2 stroke question

[donteatpeople]

How does the pre-compression of fuel/air in the crank case work for multi cylinder 2 stroke engines?

Do they partition the crankcase so that pressure is contained?

In the case of a V4 would the opposing cylinders have to operate simultaneously to allow max pressure of the mixture to be released into both cylinders?

[Pigeon]

donteatpeople said:

How does the pre-compression of fuel/air in the crank case work for multi cylinder 2 stroke engines? Do they partition the crankcase so that pressure is contained?

I believe this is the usual solution, yes. I don't think the partitions necessarily incorporate such effective seals as are used on the ends of the crank, between the crankcase and the atmosphere/gearbox, as a bit of leakage between partitions is not so important as leaking to the outside of the engine, and also it makes it easier to make.

donteatpeople said:

In the case of a V4 would the opposing cylinders have to operate simultaneously to allow max pressure of the mixture to be released into both cylinders?

I'm not totally sure about what V two-strokes do but I don't think they do what you describe as this would require a rather weird crank to get the two pots phased together (not that such cranks don't exist, but they're not a favoured solution). There are various options:

- Don't rely [much] on crankcase pressure to drive the scavenging flow, but instead rely on exhaust tuning to create negative pressure at the exhaust port while the transfers are open and thereby suck the gases through the pot - most two-strokes do this anyway to a greater or lesser extent; it's especially important at low throttle settings where the crankcase compression won't be creating much in the way of pressure to start with. (Having a multi-cylinder engine gives you more scope for playing clever tricks to make the effect work over a wider rev range, by arranging that as the negative pressure pulse from one cylinder moves out of the useful region, a differently-timed one from another cylinder takes over.) You would also do things like tweak the port timing of the two cylinders such that the later cylinder has longer transfer timing to compensate for the lowered crankcase pressure it sees.

- With a loop-scavenged two-stroke it is in any case difficult to pack the cylinders as closely together as in a four-stroke with the same bore, as the transfer ports sticking out of the side of the pots like ears tend to get in the way of each other. Since you're forced to have a wider crank pin spacing anyway, it can make sense to have more of a stagger between the banks - have it so that a pot on one side is half way between pots on the other side - and not do the sharing-crankpins thing, instead giving each crank throw its own partition, so it's basically the same setup as for an inline engine but more compact.

- Abandon crankcase scavenging altogether and supply the scavenge flow from a blower. Two-stroke diesels tend to do it this way, as the scavenge volume is not limited by the cylinder capacity so you can achieve better scavenging, which is more important on a diesel, and also with a diesel you don't get the problem of wasting fuel out of the exhaust due to overenthusiastic scavenging, nor do you get a scavenge gallery full of pressurised fuel/air mixture with the potential for highly destructive backfires

[donteatpeople]

Interesting. I thought of supercharging as an option last night.

I am thinking of designing a multi cylinder nitro engine for my final year project at uni, either a V twin so I can use standard heads or a V4 for a model NSR500.

Supercharging would simplify things as there would not have to partition the crankcase and I could Have any firing pattern I like. What implications would there be for the exhaust if any?

[Pigeon]

The main effect of supercharging on a piston-ported design is to reduce the transfer times necessary because there is a steady pressure feeding the transfer ports. As far as the exhaust goes there is less need to arrange for a negative pressure pulse to arrive at the exhaust port during the transfer period - but it still helps, and you still need the positive pulse between transfer closing and exhaust closing, so there's not much difference in the exhaust design you end up with.

A nitro engine is more like a petrol engine than a diesel, in that it breathes through a carburettor or equivalent and throttles its inlet to control power. Since you may not be getting a full charge, at part throttle, you do still need a negative pressure pulse. Since it's breathing a combustible mixture you'll need a blowoff valve in case it backfires.

The exhaust for something that small and high-revving will be fun to make...

A V4 could still use standard heads - I'd be inclined to bolt four standard barrels onto a common crankcase - and gives you the advantage of equal firing intervals.