One for tech guys, and help needed by me Latestarter/VP/Portofino etc

[MapisM]

You also said STW will not change, and I'm afraid that is never correct. STW will fall because prop slip increases - the amount is pretty small of course.

Actually, I didn't say that STW does not change: even in my first contribution to this thread, after saying that "STW remains constant", I also specified that it can decrease, proportionally to the slip increase - if any.
Now you are saying that this effect is "pretty small" while I said "marginal (if not meaningless)".
We are slipping into a semantic rather than a substantial disagreement, I reckon...
Unless you accept what that folk said about a boat gaining 2+ knots at constant rpm after cutting the skier rope, because 2 knots out of 30 or so can neither be called small nor marginal.
The only adjective I can think of for such theory is ridiculous, and I'm afraid I could only agree to disagree with anyone who buys it.

if RPM stays constant then the only factor affecting that force is AOA, aka lift, aka prop slip.

Sorry, but aside from disagreeing on the two "aka", it's the "only factor" which is very misleading imho, and this isn't just a semantic matter.
In fact, I can't think of any reason why a boat cruising at a steady P speed should necessarily change its AoA depending on its load, unless that load is unbalanced.
What happens when a boat is heavier is simply that she sits deeper in the water, which in turn means a larger wet surface and a higher drag, even if the AoA and the prop slip does not change.
Btw, if the additional load would be seriously unbalanced towards the bow, the AoA could even decrease, hence reducing the prop slip, according to your "aka" above. Not that I think such effect would be meaningful, mind.
And as I'm sure you'll agree, the wet surface/drag/fuel burn would all increase, regardless of the AoA, anyway.
That said, I accept that load (even if balanced) can affect significantly a boat behaviour/AoA during the D to P transition, or while cruising at a borderline speed, which could be just enough to keep the boat steadily on the plane when light but not when loaded, but that's yet again another matter...

 

[MapisM]

Prop slip if its not obvious, is simply the difference between the theoretical amount of travel per prop rotation vs actual travel. At slow speeds water acts fluid and is moves around the blades, much energy is simply wasted and ultimately dissipated as heat due to friction in the water. As speed increases the viscous effect of the water starts to act and it begins to act as a solid around the prop that slices through it pushing the boat forward. Typically slippage % are somewhere around 10%. Knowing the prop pitch, engine RPM and gearbox ratios, its possible to calculate prop slip.

100% agreed - 'cept that you also need STW on top of pitch, rpm and ratio.
Mind, I'm sure you are aware of it, and just gave it for granted, but just in case anyone would wonder how to calculate the slip without throwing also the actual speed into the equation... :encouragement:

 

[jfm]

Actually, I didn't say that STW does not change: even in my first contribution to this thread, after saying that "STW remains constant", I also specified that it can decrease, proportionally to the slip increase - if any.
Now you are saying that this effect is "pretty small" while I said "marginal (if not meaningless)".
We are slipping into a semantic rather than a substantial disagreement, I reckon...
Unless you accept what that folk said about a boat gaining 2+ knots at constant rpm after cutting the skier rope, because 2 knots out of 30 or so can neither be called small nor marginal.
The only adjective I can think of for such theory is ridiculous, and I'm afraid I could only agree to disagree with anyone who buys it.


Sorry, but aside from disagreeing on the two "aka", it's the "only factor" which is very misleading imho, and this isn't just a semantic matter.
In fact, I can't think of any reason why a boat cruising at a steady P speed should necessarily change its AoA depending on its load, unless that load is unbalanced.
What happens when a boat is heavier is simply that she sits deeper in the water, which in turn means a larger wet surface and a higher drag, even if the AoA and the prop slip does not change.
Btw, if the additional load would be seriously unbalanced towards the bow, the AoA could even decrease, hence reducing the prop slip, according to your "aka" above. Not that I think such effect would be meaningful, mind.
And as I'm sure you'll agree, the wet surface/drag/fuel burn would all increase, regardless of the AoA, anyway.
That said, I accept that load (even if balanced) can affect significantly a boat behaviour/AoA during the D to P transition, or while cruising at a borderline speed, which could be just enough to keep the boat steadily on the plane when light but not when loaded, but that's yet again another matter...

