Tài liệuSửa đổi

Nhược điểm ĐC2KSửa đổi

Contrary to four-stroke engines, the fresh fuel/air mixture in a classical two-stroke scavenges the cylinder after combustion. This causes about 30% of the fresh mixture to be exhausted as unburned oil mist. Along with the only partial burning of the oil, two-stroke engines generate high emissions and cause severe odour, smoke and noise pollution.[1]

Nguyên lý hoạt động ĐC2KSửa đổi

Two-Stroke Cycle CI Engine The two-stroke cycle engine is sometimes called the Clerk engine. Uniflow scavenging occurs with fresh charge entering the combustion chamber above the piston while the exhaust outflow goes through ports uncovered by the piston at its outermost position.

Low- and medium-speed two-stroke marine CI engines continue to use this system, but high-speed two-stroke CI engines reverse the scavenging flow by blowing fresh charge through the bottom inlet ports, sweeping up through the cylinder and out of the exhaust ports in the cylinder head. The three characteristic phases of the two-stroke CI engine are [16]: scavenging phase, compression phase, and power phase.

With the two-stroke diesel engine, intake and exhaust phases take place during part of the compression and power stroke, respectively, so that a cycle of operation is completed in one crankshaft rotation or two piston strokes. Because there are no separate intake and exhaust strokes, a blower is necessary to pump air into the cylinder to push out exhaust gases and to supply the cylinder with fresh air for combustion. [2]

Ưu điểm động cơ dieselSửa đổi

The heavier construction, higher compression ratio, nonthrottled air supply, and greater heat value per pound of diesel fuel, as well as the ability to produce rated horsepower (kW) at a lower speed, develop substantially higher torque curves, return superior fuel economy (better thermal efficiency), and offer “life-to-overhaul” typically three to four times longer than that of a gasoline engine, are major attributes of the diesel engine.[3]

Tỷ số nénSửa đổi

ĐNSửa đổi

CR is the ratio of the cylinder volume at BDC to the volume at TDC.[4]

Compression ratio (CR)Sửa đổi

The compression ratio (CR) is defined as the ratio of the volume of the cylinder and its head space (including the pre-combustion chamber, if present) when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel (‘top dead centre’, tdc). Typically, petrol engines have a CR of 8–10, while diesel engines have a CR of 15–20. The CR of petrol engines is limited by the requirement that the fuel burns uniformly in the cylinder and does not ignite thermally prior to the spark (so-called ‘engine knocking’). In a spark-ignition engine, the CR at which pre-ignition takes place is determined by the octane number of the petrol; see Box 4.2. High-octane fuel permits a high CR. Until about 30 years ago, lead tetraethyl was added to petrol as an anti-knock agent. This was phased out for environmental reasons and non-toxic additives are now sometimes used. Improvements in engine design over recent years have, however, led to satisfactory compression ratios with lower octane fuel.[5]

16.6.1 Effect of Compression Ratio on Engine CombustionSửa đổi

The compression ratio, in this case for a realistic engine it is the effective one, has two effects on combustion. The first, and obvious effect, is on the thermodynamic cycle. The pressure and temperature at the end of compression will be affected by the compression ratio, with a higher compression ratio increasing both these parameters. Compression ratio will also have a significant effect on the geometry of the combustion chamber, and a higher compression ratio will often result in a combustion chamber of narrower aspect ratio. This means that the flame will contact the piston earlier (see Fig. 16.11(b)), and this will tend to reduce the rate of heat release. The compression ratio of the base engine was both increased and decreased as shown in Table 16.2.[6]

13.2.1 Engine Design FactorsSửa đổi

The compression ratio is a decisive factor for the thermodynamic efficiency of the cycle. However, it affects the exhaust gas composition by two means: A high compression ratio increases the maximum temperature in the combustion chamber prior to combustion and thus enhances the NOx formation during combustion; however, preignition of some portions of the cylinder charge may result. Preignition is associated with extremely high temperatures, which further increase the NOx formation. Alternatively, demands for a higher-octane-number fuel may introduce other pollutant components to the fuel and thus to the exhaust gases.

