NS Dev
|
| posted on 13/1/06 at 12:34 PM |
|
|
Interesting aside I have just thought of!
Just answering a post on the exhaust length thread and remembered an interesting engine setup that a few autograssers have run in the past (and
now)
So called "Big Bang" engines. Basically usually run on 4 cyl or 6 cyl engines, pairs of cylinders are fired together.
I knew of a chap (actually Paul Exon of Quorn Engine Developments) running a Vauxhall XE big bang engine a few years ago.
The camshafts were made specially to give identical valve events on cyls 1-4 and 2-3, and the engine management set up that way also. The engine then
runs as a big twin cyl.
The principle was to give huge lowdown torque to get off the startline and out of corners.
Sounded VERY interesting when revving at 8000 revs!!!!!!
Problem was the shock torque loading continually destroyed gearboxes, and this was why he abandoned the idea.
Was still producing over 250hp from 2.0 XE engine though!!
There was another chap running a Honda 2.7 V6 as 3 twins in a similar fashion, but I have not seen that car for ages.
No use at all to us locosters but an interesting point all the same!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
|
|
Volvorsport
|
| posted on 13/1/06 at 12:47 PM |
|
|
scat V4 were the same incidentally .iirc
half a nascar v8 .
the bigger the bang , the more torque - hence why lots of cylinders will make hp if you rev it enough , not necessarily torque .
the volvo 4 cyl is preferred at 2.8L in rallycross purely because of this , even tho 5/6 cyl engines at that displacement are common .
www.dbsmotorsport.co.uk
getting dirty under a bus
|
|
|
dnmalc
|
| posted on 13/1/06 at 01:21 PM |
|
|
When they forst introduced these on 500grand prix bikes all 4 cyliders were being fired at once. the riders initially found them to be unridable but
after a couple of years they were reintroduced I suspect with a slightly softer power profile as everone switched over to them.
|
|
|
Cazzy R
|
 posted on 13/1/06 at 01:23 PM |
|
|
"Big bang" has been used in Moto GP for some time originally on the V4 500 two strokes to make the power delivery less vicious and to
improve tyre life.
Odd ball crank phasing is still used today on Rossi's 4-stroke Yamaha to make his inline 4 cyl motor sound and behave like a
'V4'.
Good eh? 
Cazzy R
|
|
|
garyo
|
| posted on 13/1/06 at 03:05 PM |
|
|
Hows do the physics work here - the flywheel acts as a large damper, so my intuition says whether it sees 2 power strokes per rev or 1, it
wouldn't really make much difference.
Gary
|
|
|
bernie955
|
| posted on 13/1/06 at 03:13 PM |
|
|
Even fire engines have power pulses evenly spaced apart. By having cylinders fire close together ther is a longer pause before they fire again. This
allows the tyres to regain any lost traction. This is the reason v-twins did well in superbike. They could get more power to the ground as poser
pulses were further apart allowing the tyre to regain grip more easily.
|
|
|
NS Dev
|
| posted on 13/1/06 at 04:16 PM |
|
|
quote:
Originally posted by garyo
Hows do the physics work here - the flywheel acts as a large damper, so my intuition says whether it sees 2 power strokes per rev or 1, it
wouldn't really make much difference.
Gary
think about what you have just said!
Two power strokes mean twice the mixture burnt and thus twice the torque in theory!
Yes, the power will be little different because there is a big torque impulse and then a ruddy great gap, but the torque will be greatly increased.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
NS Dev
|
| posted on 13/1/06 at 04:18 PM |
|
|
quote:
Originally posted by bernie955
Even fire engines have power pulses evenly spaced apart. By having cylinders fire close together ther is a longer pause before they fire again. This
allows the tyres to regain any lost traction. This is the reason v-twins did well in superbike. They could get more power to the ground as poser
pulses were further apart allowing the tyre to regain grip more easily.
I had this explanation in grasstrack, but I then pointed out that just as the tyre regains grip between pulses, surely it loses grip more quickly too
as a result of the torque impulse???
I have no idea which way the balance tips, in or out of favour of "big bang" theory, but people seem to have made it work!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
|
|
|
garyo
|
| posted on 13/1/06 at 04:44 PM |
|
|
it must work, or race teams wouldn't use it, but it does strike me as an agricultural version of traction control, and the best way of
transmissing the most torque to the ground would be *just* maintaining grip at a contant level, rather than delivering spikes of torque which
continually break and regain traction. Maybe this is a way of tuning the power stroke frequency and drivetrain mass so that they're in the
sweet spot with simulates a very good traction control system?
|
|
|
blueshift
|
| posted on 13/1/06 at 07:10 PM |
|
|
tyres don't work in terms of on/off maintaining grip, it's all about slip angles and friction curves.. fairly complicated physics. with
normal tyres if memory serves you get most force with some percentage of slip, in the teens of percent or something. depends on the tyre and surface.
|
|
|
jambojeef
|
| posted on 13/1/06 at 10:49 PM |
|
|
I must say I agree with the theory Blueshift but I have read of the pulsing v-twins of the ducatis etc allegedly being better at delivering their
power to the tarmac particularly out of corners at GP power levels.
I guess the way I was thinking about it was ABS in reverse with a level of slip in between pulses of drive....
Be interesting to read something definitive on the subject though
Geoff
|
|
|
MikeRJ
|
| posted on 13/1/06 at 11:06 PM |
|
|
quote:
Originally posted by NS Dev
Yes, the power will be little different because there is a big torque impulse and then a ruddy great gap, but the torque will be greatly increased.
But you can't increase the average torque at any given RPM without the power increasing proportionaly!
I guess the "big bang" engines give a high peak torque, followed by no torque due to long interval between firing strokes, so average is
around the same.
I share your thoughts that a smooth torque delivery would intuitively be preferable for controlling power delivery, rather than high torque impulses.
|
|
|
gazza285
|
| posted on 14/1/06 at 12:01 AM |
|
|
Big bang Moto GP engines solved the traction problems for the riders without the necessary throttle control. Mick Doohan didn't need a big bang
motor to win his championships; neither did Rossi when he rode the 500s.
The theory behind the big bang was that it might not give you the ultimate torque all the time at the back wheel, but that it gave you as much as you
needed without ruining through overheating the tyre with wheel spin.
As above Doohan, and to a similar extent, Rossi, have shown that careful use of the right hand (foot!) with a (what are they called now? The two
strokes were called screamers, not sure about the four bangers.) normal power distribution motor can beat a big bang motor. At our level it is much
easier to just fit traction control, and it's much cheaper than custom cams.
DO NOT PUT ON KNOB OR BOLLOCKS!
|
|
|
jon_boy
|
| posted on 14/1/06 at 04:09 PM |
|
|
This is like some v8 and v12 engines. They are called flat plane crank ie if its a v8 they have two cylinders firing at the same time giving four
pairs of firing cylinders. Look up falt plane crank in google and you'll find loads. Some tvr's i believe had them. Most american
V8's dont though.
|
|
|