Ref second half of above, we are at X-purposes. By AoA I meant only the AoA of the prop blades as they move through the water, not of the hull. That AoA of the blades increases if the boat is loaded up, which is the same thing (ie "aka") as increasing prop slip. Lift and force therefore increase, and STW must always reduce. Always. I said AoA "aka prop slip" but I apologise if I wasn't clear on that and led readers to think I meant hull AoA. We're therefore agreeing.

Ref first half of your post yup I agree the 2knts waterski thing is daft - I never agreed it or bought it. Indeed that whole article was pretty poor. Still on the subject of buying things, in your post 14 you clearly and incorrectly said that stw doesn't change and I don't buy that your mention of prop slip in the following para was related- it was another point altogether but you are now clinging to it in an effort not to admit you were mistaken on your basic "stw doesn't change" thing. I couldn't care less, but I'm not buying it!

 

[MapisM]

Mmm... I'm afraid you lost me, on the blades AoA.
The way I see it, it's the other way round: only IF for any given rpm the STW would decrease with load, then the prop AoA/slip would by definition be higher.
But the STW doesn't actually decrease (I mean, not in a meaningful way), and I posted numbers to prove it!

In this sense, you are right, my mentioning of prop slip was incidental in my first post, and only meant to include a completely marginal effect, in the contest of OP point #11.
In fact, the OP said that 3000 revs may give a speed from 18-22 knots (using more or less hp depending on load etc.), to which you replied "exactly correct", while imho that's plain wrong, similarly to the 2+ knots of the skier, as you also agreed.

My personal answer to #11 would have been yes, at 3000rpm you can indeed use more or less hp and burn more or less fuel.
But if you want to measure the STW effect, forget the 18-22 completely (unless you are thinking to use your boat as a tug or an icebreaker!), and be prepared to install a high precision ultrasonic sensor, keeping an eye on its decimal numbers.

 

[Portofino]

//

Sorry this is not right, very quickly as its late, and I need sleep.

Petrol engiens have max egt when under moderate cruise loads whilst still maintaining around 14-14.5:1 AFR (air fuel ratio by weight).

Unless designed as a 'lean burn engine' when under WOT loads fuelling will be somewhere around 12:1-13:1 and EGT will be lower.

An engine (any engine) does not know the load placed upon it. There are just three conditions.
Engine power > Load, RPM rises.
Engine power < Load, RPM falls.
Engine power = Load, RPM stable.

'Overloading' by over propping/wrong gear on a (petrol) engine will mean that its power curve will be incorrectly matched to the application (obviously) and full power output will not be reached. However, an engine with a correctly programmed engine management system will take this without issue. EGT's will not rise to damaging levels, even on a petrol turbo, unless the engine ran lean (they almost always run a little rich, especially carb engines).

The only potential problem that may arise is if due to so called 'overloading' lets say there is not enough air flow through a rad, and the engine is flat out at half rated rpm pulling say a heavy load up a hill, that on a weak system the thermal cooling capacity of the cooling system may be exceeded. But this is a cooling issue, not an engine overloaded one.

As for engine loading/wear due to 'overload', RPM is a much bigger player than 'overloading'. As example, Turbo charge a 2.0 car engine which for example would make 150 HP without a turbo, I can make one of those reliably make 250HP with a turbo, keeping the same power band, where as trying to do the same with NA tuning would mean revving the engine to 8000+ rpm where loads on everything are so much more.

Diesels are different, their EGT goes up as more fuel is applied (up to a point) and yes - enough to cause damage, but any moden electronically controlled/common rail engine should again be programmed so that under all conditions there are none that put the engine into a situation where thermal capacity of any of the parts is exceeded.


Simply not true in an engine with electronic fuel control.

