以下是之前的答案轉一些相關的英文媒體內容，供大家參考：VW caught cheating on diesel emissions standards, ordered to recall 500,000 cars-emmissions-standards-ordered-to-recall-500000-cars這個文章分析了VW如此作弊的可能原因：
「One possible partial explanation is that VW introduced these changes to cutdown on maintenance costs. As we previously noted, a number of the cleandiesel vehicles use an additive which must be periodically replaced in order to keep the vehicle』s emissions within range. Critically, the rate ofadditive consumption appears to be roughly equal between European andAmerican vehicles, even though the European cars are held to a significantly less-stringent NOx emission standard. While this is far from a smoking gun, it makes sense that the US version of a clean diesel would require
significantly more fluid to reach a commensurately lower emission target of
0.05g/mile. The Euro-5 standard, in contrast, allows for up to 0.25g/km — a much less difficult target.大致翻譯，原因之一是vw為了降低維護成本。如果要保證符合美國的氮氧化物排放標準（要求0.05g/mile,對比歐-5隻要求0.25/mile），通常使用的方法是加基於尿素的添加劑（比如美國這邊賣的賓士和寶馬的柴油車那樣）。但這添加劑其實不便宜，而且添加頻率也不算低（印象里大概一萬多公里或者一年加一次或者更頻繁），那麼放在VW這些用四缸增壓柴油機的經濟車型上就會導致擁有成本更高並降低產品競爭力，結果就是vw選擇不走這條路。Since not all the recalled vehicles actually use the urea-based fluid,however, maintenance costs can』t be the entire reason. Other researchsuggests that VW programmed the vehicle to switch its EGR (Exhaust Gas
Recirculation) valve between high and low states. When set to high EGR, the
vehicles』 NOx emissions would be significantly reduced, but total vehiclepower would be lower and the car might not respond as quickly. Set to lowEGR, and the car』s performance would increase at the cost of much higheremissions. VW, in other words, was concerned that Americans would see itsdiesel technology as lacking power compared to traditional US gasolinevehicles, and wouldn』t be as interested in buying the cars.」第二個理由就是為了保證性能。上文大致說VW把它的尾氣再循環（也就是把尾氣導入進氣中參與再次燃燒做功）系統設置為高低兩個工況。在高工況時排放會降很多，但代價是發動機油門響應變差和峰值輸出降低。而柴油機本身在功率上就相對汽油機吃虧，同時美國市場是一個對動力要求相對高的市場。為了避免給消費者留下自己柴油車動力不行的印象進而對自己的柴油車失去興趣，vw就搞了這種損招。
Volkswagen AG』s admission that it cheated to make nearly half a million diesel cars appear cleaner-burning than they are leaves the automaker facing billions in fines, its executives risking criminal charges and its U.S. expansion plans in peril.
VW admitted systematically cheating on U.S. air pollution tests for years, the Environmental Protection Agency announced Friday in citing violations that could add up to $18 billion in fines. The company said it has also heard from the Justice Department, which the EPA said could pursue criminal prosecution.
這裡提到VW不僅面臨巨額罰款，還面臨刑事指控。
The German automaker has struggled to gain a foothold in the world』s
second-biggest car market with a strategy built in part on touting the efficiency of fun-to-drive 「clean diesel」 vehicles now shown to be anything but.
「It』s a huge black eye for Volkswagen,」 said Matt DeLorenzo, managing editor for news at Kelley Blue Book in Irvine, California. Consumer Reports magazine reacted by suspending its 「recommended」 rating of two diesel models.
Diesel versions of the popular Beetle, Golf, Jetta and Passat comprise more than a quarter of the brand』s sales in the U.S. and are a vital part of the company』s strategy for meeting tougher U.S. fuel economy standards going into effect in coming years. More than other carmakers, VW has chosen to focus on diesel technology instead of electrics or hybrids.
「They were counting heavily on diesels to meet the fuel-economy numbers,」 DeLorenzo said. 「This brings that whole strategy into question.」
這裡提到，柴油版的甲殼蟲，高爾夫，捷達，帕薩特佔了大眾美國銷售的1/4以上，並且和其他考慮全電或混動方案的廠商不同，大眾選擇走柴油機的道路來提高燃油經濟性，從而滿足政府對車廠平均油耗值的要求（這裡要提到，美國政府有一個CAFE也就是corporate average fuel economy要求，也就是每個車廠在美銷售車輛的加權平均經濟性（以每加侖可以跑多少英里記）要高於某個值。達不到標準的話要對車廠罰錢。典型的例子就是賓士因為實在達不到這個標準，一直在認罰。
Defeat Device
Volkswagen admitted it sold 2009-2015 diesel Volkswagen and Audi cars with software that turns on full pollution controls only when the car is undergoing official emissions testing, the EPA said, calling the algorithm a 「defeat device.」 During normal driving, the cars pollute 10 times to 40 times the legal limits, the agency estimated.
Volkswagen, based in Wolfsburg, Germany, said it』s cooperating with the investigation and unable to comment further.
The EPA, working with the Justice Department, is likely to push for a stiff fine because there are clear violations of the law and harm to the environment, said Margo Oge, former director of the agency』s Office of Transportation and Air Quality. VW』s competitors were spending more money on systems to comply with the law and help the environment.
「My hope is the agency will send a strong message to the rest of the industry,」 Oge said. 「You want to make it clear that this isn』t acceptable.」
EPA一前官員表示希望EPA能藉此對VW以外的車廠業界釋放強硬信號，此種行為決不能接受。
The closest parallel to the Volkswagen case was a group of truck makers who used devices to suppress diesel-pollution controls to improve fuel economy, Oge said. That case, settled 15 years ago, resulted in fines of more than $1 billion, she said.
