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single vs twin vs four
- Thread starter Chris-CJ
- Start date
J_F
Guest
been thinking on this for a bit
a lot of wisdom in the post
but I need to point out the inertia of moving parts
and how their mass can affect changes in the inertia
and how these characteristics lend themselves to bottom end torque or high rev HP
an engine with few (or one) large cylinder is good at maintaining speed
an engine with several smaller cylinders can be tuned for fast RPM gain
of course you can tune an engine any away you like
but the inertia of large mass stuff is hard to overcome
Chris-CJ
Well-known member
Here is another "wrinkle" to consider.
The inertia of the recoprocating mass is conveyed to the crankshaft. So, can a twin with really small crank journals equate to a four with large and heavy journals?
(only asking and I claim no expertise in the matter and the question is hypothetical, as other physics would make that type of design impractical).
"four wheels good, two wheels better"
J_F
Guest
interesting question
what I'm considering, and I think most here are
is engines of the same displacement
for example, 1,000 cc
a twin of that size - whether a V or parallel twin or a boxer
is going to have larger mass reciprocating and rotating parts
than a 4 cylinder of the same displacement
tuning aside
an engine with 2 large pistons and the other stuff required to turn pressure into rotation
is going to have higher inertial forces to overcome
than a 4 cylinder of the same displacement - particularly when there are running pairs
and I think this has a lot to do with twins being in cruisers
and 4's being in sport bikes
Chris-CJ
Well-known member
Makes sense and the proof is in the actual on-road design execution of today's bikes. Stratospheric rpm + multi cylinder = high HP. Tho' to make a bike easier to ride, there has to be "torque". This brings us to the oft repeated controversy of torque vs HP aka twins vs fours.
Anyone here ride/own a Honda CBX or Benelli six?
Those are six cyl bikes, did they need winding up to get off the line and a constant swapping of gears to stay in the power band?
And with reference to a reduced weight power train, how does the 'Vette C8 do it with push-rods?
Actually, scratch the above, it belongs to a car forum.
"four wheels good, two wheels better"
Anyone here ride/own a Honda CBX or Benelli six?
Those are six cyl bikes, did they need winding up to get off the line and a constant swapping of gears to stay in the power band?
And with reference to a reduced weight power train, how does the 'Vette C8 do it with push-rods?
Actually, scratch the above, it belongs to a car forum.
"four wheels good, two wheels better"
Roadghost
Well-known member
I prefer counterbalanced engines if they are not V-twins. The upright engine is my preference for singles to fours. I prefer singles on smaller displacement motorcycles. Twins if middle displacement. Not a big fan of fours because of the buzz, but if I had one it would be 600cc or higher. My preference on all bikes is to have low-end torque, where it matters.
Re crank inertia etc ...
Obviously you can make a crankshaft heavier (crankshaft mass is one of the things fiddled with at the MotoGP or WorldSBK level) but there are limits to how light you can make it, because the inertia of the crankshaft and all the associated rotating bits at the minimum operating speed (idling) has to be sufficient to carry the next cylinder in the firing order through its compression stroke. This trends towards an engine with fewer bigger cylinder needing a proportionately heavier crankshaft, with a single needing the proportionately heaviest crankshaft of all. Or you can raise the idling speed. In the H-D community it seems to be "a thing" to set the idling speed as low as possible. Chug, chug-chug, chug. You can do that, if the flywheel (relatively speaking) weighs a ton.
But ... There's another complication, because the speed of the crankshaft varies through each operating cycle of the engine, and at a high level (MotoGP, WorldSBK) it affects traction feel through the rear tire. This is behind Yamaha's cross-plane crankshaft design for their inline four. It rotates more evenly through each cycle (because only half of the pistons are approaching TDC/BDC at a time with the others halfway between, unlike in a conventional inline four which has all pistons approaching TDC/BDC at a time and all halfway between at the same time) but it has an uneven torque output pattern, which has been found to give better traction feel.
