EXPERIMENT: CONSTANT-SPEED PERFORMANCE TEST
OF A DIESEL ENGINE (KUBOTA ENGINE)
OBJECTIVES
The purpose of this experiment is to measure the power, specific fuel consumption (SFC), torque and
thermal efficiency (𝜂) of a diesel engine operating at a constant-speed setting and develop a good
understanding of the relationships that exist between them.
TEST PROCEDURE
For this experiment, the diesel engine was initially started and run idle for five minutes. The idling
condition was a load of 1600 rpm and 5 lb.ft. At idle point, the fuel flow, engine power, load and engine
speed are recorded. The throttle was then increased to achieve a speed of 2800 rpm and the readings
were taken at this point. The speed was then held constant and the load was increased to 10 lbf.ft. The
speed of the engine decreased due to “governor’s droop” and it was adjusted to maintain a constant
speed of 2800 rpm. The load was successively increased to 15, 20, 25, and 30 lbf.ft with the engine
speed maintained at 2800 rpm while readings were taken in each case. The engine operation was
reduced to the idling conditions as in the beginning of the experiment by reducing the load first and
then slowly reducing the engine speed. The final readings were taken and then tabulated in the data
table.
ENGINEERING SCHEMATIC
Figure 1: Simple Control Volume Schematic for the engine [1]
̇
Where 𝑊 is the work rate of the engine
𝑄̇𝑜𝑢𝑡 is the rate of heat loss to the surroundings
DATA TABLES
Table 1: Data Set #3 for Experiment 2
Test#
Load Scale
Reading (lbf.ft)
1 (idle)
5
2
5
3
10
4
15
5
20
6
25
7
30
8 (idle)
5
Engine Speed
(RPM)
1600
2800
2800
2800
2800
2800
2800
1600
Table 2: Results from Calculations
𝑙𝑏𝑚
Test#
Mass flow rate ( )
ℎ𝑟
1 (idle)
2
3
4
5
6
7
8 (idle)
1.4414
1.1088
2.2176
2.9938
4.1026
4.7678
5.9875
1.3306
Fuel Flow Reading
(ml/min)
13
10
20
27
37
43
54
12
Power Output of
engine (hp)
Displayed Engine
Power (hp)
1.2
1.9
4.2
6.3
9.2
10.9
13.1
1.1
Specific fuel
1.2
1.9
4.2
6.3
9.2
10.9
13.1
1.1
CALCULATIONS, GRAPHS AND RESULTS
1.Calculating the mass flow rate (𝒎̇ 𝒇 , [
𝜌𝑓𝑢𝑒𝑙
Specific gravity = 𝜌
𝐻2 𝑂
𝒍𝒃𝒎
]):
𝒉𝒓
𝜌𝐻2 𝑂 = 1.0𝑔/𝑚𝑙
𝜌𝑓𝑢𝑒𝑙 = 𝑆. 𝐺.× 𝜌𝐻2 𝑂 = 0.84 ×
1.0𝑔 10−3 𝑘𝑔
1𝑚𝑙
×
× −3 = 0.84𝑘𝑔/𝐿
𝑚𝑙
1𝑔
10 𝐿
Using data from test 1 as the sample,
Fuel flow rate = 13 ml/min
13 𝑚𝑙 13 𝑚𝑙
1𝐿
60 𝑚𝑖𝑛
𝐿
=
×
×
= 0.78
𝑚𝑖𝑛
𝑚𝑖𝑛
1000 𝑚𝑙
1 ℎ𝑟
ℎ𝑟
𝐿
0.78 𝐿 0.84 𝑘𝑔
𝑘𝑔
0.78 =
×
= 0.6552
ℎ𝑟
ℎ𝑟
𝐿
ℎ𝑟
1kg = 2.2lbm
𝑘𝑔 0.6552 𝑘𝑔 2.2 𝑙𝑏𝑚
𝑙𝑏𝑚
0.6552
=
×
= 1.441
ℎ𝑟
ℎ𝑟
1 𝑘𝑔
ℎ𝑟
𝑙𝑏𝑚
𝑚̇𝑓 = 1.4414
ℎ𝑟
𝑙𝑏𝑚
consumption (ℎ𝑝∙ℎ𝑟)
1.2012
0.5836
0.5280
0.4752
0.4459
