Applying Power and Torque
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More Articles From Online Book: Engines For Cruising Boats:
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- Understanding Power and Torque
- Applying Power and Torque
- Propeller Efficiency
- How To Stop Killing Your Engine With Kindness
- How To Select The Best Power and Propeller Settings For Your Engine
- Controllable Pitch Propellers (CPPs)
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- New Engine For “Morgan’s Cloud”—What We Chose
- Engine Installation—The Devil Is In The Details
- Perkins M92B, Initial Report Card
- New Engine, The Proof is in The Voyage
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Hi Eric
two excellent articles, clearly explained and to the point. Lots of would be boat buyers think that adding a turbo is the answer to everything….
Best wishes
Colin
Hi Colin,
Interesting, I was not aware that a lot of people think that adding a turbo is that important. I wonder if it is just the fascination with fancy stuff or the fact that all diesel cars have turbos or something else? To me, unless I was outfitting a very light boat and weight conscious beyond simply not oversizing the engine, I would prefer not to have a turbo on a cruising boat simply for reliability. However, I can’t imagine having a diesel car without one as you would either have very little power or an enormously heavy car, planing powerboats fall into this category too.
You must see plenty of interesting views in your line of work.
Eric
Hi Eric,
I learned a huge amount from these two articles, but one thing I’m still confused about.
You write that maximum fuel efficiently is a:
And in part one you write:
But in my lay person world I think of throttle setting and RPM as being linked so that, unless over propped, max throttle=max RPM. More throttle = more RPM.
Clearly I’m thinking too simplistically. Perhaps what I should be thinking is maximum throttle at a given RPM, or maximum amount of fuel injected that can be burned efficiently (not lugging) at a given RPM?
Could you please expand a bit on the relationship between throttle setting and RPM and how that relates to your two points above.
I’m not Eric but I can chime in:
In a non-turbo diesel engine, there is no throttle. The amount of air in each stroke of the cylinder is always roughly the same regardless of load, and the total rate of air flow through the engine is purely a function of RPM. The lever we call the “throttle” controls the governor that controls the fuel injection metering.
If I advance this lever, more fuel will be injected in each pulse of the injector. Thus, more heat energy is released during each combustion event. Since the cylinder volume and the number of moles of air per cylinder are (approximately) unchanged, then by the ideal gas law P*V=n*R*T a higher temperature must cause a proportionally higher pressure. Higher pressure causes more torque to be applied to the crankshaft. Since the load that’s absorbing this torque (i.e. the propeller) is unchanged, the crankshaft speed must increase.
Increasing the crankshaft speed increases the propeller speed, which increases the torque applied by the propeller on the water and vice-versa. At some new, higher speed, the higher torque of the propeller will balance the higher torque that the engine produces at that governor setting, and RPM will stop increasing.
Many marine diesels have an RPM-sensitive component to the governor. If I advance the throttle to the “2500” position, the governor will increase the fuel volume per pulse until the engine reaches 2500 RPM. If I then unload the engine (eg. by sucking air into the propeller), the reduction in absorbed torque will cause RPM to increase. The governor will then reduce fuel flow to maintain 2500 RPM.
Engine efficiency depends very strongly on the difference between the hottest temperature achieved during combustion (Th) and the temperature at which cold exhaust gas is discharged (Tc).
The diesel will get its best fuel efficiency when the peak temperature in the cylinder is at the highest temperature it can reach during a very quick high-pressure combustion event without leaving unburned or partially-burned fuel behind (high Th, low Tc).
It will get its worst fuel efficiency when it is spinning fast at low load, i.e. when it is pumping a tremendous mass of cold air and burning relatively little fuel per combustion event to produce relatively little torque (low Th, low Tc)
Its fuel efficiency will also be poor when it reaches its maximum possible torque at any given RPM, i.e. when there is so much fuel in each shot that some of this fuel doesn’t encounter enough oxygen to completely burn before the power stroke has ended and the exhaust stroke begins (high Th, high Tc).
