Cylinders, Torque, and RPM query?

[Wapping]

How's that?

Put simply.

Piston speed is a product of two parts:

(1) Stroke (the motion of the piston up and its subsequent motion back downwards)

(2) RPM

You could play around with the two factors to produce all sorts of numbers.

 

[Engineer]

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?

Yes - the piston in a long-stroke single has to travel further for a given RPM so its speed is higher that a shorter stroke engine, that could be running at a higher RPM

The piston will have a maxmilum safe speed that it can travel at before the engine starts to tear itself apart!

 

[snerkler]

Thanks for all the help guys

 

[Rasher]

I remember back in the late 90's Ducati WSB machines were achieving higher piston velocities than F1 cars! (Long stroke + High Revs)

Very impressive for a "road based" engine. (Albeit it with a 300mile life span)

 

[Wapping]

The clever bit is not so much the speed of the pistons. It is the speed and accuracy of the valve timing.

 

[snerkler]

I remember back in the late 90's Ducati WSB machines were achieving higher piston velocities than F1 cars! (Long stroke + High Revs)

Very impressive for a "road based" engine. (Albeit it with a 300mile life span)

That is impressive
It must still be relatively quick in the 1198 revving to nearly 11,000?

 

[snerkler]

1 cylinder firing gives the tyre time to recover grip between firings (once every 2 revolutions) where a 4 fires 2 times. !

Just re-read this, doesn't a 4 cylinder fire twice every revolution, so four times for every 2 revolutions (or one full cycle)? A cycle is 720 degrees (2 revolutions), and a four cylinder fires every 180 degrees, a 6 cylinder every 120 degrees and an 8 cylinder every 90 degrees.

 

[Rasher]

Assuming a 180 degree crank yes, are we about to enter a discussion on big-bang engines now, or how valvetrain copes (and the Ducati desmo system vs springs)

 

[snerkler]

Assuming a 180 degree crank yes, are we about to enter a discussion on big-bang engines now, or how valvetrain copes (and the Ducati desmo system vs springs)

Haven't moved onto that area yet, but I'm sure when I do I'll have a few questions

 

[Deleted account 190920001]

the more pistons you have the more valve area you can get so the more fuel air you can get in (its called higher volumetric efficiency) so a 4 cylinder will naturally take more fuel unless you throttle it of course. With this it would make more power than a single and get over the extra mechanical loses but at the cost of fuel consumption.
An interesting engine that shows this point was the Honda NR 750 engine. Racing rules said a maximum of 4 cylinders and Honda couldn't get it to rev high enough to get enough air in so they made a V4 cylinder engine with oval pistons. Each piston had 2 conrods and above it were 8 valves. Effectively it ran like a straight plane crank V8 engine (same as Ferrari use) but of course only had 4 cylinders. It was a master piece of engineering but was hugely expensive to machine and had poor piston ring sealing. http://en.wikipedia.org/wiki/Honda_NR

I can see this going of at a tangent with a whole manner of weird and wonderful engine configurations but its all good stuff.

 

[snerkler]

the more pistons you have the more valve area you can get so the more fuel air you can get in (its called higher volumetric efficiency) so a 4 cylinder will naturally take more fuel unless you throttle it of course. .

Now you've gone and confuse me all over again .

I assume when you say getting more fuel/air in your talking without necessarily increasing the size of the cylinder. If so this would give a relatively bigger bang would it not?
Therefore if you have a 4 cylinder giving 4 relatively bigger bangs per cycle, would the sum of this not be bigger than the bang of a single cylinder of the same overal capacity within a cycle. Therefore on this basis a 4 cylinder should give more torque
Now if the throw of the single was bigger and therefore a bigger lever I could understand that the single could produce more torque (torque = force x distance), but I'm sure I read somewhere (maybe even on here) that long levers were an 'out of date' design and larger cylinders are now wider rather than longer and given the term square (stroke = bore) or undersquare (stroke < bore)?

