Load versus no load fuel consumption test.

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sdowney717

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I just refinished rebuilding my starboard engine.
I did a test at 1000 rpm and it used 1/2 gallon in 20 minutes run time.
I repeated this twice, so that is 1.5 gph.
How do you think that will relate to actually running the boat in gear?
I wont do it in the slip, that would create too much pressure on the dock.
Engine is a 392 IH Palmer marinized V8.
I know when I have run the boat on the water before, 1000 rpm is like barely any load on the motor.
 
In gear the fwd motion of the boat will to some extent unload the prop and allow it to turn faster so more fuel will be burned because of increased injections.
 
No relation whatever. Running the boat will require HP. HP require fuel. for diesel tht means about 18 gallons per HP required to move the boat.
 
Bay,
Not even w mechanical engines?
 
I am surprised that the engine required 1.5 gph to turn 1,000 rpm with no load, ie not in gear. I would have expected less than half of that. I don't know a thing about that engine, it must be a "big iron" V8.


And as the previous poster noted, there is no relationship between fuel burned in gear vs out of gear.


Eric- It doesn't matter if it is mechanical or common rail. The injection pump will inject enough fuel to balance the load at the set rpm in either case, mechanical via weights and springs, common rail by electrons.


David
 
Dave,
I thought on a mechanical engine the "throttle" position controled the amount of fuel injected per revolution. Thus in the above example the throttle position not being moved the fuel burn would increase if rpm increased. Not so?

Obviously I don't understand the governor.
Thanks ... I needed to know that.

Edit:
717 I see this is a gas engine. This explains the high burn rate for Dave. So I may be right after all ... just by accident. For awhile I thought this engine was the one FF has talked about for years ..... I assume that's diesel.
 
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manyboats said:
Dave,
I thought on a mechanical engine the "throttle" position controled the amount of fuel injected per revolution. Thus in the above example the throttle position not being moved the fuel burn would increase if rpm increased. Not so?

Obviously I don't understand the governor.
Thanks ... I needed to know that.

Edit:
717 I see this is a gas engine. This explains the high burn rate for Dave. So I may be right after all ... just by accident. For awhile I thought this engine was the one FF has talked about for years ..... I assume that's diesel.
Click to expand...

I like all kinds of engines. I work with what I have.
For the use of this boat is mostly short day trips out on the bay.

A governor will compensate for load. On a gas engine throttle position relative to rpm relative to load is useful to notice, like a car going uphill take more throttle so more fuel used.
Although hard to imagine that much more fuel at 1000 rpm would be needed for my boat.
I was kinda hoping the new rebuild will mean less fuel used at lower rpm where I usually run.
I am almost always from 1200 to 1500 rpm.
 
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OK, I see it is a gas engine. Forget everything I said above. Gas engines are very inefficient at low or no load because the throttle plate is almost closed. That means there is a lot of pressure drop across the plate and not much air is being sucked in, unlike a diesel which always runs un-throttled.


Eric: On a diesel engine the "throttle" controls the rpm set point of the governor. If more load is applied without moving the throttle, then the governor tells the injection pump to squirt more fuel to produce more power to accommodate the increased load.


David
 
Give it some time for things to settle in....it should get better fuel mileage as the engine is broken in.
 
manyboats. Not with any engine. engines use fuel according to how much work they are doing. At any speed an unloaded engine is not doing any work.
 
Some of the inefficiency comes from pumping losses, but most comes from how low the effective compression ratio is when throttled. Efficiency is highly correlated with comp ratio. Throttle the inlet and CR drops way down.

One boat on the dock has a carb'd gas Chrysler 318 with a flowscan on it. When rev'd to 1000rpm in neutral, it is burning 1.1-1.2gph. Owner says his flowscan has proven pretty accurate. So 1.5gph on a bigger engine that is still "tight" from rebuild sounds about right.
 
I really don't think CR changes, regardless of load or RPM. But I have been wrong before.
 
Basic compression ratio numbers are either static and also dynamic.

Static is the plain space of cylinder volumes with piston down at BDC and up at TDC, not adjusted for the camshaft.

Dynamic figures in the camshaft closing degree of the intake valve, later closing means less compression. That could be adjusted real time by variable valve timing.

Then turbo or supercharger boosts also affects amount of air crammed into cylinder which changes the CR ratio dynamically as it runs.

So most specs you see listed are showing the static CR. Not the Dynamic CR which really is more meaningful. On gas engine, your static could be 9.5 to 1, and the DCR 7.5 to 1 simply depending on the intake valve closing degree from after BDC.
 
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Ski in NC said:
Some of the inefficiency comes from pumping losses, but most comes from how low the effective compression ratio is when throttled. Efficiency is highly correlated with comp ratio. Throttle the inlet and CR drops way down.

