一些小型封閉區(qū)域，比如車內(nèi)的CO2積聚，可能給乘客帶來潛在健康威脅。多年以來，絕大多數(shù)車輛空調(diào)系統(tǒng)的外部空氣吊門均采用了缺口設(shè)計(jì)，因此還有一些“新鮮”空氣可以進(jìn)入車內(nèi)。而且即使沒有缺口，由于車身的密封性本身相對較差，而且乘客無法關(guān)閉送風(fēng)開關(guān)（最低只能調(diào)至低速凈化模式），因此車內(nèi)空氣問題并非如此嚴(yán)峻。
然而，為了進(jìn)一步優(yōu)化空調(diào)性能，一些較新款車型已經(jīng)可以關(guān)閉空氣循環(huán)（或開至Max A/C檔位），隔絕外部空氣進(jìn)入車內(nèi)。目前，這種措施甚至還可以作為一種燃料經(jīng)濟(jì)性優(yōu)化手段，在美國EPA的燃料經(jīng)濟(jì)評(píng)分中獲得加分。無論如何，哪怕僅僅為了乘客的舒適度，在一些溫度較高的州內(nèi)，直接隔絕車內(nèi)空氣流通的情況非常常見。
當(dāng)人們在炎熱天氣下的選擇“Max A/C”模式，即空調(diào)開到最大時(shí)，車輛的HVAC空調(diào)將隔絕車輛的內(nèi)外空氣流通。
為滿足舒適度要求，CO2含量不得超過0.1%
不過，這種做法將給車艙內(nèi)的空氣質(zhì)量帶來顯著影響，CO2積聚將影響乘客的呼吸系統(tǒng)健康。正如康奈可（Calsonic Kansei）北美公司高級(jí)測試研發(fā)經(jīng)理G.D.Mathur博士在WCX17 – SAE 2017全球汽車年會(huì)上的發(fā)言中所指出的，為了保證乘客的舒適度，車內(nèi)的CO2濃度不能超過0.1%。按照美國環(huán)保署目前的規(guī)定，車內(nèi)的CO2濃度在短時(shí)間內(nèi)（15分鐘）的積聚濃度不得超過3%，最大不得超過4%。但事實(shí)上，環(huán)保署的此項(xiàng)規(guī)定是專門針對使用R-744空調(diào)系統(tǒng)（CO2為制冷劑）的車型，主要目的也是為了判斷車輛是否存在CO2制冷劑泄露，而非由乘客呼吸引起的CO2積聚。
美國供暖制冷空調(diào)工程師學(xué)會(huì)(American Society Heating, Refrigeration and Air ConditioningEngineers, ASHRAE)表示，環(huán)境中的CO2濃度通常為400 ppm，而為了保證舒適度，車艙內(nèi)的CO2濃度不應(yīng)超過環(huán)境濃度以上700 ppm，也就是說車內(nèi)的CO2濃度最高不得超過1100 ppm (0.11%)。
CO2傳感器可以用于監(jiān)測車內(nèi)的CO2濃度積聚，但汽車行業(yè)更多將這種設(shè)備用作一種定時(shí)措施（比如10到20分鐘循環(huán)一次）。然而，汽車廠商還需要保證最大循環(huán)，從而滿足EPA的要求，拿到相應(yīng)的得分。
Mathur認(rèn)為，盡管行業(yè)已經(jīng)針對該領(lǐng)域進(jìn)行了一些建模和極限測試，但為了充分覆蓋車輛老化帶來的影響，我們還需要更多、更好的數(shù)據(jù)。通常來說，大多數(shù)新車的車門和車窗邊緣均采用了三重密封設(shè)計(jì)，但密封效果將隨著時(shí)間的推移而退化。研究人員可以針對車上人員數(shù)量、車內(nèi)空間大小、空氣泄露情況和出風(fēng)口流量等因素，建立CO2積聚模型。不過，車內(nèi)人員的活動(dòng)狀態(tài)也會(huì)對CO2的積聚帶來很大影響，比如幾位乘客都保持安靜，和父母大聲管教孩子，這兩種情景下的CO2水平肯定不同。
此外，不同乘客的肺活量也存在較大差異，一些圍繞R-744空調(diào)系統(tǒng)的研究顯示，人類呼吸系統(tǒng)產(chǎn)生的CO2在3.8%到5.8% (38,000-58,000 ppm)之間。Mathur已經(jīng)將人員的平均肺活量量化為1.65升/分鐘，他說這個(gè)水平也與自己之前的研究相一致。實(shí)驗(yàn)表明，車內(nèi)的CO2濃度在短短4到5分鐘內(nèi)即會(huì)達(dá)到1,100 ppm，也就是超過舒適范圍。這也就是說，對于行程超過500英里/800公里的長距離行車來講，8小時(shí)的旅程很可能讓車內(nèi)的CO2濃度到達(dá)到危險(xiǎn)水平。
CO2對車禍的影響
Mathur指出，美國亞利桑那州交通運(yùn)輸部已經(jīng)記錄了多起因CO2濃度積聚引起的交通致死事故。這個(gè)判斷是通過遇難者的血液檢測得出的。
Mathur表示，雖然他沒有具體的數(shù)據(jù)，但該研究還需要考慮一氧化碳 （CO）對結(jié)果的可能影響。他觀察到，當(dāng)車艙內(nèi)沒有正壓時(shí)，CO很容易通過車輛的排氣和底部接縫進(jìn)入車艙，具體水平根據(jù)具體車型及相應(yīng)排氣系統(tǒng)有很大差異。不過，一旦CO濃度達(dá)到30 ppm，就有可能導(dǎo)致乘客頭痛。
在Mathur的演講之前，SAE室內(nèi)氣候控制標(biāo)委會(huì)(SAE Interior Climate Control Committee)也在最近一次會(huì)議上討論了相關(guān)問題，并呼吁成立專門工作小組。該工作小組將僅關(guān)注車上人員的呼吸產(chǎn)生的CO2問題，而非R-744系統(tǒng)可能發(fā)生的泄露問題。參與人員將針對汽車內(nèi)部空間大小、乘客數(shù)量、空氣交換率、駕駛循環(huán)及發(fā)動(dòng)機(jī)熄火/閑置等條件下的可接受CO2濃度達(dá)成共識(shí)。相關(guān)測試將在一個(gè)CO2氣缸中完成，具體設(shè)計(jì)將根據(jù)不同車輛的傳感器和空調(diào)設(shè)置而進(jìn)行調(diào)整。
High carbon dioxide concentration in a small area, such as a passenger car cabin, is a health hazard. For many years the outside air flap door on most HVAC systems was notched, so that in recirculation there was some “fresh” air flowing into the cabin. Even without the notch, the car body was relatively leaky and the blower switch didn’t have an off position, only a low speed to purge stale air.
