為了應(yīng)對(duì)新的“真實(shí)世界”駕駛挑戰(zhàn)，汽車廠商必須拿出新的發(fā)動(dòng)機(jī)潤(rùn)滑系統(tǒng)，優(yōu)化車輛發(fā)動(dòng)機(jī)的性能，其中快速預(yù)熱就是大家關(guān)注的重點(diǎn)之一。英國政府的一項(xiàng)近期調(diào)查顯示，如今，英國民眾開車出行的平均行程已經(jīng)縮短至22分鐘，大約在12公里左右，其中絕大部分均集中在市內(nèi)區(qū)域。除了英國，世界其他地區(qū)民眾的用車特點(diǎn)也呈現(xiàn)出類似趨勢(shì)。很顯然，由于行程本身相對(duì)較短，這就要求車輛必須實(shí)現(xiàn)快速預(yù)熱，從而降低排放、提高燃料經(jīng)濟(jì)性、加快車艙制冷制熱速度，并降低維護(hù)保養(yǎng)需求。
專家指出，事實(shí)上，機(jī)油加熱時(shí)長(zhǎng)對(duì)發(fā)動(dòng)機(jī)預(yù)熱的效果有很大影響。
“機(jī)油的真實(shí)熱容其實(shí)遠(yuǎn)遠(yuǎn)高于其他發(fā)動(dòng)機(jī)元件中使用的金屬材料，很多人可能想象不到這一點(diǎn)。”嘉實(shí)多（BP Castrol）首席工程師Oliver Taylor表示，“減少3L機(jī)油（約2.6kg）的節(jié)能效果大約相當(dāng)于車輛減重6.4 kg鋁，或近12 kg的鐵。這個(gè)角度來看，減少潤(rùn)滑油的用量明顯可以加速發(fā)動(dòng)機(jī)的預(yù)熱。”
減少油底殼中的機(jī)油
在汽車?yán)鋯?dòng)階段，發(fā)動(dòng)機(jī)的內(nèi)部摩擦很大，這是產(chǎn)生油耗和排放的主要原因，而且還會(huì)極大地拉低車輛在整個(gè)行程中的平均燃油經(jīng)濟(jì)性。Taylor解釋說，在新的WLTP測(cè)試循環(huán)下，目前最常見的發(fā)動(dòng)機(jī)可能不得不消耗相當(dāng)比例（有時(shí)可能高達(dá)20%）的燃油，僅僅是為了對(duì)發(fā)動(dòng)機(jī)的金屬部分、冷卻劑和機(jī)油進(jìn)行加熱。
為了適應(yīng)如今大部分民眾的用車特點(diǎn)，發(fā)動(dòng)機(jī)必須保證啟動(dòng)階段的機(jī)油量。此外，除了保證潤(rùn)滑系統(tǒng)內(nèi)的機(jī)油（比如油道用量等）足量之外，油底殼內(nèi)也必須保證有充足的潤(rùn)滑油，這樣才能確保發(fā)動(dòng)機(jī)在長(zhǎng)時(shí)間的高速運(yùn)轉(zhuǎn)下可以充分排氣，并達(dá)到延長(zhǎng)機(jī)油換油期的目的。
此外，由于當(dāng)今市場(chǎng)對(duì)發(fā)動(dòng)機(jī)尺寸的要求越來越高，廠商不得不在機(jī)油中增加更多添加劑，從而提高小尺寸發(fā)動(dòng)機(jī)的效率。很顯然，機(jī)油成分越復(fù)雜，機(jī)油預(yù)熱就必須迎接越多挑戰(zhàn)。Taylor表示，添加劑可以“提高機(jī)油粘度、防止機(jī)油無限制稀釋，但同時(shí)保證基礎(chǔ)油不會(huì)過于粘稠。”他告訴《汽車工程》雜志，通過電子手段控制油底殼中的機(jī)油量，可以讓發(fā)動(dòng)機(jī)啟動(dòng)時(shí)無需再加熱如此大量的機(jī)油。
嘉實(shí)多的測(cè)試表明，對(duì)于一款2.0L的高增壓直噴汽油發(fā)動(dòng)機(jī)而言，借助油底殼的機(jī)油噴灑來潤(rùn)滑曲軸系統(tǒng)，發(fā)動(dòng)機(jī)油路中的機(jī)油用量最少可以減少2L，這可以有效降低車輛的寄生阻力或風(fēng)阻，從而在大多數(shù)情況下降低車輛排放，并提高燃料經(jīng)濟(jì)性。
嘉實(shí)多表示，廠商可以選擇將機(jī)油存放在Taylor口中一個(gè)位于發(fā)動(dòng)機(jī)外的“智能容器”中，這有利于控制管理機(jī)油中的關(guān)鍵添加劑。Nexcel就是嘉實(shí)多推出的一種電子控制密封型機(jī)油存放解決方案。Taylor表示，Nexcel可通過一個(gè)對(duì)接系統(tǒng)安裝在車輛的發(fā)動(dòng)機(jī)艙內(nèi)，整個(gè)過程非常簡(jiǎn)單，90秒內(nèi)就能完成快速換油。
管理機(jī)油添加劑
Taylor堅(jiān)稱，由于可以解決車輛起步階段的復(fù)雜熱管理問題，未來這種獨(dú)立的密封型可替換機(jī)油模塊將獲得更大的市場(chǎng)。此外，Taylor和他的同事還展示了一份2016年SAE技術(shù)論文中針對(duì)機(jī)油加熱對(duì)CO2減排效果影響的調(diào)查結(jié)果（http://papers.sae.org/2016-01-0892/）。
