Nope, not by a long shot. I’m sure some of that is lack of investment in the technology compared to four-strokes, but as far as real efficiency goes, the big diesels in large ships have them beat handily. I’m talking about those gigantic diesels with cylinders you can climb into, and have a peak rated speed of 100 rpm.
small two strokes simply can’t breathe well enough to make those kind of efficiency numbers. A four-stroke has ~180° of crankshaft revolution for intake, and another 180° to clear the cylinder of spent gases (exhaust.) A two stroke, depending on design, has maybe 45-90° of crankshaft revolution to do both at the same time. those little radio control nitro engines get around this by burning fuel with a lot of oxygen contained in it (via the nitromethane.)
Some of the newer developments in opposed-piston two-strokes look promising. I can’t think of the name of that concept engine design that’s being shown off right now, but they’re making reasonable claims and have credible people involved. I’m not sure how they’re going to deal with all that reciprocating mass, but we’ll see.
Two-strokers are better supercharged (vs turbo charged), because a two-stroke is free of valvetrain and will generally spin like the dickens, so it has a power curve that it top heavy. Supercharging is most effective at boosting low-end power, which is precisely where you’d want it in a two-stroker.
A four-stroker invariably moves power down in the power curve. Better breathing makes this happen. You get better efficiency and you give up screaming RPM and wicked throttle response that two-strokers enjoy.
In R/C engines, the two strokers light up so fast it is frightening, as they come to bat with few moving parts, and a nitromethane mix of anywhere from 10-40% (or more in some applications). The fuel is over 30 bucks per gallon.
Switching to four stroke engines has been tried, but nothing matches the throttle response of a 2-stroker.
I think what you are referring to are 2 stroke engines, as are many (most?) diesel engines. In the case of low RPM engines, there is plenty of time to eliminate exhaust and compress fresh air and fuel intake within a single stroke. For high speed 2 stroke engines, this portion of the cycle is a compromise. But I repeat, fuel efficiency is only one measure of engine efficiency. Weight, cost, duty cycle, lifetime, and performance under specific conditions alter the specific definition of efficiency.
from a physics standpoint, it’s the only one. small two-strokes consume more fuel for a given amount of power with all else held as equal as possible. you can take other characteristics such as power-to-weight ratio, mean time before overhaul, or whatever and try to call them “efficiency” but that’s a colloquialism at best.
No, I shouldve clarifiied better. What I meant is that the cylinder ports were responsible for the intake and exhaust, unlike the diesels that use a poppet for the exhaust. Having the exhaust port at the bottom of the cylinder is both its greatest asset and biggest problem, especially when it comes to positive aspiration.
What I originally meant was that the use of poppet valves makes it not a true two cycle in my opinion, regardless of how many power strokes per cycle.
Those super huge diesels in ships, ARE two cycles. The largest engine in the world is a two cycle. I was just watching something about them on Discovery channel a couple days ago. Apparently some use a slave cylinder to charge the chambers.
And aspiration is definitely the problem with two strokes, that’s why I want to find a way to charge them. I think I may have found a solution, but it’s impossible to test with no funds, so I guess we’ll never know.
but what causes them to lose efficiency? Is it the speed of the reciprocating parts? It just doesn’t make sense that something with like, 5 moving parts would be less efficient than something with over two hundred? The only thing I can think is design of the exhaust ports and the reed valves.
Not a colloquialism. Those are engineering terms. And even in physics fuel efficiency is meaningless for a hypothetical engine. Once an engine is used to do actual work (which is what makes it an engine and not a theory) and the definition of efficiency varies. Real engines have produce heat and friction, and also require oxygen and (usually) lubrication, not just fuel.
Fuel efficiency in engines is governed by the thermodynamic cycle. Very efficient engines often have more moving parts in order to more closely conform to an ideal cycle.
Would love to go in depth on this, but the phone is ringing. Not the good phone either.
Well, there is no great way to create the proper duration for intake and exhaust, because as the piston is coming up to compress the fuel air mix, it’s also exhausting the latest exhaust.
That’s kind of sloppy to say the least. Out the exhaust invariably goes some am’t of fuel/air mix. On aggressive 2-stroke engines, the exhaust can even be dripping wet.
On a four stroke engine, the fuel/air mix is compressed with the combustion chamber sealed.
On the exhaust stroke of a 4-stroker, the intake is sealed and the exhaust is open, sometimes until just a moment after the piston reaches top dead center. Getting twin-cams takes this farther, because exhaust and intake timing are working on their own cam, so you can get aggressive on the intake stroke, but not have it create too long of a duration on the exhaust stroke (because too long of an exhaust duration can create a sucking action back from the exhaust).
These are just some of many examples that demonstrate how a 4-stroker gains in efficiency over a 2-stroker and how twin cams are better than single cams. You want a good fuel/air mix, you want good compression and timing, and you want to fully expel exhaust gases.
Aspiration is a deffinate problem with a crankcase asperated engine. Two problems. The intake and exhaust port are both at the bottom of the cylinder the intake air has to blow the last of the exhaust gases out of the cylinder. With the ports at the bottom of the cylinder it isl harder to clear the whole cylinder. The second problem is the small volume and low pressure of the air coming from the crankcase.
Also another problem is with no oil in the crankcase the bearings have to be made of softer material. That means lower mean effective pressures.
I believe a lot of two cycle marine engines are uniflow design. With the intake valve at the top of the cylinder and the exhaust port at the bottom. The air enters the top of the cylinder and blows out the botttom, less eddy currents and a better purging of the cylinder. With a high boost pressure better cleaning and with a cam operated intake valve the intake valve closes after the exhaust valve.
A 2 cycle engine per stroke is not as efficient. On the power stroke the exhaust valve has to open sooner before the end of the power stroke while pressure is still elevated. On ported engines the exhaust valve closes after the intake port. That means lower initial cylinder pressures. But a 2 cycle engine has twice the power strokes for a give RPM. For the same engine power less weight.
Most diesels I have worked with stationary have been 4 cycle.
Do not know what I was thinking the intake is ported and the exhaust is valved. Sorry brain fart.
Use to be that the way the rings were checked on a marine engine was to crawl in the intake maifold. Use the jacking gear to roll the engine over until the rings are at the intake port.
Well since this thread was revived and has evolved into a general discussion about two-stroke engines, I have another question. Why aren’t small diesels two-strokes? Gasoline two-strokes are less efficient because they can blow part of the intake charge straight out of the exhaust port, but with a diesel, it’s just air blowing in. I would think a two-stroke diesel with forced induction would be just as efficient as a four-stroke diesel and could be much smaller, but obviously I’m missing something. What is it?
GM/Detroit Diesel truck engines were two-strokes from the late '30s to about the mid-'90s. The 6-71 was probably the most popular version; the V6 and V8 versions were very common in city buses and fire trucks up until the late '80s or so. As I recall, even with DDEC electronic controls, they ran dirty and were deemed not worth trying to bring forward into compliance; and Detroit eventually discontinued them in favor of four-strokes.
plus, even by diesel standards they were horribly loud; they had the nickname “Screamin’ Jimmy” for a reason.