9 replies to this topic

[DeadSquare]

Improving the gas-flow of siamesed inlet ports.

 

When Austin decided that pairs of cylinder would share an inlet port on their new  A series engine, I wonder why they chose an "Up-Down-Down-Up" crankshaft ?

 

This imposed the 1, 3, 4, 2, firing order which results in the 3-4 port having to cope with two sucks in quick succession, then "resting" while the 2-1 port struggles with two sucks.

 

If we cut between 2 and 3, and join the crank and cam together again at 180 degrees to make an "Up-Down-Up-Down" crankshaft, we could have a 1, 3, 2, 4 firing order.

 

This would allow 1-2 port to devote all its gas charge to number ONE cylinder, followed by number THREE cylinder having a good suck through the 3-4 port without having to share the end of its gas charge with FOUR, because TWO would have been filling before FOUR started to suck.

 

 

I will have to ask Father Christmas for a bigger soldering iron. !

 

 

[imack]

I seem to remember David Vizard mentioning this. One problem was that it would bugger up the cylinders 2 & 3 siamesed exhaust port pulses.

[DeadSquare]

I seem to remember David Vizard mentioning this. One problem was that it would bugger up the cylinders 2 & 3 siamesed exhaust port pulses.

 

 

Oh, we can't have that.  Puffin Books would never allow B*ggering.  Not sure that they'd allow s*cking, either.

 

Fortunately, I'm  "PIEKOW"  ............  Politically incorrect and the reverse of woke.

[Steve220]

I seem to remember David Vizard mentioning this. One problem was that it would bugger up the cylinders 2 & 3 siamesed exhaust port pulses.

Absolutely! This was discussed in depth a whole ago, basically 2&3 are fighting for their exhaust strokes.

[Ethel]

I've always thought it had more to do with reciptocating mass balance. It'd be a bit of a rocker with the outer pistons going in opposite directions and the inners being short by half a crank length to counterbalance them? You could also think of the Siamese ports as a very crude form of VVT - the inners & outers will hit peak torque at different rpm, making the combined torque band wider.

[Pete649]

I thought it might have something to do with the forces on the crankshaft. You would have 1 and 4 firing sequentially i.e. at the opposite ends of the crank (which only has 3 bearings). 1,3,4,2 ensures that the bores firing are no more than 2 cylinders away.

[Spider]

As Pete touched on, it's a common firing order for many straight 4 cylinder engines to reduce the effects of the torsional vibration on the crank.

[timmy850]

I think the best way to solve the Siamese inlet ports is to get rid of them & get a nice 7 or 8 port head

[Ethel]

.... if they need solving. It's still a great little lump for what it does best.  Low initial cost, compact, good tractability & great economy.

[Ethel]

Thinking about it, I wonder if the Siamese ports contribute to the fuel efficiency? It's probably too complex to answer, but one bigger port should  allow less thermal transfer than two smaller ones & "charge robbing" ought to fill one cylinder better for a more efficient burn. No idea if it would gain more than the other loses, but a "two pushing, two idling" configuration sounds a not dissimilar concept to the Cylinder Deactivation Systems on modern highfalutin enjins.