A survey on low compression ratio diesel engine

Abstract

A review on the behaviour of the low compression ratio diesel engine is presented in this work. To incorporate new strategies which improve the performance of the diesel engine, the dynamic interaction between engine subsystems and their impact on combustion phenomenon has to be retrieved. Several studies have investigated the impact of compression ratio on the performance of diesel engine and its implications on the emissions. In general, diesel runs at higher compression ratios. At higher compression ratios, the NOx emission is high even though brake thermal efficiency is high. Low compression ratio technology can be the solution to this problem. It is proposed that the reduction in compression ratio would be beneficial to reduce the NOx emissions, thermal and mechanical stresses on the components of the engine. The main objective of this investigation was to understand the impact of lowering the compression ratio on the brake thermal efficiency and brake specific fuel consumption on the diesel engine by reducing the compression ratio from 17.5:1 to 13.7:1 in two steps by using thicker head gaskets. The test results revealed that as the compression was lowered, the NOx emission got reduced with a little penalty in HC and CO. The brake thermal efficiency is as well reduced and brake specific fuel consumption was increased when compared with the standard compression ratio of the engine.

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VOL. 10, NO 22, DECEMBER, 2015 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.
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10584
A SURVEY ON LOW COMPRESSION RATIO DIESEL ENGINE
Bridjesh P. and Arunkumar G.
Department of Mechanical Engineering, Saveetha School of Engineering, Saveetha University, Chennai, Tamilnadu, India
E-Mail: meetbridjesh@gmail.com
ABSTRACT
A review on the behaviour of the low compression ratio diesel engine is presented in this work. To incorporate
new strategies which improve the performance of the diesel engine, the dynamic interaction between engine subsystems
and their impact on combustion phenomenon has to be retrieved. Several studies have investigated the impact of
compression ratio on the performance of diesel engine and its implications on the emissions. In general, diesel runs at
higher compression ratios. At higher compression ratios, the NOx emission is high even though brake thermal efficiency is
high. Low compression ratio technology can be the solution to this problem. It is proposed that the reduction in
compression ratio would be beneficial to reduce the NOx emissions, thermal and mechanical stresses on the components of
the engine. The main objective of this investigation was to understand the impact of lowering the compression ratio on the
brake thermal efficiency and brake specific fuel consumption on the diesel engine by reducing the compression ratio from
17.5:1 to 13.7:1 in two steps by using thicker head gaskets. The test results revealed that as the compression was lowered,
the NOx emission got reduced with a little penalty in HC and CO. The brake thermal efficiency is as well reduced and
brake specific fuel consumption was increased when compared with the standard compression ratio of the engine.
Keywords: low compression ratio, diesel engine, performance, brake thermal efficiency.
1. INTRODUCTION
Environment pollution issues regarding exhaust
gas emissions and fuel economy has become the prime
concern of the present day diesel engine[1]. To meet the
emission regulations and increase the thermal efficiency of
the engine, technology needs to keep upgrading. One of
the most promising research ways to increase the power at
full load with reduced emissions is the reduction of
compression ratio [2]. The parameters that affect thermal
efficiency of diesel engine are losses of heat, friction
losses, quality of fuel, compression ratio, fuel injection
pressure, fuel injection timing and ratio of specific
heats[3]. Thermal efficiency can be increased by
squeezing out the maximum possible work out of the
every drop of fuel that is injected into the combustion
chamber.
Carlo performed experimental analysis on the
effect of the compression ratio on the performance of a
single cylinder diesel engine operating with conventional
combustion and low temperature combustion mode for
low NOx emissions. The compression ratio was reduced
from 16.5:1 to 14.5:1 and the engine performance was
evaluated in terms of thermodynamic parameters,
emissions and fuel consumption. The results of
compression ratio reduction evidenced a strong
improvement in NOx–particulate trade-off coupled with
penalties in unburned compounds emissions and fuel
consumption [4].
Cursente, in his article describes the combustion
effects of the reduction of compression ratio and quantifies
improvements obtained at full load and part load running
conditions on a high speed diesel engine of a reduced
compression ratio from 18.1:1 to 14:1. The experimental
results showed an increase of 12% of power at 4000 rpm
as well as near zero NOx and PM emissions at 1640 rpm.
The brake mean effective pressure of 3, 7 bar with yet a
significant increase of CO and HC emissions [5]. At full
load, performance has been achieved with large bowl with
reduced number for the nozzle injector and finally a lower
swirl level. At low load, the reduction of emissions
promoted by a thin pulverization of fuel and the best
homogenization of fuel/air mixing has required an
antagonist design [6].
David J.MacMillan conducted experiments to
assess the effect of compression ratio on indifference to
variation in injection and air fuel ratio at low and medium
speeds by using different bowl sizes of the piston. It was
found that at low compression ratio CO, HC and ISFC
were higher with improved Soot/NO trade-off. Reducing
the compression ratio from 17.9:1 to 13.7:1 marked a
degradation of performance at low load, producing high
CO emissions and a fall in combustion efficiency [7].
To improve the specific power while minimizing
the increase in maximum cylinder pressure, a pent-roof
combustion chamber and straight ports was used on a
diesel engine [8]. The simulation of the engine cycle was
investigated. It was found that the changes of
specifications worsened combustion, however, the gross
indicated mean effective pressure was found to be lower
than that of the baseline engine [9]. The causes for the
worsening of the combustion in experiments were
analysed and the shape of combustion chamber and
specifications of fuel injection system were identified for
better combustion [10]. The factor contributing to
reduction of the maximum pressure and exhaust
temperature was increased intake air mass flow [11].