 

[Deleted account 190920001]

a four can breath more than a single for the same CC. For the same reason that 4 valves flow more than 2, 16 in a 4 cylinder engine flow more than 4 in a single. Yes it will give a bigger bang but generally not at low revs. Couple of reasons for this and your really getting fairly deep into engine dynamics here. Better breathing engines have lower air velocity which means the fuel can drop out of the mixture and it doesn't burn as well. Put some revs on and it all stays where it should and you get better combustion. This is partly why race engines run like crap at low revs. Think of the air speed of 100CFM going through a tunnel to a garden hose, extreme but you will get the idea. This has improved a lot with fuel injection but it still applies. The other reason is that fours are configured for high power, high revs so the cam doesn't work well at low revs. You could of course configure a four to give low down torque. Take a 1300cc corsa engine and a 1300cc Hayabusa. Both 1300cc 16 valve engines but very very different characteristics. Corsa being a car and heavy needs torque low down to be drivable. Put a Busa engine in a corsa and you would burn the clutch out getting it to move all the time.

 

[Rasher]

Put a Busa engine in a corsa and you would burn the clutch out getting it to move all the time.

You obviously have not seen the car stunt show at Disney Florida, they have small hatchbacks with Busa engines in for the stunt driving, see also the Smart Diablo... but I get the point.

I loved the NR stuff Honda did, they raced an Endurance version in the late 80's, back then ZXR750RRRRRRR'RR's and suchlike made about 135BHP from 11k-12k or something like that, the NR750 racer had between 135bhp and 145 bhp from 8k-15k - a 7,000rpm power band, totally awesome.

The problem was although it broke no rules, Honda were told if they raced it again (it broke down in the race whilst making every other bike look decidedly slow) it would get banned - the other manufacturers claimed it was really a V8.

 

[snerkler]

a four can breath more than a single for the same CC. For the same reason that 4 valves flow more than 2, 16 in a 4 cylinder engine flow more than 4 in a single. Yes it will give a bigger bang but generally not at low revs. Couple of reasons for this and your really getting fairly deep into engine dynamics here. Better breathing engines have lower air velocity which means the fuel can drop out of the mixture and it doesn't burn as well. Put some revs on and it all stays where it should and you get better combustion. This is partly why race engines run like crap at low revs. Think of the air speed of 100CFM going through a tunnel to a garden hose, extreme but you will get the idea. This has improved a lot with fuel injection but it still applies. The other reason is that fours are configured for high power, high revs so the cam doesn't work well at low revs. You could of course configure a four to give low down torque. Take a 1300cc corsa engine and a 1300cc Hayabusa. Both 1300cc 16 valve engines but very very different characteristics. Corsa being a car and heavy needs torque low down to be drivable. Put a Busa engine in a corsa and you would burn the clutch out getting it to move all the time.

Thanks for this. This is more of the kind of explanation I was looking for. Using my fairly decent knowledge of physics it didn't make sense that one big bang makes more torque that 4 bangs of 1/4 size, assuming lever size is similar, as seems to be more the case these days.
Is it therefore fair to say that if tuned in that way multi cylinders can give just as much low down grunt as a cylinder of the same cc (assuming the extra valves negate the extra friction of the extra components)?
This then begs the question again as to why not use a four cylinder engine tuned for low down torque in offroaders and bikes like the GS and Harley's?
For offroaders I would guess a lot of it is due to size and weight? I would also say cost, but then racing offroaders are still singles and cost won't be too much of an issue here. Also someone said already that having one bang per 2 crank rotations helps with traction compared to 4 bangs.
But why use Twins in bikes such as a GS and Harleys. Although twins can be tuned to rev relatively high, and 4's tuned to give low down torque, do twins naturally lend themselves to lower down grunt, and 4's to higher revving and therefore it's an ease of manufacturing issue?
Or could it be that although the idea of long levers is old hat, the stroke/lever of a single or twin will still be more than that of a 4?
You mention that we're getting deep into engine dynamics, is there a book you could recommend on this subject as I would like to learn more about it?

 

[snerkler]

So after re-reading all of the posts and doing a lot more research it seems as though there are a number of myths regarding the number of cylinders, and so the replies to my initial question don't seem to add up.

Why do single and twin engines produce more low down torque than 4’s and 6’s?

