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Types of Engine Vibration

Question:

Write a Literaure Review on "Effect of Piston Vibration on Engine Performance".

Technological advancement has greatly contributed to the stiff competition currently being experienced in the automotive manufacturing industry. Fuel economy and engine efficiency have become critical design concerns in the industry. Admittedly, demand for efficient automobiles has pushed engineers to rethink harder.

One of the topics in which a lot of research is currently being undertaken is the engine vibration. There is need to produce efficient and vibration-free cars.  Theoretically, this is possible but practically, there would always be some form of vibration as the engine operates (Johnson, 2003). However, the unwarranted engine vibration has often been a draw back on maximizing the performance of the engines. Most research engineers agree wholeheartedly that engine vibration is among the major issues of concern in automobiles.

Notably, there are two main types of engine vibrations; namely, the torsional and longitudinal vibration (Ramachandran & Padmanaban, 2012). The former is caused by the fluctuating pressures on the crankshaft and its mountings while the later is produced due to the piston and cylinder reciprocating movement and their corresponding weights. Therefore, in a nutshell, according to Ramachandran& Padmanaban (2012), an engine would mostly vibrate due to the unbalanced reciprocating parts; the frequent change in gas pressure inside the cylinder; the resulting forces due to the engine parts loads and the material features of the pistons.


Now, many a researchers have greatly made attempts to point out at the possible effects the engine vibration, specifically pistons, have on the engine performance. In fact, the piston vibration can be attributed to the frequent changing of the mass moment of inertia due to the cyclic fashion in which the center of mass of the piston moves (Guzzomi, 2007). Consequently, the performance of the engine is grossly affected. In fact, according to Guzzomi (2007), about a half of engine losses are attributed to the piston rings, skirt and connecting pins.

But what exactly causes these vibrations? As mentioned earlier, Ramachandran&Padmanaban (2012) opine that the vibrations are due to: (1) the unbalanced reciprocating parts; (2) the frequent change in gas pressure within the cylinder; (3) the forces due to the engine reciprocation; and (4) the material features of the piston parts. However, to cement our understanding of the phenomenon, it is crucial to illustrate the structural configuration of the pistons. The piston is normally the moving part of the engine; its movement is actuated by the forces produced by the fuel combustion. It normally moves up and down from the bottom dead centre to the top dead centre. During the power cycle, a lot of heat is produced, which causes the piston to expand, hence some dimensional clearance is always allowed between the piston and the cylinder wall. Consequently, the pistons must often be made of lighter and thermally conductive materials like Aluminum. In fact it is the most commonly used piston material. However, like with all other materials, Aluminum expands when heated; therefore, provisions are often made to allow smooth movement of the pistons up and down within the cylinder bore without allowing excessive power loss due to compression loss or for the piston to cease in the cylinder (University of Windsor, 2016). Notably, according to University of Windsor (2016), the piston features include: piston pins, head, skirt and rings (ring grooves, ring lands, and pistons ring). The piston pin connects the piston to the connecting rod. The skirt, located near the crankshaft, aligns the piston as it moves in the cylinder bore. The groove ensures that the piston rings are restricted especially during engine operation. The modern piston heads are bowl-shaped unlike the traditional flat designs. This often creates a vortex flow of fuel hence allowing perfect mixing of air and fuel (George, 2017). Consequently, the perfect air-fuel mixing has a considerable improvement in the mechanical power of the engine as all fuel injected into the cylinder get burnt. Besides, such piston designs are now being used in the gasoline powered engines as it encourages direct mixing as well (George, 2017).

Causes of Engine Vibration

Additionally, as mentioned earlier, the piston material greatly affects the performance of the engine. The piston is normally subjected to fluctuations in pressure, thermal stress and mechanical loads. Therefore, the piston material selected is often one with greater structural integrity. More often, it must withstand those constraints vis-à-vis the manufacturing cost. Cast Aluminum alloy are often the compromised material. For instance, the lightweight property of Aluminum would reduce the loads hence ultimately the vibrations during operation are minimized.

Admittedly, however, operational performance of most of these top-class piston designs often uncovers loopholes which would otherwise largely remain hidden during design. The most annoying engine problems, according to Carley (2013) include: noise, vibration and harshness (NVH). These three operational issues must be minimized to ensure efficient and safe performance of the engine. Now, as earlier mentioned, one of the contributing factors in engine vibration is the unbalanced forces due to the rotating parts of the piston Al Guzzomi (2007). Carley (2013) agrees with this notion and even goes ahead to confirm that the speeds of the engines with unbalanced parts can greatly magnify these vibrations as the centripetal forces increase. In fact, Carley (2013) bluntly states that if the rotational speed is doubled, for instance, then the centripetal force would quadruple causing more vibrations in the engine.


