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-   -   Power characteristics of long and short stroke engines: Cast in stone? (https://www.team-bhp.com/forum/technical-stuff/80106-power-characteristics-long-short-stroke-engines-cast-stone-8.html)

A long stroke engine inherently is capable of producing more toque. Power is nothing but the torque multiplied by the angular velocity.
Now a long stroke engine will have a bigger lever arm, as the webs have to be longer to cater for the increased stroke length. The torque that is transmitted to the crankshaft is F*RSin(theta). Since a longer stroke engine will have a higher R, we get higher values of torque for the same force acting on the piston vis a vis a short stroke engine.

Higher torque translates to higher power for the same RPM.

Got it.



By inertial forces do you mean centrifugal force or the force required to move the piston?

Can you tell me which component is the most critical - that will be a clue for me.



Any comments on this? you didn't write anything about burn characteristics of diesel. Does it burn fast enough to allow a fast ignition stroke (And hence higher rpm)?

The long stroke engine with have higher FHP (which is negative work) Goes up exponentially with rpm IIRC.

Long stroke engines, especially the low speed variety have more time to complete the combustion. Hence they are inherently more efficient. That is why where efficiency matters - in high HP industrial applications : Power Generation, Ship propulsion, long stroke, low speed engines are used.

The torque increases with the piston area, as there is more volume hence more fuel-air mixture can be accommodated.

Short stroke engines can run at higher RPM, hence can produce more HP for a given swept volume, and thus preferred where higher RPM and a lighter power plant is required. For a given peak HP shorter the stroke (to a reasonable extent), the lighter the engine, the higher RPM, and the lower its specific fuel efficiency.

Where the weight matters less than the specific fuel efficiency - generators and ships, long stroke low speed engines are the norm, as neither the generator nor a cargo ship requires rapid acceleration.

As there is very little space for the valves to open fully in a short stroke engine (to make the dead volume low), there are multiple valves any where from two to four per function - inlet and outlet, thus you graduate from 2 valves per cylinder to 3, 4, 6 and 8 valves or more per cylinder for high performance engines.

Centrifugal force not a problem.
The piston stops and starts -> accelerates -> has forces acting on it. These are significant. And these can work in addition to or opposition to the forces generated by the gases. You take worst case into account. (and dont forget fatigue failure!)

Simple answer with zillions of qualifiers:- For a homogeneous charge (petrol engine), not really. Diesels, well heavy fuel stock -> lower engine speed. Nature of combustion is very different.

Regards
Sutripta

Do the short-stroke engines use lighter materials? Because a quick calculation indicates that max. force/stress on piston will be at TDC both due to its motion and gases. Also since gas pressure will be same in short/long engines, the piston in short engines is likely to be heavier (can't make it thinner, have to make it larger - more mass)

Taking all that into account, the advantages of having shorter stroke (hence low velocity, hence low acceleration) are more than wiped out by extra mass of the piston and higher force on it in the short stroke.

I guess at high speeds things may not be simple for a hand calculation.


Does engine breathing have anything to do with it too?

Hi,
No.

Comeon Vina, IIRC, you were an active participant in this thread. :)

Regards
Sutripta

I was, but then I was hounded out of TBHP and later got busy with other things (like my 6months old daughter).

Anyway, I'll do a detailed calculation next month if I still have the enthusiasm.

@vina & co, We have to accept the fact that short stroke engines (over-square) develop a higher hp albeit at higher rpms'. What they often lack is the torque at lower revs. Otherwise racing engines will not be such high revving. F1 now limits them to 18,000rpm. T one time not so long ago some were going to 20k+.

I don't think anyone is disputing that the short stroke engines generate more power.

The questions is how do short stroke engines manage higher rpm in the first place.

There may be two factors. Pistons may be lighter since they are far shorter (I guess this overtakes the higher cross section). I agree area has a dimension of r^2 while length is only l. Also, the con-rods are far shorter. Even the FIAT 128 engine in the 1970's (8300rpm redline) had a very short stroke and very short pistons.

As I wrote earlier a simple back of the envelop calculation didn't yield satisfactory results. I didn't have time for a detailed calculation, perhaps will do it next weekend and come back. Stay on.

The question of course had more to do with diesel engines in the first place.

Piston may have higher mass but crank has lower mass because the throw is shorter. Note piston is aluminum and crank is cast-iron.

I considered that in my simple model

While the throw is shorter, the crank has to endure higher compressive and tensile stresses and hence have to be stronger. If same materials are used, this means roughly the same proportion of increase in cross-section area as decrease in length - i.e. the weight remains about the same.


Anyway, I did a quick search, it appears I asked the same question a year ago and there is an entire thread on it - silly me for forgetting. I'll go through it again.

If the throw is shorter, then the lever arm of the crank throw is shorter and therefore the bending moment and the resulting stresses are lower!

Secondly since the displacement is the same, the force on the piston is the same so even though the dia is larger the sections can be made smaller minimizing the weight gain.