Diesel vs miller cycle

[PMD]

click on the link, Stanislavz. It IS an opposed piston design - as was Prof. Junkers' fabulous 205 and 207, IMHO the best piston engines ever to fly.

 

[PMD]

It's statements like these on new engine design websites that make me question if they even understand fundamental engine operation as they make a case for their advantages over existing designs.

The designer is an engineering prof. His consultant is a company that has been designing and building leMans winning engines and parts for the last 90 years.

The main feature of diesels in their early days was their "constant pressure" characteristics. Very low RPM, extremely long stroke stationary engines were able to exploit the ability to continue generating and increasing cylinder pressure so that the bulk of the area under the curve occurred when rod and crank angles were most favourable. Having peak pressure around TDC does very little work. BUT, you already knew/stated that. There is a school of design thought that still attempts to achieve these goals in high speed diesels - i.e. "rate shaping" the injection events. Again: one of those things the Otto cycle just can't accommodate.

Thus, why the statement "PCP is NOT generated at TDC".

 

[rv6ejguy]

click on the link, Stanislavz. It IS an opposed piston design - as was Prof. Junkers' fabulous 205 and 207, IMHO the best piston engines ever to fly.

Serviceability rates were low and they didn't become mainstream in front line fighter or bomber aircraft.

 

[rv6ejguy]

Thus, why the statement "PCP is NOT generated at TDC".

I think you missed the point of my post- you wouldn't want PCP developed at TDC in any piston engine. Have to have some crank pin angle to perform useful work.

 

[stanislavz]

click on the link, Stanislavz. It IS an opposed piston design - as was Prof. Junkers' fabulous 205 and 207, IMHO the best piston engines ever to fly.

It was an idea o mine, even get two honda gv100 to play with. Side valve opposed piston engine. Just different. Advantages have to be - half of combustion area, and better cylinder filling. And with 15 degree difference betwen crankshaft you get that continous combustion.

 

[PMD]

I think you missed the point of my post- you wouldn't want PCP developed at TDC in any piston engine. Have to have some crank pin angle to perform useful work.

yeah, I did miss it. Trying to catch a glimpse of the website while trying to get a few things done while we have the incredible weather (3 days left). Sorry for that. Problem might start with Dr. Fair being a fair bit British and using the language a bit differently from we colonials. I know what he meant as I converse with UK constantly.

 

[Martin W]

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Another main advantage of the opposed piston engine is that instead of heat and energy being wasted against a cylinder head it is directed to a piston and turned into rotating power..

This is ideal compared to typical turbo charged engine which capture a bit of the wasted energy in the exhaust pipe ... Much better to extract the energy inside the engine.

Drawbacks are the convoluted crankshafts and wobble-plates required to make it work.

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[rv6ejguy]

I like off beat, new engine designs and we have seen various types of opposed piston, rotary engines over the years but none have "made it". The theoretical gains are often quashed by the realities of making them work reliably and to be as durable as existing conventional engines with conventional crankshafts. The first company to prove they can do it all will probably find markets for their engines but have a mountain to overcome if they wish to push existing engines out of the big markets. Would take a couple decades if it ever happens.

 

[PMD]

I like off beat, new engine designs and we have seen various types of opposed piston, rotary engines over the years but none have "made it". The theoretical gains are often quashed by the realities of making them work reliably and to be as durable as existing conventional engines with conventional crankshafts. The first company to prove they can do it all will probably find markets for their engines but have a mountain to overcome if they wish to push existing engines out of the big markets. Would take a couple decades if it ever happens.

Opposed piston, opposed crankshaft (OPOC) engines long ago "made it" starting from the earliest days of the last century (the first car to reach 100 MPH around 1902 (Gobron Brille), the justifiably famous Junkers 205 and 207 flying through the 30s into the 40s, many marine, rail and military versions, the iconic Commer knocker and continuing today in submarines (Fairbanks-Morse). They are making a strong comeback for some very good reasons (see many articles on Achates Power). Who knows - you might even see one in a little airplane one day soon.

 

[rv6ejguy]

Opposed piston, opposed crankshaft (OPOC) engines long ago "made it" starting from the earliest days of the last century (the first car to reach 100 MPH around 1902 (Gobron Brille), the justifiably famous Junkers 205 and 207 flying through the 30s into the 40s, many marine, rail and military versions, the iconic Commer knocker and continuing today in submarines (Fairbanks-Morse). They are making a strong comeback for some very good reasons (see many articles on Achates Power). Who knows - you might even see one in a little airplane one day soon.

Sorry, have to disagree in the realm of numbers. OP engines don't even represent 1% of 1% of 1% of the total of engines ever made in this world. Not successful on numbers produced. Mainly used where weight didn't matter- marine, trains etc. The Jumo 205/207s were unreliable, high maintenance, finicky and unsuitable for their intended purpose. The early 205s were replaced by BMW radial SI engines in the JU86E because of their issues and dislike by the pilots who flew them. JU86A and Ds powered by the 205, were retired early. Historical fact. The JU86P was dealt with and quickly relegated into history.

Strong comeback? Not today. Show me where they are pushing existing engines out of their markets in any sort of numbers today. They are too wide/tall for existing aircraft and are likely to end up mainly in applications where their ancestors were used- marine, rail, trucking etc. where weight and volume aren't concerns.

OP engines do have some advantages over existing designs but I don't see them replacing conventional engines in any sort of numbers for at least a decade and that will only be in certain industries.

