環(huán)境壓力和總擁有成本的考量，開始加速影響非公路車所采用的發(fā)動(dòng)機(jī)類型。非公路設(shè)備行業(yè)包括許多小眾市場，比如固定式發(fā)電設(shè)備發(fā)動(dòng)機(jī)（包括熱電聯(lián)產(chǎn)應(yīng)用和天然氣泵），以及用于海運(yùn)、鐵路運(yùn)輸、建筑和采礦設(shè)備的發(fā)動(dòng)機(jī)等。市場在過去大多選擇柴油動(dòng)力，但一些因素導(dǎo)致部分客戶開始轉(zhuǎn)向天然氣燃料發(fā)動(dòng)機(jī)，這就產(chǎn)生了對點(diǎn)火技術(shù)的需求。輝門動(dòng)力系統(tǒng)已經(jīng)開發(fā)出了能夠滿足新一代發(fā)動(dòng)機(jī)特定需求的火花塞技術(shù)。
柴油發(fā)動(dòng)機(jī)面臨的壓力
柴油發(fā)動(dòng)機(jī)在若干方面正面臨著越來越大的壓力。減排要求使得非公路設(shè)備所用的柴油發(fā)動(dòng)機(jī)變得更加昂貴，由于需要滿足氮氧化物和顆粒排放的強(qiáng)制性法規(guī)要求，越來越復(fù)雜的燃燒控制和后處理成為必備技術(shù)。與此同時(shí)，設(shè)備運(yùn)營者和車隊(duì)所有者出于成本壓力，需要進(jìn)一步減少燃料消耗、并延長維修間隔期。
多年來，天然氣一直是眾所周知的環(huán)保型燃料，但在效率方面，氣體燃料發(fā)動(dòng)機(jī)始終無法與柴油發(fā)動(dòng)機(jī)相媲美。如今，系統(tǒng)和組件技術(shù)的進(jìn)步使得米勒或阿特金森循環(huán)及高汽缸壓等先進(jìn)的燃燒策略得以應(yīng)用，使發(fā)動(dòng)機(jī)能夠?qū)崿F(xiàn)與柴油機(jī)接近的熱效率。
雖然柴油發(fā)動(dòng)機(jī)仍占主導(dǎo)地位，但在許多領(lǐng)域已經(jīng)出現(xiàn)了向天然氣和其他替代氣體燃料發(fā)動(dòng)機(jī)轉(zhuǎn)移的趨勢，比如開始采用柴油和天然氣雙燃料發(fā)動(dòng)機(jī)。在商業(yè)經(jīng)濟(jì)性適合的領(lǐng)域，設(shè)備運(yùn)營方已經(jīng)完成了向全火花點(diǎn)火技術(shù)的過渡，比如在天然氣供應(yīng)充足的條件下作業(yè)的船舶系統(tǒng)。未來10年，我們預(yù)計(jì)在北歐地區(qū)、澳大利亞和北美沿海地區(qū)，船舶系統(tǒng)將迎來快速發(fā)展，但海上燃料加注仍然面臨挑戰(zhàn)，這可能會限制洲際運(yùn)輸應(yīng)用的潛力。
除了經(jīng)濟(jì)因素的考量外，基礎(chǔ)設(shè)施條件也會影響燃料的選擇。對于發(fā)電應(yīng)用來說，生產(chǎn)現(xiàn)場通常有非常充足的天然氣供應(yīng)；而與此不同的是，鐵路運(yùn)輸和采礦業(yè)應(yīng)用的基礎(chǔ)設(shè)施發(fā)展并沒有十分成熟。采礦現(xiàn)場并沒有可用燃料，所以必須不斷進(jìn)行燃料運(yùn)輸和補(bǔ)給，而對于鐵路運(yùn)輸應(yīng)用來說，只有一兩節(jié)車廂能夠攜帶運(yùn)行大型火車所需的氣體燃料。
天然氣發(fā)動(dòng)機(jī)的設(shè)計(jì)挑戰(zhàn)
因地質(zhì)條件和燃料來源不同，天然氣或替代氣體燃料種類繁多，在成分上有很大的差異，而燃料構(gòu)成又對燃燒有著重大影響，從而影響火花塞的設(shè)計(jì)。例如沼氣和生物燃?xì)?，雖然主要成分甲烷沒有污染，但即便經(jīng)過脫硫處理，其成分仍含有大量的硫化氫，在燃燒過程中易形成高腐蝕性化合物。這就需要優(yōu)化火花塞設(shè)計(jì)，如從鉑轉(zhuǎn)為銥電極材料，從而實(shí)現(xiàn)更高的耐蝕性。
最新的發(fā)動(dòng)機(jī)設(shè)計(jì)提高了效率、降低了排放和燃料消耗，但這些優(yōu)化本身卻為火花塞設(shè)計(jì)帶來了挑戰(zhàn)。為了減少氮氧化物排放、降低燃料消耗，空燃比要求即將提高至2.0λ；這些超稀混合物的充分燃燒，需要湍流預(yù)燃條件以及較高的汽缸壓力。J-plug設(shè)計(jì)等老式的火花塞技術(shù)不能有效地處理如此高強(qiáng)度的湍流——氣體可能會像吹滅蠟燭一樣熄滅火花。由于點(diǎn)火時(shí)的氣缸壓力和流動(dòng)能上升到了新水平，我們也需要提高點(diǎn)火能量來保證持續(xù)點(diǎn)火，以實(shí)現(xiàn)燃燒。
更為積極的新燃燒策略也推動(dòng)了對多重點(diǎn)火、高能量輸出的高級點(diǎn)火系統(tǒng)的需求。兩至三次的多重點(diǎn)火模式可加快燃燒并提高效率，但同時(shí)也增加了電極腐蝕。相比于單次點(diǎn)火的一次腐蝕，多重點(diǎn)火和更長的火花持續(xù)時(shí)間不僅增加了點(diǎn)火次數(shù)，每次點(diǎn)火的腐蝕度也會更高。我們在實(shí)驗(yàn)室利用臺架試驗(yàn)?zāi)M了不同的點(diǎn)火模式及其特點(diǎn)，研究對電極材料的影響并制定合適的解決方案。
不同的工作循環(huán)對火花塞設(shè)計(jì)也有很大影響。發(fā)電設(shè)備所用的發(fā)動(dòng)機(jī)需要在穩(wěn)定負(fù)載下長時(shí)間運(yùn)轉(zhuǎn)；但在輪渡或拖船等運(yùn)輸應(yīng)用中，發(fā)動(dòng)機(jī)將在滿載和無負(fù)載工況之間交替運(yùn)行，循環(huán)疲勞可能導(dǎo)致另一種失效模式的出現(xiàn)。
天然氣發(fā)動(dòng)機(jī)的獨(dú)特解決方案
高電壓需求、汽缸內(nèi)壓力和溫度升高、進(jìn)氣口湍流現(xiàn)象的出現(xiàn)，以及較長的使用壽命要求，都對火花塞設(shè)計(jì)提出了挑戰(zhàn)、激發(fā)了新技術(shù)的發(fā)展，從而帶來產(chǎn)品設(shè)計(jì)和制造工藝的進(jìn)步，如選擇最佳的零部件幾何形狀、基礎(chǔ)金屬和貴金屬含量的配比等，最終開發(fā)出適用于火花塞尺寸和應(yīng)用的最佳解決方案。
