Steel pistons...

Fast Eddie · Jun 20, 2015

Seems bizarre doesn't it?

I was at Mahle in Stutgart this week and saw some of their steel pistons. They are primarily used in new generation Diesel engines due to the extreme heat and pressure stresses.

Apparently, the use of stronger material allows a greatly reduced compression height (distance from top of crown to centre of wrist pin), partly allowed by much smaller space between rings.

Also thinner material can be used. So, often, the pistons are lighter than alloy, resulting typically in fuel and emission savings. They also have much greater service life (service life in commercial vehicle engines is staggeringly high).

Mercedes use them in alloy blocks, but others suggest they are best for iron blocks where the more similar coefficient of expansion allows for much smaller tolerances.

Naturally, I could not help thinking that this sounded like a very appropriate approach to old Brit bikes, especially Commandos!

Triton Thrasher · Jun 20, 2015

Cast iron pistons were common on British two strokes, until the 1930s.

I think they were still used on American cars later than that.

I'm told they had close tolerances and long life, but were fragile on the workbench.

Rohan · Jun 20, 2015

Triton Thrasher said:
Cast iron pistons were common ...

Indeed - cast iron pistons were used on near ALL engines, until the mid 1920s at least.
Cars, trucks, motorcycles, aircraft engines, ships, lawnmowers, you name it was used in them.

Apart from the odd experimental and racing engines, (aluminium) alloy pistons were not well understood and often troublesome,
even into the 1920s.
Iron had the HUGE benefit that they were reliable when lubrication was scanty - iron on iron will run quite happily, with very little wear,
and almost no chance of seizure. Important in drip feed or constant loss oiling systems....

I understand that Bedford trucks used iron pistons until well into the 1950s, and they were not alone.
A bit of piston weight, in low revving engines, gives more flywheel effect too, without needing a heavy flywheel.

How do they get the steel pistons thin, thats not so easy to cast or forge.
Machine them ?

Triton Thrasher · Jun 20, 2015

Metallurgy is always advancing.

Rohan- your point about flywheel effect: I don't think reciprocating masses have the sort of inertia you want more of in an engine. If a designer wants more flywheel mass, he can easily put more mass on the flywheel.

Rohan · Jun 20, 2015

My old Triumph 1910 (project) has an enormously heavy iron piston - I was just looking for it to weigh it.
Think it was of the order of 2 lbs or so.
Far heavier and thicker than it would otherwise need to be.
The flywheels are also pretty solid. Too heavy for my kitchen scales, it only weights to 4kg.
Even the belt drive pulleys are heavy.
It has more flywheel weight than a Commando ( not by much). Its only 500cc too.

I understand from riders accounts of these things that it "will chuff down to countable revs" (~200 rpms),
and still chuff away and accelerate once back on the flat.
Essential for climbing hills - when you have only one gear, no clutch, and need to pedal it to assist.

Pistons have got lighter over the eons.
Flywheel affect was moved to the flywheel......

BTW, the 1910 Triumph was said to be the 1st (brit) bike that could take more punishment than the rider.
As several long distance rides proved - Lands End to John o Groats, where the bike was still running strongly,
but the rider less so. They did it in wintertime, it must be said.
It was also the best selling bike of that era, they sold 3000+ based on these performances and testimonials.

texasSlick · Jun 20, 2015

Fast Eddie said:
Seems bizarre doesn't it?

I was at Mahle in Stutgart this week and saw some of their steel pistons. They are primarily used in new generation Diesel engines due to the extreme heat and pressure stresses.

Apparently, the use of stronger material allows a greatly reduced compression height (distance from top of crown to centre of wrist pin), partly allowed by much smaller space between rings.

Also thinner material can be used. So, often, the pistons are lighter than alloy, resulting typically in fuel and emission savings. They also have much greater service life (service life in commercial vehicle engines is staggeringly high).

Mercedes use them in alloy blocks, but others suggest they are best for iron blocks where the more similar coefficient of expansion allows for much smaller tolerances.

Naturally, I could not help thinking that this sounded like a very appropriate approach to old Brit bikes, especially Commandos!

Diesel engines do not rev very high .... most red line at only 2500 rpm, and 3500 is tops. Thus the recip mass inertia is less of a factor. Designers of Diesel engines are concerned with longevity .... engines in trucks and busses go more than a million miles before overhaul.

Slick

Hobot · Jun 20, 2015

Cool would be just the thing for a 27 CR 3000 rpm Commando until the cermaics come of age.

Fast Eddie · Jun 20, 2015

To answer some of the points raised:

The steel Mahle pistons are usually lighter than comparable alloy ones, which is (they say) the primary reason for a circa 5% fuel efficiency gain.

The blanks are forged steel. I imagine it is an exotic mixture / specification.

