Has anyone chaged there engine size?

[Alan Wheeler]

Ummmm, let me comment.
Thick diesel body handles heat transfer: Ummm yeees but, it's not all to do with engine weight. It is to do with the design of how heat is transmitted. Actually that comment above is a little too loose to be considered totaly correct. Many commercial Vehicle engines are indeed used as Marine versions as well. With a few minor mods.

Burns fuel better meaning..: No not entirly true either. Fuel economy is derived by fuel control. Diesle by their inherint design control the fuel dosage very accuratly by injecting a measured dose of fuel. The speed/power is governed by the dose of fuel. In a petrol engine, the injected engine is a resonably late commer to injection, but it is why we have seen a dramatic economy improvement in more modern Petrol engines. The big BIG difference is is when both types of engine are under load. Petrol engines burn 80%'ish more fuel under load and Diesel engine only burns 20% more fuel under load.
Diesel engines have more power: No not true at all. Due to the way Diesel fuel burns, it has a boatload more torque. Diesel does not give the same "bang for buck", so the Hp for a given CC size is not as great. However it takes longer to burn so the power stroke is over a longer period which means power is delivered over a longer power cycle resulting in more torque. Petrol detonates reletively quickly meaning the power is given in a short cycle period resulting in less torque. But the Hp for the given CC size can be dramaticly greater.
One reason why power output in a petrol is gained mostly in the higher RPM range. Diesel can maintain power down in lower rev ranges.
Diesel fuel is safer than Petrol: YES!!!!!!!!!

[SkiprJohn]

Aloha Wayland et al,

Is the engine you have a Sabb or Saab? Is this a different company than the Swedish auto maker? Just curious.

Regards,

JohnL

[hellosailor]

"Thick diesel engine body handles heat transfer better. Burns fuel better meaning better fuel mileage than gas engines."

Diesel engines are typically built to run at 22:1 compression, versus about 8.5:1 in gasoline engines. If you built a gasoline engine as heavily as the typical diesel engine, and ran it at 22:1 compression....it would get an equally great increase in fuel mileage.

Similarly, if you put the highly energetic gasoline fuel in a diesel engine, and let that diesel try to combust it at 22:1...The diesel engine blows itself to Kingdom Come, because there's so much more energy available from the gasoline explosion.<G>

Horse for courses, that's why they don't race pack mules alongside quarterhorses. Neither one can win a "fair" race against the other.<G>

[Talbot]

Most Diesel car engines are designed (like petrol car engines) to achieve maximum power at high revs, which is the wrong torque curve for boat use.

However, car engines that are designed for automatic gearboxes have a very different torque curve, and achieve max hp at MUCH lower revs. these would be much more suitable as a boat engine. Unfortunately there are not a lot of automatic diesel cars available. (I have a vauxhall Omega that is auto, and I know mercedes and BMW do one)

[Brent Swain]

I went from a 10 HP aircooled to a 22 hp watercooled . Much heavier , quieter and not much increase in speed. I'd be inclined to go for a ten hP watercooled if I did it again for fuel economy and lighter weight.
Brent

[wayland]

Originally Posted by SkiprJohn

Aloha Wayland et al,

Is the engine you have a Sabb or Saab? Is this a different company than the Swedish auto maker? Just curious.

Regards,

JohnL

Hello John, The engine is an Old Sabb. 27 hp Wayland

[Alan Wheeler]

Hellosailor, not entirely correct mate.
The heat transfer within an engine body really has nothing to do with how efficiently the fuel is burned. You will also find many Marine engines are the same "body" as their automotive counterparts.
Engine Compression has absolutely no bearing on power achieved, fuel economy or the price of fish :-) Compression is all about octane or the speed at which a fuel combusts. High compression means higher Octane or slower burning of fuel in the combustion cycle int he cylinder. As I said earlier, Petrol has more "bang for buck" I should have made that clearer. Petrol burns very quickly. As the octanes are increased, the burn is not so volatile and the power is released over a much longer time interval, realising more power over a greater stroke length and thus equaling more torgue. It's kinda like this. Think of you being the fuel and you can strike the top of the piston with a large sledge hammer and belt the piston down, or you can place your hands on the top of the head and feet on the piston top and push it all the way down. The energy created in the fuels detonation is the same, but the release of the power is different. The very high compression in a Diesel can be achieved because of the slower combustion of Diesel yes, but it is mainly done to provide the heat that is required for the diesel to actually detonate.
All in all, Diesel and Petrol produce ruffly the same energy, but it is the speed and way that the energy is released that is the really big difference.
Running a Diesel on Petrol is a scary thing to do. I have done it and it is not a pretty noise. But the issue is not so much the "more energy of petrol". There are so many other aspects totaly out of control that you get wild detonation and you will eventually melt or break the tops of the pistons. I have heard tails of heads lifting, but I have not experianced that myself, nor found anyone it has actualy happend to and have wondered if that is just a tale. To lift a head off an engine block would take so much energy, many other things would have broken long before, but that is my opinion at the mo and an happy to hear from someone it has actually happend for real and not just a hearsay story.

