Thermodynamics Review Problems for Mechanical Engineering Students | Series 21: Fourth Series for Diesel Cycle
This is the 22nd series of my "Thermodynamics Review Problems for Mechanical Engineering Students". If you've missed the previous series you may try scrolling this blog and head over to the "Curriculum". Additionally this series happens to be the 4th that tackles the Diesel Cycle. This time around, we will be computing review problems with respect to the operating conditions of each given Diesel cycle.
Review Problem 1
Determine the air-standard efficiency of an engine operating on the Diesel cycle with clearance of
6 % when the suction pressure is 100 kPa and the fuel is injected for 5 % of the stroke.Solution
The key to solving this review problem is to be familiarize of the pressure-volume (P-V) diagram of a Diesel cycle. Looking at the above photo, the space between the Point 2 and the axis for pressure, the horizontal distance is expressed as
cVD for which c represents percent in clearance, whereas VD represents the volume displacement or the stroke, which is equal to the difference of V1 to V2. So let us start obtaining the compression ratio (rc) of the cycle which is equal to the V1 divided by V2. Using that relation we’ve found out that rk is equal to 17.67. For the computation of cut-off ratio (rc), we need to bear in mind that it is equal to V3 divided by V2. And since the fuel was injected at Point 2 to Point 3, for which the swept volume is expressed as a function of stroke thru a percentage or portion of VD, thereby making the total horizontal distance for V3 equal to cVD + %iVD (i stands for fuel injected), we can now compute for the rc, by taking the horizontal distance of point 2 to the axis of pressure which is equal to cVD and that is our V2. By using those relation, we’ve found out that rc is equal to 1.83. Finally, we can now obtain the cycle efficiency by using the shortcut formula and with that, we’ve found out that the Diesel cycle is 0.6368 ~ 63.68 % efficient.rk |
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rc |
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e |
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Review Problem 2
A Diesel cycle has a cycle efficiency of
58 %. If heat added is 1600 kJ/kgm, find the work.Solution
Well, this review problem is pretty simple. In obtaining the work (
WN), we only need to multiply the cycle efficiency (e) to the heat that is being added (QA) to the Diesel cycle. Thus, we’ve found out that the work that is being done by the Diesel cycle is equal to 928 kJ/kgm.WN |  |
Curriculum
Reference
- PIPE - PROBLEM SET #3 by Alcorcon Engineering Review Center
- Sta. Maria, H. [1990] 2012. Thermodynamics 1. Mandaluyong City: National Bookstore Inc..