The design of the combustion chamber shape also determines crevice volumes and, therefore, has an important influence on the emission level of HC. For this reason, compact combustion chambers with high volume to surface area ratios are preferable. Combustion chamber and scavenge port-entrance assembly are also important in the determination of the turbulence intensity prior and during combustion. High turbulence intensity ensures good fuel-air and residuals mixing and high flame speed, which are significant to minimizing cycle-by-cycle variations and, hence, HC emission.

The position of the spark plug in the combustion chamber affects the flame travel distance and, hence, the combustion duration and the formation of various species. A longer combustion duration is associated with lower maximum pressure and temperature and, thus, lower NOx emission. At the same time it results in higher HC emission due to relatively uncompleted combustion. For this reason, dual spark plugs contribute to a lower HC emission and a higher NOx emission.[7]

The History of Science in the United States: An EncyclopediaSửa đổi

– edited by Marc Rothenberg - pp.176-177

Kỹ thuật hóa học là chuyên ngành kỹ thuật liên quan đến những ngành công nghiệp quy trình hóa học và thường được xem là một trong bốn nhánh chính của kỹ thuật.

Cho đến giữa thế kỷ 19, nhân loại chưa có định nghĩa về lĩnh vực kỹ thuật hóa học. Việc thiết kế quy trình hóa học từ quy mô phòng thí nghiệm, với ống nghiệm và beaker, đến quy trình công nghiệp lớn, với đường ống và tháp phản ứng, thường được thực hiện bởi những nhà hóa học và kỹ sư cơ khí.

Đến khoảng năm 1880, nhà hóa học người Anh, George Davis đã sử dụng thuật ngữ "kỹ thuật hóa học" nhằm nói về lĩnh vực kết hợp giữa kiến thức về hóa học và kỹ thuật, đồng thời đảm nhiệm việc thiết kế và quản lý quy trình hóa học ở quy mô công nghiệp.

Ở Đức vào cùng thời điểm, việc sản xuất hóa chất chỉ tập trung vào những sản phẩm có số lượng nhỏ và giá trị thương phẩm cao, nên họ không thấy nhu cầu cần thiết của một lĩnh vực mới như kỹ thuật hóa học. Trong khi đó, ở Hoa Kỳ, sản phẩm hóa chất chỉ tập trung vào một vài sản phẩm cơ yếu và sản xuất với số lượng rất lớn; dẫn đến nhu cầu về việc đào tạo ngành kỹ thuật hóa học trở nên cấp thiết hơn. Do vậy, những chương trình đào tạo ngành kỹ thuật hóa học đầu tiên trên thế giới được ra đời tại đại học MIT vào năm 1888 và đại học Pennsylvania vào năm 1892.

Vai trò của lĩnh vực kỹ thuật hóa học rất quan trọng trong giai đoạn bùng nổ về khoa học kỹ thuật ở nhiều nước như Hoa Kỳ. Thời điểm này, kỹ thuật hóa học tập trung chủ yếu vào những quy trình vận hành cơ sở (unit operations). Những kỹ sư hóa học đóng góp rất lớn vào việc mở rộng quy mô sản xuất các ngành công nghiệp dầu khí và hóa dầu vào những thập niên 1920–1930, để đáp ứng nhu cầu nhiên liệu cho ô tô khi đó đang trên đà phát triển. Sau Thế chiến thứ Hai, lĩnh vực kỹ thuật hóa học càng phát triển mạnh mẽ. Viện Kỹ sư Hóa học Hoa Kỳ (American Institute of Chemical Engineers) chỉ có khoảng 2.000 thành viên vào thập niên 1940. Nhưng đến năm 1960 đã lên tới 20.000 thành viên, và đến năm 1980, con số này đã là gần 50.000 người.

Vào thời hậu Thế chiến thứ Hai, lĩnh vực kỹ thuật hóa học bắt đầu giảm sự chú trọng vào vận hành cơ sở và xuất hiện sự phân hóa giữa nhóm kỹ sư chuyên nghiên cứu và nhóm kỹ sư công nghiệp. Nhóm kỹ sư nghiên cứu tập trung vào những lý thuyết và mô hình toán học phức tạp, vốn thường ít liên quan đến những vấn đề trong sản xuất công nghiệp. Những lý thuyết về hiện tượng vận chuyển (transport phenomena) dần thay thế lý thuyết vận hành cơ sở vào thập niên 1960. Hiện tượng vận chuyển tập trung vào những khái niệm lý thuyết tổng quát hơn như chuyển khối, truyền nhiệt, truyền động năng. Nhóm kỹ sư hóa học công nghiệp lại tập trung vào lĩnh vực thực tiễn như thiết kế thiết bị phản ứng và tính kinh tế của những quá trình nhiệt độ cao.

