An
engine is a machine that converts energy into useful mechanical energy. An
internal combustion engine (ICE) is an engine in which a fossil fuel and air
are oxidized in a combustion chamber. An ICE is seen every day in automobiles
and as such the need to study their parameters and their effect on its
performance and emission in other to maximize engine efficiency.
This
paper seeks to provide an overview of some basic engine parameters such as
Crankshaft, Valve timing, pistons, Connecting rods, Fly wheels and their impact
on performance and emission. These parameters are discussed in general terms
with no particular reference to any specific engine types.
A Stroke is simply the
movement of the piston from inner deed center (IDC) to outer deed center (TDC)
while a bore is the diameter of the cylinder [1]. The stroke and the bore are
of great importance to the performance and emission of an engine as they
control the engines displacements by changing overall dimensions of the engine
[2].
The overall dimension of
the bore and stroke improves the thermal efficiency and decrease the engine
emission. The longer the stroke of an engine the higher the thermal efficiency
which in turn reduces the combustion duration (faster burning rate) thereby
reduces the overall heat loss and engine emission. A shorter stroke length
decreases the engine friction and increases the maximum operating speed,
maximum power and the indicated mean effective pressure (imep) of the engine
which in turn increase engine performance [2]. In addition a longer bore would
accommodate more poppet valves thereby increasing the number of valves per
cylinder and its overall fuel efficiency [3].
The stroke to bore ratio
(S/B) is the ratio of the piston stroke and the cylinder bore diameter. This
ratio also influences the performance and emission of an engine. Lower S/B ratio
produces higher CO and HC emissions and lower NOx emission due to
the decrease in temperature [3].
Figure 1Cross-sections of different stroke
length[4].
The above figure shows 3
cylinders cross-sectional areas of a pancake shape combustion chamber with
different Stroke/Bore ratios, for SI engines the spark plug is on the head of
each cylinder [4]. It can be clearly seen from Fig 1 that the clearance height
and chamber proportions would be changed with S/B ratio. If the S/B ratio has
been translated into a Height-Bore ratio (HTDC/B), the
progressions are going to be 8.72, 12.42 and 16.36, respectively [4].
Therefore, the chamber of the long-stroke engine has a double HTDC/B
than a short-stroke engine
A research from International
Journal of Engine Research depicts that most normal combustion process happens
close to TDC and almost 50% fuel would be burnt before 10° after top dead
center (ATDC)
Figure 2 Normalized flame area of (a)
short-stroke, (b) square and (c) long-stroke engine [4].
Fig.2 shows the effect of
different S/B ratios on flame front area maps. For example; a S/B=0.7 has a
quicker contact between the piston and flame than a S/B=1.3. This flame front
area declines after the flame and piston contacts each other [4]. This decline
in the flame area reduces the fuel efficiency in an internal combustion engine.
A high S/B ratio improves the fuel efficiency thereby improving engine
performance by maintaining a large frame area.
Figure 3Normalized flame areas calculated for three
S/B ratios at the TDC piston position [4].
The above figure shows comparison of normalized flame areas
calculated for three
S/B ratios at the TDC piston position.It could be obviously
seen that in a short-stroke engine (S/B=0.7) the flame area drops drastically
after it reaches its peak point while for a long-stroke engine (S/B=1.3), it
remains close to the peak value by 5% and then it starts to drop very gently.
Therefore, long-stroke engine benefits from the comparatively greater distance
between the spark and the piston top at TDC.[4] In fact, the dimensionless
flame area for the long-stroke engines (e.g.: S/B=1.3) at 60° ATDC would be
still remaining around its peak point till more than half of the volume has
been entrained [4]. In conclusion, Stroke/Bore ratio plays a dominating role to
improve engine performances by increasing engines internal combustion
functions.
