Piston designs

Hi all, at the sale I posted about recently, I had the opportunity to look at the first steam piston I've seen. It was basically a thick disc of metal with ring grooves. It was a piston, but with no skirt, so quite different in appearance to the pistons I usually see.

Now for a display of conspicuous ignorance:

Do most steam engines have this sort of piston?

This leads me to the questions, why do IC engines have piston skirts?

Did early IC engines have skirtless pistons?

Regards, Arthur G

i think it's because most steam engines are double acting....they use the piston in both directions. therefore there needs to be a packing on the rod end. they use a 'scotch yoke' (not sure what the uk may call it) to terminate the connecting rod, so the piston doesn't rock as an internal combustion engine may. does this help? my best, sammm

first steam piston I've seen. It was

no skirt, so quite different in appearance

Arthur,

Most steam engines have this sort of piston, my own pistons on the Burrell are like that - this is photo :

As pointed out, its because steam engines are double acting. They are in fact hollow castings.

Regards

Chris Bedo Kent UK

snipped-for-privacy@tiscali.co.uk

IC engines invariably run at higher speeds and combustion pressures than those employing hot fog to push them around. The skirt is certainly there to resist side thrust caused by the varying angle of attack from the crankpin/connecting rod, but is important in providing somewhere to put extra rings in the bottom of the skirt.

Radial engines had very short skirt pistons, very little longer than the gudgeon pin bosses required. I often wondered why.

Regards,

Kim Siddorn,

Fascinating link, thanks Martin. Pity the classic car enthusiast of the future though!

Could it be as simple as all those rods on one crankpin leaving no room for a skirt? ttfn Roland

Gentlemen,

Some light reading on the subject.

Martin P

In answer to my own question -

"Pistons used to have long tail skirts (which sometimes cracked or broke off). Now most pistons have ?mini-skirts.? Instead of a 2.5? skirt length, the piston may only have 1.5? skirt. Shorter skirts reduce weight but also require a tighter fit between the piston and cylinder bore to minimize piston rocking and noise. Consequently, today?s piston clearances are much less than before (typically .001? to .0005? or less). Some have a zero clearance fit or even a slight interference fit (made possible by special low friction coatings)."

I did know that Bristols were renowned for the close tolerances they maintained on piston manufacture, so that is probably the best answer ;o))

Thanks Martin, very interesting.

Regards,

Kim Siddorn,

Also, a bit of 'piston slap' is unlikely to be a major concern on an aero engine.

More important is that a longer skirt increases heat dissipation and transfer.

-- Brian Bailey