Mar 13, 2008

TVS Flame: What is this port business

TVS Flame 125 image from tvsflame.comSince Arpan wants to know, and I am sure he isn't alone, let me explain this.

Have seen the metal tube that projects out of any on the engine side? It's just a hollow metal tube with a groove at the end so that the round clamp has a footing to hold the rubber boot (intake manifold is the formal name, I think) firmly in place.

What the TVS Flame does is horizontally split the tube into two halves. If you saw the tube head-on you would see a horizontal metal plate (parallel to the ground) which neatly splits the tube into upper and lower semi-circles. If you looked down the tube, you would see the yellow metal of the carburettor slide. When the slide starts to move up as you open the throttle, the air-fuel mixture first flows into the lower chamber. When it has moved up far enough, it will start to fill the upper half as well. With me so far?

The two parts of the tube actually route the fuel into separate intake tracts, each leading to one intake valve of the three valve engine (two intakes, one exhaust). It's these two valves that TVS refers to as swirl and tumble ports. You see a swirl discussion here. Tumble is the same but it takes place vertically. As an example, most top-loading washing machines swirl the clothes around, while most front-loaders tumble them. To cut a long explanation short, swirl promotes the completeness of the burn, while tumble tends to speed it up. In effect, that means swirl will end up burning all of the gases in the chamber, while tumble will speed up the burn rate so that the four-stroke cycle can proceed faster.

So what happens inside the engine is that at slow speeds, the swirl valve operates fully, while the tumble valve opens and closes without any gases actually coming through. This should make the Flame a fuel efficient motorcycle when ridden at slow speeds. Also remember that in general, two valve engines favour torque, so it should have grunt as well. When the slide has risen far enough, the other valve now has gases to feed in as well. This time round, the three valve configuration offers more gas flow (quantity) which promotes more power once the revs are up.

The transitional space, where the tumble starts to interfere and then take over from swirl, according to TVS lasts a couple of cycles. These cycles, at say 4,000 rpm, are pretty hard to spot, so there's no hiccup in the transition at all.

While TVS did not clarify this, the two inlet tracts operate more or less the same as the ports in a two-stroke engine with the carb slide working as a piston, which is probably why TVS chooses to call the lower tube (and the associated valve) a swirl port, and the upper one (and its associated valve) a tumble port.

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