0.4374
0.4571
1.2096
𝒍𝒃𝒎
Calculation for the specific fuel consumption (𝑺𝑭𝑪, [𝒉𝒑∙𝒉𝒓]):
𝑆𝐹𝐶 =
𝑚̇𝑓
𝑃𝑜𝑢𝑡
Using data from test #1 as the sample calculation,
Power displayed by engine, 𝑃𝑜𝑢𝑡 = 1.2 hp
𝑙𝑏𝑚
1.4414
ℎ𝑟 = 1.2012 𝑙𝑏𝑚
𝑆𝐹𝐶 =
1.2 ℎ𝑝
ℎ𝑝 ∙ ℎ𝑟
2. Calculating the power produced by combustion of air and fuel (𝑷𝒊𝒏 , [𝒉𝒑]):
SFC is the amount of fuel that the engine needs to burn per hour to produce 1hp of power. The lower
the SFC is, the more fuel efficient the engine is. Thus, the point with the best SFC is test #6 with SFC of
𝑙𝑏𝑚
0.4374 ℎ𝑝∙ℎ𝑟
Calculating the power produced by combustion, (𝑃𝑖𝑛 , [ℎ𝑝]):
𝑃𝑖𝑛 = 𝐿𝐻𝑉 × 𝑚𝑓̇
Lower Heating Value (LHV) = 18,600 Btu/lbm
𝐵𝑡𝑢
𝑙𝑏𝑚
1 ℎ𝑝
𝑃𝑖𝑛 = 18600
× 4.7678
= 88681.08 𝐵𝑡𝑢/ℎ𝑟 ×
𝑙𝑏𝑚
ℎ𝑟
2544 𝐵𝑡𝑢/ℎ𝑟
𝑃𝑖𝑛 = 34.8589 ℎ𝑝
3.Calculating the thermal efficiency (𝜼𝒕𝒉 ) of the engine at the point of the best SFC
Test #6 is the point of best SFC
𝑃𝑜𝑢𝑡
10.9 ℎ𝑝
𝜂𝑡ℎ [%] =
× 100 =
× 100% = 31.27 %
𝑃𝑖𝑛
34.8589 ℎ𝑝
4. Calculating the cost of electrical power generated at best SFC:
As of November 10th, 2020, the average retail price for diesel in Canada is 99.9 cents/litre [2].
𝑐𝑒𝑛𝑡𝑠
𝑐𝑒𝑛𝑡𝑠 𝑆𝐹𝐶
𝐶𝑜𝑠𝑡 [
×
]=
𝑘𝑊. ℎ𝑟
𝐿
𝜌𝑓𝑢𝑒𝑙
𝑙𝑏𝑚
99.9 𝑐𝑒𝑛𝑡𝑠 0.4374 ℎ𝑝 ∙ ℎ𝑟
1𝑘𝑔
1 ℎ𝑝
𝐶𝑜𝑠𝑡 =
×
×
×
𝑘𝑔
𝐿
2.2 𝑙𝑏𝑚 0.7457 𝐾𝑊
0.84 𝐿
𝑐𝑒𝑛𝑡𝑠
𝐶𝑜𝑠𝑡 = 31.709
𝐾𝑊. ℎ𝑟
Graph of Specific Fuel Consumption (SFC) vs Power Output
0.7
0.6
SFC (lbm/hp·hr)
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
12
14
Power Output (hp)
Figure 2: Graph of specific fuel consumption (SFC) against the power output at a test speed of 2800 rpm
for a Kubota engine
DISCUSSIONS AND RECOMMENDATIONS
The governor in a diesel engine governs or controls the engine speed at some fixed value while the
power output changes to meet demand. The governor does this by automatically adjusting the engine
fuel pump configurations to meet the desired load at the set speed. For instance, when the load on the
engine is high, the governor increases the fuel flow and when the load is low, it decreases the flow. The
governor can also be used to limit the maximum engine speed possible because the engine has a
maximum amount of fuel it can deliver or inject at a time. In summary, it controls the fuel flow rate
depending on load conditions. We experienced “governor droop” during the experiment. The engine
speed decreased when the load applied to the engine was increased and we had to adjust for this
change in speed. This reduction in the actual speed of the engine with respect to the reference speed
when the loading was applied is called “governor droop.”