Nowadays, engine
emissions have increasingly become of great concern to all public and the
media. the Ministry of Ontario Transportation also treats emission as a very
important index for a vehicle. One type of the emission gas is Nitrogen Oxide
(NO) which the chemical kinetics are assumed to be governed by the extended
Zeldovich mechanism [4],[5],[6]
Figure 4Predicted NO emission with S/B
calculated without and with 15 per cent EGR [4].
The three bars on the
left side are the NO emissions with different S/B ratios and on the left side
its effect on EGR (exhaust gas recirculation) systems. Long-stroke engines
would emit more NO gas than short-stroke engines because of its higher S/B ratio
engines which would lose lower heat and have higher combustion temperature [4].
On the other hand, this phenomenon would be helpful for hydrocarbon’s burning
owing to long-stroke engines have larger crevice volume so that the unburned
charge trapped in this volume would be much smaller than that of a short stroke
engines [4].
A Connecting-rod is used
to transfer the linear motion of piston to the rotary motion of crankshaft.
Practically, the length of connecting-rod is reduced to 3.2-3.8 times the crank
throw from 4.0-4.5 times. The weight of connecting-rod should be minimized in
order to reduce the overall height of engine [8]. Connecting-rods are mainly
subjected to the inertia force caused by the reciprocating motion of piston and
forces produced from gudgeon-pin. All these forces would vary according to
different configurations of the engine which may subject the connecting-rod to
compression, stretching and bending during the strokes. To ensure a proper
function of an engine, the connecting-rod should be of high stiffness.
Pistons are one of the
most essential components of an engine motion as they are subjected to high
heat, pressure and movement than any other engine component [9].
Most traditional piston
tops are flat. This easily machined flat tops have small areas which absorb
less heat than other shape of the crown. Some pistons have concave tops which
affect the fuel air ratio to suit different engine requirements. In addition,
the concave tops can adjust the compression ratio that affects the fuel
efficiency and fuel economy.
Oil rings: With the help
of oil rings, the piston can move up and down smoothly. This is because the
lubricant reaches the top of cylinder walls through the gap between the oil
rings and cylinder walls [8]. The lubricant helps to reduce the friction
between the piston and the cylinder walls, allowing more power to push the
piston downwards thereby increasing engine output power and torque.
The crankshaft is one of the most important moving parts of
the engine, because it transfers the reciprocating motion of the piston to the
rotary motion of the crankshaft which is used to drive the vehicles. Without
the crankshafts, the internal combustion engine cannot produce output power and
torque because the linear motion of piston cannot be used to drive the cars.
The function of a crankshaft is also very important for the engine, as it effectively
absorbs forces created by the combustion of fuel, transmits energy to the
flywheel which transfers energy to the output shaft which is connected to the
driven machinery. In order to maximize engine performance (high power and high
speed), the crankshaft is of great importance as it has to be rigid enough to
withstand the forces during the power stroke without excessive bending。
2.3.1 Crankshaft
Material
Crankshafts can be manufactured using a wide range of
materials, like billet steel, steel forgings, cast steel, etc. Forged cranks
seem to be the best for manufacturing [10] because the forged cranks can
satisfy the necessary requirements like adequate strength, toughness, hardness,
high fatigue strength [1]. As the engine runs, the crankshaft continuously
suffers power impulse hits which may cause damages to the crankshaft.
Therefore, the use of a good material can expand the life time of a crankshaft
which in turn increases engine performance by enabling the crankshaft withstand
the constant force and power impulse hits produced while the engine is running.
As shown in Fig 6 the crank-throw is the distance
between the main journal centers and big-end journal centers. A longer
crank-throw would automatically result in a longer stoke and the longer the
stroke of an engine the higher the thermal efficiency which in turn reduces the
combustion duration thereby reducing the overall heat loss and engine emission.
It also produces more power during the power stroke.
Crankshafts normally do not rotate at a constant
velocity due to the accelerated and decelerated motion. Based on this
situation, a heavy crankshaft absorbs more torque, energy than a light one and
this affects the engine performance. With the same fuel condition a heavier
crankshaft decreases the engine performance by reducing the output torque of
the engine. Also, the location of weight also should be considered with respect
to the crankshaft axis of rotation. For example, the moment of inertia
increases as weight is moved away from the axis of rotation [10]. Therefore to
ensure optimal performance, the weight and the location of the crankshaft needs
to be of high consideration when designing.
