Help Support MotoGP Forum:
<div class='quotetop'>QUOTE (wise_mumu @ Feb 14 2008, 11:07 PM) <{POST_SNAPBACK}><div class='quotemain'>This may be a dumb question, but here goes...
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
is it harder to optimize the 'breathing' of a big bore/short stroke twin, than to compensate for the extra weight (extra conrods, extra valves, longer crank) of a 4 big banger? AFAIK, its due to the flame propagtion speed inside the cylinder. A mixture of any certain fuel and air burns at a set speed. So, if the conbustion chamber is long, it will take more time to burn, hence lower RPM -- regardless of valve area/turbocharge/compression ratio etc. Some designer have tried to address this by using highly oversquare engines. Honda's oval piston engines was one attempt. For example back in 1979 "The final 500cc race version was capable of developing approximately 130bhp at over 20,000rpm."
but those ran into their own problems.
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
is it harder to optimize the 'breathing' of a big bore/short stroke twin, than to compensate for the extra weight (extra conrods, extra valves, longer crank) of a 4 big banger? AFAIK, its due to the flame propagtion speed inside the cylinder. A mixture of any certain fuel and air burns at a set speed. So, if the conbustion chamber is long, it will take more time to burn, hence lower RPM -- regardless of valve area/turbocharge/compression ratio etc. Some designer have tried to address this by using highly oversquare engines. Honda's oval piston engines was one attempt. For example back in 1979 "The final 500cc race version was capable of developing approximately 130bhp at over 20,000rpm."
but those ran into their own problems.
- Joined
- Sep 2, 2007
- Messages
- 1,605
- Location
- here
<div class='quotetop'>QUOTE (wise_mumu @ Feb 15 2008, 07:07 AM) <{POST_SNAPBACK}><div class='quotemain'>This may be a dumb question, but here goes...
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
is it harder to optimize the 'breathing' of a big bore/short stroke twin, than to compensate for the extra weight (extra conrods, extra valves, longer crank) of a 4 big banger?
how about the extra kgs off the minimum weight of the bikes running twins?
just curious...
Basic rule with a twin is to increase the bore size for more hp, increase the stroke for more torque. The cc is fixed, so as the bore size increases the stroke length must decrease. A very large bore causes combustion problems with dramatically decreased efficiency. This stems from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter.
Another issue is that racing engines tend to require rather extreme valve lift and overlap angles and so cavities are made in the piston crowns to prevent contact with the half-open valves. These cavities become bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high power engines.
This is what Preziosi said when Ducati first designed the 990.
<div class='quotetop'>QUOTE <div class='quotemain'>The evolution of Ducati engines has meant that in time we have succeeded in combining the advantages of the twin-cylinder engine with the smooth power delivery of a short-stroke engine. Today the twin-cylinder 998R has the same stroke as the four-cylinder Yamaha R1 engine for example.
Between 1994 and today, in the constant search for maximum performance, the stroke value of 66mm on the 916 has been reduced to 58.4mm on the new 998R. From 140HP we have arrived at almost 190 in nine years! In order to obtain even higher power output, we would have had to reduce the stroke value even more, and as a result increase the bore size to a value that would be critical for efficient combustion.
We have continued with the reduction of the stroke value and ‘doubled’ the pistons simply to avoid the combustion problems we spoke about before. For this reason the Desmosedici is an extreme version of a twin-cylinder Ducati.
LINK
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
is it harder to optimize the 'breathing' of a big bore/short stroke twin, than to compensate for the extra weight (extra conrods, extra valves, longer crank) of a 4 big banger?
how about the extra kgs off the minimum weight of the bikes running twins?
just curious...
Basic rule with a twin is to increase the bore size for more hp, increase the stroke for more torque. The cc is fixed, so as the bore size increases the stroke length must decrease. A very large bore causes combustion problems with dramatically decreased efficiency. This stems from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter.
Another issue is that racing engines tend to require rather extreme valve lift and overlap angles and so cavities are made in the piston crowns to prevent contact with the half-open valves. These cavities become bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high power engines.
This is what Preziosi said when Ducati first designed the 990.
<div class='quotetop'>QUOTE <div class='quotemain'>The evolution of Ducati engines has meant that in time we have succeeded in combining the advantages of the twin-cylinder engine with the smooth power delivery of a short-stroke engine. Today the twin-cylinder 998R has the same stroke as the four-cylinder Yamaha R1 engine for example.
