Outboard Brake Specific Fuel Consumption Map

Does anyone have BSFC Map for an outboard? I can't find anything online. I know they will all be different, but I assume that are mostly similar. Below is an example of a honda car engine.

 

It would need to be boat specific, no?

 

No. Torque, rpm and fuel are related regardless of what they are pushing.

 

Sorry, I saw speed and didn't thing rpm.

interesting subject

 

There is quite wide variation between engines, the Honda map can be taken as a "generic" example. This info is mostly available for engines used in commercial operation, where the correct loading has a direct impact on economy. You might be able to get some info on specific engines from the guys dealing with tuning and reprogramming of fuel control units, but instead of facts, the original manufacturers focus on hype and beautiful scenery.......

 

I was having a discussion with a coworker about fuel economy and I was arguing that over-proping boats is a common way to improve fuel economy. I didn't have much evidence of it. But drawing fuel propellor curves on that honda chart helps support my case.

The green line is an over-proped boat and the magenta is proper. I wonder if this is why the old crusader 454 I/Os specified max rpm at 4000-4400?

 

Outboard engines have quite different specific fuel consumption maps. Compared to this Honda, for example, they would look extremely poor. A typical automobile these days might have a six speed transmission. The Honda Odyssey has a 10 speed automatic. With the torque converter it might be compared to a 12 speed standard. That allows the engine designers to optimize performance to an amazing degree, in a very very narrow range. The map included shows optimization right around 2000 rpm. Outside that range specific fuel consumption worsens dramatically. Imagine the fuel map for an engine that has a 1 speed transmission with a very sloppy torque converter. It can't have an optimal range. It's just mediocre to poor everywhere, but at least it can run at almost any rpm. Because it has to.

 

So we need to get an outboard specific fuel consumption map.

 

If at all possible.

 

Just to quantify poor, and optimized, refer back to the Honda fuel map. It's best specific output is 60Nm at ~2000rpm - 215g/kWh Same work at 4000rpm - 275g/kWh
6000rpm- 500g/kWh

 

The BSFC map shown above is plotted on torque vs engine speed. To compare the effects of different propellers add curves of constant power, and compare the fuel consumption at different points along the constant power curves. If the propellers have the same efficiency at a given boat speed then the shaft power (not shaft torque) to the propeller will be the same. Of course different propellers pushing a boat at the same speed will probably have different efficiencies. To refine the comparison the shaft speed and torque of each propeller would need to be obtained, and then the corresponding points on the BSFC map compared.

If the power is constant then the torque will vary as power / speed
power = torque * speed
torque = power / speed

This claim is based on what information or knowledge? General assumptions about engine design?

Note that a marine engine generally operates along a single characteristic curve. A reasonable assumption would be that marine engines are optimized for efficiency along that curve. But I have not specific knowledge of how marine engines are designed.

BSFC maps show the fuel consumption / power at various combinations of engine speed and torque or equivalently at various combinations of engine speed and power. The torque and power at the engine flywheel. What the engine may be connected to does not affect the map, though it may change the operating speed and torque of the engine.

 

In rotational dynamics power = torque * rpm

Assuming a power curve of HP = C*rpm^2.7 Then… torque = C*rpm^1.7 No?

The shape of the curves are a little different but the idea is still there.

Assuming planning, Crouch formula gives Speed = C/((DISP/HP)*0.5)
Speed α HP^0.5

MPG = Speed/Burn rate
MPG α (HP^0.5)/Burn rate
MPG α ((torque * rpm)^0.5)/Burn rate

Drawing n=1.7 power curves on a BSFC you should have all the information needed to make a best guess.

I think the assumption is that the above engine was optimized for road use and therefore has very good burn at a specific spot (2000 rpm highway cruising speed) Where as a marine engine might be optimized to have a larger area of less good but decent burn.

 

Yes. A marine engine does not have a pronounced sweet spot, because it can't. ZF isn't making 10 speed automatic transmissions because they want to show off. It's because it allows further optimization of the associated engine. I wouldn't characterize it as a claim. It's knowledge.