Taylor解釋說，Nexcel系統(tǒng)同時(shí)適用于干式或濕式潤(rùn)滑系統(tǒng)，該模塊可保證發(fā)動(dòng)機(jī)的機(jī)油量充足、滿足上油需求，并同時(shí)確保模塊內(nèi)仍有機(jī)油剩余。Taylor表示，“這種作法可以大幅減少流動(dòng)在潤(rùn)滑循環(huán)中的機(jī)油量，從而減輕發(fā)動(dòng)機(jī)加熱機(jī)油的負(fù)擔(dān)。”
據(jù)Taylor介紹，機(jī)油中的添加劑可以維持機(jī)油的潤(rùn)滑效果，從而延長(zhǎng)車輛換油期。目前常見機(jī)油中的添加劑成分大約占總重量的15%。然而，添加劑的高粘度會(huì)讓基礎(chǔ)油更加粘稠，這并不利于降低發(fā)動(dòng)機(jī)摩擦。Taylor解釋說，Nexcel模塊可以幫助設(shè)計(jì)師“控制管理機(jī)油內(nèi)的添加劑成分，允許每一部發(fā)動(dòng)機(jī)都能用上經(jīng)過專門調(diào)配的‘定制’機(jī)油”。
Taylor表示，“這樣一來，我們就可以借助機(jī)油的定制化調(diào)整，更有針對(duì)性地降低發(fā)動(dòng)機(jī)摩擦，這種作法非常有吸引力，而且成本很低。”
到目前為止，嘉實(shí)多公司仍對(duì)相關(guān)技術(shù)細(xì)節(jié)三緘其口，但Taylor表示，公司正在研發(fā)一種“基于全新化學(xué)成分”的新技術(shù)，可在換油間隔之間主動(dòng)動(dòng)態(tài)控制潤(rùn)滑油的質(zhì)量。Taylor表示，“這些技術(shù)可以一起發(fā)揮作用，在整個(gè)機(jī)油使用期內(nèi)保持潤(rùn)滑油成分的穩(wěn)定。”他認(rèn)為，未來的車輛發(fā)動(dòng)機(jī)均將使用經(jīng)過專門優(yōu)化的定制機(jī)油配方，而嘉實(shí)多的努力可以讓這一天盡早到來。
Taylor表示，“由于封閉系統(tǒng)的特點(diǎn)，Nexcel系統(tǒng)中的機(jī)油成分可以保持相對(duì)穩(wěn)定的狀態(tài)，從而保證發(fā)動(dòng)機(jī)可以一直使用理想的機(jī)油，而這可以讓發(fā)動(dòng)機(jī)在軸承負(fù)載、最高工作溫度或可靠性等方面發(fā)揮更大的潛力。”他說，“由于可以在機(jī)油的使用期內(nèi)有效控制機(jī)油的質(zhì)量，廠商其實(shí)可以利用這種有力工具，進(jìn)一步發(fā)揮發(fā)動(dòng)機(jī)的潛力，縮小發(fā)動(dòng)機(jī)的尺寸，并提高發(fā)動(dòng)機(jī)效率。”
該技術(shù)有利于機(jī)油回收利用
由于不同車輛使用的機(jī)油等級(jí)不同，回收的機(jī)油很有可能受到其他不同等級(jí)油品，甚至完全不同成分油品的污染，因此二手機(jī)油的收集和管理非常復(fù)雜，需要進(jìn)行大量工作。嘉實(shí)多公司的可持續(xù)發(fā)展總監(jiān)John Ward-Zinski表示，回收機(jī)油的二次精煉其實(shí)很難實(shí)現(xiàn)，因而大部分的回收機(jī)油會(huì)直接被當(dāng)作燃料使用。然而這種作法有很大弊端，不僅效率低下，而且可能會(huì)給環(huán)境帶來不利影響。
“在二次精煉之前，保證回收機(jī)油不受污染非常重要。”Ward-Zinski解釋說，“英國石油公司的研究表明，42L原油僅能生產(chǎn)大約0.5L潤(rùn)滑油，但相同數(shù)量的回收機(jī)油，則可以提煉大約34L潤(rùn)滑油，不過這有一個(gè)前提，那就是必須避免回收過程中的交叉污染。”
Ward-Zinski表示，如果使用獨(dú)立密封系統(tǒng)就完全沒有這方面的問題，因?yàn)?ldquo;在二次精煉之前，各類機(jī)油都仍分門別類地呆在各自的密封容器內(nèi)。”
Optimizing engines for the new era of 'real-world' driving cycles will require new lubrication strategies—fast warm-up being a particular area of focus. A recent U.K. government survey showed that the average car journey time has fallen to only 22 minutes and average journey length to 12 km (7.5 m), most of it in urban areas. Such extreme duty cycles are increasingly typical in other global regions. They emphasize rapid warm-up for lowering emissions and fuel consumption, faster cabin heating and reduced maintenance.
The contribution to warm-up time due to heating the oil within the engine is often under-estimated, experts note.
