Already the first Diesel engines (see picture) had one. Only after several attempts was a specimen without a crosshead successful. Apparently, it was simpler, as far as absorbing the strain was concerned, to first of all, build the engine with a crosshead. Indeed, what is this and what purpose does it serve?
In a crosshead engine, the pistons of the individual cylinders are not directly connected through the con-rod with the crankshaft. Instead of this, a straight rod (cannot be seen in the picture!) which has the same center axis as the piston, is now connected through a joint to the shortened con rod.
The crosshead engine is nowadays, only found in large, very slow running Diesel engines. Imagine a small-end connected directly to the gudgeon pin and at the same time having three times as much stroke in proportion to the bore. Since the crankpin would still have at least half the stroke length from the center point to the main bearing, it's pivoting movement inside the cylinder wouldn't be possible. ?
Thus the mechanism, which converts a stroke- into a rotary movement is moved to outside the cylinder. Despite the fact that this construction has more parts, thus causing more internal friction, giant slow running two-strokes of this type, are considered to be the engines with the most economic consumption. The piston is placed under an enormous strain, there are however, no lateral forces. They are absorbed by the crosshead guide. Thus the piston can do it's job of sealing and transmitting the power better. Apart from that, the lack of lateral forces has a positive affect on the very important operational safety and durability of marine engines.
If the big engines were to undergo a complete oil-change, a fuel-truck carrying 38.000 liters of oil would just about be enough. This is why the oil is only partially changed. A crosshead makes the sealing of the cylinder space from the crankcase possible, where, because of the lower temperatures and under certain circumstances, cheaper lubricating oil can be used. In addition, the pressure in the crankcase would also be reduced, which is significant as far as the oil consumption is concerned. There are, or were, also crossheads which have, or had, only one mounting point on the engine casing. 10/09