Hybrid Engine Systems and Sustainability

That is a great thread, Jeremy

 

You know, I started thinking about this today...
"why no hybrid boats"

Well, let's look at cars. Cars; Can use regenerative braking, can recharge, or conceivably swap battery packs almost anywhere, can make use of hybrid power systems, and still, they are not accepted by the general population.

Why?

Cause oil is cheap.
People are not even CLOSE to compromising.

 

The real problem is the point I made earlier, the peak to average power ratio.

Hybrid power works for cars, because they have a pretty high peak to average power ratio. This means you can use the electric motor for short periods to augment the power available from a relatively small internal combustion engine, particularly at low internal combustion engine speeds where torque is low, so getting the same sort of performance you'd get from having a bigger engine, but without the penalty of worse part-throttle specific fuel consumption.

Anyone who's driven a Prius, for example, will tell you that it has a great deal more low speed torque than you'd imagine from a small 1800cc engine. The massive amount of extra low speed torque from the electric motors has allowed the internal combustion tuning to be tweaked for better efficiency, because that engine isn't ever required to lug the car away from a standstill or provide low speed acceleration.

Because boats generally don't have a high peak to average power ratio, and because boats have no need for high torque at low speed, the benefit from hybrid power is negligible, less than the efficiency loss.

If you want some rough numbers, then a typical saloon car will have an average power requirement of around 15hp or thereabouts (you can easily work this out from the fuel consumption). It may well have a peak power of around 150hp though, so the peak to average ratio could be around 10:1. The hybrid system effectively allows the peak power of the engine to be reduced, whilst maintaining the same overall feel as a bigger engined car.

A typical boat will have an average power requirement of around 50 to 70% of it's peak power requirement, so there's very little scope for taking advantage of this effect, plus boats don't have any need for high torque at low rpm.

 

exactly jeremy. Also I don't think people (at least where I live) have anything against hybrids in the sense of being acceptable. They just cost a bit much - even with our very expensive gas (eur 1.70/l ~ $8.50/gal) it takes a lot of miles to get back your investment in the more expensive model. My gf drives a Toyota Auris hybrid and its a nice car but the financial sense it makes is so so.

 

Variable pitch rim propellers

We have to keep in mind that one of the main differences between a diesel engine and an electric engine, is the way the propeller works.
For the electric motor, the propeller rpm is directly proportional to the load/kW.

The propeller is the principal efficiency loss in the system: a standard propeller has a peak 20% efficiency at calculated cruise speed, and it drops quickly for other speeds or loads.

Calder/Hymar have considered an Autoprop propeller, which variable pitch could bring 10% efficiency, but it still has a fixed pitch/rpm curve as it is "blind".

Rim propellers with electronically controlled pitch should appear soon, and they should make hybrid engines more efficient than diesel engines in formed seas. Plus they can be retracted the same way bow thrusters are.

 

Not true, I'm afraid. The propeller load is primarily a function of propeller RPM, and secondarily a function of boat speed. The type of motor driving it has no effect whatsoever. In broad terms, the power the propeller absorbs is roughly proportional to the cube of its RPM.

Variable pitch props have been around for a very long time, I have one sitting in my workshop right now that dates from the 1960s, salvaged from a small yacht that was being broken up. Variable pitch only has a modest effect on efficiency, what's ideally needed is variable diameter, which isn't very practical.

If you plot the pitch/RPM/power/efficiency of a variable pitch plot you'll find that you can make a modest gain in efficiency if the boat/ship in question has to operate under different conditions, but the gain isn't really that great. The main benefit comes from being able to increase low speed thrust for a given absorbed power and shaft RPM, which can be a benefit for vessels that need to have a high bollard pull yet still be able to cruise reasonably efficiently to get to where they are needed. Doesn't make the prop that much more efficient overall, though.

Finally, that 20% peak efficiency figure is way, way off, even for a badly matched high speed prop on something like an outboard. Most props manage around 60 to 65%, a well designed one for a point operating condition (like the one on my electric boat) gets better than 80% efficiency.

 

For a diesel motor, the rpm controlled by the governor stays the same and the load varies. When the boat is motoring in or out a wave, or motoring headwind or downwind, the rpm does not change but the load does.

For an electric motor, the kW controlled by the electric controller stays the same and the rpm varies. When the boat is motoring in or out a wave, or motoring headwind or downwind, the load does not change but the rpm does.

You are correct in terms of the definition of propeller efficiency: ratio of thrust HP to brake HP.
So I should have said (propeller/diesel) efficiency. Because the propeller efficiency is valid only at max RPM engine for which the propeller is designed.
If you add the fact that at cruising speed (say 70% of max rpm) with the diesel engine, the propeller will take only 34% (cube of 70%) of the available engine HP, then you arrive to the 20% figure. This is the ratio of effective thrust HP to maximum theoretical thrust HP.

 

Indeed there are plenty of variable pitch props available, mainly hydraulic, few electric. These are not rim propellers, the pitch is manually changed, and the change is very slow.
What I am mentioning is instant change of pitch calculated by the electronic controller depending on the measured rpm and speed, to always be at maximum efficiency.

 

Let's take engine efficiency to start with. Most boats tend to operate at a near constant speed much of the time and tend to cruise at an operating point which may not be that efficient for the engine, especially if it's a diesel.

If a boat needs more thrust to battle into a headwind then the prop speed only needs to increase by a very small amount to significantly increase the thrust delivered (because of that cube law relationship). If the engine in question is a diesel, then it will, in all probability, deliver more efficient performance under this slightly higher load condition, as diesels tend to give best SFC under high load conditions.

