Prepared by Pr. Noureddine
Ait Messaoudene
University of Hail
Based on
Yunus A. Cengel and Michael
A. Boles
Thermodynamics: An
Engineering Approach
6th Edition, McGraw Hill,
2007.
Faculty of Enginering
DEPARTMENT OF MECHANICAL ENGINEERING
Chapter 9
GAS POWER CYCLES
Lecture 6
9–6 DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION
ENGINES
9–6 DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES
The CI engine (also known as diesel engines), first proposed by Rudolph Diesel in the
1890s, is very similar to the SI engine, differing mainly in the method of initiating
combustion.
In CI engines, the air is compressed to a temperature that is above the autoignition
temperature of the fuel (no possibility of autoignition since only air), and combustion starts
on contact as the fuel is injected into this hot air.
no spark plug and fuel injection system needed
these engines can be designed to operate at much higher compression ratios, typically
between 12 and 24.
The fuel injection process in diesel engines
starts when the piston approaches TDC and
continues during the first part of the power
stroke. Therefore, the combustion process
in these engines takes place over a longer
interval and is approximated as a constantpressure heat-addition process 2-3.
This is the only difference between CI
engines (Diesel) and SI engines (Otto).
The amount of heat transferred to the working fluid at constant pressure and rejected
from it at constant volume can be expressed as:
and
So, under the cold-air standard assumptions
We now define a new
quantity, the cutoff ratio rc, as
the ratio of the cylinder
volumes after and before the
combustion process:
Utilizing this definition and the isentropic ideal-gas relations for processes 1-2 and 3-4
for the same r
Only difference with
Otto cycle and
always > 0
Also, as the cutoff ratio decreases, the efficiency of the Diesel cycle increases. For the
limiting case of rc=1, the quantity in the brackets becomes unity (proof by l’Hopital’s
rule), and the efficiencies of the Otto and Diesel cycles become identical.
But diesel engines operate at much
higher compression ratios.
The diesel engines also burn the fuel
more completely.
Thermal efficiencies of large diesel engines range from about 35 % to 40 %.
Another ideal cycle that better approaches the real case would be to model the
combustion process in both gasoline and diesel engines as a combination of two
heat-transfer processes, one at constant volume and the other at constant
pressure (this adds some complexity).
The ideal cycle based on this concept is called the dual cycle.
The relative amounts of heat
transferred during each
process can be adjusted to
approximate the actual cycle
more closely in each case (Otto
or Diesel).
100 kPa, 27°C and 1917 cm3
Properties The gas constant of air is
R = 0.287 kPa.m3/kg . K and its other
properties at room temperature are
cp = 1.005 kj/kg . K, cv = 0.718 kj/kg . K
and k = 1.4 (Table A-2a)
1917 cm3
106.5 cm3
213 cm3
(2) (106.5)
1917 cm3
300 K
953 K
100kPa
5720 kPa
5720 kPa
953 K
1906 K
(5720
213
1917
213
1917
1906 K
791 K
264 kPa
(100 kPa)(1917x 10-6 m3 )
(0.287 kPa. m3 /kg)(300 K )
= 0.00223 kg
(0.00223 kg) (1.005 kJ/kg.K) (10906 K- 953 K)
2.136 kJ
(0.00223 kg) (0.718 kJ/kg.K) (791 K- 300 K)
0.786 kJ
2.136 – 0.786
1.35 kJ
2.136 kJ
1.35 kJ
Be carful
about the unit
conversion to
m3
1.35 kJ
(1917 – 106.5) x 10-6 m3
746 kPa