FWIW people say stressed smaller engines produce less MPG then larger ones not stressed. Under the same conditions this would not be the case. As above most (petrol) engines run richer as the load gets higher, for both power and egt reasons, but match the AFR of two engines get a 4.0 and a 2.0 at WOT producing 150hp, and the 2.0 would be by far more efficient.

Another thing I noticed in this thread above, is that someone mentioned engines typically are 40% efficient. Definitely not the case for consumer available engines. I'd be surprised if any of these diesel ones exceeded 25% and the petrol 20%

We are getting in a muddle I think .
Bear with me
Vehicles have no prop slippage -a working clutch / good tyres etc AND a gear box - the tarmac is solid the gogs are solid cement mixer driver s unlikely to attempt to set off in top gear . - gears are fixed gogs thay do not slip -engine turns them .
In a known way with a gear box it's easy to predict and set up the engine management system to fuel the road vehicle .
In the boat the prop is the one fixed gear .That slips at x% , cos water is a fluid ,
In a petrol to make it go faster you via an air valve allow more air in and it In turn ,via the now engine maagement also give it more fuel .Basically air 1st
With a diesel you tell the pump to squirt oil in -something tells the oil pump to do more .it has no air valve restriction to adjust unlike a petrol .

That's the fundamental difference

.If in a petrol car -if you attempt to squirt more fuel in - willy nilly it ( without opening the air valve ) floods it ,bogs drown ( keeping it all simple after Superheat6 ,s 8ollocking-let ignore direct injection modern petrols for now please .

In a boat the Electronic Control Unit -ECU is averagely mapped -guesstimated on the x% ave prop slippage .The engine supplier has set this ,with a rough idea of what weight it can push or should push .
I think this is kinda what the propellor demand curve does ,it's a man made estimate on much power is need to shift it.
The boat builder has chosen the one gear the prop -he should work closely with engine supplier and via sea trails pick the most appropriate prop pitch .
New owner motors off into the sun-set assured that builder /supplier know best -Why should he challenge that ?
He has a certificate from the joint sea trail - showing A graph matching Hp to prop demand curve to rpm -which says at WOT speed is 32 knots -or what ever ?
How ever it's when things start to drift away from the factory calc ,nobodies told the engine and the ECU does not know ,
About the extra wieght , the rough fouled hull , the dirty prop , extra prop slippage .
Owner been told my salesman it goes xyz knots or whatever , or he read it in MBY mag etc .
It has no EGT sensor all it knows is the factory fuelling programme which it religiously follows .
Both engine supplier and boat builder know about getting over the hump in a planning boat so ECU if it was allowed to kinda protected the engine like in a road vehicle - it would back off as soon it sensed the bow wave and the momentary prop slip at or before the " sling shot " when trying to plane and get up .
So it's allowed by design to get the air fuel ratio wrong - only for a few moments - and the EGT risers -otherwise how will the hull ever get on the plane? Once on the plane any excessive EGT rise if any? -drops back or settles where it should be .
But for a few moments the ECU has not held back the pump -so air fuel ratios gone to cock -black smoke
How -ever not all engines /boats have EGT and load sensors -and if they have -and they were interlinked to to the ECU ,as a said how does it know you are trying to get it up on the plane ? -it would cut the fuel pump - we don,t want that , nor does the builder .Remember no 1st gear to set off in

Once planning - at or over 100 % load with the throttle pushed hard down (cos it went this fast before ) -poor boat diesel engine suffers -prolonged raised EGT -shortens life span ,The ECU is not cutting the fuel pump ,flow - it can,t tell .feel the excess weight, drag , prop slip -it's following it pre set fuel mapping -pump - can not differentiate between an attemp at planing or over jealous owner ,so just keeps squirting fuel in .= EGT elevates .
Sure the closed water cooling circuit will eventually heat up -catch up with the raised EGT , But there's a lag and by that the time the water /oil temp guages start to register , damage is done -sometimes it cumulative ,so boat works 'next time -dock talk says thrash em !
Hence " -my mate in the med Allways runs about at full throttle " -so should you !

