View Single Post
Old 10-16-2010, 08:33 AM   #68
RickB
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
Low Powered Cruiser

Quote:
nomadwilly wrote:I'm mostly familiar w 20 to 55 hp engines and all that I can recall have 22 - 23 to one compression. Diesel may not be so but I think gas engines are more efficient w higher compression. Something I've wondered about that you may be able to enlighten me on is the pre-combustion (swirl chambers) on many of these engines. It seems my old Yanmar w direct injection started really easily and didn't have heaters/glow plugs. What do they have to do with tier2 regs? I would think the pre-chambered engine would start better but the reverse seems to be true from my experience.
Higher CR does equate to higher efficiency. But the increase is incremental and small, and this is the bigger but, it also equates to higher loading of parts which means heavier stronger parts and greater inertial loads that in turn require larger heavier bearings and other parts.

Small engines - very small engines - have very small cylinders that don't contain much charge air and have a large surface area in proportion to the volume compared to larger engines.

Your Mitsubishi conversion for example has about 1.8 square inches of metal for every cubic inch of swept volume, that is a lot of heat absorbing surface and that 20something to 1 CR is needed to make some heat. FF's dream engine has about 1.33 square inches for each cubic inch and a CR of around 18.7:1. A large (but far from the largest) engine that produces around 1600 hp per cylinder has about .33 square inches for each cubic inch and starts very well with a CR of 14 or 15:1.

To put that in workable terms, it means that a tiny charge of air has to be very highly compressed very quickly to retain enough heat to ignite the fuel charge. That requires a high compression ratio and means the engine can be very hard to start when it is cold. Which brings us back to the precombustion chamber.

A direct injection engine requires a fuel injector that uses very high pressure to create a perfectly atomized spray of fuel. But, it also has to deliver that fuel in a manner that can penetrate the very dense air charge so that the droplets are heated quickly and burn properly. Each drop has to find an oxygen molecule before it gets too hot and simply turns into a microscopic sphere of carbon and leaves the boat as soot.

One way to get around the low temperature of the charge air and the need to have high pressure pumps and expensive injectors is to provide the injector with a little room of its own. A cheap and clunky little low pressure injector can squirt a coarse stream of fuel into a little cavern where it can be ignited by a red hot glow plug and then squirt out at very high velocity into the clean, oxygen rich environment of the cylinder. As it squirts out it creates a great deal of turbulence that mixes the "rich" fuel charge better than its high class cousins.

When the engine has run for a bit the glow plug and the little house get toasty warm and the system works pretty well on its own. But back to the CR, you can see how the cold cylinder walls (even when the engine is up to operating temperature, the walls are "cold" compared to the combustion gas) soak the heat of compression out of the charge and create all kinds of other problems with ring lubrication and cylinder wear. That is one of the reasons those high strung little engines just don't last as long.

Looking at the "man" for your engine (an S4L2, is that correct?) it can be run continuously at 3000 rpm if it is propped so that the power produced at that rpm is limited to 35.1 hp. That one hour restriction applies to engines that deliver 37 hp at 3000 rpm. So, prop accordingly and run it on the stops all day long if you want.

To tell the truth, unless you install a hp meter on that thing you won't ever know when it is delivering 37 or 35 to the prop and 3000 rpm must sound like a can recycling facility at full capacity!


-- Edited by RickB on Saturday 16th of October 2010 09:38:44 AM

-- Edited by RickB on Saturday 16th of October 2010 09:44:26 AM
RickB is offline   Reply With Quote