2.4 Gear ratios
If the gear ratio is smaller than 1, the output speed
will be greater than the input speed. Thus, the engine speed would be higher
when the gear ratio is smaller than 1 causing the the engine to have a higher
output speed and produce enough torque to drive the car. This implies that an
engine performance would be higher with a gear ratio smaller than 1.
Figure 7 Mercedes-Benz C-class sport
coupe, six-speed manual transmission [12].
The flywheel is attached
to the end of the crankshaft. The main function of a flywheel is to absorb
excess energy while the crankshaft goes on the power stroke and transfer the
stored kinetic energy to the crankshaft to overcome the turning resistance
experienced over the rest three strokes.
Figure 8Crankshaft of an inline
four-cylinder engine piston, connecting rods, and flywheel [13].
The size of the flywheel can determine the degree of
speed variation over each cycle of the piston and crankshaft. The size of a
flywheel affects the engine performance as engines with large flywheels have a
higher performance than an engine with smaller flywheels. This is because the
large flywheels cause a decrease in the speed fluctuation which causes the
engine to run more smoothly than an engine with a small flywheel. However, a
large flywheel engine is heavier than a smaller fly wheel which also affects
the engine performance as it increases the weight of the rotary parts of the
engine. The heavier rotary parts will absorb more energy that created by the
combustion, thereby reducing the amount of energy needed to run the vehicle.
Therefore, with the same fuel conditions the heavier flywheel will decrease the
engine performance than the engine with a lighter flywheel. However, the
heavier flywheel would have a larger inertia which makes it possible to reduce
the engine response that causes the variation of the output torque and speed.
It is therefore advisable that he engine should be neither too heavy nor too
light to attain high energy performance.
The
function of valve train is to open and close the inlet and exhaust valves
according to the working cycle and firing order in each cylinder. As a result,
the fresh air fuel mixture comes in and exhaust gases go out in an orderly
timing. The more fresh air fuel mixture drawn in ,
the more power produced by the engine. Volumetric efficiency (
)is the ratio of the quantity of air trapped by the cylinder
during induction over the swept volume of the cylinder under static conditions
(
) [11]. Volumetric efficiency will increase
with high pressure and low temperature. If the pressure is high and temperature
is low, the mass of gas mixture will rise. However, the volumetric efficiency
will never exceed 100% except using forced induction such as supercharging and
turbo-charging because there is resistance caused by induction system and the
fresh air fuel mixture will be heated by engine valves, piston head, combustion
chamber and high temperature exhaust gas left in cylinder.
For valve train, the
proper valve timing could guarantee a maximum intake at the inlet and exhaust.
Therefore, design needs to be focused on decreasing resistance during inlet and
exhaust period in order to get a bigger and a better engine
performance.
3.1 Engine performance comparison
between different camshaft locations in cylinder-block
The camshaft of an engine
can be mounted at a low, middle or high position in a cylinder-block [11]. This
variation of positions would exhibit different engine performances. The
advantage and disadvantage of each arrangement are shown on the figures below.
The design on the left of
Figure 9 shows a camshaft at a low position. This makes the distance between
camshaft and crankshaft short. Hence, only one pair of gear is needed to drive
it making this design very simple but with so many components and long driving
chain causing the motion of valves to be disturbed when the engine is in high
speed rotation.
For middle mounted
camshaft shown in figure 9 a pair of gear cannot be sufficient to drive it due
to its longer driven distance. Therefore, an idle gear is needed. However,
pushing-rods are not needed because the rocker arm is indirectly driven by
camshaft through tappet.
Figure 9Camshaft mounted low and middle
in cylinder-block [14].
Figure 10Three different layout for high
mounted camshaft [14].
