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
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.
While the offset piston pin can reduce piston slap thats not the prime
reason its done. An offset pin causes the piston to reach top dead center
at a different time than the pistoin end of the connecting rod, effectively
spreading the shock loading over a greater number of crankshaft degrees. In
short, the real reason for piston pin offset is that it softens reciprocal
loading, permitting lighter more power-efficient parts to be used, and the
engines to be capable of higher rpm.
On Aug 30, 4:27 pm, dances_with firstname.lastname@example.org wrote:
There was another big reason for the crosshead. The double acting
steam engine came along not long after the Watt improvements. The
double acting engine requires a way to seal that half of the cylinder.
It would be almost impossible to seal against a connecting rod. By
having a cylindrical rod passing through the end of the cylinder, and
holding it axially with a crosshead, it was easy to pack the seal in
the cylinder head.
I was under the impression that was the main reason for crosshead
I've seen at least one set of pictures of a single-acting gas engine
with a crosshead. This was from the late 1800's, early 1900's era when
folks may not have had a firm grasp of why things were done the way they
were, and there were more engine manufacturers than you could shake a
stick at and a bewildering variety of design features on engines.
I wish I had the link, I'd post it...
On Sep 1, 1:49 am, Dan_Thomas email@example.com wrote:
Nice one, thanks.
This one isn't a crosshead design but it's an interesting oldie from
WWII submarines that is still in production.
There was someone in Italy working on a modern Diesel design, with a
crosshead, it used a connecting pointing away from the piston. Since a
connecting rod and especially a short connecting rod produces non-
hamronic motion, he was able to optimze the combustion burn vs piston
Basically a short con rod will dwell the piston at the bottom of the
strke, and increase the speed at the top of the stroke. By using a
crosshead to invert the con rod, you can get more burn time vs rpm.
And more favorable valve timing. This allows more rpm from a diesel,
which is compromised from breathing at higher rpm. This then allows
more rpm, and higher specific outputs per displacement. IIRC ther was
some potential improvements in emissions also.
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.
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.
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
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.
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.
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
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
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