如果不是被日益严峻的油耗法规影响,主流欧洲车企可能也不会大规模推广涡轮增压技术。涡轮增压技术无疑能带来更强劲的动力输出,而且在发动机down size的趋势下,小排量涡轮增压发动机的燃油经济性也变得更好。既然如此,为什么有的消费者和汽车厂家仍坚持推出搭载自然吸气发动机的车型呢?
答案就是动力响应性。很多驾驶经验丰富的消费者都能感觉到,涡轮增压车型在踩下油门踏板后,加速感要过一两秒才有,这不仅使得驾驶体验变差,而且在道路上超车时也有一定的危险性。在汽车行业,工程师把动力延迟输出的时间称之为涡轮迟滞。
随着汽车工业的进步,现在主流的合资品牌都将涡轮迟滞或者动力延迟的体验感控制在很小的范围内。而在一些豪华品牌中,很多人甚至感觉不到涡轮迟滞的存在。同样是涡轮增压,为什么不同车型之间的动力响应速度千差万别呢?
要回答这个问题,首先我们要知道涡轮增压技术是如何提高动力输出的。对涡轮发动机而言,发动机的动力在很大程度上取决于增压器的增压值。而当发动机转速越高,排出的废气也越多,自然也有更强的废气推力去带动涡轮增压器旋转做功。在这个过程中,增压器达到最大的增压值需要一定的时间,而影响这一时间的因素则有很多。
首当其冲的影响因素便是增压值。由于增压器建立压力需要时间,因此涡轮设定的增压值越大,动力迟滞的时间也越明显,这就好比一个人要跑的距离越长,所需要的时间也越多。考虑到消费者的驾驶体验,有经验的车企在调校涡轮发动机的时候会将涡轮的增压值控制在合理的范围内。
除了降低涡轮的增压值,我们还可以通过减少涡轮的运转阻力和转动惯量来提高涡轮的响应速度,进一步减少建立压力所需的时间。因为涡轮是由排气驱动的,在排气压力一定的情况下,涡轮的尺寸越小,摩擦阻力越小,就越容易达到很高的转速。
还有一个比较重要的影响因素就是排气部分的设计。短行程的排气歧管设计可以在短时间内将排气压力提升到最大,但其副作用则是会令发动机的缸内温度升高,为降低排气歧管出口的温度,越来越多厂家采用集成式排气歧管技术。
在一些高端车型中,还会采用双涡管技术和变截面排气涡轮技术,前者能减少排气干涉的现象,后者能减小发动机低转速下涡轮排气端的A/R值(入风口截面积与涡轮径向尺寸的比值),最终都可以提高涡轮的响应速度。
虽然自然吸气发动机在市面上的比例越来越少,但这不能说是一件坏事。在主流车企的努力下,涡轮增压发动机的动力迟滞越来越小。而随着48V混动系统的普及,一定程度上也改善小排量涡轮增压发动机的涡轮迟滞现象。随着汽车工业的发展,相信在未来,涡轮增压车型也会拥有接近自然吸气车型的体验。
原创文章,作者:cesifu,如若转载,请注明出处:http://www.cesifu.com/article/15553.html
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