Electric Motor Efficiency Cost

I just had a chance to see a Greenline 39 at the Seattle Boat show and found it fascinating. There is a lot to like about it and I was surprised to see how many Greenline boats have been built across the whole product line.

If I understand correctly, when running on diesel, the diesel engine actually turns the electric motor, which is connected to the drive shaft. So the electric motor is now effectively a generator. The up side of this is that you get a massive current for charging, but what is the down side? It seems like you will always be paying a fuel efficiency penalty due to the increased drag on the engine from that resistence. If you can use or store that electricity, then there is effectively no/low running cost, but once you have stored all the power you can, you are still turning the electric motor for as long as you are running your diesel.

Is my understanding accurate?

I'm not sure how to think about this vs just having a big alternator or genset with a traditional diesel (non hybrid) setup. I don't own a boat, so I may simply not have enough experience to know what I don't know.

It looks like Greeline sell diesel-only versions of their boats, so do we know what the fuel cost/difference is to drive the electric motor/generator vs the non-hybrid diesel in the same hull?

I appreciate your insight and experiences. Thanks.

Interesting subject. I'll pick off some low-hanging fruit.

The alternator(s) are variable load, down to almost zero. So in a sensible arrangement adding them has little downside. I'm not intimately familiar with the GL drivetrain, but suspect they use a motor/generator that can add or subtract power from the drivetrain as needed, or run in neutral, i.e. turned off.

Over to experts!

infinite_loop said:

....but once you have stored all the power you can, you are still turning the electric motor for as long as you are running your diesel.....

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If you electricity storage is full, wouldn't you turn off the diesel and run your electric motor off of the stored energy?

Benthic2 said:

If you electricity storage is full, wouldn't you turn off the diesel and run your electric motor off of the stored energy?

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In an ideal situation, absolutely. But the they are not direct substitutes. Switching to electric is not an option if you want/need to cruise over 5-ish knots, for example. To me, that means there may be quite a bit of engine time that a typical cruiser would *want* to be on diesel to take advantage of the greater speed and during that time on diesel they would also be driving the electric engine/generator.

Jeff F said:

The alternator(s) are variable load, down to almost zero.

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Thank you. That is new information to me, I was operating under the assumption that alternators are pretty much a constant load on the system. If these electric drives do offer variable resistance with a very low overhead, that would be interesting indeed.

infinite_loop said:

To me, that means there may be quite a bit of engine time that a typical cruiser would *want* to be on diesel to take advantage of the greater speed and during that time on diesel they would also be driving the electric egnine/generator.

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Yes, but it's not much drag in the drivetrain, if that's what you're getting at. It doesn't have to be charging all the time.

I was operating under the assumption that alternators are pretty much a constant load on the system.

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Absolutely not.

Jeff F said:

Yes, but it's not much drag in the drivetrain, if that's what you're getting at. It doesn't have to be charging all the time.

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I think that gets at the heart of my question, yes. Thank you for confirming that.

Re: fuel efficiency, if you are using the alternators in diesel mode I contend you can approximate the hit to fuel consumption by looking at the generator's use compared to the kW at a certain speed/RPM.

For example, if you have a 100hp diesel that at a certain RPM "A" you use for your speed, generates 50hp, and no alternator then obviously there's zero "increase" due to alternator use.
When your alternator starts, it may be putting putting 5kW into the batteries (DC of course). It is actually taking about 6-7kW from the engine because it's not 100% efficient. So running at RPM "A" you will only be getting 41hp at shaft instead of 50hp (7kW = 9hp).
If you want to run at the same speed, you need to move to RPM "A"+"X", and through looking at BSFC curves you could work out the extra fuel consumed.

Since I'm lazy, I approximate it: "I got consumption "C" at 50hp, when the alternator runs I'll be using 9hp/50hp more, or approx 18% more fuel". BSFC isn't linear, but it's a decent approximation for my needs...

Hi,

7.2kWh and 27kWh battery models are available, the difficulty in thinking about economy vs diesel is the life of the battery bank and the cost to replace it with a new one.

Talk about big money, because you also get xxx gallons of diesel. I don't think there is a right answer, mostly a philosophical "green" idea.

At the moment, a lot is happening in the green transition and I believe that the solution can be diesel as an engine, but the fuel is not fossil or high-emitting. Exaple Neste my diesel it is -90% low emission like fossil diesel, this product is already on the market as fuel for airplanes as well.

NBs

mcarthur said:

If you want to run at the same speed, you need to move to RPM "A"+"X", and through looking at BSFC curves you could work out the extra fuel consumed.