On a related note ... Ducati may have stumbled upon this with their 90-degree V-twins, because those inherently have that feature built in. So did Honda V-fours back in the day. The new Ducati 4-cylinder engine has an irregular firing pattern, all four cylinders fire within 380 degrees and then there is 340 degrees of nothing. And only one piston is approaching TDC/BDC at a time, so within each engine cycle, the crank rotation should be quite uniform - probably even better than that of the Yamaha cross-plane design.
And, Honda (HRC) may have known this for a long time. The Honda RC211V MotoGP engine was a V5 with three cylinders on the front bank, two on the rear, and an exceptionally irregular firing order.
The new trend towards having a parallel-twin (cheaper to manufacture than a V) with a 270-degree (or close to it) crankshaft simulates a V-twin firing order ... although it needs a balance shaft. See Yamaha FZ/MT-07.
Peaky power bands are a function of extreme cam timing and extreme narrow-focused intake and exhaust system tuning. Doesn't matter the number of cylinders. On a superbike you actually want a flat torque curve, at least in the RPM range that the rider normally uses (generally the top half), to make it easier to ride. If not a "flat" torque curve, you at least want it to be smooth and predictable without holes or spikes. This is behind variable valve timing (current Suzuki GSXR1000). But VVT gets expensive. An in-line engine allows one intake camshaft and one exhaust camshaft, which means just one intake-cam VVT actuator (Suzuki doesn't use VVT on the exhaust side).
There's another way to get a smooth, predictable, and BIG fat torque curve: Supercharging.
Obviously you can make a crankshaft heavier (crankshaft mass is one of the things fiddled with at the MotoGP or WorldSBK level) but there are limits to how light you can make it, because the inertia of the crankshaft and all the associated rotating bits at the minimum operating speed (idling) has to be sufficient to carry the next cylinder in the firing order through its compression stroke. This trends towards an engine with fewer bigger cylinder needing a proportionately heavier crankshaft, with a single needing the proportionately heaviest crankshaft of all. Or you can raise the idling speed. In the H-D community it seems to be "a thing" to set the idling speed as low as possible. Chug, chug-chug, chug. You can do that, if the flywheel (relatively speaking) weighs a ton.
But ... There's another complication, because the speed of the crankshaft varies through each operating cycle of the engine, and at a high level (MotoGP, WorldSBK) it affects traction feel through the rear tire. This is behind Yamaha's cross-plane crankshaft design for their inline four. It rotates more evenly through each cycle (because only half of the pistons are approaching TDC/BDC at a time with the others halfway between, unlike in a conventional inline four which has all pistons approaching TDC/BDC at a time and all halfway between at the same time) but it has an uneven torque output pattern, which has been found to give better traction feel.
On a related note ... Ducati may have stumbled upon this with their 90-degree V-twins, because those inherently have that feature built in. So did Honda V-fours back in the day. The new Ducati 4-cylinder engine has an irregular firing pattern, all four cylinders fire within 380 degrees and then there is 340 degrees of nothing. And only one piston is approaching TDC/BDC at a time, so within each engine cycle, the crank rotation should be quite uniform - probably even better than that of the Yamaha cross-plane design.
And, Honda (HRC) may have known this for a long time. The Honda RC211V MotoGP engine was a V5 with three cylinders on the front bank, two on the rear, and an exceptionally irregular firing order.
The new trend towards having a parallel-twin (cheaper to manufacture than a V) with a 270-degree (or close to it) crankshaft simulates a V-twin firing order ... although it needs a balance shaft. See Yamaha FZ/MT-07.
Peaky power bands are a function of extreme cam timing and extreme narrow-focused intake and exhaust system tuning. Doesn't matter the number of cylinders. On a superbike you actually want a flat torque curve, at least in the RPM range that the rider normally uses (generally the top half), to make it easier to ride. If not a "flat" torque curve, you at least want it to be smooth and predictable without holes or spikes. This is behind variable valve timing (current Suzuki GSXR1000). But VVT gets expensive. An in-line engine allows one intake camshaft and one exhaust camshaft, which means just one intake-cam VVT actuator (Suzuki doesn't use VVT on the exhaust side).
There's another way to get a smooth, predictable, and BIG fat torque curve: Supercharging.
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