Hello folks!
Had a niggle to finally look at the foundational formula concerning the relationship of CR to efficiency for the air-standard Otto Cycle, i.e. E = 1 - (1 / CR ^ (K - 1)), where K = 1.4
To my surprise, we see a relatively small increase for E across a significant span of CR values. As an example, for a large increase in pressures from 8:1 (E = 56.5%) to 13:1 (E = 64.2%) we gather a relatively modest 13.6% increase in efficiency via this route. OK, fair enough.
However, if we look at real-world examples of well-designed engines which have been structurally optimized for their respective compression pressures, a much greater efficiency gain than would be indicated from the clean Otto computation is generally seen. This seems rather counter-intuitive; as the upper ceiling for efficiency gain by CR is given with this formula.
In a nutshell, it would seem as though an increase in CR is only one player in the efficiency game; and that many other factors are at work beyond simple compression pressure. To extend the reasoning here (and neglecting the painfully obvious), a well-developed 8:1 setup might see better overall efficiency levels than a somewhat good, but less-than-masterful design which is running at significantly higher CR numbers.
That being so, why is the knee-jerk response from many corners when the topic of efficiency is raised to push the loads up on a given design via increased CR, as opposed to looking for possibly better payoffs in overall efficiency through revised mixture preparation, thermal management, friction reductions, and other enhancements? Indeed, wouldn't looking at things from the standpoint of Carnot be a better first approach towards the overall goal in many instances?
Comments, thoughts, or examples?
Thanks for the interest ;o)
Had a niggle to finally look at the foundational formula concerning the relationship of CR to efficiency for the air-standard Otto Cycle, i.e. E = 1 - (1 / CR ^ (K - 1)), where K = 1.4
To my surprise, we see a relatively small increase for E across a significant span of CR values. As an example, for a large increase in pressures from 8:1 (E = 56.5%) to 13:1 (E = 64.2%) we gather a relatively modest 13.6% increase in efficiency via this route. OK, fair enough.
However, if we look at real-world examples of well-designed engines which have been structurally optimized for their respective compression pressures, a much greater efficiency gain than would be indicated from the clean Otto computation is generally seen. This seems rather counter-intuitive; as the upper ceiling for efficiency gain by CR is given with this formula.
In a nutshell, it would seem as though an increase in CR is only one player in the efficiency game; and that many other factors are at work beyond simple compression pressure. To extend the reasoning here (and neglecting the painfully obvious), a well-developed 8:1 setup might see better overall efficiency levels than a somewhat good, but less-than-masterful design which is running at significantly higher CR numbers.
That being so, why is the knee-jerk response from many corners when the topic of efficiency is raised to push the loads up on a given design via increased CR, as opposed to looking for possibly better payoffs in overall efficiency through revised mixture preparation, thermal management, friction reductions, and other enhancements? Indeed, wouldn't looking at things from the standpoint of Carnot be a better first approach towards the overall goal in many instances?
Comments, thoughts, or examples?
Thanks for the interest ;o)
