engine question: 4-Stroke v 2-Stroke

[onara]

I've got a book here and there's something I don't understand.

"Comparison of two-stroke and four-stroke cycles

The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1.8. For a particular engine power the two-stroke engine will be considerably lighter?an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke. The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil."


That's all very straight forward. But then a bit later in the book there are these next two sentences..

"Medium-speed four-stroke engines provide a better power-to-weight ratio and power-to-size ratio and there is also a lower initial cost for equivalent power."

"Better quality fuel is necessary because of the higher engine speed, and lubricating oil consumption is higher than for a slow-speed diesel."


I've thought about it a lot and it still seems contradictory. I hope I'm not just being stupid here..

[onara]

Re: engine question

Is it that the first is comparing a two-stroke and a four-stroke engine, while later is comparing a slow-speed four-stroke with a medium-speed four-stroke? Then that would mean: For power to weight ratio, two-stroke > medium-speed four-stroke > slow-speed four-stroke; For lubricating oil consumption, slow-speed four-stroke > medium-speed four-stroke > two-stroke.

[Chiefy]

Re: engine question

/me will re-read when not so full of messers bow and morgan

[dawg]

Re: engine question

Yeah, a lot of the books on the subject are pretty badly-written... I'd look at some others.
Which book is it? We should have a "book reviews thread" for each subject...

[onara]

Re: engine question

This is from Introduction to Marine Engineering 2nd edition by D A Taylor. I got the name of it from the reading list posted in the Current Cadets section here.

Actually lots of it is very easy to understand, I liked it, but then I hit this problem.

The only way I can see that the second part (p40) doesn't contradict the first part (p15) is if they're comparing slow-speed four-strokes with medium-speed four-strokes in the second part. But the reason I'm avoiding that answer is that I haven't ever read of a slow-speed four-stroke engine. Maybe that's just because it would have the worst power to weight ratio.

About the book reviews... Yes, please tell us if you know some good ones. Or link here to the new thread if you want to create a dedicated one.

[Chiefy]

Re: engine question

I'm not sure, but fairly sure that havent been any slow speed 4s engines, as the losses etc are horrific.

The 2s is ideal for low speed direct drive applications and the s in question now reaches 3mtr which alows for full power at 78rpm BUT you need the head height to make it fit In theory oil consumption is lower on the 2s as you can control the cylinder lubrication very very accurately and this run at levels so low older engineers shudder at the thought thus the sump oil is not involved in the Cylinder Lubrication (CLO) and not getting contaminated and in theory the only losses are when the purifier blows down and any weee leaks from doors etc.

The 4s uses more oil as it is used for everything and so is burnt as CLO and consequently is more contaminated especially if your injectors are a bit iffy etc. Though consumptions have been lowered over the years and consequently newer engines often need to dump 1/2 the sump every now and again (based on quarterly LO analysis by shore side) as the TBN and other properties of the oil suffer. Where as previously you topped up the sump so often that these levels remained stable for many many months (years) without actually dumping the sump.

Power to weight is in favour of the 4s just on the fact of "power density", however most ships chose their engines on application, cruisers like medium speeds for the "power station" principle as nearly everything appears to run on electrickery so makes life nice and simple. AHTS's use them for their responsiveness and size no one wants a tug the same size as the ship it's pulling. Box boats have more space and much lower power to weight ratio (engine power to ship weight) and want economy which you get off the slow speed engine, and slow speeds like to run at xx rpm all day every day (unless you have a chief who breaks things ofc!)

Not sure this truly answers your question but then I arent 100% sure what your question is....I would read the 2 parts in isolation and apply your new found knowledge

Remember marine engineering isnt rocket science (if it is you're on the wrong course!), nothing much has changed since Messers Diesel, Mercedes et all first made coal dust go bang in a confined space. The last really big innovation was the turbo charger, electronic control / common rail technology and the likes are just evolutions and honestly you are still just squirting fuel into a confined space and making it go bang and there really is only some many ways to do that

[onara]

Re: engine question

Hmm, what was my question. That textbook had me confused as to which type of engine has a better power to size ratio and which consumes more lubricating oil. I knew very little about engines so, since I'm unemployed and not in education now, I just wanted to give myself some background knowledge to help myself later on.

First I wanted to clear up what the book was saying, but the more important questions for me have been answered in what you said. So I'll just forget what's in the book about those things. Unless other people have anything to add.

As a cadet, would I ever be expected to give an oversimplified textbook answer on this topic?

[Chiefy]

Re: engine question

Fair enough

[beer-engineer]

Re: engine question

As a cadet, would I ever be expected to give an oversimplified textbook answer on this topic?

Probably not.

[dawg]

Re: engine question

which type of engine (2-stroke or 4-stroke) has a better power to size ratio?
http://en.wikipedia.org/wiki/Power-to-w ... io#Engines
...although old Emma is a bit of an unfair comparison with an ikkle boat, sounds like 2-stroke wins on P2WR
"Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution."
http://www.deepscience.com/articles/engines.html

which type of engine (2-stroke or 4-stroke) consumes more lubricating oil?
4 stroke
http://www.deepscience.com/articles/engines.html

"Rocket science is the study of rockets, most frequently studied in the discipline of aerospace engineering and related fields."
http://en.wikipedia.org/wiki/Rocket_science

I think thermodynamics counts as basic rocket science =P

[onara]

Re: engine question

"Most of what is written on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct."
- http://www.deepscience.com/articles/engines.html

maybe this quote is better..

[dawg]

Re: engine question

at least you read it, lol.

it sounds like the answer to the question is more complex; i.e.: "it depends"

http://en.wikipedia.org/wiki/Well-posed_problem

[Chiefy]

Re: engine question

</pedant>

Thermodynamics is nearly rocket science BUT I said Marine Engineering isn't rocket science

</pedant>

[dawg]

Re: engine question

</pedant>

Thermodynamics is nearly rocket science BUT I said Marine Engineering isn't rocket science

</pedant>

true

so are we anywhere near the answer to the OP's question?

[Chiefy]

Re: engine question

dunno....I think the OP has changed the question to fit the answers

Think the "it depends" is nearest to the right answer