Between 1994 and today, in the constant search for maximum performance, the stroke value of 66mm on the 916 has been reduced to 58.4mm on the new 998R. From 140HP we have arrived at almost 190 in nine years! In order to obtain even higher power output, we would have had to reduce the stroke value even more, and as a result increase the bore size to a value that would be critical for efficient combustion.
We have continued with the reduction of the stroke value and ‘doubled’ the pistons simply to avoid the combustion problems we spoke about before. For this reason the Desmosedici is an extreme version of a twin-cylinder Ducati.
LINK
- Joined
- Sep 2, 2007
- Messages
- 1,605
- Location
- here
<div class='quotetop'>QUOTE (crvlvr @ Feb 15 2008, 08:08 PM) <{POST_SNAPBACK}><div class='quotemain'>AFAIK, its due to the flame propagtion speed inside the cylinder. A mixture of any certain fuel and air burns at a set speed. So, if the conbustion chamber is long, it will take more time to burn, hence lower RPM -- regardless of valve area/turbocharge/compression ratio etc. Some designer have tried to address this by using highly oversquare engines. Honda's oval piston engines was one attempt. For example back in 1979 "The final 500cc race version was capable of developing approximately 130bhp at over 20,000rpm."
but those ran into their own problems.
Flame speed is constant in a stationary "chunk" of homogenous fuel/air mix. However the motion of a flame in a mixture which is confined in the combustion chamber is more complex. Turbulence is necessary to help spread the flame rapidly through the mixture and this is achieved by arranging the inlet port so that the mixture enters the chamber obliquely at high speed, setting up a swirling motion that persists through the compresson stroke.
Flame speed increases very nearly proportionally with engine speed, largely due to increased turbulence. The resultant nearly constant crank angle during combustion is the only reason that spark ignition engines can run at very high piston speeds.
The ratio of flame speed to piston speed is nearly the same for different sized pistons and at a given rpm flame speed is nearly proportional to the bore size. Flame speeds are also faster with an oversquare engine, ie one where the stroke/bore ratio is less than 1.
but those ran into their own problems.
Flame speed is constant in a stationary "chunk" of homogenous fuel/air mix. However the motion of a flame in a mixture which is confined in the combustion chamber is more complex. Turbulence is necessary to help spread the flame rapidly through the mixture and this is achieved by arranging the inlet port so that the mixture enters the chamber obliquely at high speed, setting up a swirling motion that persists through the compresson stroke.
Flame speed increases very nearly proportionally with engine speed, largely due to increased turbulence. The resultant nearly constant crank angle during combustion is the only reason that spark ignition engines can run at very high piston speeds.
The ratio of flame speed to piston speed is nearly the same for different sized pistons and at a given rpm flame speed is nearly proportional to the bore size. Flame speeds are also faster with an oversquare engine, ie one where the stroke/bore ratio is less than 1.
<div class='quotetop'>QUOTE (crvlvr @ Feb 15 2008, 08:08 PM) <{POST_SNAPBACK}><div class='quotemain'>AFAIK, its due to the flame propagtion speed inside the cylinder. A mixture of any certain fuel and air burns at a set speed. So, if the conbustion chamber is long, it will take more time to burn, hence lower RPM -- regardless of valve area/turbocharge/compression ratio etc. Some designer have tried to address this by using highly oversquare engines. Honda's oval piston engines was one attempt. For example back in 1979 "The final 500cc race version was capable of developing approximately 130bhp at over 20,000rpm."
but those ran into their own problems.
Oval pistons were made to allow more valve area pr. cc than what is possible with a circular piston. more valve area -> more oxygen. As Yamaka says there are several callenges with big bore engines, and just to add to that the large flat space make it a challenge not only for compression and valve space put also the combustion it self. The area of the sqish zone become large and a wide and heavy pistons (compared to the fours) need more clearence in the zone to avoid the piston to crash into the top (Very common super sport problem) as the rods stretch at high stress. Fuel/air mix in that zone are not very productive as it is cold and reached by the flame front very late, often too late to add at all.
but those ran into their own problems.
Oval pistons were made to allow more valve area pr. cc than what is possible with a circular piston. more valve area -> more oxygen. As Yamaka says there are several callenges with big bore engines, and just to add to that the large flat space make it a challenge not only for compression and valve space put also the combustion it self. The area of the sqish zone become large and a wide and heavy pistons (compared to the fours) need more clearence in the zone to avoid the piston to crash into the top (Very common super sport problem) as the rods stretch at high stress. Fuel/air mix in that zone are not very productive as it is cold and reached by the flame front very late, often too late to add at all.