Additionally, the position of piston’s centre of gravity also has an effect on the vibration. Notably, the piston pin is normally slightly positioned outside the geometrical axis of the piston head to easily change the motion from the piston’s reciprocation to rotational movement of the crankshaft. Connectedly, therefore, any unbalanced mass in the head would trickle down to the crankshaft. The crankshaft often tries to rotate about its central axis but with inherited problems, the rotation would not be smooth thanks to the transferable vibrations. Consequently, this sets the entire engine into a massive vibration which hinders smooth ride. Additionally, the vibrations, if not checked and rectified in time, can create a mountain of problems with the ultimate one being engine shutdown. For instance, as the piston head vibrates, the rings and seals are impounded upon by the impact causing more gas leaks. These leaks are often the major cause of mechanical power losses. In a diesel engine, where a particular compression ratio must be reached before power cycle takes over, there would be possible delays leading to inconsistent firing and misfiring thanks to the leaks. Additionally, the resulting crankshaft’s torsion vibration may cause material fatigue and stresses which ultimately leads to cracks and grand-scale failure. Secondly, Al Guzzomi (2007) attributes that the structural characteristics and configuration of the cylinders can be a potential cause of engine vibration. Engineering Explained (2015) cements this notion by pointing at the inline cylinder configuration loopholes. More often, the inline type does not balance the secondary forces (Engineering Explained, 2015) leading to engine vibration.

Furthermore, according to Pente at al (2013) there is another occurring phenomenon in the piston technically referred to as piston slap. This describes a situation where the piston diameter is smaller than that of the cylinder bore. In such a case, therefore, free movement of the piston is allowed causing knocking sounds as the piston impacts on the cylinder walls (Pente et al, 2013). This traverse movement of the piston causes a lot of noise and consequently, too much vibration results. Admittedly, wear rate would normally go up. However, there have been substantive researches done in this area with the aim of actually troubleshooting the phenomenon.

Structural Configuration of Pistons

Further more, some brilliant minds discovered techniques to analyze the engine vibration. Griffiths & Skorecki (1964) discovered interesting facts about the vibration of the single-cylinder engine. They painstakingly investigated and uncovered that diesel engines would likely produce more noise when the cooling system temperature is very low. But how is this related to the piston vibration? Actually the piston slap was monitored using a motor which was connected to the engine; other sources of vibration were eliminated and focus was on the effect of the piston slap (Griffiths & Slorecki, 1964).Notably, noise results from vibrations and as mentioned earlier, the piston slap is often the common source.

Haddad & Pullen(1974) discovered a vibration monitoring technique. The phenomenon of piston slap could also be investigated using oscillographic and simulation technique to determine its relative magnitude, compared with the other noise sources of the engine (Haddad &Pullen, 1974). Furthermore, it has been discovered that this source of noise is critical when it comes to analyzing the performance of the engines hence uncovering real-time techniques is one step in improving the engine performance.


Chabot, L et al (2000) also contributed by giving the much needed insights on noise and vibration optimization of a petrol engine. In their study, they revealed how the engine noise and vibration could be optimized. They used a case to monitor the dynamic behavior of a 1.6 litre gasoline engine. According to Chabot et al (2000), the main objective was to reduce the low frequency radiated noise from the cylinder block.

Zheng et al (2001) conducted a FEM/BEM analysis of diesel piston-slap induced ship hull vibration and underwater noise. Zheng et al (2001) employed finite element method to simulate the vibration response of the hull due to the excitations of diesel piston-slap and vertical inertia reciprocating force. Gerges, S et al. (2000) agree that the piston slap is the major contributor to noise and vibration in engines. The cause of vibration, in this case, is attributed to the cyclic variation in the gas pressure. Gerges, S et al(2000) further assert that the magnitude of vibration can significantly be magnified by the increased combustion activities.

Now, there is another phenomenon, known as cycle-to-cycle variations, that occurs during piston-head-and-fuel-air-mixture engagement. According to Aydin (2011) by observing the cylinder pressure versus time measurements from a spark ignition engine, an interesting and substantial variation can be uncovered.  Admittedly, the pressure development is uniquely related to the combustion process such that increased combustion translates to higher pressures in the cylinder, which increases the impact force on the piston head. Now, as Aydin (2011) points out, the pressures also vary from cylinder to cylinder. Therefore, vibrations due to the piston movement subsequently may also vary from cylinder to cylinder.