The potential financial liability is unclear. The EPA could fine the company $37,500 per violation, said Cynthia Giles, the agency』s assistant administrator for enforcement. With 482,000 autos part of the case, the total could be more than $18 billion. EPA allegations that automakers violated environmental rules often are settled for far less than the maximum possible fine.
Hyundai Motor Co. and Kia Motors Corp. agreed last year to pay a $100 million civil penalty to resolve Clean Air Act violations based on their sale of more than 1 million vehicles that emitted more greenhouse gases that what they certified to the EPA.
Lawyers familiar with automotive law say the company could face criminal exposure if prosecutors agree with the EPA』s assertion about the defeat device. The Clean Air Act contains criminal provisions which apply to tampering with monitor devices, as well as making false statements to the EPA.
「What is so damning is that this was something actively pursued. This isn』t an oversight,」 said Bloomberg Intelligence auto-industry analyst Kevin Tynan.
Someone at VW had to decide that cheating the system was going to be a better use of time, money and resources than meeting the regulatory requirements, Tynan said.
「It sounds pretty damning from what EPA said,」 said Carl Tobias, a products-liability law professor at the University of Richmond in Virginia. 「I think they need to be worried about more than just the fines.」
The Justice Department this month said it would step up its efforts to punish individuals for corporate criminal wrongdoing.
「It may be that DOJ will pursue some kind of criminal charge, and that could be very serious,」 Tobias said. The question will then be who in the corporate chain of command knew about the deception and when did they know it, he said.
Including diesel models from the company』s Audi and Porsche brands, Volkswagen AG accounts for 63 percent of diesels sold in the U.S., he said.
The reliability of auto emissions data has also been questioned in Europe. Last year, Mercedes-Benz was accused of overstating fuel economy performance of its vehicles by 40 percent compared with real-world results, according to a study by Brussels-based lobby group Transport Environment.
Volkswagen was sued Friday in a federal court in San Francisco in a consumer class-action case alleging that the defeat device has caused vehicles to lose value.
Consumers wouldn』t have bought the vehicles or paid as much as they did for them if they had known about the defect, and stand to spend more on fuel when the cars are modified to comply with emissions standards, according to the complaint.
Tobias, the products liability law professor, said it will be easier in some states than others to establish a class of consumers who can claim injury from Volkswagen』s actions. Some drivers will have to show they specifically relied on the company』s 「clean diesel」 promises, while elsewhere it will be enough that VW didn』t live up to its representations.
But given the nature of the allegations, just bringing the lawsuit should provide suing customers with 「leverage enough to exact some reasonable settlement,」 Tobias said. 「There』s a lot that』s going to happen in the days ahead.」
While Volkswagen is a global giant with the longstanding goal of surpassing Toyota Motor Corp. as the world』s largest and most profitable automaker by 2018, it continues to struggle in the U.S., even after opening a $1 billion factory in Tennessee. VW brand』s U.S. sales fell 2.8 percent this year through August, while industrywide sales rose 3.8 percent.
Now the company』s targets are at risk, said DeLorenzo of Kelley Blue Book. It』s always been expensive and difficult to tune diesel engines in a way that meets pollution regulations, he said. VW may have found it easier to write software that passed EPA tests than compromise the way its diesels drive, he said.
The EPA and the California Air Resources Board said their investigations are continuing.
「We』re not discussing what the California fines might be at this point,」 said David Clegern, a spokesman for the California Air Resources Board, in an e-mail Friday. 「Our priority is to get these vehicles into compliance and proceed from there. A recall is really the only way to do that.」
Creating Pollution
Volkswagen was deliberately creating more pollution for the people they were trying to sell clean diesels to, said Dan Becker, director of the Safe Climate Campaign, a Washington-based environmental group.
「This is one of the companies that』s been trying to get Americans to buy diesels,」 Becker said. 「They』ve banked their future in a significant way on diesel. They assumed the EPA would never catch them at it, and that was a huge risk.」
The company』s preferred shares fell 3.2 percent to close at 162.40 euros in Frankfurt Friday before the revelations by the EPA.
Consumers haven』t yet been ordered to return to their dealers for a recall, and it』s safe to keep driving the cars, said Janet McCabe, acting assistant administrator of the agency』s Office of Air and Radiation.
Last year Ford Motor Co. was forced to lower mileage estimates and compensate more than 200,000 customers. The Dearborn, Michigan-based company sent out payments ranging from $200 to $1,050. In 2012, an investigation led to Hyundai and Kia relabeling some of their top-selling U.S. models.
這裡首先要說到汽油發動機與柴油發動機的不同。汽油發動機油氣均勻混合入氣缸，然後用火花塞點燃油氣混合物。為了防止氣體在活塞行進至上止點（top dead center）點火之前就直接壓燃了，它的壓縮比就不能很高。壓縮比小，就直接導致了熱效率低。而柴油機的壓縮比可以很大，因為柴油機是先壓縮空氣，然後在上止點附近直接噴油，油噴進去在高溫下直接燃燒。可問題是由於油氣不均勻混合，產生的顆粒物（PM）就比汽油機多很多，壓縮比大也導致燃燒溫度高，也會增加NOx的生成。
為了降低缸內燃燒溫度和氧氣濃度，柴油車往往使用排氣再循環（EGR：Exhaust gas recirculation）的方法，即把一部分廢氣導回氣缸內。廢氣再循環使得氧氣濃度變低，同時廢氣中的二氧化碳有較高的比熱容，可以有效地降低缸內燃燒溫度。同時，廢氣中的顆粒物（PM）會導致EGR管道結垢（EGR cooler fouling），增加了其不穩定性。因為一旦結垢，廢氣便無法回到缸內，整個EGR系統也就失效了。
