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Nhược điểm ĐC2K

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.

Nguyên lý hoạt động ĐC2K

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.

Ưu điểm động cơ diesel

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.

Tỷ số nén

ĐN

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

Compression ratio (CR)

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.

16.6.1 Effect of Compression Ratio on Engine Combustion

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.

13.2.1 Engine Design Factors

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.

The History of Science in the United States: An Encyclopedia

– 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.

Xupap

Valve Train System with Poppet Valves

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

• 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)

7.1 Functions 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 compressors

Maurice Stewart, in Surface Production Operations, 2019

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.

Valve angles

The degree of angle determines the valve port's flow characteristics. Valve face angles are measured from the horizontal. A 45-degree angle is common and is better at self-centering the valve at closure while wedging the valve tighter to the valve seat. (...) Flatter angles of 30 or 40 degrees can be used on diesel exhaust valve faces to provide for better heat transfer. Some exhaust and intake valve face angles are between 20 and 40 degrees to permit better flow over the edge of the seat. This flatter angle also creates less wear, resulting in fewer overhead adjustments.

Vít

Screw – Britannica

Screw, in machine construction, a usually circular cylindrical member with a continuous helical rib, used either as a fastener or as a force and motion modifier.

Although the Pythagorean philosopher Archytas of Tarentum (5th century BC) is the alleged inventor of the screw, the exact date of its first appearance as a useful mechanical device is obscure. Though invention of the water screw is usually ascribed to Archimedes (3rd century BC), evidence exists of a similar device used for irrigation in Egypt at an earlier date. The screw press, probably invented in Greece in the 1st or 2nd century BC, has been used since the days of the Roman Empire for pressing clothes. In the 1st century AD, wooden screws were used in wine and olive-oil presses, and cutters (taps) for cutting internal threads were in use.

In the Figure, which shows the main types of screws and screwheads in modern use, the cap and machine screws are used to clamp machine parts together, either when one of the parts has a threaded hole or in conjunction with a nut. These screws stretch when tightened, and the tensile load created clamps the parts together. Machine screws have various types of heads, most with screwdriver slots. They are made in smaller sizes than cap screws and bolts.

The setscrew in the Figure fits into a threaded hole in one member; when tightened, the cup-shaped point is pressed into a mating member (usually a shaft) and prevents relative motion. Setscrews are also made with conical and cylindrical points that fit in matching holes and with slotted and square heads.

A stud is a rod threaded on both ends. It is permanently screwed into one member and clamped by means of a nut on the other end.

Tiêu chuẩn quốc gia TCVN

• Quyết định 2915/QĐ-BKHCN công bố tiêu chuẩn quốc gia Bu lông Đai ốc Vít 2015
• TCVN 10864:2015 (ISO 888:2012) – Chi tiết lắp xiết - Bu lông, vít và vít cấy - Chiều dài danh nghĩa và chiều dài cắt ren
• TCVN 10865-1:2015 (ISO 3506-1:2009) – Cơ tính của các chi tiết lắp xiết bằng thép không gỉ chịu ăn mòn - Phần 1: Bu lông, vít và vít cấy
• TCVN 10865-2:2015 (ISO 3506-2:2009) – Cơ tính của các chi tiết lắp xiết bằng thép không gỉ chịu ăn mòn - Phần 2: Đai ốc
• TCVN 10865-3:2015 (ISO 3506-3:2009) – Cơ tính của các chi tiết lắp xiết bằng thép không gỉ chịu ăn mòn - Phần 3: Vít không đầu và các chi tiết lắp xiết tương tự không chịu tác dụng của ứng suất kéo

Vulnerability of Human Health to Climate

C.M. Fang, N. Chhetri, in Climate Vulnerability, 2013

1.06.6 Heat Waves

Heat can be a natural hazard. High temperatures can bring about symptoms of heat stroke, heat exhaustion, heat syncope, and heat cramps (Kovats 2008). Severe heat stroke is defined as core body temperature exceeding 103 °F and dysfunction of multiple organs. The progression to death from heat stroke can be a matter of hours, while survivors may have permanent damage to organ systems. Although the phrase ‘heat wave’ does not have a universal definition, they are considered extreme events that are associated with continuous, hot temperatures that can be fatal to humans (Meehl 2004). Examples include the 1995 Chicago heat wave, the European heat wave in 2003, and more recently the Australian heat wave. In each instance, human health and well-being suffered. From past heat wave events, health risks have become evident. In the case of Chicago, more than 500 heat-related deaths were reported. As all heat waves are considered preventable, the lack of preparedness was evident. In the European heat wave of 2003, more than 30 000 excess deaths were reported, with approximately 2000 deaths occurring in Paris. Because the heat wave occurred during the holiday season, the heat stress was exacerbated by the lack of hospital staff.