Figure 10, illustrates
high mounted camshaft. The least number of reciprocating parts is suitable for
high speed engine. The camshaft is driven by a chain or a belt. There are three
different layouts for high mounted camshaft. The left one fits large valve lift
engine and it is much easier to adjust the valve clearance than the other two.
The disadvantages are also obvious that such structure is complex and the
rocker arm has the tendency of having flexure deformation during high speed
rotation. For the middle design (which is now commonly used), the rigidity of
rocker arm is much higher than that on the left. When the rocker arm is driven
by a cam directly, it would have the least number of reciprocating parts and
this would increase the rigidity of valve to fit the upper speed engine but
rejects large valve lift and the adjustments of valve clearance.
Figure 11Valve timing diagram[8].
In Figure 11, inlet valve
opens before TDC because it ensures that the inlet valve is totally opened when
the intake stroke begins with partial depression caused by exhaust gas which
commences drawing of the fresh mixture into the cylinder.
Similarly, when piston
reaches BDC, the pressure inside is lower than that outside. The inertia of gas
flow can be used when the piston moves slow at the beginning of compression
stroke. Therefore, inlet valve closing after BDC is a merit to the intake
process. The whole intake process occupies the period as crankshaft rotates
more than 180°.
The exhaust process
occupies the period as crankshaft rotates more than 180°. When the piston moves
near BDC, the pressure in the cylinder is nearly 0.3~0.4Mpa [11]. Such pressure
has little or no importance to the power stroke. If the exhaust valve is slightly
opened, most of exhaust gas would escape. Hence, the exhaust valve can be
widely opened in order to decrease the exhaust resistance and avoid engine
overheating. Additionally, as the piston arrives at TDC, the pressure inside is
still over ambient pressure and the exhaust flow has inertia as well. In
conclusion, exhaust valve closes after TDC helps the cylinder exhaust clean.
Figure 12Variable valve lift [14].
During 1990s, Honda
Company invented VTEC mechanism, which enabled valve timing changeable through
controlling valve motion regulation. In Figure 12, when engine is at low speed,
the middle rocker arm moves separately. The valves opening and closing
movements are driven by small cams located on the left and right side. When the
engine is at high speed, the electronic unit controls the mid-synchronizing
piston moves to connect the three rocker arms together. As a result, the valves
are controlled by the big yellow cam as shown above to increase the valve lift.
After so much research it
can be clearly seen that no engine parameter is an island as they are all
connected together and all function as one unit. These parameters are of great
importance as they greatly affect the engine efficiency and as such great care
has to be taken in respect to the material selection, weight, size and the
position of these different components as they impact the efficiency either
negatively or positively.
In other to achieve maximum
efficiency these parameters must be varied according to the desired engine
specifications.
[1]
H. Heisler. Vehicle and Engine Technology. 2nd ed.,Burlington, MA 01803,
USA: Elsevier Ltd, 1999.
[2]
Z. S. Filipi and D. N. Assains. “The effect of the stroke-to-bore ratio on
combustion, heat transfer and efficiency of a homogeneous charge spark ignition
engine of given displacement”. Int. J.
Engine Res, Vol.1, pp.191–208, 2000.
[4]
D. Thornhill et al. “An experimental investigation into
the effect of bore/stroke ratio on a simple two-stroke cycle engine”. Conf.
Fuels and Combustion in Engines, Trabzon,
Turkey, 1999.
[5]
Z.S.
Filipi and D.N. Assanis. “charge spark ignition engine of
given displacement The effect of the stroke-to-bore ratio on combustion, heat
transfer and efficiency of a homogeneous” International
Journal of Engine Research, Vol.1, pp.191, 2000.
[7]
J. B. Heywood et al. “Development and
use of a cycle simulation to predict engine efficiency and NOx emissions”.
Conf. Fuels and Combustion in Engines, Trabzon,
Turkey, 1979.
[14]
Z.T. Wang. Construction of Automobile. Changchun, Jilin, China: China
Machine Press, 2008