There also many other variables too for example but not exclusive - the efficiency of the close water system , briefly a v effficent could mask elevated EGT .
That explains why a marine diesel via several owners running at brochure speed a manage to get a few 100 hrs each out it .

Another negative -potential life shortening factor that Mr Cement Mixer owner has not got s the Sea Water Air Cooler (SWAC) -in the boat diesel engine air is cooled to make it more dense . How ever the efficiency of the SWAC can be in some installations (not all ) reduced with clogging of the air side via rebreathing crank case vapour -i think with VP engines ?
And reduced by salt deposits building up on the sea water side .
So the air fuel ratio starts to gradually suffer as the SWAC starts to clogg .Sure the ECU may pick up on that -you ? notice reduced power - but I suspect it's not picked up for the reasons allready said re getting up on the plane .
Now EGT starting to elevate ,but doable with in range just -but then toss into the mix , too high pitched prop , weight , drag etc -now EGT,s are uncomfortable -ECU does not know in a boat
Hence Lionel's tech advising a sensible cruise rpm of 3000 /3800
I do 1755 /2250

That's what in my this thread is all about . Understanding how to minimise the risk of damaging your engines-or put another way avoiding the avoidable built in potential pitfalls of damaging your engines ,

 

[Beyondhelp]

We are getting in a muddle I think .
In a petrol to make it go faster you via an air valve allow more air in and it In turn ,via the now engine maagement also give it more fuel .Basically air 1st
With a diesel you tell the pump to squirt oil in -something tells the oil pump to do more .it has no air valve restriction to adjust unlike a petrol .
That's the fundamental difference.

See where your getting to, and agree to an extent (modern controls blurring the lines slightly as some diesel engines do now have throttles as well) The way I view it is it makes no difference to the required fuel demand for a given engine, whatever its used for. That is; the ECU will compute load from RPM/Manifold pressure/throttle position/airflow, and doesn't need to know nor care if the engine is on a lawnmower or boat.

In a boat the Electronic Control Unit -ECU is averagely mapped -guesstimated on the x% ave prop slippage .

I think this is where I we part! I custom map engines for a living, admittedly petrol engines, but the principles will apply - I hope!!. I don't need to know how the power generated by an engine will be used. All I care to know specifically is how much oxygen is in the engine and what rpm the engine is operating at. From that fuel and ignition can be calculated.

The engine supplier has set this ,with a rough idea of what weight it can push or should push .
I think this is kinda what the propeller demand curve does ,it's a man made estimate on much power is need to shift it.

I wouldn't need to know anything about mapping to specify an engines ability to push a boat. That I consider a separate issue, and would be calculated from the power output graphs of an engine, compared to the boat/prop choice etc.

The boat builder has chosen the one gear the prop -he should work closely with engine supplier and via sea trails pick the most appropriate prop pitch .
New owner motors off into the sun-set assured that builder /supplier know best -Why should he challenge that ?

If the engine reaches rated RPM, the prop choice is correct. The engine may still be under powered and not reaching a desired speed, but the engine is essentially matched to the drive train. If you cannot easily reach rated RPM due to being fearful of your life (like my small power boat) then arguably you probably have too much engine

He has a certificate from the joint sea trail - showing A graph matching Hp to prop demand curve to rpm -which says at WOT speed is 32 knots -or what ever ?
How ever it's when things start to drift away from the factory calc ,nobodies told the engine and the ECU does not know ,

Specifically for the computed fuel injection requirements its not even relevant. The ECU sees a load and fuels accordingly. The engine setup may be non optimal, i.e. dirty heavy hull with damaged prop meaning that the engine fails to reach its ideal full load rpm, and thus its full potential may not be released, but nevertheless, the ecu should still fuel correctly.

