Contents
Perform test of an internal combustion engine ........................................................................................... 1
Objective ....................................................................................................................................................... 1
Brief Theory................................................................................................................................................... 1
Experimental procedure ............................................................................................................................... 2
...................................................................................................................................................................... 4
Calculations ................................................................................................................................................... 4
Methodology................................................................................................................................................. 7
Conclusions and discussion ........................................................................................................................... 8
References .................................................................................................................................................... 8
Perform test of an internal combustion engine
Objective
A method of testing an internal combustion engine to detect engine defects. After the engine has
been assembled, the engine is cranked, gas under pressure is injected into the intake ports, and
the pressures developed at the exhaust ports as the engine is cranked are noted and evaluated.
The pressurized air injected into the intake ports amplifies the pressure signals developed at the
exhaust ports so as to provide more accurate identification of engine defects and detect engine
defects that might go undetected utilizing prior art technology.
Brief Theory
Internal combustion engine is the most spread motor machine in the world. The present work
shows an alternative perspective to the study of the work cycle of combustion engines. This is
not obtained by direct measurement, but is built with the data obtained in two experimental tests.
In the first, pressure variations without combustion are measured in one cylinder of the engine,
while the engine is fully loaded with the remaining cylinders. Then, during a traditional brake
power test, the combustion products are measured to obtain the data corresponding to the
combustion and expansion phase by calculation, applying the model of the indicated cycle
provided to the same number of rotations as in the first test. As an application of the principle of
overlapping effects, the result is obtaining the indicated cycle of the machine that serves to
evaluate its efficiency. At the same time, the actual working fluid and the particular system are
characterized. The text that is presented below focuses on the experimental validation of the
described trials, presenting the methodology followed from the metrological and statistical point
of view. It is expected that the results of the application of the model of the indicated cycle will
provide data to characterize alternative hybrid fuels to be used in existing engines.
The indicated cycle of an internal combustion engine is the most usual tool used to obtain the
work of the cycle and the indicated power of the machine. The work performed by the fluid that
evolves in the machine is called indicated power. With the engine in test phase, the indicated
power is the reference against which the mechanical performance of the motor is determined. In
this article we will illustrate a practical example of how the data obtained in different tests of a
four-stroke spark ignition engine are processed for the application of the indicated cycle model
provided in an existing engine. The indicated cycle provided is a way to obtain the open cycle of
the pressures of an internal combustion engine, without measuring the entire development of the
pressures inside the cylinder directly. Although its execution is more laborious, its
implementation with industrial sensors of current use lowers the equipment costs in a remarkable
way. It is obtained as an application of the concept of superposition of effects, where the cycle is
composed of the data provided by different tests performed at the same regime of rotations and
full load.
Experimental procedure
The test is performed on a hydraulic dynamometer bench that opposes a torque resistant to the
driving torque generated by the combustion engine. The test is carried out at different rotation
speeds, and in each case the pressures in the cylinder n°4 of the open cycle without combustion
is obtained, while the engine runs at 100% load on the remaining three cylinders. To register the
pressure, a Danfoss model MBS 3000 analog sensor with proportional current output signal type
4 - 20mA is used. A position sensor is used to register the piston passage through the top dead
center (TDC), placed on the crankshaft pulley. This sensor allows determining the start and end
of each operating cycle. A Siemens model 3RG4075 - 0AJ00 inductive proximity sensor is used
for this task. Both sensors are connected to individual channels of a GW Instek GDS -2062
digital oscilloscope.
Digital oscilloscope screen shot. In yellow, channel 1 register the data from the pressure sensor.
In green, channel 2 shows the TDC sensor. (b)Pressure sensor installed in the motor. The
oscilloscope screenshot when measuring the 2290 rpm series. Once the experimental register has
been obtained, the data is transferred to the spreadsheet for first analysis. This consists on
determining the good measurements that will intervene in the most representative cycle
calculation, and establishing with precision the rotation regime at which the test was carried out.
