The Function of the Engine - Term Paper Example

Super Eta Analysis and Report Name Institute of affiliation Date Abstract There has been very many phenomena which are associated with the working principles of internal combustion engine. Interesting models have been developed to demonstrate how every part in an engine operate and how it affects the performance of the engine. The engine is a very interesting assembly which possesses fascinating challenges to both engineers and scientists. Working fluids, fuel, moving and stationary parts have complicated working principles and a slight adjustment in the dimension or physical property would affect how the whole engine' performs. Adjustment of the length of the connecting rod would have an effect on the torque produced, number of revolution, friction force and fuel consumption of the engine. Efficiency is of great essence in the engine because of fuel cost and the wearing of expensive parts. If the size of the cylinder is adjusted to the slightest distance, fuel consumption is either increased or decreased. Different engines have different engine capacities and so is the power output of the engine. Some engines are required to give a lot of torque but at very low speed; tractors are a good example. Other engines are required to have very high speed. Such vehicles have low power requirement. To achieve the speed, power and efficiency, adjustment on the cylinder size, size of connecting rods and the number of cylinders would be necessary. The main objective is always to have the highest efficiency possible; maximum power output with least fuel consumption. This causes deep analysis on the every part of the engine to be very critical. Analysis has proved to very productive for the all the year man have had cars. Table of Contents Abstract 2 Table of figures 3 Analysis 6 1.Sliding load on cylinder walls 6 2.Maximum force on dungeon pin and connecting rod length 8 3. Kinematics of engine components 9 Short connecting rod 15 1. Advantages of short connecting rods 15 2. Disadvantages of short connecting rods 15 3. Opinion 16 Limitations and assumptions in the analysis 17 1Assumptions 17 4.Limitations 18 Conclusion 18 Recommendation 18 References 19 Table of figures Figure 1: A diagram of engine connecting rod and crankshaft waorking 5 Figure 2: mat lab diagram of the working piston rod 7 Figure 3: Animated connecting rod, piston and crank 9 Figure 4:Position of the piston are the crank revole 11 Figure 5: velocity of the piston are the crank revole 12 Figure 6: Acceleration of the piston are the crank revole 13 The function of the engine is to have chemical energy converted to heat energy. The heat energy is then converted to mechanical energy; kinetic energy. When the fuel explodes in the cylinder, it pushes pistons downwards rotating a crank shaft. The power from the crankshaft rotates a shaft which in turn rotates the wheel. To capture maximum energy from the engine, every component is therefore analyzed critically to ensure its kinematic and dynamic properties do not affect the performance of the engine negatively. Among the most important components of the engine are the pistons, wrist pins, crankshaft and the connecting rods (Pianese, 2005). With the aid of the above mentioned components, a portion of the energy given out during combustion is turned into very useful rotary motion. Rotary motion is useful since it possesses ability to do work. A crankshaft is made up of coaxial journals which are cylindrical. These journals can be two or more. On the crankshaft, there is also cylindrical crankpins journal. These crankpins are offset and can be more than one. The main journals of the crankshaft are mounted on bearings, and this causes the offset journals of the rods to rotate in a pattern that is circular. The stroke of the engine is equal to the diameter of that path. It is the distance moved by the piston from the highest point it touches to the lowest distance it can move. The big ends of the con-rods are connected such that they are on the offset journals. Figure 1: A diagram of engine connecting rod and crankshaft waorking However, the problem at hand could only be approached using three methods. The first is using original connecting rods of the eta engine. The other method would be to use 2.5 l engine rods which are slightly longer than the original rods by 5mm. The third option is to connect rods from a new version of BMW which are fitting only that they are slightly lighter and stronger. Weight and length of the connecting rods play a very crucial role in the engine design as it shall be seen in the discussion that follows. Analysis 1. Sliding load on cylinder walls Effect of length of the connecting rode on sliding load on walls of the cylinder Effect of length of the connecting rode on sliding load on walls of the cylinder Among the many fundamental mechanisms at the core of every car engine is connecting rods. On the connecting rods, there are components which are reciprocating and others are rotating. These different parts are affected differently by force exerted by the expansion stroke. For the connecting rode mechanism, the main objective is always to have the mass of piston assembly being at its minimum (EUCOMES (Conference), In Flores & In Viadero, 2014). When the mass of the piston assembly is at its minimum, the inertia of crankshaft is also at its minimum. This would, therefore, cause engineers always to tend to construct connecting rods with minimum length and with materials