Fuel consumption a linear function of engine RPM?

[goldenstate]

My boat engine has fuel injectors that squirt diesel fuel into cylinders that are compressed and then combust.

The cylinders are attached to cams that further turn my prop shaft and push the boat.

Do my injectors squirt the same amount of fuel into the cylinders on every rotation, regardless of rpm speed?

If so, then I think that means one hour of use at 4,000 rpm should burn as much diesel as 2 hours of use at 2,000 rpm, (right?)

This matters a lot in terms of range, if I can get 4kts/hr at 2,000rpm but only 5kts at 4,000rpm (decreasing returns to fuel consumption)

Gracias,

Tom

 

[Christian Williams]

Real world experience showed a fuel burn on my 4-cylinder Universal of .5 gallons at 1700 rpms, a throttle settling which maintained 5 knots continuous in a flat calm for about 500 miles. The usual fuel burn for my engine, maintaining 6-6.5 knots at 2100-2300 rpms, is .75 gallons per hour.

I was surprised at the reduction in burn at the low throttle setting, which I chose more to reduce engine noise than to stretch the fuel.

This was a dead calm and flat seas, so I don't know how "real world" it really is.

 

[Frank Langer]

Real world experience showed a fuel burn on my 4-cylinder Universal of .5 gallons at 1700 rpms, a throttle settling which maintained 5 knots continuous in a flat calm for about 500 miles. The usual fuel burn for my engine, maintaining 6-6.5 knots at 2100-2300 rpms, is .75 gallons per hour.

I was surprised at the reduction in burn at the low throttle setting, which I chose more to reduce engine noise than to stretch the fuel.

This was a dead calm and flat seas, so I don't know how "real world" it really is.

My 1984 Ericson 30+ will cruise at 5.5 knots comfortably, burning 1 litre per hour. At 6 knots it uses about 10% more fuel.
Frank

 

[Loren Beach]

As a starting point, there should be a graph in your owners' manual for fuel consumption vs rpm, and the info should be available on line, as well.

 

[1911tex]

Whatever the the Linear function is, these Kubota diesel tractor engines are so incredibly stringent in fuel consumption and durable...I just pick the most comfortable noise level and coolest temps and go with it until out of the marina and the sails go up. As recommended by more knowledgeable sailors, I do operate at 80% for short periods to blow the soot out, change the fluids often and keep a happy engine.

 

[Tin Kicker]

Not a linear function, as the prop becomes less efficient with higher rpms. As Loren wrote, it should be in the manual.

 

[Joliba]

Do my injectors squirt the same amount of fuel into the cylinders on every rotation, regardless of rpm speed?

No. My understanding is that the cause of the engine rotating faster (achieving higher RPM) is that the injector pump squirts MORE fuel into each cylinder each time —> causing a more powerful fuel burn —> pushing the piston faster —> leading to higher RPM. Thus by definition the answer would be NO.
Someone please correct me if I don’t understand this properly.
Mike Jacker

 

[goldenstate]

No. My understanding is that the cause of the engine rotating faster (achieving higher RPM) is that the injector pump squirts MORE fuel into each cylinder each time —> causing a more powerful fuel burn —> pushing the piston faster —> leading to higher RPM. Thus by definition the answer would be NO.
Someone please correct me if I don’t understand this properly.
Mike Jacker

Mike - We may be saying the same thing in different ways, but I'm 99% sure that the number of cylinder explosions and the rotations per minute of the drive shaft are literally directly connected as there are metal parts (cams) in the bottom end of the engine that convert the up-and-down movement of the cylinders into rotational force on the prop shaft.

In order to increase the RPM of one's engine from 1500 to 3000 per minute (for example) there have to be twice as many cylinder explosions. So, the increase in fuel consumption at higher revolutions is because there are more cylinder explosions happening, not because the engine is somehow getting more "oomf" from a particular more-highly-fueled explosion.

Of course, I'm the guy asking the question in the first place, demonstrating a lack of confidence in my own understanding

 

[Joliba]

Mike - We may be saying the same thing in different ways, but I'm 99% sure that the number of cylinder explosions and the rotations per minute of the drive shaft are literally directly connected as there are metal parts (cams) in the bottom end of the engine that convert the up-and-down movement of the cylinders into rotational force on the prop shaft.

In order to increase the RPM of one's engine from 1500 to 3000 per minute (for example) there have to be twice as many cylinder explosions. So, the increase in fuel consumption at higher revolutions is because there are more cylinder explosions happening, not because the engine is somehow getting more "oomf" from a particular more-highly-fueled explosion.

Of course, I'm the guy asking the question in the first place, demonstrating a lack of confidence in my own understanding

I don’t think we are saying the same thing.
I agree with the first part, that after all the cylinders each fire once, the engine makes one complete revolution. By mechanical linkage that cannot change.
However, you need to consider something else. How does advancing your throttle “command” the engine to speed up? The common rail or other metering mechanism permits MORE fuel to be injected PER SQUIRT. This is actually what directly CAUSES the engine to speed up. More fuel combustion in the cylinder is precisely what makes that happen by causing a fuel burn with greater force. Therefore there is always MORE fuel with EACH squirt when the engine is going faster.
It gives you a new understanding of diesels once you realize that there are no electronics, computers, or other gizmos telling the engine to go faster ... only a throttle to permit the injector pump to feed more fuel for each injector squirt.
Mike

 

[Christian Williams]

I have heard it said that a diesel doesn't really have a "throttle," a throttle being used to also control the flow of air (That flap so noticeable to a 14-year-old on a 1957 carburetor). The injectors on our diesels apparently control only fuel rate of flow.