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Thank you. This makes sense to me. I appreciate the detailed response.

North Baltic sea said:

7.2kWh and 27kWh battery models are available, the difficulty in thinking about economy vs diesel is the life of the battery bank and the cost to replace it with a new one.

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Thanks. You make a good point that there is a replacement cost for the batteries that also needs to be considered.

As for alternative (cleaner) diesel, I am familiar with "biodiesel" as a concept. But looking at airplane fuel is new to me. I think I will need to learn more about that.

Thank you for your thoughts.

infinite_loop said:

Thanks. You make a good point that there is a replacement cost for the batteries that also needs to be considered.

As for alternative (cleaner) diesel, I am familiar with "biodiesel" as a concept. But looking at airplane fuel is new to me. I think I will need to learn more about that.

Thank you for your thoughts.

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Hi,

Neste as a company is familiar around here because its headquarters and development are in Finland. Their low-emission (-80%) aviation fuel has been tested in cooperation with Boeing, it has been on sale in Europe for some time and also in the USA. link to additional information.

Link Neste-Boenig test drive

https://www.neste.com/neste-and-boeing-lead-industry-commercialization-renewable-aviation-fuels

Neste deliver US market link.

https://www.neste.com/releases-and-news/renewable-solutions/neste-delivers-more-500000-gallons-sustainable-aviation-fuel-los-angeles-international-airport

NBs

I am a Greenline 39 owner. I am not sure the primary advantages of the hybrid setup are actually about fuel efficiency from cruising on the electric motor. The electric motor is on the same shaft which is spinning regardless and it is in either charge mode (when running on diesel) or drive mode (when running off electricity.) I don't think charge mode makes any difference on overall drag and therefore has little or no impact on fuel economy. There are also no additional belts that increase friction. The Volvo D3-220s on the current hybrid drivetrains are inherently pretty efficient already.

In my view, the hybrid drivetrain offers other significant advantages:
- yes you can cruise at around 5 knots, but we use our electric motor primarily to go in an out of harbors, marinas and anchorages. Apparently that is usually 15-20% of the typical operating engine hours. So your diesel engine hours don't accumulate during this period and you don't use any diesel fuel either. It's similar to the logic on a PHEV (plug-in hybrid) car. You run a portion of your time on electric only and therefore your average mpg is significantly higher.
- the large battery bank powers most systems (except thrusters) through an inverter. Therefore there is no need for a separate generator and the associated maintenance/sound & vibration.
- the large array of solar panels usually power all systems (lighting, refrigeration, AC, etc) during the day and then you can run off the battery/inverter overnight. (AC use is by far the biggest energy user.)
- the electric motor can run at any RPM range (100+) and therefore you can fine tune any docking with your throttle as opposed to bumping in and out of gear due to the need to maintain idle RPMs when on diesel.

They have sold a tremendous number of Greenlines. In addition to the advantages of the hybrid systems, they are extremely livable in their design and offer great storage. There have been very few comparable boats on the market. You will definitely see more innovation in the coming years.

Jon

GL33 Hybrid efficiency

John,
Your understanding of the hybrid setup is correct.

Capital cost of a hybrid vs a diesel only is significant. We bought our 2011 GL33 in 2018 with ~600 hrs on the engine. Has everything we wanted.

These posts may assist.

River / canal cruising
https://martinandbelinda.blogspot.com/2022/07/fuel-and-average-speed-update.html

In the Adriatic one way speed test and fuel economy on diesel
https://martinandbelinda.blogspot.com/2019/06/diesel-speed-test.html

And no don't buy a hybrid to save on fuel costs, buy it for the peace and quiet. Ideally suited to rivers and canals in Europe, waiting for locks or bridges to open, puttering along is what I love about it.
Cheers, Martin

Mmm, not an expert in hybrid drive trains, but I am a Mechanical Engineer and the basic laws of thermodynamics (and/or physics) suggest that spinning the electric motor as a generator MUST cost fuel economy.

There is no free ride, there is absolutely going to be a fuel cost for generating electricity off the drive shaft, and there is definitely an efficiency loss in the electric motor (it might be low, but it is there for sure). Accordingly, producing electricity with the drive motor will decrease fuel efficiency of the drive motor alone.

If producing electricity in the manner done by the GL hybrid system were "free", everyone would have adopted hybrid propulsion systems.

I am sure there are interesting advantages to the hybrid drive system, and I can't wait until battery technologies reach the point of making electric propulsion system viable for cruising boats - but there is no "free" power as suggested by some of these posts.