Is that the reason for your head burn chamber vs a piston burn chamber?
Partly. A precombustion chamber engine is also much easier to design than a direct injection one, I didn't have to worry about things like swirl ratios in the cylinder or the exact piston bowl geometry, just a spherical pre-chamber in the head and a passageway of appropriate size to connect it to the cylinder. Also I just wanted to be different from the engines other people had already built!
I think for making an engine that just runs it's all a bit immaterial, like Minh said his engine still runs well with several types of injector, and it has neither a bowl in the piston nor a prechamber. But if one wants to extract significant amounts of power without emitting excessive quantities of black smoke the combustion system needs some consideration. I myself don't have any particular plans on that front other than to drive something like a little generator (light some bulbs, charge a phone, etc) or a pump to demonstrate that the engine is 'capable of work.' But I enjoy the design engineering part of the project as much as building it, so I always will tend to want to do these sorts of things.
I think a piston-burn chamber was useful for also keeping te piston crown cool, as it extracted heat in vaporising the fuel - and the "wet" fuel shielded the piston locally from flames....
But with the disadvantage of a THICK crown - and consequently much more mass to accelerate and decelerate, etc. = lost work.
Head pre-combustion chambers as easier to control for temperature (inside the water cooling of the head), and fixed, so permit pistons to be optimised to a much lower Mass, Balance, etc.
Just my ideas - for what they are worth?
K2
I'm going to take an opportunity to 'Nerd out' a bit here. Diesels have one big fundamental problem: Only a mixture of gaseous fuel and air can burn, but the diesel injects liquid fuel into the cylinder at the very moment that we want ignition to occur. So we require some method to vapourise the fuel extremely rapidly, and we also need to very rapidly mix the resulting vapour with the air. All of this has to happen while combustion is already starting!
Most diesels solve the vapourisation problem by atomising the fuel into a very fine mist, the resulting high surface area to volume ratio allows the fuel to be very quickly heated above its boiling point by the air. The exception to this is the M-system I mentioned in a previous post, which instead relies on surface evaporation from the piston. Anyway, even if we were injecting gaseous fuel it can't burn if it all winds up in one little spot in the cylinder with no air around to react with. So there are basically two ways to get the fuel in contact with as much air as possible: the first is to spread the fuel mist around by firing jets of it in all directions, such that some fuel gets to every part of the combustion chamber and mixes with the air that is already there. This is the approach you might encounter in a very large low speed diesel, like in a ship. Unfortunately manufacturing constraints on the tiny holes in the injector make this infeasible for smaller engines, you just can't have enough holes of small enough size in the injector tip to have a jet aimed everywhere. This brings us to the second approach: if we can't bring the fuel to the air, let us bring the air to the fuel. By making the air move around the combustion chamber really fast, we can ensure that air goes to where the fuel is. So the opposite extreme is the precombustion chamber: there is only one hole in the injector, but we place it in a chamber in the head that is connected to the cylinder by a narrow orifice. During the compression stroke air is forced into the prechamber through this narrow orifice, resulting in it flowing around the prechamber at very high speed and mixing with the fuel which is only being introduced in one spot. Unfortunately, it isn't feasible to get all of the air into the prechamber, but on the other hand it won't take long for the pressure in the prechamber to start forcing already vapourised fuel out through the orifice at high speed to mix with the remaining air in the cylinder once combustion begins... Mercedes Benz notably leant into this fact by having multiple orifices aimed around the chamber in their precombustion chamber engines, but many others (including myself) use just a single hole offset to one side.
Most direct injected diesels use a bit of each method. So we have 3-5 holes in the injector, and a bowl shaped combustion space in the piston crown. (Why in the piston? Mostly because the valves are occupying the prime real estate in the head directly above the piston, if we were building a loop charged 2-stroke diesel we could put it in the head instead). The intake ports are carefully shaped (usually in a spiral) to induce a fast swirling motion of the air as it enters the cylinder, as the piston approaches TDC on the compression stroke the air is forced into the bowl and (thanks to conservation of momentum) the rotation becomes even faster. This motion is nowhere near as fast as what the prechamber method would produce, but we have multiple fuel jets so we don't need to get all the mixing to happen by air motion. Each jet from the injector reaches a sector of the rotating air mass, eventually leaving us with a roughly even mixture! Once again the M-system is the weird exception, it relies on very fast swirling motion (induced by its inlet ports and very deep and narrow bowl) to bring the air to the fuel film, honestly it's more like the prechamber method in that respect.
So why choose the direct injected 'mix of both' method rather than prechamber? After all the prechamber design has a simpler one hole injector, which could save us some difficulty, and has no need for funny shaped intake ports or heavy bowl shaped piston crowns. The main answer to this is efficiency. The prechamber design forces ~50% of the gases in the cylinder through a rather small hole (in my engine, a 4mm hole connects the prechamber to the cylinder. Compare that to the 16mm diameter of the inlet valve) on every compression and power stroke, which results in a lot of extra pumping work. It also has about 50% more surface area exposed to the compressed charge at TDC, resulting in greater heat losses to the cooling system. So the direct injected engine has a higher potential for fuel economy and (to a certain extent) power. Once we get to really big engines (marine diesels) we can even dispense with both, and just use an injector with a large number of holes- this is even more efficient, as the funky port shapes and fast mixture swirl of the 'mix of both' method are also a source of losses.
