crankpin can send transverse forces to the connecting rod?

Well you lost me there. How does 250,000 miles on a car equate to hours on a ship engine?

Reply to
Chas Hurst
Loading thread data ...

Here's a link to a huge crosshead engine, single-acting cylinders, still in production. The large and slow side-forces likely mandate the crosshead design.

formatting link
Dan

Reply to
Dan_Thomas_nospam

Assume 50mph average speed...

Mark Rand RTFM

Reply to
Mark Rand

RCM only

On Sat, 01 Sep 2007 12:05:36 +0100, with neither quill nor qualm, Mark Rand quickly quoth:

Wow, you have fast ships over there. ;)

-- "Not always right, but never uncertain." --Heinlein -=-=-

Reply to
Larry Jaques

If you assume an average auto speed of 35 MPH it is about equal to 10 Months steaming.

Bruce in Bangkok (brucepaigeATgmailDOTcom)

Reply to
brucedpaige

Why does a non-harmonic motion change the efficiency of burning? I don't see that. My understanding is that few Diesels are truly constant pressure burn anyway- they cannot control the injection well enough. Certainly high speed car and truck Diesels are not.

And since the valves are closed during the entire burn time, I fail to see how the crosshead geometry would affect the valve timing.

Reply to
Don Stauffer in Minnesota

On the other hand, container ship engines operate in an almost IDEAL environment- constant power output for weeks on end without throttle changes, constant RPM, almost no vibration, a prop in water providing an almost perfectly constant load on the engine, abundant cooling to seawater, etc.

Whereas the car engine operates under constantly changing loads, constantly changing speeds, constantly changing G-loads from road vibration and handling of the car, varying ambient temperatures, etc. etc. etc.

Reply to
Steve

I would say that it changes the efficiency of the coupling of the burn pressure to the reciprocating assembly more than it affects the burn itself. I

The ideal diesel is a constant pressure burn (continuous burn as the piston moves down), the ideal Otto cycle is a constant volume burn (instantaneous burn at TDC followed by adiabatic expansion). In the real world, neither one follows that ideal cycle, but the diesel is CLOSER to constant pressure and the Otto is CLOSER to constant volume than vice-versa.

The valves aren't actually closed during the whole burn time, really. Exhaust valves, for example, tend to open before the burn is complete, because you get a bigger gain in efficiency from the extra time to purge the cylinder than the last few joules of energy out of the last little bit of expansion.

But I agree, any time the rod length/stroke ratio is greater than about

1.7 or 1.8:1, the motion is close enough to ideal that it doesn't matter. Some engines work with a shorter rod, the biggest small-block Chevies, for example had a rod ratio of something really sucky like 1.5:1, but rod ratios that bad are the exception more than the rule.
Reply to
Steve

Nice one, thanks.

This one isn't a crosshead design but it's an interesting oldie from WWII submarines that is still in production.

formatting link
jw

Reply to
Jim Wilkins

The ship engine turns 100 rpm vs 2-3000 for the car engine. I'd say things are equal.

Reply to
Chas Hurst

And an engine with really offset rods:

formatting link
Dan

Reply to
Dan_Thomas_nospam

I agree that the valve is not closed for the whole EXPANSION STROKE, but the fuel in either a Diesel or SI engine does not normally burn during the whole expansion stroke. SI engines burn only a few degrees of crank rotation, not 180 degrees. Diesels burn during injection time and a few degrees after (especially in high speed or truck engines), but not THAT many degrees after end of injection. Expansion cooling is freezing the process.

Reply to
Don Stauffer in Minnesota

Wow. What a hard way to turn fuel into motion. Must have been one of the last opposed piston two-stroke diesels ever designed, two cranks was a much more common approach and probably worked better.

I posted a link for the new 6V53 Detroit in rec.crafts.metalworking recently: HP:displacement in cubic inches is over 1. Pretty mighty two-stroke diesel..

John

Reply to
JohnM

Given the huge success and continued production of the Fairbanks-Morse engine 38 series, I'd say that two cranks did work better.

formatting link

The same could be said for THREE cranks, ala the Napier Deltic... with the sole exception that it is no longer in production.

formatting link
formatting link

Reply to
Steve

formatting link

Hey, I didn't know they were still in production, cool.

Great stuff, thanks for the links.

John

Reply to
JohnM

The design I refered to uses more non-harmonic motion than normal, IOW a shorter than normal rod. Now, with a short rod, you would have excessive piston dwell at BDC, and high piston speeds at TDC. By adding a cross head, and inverting the rod, you get longer dwell at TDC, and faster psiton motion at BDC. That is the basis for the invention. He was able to achieve a better timing relationship for a higher rpm diesel, and thus more power per displacement/size tradeoff. Normal diesels are speed limited, due to timing considerations. You don't see a lot of 7 liter diesels turing a lot of rpm do you? The way ti was explained to me, was that getting the diesel to develop power and to start at low rpm limited it's rpm range. The inversion of "piston dwell" timing allowed a faster rpm, by allowing more time at TDC. FWIW.

Reply to
Half-Nutz

and thus more power per displacement/size tradeoff.

OK..

Here is the thread where the inventor describes the advantage of the non-harmonic motion for a high speed diesel motor.

formatting link

Reply to
Half-Nutz

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.