It has no EGT sensor all it knows is the factory fuelling programme which it religiously follows .
Both engine supplier and boat builder know about getting over the hump in a planning boat so ECU if it kinda protected the engine like in a road vehicle - it would back off as soon it sensed the bow wave and the momentary prop slip at or before the " sling shot " when trying to plane and get up .

I can absolutely say this does not happen, and there is no backing off on an ecu to get it over the hump so to speak. There is no protection in a road map for say going up a hill in 5th when maximum power transfer may happen in 3rd. There are additional calculations that improve drive ability, such as acceleration enrichment (TPS based) or/& RPM based (rate of change) they ordinarily make up for transient loading which other sensors such as MAP/MAF are not quick enough to catch. There may also be additional settings for turbo spool assistance etc, egr, and the rest.

This image above is the best way I can explain it. The engine power curve is the top black line, the bottom line is the theoretical load the prop places on the engine taking into account a constant viscous fluid (could be air or water). The difference between the two (1) is extra power available. Put simply you might only need 40% throttle to maintain an engine RPM of 2800. Applying more throttle would simply cause the prop to spin faster, and accelerate the boat until the prop power = engine power. This applies all the way up until 100% Throttle which is where the prop power and engine power are the same at WOT.

So then we come onto reality, obviously a planing hull does not produce a constant coefficient of friction with respect to the water, with the least efficient (fuel burn vs velocity) being just as the boat comes onto plane. I have drawn this on a google image as a rough example. Lets say (2) is your clean hull with a well matched engine. It reaches a sensible max RPM around where the engine makes max power. Line (3) however is a heavy dirty hull full of people and kit. In this extreme example, prop power required to move the boat onto plane exceeds that of the engine, and it would achieve it. Fuel consumption etc etc would be horrendous, due to the slow speed the boat would do. If it was a bit lower, it would eventually get onto plane, would feel 'laboured'

In all cases however, you remain within the engines ecu control, and unless the engine is programmed dangerously it would be safe to run in any condition at all under the power curve. There may be ancillary issues, such as turbos, which are often undersized for a given engine displacement to enable low down power and torque to be produced. VNT/VVT turbos have however helped this, so that WOT isn't absolutely thrashing the turbo so much.

So it's allowed by design to get the air fuel ratio wrong - only for a few moments - and the EGT risers -otherwise how will ever get the hull on the plane?

Respectfully don't agree The hull gets onto plain as explained in my graph above using the remaining available power to get it there, already available and waiting on demand

I do agree that external maintenance issues, blocked heat exchangers, etc etc have a big part to play in all this, and EGT sensors might offer assistance, but really for a modern ECU measuring air flow/manifold pressure/ baro pressure / water and inlet temp is just as effective at stopping big problems developing provided they can be understood by people.

 

[jfm]

Thanks for all that explanation Beyondhelp

One further comment, as a bit of an untechnical aside in response to Porto's comments about boatbuilders wanting to publish 32 knots or whatever in their brochure and real-life boats being much more heavily loaded than the brochure boat and therefore over propped. In the sea trials I have done the boats tanks were filled with fuel/water, all holding tanks were filled with water, and the builder placed a series of water bladders on the flybridge and aft deck representing people and general loads. Then they test that max rated rpm is reached at WOT in that condition, and tweak the props as needed. That aint as bad a scenario as Portofino describes, so some if not plenty of builders have a bit of integrity on this point

 

[MapisM]

Thanks for all that explanation Beyondhelp

Positively +1. :encouragement:

Just one question for BH if I may, re...

The ECU sees a load and fuels accordingly.

Could you possibly elaborate on what "sees" actually means, technically?

I half remember an explanation from LS1 on this matter, but I'm curious to hear also the view of a petrol engines expert.

