Hydraulic Drive vs. Direct drive vs, Diesel Electric Drive -- Overall Efficiency Comparison 3

[sbmar]

Hopefully I came to the right place for some qualified information on hybrid drive system comparisons. I am looking for opinions and information from "others" based on their experiences.

Our business is all based around marine power systems for both vessel propulsion and auxiliary/ancillary hydraulic systems. We also have designed, and actually put into , two diesel-hydraulic vessel propulsion systems. One was in the 10 HP range, and one in the 80-100HP range. Both worked extremely well for the intended use but both were in applications with low annual hour light duty service.
We are now looking at a system design in the 200 HP range for high annual hour heavy duty service .

All is on the drawing board at this stage, but because of the overall design flexibility of going "hydraulic" we hope to come up with a viable efficiency comparison to the tried and proven "diesel electric" drive systems that have been in use for decades in trains and large ocean vessels.
It really all comes down the heat losses and mechanical losses in any power conversion system. What hydraulics can offer us is an easy solution in smaller systems to load the diesel engine close to its peek BSFC range and most any RPM, and still control overloading it; and an easy way to install the engine in an optimum location in the vessel that allows better use of space and weight distribution.. I have attached a chart ( maybe overly simplistic, but at this point it work to get the point across) that I found on the internet and added at the bottom "hydraulic example." The numbers for the Direct Drive & Diesel Electric Drive were there--As to whether they are right or wrong, I do not know, but they seem "about right" to me.

Your thoughts and input will be much appreciated.

Thanks
Tony ---------------

http://www.sbmar.com

Tony Athens

http://www.sbmar.com

 

[TugboatEng]

Google says 80% efficiency.

http://www.hydraulic-pump.info/hydr...ons/hydrostatic-transmissions-advantages.html

I wouldn't doubt this. Hydrostatic drives require substantial cooling which is indicative of their poor efficiency.

 

[GregLocock]

The efficiency of the electric and hydraulic drive will vary with rpm and load. So the tradeoff will vary with usage profile.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[EdStainless]

While the drive line losses may be different you also need to factor in prime mover eff.
If you are direct coupled though a gear box you may spend a lot of time running the engine in very low eff conditions.
With the other two options you can keep the engine running at high eff and suffer losses other places.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

The diesel engine power curve and propeller curve are usually quite well suited to each other. Fixed pitch prop with variable speed engines are the most fuel efficient.

Variable pitch props often get operated at constant speed which leads to wet stacking under light loading.

Variable pitch props allow for greater utilization of the engine power. A key example would be a 10,000 horsepower ocean towing tug. With a heavy tow that can't come up to speed will get stuck at a lower portion of the propeller curve meaning you're only getting 5000 HP when you paid for 10k. Variable pitch allows you to pull pitch until the engine can come up to turns maximizing horsepower.

Variable pitch has a key characteristic for dynamic positioning which is rapid thrust reversal.

 

[Steven_D_P]

The hydraulic option should not just picture the losses because of the pump and motor but because of the lines, fittings and any valves in-between as well as these losses will be higher compared to "transporting electricity". Furthermore, if you're using a variable displacement pump or motor the efficiency will be much lower when the displacement isn't maximal (when the displacement goes to zero the efficiency goes to zero as well). So by using diesel-hydraulic your diesel engine can run at optimal efficiency but your sacrificing this by lowering the efficiency of the variable displacement pump or motor.

Edit: did some google searching. Here are some rough numbers for efficiency op hydraulic motors and pumps:
Axial piston pump: 91%
Axial piston motors: 90%
So the maximum total efficiency which these motors and pumps is 82%, not taking into account their lower efficiency at lower displacements and any other losses due to lines, fittings and valves.

Without a doubt diesel-electric will be more efficient than diesel-hydraulic.

 

[sbmar]

Steven,

Thank you for taking the time to post some input. You are 100% right, every component in the system that could generate any pressure drop will also create some heat and also means some loss of efficiency.. It really all come down to whether the basic simplicity of a Hydraulic system ease of varying engine loads at different RPM's out-weights the lower overall efficiency levels.

We all know that their is more to efficiency than just using the highest efficiency level of heat conversion to rotation movement, other wise we would not have diesel electric trains or any diesel elec ships.

I might even go so far and suggest that even a newer electric car uses more energy overall to go from point A to Point B if look at it in "TOTAL".. But that would be for another topic.

Tony Athens

http://www.sbmar.com

 

[Steven_D_P]

Well, you asked about the efficiency of the diesel-hydraulic option so I replied w.r.t. the efficiency of a diesel-hydraulic system. Whether it's lower cost is worth it's lower efficiency is a trade-off you or your client will have to make.

W.r.t. the ease of varying engine loads at different rpm's, a diesel-electric solution will also allow for this. The diesel-electric solution will have a higher efficiency than diesel-hydraulic. Like you said there's a reason trains and ships are usually diesel-electric, but diesel-hydraulic is exceptional for a machine that operates the whole day (Edit: unless weight is important, in that case hydraulic has an advantage over electric).

Idk if you've seen the edit to my previous post but I found that an axial piston pump has about 91% efficiency and an axial piston motor about 90%. So I estimate that the overall efficiency of a hydraulic solution will not be more than 77%. This is with the motors running at maximum displacement, at lower displacements the efficiency will be lower. In any case I think the 5% loss for each in the original picture is too optimistic.

Another problem with the original picture is that it doesn't take the engine efficiency into account. With a direct drive the direct drive itself is more efficient compared to an electrical system but with an electrical system the engine runs at a higher efficiency which will result in an overall efficiency that about the same. You really need to look at the total system to make an educated decision.

 

[Zenithund]

thanks for sharing