[GordMay]

Higher compression ratios are more thermally efficient:
The higher the compression ratio, the higher the air temperature in the cylinder at the end of the compression stroke. Hence, higher compression ratios (to a point) lead to higher thermal efficiencies and better fuel economies. Diesel engines need high compression ratios to generate the high temperatures required for fuel auto-ignition (diesel cycle). In contrast, gasoline engines (auto cycle) use lower compression ratios in order to avoid fuel auto-ignition (detonation), which manifests itself as engine knock or ping. The upper limit on compression ratio is set by detonation, where the fuel/air charge is heated sufficiently in the compression stroke to self ignite (auto-ignition), and by the mechanical strength of the internal components (rods, pistons, etc). Higher compression ratios can be used by using higher octane fuel.

[Alan Wheeler]

Thanks Gord.
For all those reading, this particular subject is extremely long and complex and none of us have even scratched the surface of this topic of engine design. To understand or to describe the complexity of this topic would take a board on it's one and it is the heart of the design of an engine. All other aspects of an engine design revolve around this matter.

[hellosailor]

Wheels, we must be loosing something in translation, I didn't mention heat transfer, or the question or marine/vs/automotive engines. Plain fact is, when pollution became an issue and car engines in the US plummeted from 9.5 or 10:1 down to 8:1, we lost power from those engines and we've never gotten it back. Despite a lot of improvement in swirl combustion and computer management and everything else--we still run low compression engines in our cars, because they burn cleaner. And, produce less power.

I remember hearing someone say that if diesel engines were such a good thing (meaning, efficient and powerful) you'd see aircraft using them and ofcourse we all know aircraft don't use diesel engines. Well...There's a couple of crazy Irishmen building short-haul cargo haulers with a nice boxy shaped fuselage, and don't you know...they run the only "aviation diesels" in the air, too.<G> I think the logic was, if you can't find a good airfield, you probably can't find reliable avgas, so diesel was their choice.

I think we can agree to disagree while we all agree that the subject IS extremely complex. It is, after all, combustion engineering, better known as "rocket science".<G>

[never monday]

Diamond DA42 Twinstar uses diesel
http://www.diamondair.com/mainpage.php

Centurin currently has an STC for the Cessna 172 conversion to diesel/jetA and is in proscess for the Cessna 182.

The problem with diesels in aircraft ahs been weight. now that is comming down and power up thru turbos. In the next 20y every new light a/c will be diesel/jet-A

[Steve Kidson]

While not contemplating re-powering with a land based vehicle diesel motor, I need to add my 2c worth in relation to rev range.

My wife has a Jeep Cherokee Turbo diesel. Motor is a 2.8 litre, and at 100 kilometres on the freeway revs at just over 2000 RPM. We had a problem with it when we first had it where, for the same road speed, was revving at 3500, and virtually at the end of its power band. It was using significantly more fuel, as well as being more noisy than neccessary. (Problem turned out to be a loose wire in transmission preventing shifting into final gear - overdrive).

I have never heard of or driven a diesel that would have an efficient rev range up to 5 or 6000 RPM. A furniture delivery truck I used to drive had an optimum gear change level of approx 1800 RPM. Diesels make their power at low revs, and like any type of motor, over revving will both be wasting fuel and accelerating wear and tear on internals and other parts of driveline.

FWIW.

Fair winds

Steve

[Alan Wheeler]

Hellosailor, I apolagise. I just realised your sentance at the head of post No.18 was in fact a quote. I thought it was a comment from you, not a quote. Sorry, it was a translation issue.
Steve, RPM all depends. It depends on the engine manufacturer, the size, but all in all, it depends on the intended use the manufacturer designed it for. There are high reving Diesels and low. I have a Nissan TD42 in a 4WD that revs to 5K. But when I drive it on the road, it sits very happily at 2K and I change gear at 3K. However, it's a really big engine in comparison to the original posters question and is used from light 4WD to light commercial.
But I think we are all going round and round in circles here with this argument. It isn't really an argument, but we are making it such. Lets keep it simple folks, as to the original posters questions. Otherwise we can take this on forever and come up with arguments till the cows come home. I am generalising from the point of view of what I teach apprentices every day from an automotive engineering stand point.

[GordMay]

From ”Good Old Boat” magazine:
“Repowering & Installing a New Engine” ~ By Don Casey, Parts 1 & 2

1. http://www.boatus.com/goodoldboat/caseypt1old.htm
2. http://www.boatus.com/goodoldboat/caseypt2old.htm

[SkiprJohn]

Aloha Wayland,
You probably have gotten way more posts than you ever thought you would on this subject.
The original engine in my boat was a converted Mercedes 180d car engine. It had a pneumatic controlled accelerator. It did not have a heat exchanger. Now it is a converted MB OM636 from a ThermoKing refrigeration unit using parts and pieces from all over the world. It has a governer and an exhaust mounted heat exchanger.
My suggestion to just buy a good used engine is from hours and expense of gathering parts and pieces from everywhere just to make something work because you don't want to abandon a project.
My engine has about 35 hp at 3000 rpm which is perfect for a 42 foot 10ton boat.
Kind Regards,
JohnL