which are light in weight. However, mitigation of forces which are secondary, causes the connecting rod to be made longer. Such forces might be having twice as much effect as the speed of the engine has. These secondary forces are directly proportional to the ratio of connecting length to crank throw. It is advisable then to reduce this ratio. This is only possible when the length of the connecting rod is increased. The other reason which makes it hard to use short connecting rod is the effect it has on its angularity. When the connecting rod is long, it will have lower angle which it makes with the axis of the cylinder. Every angle made between the con-rod and the axis of the cylinder will be lower when con-rod is long as opposed to when it is short. This means then that the sliding force between the piston and the cylinder wall is reduced. For one to acquire lower friction drag from the lower con-rod angularity, the bore of piston is slightly offset from the axis of the same piston. The other method to achieve this would be to make engines with axes of cylinders which are offset from crankshaft`s axis. The two methods have complexity and difficulties when it comes to designing them. On the other hand, the elongating connecting rod has minimal effect since long piston connecting rod is compatible with every other component in the engine. When the pistons are in motion, the piston rings are the parts of the pistons which are in contact with the cylinder walls since they act as seals. Figure 2: mat lab diagram of the working piston rod 2. Maximum force on dungeon pin and connecting rod length The main function of the journal bearing is to provide support for the rotating axial and shaft. There is friction that exists between the bearing and the axle. When the connecting rod is long, both advantages and disadvantages affect each and every component of the engine including journal bearing. Long connecting rod enhances better combustion. This also ensures there is very high cylinder pressure. This is realized only after the first few degrees past top dead center (TDC.) There is also high temperatures within the combustion chamber. Upper and mid RPM are very good. Reduced friction with the engine is realized. Since the angularity of the rod is reduced, every angle the rod makes with the axis of the cylinder is also reduced. The surface of the piston, therefore, experiences less friction. However, this is only good for a vehicle which is light. Air flow velocity is reduced when the rods are very long. Low air flow velocity results into low volumetric efficiency. When rods are short, exhaust and intake velocity are realized. This is so only at low speed and therefore the engine can give low-end torque which is good. Figure 3: Animated connecting rod, piston and crank 3. Kinematics of engine components i. Kinematics of crank shaft a. Piston instantaneous position c= (l +r) – s equation 1 c is the instantaneous position of the piston after it moves from TDC r1 is the crank radius r2 length of the connecting rod x=r2cosӨ + r1sinα equation 2 sinα= r/x=y sin α=ysinӨ cosα= = equation 3 1-cos α= 1- =cos α equation 4 x=r2cosӨ + r1sinα x = r1(y2sin2 +r(1-cosӨ) equation 5 Figure 4:Position of the piston are the crank revole Piston position in regard to rotation of crankshaft b. Piston instantaneous velocity The velocity of cylinder and acceleration can be calculated using equations below Equation 6 . Equation 7 And do the same again to achieve acceleration of the piston Equation 8 Equation 9 A diagram of velocity position and acceleration in mat lab Figure 5: velocity of the piston are the crank revole c. Piston instantaneous acceleration a= equation 10 = . Equation 11 a=rῳ2(cosӨ +ycos 2Ө) equation 12 Figure 6: Acceleration of the piston are the crank revole d. Piston pin position Position of the pin of the piston can be derived using trigonometry. After differentiation, velocity and acceleration can be obtained. Using cosine rule, piston position can be formulated. X2=r12 + r22-2r1r2cosӨ equation 13 R2=r1cosӨ ± equation 14 e. Piston pin velocity After finding derivative of equation14 one gets. V=-r1sinӨ - equation 15 V=ῳ equation 16 f. Piston pin acceleration V=-r1cosӨ - - equation 17 ii. Kinematics of connecting rods a Instantaneous velocity of connecting rod V== equation 18 =y equation 19 ≈ 1 =ῳ*y*cosα equation 20 V= ῳ*y*cosα equation 21 Instantaneous acceleration of the connecting rod a== equation 22 a=-ῳ2ysinӨ equation 23 From the velocity, acceleration and position diagrams drawn, the length of the connecting rod has effect profile significantly. Short connecting rod 1. Advantages of short connecting rods When the connecting rods are short there is good exhaust and intake velocity for low and moderate speeds of engines. The effect of having velocities for intake and exhaust that are good is that there is a production of sufficient low-end torque (Mashadi, & Crolla, 2012). This is a result of a very big vacuum at the start of intake cycle. The piston moves very fast away from the TDC which causes the stroke to have a greater displacement for every point of rotation made by the crank. The higher exhaust and intake velocities cause the system to acquire a more homogenous mixture of air and fuel in the chamber. Greater power output is therefore realized from the engine. When the piston can move at very high speed from the TDC, the volume of the combustion chamber increases equally fast unlike when long connecting rods are used. The point of maximum pressure