Spark-ignition engines maintain a constant air/fuel ratio, and adjust their torque by changing the number of moles of air (n, in the ideal gas law) used per cycle. This, in turn, has an effect on the combustion temperature. Your control lever throttles the airflow at the intake, and then the fuel system adapts in real time to match the actual airflow. The net result, after combustion, is similar to the diesel: higher combustion pressure yields higher torque.
So yes, “throttle” setting and RPM are linked…. they’re linked by the relationship between the heat energy released in each combustion event, the pressure produced in the cylinder by that heat, the torque exerted on the crankshaft by that pressure, and the torque absorbed by the load (i.e. propeller) at that RPM.
Hi John,
To add a little to what Matt has already said, more throttle means more rpm is usually true. Physics tells us that the sum of all the forces is equal to the mass times the acceleration (F=ma). Since we are talking rotational motion, we substitute torque, inertia and angular acceleration but it is still the exact same concept. When you push harder on the throttle, the net torque is no longer zero which means there is an angular acceleration. The engine will continue to speed up until the net torque is zero again. On a boat, the fact that the propeller curve and engine power curve are so mismatched mean that this is quite stable and the user barely thinks about it as the deceleration is quick and stable.
I wouldn’t put too much into the max efficiency at WOT being around max torque other than it being useful to get a rough understanding of performance. This is not a typical operating point. For a sailboat, you couldn’t get there without a CPP, multi-speed gearbox or wildly too large/too much pitch prop. Even if you could get there, you probably wouldn’t chose to, it isn’t max efficiency for the power you need, it is only max efficiency at max power for that specific rpm and if you simply followed a constant power curve out to a slightly higher rpm, you would get a little better efficiency and be kinder to the engine.
As Matt has mentioned, different types of engines have different types of throttles/governors. In boats, they use constant speed ones which approximate the engine doing its best to hold a speed setting that you create. Contrast this to a car where it is much closer to a torque control but not exactly that. If you have ever seen an incomplete conversion of an automotive diesel into a boat in big waves, you will notice that the rpm is all over the place at constant throttle and that is because it is set up differently. Because of the type of governor on a marine engine, more throttle won’t mean more rpm with an overpitched prop once you hit a certain point but this is the only real exception I can think of to your statement.
One other little thing, the Hymar fuel island plot picture seems to have become uncropped somehow.
Eric
Hi Eric,
Thanks for the heads up on the graphic. Fixed now.
Hi Matt and Eric,
Well, as Rob wrote, that took two cups of tea and three read throughs, but I think I now get it, thanks to both of you.
Hi John,
I realized maybe an easier way to think about this in the sailboat context where we have fixed gearing. The picture with the added curve drawn on shows that the only way to get to torque peak is to be wildly overpropped. Once the prop curve hits the power curve, it can’t go any higher as if it did, there would be a negative angular acceleration due to the torque balance discussed and it would come back to the max power curve. It is worth noting that the prop curve actually passes through an area of higher efficiency before getting to max torque.
So if we take the 2 statements, in the first, we need a load such as the prop curve drawn here that lets you get to the torque peak and this is by definition at wide open throttle. Wide open throttle is not necessarily max fuel for the whole fuel map, just max fuel for that rpm. In the second statement, when we give it more throttle, the engine will already be injecting all the fuel it can for that rpm so nothing will change. The vast majority of time, we are not at wide open throttle so increases in throttle do increase rpm.
Eric
Hi Eric,
Great, that nails it for me. Are you OK if I change the article to read something like:
Instead, we can approximate by knowing that peak efficiency:
I think this will become a reference article going forward and so I want to make it as easy as we can to understand.
Hi John,
How about something like:
Instead, we can approximate by knowing that peak efficiency:
· At no load is at idle rpm.
· At max load, occurs at the torque peak of the engine. This means that the engine is delivering max fuel for the rpm and is at the rpm of the torque peak. To operate at this point, we need an oversized or controllable pitch prop that fully loads the engine at this rpm, normal cruising boats won’t reach full load until a much higher rpm.
