Introduction of Engine Parameters with respect to Performance versus Emission

Executive Summary

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.

1. Stroke and Bore ratio (S/B)

1.1 Engine performance with the change of Stroke/Bore ratio

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 NO_x 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 (H_TDC/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 H_TDC/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.

1.2 Emissions with the change of Stroke/Bore ratio

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].

2. Crankshaft, Connecting-rod, Piston, Flywheel and gear ratio

2.1 Connecting-rod:

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.

2.2 Piston:

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].

Figure 5Piston layout [10].

2.2.1 Piston crown:

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.

2.2.2 Piston rings:

The compression rings: Compression rings are located between the piston and cylinder wall, the function of compression rings is to seal the gap between the piston and cylinder wall so as to prevent the escape of the fuel and air from the combustion chamber to the crankcase. In this way, the compression rings helps to store enough pressure in the combustion chamber to push the piston downwards during the power stroke [8]. Without the compression rings, the pressure of the combustion chamber will decrease during the power stroke. This means that less force will push the piston downwards thereby causing the engine to produce less output torque and speed which would decrease the engine efficiency.

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.

2.3 Crankshaft

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.

Figure 6Crankshaft [1].

2.3.2 Crank-throw

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.

2.3.3 Weight of the crankshaft

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].

2.5 Flywheel:

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].

2.5.1 Size of the flywheel

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.

3. Valve train

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.

3.2 Valves timing diagrams

3.2.1 Inlet valve opens before TDC and closes after BDC

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°.

3.2.2 Exhaust valve opens before BDC and closes after TDC

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.

3.2.3 Honda variable timing electronic control(VTEC)

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.

Conclusion

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.

References

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