XupapSửa đổi

Valve Train System with Poppet ValvesSửa đổi

There are five common types of valve train systems with poppet valves:[8]

  • The direct-acting OHC valve train (Fig. 2.6A), also known as the bucket-style follower OHC: It is used, for example, in Ford Ztec and Olds Quad 4. This type features high rigidity during functioning that enables it to be used at high engine speeds, high friction as a result of contact between the cam lobe and tappet surface, and high values of inertial masses [25].
  • The end pivot rocker arm OHC valve train (Fig. 2.6B), also known as the finger-type follower OHC: It is used, for example, in Ford Modular, Mitsubishi 4G63, and Chrysler 2.2 L. This type is characterized by low friction because of rolling contact between cam and rocker arm, high friction for sliding contact, high sensitivity at rocker arm oscillation, low values of acceleration due to cam concavity that does not allow for usage at high engine speeds, and small cam profile due to rocker ratio [25].
  • The center pivot rocker arm OHV valve train (Fig. 2.6C), used in the Honda B18 and by Porsche, for example: It is characterized by low friction for rolling contact between cam and rocker, high sensitivity at rocker oscillation, and low stiffness as a function of rocker ratio [25].
  • The center pivot cam follower OHV valve train (Fig. 2.6D), used in Ford Escort CVH: It has similar characteristics as those of the center pivot rocker arm OHV valve train [25].
  • The pushrod OHV valve train (Fig. 2.6E), used in the GM 556-hp 6.2 L LSA V8 OHV engine of the Cadillac CTS-V: This type is very flexible because of the length of the pushrod and cannot be used at high engine speeds [25].

The Valve Spring (lò xo xupap)Sửa đổi

7.1 Functions[9] Figure 7.1 shows a valve spring. The valve spring is a helical spring used to close the poppet valve and maintain an air-tight seal by forcing the valve to the valve seat. A spring accumulates kinetic energy during contraction and the energy is dissipated upon expansion. There are many types, shapes and sizes of steel springs.

Fig 7.1. Valve spring. Generally, coil springs of a wire diameter below 5 mm ϕ are cold-formed at room temperature, while wires above 11 mm are normally hot-formed. Compression valve springs are provided with the ends plain and ground.

The valve train consists mainly of valves, valve springs and camshafts. At low camshaft revolutions, the valve spring can follow the valve lift easily so that the valve moves regularly. By contrast, at high revolutions, it is more difficult for the valve and valve spring to follow the cam. Valve float is the term given to unwanted movements of the valve and valve spring due to their inertial weights. To avoid this, the load of the valve spring should be set high. The load applied at the longest length is called the set load, and the valve spring is always set to have a high compressive stress above set load conditions. Figure 7.2 shows double springs, which are used to raise the set load while minimizing the increase in height.

Fig 7.2. Double springs installed in a bucket type valve lifter.

Another resistance phenomenon that occurs at high revolutions is surging, due to resonance. Surging occurs when each turn of the coil spring vibrates up and down at high frequency, independently of the motion of the entire spring. It takes place when the natural frequency of the valve spring coincides with the particular rotational speed of the engine. Generally, surging occurs at high revolutions, and the surging stress generated is superimposed on the normal stress. The total stress is likely to exceed the allowable fatigue limit of the spring material and can break the spring. A variable pitch spring reduces the risk of surging. This spring has two portions along the length, a roughly coiled portion and a densely coiled portion, which ensures that the natural frequency of the spring is not constant and therefore not susceptible to resonance.

Poppet valves in Reciprocating compressorsSửa đổi

Maurice Stewart, in Surface Production Operations, 2019[10]

Poppet valves (Figs. 9.62 and 9.63) are the most expensive but are also the most efficient. They have an effective lift area approximately 50% greater than that provided by the same size concentric ring valve. Poppet valves can operate with lifts as great as ¼ inch and are used extensively in natural gas pipeline booster applications. As the pressure builds above each of the individual poppets, they open, allowing gas to pass through the openings in the lower portion. The greater the distance between the upper and lower portions, the smaller the resistance to flow.

