Cylinders, Torque, and RPM query?

[Engineer]

Not strictly true, if more fuel meant more torque then my old for Granada would outpace my Audi but it doesn't

as for 4 cylinders putting out the same torque as 1 of the same capacity that only applies to the theory, it don't reflect reality - a bit like the bumble bee who cant theoretically fly.

The general restriction in speed isn't about piston size, it's about the weight of the piston and the load that this puts on the con rod bearings, combustion also has an effect on this and a bigger piston will generally allow for a bigger bang.

A smaller piston in the same material will be much lighter (and will have a shorter stoke and a smaller bang) and can therefore run at higher revs without having a massive crankshaft that also slows the increase in revs.

4x 300cc = 1200 cc the same as 2 x 600cc.

Agree, but I did say provided they both burn the fuel efficiently - the old Grandad might have been a bit lacking in that department. Yes accelerating and de-accelerating large lumps of metal restricts the speed at which you can spin a large capacity single or twin.

 

[Engineer]

Whilst this makes sense in theory, one of my questions was why singles and twins produce more low down grunt. From reading the various answers it would appear that it is due to the pistons in singles and twins giving a bigger 'bang'. But if bigger pistons have a bigger bang how can a four cylinder produce the same torque as a twin of the same cc, albeit further up the rev range? Surely a bigger piston always produces a bigger bang? Or is it a similar reason as why they can't rev as high ie inertia and weight of the pistons?
I really need to buy that book

A bigger piston (cylinder) does give a bigger bang, provided you fill it and burn the fuel efficiently. Four smaller pistons pushing around the crankshaft can give the same torque if they are buring the same amount of fuel as efficiently.

 

[Steptoe]

, but can't rev as high as engines with 4 cylinders?

Maximum piston speed.

 

[Deleted account 190920001]

Rasher

Couple of routes you can take, you can do as per Volkswagen and use both a supercharger and a turbo or you can go for a variable vane turbo. Both of which give you loads of torque from virtual tickover. Again it depends on how the other parts of the engine are put together as to how it works. Superchargers are good but they are very inefficient. It takes a lot of energy just to run them where a turbo is pretty much free to run. Now that variable vane has more or less been sorted on petrol engines (been on diesels for years) they are the better bet. Once the prices have come down they will be on the common cars and not just the Porches of the world.

 

[snerkler]

My favourite engine for delivery is actually the Triumph triples, a great mix of power and torque and I think a great comprimise between twin cylinder grunt and four cylinder smoothness, if they ever get the comfort / ergonomics and handling to match the GS as an allrounder I would be trading in immediately.

I swapped from the 1050 Tiger to the GSA and I have to say that although the triple is a great engine it lacks character compared to a Boxer Twin imo. Character is a very difficult thing to describe, and I'm sure it will be different for each individual, but I would rather have a less powerful, and lumpier engine wit a lot of character than one with more HP, is smoother but has no character. I've found that if I've had a bike with plenty of HP and is smooth but bland, once I get over the initial thrill of being able to accelerate fast and reach silly speeds I find there's no real 'substance' and get bored and ultimately want to change the bike.
The GSA is probably the slowest bike I've ever owned, and the lumpiest, but it also has the most character, and is also only the 2nd bike I've owned that my 'bond' with it has grown stronger the more I ride it rather than weaker

 

[snerkler]

After looking into torque curves etc I have to say that I'm quite surprised at how uneven the torque curve is on the GS/GSA

Obviously Im only just learning about this stuff and don't know how difficult it is to produce a smooth curve, but if you compare it to my car below it's miles behind. This is smooth, reaches peak torque lower down the range, and the trail off is negligible until right near the top of the rev range. I guess this is the trouble with using such an old engine in the GS/GSA

 

[Engineer]

After looking into torque curves etc I have to say that I'm quite surprised at how uneven the torque curve is on the GS/GSA

Obviously Im only just learning about this stuff and don't know how difficult it is to produce a smooth curve, but if you compare it to my car below it's miles behind. This is smooth, reaches peak torque lower down the range, and the trail off is negligible until right near the top of the rev range. I guess this is the trouble with using such an old engine in the GS/GSA

The GS torque is all over the place - you can easily feel it too if you play with the throttle at different revs and in different gears. I guess that it is very difficult to accurately fuel and efficiently burn the fuel in a large cylinder over a wide rev range, whilst maintaining emissions and noise regulations.

I would have thought that lots of small cylinders are likely to give a smoother torque curve than a couple of big ones - bet a V12 has a lovely smooth curve.

The best place to get a first understanding about torque is go to to Wiki - but you really have to get your head around some maths if you want a clear understanding.

 

[snerkler]

The GS torque is all over the place - you can easily feel it too if you play with the throttle at different revs and in different gears.

I can certainly feel the drop between 500 and 6000rpm

I guess that it is very difficult to accurately fuel and efficiently burn the fuel in a large cylinder over a wide rev range, whilst maintaining emissions and noise regulations.