The heat wave that Russia experienced during the summer of 2010 is an example of potential impacts of climate on human health. The heat wave killed approximately 11 000 people in the city of Moscow, helped spread wildfires, and took out more than one-third of the country’s grain harvest, which drove up food prices globally. Climatologists in Russia noted that no similar event had been recorded in at least a thousand years; however, more recent studies disagree that it was so unprecedented (Dole et al. 2011). The extreme temperatures were due to a major high-pressure ridge that stopped the normal movement of cooling storms that come from the west, which allowed warm air to flow north from the tropics. These anomalies are common and result from natural instances; however, the intensity of the occurrence over Russia was unusually strong. Because of the complexities behind the science of blocking events, more research is needed to determine the role of climate variability and change in these events.

In 2009, many regions in Australia reached record temperature highs. The heat wave exacerbated fire conditions during the 2008–09 Australian bushfire season, causing many bushfires in the affected region and leading to the Black Saturday bushfires, which killed 173 people in Victoria (Nitschke et al. 2011). Many people, a majority being elderly, were treated for heat-related illnesses. In addition, many homes and businesses lost power for days. Railway lines were stopped, and many acres were destroyed. The state’s power supply also collapsed as many residents tried to use air conditioning. In a matter of barely 3 days, Hurricane Mitch in 1998 brought some 6 feet of rain on Central America. In the ensuing days, the incidence of malaria, dengue fever, cholera, and leptospirosis soared (Epstein 2005). In Mozambique, three cyclones inundated the country for 6 weeks in 2000, whereby the incidence of malaria rose fivefold. These cases are clear indicators stressing the need for a strong response and recovery unit within a working public health system (Nitschke et al. 2011).

While many heatstrokes result from activity in hot weather (e.g., working outdoors, playing sports), in the case of individuals who are less fit, these heat illnesses can occur even when performing low levels of activity. This is due to the low cardiovascular reserve associated with lack of exercise, which leads to low heat tolerance. There is a clear limit to which the human body can tolerate heat. From studies done in Saudi Arabia, evidence has shown that after a severe heat wave, heat stress proteins were created and reactions of the immune system were significant in causing deaths (Bouchama 2002). Humans do have the capacity to adapt to changing climates and environments. We have seen differences among cultures in physiological, architectural, and behavioral aspects because of differences in climate. People who live in warmer climates have been shown to have higher thresholds to hot weather, in terms of adaptability. Examples of this include those who have moved to hotter countries, including soldiers and athletes. This physiological adaptation takes place over a few days to weeks. The body’s response is to increase the number of sweat glands and improve cardiovascular stability to heat exposure. Behavioral changes also play a part, including clothing, diet, activity, and drinking (McGeehin 2001). Heat waves occur in all countries; however, some countries are more vulnerable than others. The two main reported groups of people at highest risk are young children and the elderly. It is the changes in the thermoregulatory system that makes elderly people more vulnerable to heat (Meehl 2004). Children and babies also have a higher risk of dehydration and lower thermoregulation capabilities. Self-tapping screws form or cut mating threads in such materials as metals, plastics, glass fibre, asbestos, and resin-impregnated plywood when driven or screwed into drilled or cored (cast) holes. The self-tapping screw in the Figure forms threads by displacing material adjacent to a pilot hole so that it flows around the screw. Thread-cutting tapping screws have cutting edges and chip cavities that produce a mating thread by removing material.

Wood screws are made in a wide variety of diameters and lengths; when using the larger sizes, pilot holes are drilled to avoid splitting the wood. Lag screws are large wood screws used to fasten heavy objects to wood. Heads are either square or hexagonal.

Screws that modify force and motion are known as power screws. A screwjack converts torque (turning moment) to thrust. The thrust (usually to lift a heavy object) is created by turning the screw in a stationary nut. By using a long bar to turn the screw, a small force at the end of the bar can create a large thrust force. Workpiece tables on machine tools are moved linearly on guiding ways by screws that rotate in bearings at the ends of the tables and mate with nuts fixed to the machine frame. A similar torque-to-thrust conversion can be obtained by either rotating an axially fixed screw to drive a rotationally fixed nut along the screw or by rotating an axially fixed nut to drive a rotationally fixed screw through the nut.