Anything to do with bikes

[sheepshearer]

i ride a '98 carbed Ducati Monster (750 4 stroke V twin)

i understand the concepts of engine braking in that with the throttle closed,  the energy of the bike moving forwards is now responsible for the energy going into the compression stroke and that energy is significant enough to cause braking. but...

although both are 4 stroke, can anyone explain to me why engine braking is more pronounced on a twin than on a 4 cylinder?

[Weerwolf]

I think it has to do with the larger swept volume of the twin. In a 4 cylinder 750cc engine, each cylinder would be 187.5cc , with a 750cc Twin , each cylinder is 375cc. So there is a lot more air to compress.
Pretty much the same reason why a 4 cylinder is smoother.

[sheepshearer]

over 4 engine revs, the 2 cylinder will have compressed 2 x 375cc, but the 4 cylinder 4 x 187.5cc. is it non-linear - in other words does it take more energy to compress a large amount once than half that amount twice?

some long forgotten physics class make me think it's Boyle's Law. wikipedia here i come...

[Chris Longhurst]

In it's simplest form, it's because you have less cylinders. Engine braking on a single cylinder bike is even more pronounced.
Imagine a 600cc single, a 600cc twin and a 600cc inline 4. The size of each cylinder respectively is 600cc, 300cc and 150cc. Typically only one cylinder is under compression at once, so with a 4-cylinder bike, you're trying to manually compress 150cc, whereas with a twin, you're trying to manually compress 300cc.
I'm sure there's a highly technical explanation of that, but my answer ought to steer you more or less in the right direction :-)

[sheepshearer]

sorry guys - i'm not convinced. the same amount of air is compressed over the same number of engine revs, end of story.

after drawing cylinder strokes out on a scrap of paper, i think i have sussed this for myself though...

consider a 4 cyl 4 stroke (straight 4):

• cyl1 - compression (valves closed, piston at top of cylinder)
• cyl2 - induction
• cyl3 - expulsion (exhaust valve open, piston at top of cylinder)
• cyl4 - ignition

the pattern repeats every 2 engine revs (not 4 as i said above) - compression, ignition, expulsion, induction (each cylinder does one of those)

now consider a V twin - the pistons will both be at the top of the cylinder simultaneously or the thing would shake apart hence:

• cyl1 - compression
• cyl2 - expulsion

half rev

• cyl1 - ignition
• cyl2 - induction

half rev

• cyl1 - expulsion
• cyl2 - compression

half rev

• cyl1 - induction
• cyl2 - ignition

half rev (repeat pattern)

in the twin, ignition never occurs at the same time as compression, unlike in the 4 cylinder the compression (engine braking) on one cylinder will always be counteracted by the ignition on another. hence 4's are smoother and have less pronounced engine braking.

that has to be the answer doesn't it?

[Weerwolf]

That might be it.
But , you do have to keep in mind , that with the throttle closed , the force of the ignition in that cyliner will be very small. With a bike that uses fuel injection , there won't even be any fuel injected at that time.

[sheepshearer]

> But , you do have to keep in mind , that with the throttle closed , the force of the ignition in that cyliner will be very small.

true - but on my carbed motor it'll still be the idle mix going in.

on an injected engine, do you know if the ECU disables the spark when the injector shuts off? i guess it does