snerkler
Registered user
Your next project is to find out about V fours and why they create such wonderful mid range power .......
I'll do my best, my next project is flywheels and alternators though
Cylinders, Torque, and RPM query?
- Thread starter snerkler
- Start date
I'll do my best, my next project is flywheels and alternators though
snerkler
Registered user
I didn't say it was, I was talking in general. I know BMW's boxer twin goes way back
E. Buygum
Registered user
I suspect the full answer is as complicated as engine design itself but a four stroke twin will generate a bang every revolution. A four stroke single will produce a bang half as often but it will be about twice as big (assuming same cc). This bigger bang means more force on the piston which means more rotational force on the crankshaft i.e. more combustion torque. The engine geometry could also make a contribution as above.
As above, size and weight of pistons, difficulty of evacuating a larger chamber, potentially heavier duty crankshaft and flywheel (for smoothness) will all slow a single down.
Horsepower is related to torquexRPM so in a massively simplified universe, you could get roughly the same power from an engine at low revs, high torque (single) or at twice the revs, half the torque (twin).
It is less to do with piston size than piston velocity, CC is just a combination of bore and stroke.
How high an engine can rev tends to have more to do with piston speed...
With a short stroke the piston actually travels less distance at the same RPM and therefore is moving slower, high piston speeds increase friction (exponentially) so the same engine will have four times the piston resistance at 6,000rpm as it had at 3,000 rpm.
With a long stroke the piston moves much faster, also it has to be stopped at top and bottom dead centre from these higher speeds.
The bigger the piston the more inertia it will have at the top and bottom of the strokes again increasing stress.
Beyond that cam profiles (valve overlap), exhaust shape / length and air intake size / length also has an effect.
For the same cc a short stroke engine will most likely rev higher and produce less bottom end power (or torque) than a longer stroke engine, but there are many other factors, for example I once rode a K75 whilst working for a Honda dealer, it was low revving yet had no more low down grunt than the VFR, but the VFR was still comparitively "revvy" as when the K75 ran out of puff the VFR still had 3,000 rpm to go.
Similarly a Ducati Multistrada has far more power and torque throughout the rev range than the similarly sized 1200GS, probably has bigger valves, higher compression, lighter pistons, water cooling
Also torque at the engine is not the same as at the wheels, by revving higher you can use lower earing, the lower gearing amplifies the tirque reaching the rear wheels - think about a modern superbike, it may have 80ft/lb of torque, well you can apply that tightening your car wheel nuts - so why doesn't the wheel spin whilst you try to do this? Quite simply because even 100ft/lb is nothing, your superbike engine may produce this torque at 10,000rpm, but the wheels may only be spinning at 1000 rpm or less as the engine rpm is reduced via gearbox and output shafts, this is also why you get more acceleration in lower gears as the engines torque has been amplified by the lower gearing.
Rule of thumb:
Long Stroke = More Torque
Short Stroke = More Revs (Top end Power)
More Cubes = More power / torque
The saying "you cannot beat cubes" is pretty true when comparing like for like.
You cannot compare a bike from 1970 with a bike from these days, but say you compare a Hayabusa to a GSXR1000 and you will see the Busa's extra cubes give it a load more bottom end and a bit more top end, you could further tune the 1000 for the same top end but probably lose more bottom end, but if you then tune the Busa a bit more it will still have more power everywhere.
Come back when you have designed a revolutionary new linear drive engine for us all to enjoy!
How high an engine can rev tends to have more to do with piston speed...
With a short stroke the piston actually travels less distance at the same RPM and therefore is moving slower, high piston speeds increase friction (exponentially) so the same engine will have four times the piston resistance at 6,000rpm as it had at 3,000 rpm.
With a long stroke the piston moves much faster, also it has to be stopped at top and bottom dead centre from these higher speeds.
The bigger the piston the more inertia it will have at the top and bottom of the strokes again increasing stress.
Beyond that cam profiles (valve overlap), exhaust shape / length and air intake size / length also has an effect.