 

[kashurst]

my understanding of it is that the boat "throttles" are not actually controlling the air input or the fuel input. They are telling the engine management what RPM you want. The fuel injection governor (mechanical or electronic) increases the amount of fuel injected until the engine RPM matches the throttle position setting, Like a car cruise control. If the engine load decreases the fuel injection quantity reduces to maintain the set RPM

So the engine perceives a load by varying the amount of fuel injected to run at a desired RPM. I have a fuel flow read out on my engines and as you go up and down big waves you can see the fuel rate varying accordingly but the RPM doesn't change.
Excellent explanation from Beyondhelp by the way.

 

[MapisM]

my understanding of it is that the boat "throttles" are not actually controlling the air input or the fuel input.

Agreed & understood, but only as long as you're talking of diesel.
With engines for girls (as Latestarter would call them - aka petrol ), afaik it's an entirely different kettle of fish - hence my question to BH.

 

[boatingdave]

What a corker of a thread. I definately feel 'too stupid' to comment and I used to be a qualified aeronautical engineer (Ive changed careers many times). . keep them coming folk - enjoying reading this one....

 

[lionelz]

//
That's what in my this thread is all about . Understanding how to minimise the risk of damaging your engines-or put another way avoiding the avoidable built in potential pitfalls of damaging your engines ,

That's exactly what its about! I am a Very simple guy I has a set of skill, but maths, Physics and mechanics are not one of those skills. I am trying to understand the very basics of how and engine works, and the interaction of the engine, propeller, boat design, WHY because I want to get the most out of my money!! I have a very limited budget to spend on my boat so if I can get the most out of my Fuel and engine that is what I want to do. How many times have we been to the salesperson and they tell us a complete load of PISH. How many times have we been stood on the pontoon, and you know what someone is saying just does not make sense but you just don't know how to explain to then why they are talking PISH! I fully understand that with this debate there are many variables, and these engines should be and I am sure are tested under exacting standards! but when you get on the water it becomes a different ball game. I now have a basic understand of what WOT is and what it can do to your engines and why, I understand a bit better about loading of boats I understand a bit more about how important the right prop is, If nothing else it has made me understand that sometimes it pays to get the right person in for the job who actually knows what they are doing. pay to have the boat serviced when it should be have the right oil and lubes fitted, I have also learnt don't be afraid to as questions because there always someone else who wants to ask it but just wont for fear of embarrassment.

A much better informed

Lionel

 

[BartW]

In fact, the OP said that 3000 revs may give a speed from 18-22 knots (using more or less hp depending on load etc.), to which you replied "exactly correct", while imho that's plain wrong, ....

Mapism, when You and Jfm are discussing I need to use all my brain cels to keep on track and even then its hard sometimes, so I might miss something but have a look at this real live example:

when on the plane, I run the engines at 2000RPM most of the time.
with a full tank I get than 19kn, and with a almost empty tank, 2000RPM acheeves 21kn

similarly at displacement speed,
1100RPM gives 9kn with full tank, but approx 10kn with a almost empty tank

isn't that contradicting what you are saying ?
excuses when I am missing something

 

[MapisM]

Yes B, your experience does contradict what I said, because the speed differences you are reporting are definitely meaningful.
Not so much at D speed actually, 'cause 10 kts is already a borderline D speed, also for a 70'+ hull, where the difference between a light or loaded boat can introduce a drag difference big enough to affect the props slip significantly, also because the lower the speed, the "softer" the water, and the easier it is for the props to slip much more (just think of when you engage the gear from zero speed: prop slip is practically 100% in that instant, before the boat starts moving).
For sake of comparison, also in my old tub there's a big difference between light and full load (5.5T out of 32), but I never experienced a meaningful STW change at 1500rpm, which is her usual cruising speed. Otoh, her 1500rpm speed is 8.5kts or so, which is firmly into D mode. And the whole boat setup (prop pitch+diameter, g/box ratio, etc.) is obviously optimised for D speed, as opposed to BA which is designed as a P boat, even if not a fast one.

I'm actually more surprised by the 2kts difference at 20 kts, but I take your word for it of course.
Do you know by chance the pitch of your props and the g/box ratio?
I'd be curious to calculate the slip increase, because by heart that sounds huge!