 

[PMD]

Junkers OPOC diesels were very much underdeveloped. Prof Hugo Junkers had a long history with OPOC engines before going into aviation and re-kindled his interest near the end of WWI. He had a flyable engine, but war not exactly going Germany's way in 1918. Somewhere along the line as I understand it, he was not "politically correct" for the government going into WWII, so they stripped him of his airframe company and pushed him into his engine supply role. The OPOC was dear to his heart, but when he decided to do the aviation version once again, he was not exactly rolling in favours/cash from the administration of the day and the 205 and later 207 got the short end of the stick, development wise. The layout has a lot going for it and with large engines the reason they were and are very useful is they can be considerably lighter for any given power - especially when going from one bank to three (Napier Deltic). Again, have a look at what Achates has been doing and you can see 270 HP can be very compact. You could even build one on a horizontal plane and have about the same footprint and weight as a Lycosaurus of similar power. The other special feature is fantastic balance with near-identical motion on each side (offset by 12-15 degree lead of exhaust crank).

 

[wsimpso1]

Who knows - you might even see one in a little airplane one day soon.

Gemini :: Aviation

www.geminidiesel.aero

 

[stanislavz]

The layout has a lot going for it and with large engines the reason they were and are very useful is they can be considerably lighter for any given power - especially when going from one bank to three (Napier Deltic).

Comparing three cylinder / six piston in inline 3 and delta :

One extra pair of gears, and three single throw crankshafts in place of two with there throws + three master conrod + three articulated ones (due to 15degree difference between opposite piston, one piston transfer up to 70% of power) ?

And big pain in machining that three bores at 120degree each separation. In inline one - it is just much easier

For 90 degree it is still easier (done by my, hobbiest machinist with machinis angle and parallel cylinder.. ). :

 

[rv6ejguy]

Gemini :: Aviation

www.geminidiesel.aero

Well the website was last updated in 2015 and last I heard, this project was dead for now anyway. This one looked pretty cool actually and like it was going to be produced by Superior. The project dragged out for many years from the Westlake development and soaked up a lot of cash. The new Chinese owners of Superior don't seem too interested in engines for Experimentals, especially so after the triple debacles of their 382, 400 EXP engines and the latest crankshaft nightmare. These have devastated their reputation and probably cost them millions.

 

[PMD]

Comparing three cylinder / six piston in inline 3 and delta :

One extra pair of gears, and three single throw crankshafts in place of two with there throws + three master conrod + three articulated ones (due to 15degree difference between opposite piston, one piston transfer up to 70% of power) ?

And big pain in machining that three bores at 120degree each separation. In inline one - it is just much easier

Deltics did not used master rods (would bugger up timing on other cylinder) but fork and blade type. Junkers was developing a 4 bank engine since they couldn't figure out how to do 3 bank. Napier did, and 3rd crank rotates opposite direction on Deltics.

I am also a very amateur machinist and 60 degrees is pretty easy.

 

[stanislavz]

I am also a very amateur machinist and 60 degrees is pretty easy.

Could you explain it in pm ?

 

[lelievre12]

There is a school of design thought that still attempts to achieve these goals in high speed diesels - i.e. "rate shaping" the injection events. Again: one of those things the Otto cycle just can't accommodate.

Modern auto Otto cycle now have rate shaping with timed injections preceding and post the ignition spark. Many are now direct injected like common rail Diesel to add this capability.

I know my own Audi A8 with a 4.2FSI (direct injection) engine has an insanely high 12.5:1 compression ratio. The ability to run such a high compression comes from the direct injection. It permits higher compression by suppressing detonation and promoting swirl. This from Audi literature:

" the directly injected fuel swirls intensely in the combustion chamber, which in turn cools the walls of the combustion chamber. This solves an old problem ....– the tendency toward early spontaneous ignition of the mixture at hot spots in the combustion chamber due to strong heat build-up at high compression, a phenomenon known as knocking. With its TFSI engines, Audi can achieve high compression ratios, to the benefit of combustion quality and thermodynamic efficiency – and consequently fuel efficiency. "



You can see the cam driven high pressure (~3000 psi) pumps feeding the common rail on each bank of the engine.

There are come aero direct injected engines on the market now (Viking Engines) however as the factory Honda ECU programming is replaced with a custom ECU and the compression ratio is only 10.5:1, I expect the 'advantages' of direct injection have been mostly lost.

 

[Martin W]

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Keep in mind that high compression itself doesn't really do anything .

It is the fact that more oxygen has been brought in to aid fuel burn.

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[Vigilant1]

Keep in mind that high compression itself doesn't really do anything .

It is the fact that more oxygen has been brought in to aid fuel burn.

For a given engine, a higher compression ratio doesn't mean there's more oxygen in the cylinder. The amount of O2 in the jug is set when the intake valve closes, before the compression really begins.
Higher CR produces higher efficiency because it allows the same temperature to be achieved with less fuel, and because the expanding gasses do work over a longer distance.

 

[Martin W]

For a given engine, a higher compression ratio doesn't mean there's more oxygen in the cylinder. The amount of O2 in the jug is set when the intake valve closes, before the compression really begins.
Higher CR produces higher efficiency because it allows the same temperature to be achieved with less fuel, and because the expanding gasses do work over a longer distance.

It means the oxygen is more concentrated (smaller space)

By the way ... gasoline will not burn without oxygen ... but oxygen will burn without gasoline.

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