相比于發(fā)電設(shè)備發(fā)動(dòng)機(jī)火花塞（M18螺紋），運(yùn)輸設(shè)備發(fā)動(dòng)機(jī)通常使用較小的火花塞（M14螺紋），因?yàn)檩^小的橫截面能夠更好地應(yīng)對強(qiáng)度、介質(zhì)材料和耐久性方面的挑戰(zhàn)，也推動(dòng)了增強(qiáng)材料和獨(dú)特電極設(shè)計(jì)的發(fā)展。
為了提高點(diǎn)火效果，輝門動(dòng)力系統(tǒng)在電極周圍增加了屏蔽裝置，例如在環(huán)形間隙中加入環(huán)形電極，以降低燃燒點(diǎn)火時(shí)火花熄滅的風(fēng)險(xiǎn)。另一種點(diǎn)燃超稀混合物的方法是使用預(yù)燃室火花塞，點(diǎn)火時(shí)用封閉的蓋子來改善火花隙附近的混合物和流動(dòng)條件。在預(yù)燃室內(nèi)成功啟動(dòng)點(diǎn)火裝置后，火焰會通過噴流進(jìn)入主燃燒室?；鹧鏀y帶著超高能量源，在實(shí)現(xiàn)可靠點(diǎn)火的同時(shí)可確保最高燃燒效率和最低氮氧化物排放。
另一個(gè)解決天然氣相關(guān)挑戰(zhàn)的具體技術(shù)，是輝門動(dòng)力系統(tǒng)專門為14毫米應(yīng)用而設(shè)計(jì)的Pokal火花塞。隨著發(fā)動(dòng)機(jī)越來越接近爆震邊緣，我們開發(fā)了一種獨(dú)特的陶瓷幾何絕緣層，以抵抗在這些條件下運(yùn)行帶來的負(fù)荷。傳統(tǒng)絕緣層的錐形頭限制了火花塞的熱量范圍和熱電性能。Pokal火花塞的頭部在中央電極周圍有一個(gè)杯型腔，可以同時(shí)改善電氣強(qiáng)度和機(jī)械強(qiáng)度，尤其是后者。該設(shè)計(jì)使火花塞能夠更好地承受高電壓，確保在汽缸壓力峰值或異常燃燒條件下的安全操作。
為了在18毫米的預(yù)燃室燃料發(fā)動(dòng)機(jī)中降低電極溫度、抵抗腐蝕，我們開發(fā)了“冷塞”技術(shù)，這一解決方案可將接地電極溫度降低200°C（360°F）以上。在23巴（334 psi）BMEP以上的預(yù)燃室燃?xì)獍l(fā)動(dòng)機(jī)中，“冷塞”可生成超大的火花表面積，使用壽命最高達(dá)到標(biāo)準(zhǔn)銥材料J-gap火花塞的四倍。為了確保電極耐久性，我們成功地突破了制造工藝上的挑戰(zhàn)，通過專門的激光焊接技術(shù)實(shí)現(xiàn)了鎳合金與銥-銠合金的結(jié)合。
以上成果展示了輝門動(dòng)力系統(tǒng)為應(yīng)對行業(yè)挑戰(zhàn)所作的努力。作為全球OEM基礎(chǔ)發(fā)動(dòng)機(jī)組件技術(shù)的主要供應(yīng)商，我們了解客戶面臨的商業(yè)和監(jiān)管壓力，并堅(jiān)持不懈地致力于開發(fā)創(chuàng)新型技術(shù)，為客戶提供具有成本效益的解決方案，幫助客戶應(yīng)對技術(shù)挑戰(zhàn)。
本文作者是輝門動(dòng)力系統(tǒng)公司的首席技術(shù)官Gian Maria Olivetti，文章屬于《卡車與非公路工程》“高管視角（Executive Viewpoints）”年度系列報(bào)道的一部分。
Environmental pressures and total cost of ownership are beginning to change the types of engines used for off-highway applications, and this trend is likely to accelerate. The off-highway sector includes many niche markets, such as stationary engines for power generation including combined heat-power applications, natural gas pumping, and transit engines for marine, rail, construction and mining applications. Historically, the preferred choice for most of these has been diesel power, but a combination of factors is leading some to switch to natural gas-fueled engines, requiring spark ignition. Federal-Mogul Powertrain has developed a number of spark plug technologies that meet the specific needs of this new generation of engines.
Pressures on diesel engines
Diesel engines face growing pressures on several fronts. Emissions reduction is making diesel engines more expensive in many of the off-highway sectors because increasingly sophisticated combustion control, allied to more complex aftertreatment, is becoming necessary to meet the mandatory limits for NOx and particulate emissions. At the same time, cost pressures on operators and fleet owners mean demand continues for further reductions in fuel consumption and extended intervals between servicing.
