Many problems can cause power loss in a diesel engine after repair. The primary
problem is increased leakage between the cylinder and piston, which decreases
the cylinder working pressure and engine compression ratio. The leakage increase
is related to quality and fit precision of the parts and has been viewed as
the key problem for improving engine power after repair over many years.
Most existing studies focus on engine motion simulation and structural parameter
design optimization. For example, Zhang et al. (2011)
simulated the kinematics and dynamics of the crank connecting rod by performing
a stress analysis on the dynamics and kinematics of an 8V150 diesel engine to
determine the torque output characteristics and provide references for optimizing
diesel engine-related parameters. Qiao and Huang (2012)
determined the motion curve and force curve of the piston, the tangential force
curve of the crankshaft and the output characteristics of the diesel engine
by simulating the kinematics and dynamics of the crank-connecting rod mechanism
to support an optimized design and finite element analysis of the crank-connecting
rod mechanism. However, relatively few studies on improving the power of a diesel
engine after repair have been reported. Bayrakceken et
al. (2007) analyzed the reasons that certain diesel engine components
prematurely fail and proposed that improper heat treatment was an important
problem that caused crank failure. Yue (2008) analyzed
the causes of power and fuel economy losses in a type 195 diesel engine after
overhaul. They proposed that several adjustments were needed to restore the
technical specifications of these diesel engines and to maintain the cylinder
compression pressure. These included adjustments to the fuel injection advance
angle and the oil pressure of the auxiliary plunger fuel pump and delivery valve
seal. Wang and Wang (2010) noted that the major problems
that cause insufficient power in a repaired engine include poor sealing, low
fuel injection, low volumetric efficiency and incomplete combustion. Further,
such problems are caused by a broken valve spring, too large or too small valve
clearance, exhaust pipe and muffler clogging, carbon residue deposition in fuel
injectors, diesel engine filter clogging and air filter clogging, among others.
Yang (1993) noted that poor engine seals are related
to the upstroke speed of the piston; leakage decreased with increasing upstroke
speed and thus the thermal efficiency of an old diesel engine is higher under
high-speed operation than low-speed operation. Therefore, if the fitting between
the cylinder and the piston or the piston rings deteriorates, then an improved
piston upstroke speed is conducive to reducing the leakage between the cylinder
and piston, which produces improved power and economy in an engine. Zhao
et al. (1997) proposed to increase the crank radius to increase the
piston upstroke speed without changing the speed of the engine.
In the study, the crank-lengthening method was adopted. The impact of this
method on the diesel engine power after repair was studied through investigations
and methods to restore the original power, economy and technical indicators
in an overhauled diesel engine were explored.
MATERIALS AND METHODS
An overhauled diesel engine with a lengthened crankshaft and the test methods:
An overhauled Dongfanghong type 4125A diesel engine (YTO Group Corporation,
China) was studied. The piston-cylinder liner clearance, clearance between the
piston ring and ring groove edges, journal-bearing clearance, technical status
of the crankshaft as well as connecting rods for the engine met the technical
requirements for repair (Wang, 2011). The fuel injection
advance angle of this diesel engine was 17.5° before the top dead center.
The fuel injection pressure was 12.3 MPa. The calibrated power/rotation was
40 kW/1300 r/min. The standard size of the crankshaft connecting rod journal
of the 4125A diesel engine was ground to +0.75 mm with a crankshaft grinder.
The rod journal was fitted with +0.75 mm bearings and was offset-ground outwardly
along the crank (i.e., the crank was lengthened by 0.75 mm).
Methods compliant with the Performance Test Methods for Reciprocating Internal
Combustion Engine-Test Methods (GB-T 1105-1987) were used to test the diesel
engine. GB-T 1105-1987 specifies methods for rig testing the performance of
general-purpose reciprocating diesel engines and reciprocating petrol engines.
The fuel oil and engine oil in the test should be used in accordance with the
provisions in the manual for the internal combustion engine and the oil quality
should comply with the provisions for related petroleum product standards. Smoke
suppressants were not added to the fuel oil. Accessories compliant with professional
standards were installed in the internal combustion engine during the test,
but only the internal combustion engine was tested. During the test, the internal
combustion engine was not adjusted. The fuel oil, engine oil and cooling medium
temperatures complied with professional standards or product manual provisions.
Test equipment and methods: The diesel engine power was measured with
a hydraulic dynamometer (model D350, Jiangsu Qidong agricultural machinery repair
and manufacture factory). The D350 hydraulic dynamometer is an energy-absorption
type hydraulic dynamometer with water paddles. During operation, the water-resisting
rotor connected to the engine propels the water entering the internal cavity
of the hydraulic dynamometer, throws the water to the inner wall of the stator
through the centrifugal force and forms a rotating water ring. The rotational
movement of the water ring is impeded by the water-resisting columns arranged
on the inner wall of the stator housing. Therefore, a strong water vortex is
generated between the stator and rotor. Such liquid-solid friction and collision
consumes power from the engine and converts much of the energy into heat, which
is then absorbed and carried away by the flowing water. A small portion of the
heat is conducted through the housing. Because the heat capacity of water is
large and the water flow rate can be adjusted over a wide range, the power absorption
range is relatively large.
