"Long stroke" engines?

[clockworks]

Back in the day, I seem to remember that "performance" engines seemed to be square, or short stroke (bore same as or bigger than than the stroke). Looking at modern designs, many seem to be long stroke.

What's the reason for having a smaller bore, longer stroke, on modern turbo engines? The 2 litre turbo in the current BMW 135i has a bore of 82mm, stroke is 95mm, for instance. The 1.2 PSA 3 cylinder engine is 75mm bore, 90mm stroke.
Efficiency, torque, something else?

[Draxindustries1]

It's for increased fuel efficiency , torque and in turn lower emissions. Increased stress on the crankshaft though..

[Skyedriver]

"Over-square" I believe was the term and they revved a lot easier and faster. Long stoke engines such as the 1098cc A-series were not noted for their power outputs compared to the 998/970 engines. Of course there were other factors involved.

[Greenbot35]

I think many English cars used to be taxed by cylinder bore size so long stroke was the only way to Increase capacity.

Long stroke also works well for turbocharged engines.

[Harrison Bergeron]

SKYACTIV and 50mpg in my miata

[tendown]

A longer stroke (and therefore smaller bore) leads to a less flattened disc combustion chamber and more towards the ideal spherical shape. This is better for heat loss, burn duration.... efficiency.

NA engines compromised on this shape to fit large valves in the bigger bore, and to avoid high piston speeds of the longer stroke. With lower revving boosted engines where efficiency is more critical, strokes get longer.

[stevieturbo]

clockworks said:

Back in the day, I seem to remember that "performance" engines seemed to be square, or short stroke (bore same as or bigger than than the stroke). Looking at modern designs, many seem to be long stroke.

What's the reason for having a smaller bore, longer stroke, on modern turbo engines? The 2 litre turbo in the current BMW 135i has a bore of 82mm, stroke is 95mm, for instance. The 1.2 PSA 3 cylinder engine is 75mm bore, 90mm stroke.
Efficiency, torque, something else?

They are all different...some are longer than others, and some have larger bores than others. They just do.

They do not all have longer stroke and smaller bore.

[clockworks]

Looking at the various BMW 6 cylinder petrol engines, each new generation had a longer stroke/smaller bore. Improved efficiency makes sense.

I guess inlet valves can be smaller with forced induction, and may even help efficiency by increasing flow speeds for the same volume of air/fuel?

[LimaDelta]

Longer stroke means a longer crank throw. The crank is essentially a lever arm so the longer you can make it the more torque is applied for a particular force. The long stroke also means there is more time for complete combustion, so better for emissions.

In very basic terms, long stroke, low revving, high torque, slow throttle response. Short stroke, high revving, low torque, fast throttle response. Different designs for different applications.

[littleredrooster]

If you want to play with the variables in an engine, there's a good resource Here

[AW111]

LimaDelta said:

Longer stroke means a longer crank throw. The crank is essentially a lever arm so the longer you can make it the more torque is applied for a particular force. The long stroke also means there is more time for complete combustion, so better for emissions.

In very basic terms, long stroke, low revving, high torque, slow throttle response. Short stroke, high revving, low torque, fast throttle response. Different designs for different applications.

Check your physics.
Long stroke = longer crank throw. Correct.

BUT: for the same capacity, a longer stroke means a smaller bore, so less piston area. For a given cylinder pressure (BMEP), the force on the piston is proportional to the surface area of the piston. So what you gain in crank leverage is balanced by what you lose in downward force on the piston.

[LimaDelta]

AW111 said:

LimaDelta said:

Longer stroke means a longer crank throw. The crank is essentially a lever arm so the longer you can make it the more torque is applied for a particular force. The long stroke also means there is more time for complete combustion, so better for emissions.

In very basic terms, long stroke, low revving, high torque, slow throttle response. Short stroke, high revving, low torque, fast throttle response. Different designs for different applications.

Check your physics.
Long stroke = longer crank throw. Correct.

BUT: for the same capacity, a longer stroke means a smaller bore, so less piston area. For a given cylinder pressure (BMEP), the force on the piston is proportional to the surface area of the piston. So what you gain in crank leverage is balanced by what you lose in downward force on the piston.