Natural gas has been well-known as an environmentally friendly fuel for many years, but gaseous-fueled engines have traditionally been unable to match the efficiency of diesels. Now that advances in system and component technologies enable advanced combustion strategies like Miller or Atkinson cycles and higher cylinder pressures, they can reach a thermal efficiency closer to a diesel.
While diesel engines still remain dominant, in many sectors a migration to natural gas and other alternative gaseous fuels began with the use of dual-fuel engines able to run on diesel and gas. Some operators have already completed the transition to full spark ignition operation where the business economics support the change, such as marine systems operating at a site with a plentiful gas supply. We anticipate rapid growth in marine in the next 10 years in the coastal regions of Scandinavia, Australia and North America, but the challenge of mid-ocean refueling is likely to limit the potential for inter-continental applications.
Beside economic considerations, infrastructure requirements also influence the choice of fuel. For power generation applications, there is often an unlimited natural gas supply on-site; for rail transport and mining applications, the infrastructure is less mature in its development. With mining, there is no fuel available onsite, so it must be transported and refilled constantly, while for rail applications, only one or two railroad cars can carry the gas needed to operate a large train.
The challenges of designing for natural gas
Natural gas or alternative gaseous fuels include a wide range of variations from different geological regions and derived from alternative sources, leading to considerable differences in composition that can have a major influence on combustion and consequently on spark plug design. For example, while the main constituent, methane, is very clean, some of the compositions of digester gas or biogas contain significant hydrogen sulphide levels even after sulphur scrubbing, from which highly corrosive compounds are formed during the combustion process. This necessitates design changes on spark plugs, such as switching from Platinum to Iridium electrode materials for greater corrosion resistance.
The higher efficiency, lower emissions and reduced fuel consumption of the latest engine designs have been achieved through various measures that, in themselves, create challenges for the spark plug. To reduce NOx and fuel consumption, air:fuel ratios as high as Lambda 2.0 are being introduced; satisfactory combustion of these ultra-lean mixtures requires turbulent pre-combustion conditions combined with higher in-cylinder pressures. Older plug technology such as J-plug designs do not work effectively with such high levels of turbulence—akin to blowing out a candle. As cylinder pressures and turbulent kinetic energy rise to new levels at the time of ignition, we also need higher ignition energy to ensure consistent combustion initiation.