I am staying away from lugging here as that is a little design dependent for the engine. It is an important topic and I will need to cover it when talking about whitespace programming for alternator regulators.
Eric
Hi Eric,
That’s great, thanks. I will change it.
Hi Erik,
Thank you so much and again it shows your love for your profession.
However, like John, I too was a little surprised with your comment re:
max torque near WOT.
Many years ago I remembered that max Torque was always at an rpm lower than max rpm and that max fuel economy was near max Torque rpm, but this was not always confirmed in practise, HD trucks.
Also happy to read your comments on turbos.
My 1984 Volvos have none and no computers, just the way I like it !
One more question, please. What is your comment re: the Sharrow propeller?
I find the design fascinating and whoever came up with this idea, brilliant.
Did listen to the comment of a customer who had bought one for his outboard.
Overall he was very happy with its performance, but the fuel efficiency was not what the factory had promised.
Thanks again.
Hi Rene,
Regarding the WOT comment, I think it can be confusing as to whether that means max power or max load. In truth it means max load meaning that the governor is putting in the max amount of fuel it is allowed for that rpm but it can be at any rpm including idle. This means that the load on the engine is equal to the engine’s output or the engine is accelerating through this point quickly and not staying there. On a boat, WOT typically also means max power as that is how we set up props but for engines in general WOT just means max load for any rpm. If you drive a manual transmission, just think of putting it in a high gear going up a hill so that the engine is at a lower rpm but your throttle foot is to the floor.
To the Sharrow propeller, I don’t have any specialized information and I have not gone through all of their marketing materials. Props are generally ~50% thermodynamically efficient which means that it is possible to improve on them. My specialty is machine design meaning mechanisms and structures. I have taken the basic courses in fluid dynamics and have worked with true experts professionally but I am not a fluids expert myself. In general, it looks like a clever way to deal with tip losses when you are diameter constrained which describes fast outboard boats. Similar to winglets, it isn’t clear to me that it is any better than if you could fit a slightly larger diameter prop (this isn’t just a space thing, it is tip speed, slip ratio, etc.). It does look quite expensive to manufacture and I don’t think you can re-pitch so that creates interesting problems. Probably the biggest thing for us sailors is that I don’t see a way to make it folding or feathering so it isn’t applicable for inboard installations. I would really like to see performance data for different rpm, slip ratio, shaft angle, etc. to see how sensitive it is to those, sometimes things work great in an ideal case but can really lose performance if you change any of the variable much at all. I keep dreaming of a multi-speed transmission to let us run slightly larger and more efficient props (you don’t become too sensitive to slip ratio as your speed drops) as well as run the engine at a more efficient operating point.
Eric
Hi Eric,
Thanks for the fill on that, I looked at the Sharrow but as always good to have your balanced view on it.
And I too, thanks to the explanations over the years from you and Matt, would love to see a fully automated multi speed transmission for yachts. It always struck me as a huge pity that the HYMAR project started with the premise of electric drive and then tried to make it work, rather than starting with “how can we make yachts more fuel efficient”.
Thank you John and Erik for all your interesting comments.
Is there a study comparing the efficiency of propeller with that of the tail of a ie dolfin. In Holland there was a barge fitted with a dolfin like propulsion, but cant remember how reversing was done.
As for me, the more I study the more I realise how little I know.
Hi Rene,
I am unaware of a study that compares those efficiencies. There are a few studies on propeller efficiency such as “Comparison of 10 Sailboat Propellers” which was done in conjunction with MIT although the props chosen are dated due to being published in 1994. As a study like this shows, you can’t just compare a single efficiency number, you need to think about what you want out of the prop and the overall drivetrain. As a general rule, props are something like 50% efficient so there is definitely room for improvement there. I hope to explore some of the practical things that we as sailors can do to improve this in future articles without having to invent new technologies.
The trouble is that I don’t know the efficiency of a tail type propulsion, maybe someone has studied this? You would also need to understand the other aspects such as the efficiency of a drivetrain to drive it, the drag under sail, etc.
Eric