For the same cc a short stroke engine will most likely rev higher and produce less bottom end power (or torque) than a longer stroke engine, but there are many other factors, for example I once rode a K75 whilst working for a Honda dealer, it was low revving yet had no more low down grunt than the VFR, but the VFR was still comparitively "revvy" as when the K75 ran out of puff the VFR still had 3,000 rpm to go.
Similarly a Ducati Multistrada has far more power and torque throughout the rev range than the similarly sized 1200GS, probably has bigger valves, higher compression, lighter pistons, water cooling
Also torque at the engine is not the same as at the wheels, by revving higher you can use lower earing, the lower gearing amplifies the tirque reaching the rear wheels - think about a modern superbike, it may have 80ft/lb of torque, well you can apply that tightening your car wheel nuts - so why doesn't the wheel spin whilst you try to do this? Quite simply because even 100ft/lb is nothing, your superbike engine may produce this torque at 10,000rpm, but the wheels may only be spinning at 1000 rpm or less as the engine rpm is reduced via gearbox and output shafts, this is also why you get more acceleration in lower gears as the engines torque has been amplified by the lower gearing.
Rule of thumb:
Long Stroke = More Torque
Short Stroke = More Revs (Top end Power)
More Cubes = More power / torque
The saying "you cannot beat cubes" is pretty true when comparing like for like.
You cannot compare a bike from 1970 with a bike from these days, but say you compare a Hayabusa to a GSXR1000 and you will see the Busa's extra cubes give it a load more bottom end and a bit more top end, you could further tune the 1000 for the same top end but probably lose more bottom end, but if you then tune the Busa a bit more it will still have more power everywhere.
Come back when you have designed a revolutionary new linear drive engine for us all to enjoy!
TeaPig
Registered user
snerkler
Registered user
It is less to do with piston size than piston velocity, CC is just a combination of bore and stroke.
How high an engine can rev tends to have more to do with piston speed...
With a short stroke the piston actually travels less distance at the same RPM and therefore is moving slower, high piston speeds increase friction (exponentially) so the same engine will have four times the piston resistance at 6,000rpm as it had at 3,000 rpm.
With a long stroke the piston moves much faster, also it has to be stopped at top and bottom dead centre from these higher speeds.
The bigger the piston the more inertia it will have at the top and bottom of the strokes again increasing stress.
Beyond that cam profiles (valve overlap), exhaust shape / length and air intake size / length also has an effect.
For the same cc a short stroke engine will most likely rev higher and produce less bottom end power (or torque) than a longer stroke engine, but there are many other factors, for example I once rode a K75 whilst working for a Honda dealer, it was low revving yet had no more low down grunt than the VFR, but the VFR was still comparitively "revvy" as when the K75 ran out of puff the VFR still had 3,000 rpm to go.
Similarly a Ducati Multistrada has far more power and torque throughout the rev range than the similarly sized 1200GS, probably has bigger valves, higher compression, lighter pistons, water cooling
Also torque at the engine is not the same as at the wheels, by revving higher you can use lower earing, the lower gearing amplifies the tirque reaching the rear wheels - think about a modern superbike, it may have 80ft/lb of torque, well you can apply that tightening your car wheel nuts - so why doesn't the wheel spin whilst you try to do this? Quite simply because even 100ft/lb is nothing, your superbike engine may produce this torque at 10,000rpm, but the wheels may only be spinning at 1000 rpm or less as the engine rpm is reduced via gearbox and output shafts, this is also why you get more acceleration in lower gears as the engines torque has been amplified by the lower gearing.
Rule of thumb:
Long Stroke = More Torque
Short Stroke = More Revs (Top end Power)
More Cubes = More power / torque
The saying "you cannot beat cubes" is pretty true when comparing like for like.
You cannot compare a bike from 1970 with a bike from these days, but say you compare a Hayabusa to a GSXR1000 and you will see the Busa's extra cubes give it a load more bottom end and a bit more top end, you could further tune the 1000 for the same top end but probably lose more bottom end, but if you then tune the Busa a bit more it will still have more power everywhere.
Come back when you have designed a revolutionary new linear drive engine for us all to enjoy!
