Relative advantages of 4-Stroke Auto Cycle compared to 2-Stroke, as well as Diesel Cycle?

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Oct 21, 2021
I'm aware this is likely a silly sounding question, but I still don't understand the relative advantages and disadvantages of different piston engine types that well, particularly not in relation to their potential uses in WWII aircraft. I know that there WERE some experiments with both: the Rolls Royce Crecy was a 2-stroke engine design that promised power similar to a late war R-2800 in the relative packaging of a Merlin, and the Germans had a turbocharged Diesel engine on the Ju-86P for trying to squeeze more range and altitude out of the design

Here's what I DO know:
-2 Stroke engines have a power stroke and a compression stroke: exhaust and intake happen during these, rather than sequentially. I could see some potential loss of efficiency from this, but I don't know for certain
-Diesel Cycle engines don't use a spark plug: instead, the "raw" air is compressed by itself, and fuel is sprayed directly into the cylinder, where it combusts. This means, as far as I know, no risk of engine knock, and possibly no risk of detonation
-2 strokes need to have Oil mixed into the fuel, and operate on a "lossy" system. In present day this raises emission concerns, but I don't see this being an issue for planners of the time. I've heard that 2 stroke engines often use a 32:1 mix, which would mean a 2 stroke plane at this ratio would be using around 3% of its effective fuel tank space for oil storage. That actually sounds like the same amount that planes carry in their dedicated oil tank, though…

So what am I missing here that makes the near unanimous of 4-stroke Otto so obvious to designers of the period? I KNOW that there must be some other serious tradeoffs that I just haven't noticed, or haven't fully appreciated yet, and I can't hope to even begin to understand engines if I can't understand something like this
This is a big topic, but just a few random points to get some discussion going..

  • Blending oil with fuel is necessary in simple low tech 2 stroke engines (think chainsaws, mopeds etc) that use the crankcase to pressurize the air-fuel mixture and force it into the cylinders. Bigger 2 stroke engines tend to use separate compressors, thus no need for oil blending in the fuel.
  • 2 stroke diesels with uniflow scavenging are successful to this day in marine engines. They were also successful in faster revving smaller diesels e.g. for trucks, although eventually tightening emissions regulations killed them. I'm not sure why it's harder to meet the emissions regulations with a 2 stroke diesel than with a 4 stroke. Maybe the 4 stroke just has more time to burn off the last remnants of the fuel instead of exhausting it.
  • Otto engines generally provide better power/weight as the much higher cylinder pressure in a diesel requires sturdier construction.
  • 2 stroke Otto engines tend to have poorer fuel economy than a 4 stroke since some of the air fuel mixture tends to escape through the exhaust port before having a chance to contribute to making king the engine turn. This can be alleviated by direct fuel injection and not starting the fuel injection until the exhaust ports have been covered by the piston.
  • To the extent power is limited by cylinder cooling, a 2 stroke not "wasting" every other stroke doesn't provide a benefit.
An observation/life experience by an old timer. Model airplane glow plug engines are Otto cycle therefore R.P.M. is usually desired rather than fuel economy. Once, I was given a two cycle lawn mower because it did not have the power to cut the thick grass in southern Louisiana. After using it a couple of times, it was obvious that full throttle was necessary to cut, and if rpm dropped, retreat, allow rpm to build to full throttle before reentering combat. Upon closer examination, I realised this was simply a large model air plane engine with magneto ignition. The exhaust ports were three 1/2 inch holes. I applied simple model engine hop up techniques by filing the spaces between the holes to leave a simple guides to keep the ring on the piston. My estimate was about double exhaust area. Then I noticed a throttle limiting device, which was removed allowing full intake opening. After reassembly, the next grass cut was much easier, as it was possible to add throttle as needed. Finished, just before clean up, I stood revving the engine open and closed as in a motorcycle, when there was a loud crash and I found myself standing in fire up to my ankles. The rod had broken about 1/2 it's length, allowing the shaft to swing the remaining piece of rod through the aluminum crankcase, cutting it nearly in half. Years later, at a garage sale, I saw a fellow looking at the only other one I have ever seen, and advised him not to get it. With the muffler off, I showed him that only a small diameter was open in the center exhaust hole, the others crusted over with the burned oil from the mix, and warned him of the urge to increase it's power.
Here's an apropos saw from Stanley Hooker (yes, the R/R supercharger/turbine bloke) on the subject:

"4-Strokes? That's one stroke for power, & three to wear the engine out".

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