Poppet valves utilize a mushroom-shaped element made from a variety of materials. The sealing element material determines the range of application. Valves with metallic poppets can withstand pressures up to 3000 psi (20,684 kPa) and temperatures up to 500°F (260°C). However, metallic poppets are seldom used due to inertial effects. Nonmetallic poppets are limited to 450°F (232°C) and 800 psi (5516 kPa), with speeds up to 1800 rpm. Nylon, Torlon, and PEEK are used for the poppet material due to their lightweight and conformability to the valve seat.

Figs. 9.64 and 9.65 show the Dresser-Rand “Magnum” valve. Unlike other poppet valves the Magnum can be used at high compressor speeds. The valve's unique element design minimizes tensile stresses. The streamlined flow path with optimized seat, guard and lift areas, maximizes valve flow area and is more tolerant of particles and liquids in the gas. The valve is specifically designed for high molecular weight applications at both low and high compressor speeds.

Tham khảoSửa đổi

  1. ^ Bart, Jan C.J.; Gucciardi, Emanuele; Cavallaro, Stefano (2013). “Biolubricant product groups and technological applications”. Biolubricants. Elsevier. tr. 565–711. ISBN 978-0-85709-263-2. doi:10.1533/9780857096326.565. 
  2. ^ Siczek, Krzysztof Jan (2016). “Principles of valve train operation”. Tribological Processes in the Valvetrain Systems with Lightweight Valves. Elsevier. tr. 3–18. ISBN 978-0-08-100956-7. doi:10.1016/b978-0-08-100956-7.00012-6. 
  3. ^ Brady, Robert N. (2004). “Internal Combustion (Gasoline and Diesel) Engines”. Encyclopedia of Energy. Elsevier. tr. 515–528. ISBN 978-0-12-176480-7. doi:10.1016/b0-12-176480-x/00089-9. 
  4. ^ Kreith, F. (1998). The CRC Handbook of Mechanical Engineering, Second Edition. Handbook Series for Mechanical Engineering. Taylor & Francis. tr. 8-PA53. ISBN 978-1-4398-7606-0. 
  5. ^ Dell, Ronald M.; Moseley, Patrick T.; Rand, David A.J. (2014). “Development of Road Vehicles with Internal-Combustion Engines”. Towards Sustainable Road Transport. Elsevier. tr. 109–156. ISBN 978-0-12-404616-0. doi:10.1016/b978-0-12-404616-0.00004-9. 
  6. ^ Winterbone, Desmond E.; Turan, Ali (2015). “Reciprocating Internal Combustion Engines”. Advanced Thermodynamics for Engineers. Elsevier. tr. 345–379. ISBN 978-0-444-63373-6. doi:10.1016/b978-0-444-63373-6.00016-2. 
  7. ^ Ikeda, Yuji; Nakajima, Tsuyoshi; Sher, Eran (1998). “Air Pollution from Small Two-Stroke Engines and Technologies to Control It”. Handbook of Air Pollution From Internal Combustion Engines. Elsevier. tr. 441–476. ISBN 978-0-12-639855-7. doi:10.1016/b978-012639855-7/50052-1. 
  8. ^ Siczek, K.J. (2016). Tribological Processes in the Valve Train Systems with Lightweight Valves: New Research and Modelling. Elsevier Science. tr. 3–18. ISBN 978-0-08-100973-4. doi:10.1016/B978-0-08-100956-7.00012-6. 
  9. ^ Yamagata, H. (2005). The Science and Technology of Materials in Automotive Engines. The Science and Technology of Materials in Automotive Engines. Elsevier Science. tr. 152–164. ISBN 978-1-85573-742-6. doi:10.1533/9781845690854.152. 
  10. ^ Stewart, M. (2018). “9–Reciprocating compressors”. Surface Production Operations: Volume IV: Pumps and Compressors. Elsevier Science. tr. 655–778. ISBN 978-0-12-809895-0. doi:10.1016/B978-0-12-809895-0.00009-0.