Thanks for this, makes it much clearer
I can understand why on a torque curve it rises to a peak and then more often than not gradually fades away again, but I've seen some torque curves that peak, fade and then peak again. Any idea why this is?
TeaPig
Registered user
Thanks for this, makes it much clearer
I can understand why on a torque curve it rises to a peak and then more often than not gradually fades away again, but I've seen some torque curves that peak, fade and then peak again. Any idea why this is?
There are many variables that influence torque, not just the bore and stroke but also stuff like the cam timing, induction and exhaust design and fuelling. By playing around with the variables, the revs at which max torque is produced will alter. Modern road engines are stifled by noise and emission reg's and if you look at many dyno curves you'll see a dip in curve around 4000rpm which is where they have to pass noise tests. In other words they are deliberately de-tuned to meet the regulations. Poor design, or a design where ultimate power rather than a flat delivery of torque is the goal may also produce wobbly torque curves
a gsxr 750 produces more torques than a 1340 harley. 
just a different point in the rev range.
BMW K bikes have more torques than the twins.........
http://en.wikipedia.org/wiki/Torque
Derivation of the Power Equation
HP = RPM x TORQUE ÷ 5252, then where does the 5252 come from?"
By definition, POWER = FORCE x DISTANCE ÷ TIME
where a constant tangential force of 100 pounds was applied to the 12" handle rotating at 2000 RPM, we know the force involved, so to calculate power, we need the distance the handle travels per unit time, expressed as:
Power = 100 pounds x distance per minute
how far does the crank handle move in one minute? First, determine the distance it moves in one revolution:
DISTANCE per revolution = 2 x π x radius
DISTANCE per revolution. = 2 x 3.1416 x 1 ft = 6.283 ft.
Now we know how far the crank moves in one revolution. How far does the crank move in one minute?
DISTANCE per min. = 6.283 ft .per rev. x 2000 rev. per min. = 12,566 feet per minute
Now we know enough to calculate the power, defined as:
POWER = FORCE x DISTANCE ÷ TIME
so
Power = 100 lb x 12,566 ft. per minute = 1,256,600 ft-lb per minute
...but how about HORSEPOWER? Remember that one HORSEPOWER is defined as 33000 foot-pounds of work per minute. Therefore HP = POWER (ft-lb per min) ÷ 33,000. We have already calculated that the power being applied to the crank-wheel above is 1,256,600 ft-lb per minute.
How many HP is that?
HP = (1,256,600 ÷ 33,000) = 38.1 HP.
Now we combine some stuff we already know to produce the magic 5252. We already know that:
TORQUE = FORCE x RADIUS.
If we divide both sides of that equation by RADIUS, we get:
(a) FORCE = TORQUE ÷ RADIUS
Now, if DISTANCE per revolution = RADIUS x 2 x π, then
(b) DISTANCE per minute = RADIUS x 2 x π x RPM
We already know
(c) POWER = FORCE x DISTANCE per minute
So if we plug the equivalent for FORCE from equation (a) and distance per minute from equation (b) into equation (c), we get:
POWER = (TORQUE ÷ RADIUS) x (RPM x RADIUS x 2 x π)
Dividing both sides by 33,000 to find HP,
HP = TORQUE ÷ RADIUS x RPM x RADIUS x 2 x π ÷ 33,000
By reducing, we get
HP = TORQUE x RPM x 6.28 ÷ 33,000
Since
33,000 ÷ 6.2832 = 5252
Therefore
HP = TORQUE x RPM ÷ 5252
Note that at 5252 RPM, torque and HP are equal. At any RPM below 5252, the value of torque is greater than the value of HP; Above 5252 RPM, the value of torque is less than the value of HP.
just a different point in the rev range.
BMW K bikes have more torques than the twins.........
http://en.wikipedia.org/wiki/Torque
Derivation of the Power Equation
HP = RPM x TORQUE ÷ 5252, then where does the 5252 come from?"
By definition, POWER = FORCE x DISTANCE ÷ TIME
where a constant tangential force of 100 pounds was applied to the 12" handle rotating at 2000 RPM, we know the force involved, so to calculate power, we need the distance the handle travels per unit time, expressed as:
Power = 100 pounds x distance per minute
how far does the crank handle move in one minute? First, determine the distance it moves in one revolution:
DISTANCE per revolution = 2 x π x radius
DISTANCE per revolution. = 2 x 3.1416 x 1 ft = 6.283 ft.
