Squish band

[crmachineman]

Yeah, I could go on and on about combustion chamber design, and since Jaguar started this thread, giving terrible recommendations, I feel obligated to correct this misleading talk.

You see, he wants to sell those ignition retard devices. A well designed cylinder head is the correct solution for the problem which his ignition device attempts to deal with.
So Jaguar is attacking a competing solution to the problem, which is a properly designed cylinder head.

Retarding the ignition timing away from the optimal factory setting is a compromise at best.
The correct solution is what all the leading racers apply to the situation; a properly designed cylinder head.
This solution to the problem of detonation allows you to maximize your compression for whatever configuration you are running.

Whether it is a fully ported race engine, running 110 octane fuel, or a restricted stock engine running 91 octane fuel, bolting on a appropriately designed cylinder head to maximize the compression while minimizing detonation is the simplest and most efficient way to make the most out of your fuel.

The rigors of racing is the most effective way to improve designs for ordinary consumers. This is why Toyota and Honda are heavily involved in racing.
-Fred

 

[Fabian]

I feel obligated to correct this misleading talk. You see, he wants to sell those ignition retard devices. A well designed cylinder head is the correct solution for the problem which his ignition device attempts to deal with. So Jaguar is attacking a competing solution to the problem, which is a properly designed cylinder head. Retarding the ignition timing away from the optimal factory setting is a compromise at best. -Fred

I wouldn't say that is a true statement regarding the Jaguar CDI, because the standard CDI supplied with Chinese bicycle engines uses a 4-stroke ignition advance curve.
Since changing to the Jaguar CDI i have had no more big end connecting rod bearing failures; in fact the bottom end in my last engine gave me 10,000 kilometers before the connecting rod big end bearing became too noisy to put up with.
I am absolutely confident to say that the standard CDI would have pounded out the big end bearing well before 2,000 kilometers under high duty cycle operation.

Most definitely i do endorse the Jaguar CDI with it's adjustable ignition curves, and i am currently using the CR Machine Manufacturing 3 hole cylinder head.

 

[Fabian]

And the factory setting of the standard Chinese bicycle engine CDI does not appear to use an optimal setting for a 2-stroke engine.

 

[Deleted member 12676]

Fabian, thanks for another voice of reason.
Fred, you are making yourself look ridiculous now you are so far off in left field. High compression with squish band does not solve any problem at all, but rather creates a new problem. Pre-detonation can occur even with a squish band if the ignition is too far advanced (as is the case with the standard CDI). With or without a special head the Grubee timing needs to have a normal ignition timing retard at high rpm to reduce mechanical stress and improve power and reduce excess heat. Raising compression raises heat which is why more generous finning is useful to counteract that effect. If you raise compression you need to re-establish a normal ignition timing curve. It is not an either/or situation. There is no battle between the two parts, they work together. Here are some sections of the research paper "The Effect of Higher Compression Ratio in Two-Stroke Engines" that says the same thing:

 

[Fabian]

It's interesting to note that Honda spent a great deal of effort in refining gas locomotion and combustion effects as it moved through a 2-stroke engine when developing their 500cc Grand Prix engines, before they moved to the current 4-stroke formula which is called MotoGP

It's also interesting to note that Honda reduced the compression ratio in the efforts to raise rpm because the engine was effectively fighting a gas spring as compression was incrementally increased, which wasn't fully recovered by any improved combustion efficiency.

Sometimes you can have too much of a good thing, hence the reason why i went with the 2 hole and 3 hole CR Machine Manufacturing cylinder heads, because the extra internal volume of the spark plugs allows you to play around with compression ratio (reducing the ratio), especially if using a third spark plug in the 3 hole CR Machine Manufacturing Billet cylinder head, which is exactly what i have done.

Now that CR Machine Manufacturing has introduced their low compression cylinder head with a designed squish band, it will be the next thing on my order list.

My ideal cylinder head would be a 4 hole cylinder head with three radially spaced spark plugs at 120 degrees and a centrally located spark plug for the conventionally accepted method of combustion initiation.

I've always toyed with the idea of a variable ratio compression ratio 2-stroke cylinder head by having adjustable gas chambers located behind the cylinder head combustion chamber wall. It's a bit hard to get your head around the idea or to visualise it, but i would love to have access to a workshop like CR Machine Manufacturing to create one of these designs.

 

[crmachineman]

There is a bunch of magnets that have been recently coming into the US in which the keyway is broached in the wrong position, making the ignition timing 10-20 degrees more advanced than older ones.

When one of those magnets was installed in one of my test motors, it could hardly stay running, so I can't use them. I have a dozen of them sitting on the shelf.

There may be a lot of those magnets around. In this case, a CDI, like the jaguar one would be the only way you could make the engine run properly. So for those cases, I agree, it would be extremely helpful.
This was a factory mistake and I believe that it will be corrected.

But according to the current leaders in the motorized bicycle racing, the stock advance curve is very close to ideal. I have also run my own tests using various scooter CDI's and really didn't find any performance gain. I even briefly ran a twin ignition cylinder head, powered by a scooter CDI, which ran very well.

Sure, I have retarded the ignition timing on vintage cars because of poor quality fuels.
It would not be convenient to change or modify all of the pistons, or re-machine each combustion chamber, so retarding the ignition timing is the only course of action. It is however, still a compromise, and not ideal for maximizing output for the given engine displacement.