“The specific thermal capacity of oil surprises many people because it’s substantially higher than for the metals used for the high-mass engine components," said Oliver Taylor, a chief engineer at BP Castrol. "Saving three liters (approximately 2.6 kg/5.7 lb) of oil is equivalent to shedding 6.4 kg (14 lb) from an aluminum block, or nearly 12 kg (26.5 lb) from an iron block. With that perspective, it becomes very clear how reducing the lubricant volume helps an engine to warm up more quickly.”
Less oil in the sump
High levels of internal friction during cold-start conditions are the dominant reason for increased fuel consumption and emissions during warm-up, which can comprise a significant proportion of the average journey. Taylor explained that in the new World Harmonized Light Vehicle Test Procedure (WLTP), up to 20% of the fuel energy is lost into warming up the metal parts, coolant and oil of a typical current-generation engine.
That engine’s oil volume throughout this period has to be sufficient to cater for the extremes required by a number of factors. In addition to that actually required for steady-state lubrication (i.e., the oil gallery requirement), the sump must contain sufficient lubricant to accommodate operation at a typical inclination of up to 30° from vertical to allow de-aeration when the engine is working at maximum speeds for prolonged periods and to achieve increasingly long oil-change intervals.
Another challenge comes from the increasingly complex lubricant additives required by today’s significantly downsized boosted engines. Additives "push up the viscosity, imposing a limit on viscosity reduction, however thin the base oil,” said Taylor. He told Automotive Engineering that electronic control of sump-oil volume can remove the need to always heat the full capacity required to accommodate the outer limits of these requirements.
BP Castrol testing shows that on a 2.0-L, highly-boosted, direct injection gasoline engine, more than two liters of oil can be removed from the engine lubrication circuit—effectively reducing the parasitic drag, or windage, that results from sump oil splashing on the cranktrain during operation. The reduction can significantly improve emissions and fuel consumption during most journeys.