Conserving only the complete cycles comprised between the TDC marks and counting the
number of complete cycles in the time interval in which they have been recorded, the number of
rotations to which the test was actually performed can be accurately established. Once the
rotation regime has been established for each series of data and the number of cycles that make
up each series, it is necessary to establish the most representative cycle of each events series. To
do this, the x-axis it is changed to indicate degrees of crankshaft rotation instead of time; and the
y-axis to indicate pressure instead of voltage, and all the events that make up each series are
superimposed. For the present article, we have chosen to work with the measurements obtained
at 2715 rpmthat is very close to the number of rotations where the engine has the maximum
torque.
Calculations
Fuel Consumption
Fuel volume flow is figure out by using drained fuel volume and its drain time. As fuel volume
flow is find out we can find fuel mass flow rate by multiplying it with its density.
Air Consumption
Air mass flow rate can be finds by using coefficient of discharge, diameter of orifice, Gas
constant, ambient pressure and pressure difference between orifice and by using ambient
temperature. All these are put in formula to find air mass flow rate
Brake Power
By multiplying angular velocity and torque brake power is figure out.
While angular velocity is calculated by using speed multiply with pie and 2 and then divide by
60.
Specific Fuel Consumption
From fuel mass flow rate and brake horse power specific fuel consumption is finds out. Fuel
mass flow rate is multiples with 3600 and then divide with the ratio of brake power and 1000.
Air Fuel Ratio
By dividing air mass flow rate to fuel mass flow rate air fuel ratio is calculated.
Heat energy by Combustion
Fuel mass flow rate is multiple with calorific value to find combustion energy.
Inlet Air Enthalpy
Air mass flow rate is multiple with specific heat at constant pressure and ambient temperature to
finds out inlet air enthalpy.
Thermal Efficiency
Brake power is divides by the multiple fuel mass flow rate and calorific value to calculate
thermal efficiency.
Volumetric Efficiency
Volumetric efficiency is calculated by multiple of 100 and division of measured volume and
calculated volume. Calculated volume is the ratio of multiple of engine capacity and speed and
multiple of half of stroke and 60.
While measured volume is the multiple of air mass flow rate, gas constant and ambient
temperature and divides by ambient pressure.
Brake Mean Effective Pressure
It is calculated by division of multiple of 60, brake power and half of stroke to multiple of 0.1,
speed and engine capacity.
Methodology
For the first test, a pressure sensor will be installed in the spark plug hole, corresponding to the
cylinder number four. The engine will compress the working fluid, composed by the mixture of
air and gasoline provided by the carburetor or injection system. Since the cylinder number four
spark plug is missing, the mixture will not ignite, allowing its expansion and exit through the
exhaust pipe. All these events will be registered by the pressure sensor. This situation is
illustrated in Figure 1 with a full line and we will call it the indicated cycle without combustion.
This test will allow obtaining the real pressure variation curve for the engine intake and
compression races, depending on the angle of rotation of the axle.
The effect that the combustion process has on the indicated cycle can be seen in figure 1 in
broken lines. To obtain the value of the maximum combustion pressure, a second test will be
carried out, also at full load and at the same rotation regime, in which the specific fuel
consumption of the engine and the elemental composition of the combustion gases will be
determined. With the help of the combustion products enthalpy diagram, the temperature and
pressure values corresponding to the maximum pressure developed by the cycle will be obtained.
The expansion curve will be modeled with the average polyropic coefficient in the expansion
phase taking as reference the maximum combustion pressure and the final pressure in that phase
observed in the previous test.
Open cycle of an SI-FSE motor. The cycle without combustion phase is shown in continuous
line. The combustion effect is shown in broken line.