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In order to find the optimum compression ratio,
experiments were conducted on a single cylinder variable
compression ratio diesel engine at compression ratios of
13.2, 13.9, 14.8, 16.9, 18.1 and 20.2. Results showed a
significant improvement in performance and emission
characteristics at a compression ratio of 14.8. The
compression ratios lower than 14.8 and higher than 14.8
indicated a drop in thermal efficiency, rise in fuel
consumption along with the increased smoke densities
[12].
The engine’s effective compression ratio can be
estimated with flexible intake valve actuation without the
need for in-cylinder pressure data. The experimentation
scheme was able to converge within 3 engine cycles after
a transient event with less than 6% average steady state
error compared to experimental data [13].
Reducing soot emission with engine technology
is effective for reducing Particulate emission. It results in
minimizing extra fuel consumption and downsizing [14].
Soot emission level mainly depends on excess air ratio and
can be reduced by keeping excess air ratio high. Lean
combustion under the limited amount of air and maximum
in-cylinder pressure requires decrease in fuel injection
quantity and yields decrease in engine power [15]. In order
to achieve low soot emission without decreasing engine
output, low soot combustion with minimum excess air
ratio without a significant increase in soot emissions is
required [16]. Low compression ratio encompasses
increase in power density under the limited maximum in-
cylinder pressure [17]. On a low compression ratio diesel
engine and in high EGR rate operating conditions to
evaluate the benefits of multiple injection strategies to
improve the trade-off between engine emissions, noise and
fuel economy[18]. It was found that by decreasing the
peak heat release process appears to be satisfactory for
controlling the combustion noise. Multiple injections are
used in the appropriate thermodynamic and auto ignition
delay conditions in order to reduce the instantaneous fuel
burning rate [19]. To enhance the fuel spray distribution
and air use in the combustion chamber, multiple injection
strategies are used. The cooling effect associated with fuel
vaporization lowers locally and globally the temperature
of the gases contained in the combustion chamber [20].
This phenomenon can be applied to increase the ignition
delay allowing for a longer mixing period and thus a more
homogeneous fuel/air mixture to modify the rate of heat
release in the early stage of combustion [21].
1.1 Compression ratio
Compression is a process in which charge is
confined and pressed into a smaller volume within the area
of a cylinder. Compression forces all of the molecules to
be pressed together under high pressure [22]. Static
compression ratio is the ratio derived from the sweep
volume of the cylinder using the full crank stroke from
bottom dead centre to top dead centre. Dynamic
compression ratio uses the position of the piston at intake
valve closing rather than bottom dead centre of the crank
stroke to determine the sweep volume of the cylinder [23].
Dynamic compression ratio is always less than the static
compression ratio. The actual compression and expansion
processes in engines depend on valve timing details and
the importance of flow through the valves while they are
opening and closing which depend on engine speed.
1.2 Calculation of compression ratio (rc)
The geometry of cylinder, piston, connecting rod and crankshaft is shown in Figure-1.
The compression ratio of a reciprocating engine can be calculated using the following formulae,
Compression ratio rc= maximum cylinder volume/minimum cylinder volume
= (Vd+Vc)/Vc
Where Vd= displaced or swept volume and Vcis the clearance volume.
Ratio of cylinder bore to piston stroke, Rbs= B/L
Where B = bore and L=stroke
Ratio of connecting rod length to crank radius R=l/a
Where l = connecting rod length and a = crank radius
Stroke and crank radius are related as , L = 2a
The cylinder volume V at any crank position Ɵ is,
V = Vc + Π B2 (l+a-s)/4
Where ‘s’ is the distance between the crank axis and the piston pin axis and is
given by,
S = a cosƟ + (l2 – a2sin2Ɵ )1/2
Figure-1. Geometry of piston cylinder arrangement.