PS: many thanks for your email on the other matter, I'll follow up asap in detail. :encouragement:
PPS: no need to be worried about your difficulties to keep up with these debates: I also struggle with my own train of thoughts, at times...

 

[kashurst]

re BartW have a look at this:
http://www.mercuryracing.com/prop-slip-calculator/

 

[BartW]

I'm actually more surprised by the 2kts difference at 20 kts, but I take your word for it of course.
Do you know by chance the pitch of your props and the g/box ratio?
I'd be curious to calculate the slip increase, because by heart that sounds huge!

best way to check the data is to come on board and experience by yourselve ;-)

I've found some info about my setup,

according the dutch company that did a overhaul on my props in 2013,
prop diameter is 810mm
and pitch is 970 (he didn't specifie the unit, I assume it is mm , is this possible ?)

the gearbox is a ZF BW165P,
online I found that the gearratio of such a model is 1.5 (I'm not sure if there are other variants of this gearbox)

our cruising speed(s) are:
2000RPM / 20kn
1100RPM / 10kn

I have used Kashurst link to calculate propslip,
but didn't get a sensible outcome, have to make sure that I use the correct units

indeed 10kn, is slightly above displacement speed, still generate quite some waves,
real displacement speed is around 8.5kn (no waves behind the boat)
but for us, 10kn is a pleasant speed in every aspect


re the water skier
iirc you have experience with "real" waterski boats,
these have almost Always petrol engines

you might remember that we have done quite a bit of (family) water skiing,
first with different outboard and inboard petrol boats,
but eventually also with our diesel engined Karnic,
On that boat, we had the symptom as discribed in that article iirc,
when one or more skiërs fall, boat speed increases, (might have been up to 2 kn) and RPM didn't change
we have been towing up to 4 skiërs at a time
so towing with a diesel engine, that probably is the experience the article is refering to ???

 

[Beyondhelp]

Positively +1. :encouragement:

Just one question for BH if I may, re...

Could you possibly elaborate on what "sees" actually means, technically?

I half remember an explanation from LS1 on this matter, but I'm curious to hear also the view of a petrol engines expert.

Sorry been busy, so couldn't reply sooner - and apologies if this is a bit long winded.

Calculating engine load is all about establishing the amount of air entering the engine at any given rpm. There are essentially three ways in which this is done on any EFI diesel or petrol engine:

MAF (Mass Air Flow), this is usually a hot wire sensor, they sit in line with the air flow and output a signal that is directly proportional to the mass/weight (not to be confused with volume) of air entering it. Therefore they compensate for air density and air temperature. These are very common, and work very well. You just need to know engine RPM and crank shaft position in order to calculate required fuel and ignition timings. Downsides in my line of work is in some situations they are restrictive (as they sit in the path of the air entering the engine), they don't like to be on the boosted side of a turbo, and if you have a split pipe that allows air to escape after it, it will incorrectly measure fuel entering the engine and it will run like a complete pig.

MAP (Manifold Absolute Pressure) If you decide not to measure the mass of air to determine load, then one alternative is to use a MAP sensor. This is a pressure sensor that measures manifold inlet pressure however on its own it cannot accurately establish load. You must also determine baro (atmospheric pressure) and an inlet air temp (entering the engine, not pre turbo. Both of these are used to correct for air density. Some ECUs will use the MAP sensor reading as the BARO and correct for air pressure when you power up the ecu, however this may not work so well if you then drive up the side of a mountain where the air density is less - many systems now will therefore have two pressure sensors, BARO and MAP and will constantly update during running. Benefits of this system are its more compact & if a boost pipe leaks, the engine will still run fine as its still possible to correctly measure engine load. There is also less of a delay from throttle opening and the load being measured (MAF sensors can be a long way from the inlet valves in some cases, with turbos/intercoolers etc in the way.

Both of the above are suitable for turbo or N/A (normally aspirated) engines, but there is one final way in which you can calculate load, and its the most basic.

