Désaxé definitions

[Charles Lamont]

Checking the timing diagrams for my Westbury Seagull engine
it seems ETW did not take into account the fact that this is a désaxé (desaxe, if the e-acutes does not come out on your screen) design. Is Top Dead Centre when the crankpin is at the top or when the piston is at its highest point (3.42° later)?

 

[trumpy81]

Charles, to my mind, TDC relates to the piston position irrespective of the crank position. The center line of the cylinder can be offset in respect to the center line of the crank and this is often done to reduce piston 'slap'.

 

[canadianhorsepower]

Checking the timing diagrams for my Westbury Seagull engine
it seems ETW did not take into account the fact that this is a désaxé (desaxe, if the e-acutes does not come out on your screen) design. Is Top Dead Centre when the crankpin is at the top or when the piston is at its highest point (3.42° later)?

this is awquard to answer without the instruction sheet
here's the definition of TDC

In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).
More generally, the dead centre is any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, including unicycles, bicycles, tricycles, various types of machine presses, gasoline engines, diesel engines, steam locomotives, and other steam engines. Crank-driven machines rely on the energy stored in a flywheel to overcome the dead centre, or are designed, in the case of multi-cylinder engines, so that dead centres can never exist on all cranks at the same time. A steam locomotive is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.
with this example TDC is wrong
In a reciprocating engine, top dead centre of piston #1 is the datum point from which ignition system measurements are made and the firing order is determined. For example, ignition timing is normally specified as degrees before top dead centre (BTDC) although a very few small and fast-burning engines require a spark just after top dead centre (ATDC
with this one it should be top center only

I would say if he's talking about ign timing what you have is good before TDC

 

[trumpy81]

Luc, désaxé means that the cylinder is not inline with the crankshaft center line. As you may know, in a piston engine all events relate to the position of the piston (especially two stroke engines). It does not matter what position the crank is at as long as the piston is in the proper position for the event. In this case there is a difference of 3.42° between the piston being at it's upper most point of travel and the crankshaft throw being at it's outer most position relative to the cylinder/piston.

In all cases when talking about ignition, the events must relate to the piston position. If the spark event is specified at 10° BTDC then the piston must be at that point in it's travel in order to achieve the correct timing. If you were to use the crankshaft position, then the spark event would take place 3.42° later (or earlier as the case may be) which would result in the piston being in the wrong position for the event. The same applies for valve events etc...

The best method of obtaining true TDC is to place a dial indicator on top of the piston. You then 'rock' the crankshaft through approx. 10° BTDC to approx. 10° ATDC and note the readings of the dial indicator. True TDC will be at a point where the pointer reads the highest reading and the number of degrees of crankshaft rotation that is observed at that highest reading will be divided by two to give true TDC.

 

[canadianhorsepower]

Yes I know this. but the plans should have been clearer in whgy TC or TDC
in two stroke it is common to have it "desaxé" but on the exaust stroke

 

[Charles Lamont]

Thanks for the confirmations that I was thinking on the right lines. It was concerned with timing the camshaft, as I was about to cut the keyway in the crankshaft pinion. This is where I have decided to put the necessary offset, everything else being 'square'. Having drawn it out in Autosketch with the crankpin vertically above the crankshaft axis I suddenly realised that would not be the dead centre, and doubts crept in, as Westbury does not make it clear in the construction series. I have gone with top dead being when the crank and con-rod are in line, which, as it worked out, required the keyway to trail the centre line of a tooth by 1.2°.

Cut it yesterday. If it turns out I have boobed, I have a get-out in that I have not fixed the cams to the camshaft yet - not having made the cams.

An interesting effect of the désaxé layout is that the strokes are not equal in crank rotation. If top dead is at 3.42° crank rotation, bottom dead is at 185.10°. In the unlikely event of anyone wanting to have a play around with this, the crank throw is 11/32", the cylinder centreline is offset 1/8" and the
con-rod is 1-3/4".

 

[trumpy81]

Charles, that is a lot of offset. usually it is in the order of 1/64 inches or less and only has a small impact on timing events. In all honesty though, I think you should have waited until you tested TDC using a dial gauge. That would find true TDC. What is the compression ratio? I'm betting it is pretty high.

