effect of resistance to tuning

[Mark944turbo]

I am curious about the effect that being in a higher gear, or driving up a hill, has on tuning fuel and timing.

Obviously when you spend more time at high load the detonation threshold is lowered and temps increase, but what is the effect of the piston having more resistance to acceleration? Is there one that can be explained and taken advantage of?

Any discussion on this topic is interesting to me.

 

[tbuelna]

Mark944turbo,

Per your question regarding "the effect of the piston having more resistance to acceleration" and its ultimate impact on power output, it's not significant. While inertia forces on the reciprocating masses, and their resultant mechanical losses, increase at the square of crank speed, it is still not a significant loss at the speed most engines operate at- 6000 rpm or less.

As for the thermodynamic limits for recip piston engines, SI (Otto cycle) engines are detonation limited, and CI (Diesel cycle) engines are "smoke" limited. With modern ECU's, closed-loop feedback and electronic fuel injectors, both types of engines can be run at their most efficient operating point, regardless of speed or load.

An Otto cycle engine always operates most efficiently at WOT (wide-open-throttle) and a stoichiometric A-F ratio, with ignition timing set for MBT conditions. If detonation is encountered, the A-F ratio is richened and the ignition timing is retarded, until detonation stops. And of course, this adversely affects engine efficiency.

Of course, with regards to maximizing fuel economy, every little bit helps. So reciprocating engine components are designed to have as little mass as practical. At least within the cost and durability constraints dictated by commercial mass production.

Hope that answers your question.
Terry