Notably, however, Mechanical Engineers continue to grapple with these phenomena in the engines. There are a dozen designs of the pistons alone. Each with design features that boost the operational efficiency and safety of the engines. Thanks to the modern design tools, modern and sophisticated pistons are being manufactured. For example, the current design has a wave-like head in the crown (Lockridge, 2016). According to Lockridge (2016) there are six ridges at the centre of the piston such that spraying occurs between the ridges. This ensures that less soot is formed and the fuel is burnt completely. The technology is seen to be a game changer in the industry. However, even as concerns are being shifted to the design and development of pistons that integrate fuel economy and engine efficiency, manufacturing cost is normally not ignored. Actually, there have been a dozen proposals but after considering the cost implications, more often, these ideas are abandoned. Importantly, however, improvement of piston designs would still require these innovative approaches. Besides, alternatively, the search for piston materials which are more superior to the existing ones, in terms of properties, is still a work-in-progress. Furthermore, the electric car technology, as has been pointed out by many scholars, would still not provide a stiff competition to the ICE engines as the technology is still at an infantile stage. Hence, the automakers still have the grace period to make the necessary improvements in the existing technology, that is, the ICE engines. However, certainly, as far as internal combustion engines are still here with us, more sophisticated designs would still be realized in the future and the engineers will never stop to produce more efficient car engines.

References

Ramachandran, T & Padmanaban, K. (2012). Review on Internal Combustion Engine Vibrations and Mountings. Dindigul, Tamilnadu. Available at: https://www.ijeset.com/media/8N5-IJESET0202516.pdf  [Accessed: 9/4/2017]

Guzzomi , A.L et al. (2007).Variable Inertia Effects of an Engine Including Piston Friction and a Crank or gudgeon pin offset. The University of Western Australia. Available at: https://research-repository.uwa.edu.au/files/1489255/11941_PID11941.pdf

University of Windsor.(2016). Piston and Piston Rings. Available at: https://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20and%20Piston%20Rings.htm

George, P. (2017). How does piston shape affect combustion? Available at: https://auto.howstuffworks.com/piston-shape-affect-combustion1.htm

Carley, L. (2013). Maintaining Your Balance: Engine Building Tips to Reduce NVH and Increase Life. Available at: https://www.enginebuildermag.com/2013/11/maintaining-your-balance-engine-building-tips-to-reduce-nvh-and-increase-life/

Engineering Explained. (2015).The Pros And Cons Of Different Engine Types. Available at: https://www.carthrottle.com/post/engineering-explained-the-pros-and-cons-of-different-engine-types/

Pente, S …et at.(2013). Vibrational Analysis of a VCR Diesel Engine: A Review. Available at: https://www.ijetae.com/files/Volume3Issue6/IJETAE_0613_74.pdf

Griffiths, J & J.Skorecki, J. (1964). Some aspects of vibration of a single cylinder diesel engine. Journal of sound and vibration vol.1 (345-364).

Haddad, D  & Pullen, L.(1974). Piston slap as source of noise and vibration in diesel engine.  Journal of sound and vibration 34(2), (249-260).

Chabotm, L.. et al. (2000). Noise and Vibration Optimization of a Gasoline Engine. Fifth Ricardo Software International User Conference, Detroit.

Zheng, H…et al.  (2001). FEM/BEM analysis of diesel piston-slap induced ship hull vibration and underwater noise.

Gerges, S…et al. (2000). A Literature Review of Diesel Engine Noise with Emphasis on Piston Slap. Online journal. Available at: https://iiav.org/ijav/content/volumes/5_2000_1108041287056607/vol_1/307_firstpage_164321287057558.pdf

Aydin,K. (2011).Effect of Engine Parameters on Cyclic Variations in Spark Ignition Engines. Available at: https://web.firat.edu.tr/iats/cd/subjects/Automotive/ATE-14.pdf

Johnson, L. (2003). Theory: Reciprocating Engine Vibration. Available at: https://www.dssmicro.com/theory/th_recip_vibs.htm

Lockridge, D. (2016). Volvo Talks about Piston Technology in Latest Engines. Available at: https://www.truckinginfo.com/channel/fuel-smarts/article/story/2016/06/volvo-talks-about-piston-technology-in-latest-engines.aspx

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