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2. LOW COMPRESSION TECHNOLOGY
Low compression technology diesel engines have
become recognized as viable solution. The slower ignition
that results will allow time for better mixing of air and fuel
and, therefore, more complete combustion occurs to
reduce soot and NOx. The reduction in compression ratio
lowers the cylinder pressure which gives rise to a virtuous
cycle of beneficial effects that were previously available.
The lower cylinder pressure reduces the thermal and
mechanical stresses in the engine. So that, the heavy cast
iron block traditionally needed to stop a diesel ripping
itself apart can be replaced with lighter materials, which
increase the fuel economy. The vibrations caused by the
conventional diesel engine substantially get reduced due to
low cylinder pressures. As lighter moving parts can be
used on the engine, internal friction and inertia is reduced
and the engine spins faster and more freely. The low
compression ratio engine reduces the combustion chamber
temperature and therefore NOx emissions get reduced.
The low compression ratio helps to extend the engine load
range [24].
3. PARAMETERS CONSIDERED FOR REDUCING
THE COMPRESSION RATIO
3.1 Thicker head gaskets:This is by far the
suitable method and has dramatic effect in lowering the
compression ratio in an engine.
3.2 Low compression pistons: The pistons are
much shorter than conventional ones. The advantage is
that they are also often lighter so the engine will run a
little more freely. It is recommended that combining low
compression pistons with a shorter stroke to get advantage.
The shape of the piston crown will also have a bearing on
the amount of compression that takes place in the engine.
3.3 Shorter rods and reducing the stroke:A
shorter stroke will have a dramatic effect on the
compression ratio. By combining this method with low
compression pistons, high boost pressures can be attained
when adding a turbocharger. The crank will also have
some impact on the throw of the engine and the crank,
piston crowns and rods should ideally all be matched up.
3.4 Head work: head work increases the volume
of the cylinder but the effectiveness depends a lot on how
the intake and exhaust valves are seated, and how much
space there is to work with. Removing the head is
relatively simple and does not require as much effort as
other compression lowering methods.
3.5 Decompression plates: They are essentially
an extension to the head and can be very effective at
reducing the compression ratio. The block side needs a
conventional gasket seal but the head side generally only
requires a non setting high temperature sealant (in the case
of aluminium decompression plates). Plates can be made
of a variety of metals. The decompression plates may fail
prematurely in high boost applications where high
temperatures are involved. It is viewed that this is a good
thing as replacing a decompression plate is a lot easier to
do than replacing pistons and heads should they go, and in
these extreme conditions this can be quite likely and the
plate failure will have flagged up the potential problem.
3.6 Long duration cams: Long duration cams
delay the closing of the intake valve and substantially
reduce the compression ratio. The cam specification to
determine the compression ratio is the intake valve closing
time angle. Changing the intake center line changes the
compression ratio. Retarding the cam delays intake closing
and decrease the compression ratio. It is necessary to
determine the position of the piston at intake valve closing
to calculate the compression ratio.
4. EXPERIMENTAL METHODS
The engine used is a four stroke single cylinder,
vertical, water cooled, natural aspirated, direct injection
diesel engine. The specifications of the engine are given in
Table-1.
Table-1. Specifications of engine test rig.
Component Specification
Make Kirloskar Engines Ltd, Pune
Type of engine Four Stroke Single Cylinder
Water Cooled Engine
Bore and Stroke 87.5 mm & 110 mm
Compression ratio 17.5 : 1
BHP and rpm 4.4kW & 1500 rpm
Fuel injection
pressure 200 N/mm2
Fuel injection timing 230 BTDC
Dynamometer Eddy Current Dynamometer
A pressure transducer is used to monitor the
injection pressure. The engine apparatus was interfaced
with an emission measurement device AVLDigas 444 a
five gas analyser, and also the setup is provided with
necessary instruments for measuring combustion pressure
and crank angle. These signals are interfaced to the
computer through engine indicator for P-V and P-Ɵ
diagrams with AVLINDIMICRA 602 -T10602A software
version V2.5. Atmospheric air enters the intake manifold
of the engine through an air filter and an air box. An air
flow sensor fitted with the air box gave the input for the
air consumption to the data acquisition system. All the
inputs such as air and fuel consumption, engine brake