New, more aggressive combustion strategies also drive the need for advanced ignition systems featuring multi-strike modes and high energy output. A two- or three-strike ignition mode produces faster combustion and better efficiency, but increases erosion of the electrodes. Whereas a single strike creates a single erosion event, multiple strikes combined with longer spark duration cause several events with higher erosion rates per event. We simulate in the laboratory the different ignition modes and their characteristics using rig tests, in order to investigate their effect on electrode materials and to develop suitable solutions.
Differences in the duty cycles for alternative applications have a major impact on spark plug design as well. An engine used for power generation will operate for prolonged periods at a steady load; a transit application, such as a ferry or tugboat, will alternate between full-load and no-load running much more often. This introduces another potential failure mode through cyclic fatigue.
Unique solutions for natural gas operation
The combined effects of higher voltage demands, increased in-cylinder pressures/temperatures and turbulent charge flow conditions, coupled with longer service life requirements, presents challenges to spark plug design and stimulates the evolution of new technologies. This has enabled developments in both product design and manufacturing processes, including the choice of physical geometry, base metallurgy and precious metal content, with optimum solutions tailored to the size of the plug and to the application.
The engines for transit applications typically use smaller plugs (M14 thread) than the power generation sector (M18 thread), which subjects the plug to greater challenges of strength, dielectric and durability because of the inherently smaller cross sections. This drives the development of enhanced materials and unique electrode designs.
To increase ignitability, Federal-Mogul Powertrain has added inherent shielding around the electrodes, such as an annular gap with a ring electrode to reduce the risk of spark blow-out during combustion initiation. Another means of igniting ultra-lean mixtures is the use of a pre-chamber type plug with an enclosed cap to optimize mixture and flow condition around the spark gap at time of ignition. Successful initiation of ignition in the pre-chamber results in flame torches blowing over via jets into the main combustion chamber. These torches carry an ultra-high energy source, allowing for reliable ignition, which results in highest efficiency and lowest NOx.
Another good example of a specific technology that addresses natural gas-related challenges is Federal-Mogul Powertrain’s Pokal plug, specifically designed for 14-mm applications. As engine manufacturers are pushing combustion closer and more frequently towards the knock threshold, we developed a unique ceramic insulator geometry to resist the loads associated with operating under these conditions. Conventional insulators have a conical nose, the shape of which defines the heat range of the plug and ultimately limits the thermal and electrical properties. The nose of the Pokal spark plug incorporates a cup-shaped cavity around the center electrode, improving both the electrical and especially mechanical strength. This shape allows the plug to better withstand increased voltage demands and allows for safe operation under high peak cylinder pressure or abnormal combustion conditions.
To reduce electrode temperatures and resist erosion in the 18-mm size for fuel-fed pre-chamber engines, our “cold” plug technology is a solution which reduces ground electrode temperatures by more than 200°C (360°F). It produces an exceptionally large spark surface area and provides up to four times the service life of a standard iridium J-gap spark plug in active pre-chamber gaseous-fueled engines operating at over 23 bar (334 psi) BMEP. To ensure the required electrode durability, we applied our specialized laser welding technology, which enables the combination of nickel alloys with iridium-rhodium alloys that would normally present significant manufacturing challenges.
These developments illustrate Federal-Mogul Powertrain’s approach to the industry’s challenges. As a leading supplier of base engine component technology to the world’s OEMs, we understand the commercial and regulatory pressures facing our customers and continue to focus on innovative technologies that provide cost-effective solutions to their challenges.
Gian Maria Olivetti, Chief Technology Officer, Federal-Mogul Powertrain, wrote this article for Truck & Off-Highway Engineering as part of our annual Executive Viewpoints series.
Author: Gian Maria Olivetti
Source: SAE Truck & Off-highway Engineering Magazine
優(yōu)選材料和最大電極面積，使輝門動(dòng)力系統(tǒng)的新型M18冷塞具備了更強(qiáng)的抗火花腐蝕和化學(xué)腐蝕能力。（圖片來源：輝門）
“由于系統(tǒng)和組件技術(shù)的進(jìn)步，米勒或阿特金森循環(huán)及高汽缸壓（氣體燃料發(fā)動(dòng)機(jī)）等先進(jìn)的燃燒策略能夠?qū)崿F(xiàn)與柴油機(jī)類似的熱效率，”輝門動(dòng)力系統(tǒng)的首席技術(shù)官Gian Maria Olivetti說。
輝門的新型Pokal工業(yè)火花塞在絕緣體上設(shè)置了U型芯，從而實(shí)現(xiàn)了前所未有的機(jī)械和電氣性能。（圖片來源：輝門）