 

[Charles Lamont]

Charles, that is a lot of offset. usually it is in the order of 1/64 inches or less and only has a small impact on timing events. In all honesty though, I think you should have waited until you tested TDC using a dial gauge. That would find true TDC. What is the compression ratio? I'm betting it is pretty high.

I don't mean to be difficult but I'm afraid I have to disagree on all three points.

In a 4-stroke engine the main point of making the engine désaxé is to reduce the angle of the con-rod relative to the piston on the power stroke and thereby reduce the lateral thrust on the piston. 1/64 offset on an 11/16 stroke would hardly make any difference.

The thing about a dead centre is it's dead: the piston stops moving. This means it is actually quite difficult using a dial gauge to be certain at exactly what position of the crank it occurs. My CAD program and I can cope with the geometry and calculate the position to a far higher level of accuracy. Any variation in crank angle resulting from feasible manufacturing inaccuracies would not be measurable.

Westbury tended to avoid stating compression ratios for his engines. I think he expected brickbats for making them so low. He describes the Seagull as being 'docile' and 'flexible', which points to low compression ratio. I have done a reasonably accurate calculation working from the original design shape of the combustion chamber and get it to 3-1/2 to 1. The castings supplied by Hemingway do not follow the design exactly, and I estimate those as giving about 5:1.

 

[trumpy81]

In a 4-stroke engine the main point of making the engine désaxé is to reduce the angle of the con-rod relative to the piston on the power stroke and thereby reduce the lateral thrust on the piston. 1/64 offset on an 11/16 stroke would hardly make any difference.

I guess it depends on the designers intent, but as I previously stated, the désaxé design was employed mostly to control piston 'slap'. Changing the con-rod angle can give the benefit you describe but it also shortens the power stroke by an equal amount. I don't know if the benefit outweighs the negative or not?

Then there is the increased piston to cylinder pressure to consider on the compression and exhaust strokes.

The thing about a dead centre is it's dead: the piston stops moving. This means it is actually quite difficult using a dial gauge to be certain at exactly what position of the crank it occurs. My CAD program and I can cope with the geometry and calculate the position to a far higher level of accuracy. Any variation in crank angle resulting from feasible manufacturing inaccuracies would not be measurable.

Yes you are correct. But, that is why we use the rocking method of determining where TDC is, which takes into account all the machining inaccuracies etc... We don't simply stick a dial indicator on top of the piston and call it TDC, as you've pointed out, that would not work.

Granted, it is still not the most accurate method, but it has proven to be the most effective.

Westbury tended to avoid stating compression ratios for his engines. I think he expected brickbats for making them so low. He describes the Seagull as being 'docile' and 'flexible', which points to low compression ratio. I have done a reasonably accurate calculation working from the original design shape of the combustion chamber and get it to 3-1/2 to 1. The castings supplied by Hemingway do not follow the design exactly, and I estimate those as giving about 5:1.

Charles, clearly I was wrong in my assumption RE: compression ratio, I expected a high CR to take advantage of the désaxé design. It appears that E.T.W had different ideas about this.

In regards to the castings, I fail to see how the castings effect the compression ratio?

 

[Charles Lamont]

In regards to the castings, I fail to see how the castings effect the compression ratio?

The combustion chamber is cast into the underside of the head.

 

[trumpy81]

Could you not machine the combustion chambers at least enough to cleanup and level out the castings and then use gasket thickness to adjust the compression ratio?

 

[Charles Lamont]

Could you not machine the combustion chambers at least enough to cleanup and level out the castings and then use gasket thickness to adjust the compression ratio?

Why would I want to make the compression even lower? I cannot machine more off the bottom of the casting as I would
end up with the plug hole breaking into the water space. It is
all very tight. I do intend to smooth the cast surface of the combustion chamber.

 

[trumpy81]

I was not suggesting that you lower the compression ratio, I simply outlined a means of adjusting it after cleaning up the combustion chambers and the mating surfaces.