Containing the oil within what Taylor terms an “intelligent cell” remote from the engine, also permits new approaches to the management of vital additives included within the oil formulation. One such approach is BP Castrol's self-contained, electronically managed sealed-cell system called Nexcel, is installed via a docking system (http://articles.sae.org/14426/) and can be swapped out within 90 s, according to Taylor, who led system development.
Governing the oil-additive content
The concept of a sealed-cell, easily changed engine oil module is increasingly viable within the complex issue of thermal management during warm-up, Taylor argues. He and colleagues presented results of an investigation into the effect of oil warm-up on CO2 emissions in a 2016 SAE Technical Paper (http://papers.sae.org/2016-01-0892/).
Nexcel has the capability to operate in a dry-sump architecture, but it can also be applied to a wet-sump installation, he explained. The system maintains sufficient oil in the engine’s oil pan to ensure adequate coverage of the oil pick-up, but retains the surplus within the cell. This "substantially reduces the thermal capacity of the oil circulating in the engine,” he said.
Contemporary engine oils contain up to 15% additives by weight, enabling them to remain effective for the extended oil-change intervals required by OEMs. But the additives' high viscosity contradicts the use of low-viscosity base stock to reduce engine friction. Taylor explained that the Nexcel unit "can be arranged to govern the additive system, allowing the engine to be supplied with oil containing 'tailored' additive content.
"That makes advanced oils a very attractive, low-cost route to reducing friction,” he said.
So far, BP Castrol is keeping secret details of the technique, but Taylor indicated that it is related to a new technology the company is developing based on "unique new chemistries” to actively control lubricant quality over the oil drain interval. “These techniques together will allow a precise and stable composition of the lubricant throughout the change interval,” claimed Taylor. He believes this is one of the major factors that will enable a significant further step towards highly optimized, vehicle-specific oils.
“Because the closed nature of the Nexcel system ensures the engine always receives the oil specified by engine designers, they can extend the envelope of possibilities in areas like bearing loads and temperatures while retaining robust durability margins," Taylor said. "Add the ability to manage oil quality through the change interval and you have a very powerful new tool for enabling new generations of downsized, highly-efficient engines.”
Oil-recycling benefit
The complexity and care needed to efficiently collect and manage the various oil grades drained from vehicles means that the vast majority of used oil becomes contaminated by other grades, or even by completely different fluids. This makes it impractical to re-refine, leading to a high proportion of recovered lubricants being used as fuel for burners that are “often of poor efficiency and questionable” environmental performance, noted BP Castrol Sustainability Director John Ward-Zinski.
“The benefit of controlling the feedstock for re-refining will be very significant," he explained, noting that just one-half liter of lubricant can be produced from 42 L of crude oil. BP’s research suggests 34 L of lubricant can be extracted from 42 L of recycled oil — "but only if you eliminate cross-contamination during the recycling process," Ward-Zinski said.
The sealed-cell system enables this "because it keeps individual oil types protected within their cells up to the point of re-refining.”
Author: Stuart Birch
Source: SAE Automotive Engineering Magazine
加熱時(shí)間的重要性：圖表顯示了使用歐洲三大發(fā)動(dòng)機(jī)進(jìn)行模擬WLTP（全球統(tǒng)一的輕型車輛測(cè)試程序）周期測(cè)試結(jié)果
“通過電子控制油底殼內(nèi)的機(jī)油量，根據(jù)外部需要的量來進(jìn)行加熱而不是全部?！?Nexcel總工程師 Oliver Taylor表示。
Nexcel潤(rùn)滑油模塊已經(jīng)成為阿斯頓·馬丁的標(biāo)配，同樣的，這一概念也已經(jīng)被運(yùn)動(dòng)到其他乘用車、輕型及重型卡車及設(shè)備之中。
Nexcel機(jī)油模塊被放置在發(fā)動(dòng)機(jī)艙，更換一下僅需90秒。
加熱時(shí)間的重要性：圖表顯示了使用歐洲三大發(fā)動(dòng)機(jī)進(jìn)行模擬WLTP（全球統(tǒng)一的輕型車輛測(cè)試程序）周期測(cè)試結(jié)果