Cylinders, Torque, and RPM query?

[snerkler]

Why is it that engines with fewer cylinders produce more grunt/low down torque, but can't rev as high as engines with 4 cylinders?

 

[(RIP) Mark H]

I give up ....Why ?

 

[snerkler]

I give up ....Why ?

Dunno, that's why I'm asking

 

[Spout]

Big cylinder = lots of petrol/air = big bang.

Big piston = big weight = slow piston speed.

Roughly

 

[cookie]

probably has something to do with piston speed

twins also tend to have bigger, heavier flywheels.

 

[snerkler]

Big cylinder = lots of petrol/air = big bang.

Big piston = big weight = slow piston speed.

Roughly

Thanks. So you're saying that the fewer the cylinders the bigger the cylinders/pistons have to be?
So in theory if you had a 4 cylinder engine with the same size pistons as a single cylinder bike would it produce massive low down torque ie 4 times as much as a single cylinder?

 

[DavidJames]

Thanks. So you're saying that the fewer the cylinders the bigger the cylinders/pistons have to be?
So in theory if you had a 4 cylinder engine with the same size pistons as a single cylinder bike would it produce massive low down torque ie 4 times as much as a single cylinder?

and you would also increase the capacity of the enginer 4 times

 

[MalcolmA]

Thanks. So you're saying that the fewer the cylinders the bigger the cylinders/pistons have to be?
So in theory if you had a 4 cylinder engine with the same size pistons as a single cylinder bike would it produce massive low down torque ie 4 times as much as a single cylinder?

No because there is no need of a heavy flywheel cos the engine is more in balance.

 

[autogs]

Thanks. So you're saying that the fewer the cylinders the bigger the cylinders/pistons have to be?
So in theory if you had a 4 cylinder engine with the same size pistons as a single cylinder bike would it produce massive low down torque ie 4 times as much as a single cylinder?

you didnt read that book i suggested did you

 

[snerkler]

you didnt read that book i suggested did you

You got me
Been Internet searching but can't find all the answers I want. Maybe I'll have to bite the bullet and spend a bit of cash
Would you be kind enough to remind me of the title of the book again please?

 

[Deleted account 190920001]

fewer cylinders for the same engine size usually means a longer stroke. A longer stroke puts more leverage on the crank so you get more torque. Think of you as the piston, the conrod as a lever and the crank as a pivot. If you get a longer lever (longer stroke) you will be able to pry a larger object at the end of the lever because you have more torque. Also because of the longer stroke the piston has further to travel but in the same time so the piston speed has to increase. You can only move a piston so fast without it being damaged to the net result is you have to keep the engine revs lower.
You get less power basically because you cant get as much fuel/air in as you can with more cylinders. There isnt the same space to get the same area of valves in as you have with 4 cylinders. Same reason that 2 valve engines make less power than 4 valve engines.

 

[autogs]

You got me
Been Internet searching but can't find all the answers I want. Maybe I'll have to bite the bullet and spend a bit of cash
Would you be kind enough to remind me of the title of the book again please?

fundamentals of motor vehicle tecnology,if you really want i will dig my copy out at the weekend.

 

[snerkler]

fewer cylinders for the same engine size usually means a longer stroke. A longer stroke puts more leverage on the crank so you get more torque. Think of you as the piston, the conrod as a lever and the crank as a pivot. If you get a longer lever (longer stroke) you will be able to pry a larger object at the end of the lever because you have more torque. Also because of the longer stroke the piston has further to travel but in the same time so the piston speed has to increase. You can only move a piston so fast without it being damaged to the net result is you have to keep the engine revs lower.
You get less power basically because you cant get as much fuel/air in as you can with more cylinders. There isnt the same space to get the same area of valves in as you have with 4 cylinders. Same reason that 2 valve engines make less power than 4 valve engines.

I thought I read somewhere that increased torque isn't anything to do with length of stroke these days, and that was more with the older engines?

 

[Sgt Bilco]

Don't know that answer but in the truck world, we always say there is no sustitute for cubes

Big old high capacity, slow revving diesels make mountains of torque where the more modern lower capacity, highly tuned engines produce higher BHP but less torque. Much better on fuel and emissions though

 

[oldchris]

Hi,

Think of it like a small twig and a tree trunk both swinging on a rope. For the same amount of power (fuel) you can move the twig a lot faster, but you could probably put your hand out and stop it, (stall). once the tree trunk is swinging if you stood in front of it would knock you over, (torque).

For the same amount of power you could get 4 twigs swinging at a fast rate quicker than you could a trunk, (acceleration). Also, remember a piston has to stop and change direction at the end of every stroke, with a trunk this is going to take a lot more effort, (vibration), so better to keep the revs lower.

Hope this helps more than it confuses.

 

[Giles]

.......... which (following on from your clutch / slipping / off roading queerie) is why singles make good off roaders (and dumper trucks..!!). Low down torque, difficult to stall, they keep thump thump thumping away .....

 

[Giles]

Your next project is to find out about V fours and why they create such wonderful mid range power .......

 

[Deleted account 190920001]

What makes you think a 1200 air cooled boxer engine is a modern engine?

 

[snerkler]

Don't know that answer but in the truck world, we always say there is no sustitute for cubes

Big old high capacity, slow revving diesels make mountains of torque where the more modern lower capacity, highly tuned engines produce higher BHP but less torque. Much better on fuel and emissions though

Nope, there's no substitution for cc. but in theory you can have a twin cylinder with the same cc as an eight cylinder, and the twin will produce more low down torque.

From what I can gather, older engines relied on longer strokes, therefore longer 'levers' and thus more torque. From what I can gather now it's more to do with the size (ie diameter) of the pistons, and therefore the bigger the pistons the more of a 'bang' for each firing and thus more torque. Does anyone know to the contrary? The reason they can't rev as quickly is due to the size and inertia.

 

[snerkler]

Hi,

Think of it like a small twig and a tree trunk both swinging on a rope. For the same amount of power (fuel) you can move the twig a lot faster, but you could probably put your hand out and stop it, (stall). once the tree trunk is swinging if you stood in front of it would knock you over, (torque).

For the same amount of power you could get 4 twigs swinging at a fast rate quicker than you could a trunk, (acceleration). Also, remember a piston has to stop and change direction at the end of every stroke, with a trunk this is going to take a lot more effort, (vibration), so better to keep the revs lower.

Hope this helps more than it confuses.

Yeah thanks, I think I just said something with the same pricinple above