Bloody emissions regs , and noise for that matter. Is it just me that thinks they've gone OTT with this nowadays?

I would have thought that lots of small cylinders are likely to give a smoother torque curve than a couple of big ones - bet a V12 has a lovely smooth curve.

I knew they were smotther running, I didn't realise they generally produce smoother torque curves as well

The best place to get a first understanding about torque is go to to Wiki - but you really have to get your head around some maths if you want a clear understanding.

I think I pretty much fully understand torque now, but what I don't understand is how the different aspects of the engine affect the torque as the revs get higher, such as cams, and valve timing, inertia etc. In my 'idealistic' head I just assumed that with all the electronic gizmos now they'd be able to advance/retard timing constantly so that the max combustion and max efficiency of fuel burning was acheived at all times, therefore producing a nice linear line up to max torque and then never dropping off. I obviously realise how naive this was, and how complicated it is to acheive the perfect curve.
On recomendation I've bought the fundamentals of auto vehicle technology so hopefully that'll help me understand all of this much better

 

[Rasher]

The GS has by far the lumpiest delivery I have ever seen, I have had most my previous bikes on a dyno and most have had pretty linear curves, the odd waver here and there but the GS ones are proper up and down.

I think most is down to mega lean running to get them through emmisions, someone posted some from a remap a while back and they were much better (still crazy compared to any JAP / Italian Bike) across the range.

 

[snerkler]

The GS has by far the lumpiest delivery I have ever seen, I have had most my previous bikes on a dyno and most have had pretty linear curves, the odd waver here and there but the GS ones are proper up and down.

I think most is down to mega lean running to get them through emmisions, someone posted some from a remap a while back and they were much better (still crazy compared to any JAP / Italian Bike) across the range.

Yet the GSA is arguably the best bike I've ever owned IMO. It's a toss up between this and the K6 Gixxer thou.

 

[snerkler]

So I've been reading the "fundamentals of motor vehicle technology" by Hillier, what a great book, thanks for the recommendation Autogs

The trouble is I still can't get my head around why singles and twins produce more low down torque?
Comparing a 600cc single and a 600cc four cylinder, I understand that the single cylinder is roughly 4 times the size of the 4 cylinder and so for each firing the single cylinder will give a bigger bang and so more torque per fire, for simplicity terms we'll say 4 times as much. However, the four cylinder will fire 4 times as many times and so the overall torque for a given time will be equal will it not?
After reading the book it appears as though you can tune and engine to have whatever characteristics you want (within reason), ie you can shift the torque curve up and down the rev range depending on what characteristics you want.
I understand why you have to use higher cylinder engines for high end performance as singles can't rev high enough. But why don't they use four cylinder engines in off-road bikes and just tune the engine for low down power and sacrifice top end? They'll be smoother and give a more consistant torque/power delivery?
Is it a size and weight issue ie the single cylinder is smaller and lighter, or have I missed something and singles will always produce more low down grunt, perhaps due to the size and inertia of the flywheel?

 

[Engineer]

So I've been reading the "fundamentals of motor vehicle technology" by Hillier, what a great book, thanks for the recommendation Autogs

The trouble is I still can't get my head around why singles and twins produce more low down torque?
Comparing a 600cc single and a 600cc four cylinder, I understand that the single cylinder is roughly 4 times the size of the 4 cylinder and so for each firing the single cylinder will give a bigger bang and so more torque per fire, for simplicity terms we'll say 4 times as much. However, the four cylinder will fire 4 times as many times and so the overall torque for a given time will be equal will it not?
After reading the book it appears as though you can tune and engine to have whatever characteristics you want (within reason), ie you can shift the torque curve up and down the rev range depending on what characteristics you want.
I understand why you have to use higher cylinder engines for high end performance as singles can't rev high enough. But why don't they use four cylinder engines in off-road bikes and just tune the engine for low down power and sacrifice top end? They'll be smoother and give a more consistant torque/power delivery?
Is it a size and weight issue ie the single cylinder is smaller and lighter, or have I missed something and singles will always produce more low down grunt, perhaps due to the size and inertia of the flywheel?

Torque is not measured over time - that is power.

'Grunt' is not an SI unit ;-)

4-cylinder engines are too big for off-road bikes.

 

[snerkler]

Torque is not measured over time - that is power.

Sorry I've mislead you there, and used bad terminology
What I meant by time was 'time' it takes for a full 4 stroke cycle.
For arguements sake lets say a 600cc single cylinder produces 80lbft torque. If you had 4 cylinder 600cc bike would each cylinder not produce 20lbft torque?
As the single cylinder only has one power stroke per 4 stroke cycle the max torque it can produce is 80lbft per cycle. The 4 cylinder has 4 power strokes in this same period, therefore producing the same 80lbft torque (4 x 20lbft).
Therefore both are producing the same torque per cycle

'Grunt' is not an SI unit ;-)

.

