Piston reversal.

[andypdq]

Its also worth noting (as I said above) that TDC will occur in a slightly different place - therefore cam timing will be slightly out as will injection timing

Needs retarding, degree cam to equal valve overlap at tdc on exhaust stroke.

[Mike]

So from beginning of this thread you've gone from being confident that this will be detrimental to power output and anyone who thinks that it will increase power is an absolute idiot, to your present position where you admit that it probably will. Hmmm.

No, I still think it won’t produce anymore power in a Transit engine, but that doesn’t mean that I’m not interested to see the progress.

[andypdq]

Some performance engines have offset cylinders, relative to crank centreline and Henry Ford used it with his ground breaking flathead V8 in 1932. The centre line of the cylinder block is offset to the right relative to the centrelline of the crank the in a clockwise rotating engine, this gives a mechanical advantage in that tdc for the piston occurs after tdc for the crank, so the conrod is more vertical relative to the piston than in a conventional engine when the power stroke initiates,and the crank is past tdc, this produces a larger vertical vector and a lessened horizontal vector, less side thrust on the piston, less friction, more torque at the crank. Reversing the piston pin offset should theoretically (Maybe... lol) do something similar in a conventional engine.

https://www.youtube.com/watch?v=8bFhc05CGcw

[bambi mk 1]

Why don't you try a triangular piston with 3 conrods as an oval piston didn't last long

[Mike]

I think you're right to look at reducing friction, it'd be more productive than reversing the pistons imo. Reducing the friction at the big end is worth doing as is machine the piston skirt.

[andypdq]

Why don't you try a triangular piston with 3 conrods as an oval piston didn't last long

Why don't you try calculating vectors?

[Jim Archer]

If I may chuck my 2ps worth in.

What you are alluding to is a DeSaxe offset. This can be crank from bore centreline (ideally) or offset piston pins (work-around fudge).

The purpose is to reduce the thrust loads on the piston bearing face during the combustion/power/expansion stroke.

Offset pins mean everything can be on the block centreline, downsides are the piston wants to tip more at T/BDC and the forces only balance at a certain cylinder pressure range, and the rod pressure angle changes more than if the bore is offset to the crank.

Bore offset is nicer mechanically, but both offset and angle need to be controlled if you don't want odd load vectors creeping in. Generally is kinder to the piston skirt at BDC - which is where it matters most.

The offset can be either way, and amount depends on what power characteristics you want to achieve, low down torque, high Rpms, etc. Diesels run different angles and offsets to petrols, etc. Rod length changes the pressure angle considerably.

TDC is TDC, it's when all 3(or 4) centres are in line, there is no leverage advantage, and no change in TDC point of crank rotation, what will change is the Sine curve the piston travel to crank angle path follows around TDC. The advantage comes from the piston being moved down the cylinder at an angle relative to the 'TDC' angle. The important piston travel/rod to crankpin angle is around maximum combustion pressure.

Just to add to the fun, you can use both offset pin, and offset bores together i.e. Mahoosive power source stuff.

Popular at parties me

Jim

[Mike]

What about the effects on piston dwell time at t/bdc?

[Jim Archer]

Depends on the geometry, rod length is the governing factor, offset just biases the Sine path before or after TDC. Dwell time isn't that critical other than trying to avoid detonation in petrols. The longer the dwell at TDC, the faster the piston moves away down the cylinder - you need the expanding combustion gases to be able to keep up rather than chase the crown. The important bit is trying to get max cylinder pressure to happen around 15° of crank angle - +/- whatever your geometry fiddling has let you fudge the rod to crank pin angle.

Not wanting to tell Nan how to suck eggs, but the ideal is for the piston to push the crank round rather than down through the main bearings - there is a trade off here obviously, as you only get so much effective crank angle verses time for the expanding gases to push on the piston with enough pressure to harness effective torque. In essence we start combustion early, and trade off forces trying to shove the piston through the side of the cylinder, and the crank out the bottom of the cases, to achieve the best average torque production at the crank.

Jim

[Mike]

That’s why lengthening the rod and raising the gudgeon pin works, and would be a much more worthwhile exercise imo, especially in a relatively low powered (hp/litre) engine.

[andypdq]

Hi Jim. From doing a bit of trig, stroke/rod ratio is critical, if you have a relatively short rod, which the engine I'm playing with has, then the reversal of the pin offset should, give more power gain than with a longer rod. As far as I'm aware the only reason manufacturers offset the pin to the left in a clockwise rotating engine was to reduce noise from piston slap in a cold engine. Can't have your expensive brand new car rattling like a bag of tip nails when it cold starts! The piston will transition on the compression stroke before tdc and ignition, so there's far less pressure on the piston and less noise than if it transitioned at tdc or after. Otherwise I don't see any advantage in the standard configuration, more side thrust, more friction, less efficiency, less power..

[Jim Archer]

Offset depends on the rod/stroke ratio in essence, longer rods invariably give more/better power in the same motor than shorter ones - which is why modern stuff has long rods, short crown heights, and oil cooled pistons to keep the piston pin bearing happy.

The reduced rod to crankpin angle then means your piston needs less support 'cos it's subject to less side thrust loadings, rock less at the around TDC transition. Less side loadings mean a shorter skirt can be used, which means less friction, which equals more energy shoving the crank round.

Give it a go, it'll either make next to no difference, or open the door to a whole world of arseache

Jim

[andypdq]

It won't make much difference, but gain a bit here and a bit there. Funny story, the guy who first tried this was renowned for pushing the rules. On one occasion, when the rules stated that you could only have a certain capacity fuel tank for the race, but the size of supply lines was not specified, he installed a BIG diameter supply pipe from the tank to the engine.

https://en.wikipedia.org/wiki/Smokey_Yunick

[Mike]

Anyone like to have a guess (educated or otherwise) why the dish in this piston is offset?

[dumper]

So you know which way round it goes or the squish effect I think if I remember it’s uesd on petrol engine with carbs pushes the mixture toward the spark plug for better combustion .