Mechanical Engineering - Thermodynamics - Discussion

1. 

For the same maximum pressure and temperature,

[A]. Otto cycle is more efficient than Diesel cycle
[B]. Diesel cycle is more efficient than Otto cycle
[C]. Dual cycle is more efficient than Otto and Diesel cycles
[D]. Dual cycle is less efficient than Otto and Diesel cycles

Answer: Option B

Explanation:

No answer description available for this question.

Ragul said: (Aug 2, 2014)  
Because diesel cycle gives more efficient.

Imran Ali said: (Nov 14, 2014)  
Diesel cycle depends on compression ratio and cut of ratio but not on temperature than how it is more efficient than Otto cycle.

Manoj Kumar Sharma said: (Feb 13, 2015)  
Otto cycle gives more efficient because Diesel cycle depends on compression ratio and cut of ratio but not on temperature.

Jain G said: (Mar 10, 2016)  
From PV diag w for diesel is greater than Otto so diesel is more efficient.

Pritesh said: (Jul 19, 2016)  
I think it should be, Otto cycle is always efficient than diesel cycle.

Sanjeev said: (Sep 25, 2016)  
I think the answer is A.

If the answer is B please do explain.

Ajay Gupta said: (Oct 2, 2016)  
I am sure that the correct answer is A.

Please don't create any doubt friends, but if ask thermal efficiency diesel and otto then diesel efficiency greater than otto cycle.

Roshan Virat Pandey said: (Nov 1, 2016)  
Diesel cycle is more efficient. Compare by making pv or TS diagram. In this condition, the diesel cycle has higher compression ratio than the otto cycle.

Roshan Virat Pandey said: (Nov 1, 2016)  
If the same compression ratio is given then the otto cycle will be the answer.

Sanket said: (Dec 7, 2016)  
A is the correct answer.

Rathod Bhavesh said: (Jan 5, 2017)  
A is right. I also agree.

Mohit said: (Mar 3, 2017)  
For same compression ratio, Otto cycle is more efficient than diesel & for any other condition, always diesel cycle is more efficient.

Vijay said: (Mar 11, 2017)  
Answer b is right.

Lova said: (Apr 12, 2017)  
According to me, the correct answer is A.

For same compression ratio, Otto cycle is more efficient than diesel cycle.

During compression changing parameters is pressure, temperature and volume.

Rangzaib said: (May 31, 2017)  
The otto cycle is more efficient at any pressure and temperature than diesel cycle. And diesel cycle's efficiency will equal to otto cycle when cut off will 0.

Basha Khan said: (Oct 2, 2017)  
The Otto cycle is efficient than diesel cycle.

Mounika said: (Nov 18, 2017)  
Can any one what is the correct and why?

Ujjawal Aggarwal said: (Aug 5, 2018)  
A is the correct answer.

Because Otto cycle only depends on temperature ratio but in diesel cycle efficiency there has also one factor which is cutt-off ratio due to which the efficiency of a diesel is less as compared to the Otto cycle.

Dhanunjay said: (Dec 17, 2018)  
1. At the same compression ratio and heat rejection.
Otto cycle efficiency>diesel cycle efficiency.

2. At the same maximum pressure and temperature the heat rejection being also the same.
Diesel cycle efficiency>otto cycle efficiency.

Sambhu Jana said: (Feb 27, 2019)  
The correct answer is A.

Swayamjeet said: (Mar 4, 2019)  
For the same compression ratio, Otto cycle efficiency is more than diesel. But for maximum temperature and pressure efficiency of diesel cycle is more than otto cycle.

Sarath said: (Apr 11, 2019)  
The entropy at 3 should be greater than entropy at 3'. It is clear that the heat rejection for Otto cycle is more than that of diesel cycle. Hence, for these conditions, the Diesel cycle is more efficient than the Otto cycle. The efficiency of Dual cycle lies between the two cycles.

Ashish Patel said: (Jun 3, 2019)  
For the same maximum pressure and heat input, the Otto cycle (1-2-3-4-1) and Diesel cycle (1-2'-3'-4'-1) are shown on the T-s diagram in the figure. It is evident from the figure that the heat rejection for Otto cycle (area 1-5-6-4 on T-s diagram) is more than the heat rejected in Diesel cycle (1-5-6'-4'). Hence the Diesel cycle is more efficient than the Otto cycle for the condition of same maximum pressure and heat input. One can make a note that with these conditions, the Diesel cycle has a higher compression ratio than that of the Otto cycle. One should also note that the cycle which is having higher efficiency allows maximum expansion. The Dual cycle efficiency will be between these two.

Therefore, ranking the three cycles on the basis on efficiencies,

Diesel Cycle > Dual Cycle > Otto Cycle

Case 2: Same Maximum Pressure and Work Output

The efficiency, η can be written as

Refer to T-s diagram. For same work output, the area 1-2-3-4 (work output of Otto cycle) and area 1-2'-3'-4' (work output of Diesel cycle) are same. To achieve this, the entropy at 3 should be greater than entropy at 3' .It is clear that the heat rejection for the Otto cycle is more than that of the diesel cycle. Hence, for these conditions, the Diesel cycle is more efficient than the Otto cycle. The efficiency of Dual cycle lies between the two cycles.

Therefore, ranking the three cycles on the basis of efficiencies,

Diesel Cycle > Dual Cycle > Otto Cycle

Note: We cannot compare Rankine cycle with Gas Power cycles (Otto, Diesel Dual), since it is a steam cycle and the working medium for a steam cycle is water. Water in the steam cycle has a much lower temperature and pressure than air standard cycles and thus cannot be compared with them.

Also, the Brayton cycle is meant for much larger heat engines and cannot be compared to the much smaller Otto, Diesel and Dual Cycle Engines.

Hardik said: (Jun 5, 2019)  
Please explain the right answer.

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