Stephen Hill · Jun 20, 2015

The new generation of automotive diesel engines are pushing higher rpms. 2500/3500 is no longer the upper limit. For example, the BMW 530 engine redlines at 5700. Interesting that even the most radical racing diesels don't turn much faster. The combustion rate of diesel fuel appears to limit rpm. However, they keep getting more torque out of the diesel engines, which is another reason for looking at steel pistons.
Here is a good short article explaining why steel pistons are becoming an option:
http://www.federalmogul.com/en-US/Media ... _Nov12.pdf

Stephen Hill

texasSlick · Jun 20, 2015

Fast Eddie said:
To answer some of the points raised:

The steel Mahle pistons are usually lighter than comparable alloy ones, which is (they say) the primary reason for a circa 5% fuel efficiency gain.

The blanks are forged steel. I imagine it is an exotic mixture / specification.

If they can be made lighter than a comparable aluminum piston, then why not? Everything becomes in steel's favor in that case.

Slick

Hobot · Jun 20, 2015

Before ya get into steel Norton pistons visions dig a bit deeper into their heat conduction issues to see All the engines they are used in will have *copious* amounts of cooling oil jets underneath and often internal piston oil paths too.

Mr.Sparks · Jun 20, 2015

Hard on rod bolts

texasSlick · Jun 20, 2015

hobot said:
Before ya get into steel Norton pistons visions dig a bit deeper into their heat conduction issues to see All the engines they are used in will have *copious* amounts of cooling oil jets underneath and often internal piston oil paths too.

I was thinking the same thing. OTH, steel can withstand higher temps so the lower heat conduction may not be a problem. Higher piston head temps can result in better combustion, providing detonation can be controlled.

With higher allowable piston temps, higher engine temps can be tolerated, and will result in increased fuel economy .... 2nd Law of Thermo .... efficiency = ( T (hi) - T(lo))/ T(hi)

Maybe if these pistons promise higher fuel economy, the oil companies will be forced to give us higher octane to control detonation, or the gubmint rats will just boost the booze level up another 5 to 10 %.

Slick

Hobot · Jun 20, 2015

Heat conduction in steel pistons prevents use in air cooled engines w/o copious oil cooling, unless antique power to weight ratio engines. Major player reviews what it takes in Alu and steel pistons, now a days.
http://www.federalmogul.com/en-US/Media ... _Nov12.pdf

Rohan · Jun 20, 2015

texasSlick said:
2nd Law of Thermo .... efficiency

The 2nd Law of Thermodynamics states that the entropy of the universe will always increase, and cannot be negative.
Efficiency doesn't come into it ??

We have had this discussion before......
e.g.
http://chemwiki.ucdavis.edu/Physical_Ch ... modynamics

Some car and bike engines can run at ~ 12:1 or 13:1 compression, quite happily,
so if a Commando cannot then its unlikely the fuel wil be improved to compensate.
Combustion chamber design has come a long way over the decades.....

Hobot · Jun 20, 2015

In daily life and times entropy always increases but not necessarily on cosmic scales or mind level which decreases entrophy with each new data point gained on a particular system observed. Commandos as most engines well tuned, are made to tolerate mild detonation for long intervals. Its water cooling and oil jets that mainly allows moderns run more CR than Nortons can take on same pump gas and ignition timing. Puterized back off of spark time and fuel leaness helps a lot too. The chamber shape/size comes into play some. Ceramics and glass-like metals may be next jump in combustion thermal efficiency. To dump the heat of sufficient oil cooling jets volumes takes a big water cooled engine radiator. Hot sports bikes have both cooling system mass and streamline issues that are educational to review.

Rohan · Jun 20, 2015

hobot said:
Commandos as most engines well tuned, are made to tolerate mild detonation for long intervals.

eh ??

lcrken · Jun 20, 2015

Rohan said:
Some car and bike engines can run at ~ 12:1 or 13:1 compression, quite happily,
so if a Commando cannot then its unlikely the fuel wil be improved to compensate.
Combustion chamber design has come a long way over the decades.....

Actually, a Commando engine can be made to run quite happily at 12:1 +, as demonstrated by several vintage racers, but those engines also typically have longer overlap cams and lower back pressure exhausts than stock, and run on high octane race gas. Not very practical for our regular street bikes.

Ken

Hobot · Jun 21, 2015

As no oil jet coolled steel pistons in our sights if ya enjoy the delights of Norton torque of a responsive tuned-set up engine then its likely often lightly detonating in a good way. Online is spare on references but studying Engine Masters and early air coolled engine studies reveals its more common than expected. Dig deeper here.
http://www.google.com/url?sa=t&rct=j&q= ... 9352,d.aWw

Rohan · Jun 21, 2015

hobot said:
its likely often lightly detonating in a good way.

Eh ???

Steel pistons...

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