is delayed on the cylinder. This has the best effect on the vehicles which use nitrous oxide, turbo boost or supercharger. For short connecting rod, cam timing becomes more radical as compared to a situation where a long connecting rod is used. 2. Disadvantages of short connecting rods Increased speed of piston away from top dead center causes an outrun in the flame front at very high rpm. The total pressure in the cylinder is reduced at the termination of a combustion cycle. This pressure is referred to as braking mean effective pressure. Total combustion is reduced since the piston moves from the top dead center very fast. This is so because of the reduced total pressure on the piston head. 3. Opinion In my opinion, a short connecting rod has many disadvantages which have a massive effect on engine and environment. This is so because, from our discussion above, there is reduced combustion as result of reduced total pressure. Reduced combustion means high emissions of CO and CO2 into the air. This would cause the engine fail in some test by the environmental agencies. At the introduction, we also saw that the shorter the connecting rod the greater effect it has on the rod angularity and the secondary forces which are almost double the effect the speed has. Long connecting rod, on the other hand, reduces the angle made by the axis of the connecting rod and axis of the cylinder. This has an advance effect on the sliding force along the cylinder; the drag force is reduced and therefore the piston can move faster. Therefore a long connecting rod is preferred to a short connecting rod. Maximum RPM for the engine with 7000rpm when using 25i and M3 Converting angular velocity to linear velocity Where v is the linear velocity is the angular velocity r is the length of the stroke v0 is the initial linear velocity So cr is the critical speed of the engine = 228.37rad/s When the using a connecting rod of 135 mm = Limitations and assumptions in the analysis 1 Assumptions The connecting rod for the first two cases was made of uniform materials. The engine under the analysis is a four stroke engine. When analyzing effects on forces, the friction force is negligible and therefore could be ignored 4. Limitations Actually, it is not only the connecting rod that would have advance effects on the engine when changed. Pros and cons of both long and short connecting rodS were almost equal. The choice of the type of rod, therefore, was dictated mainly by the power output of the engine required. Conclusion Internal combustion engines rely on the pressure above the pistons so that the rotary motion can be achieved. When this pressure is multiplied by the area below it, it produces a force. This force is transmitted through the connecting rod to the crankshaft. If the crankshaft is treated like a lever, the greatest force can be realized when the connecting rod is acting perpendicular to it. So the greatest power from engine is achieved when the connecting rod and the crankshaft are at right angles. Recommendation When the length of the connecting rod is adjusted, the time when right angle is achieved is changed also. For a short rod, the angle is arrived at sooner as compared to long connecting rods. Pressure in the cylinders is also higher for short connecting rods. When it comes to mechanical stress (for example friction), it is less for the connecting rods which are long because of the angle made by the connecting rod and the axis of the cylinder. When the angle is less, the stresses are reduced. The length of the connecting rods possess both technical and design difficulties since both short and long connecting rods have their advantages and disadvantages. My advice to engine builder would be to choose the long connecting rod since the stresses are reduced with the engine and therefore a longer life span for the engine. Sufficient combustion enhanced by the long connecting rod ensures fuel is completely used for maximum power. Long con-rod means that the angle between the cylinder axis and the con-rod is reduced. When the angle is reduced, the dragging force of friction is also reduced. This enhances performance of the engine at large. Therefore long connecting rod are better than short ones. The best choice is the connecting rod from M3 version with a length of 135 mm. This connecting rod provides a good length without having much effect on the weight of the engine. References EUCOMES (Conference), In Flores, P., & In Viadero, F. (2014). New trends in mechanism and machine science: From fundamentals to industrial applications IFAC Symposium on Advances in Automotive Control, Pianese, C., & International Federation of Automatic Control. (2005). Advances in automotive control 2004: A proceedings volume from the IFAC symposium, Salerno, Italy, 19-23 April 2004. Oxford: Published for the International Federation of Automatic Control by Elsevier. Mashadi, B., & Crolla, D. (2012). Vehicle powertrain systems. 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Figure 2: mat lab diagram of the working piston rod 2. Maximum force on dungeon pin and connecting rod length The main function of the journal bearing is to provide support for the rotating axial and shaft. There is friction that exists between the bearing and the axle. When the connecting rod is long, both advantages and disadvantages affect each and every component of the engine including journal bearing. Long connecting rod enhances better combustion. This also ensures there is very high cylinder pressure.