TPS/AlphaN This is strictly for non turbo engines* and works on the basis quite simply that engine load can be calculated by RPM*Throttle position. This is common on small race track/race cars with after market engine management systems with engines running throttle bodies (one throttle valve per cylinder). In addition to the TPS sensor it will have an IAT (intake air temp) sensor to adjust for air density, and hopefully BARO correction as well (not always - avoid those mountains). This is the most simple system, with the least amount of engine wiring. The great thing is, its very responsive to changes in throttle, so the response is very sharp, but that's about it. This system suffers horrendously when the TPS is out of alignment, or worn out sensors, as it would offset the whole fuel map with regards to airflow, and it doesn't work on a turbo engine as it cannot adjust for boost. (*Although some systems exist which also include a MAP sensor which corrects for TPS - Porsche do this on some of their cars, it can be viewed as 4D mapping).

Finally a bit mixed up, there are hybrids of the above. For example old Mercury outboard EFI 2 strokes use a combination of AlphaN at low RPM then MAP at high rpm, blended in the middle. This is because two stroke engines don't have much in the way of manifold vacuum at low speeds, and I guess they can use the MAP for BARO as well to correct for altitude. - Its a curious system.

So wonderful how does this work once you have load?

Essentially imagine numerical table from around 16x16 rows/columns one axis is RPM the other is calculated load (from the sensors above) this could be a then on the map are fuelling values. There will be another map for Ignition values. In fact there can be many many maps, for variable cam timing, boost control, idle duty and so on.

 

[Beyondhelp]

my understanding of it is that the boat "throttles" are not actually controlling the air input or the fuel input. They are telling the engine management what RPM you want. The fuel injection governor (mechanical or electronic) increases the amount of fuel injected until the engine RPM matches the throttle position setting, Like a car cruise control. If the engine load decreases the fuel injection quantity reduces to maintain the set RPM.

Actually not the case. The ECU simply computes from external sensor data. Its no where near as complicated as try to achieve an RPM vs human throttle demand. As I explained above, in most systems you don't actually need a throttle position sensor to establish load, the main purpose in most cases for one is simply to let the ECU know when the engine should be idling (TPS = closed = turn on idle control), fuel cut on over run (won't happen on a boat engine, we dont roll down hills!!!) or acceleration enrichment (sudden open of TPS, so squirt in extra fuel to prevent engine stutter in the cases of say MAF sensor which takes a few 10ths of second to 'register')

So the engine perceives a load by varying the amount of fuel injected to run at a desired RPM. I have a fuel flow read out on my engines and as you go up and down big waves you can see the fuel rate varying accordingly but the RPM doesn't change.
Excellent explanation from Beyondhelp by the way.

Diesels do blur the line a bit in all this, but modern electronic direct injection diesels will work on a similar basis, and also have air flow meters. However the approach would be different - its quite a complicated one as you are right there is no air valve/throttle, but in all cases the ECU would know the amount of air available and then compute an amount of fuel to inject, whilst maintaining efficient engine operation, and then of course correct for boost temp/baro/etc .

And actually it may well be actually that throttle may target RPMs and its something that can be played with on DBW (drive by wire) petrol engines, where you can have a hybrid accelerator and target an RPM vs TPS. I do this as well for customers with race cars where boost level can be altered by TPS rather than the way they usually work which is the turbo always trying to make the pre-set boost level of the actuator.

 

[MapisM]

Calculating engine load is all about establishing the amount of air entering the engine at any given rpm. There are essentially three ways in which this is done on any EFI diesel or petrol engine......

Many thanks for your very comprehensive summary, interesting stuff indeed.
That sounds all pretty logical now, but whoever was the first who thought of managing engines by mean of ECU, electronically controlled injectors etc. must have been a pretty clever person!

 

[lionelz]

, but whoever was the first who thought of managing engines by mean of ECU, electronically controlled injectors etc. must have been a pretty clever person!

Bloody hell that counts me out then :ambivalence: