Flat 4 or 6 cyl Aircraft Engine

[Mike S]

Originally Posted by SvingenB

An otto engine with a CR of 9:1 have the same internal pressure as a diesel engine with a CR of 20:1.

Not on the compression stroke. It will be higher on the diesel, with a corresponding pressure applied to the crankshaft in opposition to the direction of rotation. This is what I was referring to above--------this resistance to the crank rotation tries to causes flex in the crank, and is part of why diesels have such massive cranks, and flywheels, as compared to gas engines.

[rv8or]

Diamond have recently flown their own 170hp diesel

[asav8tor]

The price of fuel (all fuels) will make this conjecture irrelevant before any significant engine designs could be brought to market. Hopefully the gold rush investment in batteries (for the auto industry) will provide a viable solution we could use in light aircraft.

[Andy_RR]

Originally Posted by SvingenB

An otto engine with a CR of 9:1 have the same internal pressure as a diesel engine with a CR of 20:1.

I hope you don't spend too much money designing your diesel engine Mr Svingen, because you will be in for a rude shock when it fires up!

[SvingenB]

Originally Posted by Andy_RR

I hope you don't spend too much money designing your diesel engine Mr Svingen, because you will be in for a rude shock when it fires up!

You can relax, I have no plans developing a diesel anytime soon, turbines on the other hand

I know equal pressure is a rude simplification (as my understanding of diesel engines are ), but as far as I know the "problem" with a diesel is loads of pressure at low rpm where you don't really need it in a car, and nothing at higher rpm when the pistons start to run away from the pressure so to speak due to the slow burning fuel. In the narrow rpm band where the diesel actually work fairly close to the diesel principle, I can't imagine that the pressure is significantly higher than in an otto engine. I mean the whole idea of a diesel is to transfer heat at constant pressure, that is - expand the gas at constant pressure throughout the stroke and thereby creating massive amount of torque, mostly due to max amount of force on the piston when the "lever arm" to the crank is at optimal position.

An equal displacement otto engine, to produce the same amount of work at the same rpm, will have to increase the pressure from the compression, by combustion, to a higher value than what is needed in a diesel engine because max pressure will be closer to top center than in a diesel. But I guess this is inpractical/impossible, and it is easier to simply increase the rpm in an otto engine since you are not limited by the slow burning diesel fuel.

[rv6ejguy]

I don't see any evidence that diesel engines produce "massive torque" compared to Otto cycle engines. This is a misconception constantly re-perpetuated. Hp does the work in moving airplanes or cars, not torque which is simply force. The higher working rpm on Otto cycle engines also allows us higher gearing ability compared to diesels. The result is the Otto cycle engine will easily outperform diesels in specific output and output per unit weight. Only in fuel economy is the diesel superior due to it's higher thermal efficiency.

[SvingenB]

Originally Posted by rv6ejguy

I don't see any evidence that diesel engines produce "massive torque" compared to Otto cycle engines. This is a misconception constantly re-perpetuated. Hp does the work in moving airplanes or cars, not torque which is simply force. The higher working rpm on Otto cycle engines also allows us higher gearing ability compared to diesels. The result is the Otto cycle engine will easily outperform diesels in specific output and output per unit weight. Only in fuel economy is the diesel superior due to it's higher thermal efficiency.

If you don't believe it, try it. Take a 2L Golf diesel and a 2L Golf gasoline with manual transmission (or whatever the displacements are these days, 1.9?). Preferably you should chose a diesel model before common rail, since the common rail electronics tends to increase power when the engine rpm goes below idle.

Start on a dirt road, put them in first gear and let go of the clutch fairly fast. The gasoline will stall, while the diesel will spin and run. With the diesel you can even start in third gear without touching the throttle.

Edit: Another thing you can try. Since the common rail controller tries to prevent the engine from stalling (at least on some engines), this is a good example of the low rpm torque. Just set it in first or second or even third, and the car will go steadily uphill, downhill and so on at 900 - 1000 rpm.

[leeschaumberg]

Originally Posted by SvingenB

If you don't believe it, try it. Take a 2L Golf diesel and a 2L Golf gasoline with manual transmission (or whatever the displacements are these days, 1.9?). Preferably you should chose a diesel model before common rail, since the common rail electronics tends to increase power when the engine rpm goes below idle.

Start on a dirt road, put them in first gear and let go of the clutch fairly fast. The gasoline will stall, while the diesel will spin and run. With the diesel you can even start in third gear without touching the throttle.

Edit: Another thing you can try. Since the common rail controller tries to prevent the engine from stalling (at least on some engines), this is a good example of the low rpm torque. Just set it in first or second or even third, and the car will go steadily uphill, downhill and so on at 900 - 1000 rpm.

As Ross said torque does't do any thing horse power does! A diesel will typicly have a much better BSFC. Lee

[Bevan]

Wouldn't the Subaru flat 4 diesel be a good place to start when making small batches of experimental aircraft engines....similar to what Eggenfellner is doing now with the gas engines? I would think Egg is in the best position to transition to diesel. I'm curous to hear from Ross and Jan on this.

Bevan

[Garage Guy]

Originally Posted by leeschaumberg

As Ross said torque does't do any thing horse power does!

Well, but... horsepower is just torque times RPM (times a constant). If you want to deliver 200 hp to a propeller spinning at 2600 RPM, no way to do it without applying 404 ft-lb of torque to the propeller shaft.

So if a diesel could get you 400 ft-lb of torque at 2626 RPM crank speed, you would have a 200hp engine that wouldn't need a prop speed reduction unit, and that would be nice. But making it light and reliable seems to be easier said than done.