This is realized only after the first few degrees past top dead center (TDC.) There is also high temperatures within the combustion chamber. Upper and mid RPM are very good. Reduced friction with the engine is realized. Since the angularity of the rod is reduced, every angle the rod makes with the axis of the cylinder is also reduced. The surface of the piston, therefore, experiences less friction. However, this is only good for a vehicle which is light. Air flow velocity is reduced when the rods are very long.

Low air flow velocity results into low volumetric efficiency. When rods are short, exhaust and intake velocity are realized. This is so only at low speed and therefore the engine can give low-end torque which is good. Figure 3: Animated connecting rod, piston and crank 3. Kinematics of engine components i. Kinematics of crank shaft a. Piston instantaneous position c= (l +r) – s equation 1 c is the instantaneous position of the piston after it moves from TDC r1 is the crank radius r2 length of the connecting rod x=r2cosӨ + r1sinα equation 2 sinα= r/x=y sin α=ysinӨ cosα= = equation 3 1-cos α= 1- =cos α equation 4 x=r2cosӨ + r1sinα x = r1(y2sin2 +r(1-cosӨ) equation 5 Figure 4:Position of the piston are the crank revole Piston position in regard to rotation of crankshaft b.

Piston instantaneous velocity The velocity of cylinder and acceleration can be calculated using equations below Equation 6 . Equation 7 And do the same again to achieve acceleration of the piston Equation 8 Equation 9 A diagram of velocity position and acceleration in mat lab Figure 5: velocity of the piston are the crank revole c. Piston instantaneous acceleration a= equation 10 = . Equation 11 a=rῳ2(cosӨ +ycos 2Ө) equation 12 Figure 6: Acceleration of the piston are the crank revole d.

Piston pin position Position of the pin of the piston can be derived using trigonometry. After differentiation, velocity and acceleration can be obtained. Using cosine rule, piston position can be formulated. X2=r12 + r22-2r1r2cosӨ equation 13 R2=r1cosӨ ± equation 14 e. Piston pin velocity After finding derivative of equation14 one gets. V=-r1sinӨ - equation 15 V=ῳ equation 16 f.

Piston pin acceleration V=-r1cosӨ - - equation 17 ii. Kinematics of connecting rods a Instantaneous velocity of connecting rod V== equation 18 =y equation 19 ≈ 1 =ῳ*y*cosα equation 20 V= ῳ*y*cosα equation 21 Instantaneous acceleration of the connecting rod a== equation 22 a=-ῳ2ysinӨ equation 23 From the velocity, acceleration and position diagrams drawn, the length of the connecting rod has effect profile significantly.

Short connecting rod 1. Advantages of short connecting rods When the connecting rods are short there is good exhaust and intake velocity for low and moderate speeds of engines. The effect of having velocities for intake and exhaust that are good is that there is a production of sufficient low-end torque (Mashadi, & Crolla, 2012). This is a result of a very big vacuum at the start of intake cycle. The piston moves very fast away from the TDC which causes the stroke to have a greater displacement for every point of rotation made by the crank.

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