A lot of people say it’s due to a bigger bang and bigger lever, but this doesn’t make sense as a four cylinder can make the same max torque as a twin of the same capacity. For example a Flat twin BMW GS/GSA produces a max torque of 120Nm, and a four cylinder Honda VRF1200 produces 129Nm. Taking into account the extra 70cc displacement of the Honda the 2 bikes would produce exactly the same torque. The BMW has cylinder size of 101x73mm, and the Honda 81x60mm, therefore the BMW has a bigger bang and longer lever. So the theory that singles and twins produce more torque due to a bigger bang and longer stroke goes out of the window. Also, if you look at the Super Tenere (a twin) which again has a bigger bore and stroke than the Honda and a longer stroke than the BMW, this actually produces less torque than both. However, the compression in the Ten is only 11:1 compared to 12:1 of the other 2 bikes.
So from this evidence, plus from research on the net it shows that the statement that singles and twins produce more torque than multicylinders is actually a myth.
So let’s look at the other statement. Manufacturers use singles and twins as they produce more low down torque/grunt, this is why off-road bikes use singles, and companies like Harley and BMW use twins. Well if we compare the 2010 BMW GS boxer twin 1200cc to say a Vauxhall Corsa 4 cylinder 1200cc, the BMW’s torque does not peak until 6000RPM (120Nm), whereas the Corsa’s torque peaks at 4000RPM (115Nm), so the statement that fewer cylinder engines produce more low down torque is again a myth.
So this begs the question why companies like BMW and Harley use twins as it appears that there are no benefits over 4 cylinders as 4 cylinder engines can produce just as much torque for the same displacement, and can be tuned to have this torque ‘kick in’ earlier in the rev range? The only answer that I get is that twins produce more low down torque/grunt, which from this example doesn’t ring true. I can understand why off-roaders use singles due to size and weight issues, but size and weight isn’t necessarily an issue for BMW and Harley. As the 4 cylinder can have the same advantage as a twin (ie same max torque and low down torque) but has a smoother and more consistent delivery I can’t see any benefit in Twins? I don’t think it’s fuel consumption either as four cylinder cars carrying much more weight can have better consumption than twin bikes. Nor do I think it’s cost as a BMW GS (twin) with ABS is only just under the price of a Honda VRF1200 (4 cylinder) (which has ABS), whilst the BMW GSA (twin) with ABS and Yamaha Super Tenere (twin) are more expensive that the Honda, the Ten being over £1000 more.
So based on the info and evidence above can someone please explain:-
1) Why people say that fewer cylinders produce more low down grunt?
2) Why manufacturers choose twins over 4 cylinders?

 

[Mightytharg]

Why do Harley and BMW make Twins? - because they can sell them and they have a definite market out there who express a preference for them and who don't google the specs/power outputs of a 4 cylinder sports machine as part of the purchase decision process.

Harley would have difficulty, I'd reckon, in selling a 4 to what is their established market. At the very least they'd need to recycle one of the other brands that sold 4 cylinder bikes in the US in the early days (Henderson, Indian, Ace?)

The whole engine config debate is almost pointless anyway when reduced to pure numbers unless you're purely focussed on competition and even then without understanding other factors (weight / gearing / the rules you have to conform with) the numbers alone can't tell you everything.

Also random pub-talk about torque is often rubbish - many people often say "it's torquey" when they mean "it's flexible" or "has a wide spread of power" and that can come down to a whole number of issues from how it's geared to how well the carburation/injection has been set up to pretty much anything else.

Just because an old 30's brit single can "go up 't hill in top" doesn't mean it generates a higher torque output than a modern single that might be snatching horribly at its chain and demanding you drop down 3 gears.

My first FJ1200 would go from 15mph to 150mph (on the speedo) in top gear, my K1200S wouldn't. But there's no single answer to why that should be the case.

 

[snerkler]

Why do Harley and BMW make Twins? - because they can sell them and they have a definite market out there who express a preference for them and who don't google the specs/power outputs of a 4 cylinder sports machine as part of the purchase decision process.

Harley would have difficulty, I'd reckon, in selling a 4 to what is their established market. At the very least they'd need to recycle one of the other brands that sold 4 cylinder bikes in the US in the early days (Henderson, Indian, Ace?)

The whole engine config debate is almost pointless anyway when reduced to pure numbers unless you're purely focussed on competition and even then without understanding other factors (weight / gearing / the rules you have to conform with) the numbers alone can't tell you everything.

Also random pub-talk about torque is often rubbish - many people often say "it's torquey" when they mean "it's flexible" or "has a wide spread of power" and that can come down to a whole number of issues from how it's geared to how well the carburation/injection has been set up to pretty much anything else.

Just because an old 30's brit single can "go up 't hill in top" doesn't mean it generates a higher torque output than a modern single that might be snatching horribly at its chain and demanding you drop down 3 gears.