Now we know how far the crank moves in one revolution. How far does the crank move in one minute?
DISTANCE per min. = 6.283 ft .per rev. x 2000 rev. per min. = 12,566 feet per minute
Now we know enough to calculate the power, defined as:
POWER = FORCE x DISTANCE ÷ TIME
so
Power = 100 lb x 12,566 ft. per minute = 1,256,600 ft-lb per minute
...but how about HORSEPOWER? Remember that one HORSEPOWER is defined as 33000 foot-pounds of work per minute. Therefore HP = POWER (ft-lb per min) ÷ 33,000. We have already calculated that the power being applied to the crank-wheel above is 1,256,600 ft-lb per minute.
How many HP is that?
HP = (1,256,600 ÷ 33,000) = 38.1 HP.
Now we combine some stuff we already know to produce the magic 5252. We already know that:
TORQUE = FORCE x RADIUS.
If we divide both sides of that equation by RADIUS, we get:
(a) FORCE = TORQUE ÷ RADIUS
Now, if DISTANCE per revolution = RADIUS x 2 x π, then
(b) DISTANCE per minute = RADIUS x 2 x π x RPM
We already know
(c) POWER = FORCE x DISTANCE per minute
So if we plug the equivalent for FORCE from equation (a) and distance per minute from equation (b) into equation (c), we get:
POWER = (TORQUE ÷ RADIUS) x (RPM x RADIUS x 2 x π)
Dividing both sides by 33,000 to find HP,
HP = TORQUE ÷ RADIUS x RPM x RADIUS x 2 x π ÷ 33,000
By reducing, we get
HP = TORQUE x RPM x 6.28 ÷ 33,000
Since
33,000 ÷ 6.2832 = 5252
Therefore
HP = TORQUE x RPM ÷ 5252
Note that at 5252 RPM, torque and HP are equal. At any RPM below 5252, the value of torque is greater than the value of HP; Above 5252 RPM, the value of torque is less than the value of HP.
TeaPig
Registered user
Trouble is that in an internal combustion engine FORCE is not constant.
The amount of torque produced comes entirely from the conversion of the energy released from buring the fuel; it can't come from anywhere else! The greater the amount of fuel burnt the greater the torque produced - so the bigger the capacity of the cylinder the greater the torque. One can have one big cylinder or lots of smaller ones - the torque will be essentially the same. The torque from a 1200cc 4-cylinder bike will be about the same as a 1 cylinder 1200cc engine - provided they both burn the fuel as efficiently as each other.
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.
snerkler
Registered user
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
snerkler
Registered user
Is this by Hillier? I've found loads of different ones, the latest appears to be the 5th edition published 2004. The previous edition was published 1993 so probably a bit out of date now?
Kremmen
Registered user
Showing my age hear My copy of FMVT is dated 1973... OHC engine wasn't only given a couple of pages...Plenty of pages given over to OHV & side valve
There is even reference to TVO!!....takes me back
There is even reference to TVO!!....takes me back
snerkler
Registered user
Deleted account 190920001
Registered user
- Joined
- Oct 15, 2009
- Messages
- 3,368
- Reaction score
- 0
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
You have kinda missed some of what the equations above were telling you.
HP = Torque x RPM
So you want more HP (Which sports bikes do)you need to move torque up the rev range i.e increase revs. Over square bore found in modern 4 cylinder engines means short stroke, lower piston speed and possibly lower mass which lets all this happen. In theory you could half the stroke and double the RPM effectively doubling HP. Course there are limits and you would have to go to some pretty exotic materials, think F1.
Anyway take 1x 4 cylinder engine of 1200cc with a rev ceiling of 8500rpm and 105BHP. All other things being equal where do you think peak torque would be on each engine (4 cylinder and twin)? So does that explain why the torque is in different places to you?