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10587
power, cylinder pressure and crank angle were recorded by
the data acquisition system, which is stored in the
computer and displayed in the monitor. A thermocouple in
conjunction with a temperature indicator was connected at
the exhaust pipe to measure the temperature of the exhaust
gas. The smoke density of the exhaust was measured by
the help of an AVL415 diesel smoke meter. A crank
position sensor was connected to the output shaft to record
the crank angle. The engine test rig is shown in Figure-2
and the schematic diagram of experimental setup is given
in Figure-3.
Figure-2. Engine test rig.
Figure-3. Schematic diagram of experimental setup.
1. Engine 2. Dynamometer 3. Crank angle encoder 4. Load
cell 5. Exhaust gas analyzer 6. Smoke meter 7. Control
panel 8. Computer 9. Silencer
5. EXPERIMENTAL PROCEDURE
The engine used in this study was a direct
injection single cylinder engine manufactured by
Kirloskar. The engine was run at different compression
ratios to evaluate the performance with emission
charectaristics. Initially the engine was run on no load
condition and its speed was maintained at a constant speed
of 1500 rpm. The engine was tested at varying loads of 4.5
A, 9A, 13.5A and 18 A by means of an electrical
dynamometer. For each loading conditions, the engine was
run for at least 2 min after the data was collected. By
changing the thickness of the cylinder head gasket the
compression ratio can be changed to a certain limit. In
order to vary the compression ratio of the engine in the
present study, a thin copper spacer of 1 mm thick was
inserted between the engine cylinder head and the cylinder
block. With this various compression ratios of 15.37:1 and
13.7:1 are obtained by using 2 spacers apart from the
standard compression ratio of 17.5:1.
6. RESULTS AND DISCUSSIONS
Brake thermal efficiency: Brake thermal
efficiency gives the idea of the output generated by the
engine w.r.t the heat supplied in the form of fuel. In
general, increasing the compression ratio improves the
efficiency of the engine due to the reduced ignition delay.
Figure-4 shows the variation of brake thermal efficiency
with load. The brake thermal efficiency with standard
compression ratio of 17.5:1 was found to be 27.03% at full
load of 18A and brake thermal efficiency decreases as the
compression ratio was reduced. This can be attributed that
the fuel added to the cylinder which vaporizes and mixes
with air to produce a fuel/air ratio distribution which is
non uniform and varies with time. This lead to the inferior
combustion at reduced compression ratio of 13.7:1. This
can be overcome by premixed compression ignition
combustion by controlling ignition timing based on model-
based prediction of ignition delay. Not only changing the
profile of the combustion chamber or piston bowl but also
the fuel spray pattern enhances the brake thermal
efficiency in low compression ratio diesel engine.
5 6
1
8
7
2
3
4
9

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Figure-4. Variation of brake thermal efficiency w.r.t load.
Brake specific fuel consumption: An important
parameter to measure the engine performance is the brake
specific fuel consumption. Figure-5shows the variation of
BSFC with load at different compression ratios. In
general, the BSFC decreases with the increase in load on
engine. It was found from the figure that the BSFC was
increased as the compression ratio was reduced. At higher
compression ratio lesser value of BSFC is apparent
because of better atomization which is associated with a
marginal delay in admission of fuel due to high needle lift
pressure during injection.
Figure-5. Variation of brake specific fuel consumption w.r.t Load.
7. CONCLUSIONS
Tuning a conventional diesel engine into a low
compression ratio diesel engine is demonstrated in this
work. Unlike conventional diesel engines, low
compression ratio diesel engines operate at relatively low
peak temperatures and pressures. Low compression ratio
reduces the pre mixed part of the combustion, which
reduces the cylinder pressure and therefore the
temperature, which reduces NOx production and also
allows the fuel to mix better avoiding locally rich areas
that produce soot. The downside to lowering the
compression ratio of a diesel engine is that, during warm-
up, the engine temperature can be too low to support
proper combustion.
REFERENCES
[1] Abou Al-Sood.M.M., Ibrahim.A.M. and Abdel
Latif.A.A. 1999. Optimum compression ratio
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0
5
10
15
20
25
30
0 5 10 15 20
Brakethermalefficiency%
LoadA
BTEvsLoad
CR17.5:1
CR15.37:1
CR13.7:1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20
BSFCkg/hkW
LoadA
BSFCvsLoad
CR17.5:1
CR15.37:1
CR13.7:1

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