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Static CR vs. Efficiency: Disappointing Otto Gains?

Static CR vs. Efficiency: Disappointing Otto Gains?

Static CR vs. Efficiency: Disappointing Otto Gains?


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)

RE: Static CR vs. Efficiency: Disappointing Otto Gains?

From my OP:
"Indeed, wouldn't looking at things from the standpoint of Carnot be a better first approach towards the overall goal in many instances?"

To flesh this out a bit, what about lowering the cold-sink temperature of any given design's cycle via ATDC water vapor injection? After all, if we can enhance the Carnot spread for the cycle, increased efficiency inevitably follows suit.

As a spinoff, wouldn't we see better reliability due to lower overall internal temps (if condensed H2O is kept at bay)?

Why not push work in this type of direction towards maturity?

Thanks again --

RE: Static CR vs. Efficiency: Disappointing Otto Gains?

thermal efficiency is one thing. higher CR always helps.
combustion efficiency may be another benefit unaccounted for.

part throttle BSFC is so miserable that the best mileage real world vehicles are starting to be the ones with automatics that shift into high far more often than any driver would. The end result can be Better efficiency with the same "old" engine.

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