Engine loading

If i've got a variable pitch prop, is it better for the engine to run at a low RPM with a higher loading or higher RPM with less loading. I'm interested in engine life and fuel consumption.

 

Usually running a engine at it's peak torque RPM gives the best LOADED eff. That same speed should also be the structuraly strongest and longest life speed. Least carbon, etc. But, there are no standards now. So you have to get into the company yourself or have a authorized dealer of that engine get the operating curves of your engine. If you can't get the curves. Buy or borrow a fuel consumption guage and make a set of curves at 1/4-- 1/2-- 3/4--((((((( throttle setting)))))) forget full.

 

What I learned with cars is you should run the engine at as low rpm as you can without lugging. Since you have to generate a fixed amount of power, lowest rpm possible must correspond to peak torque as cyclops said. High rpm is for acceleration, i.e. racing.

 

Generally, you get the best BSFC (power producted per comsumpted fuel) at max torque.

Than mean at FULL throttle, your engine should be so heavily loaded that it cannot run faster than the RPM where it delivers max torque. You get the absolute best efficiency of your engine.

Now, for engine life, it fully depends on how the engine has been built. At max torque, you have the max stress in the engine, with water and oil pump not running at full speed.

 

They're my thoughts. At lower RPM the internal pressures must be higher per revolution. Isn't it the higher the pressure the greater the wear? But, isn't it also true higher RPM = greater wear ??? Any thoughts.

 

With in the design limits of the Mfg running the engine at lowest RPM that will pull the load is best , from an engine wear AND usually fuel consumption .

The ONLY way to optimize your setup and be safe is with an Exhaust Gas Temp gage (about $100 or $200 depending on source) AND the mfg max EGT recomended number.

IF (usually not) you can obtain a "fuel map" from the engine producer you can see where the best operating range is .Shows HP/ fuel burn in pounds per hour

A Fuel Map looks like a series of clouds superimposed over each other.

In the center will be the lowest fuel burn at different loads and speeds , next out is next best fuel burn at other than optimum rpm , but still within engines rating , and so on for 4 or 5 combinations.

The Fuel Map is the most important tool in setting up a real long distance ocean boat , as you can stay in great fuel use area at higher rpm , needed for different sea/wind conditions.

Unfortunatly many MFG are very cagy about giving one out , and prefer the HP and idealized propeller load curve.
With this aproach the engine can only be optimized for a single speed, UGH!

FAST FRED

 

I think by now you are getting the idea that there are a ton of variables to come up with a simple answer that fits all loads and speeds. There is a REAL reason for so many gauges on the Off Shore Racers. Efficiency and breakdown avoidence. I know we really can't give you the answer you wanted. I run the boat the way it gives me pleasure. Otherwise there is no pleasure and I may as well move on to something else.

 

most inboard gas engines in heavier boats seemed to be propped for peak horsepower then you run them at peak torque. As an example a Chrysler 360 C.I. my be propped so that W.O.T. is 4400 RPM then you run it between 3000 and 3300 which should be peak torque. This is not a marine engine but you can see that peak HP is 750 higher than peak torque http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=4514&prmenbr=361 And this link shows peak torque for a 5.7 marine power coming at 3200 RPM http://www.marinepowerusa.com/eng specs/5.7.htm with full throttle coming between 4600 and 5000 rpm. So my thoughts are that one should not go to either extreme but a compromise that has a loaded engine but not to the point that it couldn't reach peak horsepower.

 

A serious word about propping a engine to reach it's maximum rpm easily. With todays gasoline quality all over the place it is most important to let the engine reach max speed with your heaviest load. Complete combustion and REASONABLE carbon buildup are tied together along with oil temperature. DON'T load your engine down. Carbon can build up very fast in marine engines. Undersized engines are common in the standard boat package when you add a near full # of people and you find it can't get on plane. How many of us can power up to 4800rpm while plowing off plane? ------------------------------------------- Give it 3/4 throttle and push a big wake for a 1/2 hour to please your guests and yourself. Later, crawl inside the head chamber and chip off the hard glazed carbon. Some makes glaze very quickly. That is one of the main reasons for engine KNOCK sensors on modern engines.