I didn't make any assumptions about same capacity, I said same force. Perhaps I could have been clearer in my explanation.

[Fiesta1.0L]

Casting my mind back to university days there were some limits on the velocity/acceleration of the piston. At the time I ran numbers for race engines (short stroke) and road engines (in those days no turbo=long stroke to improve torque) the numbers were remarkably similar for both with a pretty consistent max velocity of the piston. The short stroke race engines revved faster so could have more power cycles per second, but the max piston speed in the bore was very consistent.

Time has moved on with materials for piston rings, cranks, pistons and lubricants, so i'd be interested to see if that still holds true.
I'll try and dig my notes out.

[SystemOfAFrown]

LimaDelta said:

I didn't make any assumptions about same capacity, I said same force. Perhaps I could have been clearer in my explanation.

If you don't consider displacement then the concept of bore/stroke ratio is meaningless. You could get more force from a larger bore with the same or even a smaller stroke.

A long stroke/smaller bore design means there is less surface in the combustion chamber to absorb heat, and also means the mixture takes less time to burn due to the shorter distance the flame front has to travel.

Edited by SystemOfAFrown on Wednesday 26th October 14:02

[Evoluzione]

It's because the turbo spools earlier.

[nebpor]

Fiesta1.0L said:

Casting my mind back to university days there were some limits on the velocity/acceleration of the piston. At the time I ran numbers for race engines (short stroke) and road engines (in those days no turbo=long stroke to improve torque) the numbers were remarkably similar for both with a pretty consistent max velocity of the piston. The short stroke race engines revved faster so could have more power cycles per second, but the max piston speed in the bore was very consistent.

Time has moved on with materials for piston rings, cranks, pistons and lubricants, so i'd be interested to see if that still holds true.
I'll try and dig my notes out.

The B18C6 in the Integra type r had a longer stroke and Honda said it had the fastest production piston velocity, safely revving to 8400rpm in 1996 - they said nearly all the effort was in controlling vibration at those revs so it would be reliable

[tapkaJohnD]

Fiesta1.0L said:

Casting my mind back to university days there were some limits on the velocity/acceleration of the piston. At the time I ran numbers for race engines (short stroke) and road engines (in those days no turbo=long stroke to improve torque) the numbers were remarkably similar for both with a pretty consistent max velocity of the piston. The short stroke race engines revved faster so could have more power cycles per second, but the max piston speed in the bore was very consistent.

Time has moved on with materials for piston rings, cranks, pistons and lubricants, so i'd be interested to see if that still holds true.
I'll try and dig my notes out.

Are you thinking of "Mean Piston Velocity", Fiesta? The acceleration of the piston being more complex to calculate, this simple sum (Stroke/time) is a surrogate. If MPV is over 20 meters/sec then as you say more exotic materials may be required for reliability.

John

[Zener]

nebpor said:

Fiesta1.0L said:

Casting my mind back to university days there were some limits on the velocity/acceleration of the piston. At the time I ran numbers for race engines (short stroke) and road engines (in those days no turbo=long stroke to improve torque) the numbers were remarkably similar for both with a pretty consistent max velocity of the piston. The short stroke race engines revved faster so could have more power cycles per second, but the max piston speed in the bore was very consistent.

Time has moved on with materials for piston rings, cranks, pistons and lubricants, so i'd be interested to see if that still holds true.
I'll try and dig my notes out.

The B18C6 in the Integra type r had a longer stroke and Honda said it had the fastest production piston velocity, safely revving to 8400rpm in 1996 - they said nearly all the effort was in controlling vibration at those revs so it would be reliable

They also cracked the vibration issue when they started making muli cylinder racing bikes (RC series) along with 4 valve head benefits in the early 60's Honda are noted for being big time into piston speeds since way way back

[nebpor]

Zener said:

They also cracked the vibration issue when they started making muli cylinder racing bikes (RC series) along with 4 valve head benefits in the early 60's Honda are noted for being big time into piston speeds since way way back

Thanks for that, I forgot about the bike heritage!!

[kylos27]

Try to buy a vehicle with under square engine , you won’t need to rev the ar5e out of it -let the torque do the work,
Oversquare engines you to ring their necks and waste fuel

hope this helps