My first FJ1200 would go from 15mph to 150mph (on the speedo) in top gear, my K1200S wouldn't. But there's no single answer to why that should be the case.

Thanks for your input. Unfortunately I think you've missed the point I'm trying to get my head around though.
My initial question was why do sinlges and twins make more torque and low down torque than an equivalent cc 4 cylinder as I want to understand the workings and physics, not to quote performance figures. The answer to my own question is they don't, it's a myth. The only reason I quoted performance stats was to prove this point.
So my next question was due to the fact that singles and twins don't produce more torque, why use them if they're 'rougher' engines? After more research today it appears for bikes such as Harleys it's tradition and a V-twin is almost what defines them. Taking this away would be like taking it's soul away. So in reality has nothing to do with low down torque, which is what most people say.
The next one is off-road bikes, why use a single. Most will again say due to the low down torque, but again this seems to be a myth. The reason is size and weight, but more importantly due to the flywheel. The flywheel is larger and therefore caries more momentum and so is harder to stop. Plus the flywheel's momentum is only slowed by one compression stroke rather than 4 in a 4 cylinder and so again caries more/smoother momentum.
So onto BMW, it appears that some of it is due to tradition and the boxer being what defines BMW, but in the case of the GS is needs the combination of a large flywheel for off-road, plus good road manners so a twin is a good compromise.
This is how I now understand it all anyway.

 

[ianp5uk]

You obviously have not seen the car stunt show at Disney Florida, they have small hatchbacks with Busa engines in for the stunt driving, see also the Smart Diablo... but I get the point.

I loved the NR stuff Honda did, they raced an Endurance version in the late 80's, back then ZXR750RRRRRRR'RR's and suchlike made about 135BHP from 11k-12k or something like that, the NR750 racer had between 135bhp and 145 bhp from 8k-15k - a 7,000rpm power band, totally awesome.

The problem was although it broke no rules, Honda were told if they raced it again (it broke down in the race whilst making every other bike look decidedly slow) it would get banned - the other manufacturers claimed it was really a V8.

Nothing like racing encouraging progress and that's nothing like it!
At least Norton lasted a bit longer.

 

[Deleted account 190920001]

I get what your saying and in part your right but your not comparing like for like. A 1200 Corsa engine isnt tuned the same as a 1200 boxer. A Corsa doesn't rev to 8000-8500 rpm for a start and it only has 80bhp. Could a boxer out torque a Corsa, very definitely yes. So why doesn't it? Because the masses are fooled into thinking that the BHP figure is the be all and end all and demand more. You see it all the time in this forum. People clambering over another 5 BHP and spending vast sums of money to do it. For a manufacturer to get the extra horse power you have to either rev it harder or increase the CC and that's a trade off at the expense of torque. If you detuned a 1200 boxer to 80BHP the torque would be much greater than it is just now and a lot lower down. Its also a fact that a bike can easily trade torque for BHP because of its much lighter weight and bike riders are used to short service intervals where car drivers demand at least 1200 miles.
The part you are right about though it that packaging is far more important so long as the choice can do the job. A car driver wouldn't put up with the levels of vibration that a biker will so a twin is out. A 1200 GS-A wouldn't be the same bike with a 4 in it and it would probably be even heavier and taller than they are already. You could do a flat 4 but then you would smash your shins all the time or have a longer bike. A single cylinder crosser would just be completely unmanageable with a 4 in it due to weight and size. Whatever engine a manufacturer chooses above all it has to be able to do the job the vehicle is designed to do and fit in with what people want so they buy it.
The likes of Ducati were lucky in years gone by because WSB heavily favoured them. Not because a twin was better than a 4 but because the other manufacturers were handicapped by the rules meaning the twins were competitive. Because of patents on the Desmo valve system nobody else could make a twin that could really compete. Yes Desmo valves are that good but only in the short term as its complicated and requires a lot of maintenance. The rules were changed and the twins were on a more even par so the fours started wining again. Many a racing fan bought the big twins because they were winning races but in reality the GSXR's and the like were far better road bikes.

 

[Wapping]

Nothing like racing encouraging progress and that's nothing like it!
At least Norton lasted a bit longer.

I watched the early Honda NR racers at Silverstone, Assen and Spa, ridden by Mick Grant, Fast Freddie Spencer etal. Total junk.