No, but it would be a good one to add

 

[karlp]

Sorry I've mislead you there, and used bad terminology
What I meant by time was 'time' it takes for a full 4 stroke cycle.
For arguements sake lets say a 600cc single cylinder produces 80lbft torque. If you had 4 cylinder 600cc bike would each cylinder not produce 20lbft torque?
As the single cylinder only has one power stroke per 4 stroke cycle the max torque it can produce is 80lbft per cycle. The 4 cylinder has 4 power strokes in this same period, therefore producing the same 80lbft torque (4 x 20lbft).
Therefore both are producing the same torque per cycle



No, but it would be a good one to add

But you have twice the number of conrods to move.
Twice the number of valves to actuate.

 

[Mightytharg]

4-cylinder engines are too big for off-road bikes.

Surely you remember the FZ750 based Yamaha Paris Dakar bikes - bored out to 900cc(I think)?

Not their best idea maybe and I don't think they offered a low seat option.

 

[snerkler]

But you have twice the number of conrods to move.
Twice the number of valves to actuate.

So does this mean that torque is reduced then?

So in my explanation above if a single cylinder 600cc engine produces 80lbft of torque, a 4 cylinder 600cc engine would not produce 80lbft as it would be (4 x 20lbft) minus force used to move the extra components? So for a given engine size a 4 cylinder engine might only produce X% (where x could be 75, 80, 90 etc) of the torque of the equivalent single?

 

[Deleted account 190920001]

Many reasons not to use a 4 cylinder engine in an off road bike. Size, weight being the obvious but also cost. Other reasons would be traction. 1 cylinder firing gives the tyre time to recover grip between firings (once every 2 revolutions) where a 4 fires 2 times. Go back a good few years to 500GP racing and the big bang engines. So called because they fired 2 cylinders at the same time effectively working as a twin. When you accelerated a normal engine out a corner the tyre spun up to the point that only a few riders could effectively drive them to the potential. With the big bang engines wheel spin was reduced a lot so the bike was easier to ride harder without high siding the beast. Same goes for off road bikes riding on mud and loose dirt. You could reconfigure a multi engine as the did in the GP but you get back to the size and weight issue.

You are correct, a 4 will make less torque and power for a given amount of fuel than a single because of mechanical losses. But put more fuel in and it will produce more of everything. That's another reason why single cylinder engines are used so much i.e economy. If the bike isn't designed to rip up the road and do 200mph but instead is a small economical commuter then a single wins every time. Why stick at a 4 lets have a nice smooth 6 or what about a V12? See where I am going?

Horses for courses, you didn't see any Clydesdale horses running the national at the weekend did you? Stick a plough behind a race horse though and it wont be able to pull it. That said the one I backed must have been a donkey in a horse skin!!!!!

 

[Steptoe]

I understand why you have to use higher cylinder engines for high end performance as singles can't rev high enough.

You can have a high revving multi and a slow revving single but the piston speed can be the same....

 

[snerkler]

Many reasons not to use a 4 cylinder engine in an off road bike. Size, weight being the obvious but also cost. Other reasons would be traction. 1 cylinder firing gives the tyre time to recover grip between firings (once every 2 revolutions) where a 4 fires 2 times. Go back a good few years to 500GP racing and the big bang engines. So called because they fired 2 cylinders at the same time effectively working as a twin. When you accelerated a normal engine out a corner the tyre spun up to the point that only a few riders could effectively drive them to the potential. With the big bang engines wheel spin was reduced a lot so the bike was easier to ride harder without high siding the beast. Same goes for off road bikes riding on mud and loose dirt. You could reconfigure a multi engine as the did in the GP but you get back to the size and weight issue.

You are correct, a 4 will make less torque and power for a given amount of fuel than a single because of mechanical losses. But put more fuel in and it will produce more of everything. That's another reason why single cylinder engines are used so much i.e economy. If the bike isn't designed to rip up the road and do 200mph but instead is a small economical commuter then a single wins every time. Why stick at a 4 lets have a nice smooth 6 or what about a V12? See where I am going?

Horses for courses, you didn't see any Clydesdale horses running the national at the weekend did you? Stick a plough behind a race horse though and it wont be able to pull it. That said the one I backed must have been a donkey in a horse skin!!!!!

Thanks for the input. When you say put more fuel into an engine with more cylinders and it make more of everything, to do this I assume you have to increase engine capacity?
With regards to the smooth 6 cylinder statement, BMW have just done this haven't they, and it's apparantly a wonderful engine A bit weighty though I'd have thought.

 

[snerkler]

You can have a high revving multi and a slow revving single but the piston speed can be the same....

How's that? Engine speed (RPM) refers to the speed of the rotation of the crankshaft. As all pistons are connected to the cranshaft surely piston speed increases as crankshaft speed increases? Is it due to the fact that for every rotation on the crankshaft the single piston moves more due to the stroke length and throw so for any given RPM the piston in a single will be moving quicker than that of an equivalent multicylinder?