Engine question (for the engineers!)

[Squashed_Fly]

Why does a V engine produce more torque than an inline engine?

Is it because (on a litre bike example) as there are only 2 cylinders (I'm presuming on a twin?), they are much bigger bores so in theory 500cc each cylinder, thus producing more power, whereas the inline 4 engine of the same size only has 250ccs per cylinder?

Or have I got the completely wrong?

Can anybody explain it in laymans terms?

[Nooj]

A twin doesn't necessarily produce more torque, it just produces it sooner.

There are lots of variable involved (firing order, size and shape of exhausts and velocity stacks, valve sizes and timing etc.), but generally, bigger pistons with longer strokes (more common on twins than fours) produce more torque earlier on in their rev range.

[Snowy]

Power and Torque are two different things in much the same way as Voltage and Current. Not easy to explain in laymans language, try a quick google and see how you get on.

[jonnydangerous]

nooj has hit the nail on the head....
because of the mass of the rotating and oscillating parts on a twin being heavier, they generally have a lower rev limit, therefore you can have the same "mean piston speed" (the actual speed im metres a minute that the piston moves at, cos if it goes too fast it wears too fast) with a longer "Lever" (the crank offset from main bearing centre to big end centre x 2) to create a larger "turning moment" (Torque).
a v twin will generally have a longer stroke

if you can imagine, a short stroke 4 cylinder 1000 does 13,000 revs, where as a long stroke twin 1000 does 10,000 revs. the long stroke has further to go than the short stroke, so the mean piston speed is higher at the same revs, but ultimately the mean piston speed is the same at both redlines!!!!!....
therefore less levering effect on the 4 cylinder 1000, and more on the twin 1000 at the same revs...so more torque to the twin at one set revs.
and as nooj said, they make approx the same flat out, but at different revs.

with me so far?.....

4 stroke normally asperated engines have now reached peak volumetric efficiencies (about 97% for a good one) that means that a 1 litre bike draws approx 970cc of air in one "cycle (2 revs) as it only loses a bit to turbulence and heat absorbtion.

the only way that manufacturers can now get more power is by higher rev limits (shorter strokes as the mean piston speed cannot go any higher) or by supercharging (positive displacement or exhaust driven) as these allow a higher Volumetric efficiency than 100%.......

it wont be long before we see a return to turbo bikes OR the return to bikes with mid range power, and less top end (as soon as the public realise that top end is useless on the roads apart from about 3% of the time)......

now, ask me a hard one???...... ;-)

[Jon_W]

Ok, back to basics.

Power = energy relesaed in each fire X number of fires per second. (or to quote Keith Duckworth, "the size of the bangs times the number of bangs per minuite&quot

Torque = The force developed by each fire X the length of the crank. Imagine a lever, the more weights you pile on it the more that has to be put on the other end to balance it. The same if the weights are further from the fulcrum.

Force = pressure X area of piston

There are limits to engine design to consider as well. The piston speed up and down is limited by design due to friction. also the amount of time that the charge has to flow into the piston limits the top end performance.

Ok, here's where it gets fun. A two cylinder engine produces a few bigger bangs for a given rpm. The bigger bangs are however not as punchy (hence lower pressure) due to the larger volume. To keep the volume down and get a bit more work, the stoke in increased. This gives greater torque but at the cost of rpm due to piston speed limitations and charge inlet times.

The four cylinder engine has a smaller combustion volume and can get much better pressures, but the pistons are smaller, hence less force. The soultion is to reduce the stroke and enlarge the piston diameter, this gives a greater rpm but can cost torque and top end.

The torque differences can be settled by the gearbox...

As you can see there is a lot of difficult compromises to make. And that's just in simple terms. The complexitied of cam profile, combustion chamber shape, inlet and exhaust lenghts, pressure waves etc all have a part to play as well.

Hope that answers the question.

[Snowy]

Well, you did ask..... ;D ;D ;D

[Jon_W]

Lightweight diesel.

The future. :P

[Squashed_Fly]

So if ultimately, they make about the same power at the top of the rev range, yet V-twins have more usable power/torque lower down the rev range, and what Jaydee says is true about only getting near the top of the rev range 3% of the time on the road, then why don't more people have V-Twin engines where they can use more of the power more of the time?

There must be a good argument somewhere for having to thrash the nuts off something to get it to work as well as something that doesn't need the nuts thrashing off it...?

Or have I missed something in the process? I am trying my best to follow, understand and keep up!

[Mark_Able]

Four cylinders is smoother. V-twins, no matter how well designed, still vibrate a fair bit. Some people don't mind (Harley riders), others prefer the refinement of a four.

[Dabz]

or get the best of both worlds and go ask Triumph for one of their lovely triples