Engines are built for different reasons and in the bike world it just happens that they use twins and singles for lazier go anywhere engines and fours for fast sports bikes. Not always but most of the time, there are exceptions like Ducati who make fast twins and BMW who now use a 6 for a tourer although that's probably more to do with a 6 being a very smooth engine.
I could go on about how high rev cams have lots of overlap so they are basically crap at low rpm (variable valve timing can eliminate this think Honda VTEC) but hopefully you get the general idea.
Engine design is pretty complex but you could in theory produce an engine of any number of pistons that had the same characteristics.
Oh if your interested look up the torque curve of an old 350 small block Chevy. They only make around 200bhp from 5700cc and max out at about 5000-6000 rpm. So from what you have learnt from this thread, where and how big do you think the torque is?
snerkler
Registered user
So basically you can tune an engine to have what ever characteristics you want, within limits? So in theory you could have a twin who's peak torque come in much higher up the rev range, and a 4 cylinder that has its peak torque much lower down the rev range?
As HP = Torque x RPM I'm guessing that in an ideal world you'd design an engine who's max torque appeared low down in the rev range and never trailed off and that could still rev high, therefore you get lots of low down grunt without sacrificing horsepower.
I've ordered the book "Fundamentals of Motor Vehicle Technology" to try and understand these differences in characteristics, and also why you can't get the ideal scenario as I mentioned above.
With regards to your question about the Chevy block I've seem all different torque curves, one where it had massive torque very low down, but relatively low HP, and one with relatively low torque but with massive HP
I'm guessing from these discussions that if I could come up with an engine that can produce huge low down torque that never trails off, and produces massive HP, ie high RPM I'd be a multi-millionaire?
put it simply - its the gearing
a big piston does 10 units of work each downstroke and has gearing to allow the rear wheel to go round 10 times.
a piston of 1/4 displacement can only do 3 units, so the gearing is adjusted to turn the rear wheel 3 times
thats why a single / twin scoots you up the road farther and quicker for the same revs
a big piston does 10 units of work each downstroke and has gearing to allow the rear wheel to go round 10 times.
a piston of 1/4 displacement can only do 3 units, so the gearing is adjusted to turn the rear wheel 3 times
thats why a single / twin scoots you up the road farther and quicker for the same revs
Deleted account 190920001
Registered user
- Joined
- Oct 15, 2009
- Messages
- 3,368
- Reaction score
- 0
So basically you can tune an engine to have what ever characteristics you want, within limits? So in theory you could have a twin who's peak torque come in much higher up the rev range, and a 4 cylinder that has its peak torque much lower down the rev range?
As HP = Torque x RPM I'm guessing that in an ideal world you'd design an engine who's max torque appeared low down in the rev range and never trailed off and that could still rev high, therefore you get lots of low down grunt without sacrificing horsepower.
I've ordered the book "Fundamentals of Motor Vehicle Technology" to try and understand these differences in characteristics, and also why you can't get the ideal scenario as I mentioned above.
With regards to your question about the Chevy block I've seem all different torque curves, one where it had massive torque very low down, but relatively low HP, and one with relatively low torque but with massive HP
I'm guessing from these discussions that if I could come up with an engine that can produce huge low down torque that never trails off, and produces massive HP, ie high RPM I'd be a multi-millionaire?
Na you have just re-invented a forced induction engine.
But they have their own problems for example not being able to have such high compression which limits low down grunt and with turbo's also has lag, giving an engine that has huge midrange but relatively poor low down and top end.
Superchargers are better with regard to lag, but still are not perfect.
I like big fours, although technically relatively peaky they have so much excess power it does not matter, for example my ZZR1400 had more power and torque by 5,000rpm than the GS makes at peak, you could ride (bloody fast) without going over 6k. The tall (200mph) gearing meant that from 40 in top the GS would outgrunt it in a top gear roll on, but from 70-80 upwards the ZZR would win, bearing in mind you could still drop 5 gears back to 1st at this speed it was an impressive engine.
I think the GS engine suits the bike and the roads it is best on (i.e. sub 100mph riding) a shade more power (but only if at same RPM, not by making it revvier) would be handy for when loaded and better smoothness.
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.
Similar threads