These bicycle engines are so limited in displacement, many people are truly searching for maximum performance out of these motors; it's what keeps me very busy.

After all, wasn't this thread all about maximizing performance?

Jaguar, on the bottom of all your responses, you have tips for getting more power: Compression is one of those tips.

Well, if you study what the leaders are doing, it is squish band type combustion chambers with raised compression, adequate cooling and stock ignition.

How can you argue with the true facts.

Several times a year in Southern California, the recipe that I am citing is the one that routinely wins, and is therefore proven to be the most efficient. Looking at the matter from that point of view is simply logical.
-Fred

 

[crmachineman]

Oh, and in case you would like to get a visual idea of this performance recipe that I'm talking about, check out this you tube video I made yesterday.

This bike has 26 inch wheels and a 32 tooth sprocket. The engine is a PK-80 70cc motor with a Mikuni VM-18 carburetor(not that radical) The head is a low compression head (7.2 cc combustion chamber), and a fair exhaust system.

This engine is using 93 octane pump fuel. This bike easily reaches 50 MPH for sustained periods of time, and is very, very reliable.

In fact this engine, even with it's significantly enhanced output is much, much more reliable than lower powered units that I have tested in the past using the stock, non squish band cylinder head.

You may notice that at the end of the run, I put my hand right on the cylinder head, showing how little damaging deto there was, if any. The same holds true with my higher compression version which runs on 110 octane fuel.
http://www.youtube.com/watch?v=WEPGNUF3EAo
-Fred

 

[Fabian]

Fortunately i am able to do a real world comparison between the standard CDI and the Jaguar CDI with one black box set for the very lowest ignition curve and one black box set for the highest possible ignition curve, as i run twin Jaguar CDI's (though only a single CDI is used to fire the ignition) so i easily can flip back and forth between max and minimum ignition curves, and also flip between Jaguar CDI's and the stock CDI when i'm playing around with different cylinder heads.

What i can say is that the standard CDI makes less power than the Jaguar CDI set at the lowest possible ignition curve and the lowest curve produces power that gives significantly reduced engine vibration. The highest ignition curve gives the most power but increased engine vibration.
The standard CDI is the worst performer giving slightly higher vibration over the Jaguar CDI set to the maximum advance curve as well as delivering reduced engine power.

For this reason i do not use the standard CDI because it gives less of one thing and more of the other in the wrong manner of operation.

It's handy having the three hole cylinder head fitted with three spark plugs because i can hang 2 Jaguar CDI's and a standard CDI from the cylinder head for back to back testing.
They are all fitted with banana plugs, making it a simple manner of swapping over magneto wires to the different CDI's.

 

[crmachineman]

Okay. Well, that's interesting. Good job with that testing method. Well, I still strongly disagree with what Jaguar says about squish band combustion chambers not helping. I simply have way too much proof otherwise. Is the Jag unit hard to make work? I've heard people on forums spending a lot of time making repairs and such. Are they reliable?
-Fred

 

[Deleted member 12676]

As to how many of the top racers use the Jaguar CDI, my friend Dionel goes to most of them and he says at least 60% of the top riders have it.
As to the question of whether or not it is necessary to retard (delay) the ignition at high rpm I provide a detailed answer:

Shouldn't ignition timing be advanced at high rpm since there is less time available for combustion before 10-15 degrees ATDC (the ideal spot for peak combustion pressure to occur)?
Although there is less time available, because of greater intake/transfer turbulence, the combustion happens faster. Admittedly though the shorter combustion time doesn't shorten as much as the shorter degrees time.
There is a balancing act going on with the correct ignition delay.
As rpm rises the fuel/air delivery ratio decreases leaving less power derived from each combustion cycle, but there is more mechanical pumping losses (if the ignition timing remains the same) to counteract.
So retarding (delaying) the ignition at high rpm is a balance between reducing pumping losses without excessively delaying peak combustion pressure past the ideal 10-15 degrees ATDC (which reduces usable power).
The result is that some power is lost due to the peak pressure happening past 15 degrees but there are also two advantages to ignition delay. One is that less heat is transferred to the head/cylinder/piston for less likelihood of piston seizure, and the other is that the exhaust gases become hotter which causes the sound wave in the expansion chamber to travel faster and return to the cylinder faster, better matching the lessened cycle time which results in a broadening of the powerband due to the expansion chamber.

Why does a 4 stroke engine need the ignition timing the most advanced at high rpm?
As rpm rises the delivery ratio reduces which, contrary to how a 2 stroke is, also reduces pumping losses. That is because there are no remaining exhaust gases, as a 2 stroke has, to fill the cylinder to make up for the reduction of fresh fuel/air delivered. Also there is no pressure wave returning from an expansion chamber to cause more cylinder pressure (causing more heat). Since the power strokes happen half as often as in a 2 stroke (given the same rpm) there is less potential for piston seizure. And since they don't have expansion chambers there is no point in making the exhaust gases hotter to lengthen the engines powerband.

For anyone to question the method of delaying 2 stroke ignition at high rpm is just the result of not being educated on the subject. Any online exploration of the subject will give you the bits and pieces that I have put together for you here and on my pages about the Jaguar CDI (click on my signature link). It is just the best method, especially for high reving engines.