<div class='quotetop'>QUOTE (wise_mumu @ Feb 15 2008, 06:07 AM) <{POST_SNAPBACK}><div class='quotemain'>This may be a dumb question, but here goes...
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
I don't think any of the teams have big bang engines where a pair of cylinders actually fire together. I was under the impression that Ducati attempted to use an engine that mimmicked a twin but didn't follow it through long term. I think the usual approach is to have a pair of cylinders spaced very closeley, but not firing together. This means that the ignition stroke of the first cylinder will begin early and eliminate the crank shaft deceleration caused by the compression of the second cylinder.
Why go big bang 4 and not go for twin cylinders instead (i2/v2) ?
I don't think any of the teams have big bang engines where a pair of cylinders actually fire together. I was under the impression that Ducati attempted to use an engine that mimmicked a twin but didn't follow it through long term. I think the usual approach is to have a pair of cylinders spaced very closeley, but not firing together. This means that the ignition stroke of the first cylinder will begin early and eliminate the crank shaft deceleration caused by the compression of the second cylinder.
- Joined
- Sep 2, 2007
- Messages
- 1,605
- Location
- here
<div class='quotetop'>QUOTE (Tom @ Feb 16 2008, 12:04 PM) <{POST_SNAPBACK}><div class='quotemain'>I don't think any of the teams have big bang engines where a pair of cylinders actually fire together. I was under the impression that Ducati attempted to use an engine that mimmicked a twin but didn't follow it through long term. I think the usual approach is to have a pair of cylinders spaced very closeley, but not firing together. This means that the ignition stroke of the first cylinder will begin early and eliminate the crank shaft deceleration caused by the compression of the second cylinder.
This is what Preziosi said when Ducati first designed the 990
<div class='quotetop'>QUOTE <div class='quotemain'>Although it has four pistons, the Twinpulse maintains the typical characteristics of a twin-cylinder engine. Unlike a conventional four-cylinder unit, which has four distinct combustions for each complete cycle – that is, two turns of the crankshaft – the Twinpulse only has two. This is obtained by simultaneous combustion in the two cylinders of the same bank. The effect, which we have called Twinpulse, cannot be obtained by a four-cylinder engine by simply changing the firing order; it is necessary to design the entire engine with this function in mind, because many internal organs, which are stressed in a different and more ‘traumatic’ way, must be built to the right size.
LINK
This engine was in use throughout the 990 era, it's only with the advent of the 800s that Ducati has returned to the screamer configuration.
This is what Preziosi said when Ducati first designed the 990
<div class='quotetop'>QUOTE <div class='quotemain'>Although it has four pistons, the Twinpulse maintains the typical characteristics of a twin-cylinder engine. Unlike a conventional four-cylinder unit, which has four distinct combustions for each complete cycle – that is, two turns of the crankshaft – the Twinpulse only has two. This is obtained by simultaneous combustion in the two cylinders of the same bank. The effect, which we have called Twinpulse, cannot be obtained by a four-cylinder engine by simply changing the firing order; it is necessary to design the entire engine with this function in mind, because many internal organs, which are stressed in a different and more ‘traumatic’ way, must be built to the right size.
LINK
This engine was in use throughout the 990 era, it's only with the advent of the 800s that Ducati has returned to the screamer configuration.
<div class='quotetop'>QUOTE (yamaka46 @ Feb 16 2008, 11:34 AM) <{POST_SNAPBACK}><div class='quotemain'>This is what Preziosi said when Ducati first designed the 990
LINK
This engine was in use throughout the 990 era, it's only with the advent of the 800s that Ducati has returned to the screamer configuration.
How sure are you, because i was under the impression this was not the case. I will do some more reading this afternoon, have you got any evidence to support your claim?
LINK
This engine was in use throughout the 990 era, it's only with the advent of the 800s that Ducati has returned to the screamer configuration.
How sure are you, because i was under the impression this was not the case. I will do some more reading this afternoon, have you got any evidence to support your claim?
- Joined
- Sep 2, 2007
- Messages
- 1,605
- Location
- here
<div class='quotetop'>QUOTE (Tom @ Feb 16 2008, 12:43 PM) <{POST_SNAPBACK}><div class='quotemain'>How sure are you, because i was under the impression this was not the case. I will do some more reading this afternoon, have you got any evidence to support your claim?
Best I can find is this quote from Domenicali on the similarity of the Desmosedici RR engine to the GP6 which does indeed imply that they went away from the simultaneous firing, but still called it "twin pulse".