Wish I had taken a diesel course. But then I always have useful forum discussions like this.

 

[Joliba]

Here is the dictionary definition of throttle. I certainly see the word used for all types of engines, even steam locomotives in which neither fuel nor air are regulated by it. I have also seen people use the term for electric boat motors. Go figure!


throt·tle
/ˈTHrädl/
noun
noun: throttle; plural noun: throttles

1. a device controlling the flow of fuel or power to an engine
   
   
   

 

[gabriel]

The throttle (accelerator) on your diesel probably works by controlling the amount of fuel ‘allowed’ into the high pressure diesel pump.

The fuel consumption increases at 4000 rpm’s mainly because of drag of the hull and having to counteract that drag with more throttle/power...the power needed to push an object through a fluid increases as the cube of the velocity—small increases in speed lead to big increases in fuel consumption.

 

[Kenneth K]

I don't know the mechanics of HOW it's done, but the injector pump and throttle linkage somehow meter varying amounts of fuel out of the pump and into the cylinders, even at a fixed RPM. Without that, there would be no way to change the RPM of the engine.

Regardless, there is still no linear response between RPM and prop speed and/or boat speed. Both engines and props have "Power Curves" where they create different (and non-linear) amounts of torque and power at various RPMs.

Here's a good article on the subject: https://www.gerrmarine.com/Articles/EnginePowerCurves.pdf

 

[Kenneth K]

Actually, after some flashbacks to shop class 40-ish years ago: I remember something about a governor-like system--two little opposing weights that spin around as the engine turns. When you set the throttle position, you set a spring tension to counteract these spinning weights. When the spring tension and the force of the spinning weights is equal, the fuel valve stays in a set position. If you advance the throttle, spring tension increases, opening the fuel valve further, the engine accelerates, and the weights begin to spin faster until the spring-to-weight equilibrium is reached again (and the fuel valve stays resets its new position).

Throttle movement sets spring tension (thus equilibrium RPM), it does not directly set fuel flow. At any given setting, if the engine slows down because the load has increased, the weights spin slower and more fuel is signaled into the system to speed the engine back up to equilibrium RPM, all without a change in the throttle.

Or something roughly like that.....

 

[Joliba]

Throttle movement sets spring tension (thus equilibrium RPM), it does not directly set fuel flow. At any given setting, if the engine slows down because the load has increased, the weights spin slower and more fuel is signaled into the system to speed the engine back up to equilibrium RPM, all without a change in the throttle.

Yes, I completely agree. A governor is important to maintain a steady RPM at one throttle setting. Otherwise the engine speed would oscillate with load changes possibly causing over speed, increased wear , or other problems.
The point I was making above was a simplification—simply to point out that fuel flow per injection itself is what regulates engine speed.... even with the boat in neutral gear without any load imposed by the propeller and the boat.

 

[Kenneth K]

....the drive shaft are literally directly connected as there are metal parts (cams) in the bottom end of the engine that convert the up-and-down movement of the cylinders into rotational force on the prop shaft.

Don't mean to sound too proper, but as long as were talking shop class.....

The "bottom end" (power output) part of the engine would generally be thought of as:
piston--> connecting rod--> driveshaft

The cams (camshaft) can actually be located above or below the cylinders but are generally associated with the "upper engine" or cylinder head. Overhead cams (unlike our engines) push down on the valves directly from above. An overhead cam connects to the crankshaft via a timing chain. In our engines, a low-mounted gear-driven camshaft pushes up on pushrods and rocker-arms to open the valves.

 

[goldenstate]

Don't mean to sound too proper, but as long as were talking shop class.....

The "bottom end" (power output) part of the engine would generally be thought of as:
piston--> connecting rod--> driveshaft

The cams (camshaft) can actually be located above or below the cylinders but are generally associated with the "upper engine" or cylinder head. Overhead cams (unlike our engines) push down on the valves directly from above. An overhead cam connects to the crankshaft via a timing chain. In our engines, a low-mounted gear-driven camshaft pushes up on pushrods and rocker-arms to open the valves.

That I was chasing girls in high school chorus instead of auto shop is one of my life's regrets. (Well, I regret it partly.)

Thank you for the clarification. I suppose I mean the 'crankshaft' for the part that turns up-and-down cylinders into twisting force on the drive shaft?

 

[Kenneth K]

It's okay.

Perhaps I was too studious in my younger years (I do regret it). And, chasing girls in my mid-50s has turned out to be be even more expensive, if you can believe it, than owning a boat.

 

[goldenstate]

The question I am really trying to answer is, given a finite amount of fuel, what is the optimal RPM speed at which to operate to get the longest range. (How do I keep my wife happy that I am getting home ASAP if motoring through the North Pacific doldrums?)



The above chart shows fuel consumption in gallons per hour at a given RPM, from a table I found on the internet earlier today, (but of course now that I want to provide a link, I am unable to find again.) The previous owner of my boat said 1/2 gallon per hour, so the numbers seem reasonable. The numbers aren't quite 'linear' (the red line I'm talking about) but they are pretty close.

Then I assume I have a 20 gallon tank.
I also assume my "feelings" for how fast I can go at different RPMs. I do not motor in a lake without current, so there is fuzziness in these numbers.

Solve for maximum number of miles given 20 gallons, at X gallons per hour, traveling Y knots (knot = 1.15 miles) per hour and I calculate the 'tank of diesel range.' I think the lesson is to stay at lower RPM's, though my kts/hr estimates have substantial user input error.

How fast would you run your engine if you were trying to maximize distance given your fuel capacity?

 

[Kenneth K]

Depends on what waters you are sailing in. In the Puget Sound, max miles/gal is achieved by drifting with the tide with the engine off.