If the engine RPM is held constant by a governor then there is no way of increasing power to the prop, the prop will already be absorbing whatever power it has been designed for at that RPM. Opening the throttle more may have an effect on the engine, but if the RPM stays at the governed RPM then the prop conditions stay exactly the same.

If you want to look at overall efficiency then you have to define the conditions. For example, if you use fuel PE to thrust KE as the measure, then you do get a pretty low figure, as a diesel may run at around 35 to 40% PE to KE, the transmission may absorb another 5% and the prop may be around 65%, giving an overall fuel PE to propulsive thrust KE efficiency of around 20%.

Alternatively, if you decided to use engine rated power to effective propulsive power as the efficiency measure you'd get a very different figure, as the only losses would those in the transmission and prop, so overall efficiency might be between 60 and 65%.

Finally, propellers are not usually designed for the maximum engine power, they are normally optimised for the boat cruise conditions. This means that they drop in efficiency at maximum power, and peak at around cruise power, perhaps around 60 to 70% of maximum.

As an example, the prop on my boat was designed to deliver enough thrust to reach cruise speed in still water with best efficiency. Under these conditions it has an efficiency of about 84%. If I need to counter a headwind, so increase the prop RPM by about 10% to deliver more thrust, then the prop efficiency drops a bit to about 78%. Obviously absorbed power increases significantly, but the overall impact on efficiency isn't that great.

 

First off, prop pitch change on the 1960's unit I have is near-instant. Pull the pitch lever and the prop pitch instantly changes, as it's a mechanical linkage. Secondly, pitch doesn't have anywhere near as big an impact on prop efficiency as diameter or chord. Check with any prop modelling software you like and you'll quickly get a good feel for the way the efficiency curve changes as you vary prop parameters.

Varying pitch alone does have some benefits for commercial vessels that need to operate at two widely differing operating points, for example a tug that may need to have a high bollard pull, yet also be able to transit to a new operating area with reasonable efficiency. Even then the effect will be modest, as to get the high bollard pull needed the prop diameter will be relatively large, and coarsening the pitch to improve cruise speed efficiency won't have a big impact.

 

Jeremy, all agreed except that I do believe that constantly varying the pitch can have a sufficient impact for an hybrid system to be at least as efficient as a diesel one in a formed sea. Overall fuel PE to propulsive thrust KE efficiency of around 20%, this gives a lot of room for improvement, and propeller efficiency is very sensitive to advance coef./load & speed.

But obviously you know what you're saying, and I certainly don't have your experience. Anyway I am a strong believer of a serial hybrid system for fast sailing cats than can use regeneration, even if they consume more than a diesel. You don't use the engines that often, and the ease of use for harbor maneuvers, noise charge etc * make them worth it.

* Edit: and up-down propellers indeed!

 

Gentlemen, I'd like to clarify your terminology: What you are referring to as a "variable pitch prop" is actually a "controllable pitch prop." In the traditional naval architectural terminology, "Variable Pitch" means that the pitch varies along the blade from root to tip, and the blade itself is fixed, it does not turn on its own axis; its pitch is not controlled. Virtually all propellers are variable pitch propellers with pitch varying according to radius, being less pitch at the tip, more pitch at the root. Think of pitch as the local angle of attack of the propeller blade section to the axis of the flow into the propeller disk. When pitch is measured on a propeller, it is usually at the 70% radius.

When you actually turn the blades on their own axes by means of an actuator, that is a "controllable pitch" propeller. Controllable pitch propellers also typically have variable pitch blades.

Just trying to set the terminology straight.

Eric

 

If we are going for technical correctness, then we need to distinguish between "pitch" and "twist". Pitch of a propeller is normally defined as the mean geometric pitch along the length of the blade, so both "controllable pitch" and "variable pitch", refer to the same property; varying the mean geometric pitch.

There are variable twist props, too, where the pitch per unit blade radius (i.e. the twist) varies and is controllable. The most common control method used for these is a torsion bar running down the centre of a torsionally flexible blade; turning the root of the torsion rod varies the twist along the blade length. For a few years I owned an aircraft with a propeller like this, it had a small electric motor in the hub that twisted the torsion rods, using cams, to vary blade twist. The same principle has been used for experimental submarine propellers (as a cavitation reduction measure) but it is of limited benefit.

The reason that variable twist isn't that useful is that the practical gain over straightforward variable pitch (or controllable pitch if you prefer) props is almost unmeasurable, plus the inherent blade torsional flexibility, together with the tendency for all blades to de-pitch under high blade loading, acted to make the overall performance very little different between the two types.

 

For a diesel engine in a boat the speed of the engine depends on where the "throttle" lever which controls the governor is set.

Not always; it depends on the type of motor and controller. Some motor control systems vary power into the motor to control the speed. And the set speed or power level depends on where the control lever or knob is set.

The conventional definition of propeller efficiency is

thrust of propeller * water speed / shaft power into the propeller​

with the shaft power being the power at the given operating point, not the rated maximum engine power.

The conventional definition of propeller efficiency can be determined at any combination of engine speed and water speed. It is not "valid only at max RPM engine for which the propeller is designed".

This is a very novel definition of propeller efficiency and unlike any that I've seen previously. Can you provide a reference for it or an example of its use elsewhere? Or is it your own creation?

 

David, pity you haven't really read the posts exchanged between Jeremy and myself. Will all respect, please try again.

Jeremy, am I wrong when saying that propeller efficiency does not change with rpm, as the speed is directly proportional to rpm? If so, the apparent slip / advance coef. would be the same whatever the rpm (for a full displacement hull indeed).

Maybe we should open another thread, as we start hijacking this one...