Re: which prop to use for cruising?
Yes the 17 will likely give the best speed but may require a higher throttle setting which could reduce the mileage results.
Actually, the higher throttle setting is what delivers the better fuel mileage. A given speed requires a given amount of power. For a higher pitched prop to deliver the same amount of power at a lower RPM, it requires more torque. Only way to increase the torque of your engine is to open the throttle wider. The wider throttle opening reduces the pumping losses in your engine and delivers better fuel economy.
That's the reason Totota switched from an Otto cycle to an Atkinson cycle engine in the 2010 Prius.
http://www.allaboutprius.com/blog/1014183_the-priuss-not-so-secret-gas-mileage-secrets
The 12- to 14-percent gain in efficiency versus an Otto-cycle engine is mainly due to the reduction in pumping losses, or the amount of energy needed simply to suck air into the cylinders and then expel spent exhaust gases.
This website has a simplified explanation of how pumping losses affect an otto-cycle spark ignition engine if you've ever had a thermodynamics course and understand a P-V diagram.
http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine
When operating at part load the throttle restricts the airflow into the engine, reducing the volumetric efficiency, and as a result the air pressure in the intake manifold falls significantly below atmospheric pressure. In order to draw air from the manifold into the cylinder, the piston is required to do work against the manifold depression and this is termed pumping work (Strictly speaking, the work done by the piston is a result of the pressure differential between that of the manifold and the crankcase).
One of the major reasons a diesel is more efficient than a spark-ignition engine is because the diesel doesn't have a throttle in the air intake reducing it's efficiency. Speed regulation in a diesel is controlled solely by the amount of fuel injected into the cylinder.
From
http://www.hybrid-vehicle.org/hybrid-vehicle-ice.html
Pumping losses are caused by the way power output from a petrol "Otto" engine is regulated. It is regulated by controlling, or rather constricting airflow to the engine. This constriction of airflow creates partial vacuum (low pressure) in the inlet manifold. Maintaining this "low pressure" in the inlet manifold wastes energy.
One reason for diesel "Otto"-engines being more effective than the corresponding petrol engines is because there is no pumping loss in a diesel as power is regulated by injecting less fuel into the cylinders and not by choking the airflow to the cylinders.
http://prius.ecrostech.com/original/Understanding/InternalCombustion.htm
Sources of Inefficiency
To improve on the efficiency of the Otto Cycle engine, it is first necessary to understand where inefficiency arises.
Partial Power
A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m. and 70% to 80% of its peak torque. At higher r.p.m., friction losses in fast-moving engine parts increases. At lower torque, the engine suffers most from what is termed "pumping loss" (discussed below). At the efficiency "sweet spot", the engine produces around 40% of its rated peak power.
I realize that this concept eludes most people and it's a waste of time trying to explain it here, QC and I have gone over it a few hundred times in the last 5 or 6 years. If you're really interested take some thermo courses, or at least google "otto cycle pumping losses" or "otto cycle efficiency". But I was bored and figured I might as well copy and paste some old posts into this thread for you!