A comprehensive engine tester (model QFC-5D, JiNan Zhongbao Automobile Testing
Equipment Co., Ltd) was used to measure overall performance of the diesel engine
(Sanli et al., 2008). The QFC-5D engine tester
is a comprehensive engine analyzer that reads car computer error code and manages
the files in one system. It analyses the engine ignition system, boost system,
charging system, dynamic balance of the engine and engine guide noise, among
The diesel engine exhaust smoke intensity was measured with a smoke meter (model
FQD-102, Wenzhou Instrument Factory). The working mechanism of the FQD-102 exhaust
smoke meter was to extract a certain volume of exhaust gas from the diesel engine
exhaust pipe with a piston suction pump and pass it through white filter paper
with certain area so the carbon particles in the exhaust gas adhere to and blacken
the filter paper. Light absorption for the smoke deposit on the filter paper
is then measured with an optoelectronic transmitter to evaluate exhaust smoke
density in the diesel engine (Thu Zar et al., 2011).
Test content and indicators
Comparison test for cylinder pressure: The engine was operated at the normal
temperature (80-90°C water temperature) and then powered down before measuring
the cylinder compression pressure. The fuel injectors were removed from the
diesel engine and cleaned thoroughly. The tapered rubber head of the cylinder
pressure gauge was then pressed into the spark plug hole (injector) and the
crankshaft was turned for 3-5 sec with a starter. One person inserted the cylinder
pressure gauge plug to seal the cylinder spark plug hole. A second person fully
opened the choke and the throttle, switched on the starter motor and ran the
engine for 3 to 5 compression cycles. The first person removed the pressure
gauge when the pointer indicated zero cylinder pressure. The above procedure
was repeated three times to measure the same cylinder and the highest reading
was recorded as the measured data.
Comparison test for the engine power and economy: Depending on the application,
the speed of an internal combustion engine should be maintained at the calibrated
speed or a certain percentage of the overload speed during the test and its
load characteristics measured under such conditions. The speed of the internal
combustion engine was maintained at the calibrated speed or additional professional-standards-specified
speeds; the load was gradually increased from low to the maximum and the torque,
fuel consumption, exhaust gas temperature and additional parameters were measured
for each load (Lu and Wang, 2011).
Comparison test for the diesel engine speed control characteristics:
During the test, the internal combustion engine was operated stably under the
calibrated operating conditions or the overload condition. The entire load was
first unloaded to raise the engine to the maximum no-load speed or the maximum
overload no-load speed and then gradually increased until the above conditions
were reached. The steady speed, torque, fuel consumption and additional parameters
for the engine were measured for each load and a speed control characteristic
curve was generated under the calibrated condition or overload condition.
Diesel engine reliability test: At full throttle, the speed was evenly
raised from the maximum-net-torque speed to the maximum-net-power speed and
then evenly decreased to the maximum-net-torque speed. The above alternating
operating conditions were repeated. The throttle was then closed and the speed
was reduced to idle. It was then reopened gradually and evenly to achieve the
rated speed or maximum speed under no load specified by the engine manufacturer.
The throttle was closed to evenly slow the engine to its maximum-net-torque
speed. Until this step, one cycle was completed. The engine ran for 4000 cycles
and the duration of the run was 2000 h. The piston-cylinder liner fitting clearance,
piston ring side clearance, piston ring end gap, journal-bearing clearance and
bending and twisting of the connecting rod, among others, were then measured
and compared with the standard and repairable values (Yao
et al., 2000).
TEST RESULTS AND ANALYSIS
Cylinder pressure: A comparison of the cylinder pressure before and
after the rebuild is shown in Fig. 1.
Figure 1 shows that the pressures for the various cylinders
significantly increased after crank-lengthening. On average, the single-cylinder
pressure for the four cylinders increased by 0.31 MPa. The first reason for
the pressure increase is that the compression ratio increased. The piston stroke
increase caused the compression ratio to increase from the original design of
16 to 17.3. The second reason is the increase in piston upstroke speed. The
maximum piston upstroke speed increased from the original 11.925-12.045 m sec-1,
which is a 0.12 m sec-1 increase. The increase in piston speed generated
less leakage and is a major reason for the pressure increase. The piston stroke
increased from 152 to 153.5 mm, which is a 1% increase, the displacement volume
per cylinder increased from 1.863-1.878 L and the air intake flow rate increased
by 0.015 L, which contributed to the full combustion of fuel.
Power and economy: A comparison of the power and the economy indicators
before and after the diesel engine rebuild is shown in Fig. 2.