A third test consisting of a traditional brake power test was also carried out in order to obtain the
characteristic power curve of the engine. This test is not part of the indicated cycle provided
scheme, but is obtained in order to allow the engine mechanical performance calculation, useful
to establish the method validity. For the basic relationship between the degrees of rotation of the
motor shaft and the volumes observed inside the cylinder should be established at first. This is
obtained
from
the
geometric
analysis
of
the
crankshaft-rod
system:
𝑉𝑉𝑥𝑥=𝑆𝑆.�𝑟𝑟.(1−𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐)+𝑟𝑟
𝜑𝜑.�1−�1−𝜑𝜑2𝑐𝑐𝑠𝑠𝑠𝑠2𝑐𝑐��+𝑉𝑉2
(1)Where
Vx
represents the volume inside the cylinder for each angle α of the motor shaft rotation. The r
parameter is the crank radius, φ is the crankshaft turning ratio equal to the ratio between r and the
l length of the connecting rod between centers. S is the cylinder cross section surface of and V2
is the combustion chamber volume. Adopting the ideal gases model, the polytropic compression
curve (in this case obtained experimentally) is: 𝑃𝑃𝑦𝑦= 𝑃𝑃1�𝑉𝑉1 𝑉𝑉𝑥𝑥�𝑠𝑠𝑐𝑐 (2)Where the
value Py results the pressure corresponding to the volume Vx inside the cylinder, in reference to
V1 (cylinder total volume), and the pressure P1 at the beginning of the compression phase. The
exponent nc is called the average polytropic coefficient, and in addition to defining the shape of
the compression curve, from the thermodynamic point of view, it defines the heat losses of the
system. As the compression curve is known, nc values can be obtained for each volume (any
volume) Vx.In a real compression, the polytropic pressure increase can only be considered to the
point where the fuel mixture ignites, that situation does not occur when the cylinder volume is
V2, but in the volume corresponding to the ignition advance, which will be called V2´. In all
cases V2´ ˃ V2. The nc value does not remain constant, but varies throughout the stroke.
However, the nc coefficient can be taken as a
Conclusions and discussion
The method consisting on obtaining the indicated cycle of the engine as the composition of a
pressure diagram without combustion phase, plus a power phase modelled with the data obtained
from a second brake power test, with the same state of load and speed rotation, seems possible.
This is shown by the value obtained from mechanical performance, which, although high, is
within the expected values. I n its practical execution, the experimental method is laborious, but
from the point of view of the necessary equipment, it is extremely economical since the sensors
and instruments used are of common use in industrial applications. The indicated work
calculated with the integration method of the compression and power polytropic function, using
the average polytropic exponent for the whole stroke, is more accurate than other theoretical
methods. Even, when the air and fuel mixture, is considered as an ideal gas. Likewise, the
information obtained with the pressure sensor during the intake and compression strokes allow
plotting with precision the curve of real variation of polytropic coefficient in those phases, at the
same time that conclusions and valuable characterizations about the working fluid can be
obtained. The polytropic compression coefficient values also findsimilarity with the values
usually registered in the specialized literature.
References
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J.M. (2017).Determinación experimental de la variación del coeficientepolitrópico de
compresiónutilizandodiagramasabiertos. Ciencia y Tecnología 17, pp. 9-28. Jovaj, M.S. (1982).
Motores de automobile. Ed. Mir, Moscú. Lapuerta, M. (2002).Study of the compression cycle of
reciprocating engine through the polytropic coefficient. Elsevier Science Ltd, Valencia. Nieto
Garzon, N., MenesesZarza, H., Schroeder, F., Dos Santos, R., Oliveria, A.M. Bazzo, E (2016).
Aplicação de procedimentos estatísticos na análise da curva de pressão na câmara de combustão
de motores de ignição por compressão. Jornadas Iberoamericanas de Motores Térmicos y
Lubricación, 30 august –1 September 2016, La Plata, Buenos Aires. Torregosa, A.J.,Olmeda, P.
(2011). Pérdidas de calor. Refrigeración. En: Payri, F. y Desantes, J.M. (ed.) Motores de
combustion