V2 vs. V4 torque things that make you go hummm

J

Janu42 {?uotable}

New member
Stroke has always been the significant factor (not the ONLY factor but the most SIGNIFICANT) for torque. A longer stroke increases the leverage angle of the conrod on the crankshaft thruout its cycle and this increased leverage is the fundamental basis to create more torque.
1000cc displacement from a 2 cylinder engine requires Twice the stroke length of a 1000cc engine of 4 cylinders using the same size bore. The faster the piston must travel the 'heavier' it becomes at the change of direction from up/down down/up. Unfortunately longer strokes increase piston speeds and inertial mass for any given RPM (compared to a 4 cylinder of equal displacement) due to the size and weight of the moving parts, therefore the maximum reliable piston speed becomes the limiting factor for peak RPM and power.
To increase these limits a larger/heavier/stronger piston conrod assembly is required but then the engines rev rate is reduced due to the heavier reciprocal masses, which compromises overall performance potential. This explains why a twin cannot make the same reliable power of a 4cyl and does not rev as high.
The more fuel and air you can burn in a given time period, the more heat you can make. The more heat you generate the more power you get. A 1000 cc 4cyl that can turn 15000rpm will always pump a significantly greater volume of fuel/air/heat then a 1000cc twin at 10000rpm. When it can pump more air it can burn more fuel, thus making more heat resulting in more power.
It's simple maths. If your 1000cc 4 cylinder engine does 15000 RPM it pumps (approximately) 15000 litres of air/minute. A 1000cc twin at 10000rpm pumps (approximately) 10000 litres of air per minute. From here it is easy to understand why the 4cylinder with its reduced stroke and higher RPM limit has the greater power potential at the sacrifice of greater torque.
 
Well, you're right, but you said some strange things.

Heat isn't what you want out of the motor, that would be work. Heat is a byproduct of combustion (and friction...and so on of course). So, the more air and fuel you can put in a combustion chamber, the more force you get on the piston in the result of kinetic energy.

Also, it's not fair to talk about the power of a motor in terms of it's displacement and RPM. In a VERY simple manner, the power of a motor is the torque it produces multiplied by it's rotation speed. So, of course a motor that spins faster will create more power.

The 4's make more power than a twin for a given displacement because the ratio of engine speeds (RPM) is greater than the ratio of torques produced from each motor.

...at least I think.
 
You are absolutely correct in your understanding that heat is undesireable. The science of an internal combustion engine goes basically like this..... the atmosphere is approximately 70% nitrogen, This is a gas that expands dramatically when heated. Roughly 25% of the remainder is oxygen and the rest made up of other gas traces.
Simplified, the oxygen supports burning of the fuel which in turn produces heat to expand the nitrogen. It is the rapidly expanding Nitrogen which applies pressure to the piston to drive the piston to do the work. Unfortunately this molecular process cannot occur without rapid heat (and lots of it) and the more you make the more efficiently the engine produces power up to an ideal thermal limit. The challenge in developing any very high performance engine is to keep the intake charge as cold as possible (to improve volumetric efficiency via greater gas density) then create maximum combustion temperature in the cylinder (to get maximum work from the expanding nitrogen) and still be able to control the unwanted heat buildup in the engine itself.
Nitrous oxide injection uses this exact same science where nitrogen and oxygen are injected as an ice cold liquid with fuel to 'supercharge' the motor, making it think it has a larger displacement than what it does. All forms of supercharging simply force feed the engine a greater volume than it can aspirate (breathe naturally).

So, the more atmoshere you can pump through the engine in a given time period the more power can be made. To make more power (without the above mentioned use of supercharging) you need more capacity (bigger displacement at same RPM) to pump that nitrogen/oxygen/fuel combo or you need to pump it faster (same displacement at higher RPM).
The hp/torque characteristics are largely (but not entirely) dictated by the engines bore/stroke configuration. Small bore long stroke = lower revs greater torque. Large bore short stroke = higher revs(greater outright HP potential) and proprtionally less torque developed in a narrower rpm range.
Camshaft technologies do a lot to help overcome torque losses in short stroke engines (and higher compression also helps) but I was making direct comparisons assuming both these theoretical engines were using the same compression and valve timing.

Skipping back to engine configuration the maths get more complex. The longer the stroke at a predetermined RPM the faster the piston must move between stationary at the top and stationary at the bottom. The kinetic mass of the piston can be calculated but the key factor is, a twin of the same displacement of a four needs to have more stroke. The increased stroke means increased piston speeds (and exponentially increased piston kinetic mass) in proportion to a 4cyl at the same RPM. The RPM/HP capability is directly related to structural limits of these different designs and explains why the twin cannot rev as high simply because the internal forces would tear the engine apart at equivallent RPM to the 4cyl.
It is because of these very mathematical facts/limits that Ducati were allowed to use 1200cc twins in order to compete with production 4cylinder 1000's. A WSB spec 999 motor structurally fatigued itself to a citical failure point within 20 hours because the 12-13000rpm limits they were using to try to make enough power was simply too much stress for a twin configuration.
 
You're right Cucci, it's more than most of us will ever need to know but the topic is still enlightening.
This kind of science/engineering is best left to the wizards who design and develop what we all love to ride!
 
Amen to that..... My cheeks constantly hurt ever since i got it.... can't stop grinning like an idiot whenever I'm on it.
 
Very good explanation.

I was being very simple in my explanation but i na round about way, was alluding to just that.

The thing is, heat is a byproduct (a necessary one if you know anything about thermodynamics) of the exothermic (no, I'm not using big words to look smart) process of combustion.

Power is a given amount of energy over time, so the faster you can produce work (or torque in this instance), the more power you achieve. Conversely, the greater the torque you produce in that given time, the more power you have as well. That's the trade-off between the two motor configurations. It just so happens that the limits on rotatioanal speed (RPM) more greatly affect the overall power of a motor than the produced torque due to mechanical limitations of the stroke length (and in turn the piston speed) of the piston.
 
Did someone say mathematics ? I do love a good equation

Imperial (HP & lb-ft)

Power = Torque X RPM/5252

Metric (KW & Nm)

Power = Torque X RPM/9549

Using the imperial eqn (developed by James Watt - steam power dude), the 5252 figure is the RPM at which power and torque cross over.

EG - if a motor makes 70 hp at 5252 rpm then it will also be making 70 lb-ft of torque at the same revs.
 
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