One boat on the dock has a carb'd gas Chrysler 318 with a flowscan on it. When rev'd to 1000rpm in neutral, it is burning 1.1-1.2gph. Owner says his flowscan has proven pretty accurate. So 1.5gph on a bigger engine that is still "tight" from rebuild sounds about right.
Click to expand...

That is a useful fact, and makes relative sense to me.
I could also lean the mixture, turn in the idle mixture screws and see what that does.
My 1.5 gph sounds about right considering everything.
This engine is additionally bored out .030, which makes it slightly bigger than 392 cu inches.
I did not bore it out, this was done by a prior owner many years ago.
 
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Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.

The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.

Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.

"Above copied from net" I am not sure if there are marine diesels that use variable valve timing, I would assume most are fixed, and I would guess that most have all valves closed at top dead center of piston. If so, CR in regard to definition would be fixed, cylinder pressures another story. interesting debate.
 
At an idle the gas engine is so choked or throttled that it can't get much air so very little compression will result. Hence the high fuel burn on 717's engines at 1000rpm.

Rebel,
The numerical CR can be very accurately computed but is way different that the dynamic things actually happening in the combustion chamber. The throttled gas engine w the throttle mostly closed is a very inefficient air pump. The oxygen in the air is needed to make power from the fuel.
Rebel the cyl pressure you speak of is before ignition or after? BMEP is "Brake Mean Effective Pressure" .. I think. Is that w fire in the hole (I think so) or just pumping pressure?

With a diesel the fuel mixture can be as low as 60-1 at an idle and only at wot will the fuel mixture be about the same as a gas engine ... that is almost a constant 15-1. That's why diesels are fuel efficient at low loads. But they are amazingly similar at wot and rated rpm.

Now here's one for David or ??
Why can't a gas engine be fueled like a diesel and run lean at low loads?
And why does a gas engine run hot when lean and a diesel cool? They say you can fry eggs on a gas engine's exhaust manifold .. no problem .. when idling but no way on a diesel.
 
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You can run compression ignition engine on gasoline.
Engineers are discovering new ways of using gasoline. Diesel pollution is one reason gas is now more in favor in EU politics than diesel.
HCCI engine
Learn All About the Spark Ignition

And of course direct injection gasoline just like diesel can use compression ignition for light loads and lean mixtures, and spark ignition for heavier loads. I think I read that right somewhere.
 
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Eric:


In order to ignite with a spark, gasoline requires a fairly narrow fuel to air ratio. It won't spark ignite if it is too rich or too lean. That is why it has to be throttled to keep the fuel/air ratio in the spark ignite range. The gasoline wouldn't ignite if it were too lean. Remember in the late 60s (well I do) when carbureted low emissions car engines first came out. They were run so lean that they stumbled on acceleration because the mixture wouldn't fire. Fuel injection solved that problem a few years later.


So your next question is why can't gasoline be injected and compression ignited like a diesel. Well, in addition to diesel being cheaper (on a btu/gallon basis) I suspect that gasoline will vaporize in the injector and vapor lock it.


I am not sure that a gasoline engine runs particularly hot when run lean. Like I said, it runs in a fairly narrow fuel to air ratio. A diesel runs cool because it is sucking a lot of air in and little fuel is burned at light loads. EGTs will be in the 400 or so range at light load but get up to 900+ at heavy loads.


David
 
On lean burn, gasoline is injected into combustion chamber called stratified charge.

https://en.wikipedia.org/wiki/Stratified_charge_engine

A stratified charge engine is a type of internal combustion engine, used in automobiles, in which the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.
Click to expand...

I have read maybe, gas engines will equal diesel for efficiency or come close to it.

The green thinking governments, I hate green, but you know things are what they are, are pushing to favor gasoline engine over diesel now in Europe, due to diesel pollution versus cleaner running gas motor.
Same here with EPA, all sorts of schemes to clean the diesel engine up, and outlawing use of older diesels.
 
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All the design changes in the world will not offset the fact that diesel fuel has a higher specific energy content than gasoline.
 
Thanks very much Dave and 717,
I remember the start-up stumble very well and "fixed" a few offenders.

I also remember "sticking" a few two stroke air cooled ultralight aircraft engines from running too lean both while flying and on the ground running up. On WOT climb out one needed to run a tad on the rich side of full power .. about 200rpm. My first flight to 1000' was at max power (lean) and at 1200' it seized up. Depending on the engine about 400 degrees F cyl head temp was maximum but synthetic lube, wide ring gaps or even lower ring removal was used to increase rpm and intolerance to heat.
 
bayview said:
All the design changes in the world will not offset the fact that diesel fuel has a higher specific energy content than gasoline.
Click to expand...

But friction, heavier pistons and other features of diesels limit their ability to turn the energy of their fuel into shaft rotating power.
 
1.5 gph for one 392 cu in gas v8 is normal in my experience with gas flowscans.
 
717, is that 392 the first gen Chrysler Hemi design. If so, thats about the best looking engine ever. Even in an old Desoto.
 
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