engine that operates on a thermodynamic cycle, consisting of four distinct processes: intake, compression, power, and exhaust. These processes are repeated continuously to convert heat energy into mechanical work.

2. What is the difference between an Otto and a Diesel cycle?

The main difference between an Otto and a Diesel cycle is the type of fuel used. In an Otto cycle, the fuel is ignited by a spark plug, while in a Diesel cycle, the fuel is ignited by the high temperature and pressure caused by compression. Additionally, the compression ratio is higher in a Diesel cycle, resulting in a more efficient conversion of heat energy into work.

3. How is thermal efficiency calculated for a 4-process heat engine?

The thermal efficiency of a 4-process heat engine can be calculated by dividing the work output by the heat input. This can be represented by the equation: efficiency = work output / heat input. The work output is the net work done by the engine, while the heat input is the total amount of heat supplied to the engine.

4. What factors affect the thermal efficiency of a 4-process heat engine?

There are several factors that can affect the thermal efficiency of a 4-process heat engine. These include the compression ratio, the type of fuel and combustion process, the temperature and pressure at various stages of the cycle, and the mechanical and thermal losses within the engine.

5. Can the thermal efficiency of a 4-process heat engine be greater than 100%?

No, the thermal efficiency of a 4-process heat engine cannot be greater than 100%. This is due to the second law of thermodynamics, which states that no engine can have a thermal efficiency of 100% or higher. In real-world applications, the thermal efficiency of a 4-process heat engine is typically around 30-40%, with the remaining energy being lost as heat.

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