“The general concept and scheme is untouched [from the GP6], down to the bore and stroke and the uneven firing order,” Domenicali says, noting that just about every part of the exhaust and engine are borrowed from Ducati’s MotoGP stable. The 200+ hp 989-cc 4-cylinder engine is configured in the traditional Ducati “L” shape, with four titanium valves per cylinder, arranged in an asymmetrical Twin Pulse configuration, complete with Ducati’s signature desmodromic distribution, providing precise valve control at the highest revs with reduced friction."
LINK
Best I can find is this quote from Domenicali on the similarity of the Desmosedici RR engine to the GP6 which does indeed imply that they went away from the simultaneous firing, but still called it "twin pulse".
“The general concept and scheme is untouched [from the GP6], down to the bore and stroke and the uneven firing order,” Domenicali says, noting that just about every part of the exhaust and engine are borrowed from Ducati’s MotoGP stable. The 200+ hp 989-cc 4-cylinder engine is configured in the traditional Ducati “L” shape, with four titanium valves per cylinder, arranged in an asymmetrical Twin Pulse configuration, complete with Ducati’s signature desmodromic distribution, providing precise valve control at the highest revs with reduced friction."
LINK
<div class='quotetop'>QUOTE (yamaka46 @ Feb 16 2008, 12:42 PM) <{POST_SNAPBACK}><div class='quotemain'>Best I can find is this quote from Domenicali on the similarity of the Desmosedici RR engine to the GP6 which does indeed imply that they went away from the simultaneous firing, but still called it "twin pulse".
“The general concept and scheme is untouched [from the GP6], down to the bore and stroke and the uneven firing order,” Domenicali says, noting that just about every part of the exhaust and engine are borrowed from Ducati’s MotoGP stable. The 200+ hp 989-cc 4-cylinder engine is configured in the traditional Ducati “L” shape, with four titanium valves per cylinder, arranged in an asymmetrical Twin Pulse configuration, complete with Ducati’s signature desmodromic distribution, providing precise valve control at the highest revs with reduced friction."
LINK
I found this is Neil Spalding's Motogp Technology book
"Even though the Ducati firing sequence was called 'Twin Pulse', it didn't consist of two cylinders firing together. Instead it used a crankshaft phased to mimic as close as possible the Honda V5's firing sequence."
“The general concept and scheme is untouched [from the GP6], down to the bore and stroke and the uneven firing order,” Domenicali says, noting that just about every part of the exhaust and engine are borrowed from Ducati’s MotoGP stable. The 200+ hp 989-cc 4-cylinder engine is configured in the traditional Ducati “L” shape, with four titanium valves per cylinder, arranged in an asymmetrical Twin Pulse configuration, complete with Ducati’s signature desmodromic distribution, providing precise valve control at the highest revs with reduced friction."
LINK
I found this is Neil Spalding's Motogp Technology book
"Even though the Ducati firing sequence was called 'Twin Pulse', it didn't consist of two cylinders firing together. Instead it used a crankshaft phased to mimic as close as possible the Honda V5's firing sequence."
- Joined
- Sep 2, 2007
- Messages
- 1,605
- Location
- here
<div class='quotetop'>QUOTE (Tom @ Feb 17 2008, 02:03 PM) <{POST_SNAPBACK}><div class='quotemain'>I found this is Neil Spalding's Motogp Technology book
"Even though the Ducati firing sequence was called 'Twin Pulse', it didn't consist of two cylinders firing together. Instead it used a crankshaft phased to mimic as close as possible the Honda V5's firing sequence."
Interesting - 5 pulses from the honda, 4 from the Duke. Wonder what the "close as possible" order would be. "Asymmetric twin pulse" is a bit vague, but...
"Even though the Ducati firing sequence was called 'Twin Pulse', it didn't consist of two cylinders firing together. Instead it used a crankshaft phased to mimic as close as possible the Honda V5's firing sequence."
Interesting - 5 pulses from the honda, 4 from the Duke. Wonder what the "close as possible" order would be. "Asymmetric twin pulse" is a bit vague, but...
How did they make a four mimic a five, splitting the crank and put a gearing in between
What I've heard they all do in some way is that they offset each sylinder slightly (what they cant do in SBK) to get a longer power pulse and less stress on the crank and valve train.
What I've heard they all do in some way is that they offset each sylinder slightly (what they cant do in SBK) to get a longer power pulse and less stress on the crank and valve train.