To improve A/C performance, the recirculation switch (or Max A/C position on HVAC switch) in newer cars permits shutting off outside air. Now there’s even a U.S. EPA fuel economy credit because this approach improves A/C fuel economy. However, just for passenger comfort, in states with high ambient temperatures, shutting off outside air is common.
Comfort level just 0.1%
This has an obvious effect on passenger compartment air quality, and CO2 buildup from human respiration can affect passengers. As Dr. G.D. Mathur, senior manager for test and development at CalsonicKansei North America, pointed out in a 2017 SAE World Congress (WCX17) presentation, just 0.1% concentration is the comfort limit. EPA’s short term exposure limit (15 min) of 3% and a maximum exposure of 4% in the breathing zone was promulgated only for R-744 air conditioning (carbon dioxide used as a refrigerant), to cover a large leak, not a human-caused buildup.
ASHRAE (American Society Heating, Refrigeration and Air Conditioning Engineers) says the comfort limit for CO2 concentration is 700 ppm over the ambient level, which is approximately 400 ppm, for a total of 1100 ppm (0.11%).
CO2 sensors provide one avenue for automotive control, but more likely is the timed approach used by some car manufacturers (10-20 minutes at a time in recirculation). However, there is a need to maintain maximum recirculation to meet the intent of the EPA credits.
For all the modeling and the limited testing that has been done in this area, Mathur noted that better data is needed to cover vehicle ageing. Most new vehicles start life with triple sealing of the doors and glass areas, but seals deteriorate over time. A researcher can model CO2 buildup based on number of passengers against cabin volume, air leakage and blower flow rate. However, there is great variability in exhalation CO2 for passenger activity level (sitting quietly vs. parent screaming at youngster in high activity, for example).
There also is a major difference in human lung capacity, and work on R-744 air conditioning systems has led to studies on that subject, showing a range of 3.8% to 5.8% CO2 (38,000-58,000 ppm) in human respiration. Mathur’s research led him to quantify lung capacity at 1.65 L/min, which he said matched well with previous work he had performed. It indicates a buildup to 1100 ppm –just over the comfort level—within the first 4-5 min of a simulated test drive. With a vehicle range of over 500 mi/800 km, an eight-hour trip can raise CO2 concentration to dangerous levels.
CO2 effect on car crashes
Mathur noted several deaths recorded by the Arizona Dept. of Transportation were blamed on crashes from CO2 buildup affecting the driver. The attributions were validated by blood analysis of the crash victims, indicating the issue is real world.
Although he had no specific data, Mathur said that research also needs to consider possible contributions from carbon monoxide (CO). He observed that in recirculation there is no positive pressure in the cabin, so with exhaust and underbody seams leakage, CO can penetrate. The level would be subject to great variability based on the exhaust system and car. If it reaches a level of 30 ppm, it is likely to cause passenger headaches.
Prior to Mathur’s presentation, the SAE Interior Climate Control Committee had discussed this subject at its last meeting and a call was issued for a working group. The purpose was described as to focus only on occupants respiration, not leakage from an R-744 system. Participants would agree on vehicle interior volume, passenger volume, air exchange rate, drive cycle, also engine off and at idle. Testing would be performed with a CO2 cylinder, and specific settings for vehicle sensors and HVAC operation, including possible preconditioning.
Author: Paul Weissler
Source: SAE Automotive Engineering Magazine
在車上有4個(gè)人的情況下，車艙內(nèi)的CO2水平將在短短1.5小時(shí)內(nèi)增加至0.6% (6000 ppm)的水平。
當(dāng)人們在炎熱天氣下的選擇“Max A/C”模式，即空調(diào)開到最大時(shí)，車輛的HVAC空調(diào)將隔絕車輛的內(nèi)外空氣流通。
車內(nèi)二氧化碳水平及其對乘客的影響。