Figure 2 shows that crank-lengthening had relatively large
impact on diesel engine power.
||Maximum pressure (MPa) measured in each of the 4 cylinders
of a Dongfanghong type 4125A diesel engine before and after rebuilding and
extension of the crank by 0.75 mm
The maximum engine power increased 2.32 kW, the fuel consumption rate decreased
5 g/(kW·h), the exhaust gas temperature decreased 27°C and the smoke
density decreased 0.2 BSU. Various operation parameters of the rebuilt diesel
engine were clearly altered favorably at maximum power. The rebuilt engine could
operate at the maximum power. When the engine operated at the minimum fuel consumption
rate, the fuel consumption rate decreased 3 g/(kW·h), the power was 5.78
kW higher than before rebuild, the exhaust gas temperature increased 53°C
and the smoke density increased 0.2 BSU. The results show that after increasing
the crank radius, the diesel engine had problems, such as an excessive exhaust
gas temperature rise and increased smoke density at the minimum fuel consumption
Figure 2 also shows that, at the mean exhaust gas temperature,
the diesel engine power increased 4.7 kW, the fuel consumption rate decreased
2 g/(kW·h), the exhaust gas temperature increased 4°C and the smoke
density decreased 0.2 BSU. The rebuilt diesel engine was clearly advantageous
at the mean exhaust gas temperature. At the mean smoke density, the diesel engine
power increased 218 kW, the fuel consumption rate decreased 2 g/(kW·h)
and the exhaust gas temperature decreased 29°C. The various parameters were
favorable for operating the diesel engine.
Speed control characteristics: A comparison of the speed control characteristics
before and after the diesel engine rebuild is shown in Fig. 3.
Figure 3 shows that the diesel engine's maximum torque increased
30.4 Nm after the pressure increased. Further, a comparison of speed control
characteristics shows significant decreases in fuel consumption rate, smoke
density and exhaust gas temperature, which suggests that crank-lengthening was
conducive to improving engine power.
Reliability: The overhauled diesel engine was installed in the test
rig and loaded with the rated load after a cold/hot break-in. The engine operated
continuously for 2000 h and was then powered down. The oil consumption measurements
were 263 g/(kW·h) for diesel and 8.12 g/(kW·h) for engine oil.
The key components were examined after engine dissembling and the technical
indicators are shown in Fig. 4.
Figure 4 shows that the diesel engine oil consumption was
within the normal range for this engine. Further, the technical parameters for
the key engine components, such as the piston-cylinder liner fitting clearance
(Fig. 4a), piston ring side clearance (Fig.
4b, c and d), piston ring end gap (Fig.
4e), journal-bearing clearance (Fig. 4f and g)
and connecting rod bending (Fig. 4h) and twisting (Fig.
4i), were within the repairable range after 2000 h of continuous operation.
|| Comparison of (a) Effective power (N/ kW), (b) Fuel consumption(g/kW
h), (c) Exhaust gas temperature (°C) and (d) Smoke density (BSU) for
a Dongfanghong type 4125A diesel engine before and after rebuilding and
extension of the crank by 0.75 mm
|| Comparison of (a) Speed, (b) Torque, (c) Power and (d) Fuel
consumption rate, (e) Exhaust gas temperature, (f) Smoke density, under
different operating conditions for a Dongfanghong type 4125A diesel engine
before and after rebuilding and extension of the crank by 0.75 mm
|| Comparison of measured values of (a) Piston-cylinder liner
fitting clearance, (b) Side clearance of the top ring, (c) Side clearance
of the second and third rings and (d) Side clearance of the oil ring, (e)
Opening clearance, (f) Clearance of the main shaft neck and inside, (g)
Clearance of the connecting rod shaft neck and inside, (h) Bending of the
connecting rod and (i) Twisting of the connecting rod, in each of the 4
cylinders of a Dongfanghong type 4125A diesel engine (after rebuilding and
extension of the crank by 0.75 mm) with manufacturer-specified standard
Lengthening the diesel engine crank increased the compression ratio and average
working pressure of the diesel engine at given speeds, which is conducive to
improving the diesel engine power. The fuel consumption rate of the diesel engine
decreased significantly after crank-lengthening, which improved the diesel engine
power and economy.
After lengthening the diesel engine crank by 0.75 mm, the piston reciprocating
stroke speed was increased by 0.12 mm sec-1, which relatively improved
the dynamic sealing between the piston and cylinder liner without increasing
the engine speed. Thus, for the same power output, the diesel engine speed was
relatively decreased, which is conducive to reducing diesel engine vibration
and thus extending the working life of the various parts. Therefore, lengthening
the diesel engine crank is a relatively safe method for restoring power after
After crank-lengthening, the speed of the piston that connects the rod was
increased, but the oil consumption and the technical status of key components
were in good condition. These results suggest that it is feasible to restore
power for an overhauled diesel engine by lengthening the crank.
This study was founded by the National Science and Technology Supporting Plan
(2011BAD29B08) and the 111 Project (B12007).