The thermal efficiency of the engine at the point of best SFC was calculated to be 31.3%. This is an
acceptable value for a diesel engine as the thermal efficiency of internal combustion engines typically
fall under 35% [1]. From the schematic in figure 1, the heat loss to the environment shown by 𝑄̇𝑜𝑢𝑡 is
primarily responsible for the energy loss during the experiment. This heat loss could be due to radiation.
Heat could be lost when it is transferred to the cooling fluid and discharged using the radiator. In
addition to this, heat could be lost due to friction between the mechanical parts or components within
the engine as they move.
The specific fuel consumption is the amount of fuel consumed per KW of power developed per hour. It
could be seen as fuel efficiency and a criteria for economical power production. It is better to have a
lower SFC because this means less fuel is used to produce 1 KW of power in one hour. Fuel consumption
or the mass flow rate of fuel (𝑚𝑓̇ ) is the mass of fuel consumed per hour whereas SFC is the mass of fuel
consumed per KW per hour.
From the performance graph in figure 2, we can see that the specific fuel consumption decreases as the
power output increases. This means that the engine is more efficient when it is loaded with a higher
engine load because the power output increases with increasing load on the engine. This could be why
diesel engines are more fuel efficient at handling high loadings. This engine is thus useful for industrial
processes, electricity generation and heavy-duty machinery because it is very fuel efficient under the
high loads which are very typical in these environments.
Advantages of using diesel engines to generate electricity: Diesel engines are sometimes used as an
electrical power source for a couple of reasons. Diesel engines are highly fuel efficient as they consume
less fuel compared to other generators e.g. gasoline generators. They are durable and will hold up
through wear and tear and generally require low maintenance due to the fewer parts e.g. there are no
spark plugs in a diesel engine so they do not have to be changed. They produce much greater power
output compared to some other fuel generators and this makes them the perfect choice for places or
work sites where no power grid is available.
Disadvantages of using diesel engines to generate electricity: The biggest drawback when it comes to
using diesel engines as a power source is pollution. They produce a mixture of pollutants such as
nitrogen oxides, carbon monoxide, carbon dioxide, hydrocarbons and soot which are hazardous to the
environment and our health. Diesel engines are also very noisy, and this means they can not be placed
near working environments due to discomfort. They are also very bulky and expensive to set up since
they are made up of large and heavy components which are expensive to repair in the event of a failure.
Error Analysis:
There were several short comings to the experiment which made room for experimental error. Firstly,
the fuel system where the fuel consumption rates were taking from is based on a 30 second averaging
period. If 30 seconds was not achieved between readings when the load was increased, the values
recorded will not have averaged the same leading to incorrect values. Also, the engine consists of a
three-cylinder system which is very cyclic compared to the four-cylinder system. Thus, a lot of values or
readings fluctuate or are not stable and this makes it hard to get an accurate reading.
The experiment can be improved by improving the equipment used for measuring fuel consumption
rates to measure the instantaneous flow at a given time. This way we would reduce the wait time for
taking readings and the error from taking readings too fast. Also, this experiment would be more
accurate if more readings or measurements are taken and the averages are used for calculations. Since
the values fluctuate during the experiment, average values will provide a better idea of the actual value
that is needed for analysis.
CONCLUSION
At the end of this experiment, we were able to calculate the thermal efficiency of the engine to be
31.3% at the point of best specific fuel consumption. This means that 31.3% of the energy generated by
combustion of fuel and air in the engine translates into useful work for the engine. The SFC at this point
𝑙𝑏𝑚
was calculated to be 1.20 ℎ𝑝∙ℎ𝑟 meaning that we would have to consume 1.201 pound mass of fuel to
produce 1 hp in one hour. With this SFC, the cost of electrical power generated was 31.7 cents per KW
per hr meaning it would cost approximately $0.32 to generate one KW in one hour if diesel were used
which as fuel. From the performance graph, we can conclude that higher engine loads result in better
specific fuel consumption and better power output.
REFERENCES
[1] C. University, "MAAE 2400 Thermodynamics and Heat Transfer Laboratory Instructions," Culearn,
2020.
[2] National Resources Canada, “Average Retail Prices for Diesel in Canada”:
http://www2.nrcan.gc.ca/eneene/sources/pripri/prices_byyear_e.cfm?productID=5